Clinical manifestations

Presentation and course

The presenting signs and symptoms of Gaucher disease vary throughout childhood (181).

Common features include coagulopathy, hepatosplenomegaly, fatigue, bone pain, and frequent infections; frequent infections are common in childhood, whereas the others are typical of adolescence (114).

Based on clinical signs and symptoms, Gaucher disease has been divided into three subtypes: type 1, non-neuronopathic; type 2, acute neuronopathic; and type 3, chronic neuronopathic (99). All three types of Gaucher disease are caused by a deficiency of acid beta-glucocerebrosidase that results in the accumulation of glucocerebroside within the cells of the reticuloendothelial system. The principal differences between the subtypes are the presence of neurologic manifestations caused directly by glucocerebrosidase deficiency. Neuronopathic Gaucher disease is defined as the presence of neurologic involvement in a patient with biochemically proven Gaucher disease for which there is no explanation other than Gaucher disease (171). Because symptoms in all three types may begin in infancy, and the discrimination of subtypes depends on the evolution of clinical manifestations, especially nervous system involvement, this classification is more appropriate than one that includes reference to age of onset (ie, "adult," "infantile," and "juvenile" forms of Gaucher disease). The assignment of type, especially in children, should be made only after a careful examination for the presence and progression of related neurologic abnormalities or if a genotype study shows the presence of the N370S allele that typically is associated with Gaucher disease type 1.

Type 1: Chronic non-neuronopathic Gaucher disease. The age of onset and the severity of symptoms within this subtype vary widely and are not completely explained by the genotype. The diagnosis is frequently made later in life because of a lack of apparent clinical manifestations.

Although exceptions occur, painless splenomegaly with thrombocytopenia, anemia, and leukopenia are the usual initial signs. In many patients, these complications are not life threatening and may go unrecognized for years.

Visceral features. Although hepatomegaly is often noted at the time splenomegaly is observed, the liver may not become enlarged until later in the course of the disease. Moderate hepatic dysfunction is discerned by elevated levels of liver enzymes in serum. Histologically, all patients have some degree of hepatic fibrosis (90). Hepatic failure can occur in untreated patients. Portal hypertension leading to esophageal varices is a recognized complication, occurring in patients with severe type 1 and type 3 disease.

Hematological features. Patients may have platelet counts below 50,000 without an accompanying bleeding diathesis. Conversely, some patients with Gaucher disease who have a normal prothrombin time, partial thromboplastin time, and platelet count (greater than 100,000) may have abnormal clotting times, excessive bruising, and unexpected perioperative bleeding. This variability in hematological features necessitates an individualized approach that may require the administration of platelets and fresh frozen plasma prior to and after surgical procedures.

Skeletal features. Complex degenerative skeletal changes are the leading cause of disability in patients with type 1 disease; these may include marrow infiltration, bone pain, osteopenia, fragility fractures, and recurrent avascular osteonecrosis (15).

Some degree of osteopenia and osteolysis occurs in all patients, although the extent of bone disease is variable (184; 87).

Radiologic abnormalities are typically extensive.

Patients rarely have neither radiographic nor scintigraphic evidence of bone involvement. Most affected individuals have a substantial burden of bone disease that is progressive and leads to clinical presentation. Others have such severe involvement that they are confined to a wheelchair early in life because of pain, pathologic fractures, and/or skeletal instability. Many patients experience episodic pain in the hips, legs, back, and shoulders, referred to as "bone crises," lasting from days to months.

Splenectomy is associated with increased risks of osteonecrosis and fragility fractures (43). Controlling for gender, the hazard or risk of having a first osteonecrosis event after presentation of Gaucher disease is greater in patients who had splenectomy (hazard ratio of 3.32 [95% CI 1.74–5.00; p < 0.001]) (43).

Similarly, controlling for gender, the hazard or risk of having a first fragility fracture after presentation of Gaucher disease was also greater in patients who had splenectomy (hazard ratio of 2.83 [95% CI 1.33–5.99; p = 0.01]) (43).

The hazard or risk of having a first osteonecrosis event after presentation of Gaucher disease is significantly lower after starting enzyme replacement therapy (ERT) than before (hazard ratio of 0.20; 95% CI 0.11–0.38; p < 0.001) (43).

In a study by D'Amore and colleagues, the effects of substrate reduction therapy (SRT) and bone marrow transplantation (BMT) were difficult to assess due to the small sample size (nine and two patients received substrate reduction therapy or bone marrow transplantation as first treatment, respectively) (43). Of the two patients with bone marrow transplantation, one had their first episode of osteonecrosis after the procedure.

Pulmonary features. Pulmonary hypertension is a relatively common complication found in approximately a third of Gaucher disease patients, a direct consequence of acid beta-glucocerebrosidase storage that is worsened in splenectomized patients by the accumulation of storage cells in the lung. Fortunately, most cases are mild and subclinical with pulmonary artery pressures below 25 mmHg. The rare cases with severe pulmonary hypertension typically improve with enzyme therapy (103).

Ophthalmological abnormalities. Rare ophthalmological abnormalities may involve the vitreous, retina, cornea, uvea, and conjunctiva (50). Vitreous opacities are the most common and seen mostly in patients with Gaucher type 3 and sometimes require vitrectomy to ameliorate visual acuity.

Type 2: Acute neuronopathic Gaucher disease. In contrast to the variability seen within type 1 Gaucher disease, type 2 is somewhat more uniform in its presentation (60). It has no ethnic predilection. The average age of onset is three months, and the presenting sign is usually massive hepatosplenomegaly.

Neurologic abnormalities. Neurologic complications develop by 3 to 6 months of age. The presenting signs usually indicate involvement of cranial nerve nuclei and pyramidal tracts. A triad of clinical findings is very suggestive of the disease: (1) rigidity of the neck and trunk (which can progress to neck retroflexion and to frank opisthotonus); (2) bulbar signs (particularly poor suck and swallow reflexes with dysphagia and aspiration); and (3) oculomotor paresis (or bilateral fixed strabismus, particularly a convergent squint). These signs may be associated with microcephaly, trismus, hypertonia, hypokinesia, progressive spasticity, hyperreflexia, Babinski signs, and other pathologic reflexes. Seizures may occur. As neurologic deterioration proceeds, the child usually becomes apathetic and motionless. Death occurs either from apnea or aspiration pneumonia at an average age of nine months, with a range of 1 month to 2 years.

This form of Gaucher disease exhibits alterations in epidermal ultrastructure, which may provide an early and specific diagnostic tool (33).

Neonatal Gaucher disease. A rare form of Gaucher disease presents with hydrops fetalis and so-called "collodion skin" (congenital ichthyosis) (164; 157; 108).

A review of the clinical and hematological features of neonatal Gaucher disease found that this type is typically diagnosed based on postmortem pathological findings, with familial history suspected in 22% (115). Contrary to what is observed in type 2 disease, 22% of neonatal cases have dysmorphic features characterized by low-set ears, small nose with flat bridge, and anteverted nares. In a molecular analysis of 31 patients with this type of Gaucher disease, homozygosity for a recombinant allele that included the mutation p.Leu483Pro (formerly Leu444Pro) was associated with early lethality (157).

The characteristics of the clinical phenotype and the pregnancy are somewhat different between those newborns and fetuses affected by non-immune hydrops fetalis, in whom prematurity, fetal demise, and neonatal distress are prominent features. Hepatosplenomegaly is suspected prenatally in most cases with non-immune hydrops fetalis. Cardiomegaly has also been observed in some fetuses with Gaucher disease associated with non-immune hydrops fetalis (115).

Neonatal Gaucher disease should be distinguished from types 1 and 2. Type 1 cases are also diagnosed before 2 years of age and may have rapid progression in bone, liver, and spleen manifestations. CNS involvement must be established as an associated finding prior to making a diagnosis of type 2 disease.

Type 3: Subacute or chronic neuronopathic Gaucher disease. The clinical features of Gaucher disease type 3, apart from those referable to the nervous system, are common to Gaucher disease type 1. Patients may have a combination of systemic and neurologic abnormalities, including mild systemic disease with a progressive neurologic course, or severe skeletal and hematological abnormalities with a nonprogressive neurologic course. Hepatosplenomegaly is often the presenting feature rather than the neurologic abnormalities. In well-documented and biochemically proven cases, there is marked variation in age of onset and severity of organ involvement.

Neurologic abnormalities. A neurologic hallmark of Gaucher disease type 3 (and type 2) is marked slowing of horizontal saccades, sometimes labeled saccade initiation failure or oculomotor apraxia (171; 18). Vertical saccades are also slow, but to a lesser extent, and this deficit lags the slowing of horizontal saccades. Other oculomotor abnormalities include decreased gain of smooth pursuit and of the vestibulo-ocular reflex (30).

Other neurologic manifestations may include various degrees of intellectual disability or dementia, ataxia, dystonia (especially nuchal and facial), and, more rarely, tonic-clonic or complex partial seizures, or progressive myoclonic epilepsy.

Patients who are otherwise stable medically and neurologically, and who are on long-term enzyme replacement therapy, may develop partial complex seizures of temporal lobe origin that should not be confused with progressive myoclonic encephalopathy (07; 130).

There is significant variation in neurologic pattern among different ethnicities; for example, an oppositional behavioral abnormality is seen in Egyptian patients with Gaucher disease type 3 (02). Egyptian patients may also be more susceptible to sudden unexpected death, often with epilepsy, which has otherwise been rarely observed among Gaucher disease patients (01). Certain GBA1 mutations are associated with myoclonic encephalopathy (eg, N188S), whereas L444P/L444P rarely, if ever, is (130; 100).

A group of patients with a neurodegenerative course and cardiac valvular calcification and stenosis are homozygous for the mutation in the acid beta-glucocerebrosidase (GBA1) gene (p.Asp409His, now annotated as NM_000157.3: c.1342G > C; p.Asp448His) (37; 101). These cases have been classified as type 3c. The neurologic features occur at an older age but are no different than those of typical type 3 disease.

The neuronopathic phenotypes appear to be part of a continuum of disease severity, and some patients may have intermediate phenotypes between types 2 and 3 (20; 71). Other reported cases have novel features that may defy easy categorization (86; 101).

The Gaucher/Parkinson disease phenotype. GBA1 mutations are a strong genetic risk factor for synucleinopathies (Parkinson disease and Lewy body dementia) (140; 150). Patients with Gaucher disease and parkinsonism have typical Lewy bodies in hippocampal regions susceptible to Gaucher-related neurodegeneration (185).

A large international collaborative study of 16 centers found significantly higher odds of 5.4 for any GBA1 mutation among 5691 patients with Parkinson patients compared with 4898 controls (150). Moreover, patients with a GBA1 mutation presented earlier with Parkinson disease, were more likely to have affected relatives, and were more likely to have atypical clinical manifestations. Although the centers varied in their ability to detect GBA1 mutations, all 16 centers could detect two particular GBA mutations: L444P and N370S. Among Ashkenazi Jewish subjects, either mutation was found in 15% of patients (n=780) and 3% of controls (n=387), whereas among non-Ashkenazi Jewish subjects, either mutation was found in 3% of patients and less than 1% of controls. GBA was fully sequenced for 1883 non-Ashkenazi Jewish patients, and mutations were identified in 7%, demonstrating that limited mutation screening may identify less than half of the mutant alleles.

A prospective cohort study of patients with Gaucher disease and GBA1-mutation carriers found that, over a 2-year period, GBA-mutation-positive individuals show deterioration in clinical markers consistent with the prodrome of Parkinson disease (16).

Even patients with Gaucher disease type 1 (ie, chronic non-neuronopathic Gaucher disease) are at a higher risk of developing Parkinson disease (31).

Although loss of function of acid β-glucocerebrosidase caused by GBA1 mutations presumably contributes to the increased risk of synucleinopathies, the pathophysiological mechanism is not clear (65). Even in the absence of GBA1 mutations, patients with Parkinson disease have lower glucocerebrosidase activity in peripheral blood (06) and in the substantia nigra (69; 36), suggesting that loss of glucocerebrosidase function, whether due to genetic or nongenetic causes, contributes to the pathogenesis of Parkinson disease. Proposed mechanisms include (1) apoptosis induced by a bidirectional pathogenic loop between glucocerebrosidase deficiency and α-synuclein (113); and (2) endoplasmic reticulum stress in susceptible neurons caused by glucocerebrosidase deficiency (110). In addition, patients with Parkinson disease carry a significant burden of other rare lysosomal storage disorder gene variants, implying that multiple genetic hits may act in combination to degrade lysosomal function, thereby enhancing Parkinson disease susceptibility (138).

Prognosis and complications

Death in Gaucher type 1 patients who have never received enzyme replacement therapy occurred at a median age of 66 years and was often caused by liver disease, septicemia, malignancies, and suicide (179).

So-called "Gaucher cells" are macrophages that become full of unprocessed glucocerebroside in patients with Gaucher disease. Gaucher cells accumulate primarily in the spleen, liver, and bone marrow, causing organ inflammation and dysfunction.

Visceral involvement.

The lymphoreticular system. Painless splenomegaly is usually the earliest sign in all types of Gaucher disease. Even when the spleen is not palpable by physical examination, it usually can be demonstrated to be enlarged by diagnostic imaging techniques. The rate of enlargement of the spleen is helpful in judging the rate of progression of the disease. The preferred modality for measuring organ volume is volumetric MRI (34). The rate of splenic enlargement is often consistent for each case. Changes in that rate have been associated with malignancy or other intercurrent disease. Spontaneous rupture of the spleen is uncommon. Most cases develop hypersplenism manifested by pancytopenia and a bleeding diathesis. Red cell and platelet survival time is shortened. Splenectomy should be limited to those patients having severe bleeding diathesis, high-output cardiac failure, or mechanical interference of bowel, diaphragm, or kidney. The spleen is enlarged and firm, with pathological findings of infarcts, fibrosis, and distortion of the splenic architecture. Splenic infarction can cause abdominal pain and should be considered in the differential diagnosis of an acute abdomen in patients with Gaucher disease. The red pulp of the spleen is replaced by white collections of Gaucher cells. The surface and body of the spleen may contain dark-purple nodules that are foci of extramedullary hematopoiesis. Splenectomy is almost always followed by correction of cardiac and hematologic abnormalities. However, splenectomy poses a risk for severe, overwhelming sepsis that can be fatal within a day. For these reasons enzyme replacement therapy is the treatment of choice.

In some patients, lymph nodes may be enlarged and contain Gaucher cells. The thymus, Peyer patches in the intestine, and the pharyngeal tonsils are frequently affected.

Liver. Despite the frequent occurrence of hepatomegaly and abnormal liver function tests in Gaucher disease, hepatic failure occurs infrequently. In contrast to other lipidoses (eg, Niemann-Pick disease), hepatocytes are not involved in storage. Gaucher cells are seen within the sinusoids. In the more severely affected cases with cirrhosis, fibrosis distorts the architecture, forming small regenerating nodules that are infiltrated by Gaucher cells. Marked portal hypertension and consequent complications (eg, ascites and esophageal varices) are common in severe disease. The enlarged liver may contain sites of extramedullary hematopoiesis. Recurrent bleeding from esophageal varices has been successfully treated with a combination of aggressive medical management and sclerotherapy. Jaundice is a serious adverse prognostic sign in this disease and represents either intercurrent infectious hepatitis or hepatic failure. In cases with liver involvement, early intervention with enzyme replacement therapy is indicated. Hepatic transplantation is required in patients with end-stage liver disease.

Skeletal manifestations. In Gaucher patients types 1 and 3, skeletal abnormalities on x-ray occur in 50% to 75% of patients and include expansion of the cortex in the distal femur (Erlenmeyer flask deformity), fractures, and other abnormalities of the acetabulum and head and neck of the femur. Prosthetic hip replacement is often necessary for patients to remain ambulatory. In some cases, the hip lesions may be confused with Legg-Calve-Perthes disease. Destruction of vertebral bodies may produce collapse, gibbus formation, and spinal cord or nerve root dysfunction.

Bone pain and bone crises. Bone infarction causes so-called "bone crises." This process affects the femoral heads and distal femur more frequently than other bones. Episodes are usually self-limited and typically last approximately 2 weeks but may be protracted over many months. During the first days of crisis, patients may require hospitalization to control pain. Bed rest is always indicated until the episode is completely resolved. Typically, a region of osteosclerosis, bone deformity, or pathologic fracture develops. Although the infarcted region can become secondarily infected, producing osteomyelitis, bone crises should be treated conservatively unless infection is highly suspected. Instrumentation of infarcted bone should be avoided unless clearly indicated because instrumentation may cause secondary infection and development of a sinus tract. Gallium scans may be helpful in distinguishing infarction from osteomyelitis, whereas x-rays and technetium scans are usually not helpful.

Adequate calcium intake should be maintained. If urinary calcium is low, the diet should be fortified with calcium and vitamin D. In addition, the storage of glucocerebroside in tissue macrophages may alter the generation of competent osteoclasts and result in a failure to maintain a healthy bone matrix.

A rare occurrence in type 1 disease is the extraosseous extension of Gaucher cells to the surrounding soft tissues, a condition that may mimic a malignant process (133). Because the incidence of several malignancies, including lymphoproliferative disorders, is increased in Gaucher disease (148), a biopsy may be required for an accurate differential diagnosis.

The hypothesis that bone crises are the result of progressive vascular compromise produced directly by occlusion of vessels by Gaucher cells is not supported by scintigraphic or histologic studies. In fact, perfusion scans of bones are often enhanced. Vascular occlusion by Gaucher cells would not explain the combination of osteopenia, osteonecrosis, and osteosclerosis seen in the disorder. Other vascular complications, such as premature stroke, myocardial infarction, or renal failure, are not features of the disease, making a simple vascular occlusive process a less attractive explanation of the skeletal involvement. Metabolic and endocrinologic studies suggest an imbalance in calcium homeostasis; however, the entire skeleton is not affected uniformly. On the contrary, the lesions consist of collections of Gaucher cells scattered throughout the bone substance. Bone complications probably result from a toxic process around these foci, which then leads secondarily to edema, vascular compromise, and infarction. One study reported a correlation between the severity of type 1 disease and serum levels of macrophage derived cytokines. This correlation suggests that a cytokine imbalance may have a role in the pathophysiology of the bone lesions (81). From existing information, it is not likely that infarction is incited by vascular occlusion by Gaucher cells, but rather by the vascular compromise alluded to above.

Progressive kyphosis (sometimes with scoliosis) without any vertebral body abnormality is commonly observed in patients with Gaucher type 3. It is very rarely seen in patients with Gaucher type 1. Correction and stabilization of the kyphosis is required, and Harrington rods are often placed towards the end of the growth period (late adolescence). The mechanism of this phenomenon is unknown, but studies in a zebrafish model suggest a developmental abnormality of bone (193).

Growth retardation and delay in skeletal maturation occurs in children but is normalized by enzyme replacement therapy (93).

Other organic systems. In the kidney, Gaucher cells can be found in the cortex, medulla, or glomeruli. Renal function in Gaucher patients is usually normal. In a few cases, proteinuria or hematuria has been reported. The cause of these signs is uncertain, but they have been attributed to the infiltration of Gaucher cells.

Focal collections of Gaucher cells are found within Peyer patches and the lamina propria of the gastrointestinal tract. Patients may complain of bloating, cramps, and diarrhea. Abnormal uptake of calcium may be one of several factors affecting bone in the disorder.

The cherry-red spot that appears in the macula in some sphingolipidoses and mucolipidoses does not occur in Gaucher disease; however, white patches containing Gaucher cells have been seen in the vitreous and retina of some Gaucher type 3 patients, and they can develop while the patient is on enzyme replacement therapy (142). A case has been reported in which loss of vision due to uveitis in type 1 disease could be reversed by enzyme replacement therapy (21). There have been many reports suggesting that patients with Gaucher disease are at increased risk of developing malignancies, particularly hematopoietic tumors. A large study of 1525 patients found 2- to 3-fold risks of non-Hodgkin lymphoma, malignant melanoma, and pancreatic cancer in patients with Gaucher disease but no significant association between Gaucher disease and cancer in general or with other specific malignancies, such as multiple myeloma (102).

Because of the different clinical courses of patients with Gaucher disease, researchers have tried to develop a laboratory test that would both discriminate subtype and be useful in determining prognosis. Genotype analysis gives some guidance in this area but is not completely satisfactory. Most individuals with even one p.Asn409Ser allele do not have neurologic involvement, whereas almost all patients with the p.Leu483Pro allele have neurologic involvement. Differences in epidermal cells may provide a means to differentiate type 2 neuronopathic Gaucher disease from types 1 and 3 Gaucher disease (61; 149). Disorganized lamellar membranes in the epidermal stratum corneum are present in type 2 patients but not in type 3 patients (83; 33).

Garvey and colleagues observed that the amplitude of stretch-evoked somatosensory evoked potentials in patients with type 3 Gaucher disease positively correlated with the degree of cognitive deficit and, therefore, with neurologic disease burden (66). It is possible that measurement of somatosensory evoked potentials may be useful to monitor the response of future therapies aimed at correcting the neuropathology of Gaucher disease.

Clinical vignette

Case 1. Ashley was a 5-year-old who experienced a bladder infection that became obvious when she suddenly began to have toileting accidents. The infection was treated with sulfa drugs and cleared up initially, but reoccurred six months later. When the primary care physician examined Ashley, she found splenomegaly and a low platelet count. Ashley was diagnosed with a platelet disorder and treated with prednisone. Several days into the steroid treatment, her platelet count was found to be very low, and a bone marrow biopsy was performed. The biopsy showed Gaucher cells. Follow-up testing showed enzyme assay of 2 (NR 12-17) and genotype p.Asn409Ser/p.Leu483Pro.

Follow up and discussion. The p.Asn409Ser mutation is only associated with type 1 Gaucher disease. When p.Asn409Ser occurs with a second different mutation like p.Leu483Pro, the disease sometimes presents earlier in life. Ashley had two brothers, and because neither of them had symptoms of Gaucher disease, they did not have genetic testing. When a genetic condition, such as Gaucher disease, is known in a family, the brothers and sisters of a person with the condition are at higher risk for having the same condition and for being carriers. For this reason, it is important for parents to educate all of the children in their family about the condition. Many parents want and need help with this process. Genetic counseling is recommended for each family member when he or she becomes an adult (or before if indicated).

Case 2. At four months of age, Justin's parents noted "something not quite right with his eye movement" and mentioned this to their pediatrician at Justin's six-month well baby visit.

At this visit, the pediatrician noticed splenomegaly and heard a report from the parents that Justin had a choking episode during which he became cyanotic and was taken to the ER. Developmentally, Justin was meeting expected milestones and at six months was sitting, reaching for and grasping objects, and responding to his name. The pediatrician ordered a panel of metabolic tests including acid beta-glucocerebrosidase.

Follow up and discussion. Justin was treated with enzyme replacement therapy at a dose of 120 U/kg every 2 weeks. He did well for several months; he was walking well and saying a few words. Eventually, he failed to progress and began having more frequent choking episodes. Justin died during the night at age 22 months.

Case 3. Christopher, a 20-month-old Caucasian male, presented with the following history: at his 12-month well-child checkup, the pediatrician noted hepatosplenomegaly. Chris's hepatosplenomegaly was unchanged on follow-up at 15 months, and at 18 months he was seen because of irritability and fever that had persisted for approximately 1 month. At that time, increased hepatosplenomegaly and cervical lymphadenopathy were noted. Christopher was admitted to the hospital, at which time he also had hoarseness and diarrhea. He was evaluated for possible malignancy with results that included the following: chest x-ray, normal; abdominal x-ray, HSM; fragmented right femoral epiphysis; cervical lymph node biopsy, normal; bone marrow aspirate, Gaucher cells.

Follow up and discussion. Over the next few months, it became apparent from examination and parents’ observations that Christopher had horizontal supranuclear gaze palsy with compensatory head thrusting. Treatment with enzyme replacement therapy was initiated at a dose of 60 U/kg every 2 weeks. Years later at age 11 years, Christopher had no progression of the slow looping saccadic eye movements and no organomegaly or obvious bone disease; he played baseball and football, was an A and B student, and had the lead in the school play during the previous academic year.

Biological basis

Etiology and pathogenesis

Gaucher disease in the spectrum of glycosphingolipid diseases. Glycosphingolipids are composed of a sphingosine, a fatty acid chain (these two forming a ceramide) and a carbohydrate moiety (17).

The fatty acid attached to the sphingosine may vary in chain length, degree of unsaturation, and/or hydroxylation (17). Glycosphingolipids include ceramide, cerebrosides (galactosylceramide and glucosylceramide), lactosylceramides, and gangliosides.

The biosynthetic and degradation pathways of glycosphingolipids in the brain are complex and incorporate metabolic loci for Fabry disease, metachromatic leukodystrophy, Krabbe disease, Gaucher disease, Sandhoff disease, Sialidosis, Tay Sachs disease/Sandhoff disease/GM2 gangliosidosis AB variant, and GM1 gangliosidosis.

Glucosylceramides and galactosylceramides are hydrolyzed respectively by glucocerebrosidase (also named glucosylceramidase; the GBA gene encoding lysosomal glucocerebrosidase is associated with Gaucher disease) and galactosylceramidase (encoded by the GALC gene; that deficiency causes Krabbe disease) to regenerate ceramides.

Ceramides are further deacetylated to sphingosines that can be broken down or recycled for sphingolipid synthesis by the salvage pathway.

Genetics of Gaucher disease. Gaucher disease is a lysosomal storage disease that is transmitted as an autosomal recessive trait.

Gaucher disease results from homozygous or compound heterozygous mutations in the gene encoding acid beta-glucosidase (GBA; OMIM 606463) on chromosome 1q22.

The wide spectrum of clinical severity in patients with Gaucher disease, but in particular in Gaucher type 3 patients, is often explained by the presence of genetic modifiers. An interesting effort used animal models to identify such genes (98). The authors identify a number of variants that modify the lifespan of the affected mouse. The most interesting was Grin2b, which codes for NR2B subunit of the N-methyl-D-aspartate receptor. Pharmacological blocking of the N-methyl-D-aspartate receptor using memantine led to a significant prolongation of the Gaucher mouse model (98). Effort to identify genetic modifiers in Gaucher type 3 patients is underway.

The precise genetic and biochemical reasons for the manifestations of the disease and the phenotypic differences among patients are not completely understood. There is little doubt that additional genetic modifiers of Gaucher disease cause marked differences in the presentation of signs and symptoms related to bone marrow, liver, spleen, lung, bone, brain, and other systems involved in the disease (70).

Acid beta-glucocerebrosidase. In addition to the 5' and 3' untranslated regions, the cDNA of the GBA gene contains 1548 base pairs encoding human acid beta-glucocerebrosidase. The molecular weight of acid beta-glucocerebrosidase calculated from the 536 amino acids deduced from the cDNA sequence is 59,716, which is in good agreement with that estimated by SDS-polyacrylamide of the product of in vitro translation of human placental mRNA (57; 56). Five potential glycosylation sites (Asp-X-Ser/Thr) were identified. Quantitative carbohydrate analysis indicates that only four are glycosylated (162). This has been confirmed by direct analysis of the amino acid sequence (B Martin, unpublished data). In addition to the amino acid sequence of the structural protein, the reading frame of the cDNA codes for 39 additional amino acids upstream of the amino terminus. This signal polypeptide contains a hydrophobic core, consisting of Gly-Leu-Leu-Leu-Leu and, in addition, has glycine at the peptidase cleavage site. These features are consistent with the properties of signal peptides of other translocated proteins. Furthermore, the cDNA sequence confirms the presence of a 2-kd signal sequence identified by pulse-labeling studies. More than 300 mutations of the GBA gene have been described as a cause of Gaucher disease, including missense or nonsense mutations as well as splice mutations, deletions, and complex gene rearrangements.

Biochemical abnormalities in Gaucher disease.

Metabolic block and storage substance. With all lysosomal storage diseases, an enzyme deficiency results in the accumulation of its substrate in the lysosome. In particular, Gaucher disease is caused by a deficiency of glucocerebrosidase (GCase; or β-glucosidase). β-Glucocerebrosidase, a member of the glycoside hydrolase family 30, has three distinct domains (I-III) (106).

Domain I forms a three-stranded anti-parallel β-sheet; this domain contains two disulfide bridges that are necessary for correct folding, and a glycosylated residue (Asn19) that is required for catalytic activity in vivo. Domain II consists of two β-sheets that resemble an immunoglobulin fold. Domain III is homologous to a TIM barrel (triose-phosphate isomerase barrel, or alpha/beta barrel), a highly conserved domain among glycoside hydrolases, consisting of 8 α-helices and 8 parallel β-strands that alternate along the peptide backbone.

Domain III harbors the active site, which binds the substrate glucocerebroside in proximity to the catalytic residues E340 and E235.

The deficiency of glucocerebrosidase in Gaucher disease leads to accumulation of glucosylceramide (Glc-Cer; also called glucocerebroside), as well as glucosylsphingosine (lyso-Gb1) (156; 03; 137).

Glucosylsphingosine concentrations are markedly elevated in the brains of type 2 and 3 Gaucher disease patients, and to a lesser extent in type 1 Gaucher disease patients (123; 129; 48).

Glucocerebroside is a compound of ceramide and glucose. The glucose moiety is esterified to the C-1 of ceramide in a beta-glucosidic linkage. The compound is similar in structure and properties to the group of sugar-containing lipids isolated from the brain by Thudichum (165; 166).

Cerebrosides are composed of ceramide esterified to a variety of different substituents at C-1. This carbon may participate in reactions with phosphorylcholine to produce the sphingomyelins, an unsubstituted monosaccharide or oligosaccharide to produce the neutral glycosphingolipids, or an oligosaccharide containing one to four molecules of sialic acid to produce the gangliosides. The common unit among these compounds is ceramide. Ceramide is derived from a long-chain base named sphingosine (D(+)-erythro-1,3-dihydroxy-2-amino-4-transoctadecene, or C18 sphingosine). This lipid is joined by an amide bond at C-2 to a long-chain fatty acid to form ceramide. The fatty acid chain length varies. In general, the neutral glycosphingolipids and sphingomyelins contain C20 and C24 fatty acids, whereas the gangliosides contain C18 fatty acids. It is from sphingosine that the group of disorders of lipid catabolism obtains its name (ie, sphingolipidoses) because the accumulating lipid compounds are derived from it.

Glucocerebroside is at the end of the glycosphingolipid catabolic pathway. The higher glycosphingolipids and gangliosides are degraded in a stepwise fashion by specific acid hydrolases, resulting in the formation of glucocerebroside, which is normally degraded to ceramide and glucose by acid beta-glucocerebrosidase. The compounds that contribute to the pool of glucocerebroside in peripheral organs are globoside, globotriose, and lactosylceramide. These are derived from the degradation of membranes, the major source of which is white blood cells (94).

The glucocerebroside found in spleen, liver, kidney, plasma, and red cells contains fatty acids with chain length of approximately C22 to C24 (60). The glucocerebroside in the brains of patients with type 2 disease is composed primarily of C18 (stearic acid) (159; 124). This conclusion has been confirmed and extended to type 3 cases (123; 39). These data have been interpreted to mean that the glucocerebroside accumulating in brain derives from gangliosides within the brain itself. This is consistent with the known fatty acid content of gangliosides. Some of the glucocerebroside, in certain type 3 cases, may be derived from sources outside the central nervous system. This may have important consequences because levels of plasma and tissue glucocerebroside increase following splenectomy in Norrbottnian cases (ie, a neuronopathic form of Gaucher disease that is relatively prevalent in Norrbotten, Sweden caused by a single mutation in exon 10 of the glucocerebrosidase gene) (124; 44; 146; 167).

Although the level of plasma glucocerebroside has been shown to increase following splenectomy in some Norrbottnian cases, this has not been reported in cases of other subtypes of the disease.

Glucosylceramide forms fibrillar aggregates that accumulate in macrophages forming so-called Gaucher cells that have a characteristic "crumpled tissue paper" appearance (23; 156). Gaucher cells result from the transformation of macrophage cells and correspond to a distinct "M2" subpopulation from an alternative differentiation pathway. The M2 subpopulation has anti-inflammatory, immunomodulatory, and tissue repair properties (156).

Alternative pathway. As was identified in a mouse model of glucocerebrosidase deficiency, glucosylceramide is also the substrate of an alternative pathway, which is favored in cases of glucocerebrosidase deficiency (ie, Gaucher disease) (118; 156).

In the alternate pathway, a ceramidase transforms glucosylceramide into glucosylsphingosine (or lyso-glucosylceramide), which then diffuses into fluids due to its reduced hydrophobicity. The enzymatic deficit of glucocerebrosidase is multifactorial, resulting from intrinsic enzymatic dysfunction as well as abnormalities during transport and delivery of the enzyme to the lysosomes. In particular, enzyme misfolding during passage through the endoplasmic reticulum can lead to premature degradation by the proteasome (141; 187; 156).

Other biochemical abnormalities in Gaucher disease. Levels of acid phosphatase, angiotensin-converting enzyme, lysosomal hydrolases, lysozyme, and immunoglobulins are elevated in the plasma of Gaucher disease patients (168; 126; 107; 177; 152; 119; 85; 139). Chitotriosidase is a particularly sensitive indicator of macrophage storage in patients with Gaucher disease (82).

The severity of neurologic symptoms is associated with glycoprotein nonmetastatic B levels (194). The same was found in a mouse model of Gaucher disease. GPNMB may prove to be a useful marker to quantify neuropathology in Gaucher disease patients and treatment efficacy in future treatment trials (194).

Organ involvement in Gaucher disease.

Clotting cascade. Because of liver involvement, patients may have prolonged partial thromboplastin, prothrombin, and bleeding times. In addition, increased amounts of plasma glucocerebroside may interfere with the clotting cascade (22).

Reticuloendothelial system. A study of the gene expression profile in Gaucher disease showed enhanced expression of genes associated with inflammatory reactions in the affected spleen (121). In particular, the transcript abundance of the cDNAs representing cysteine proteinases (cathepsins B, K, and S) was greatly increased. These proteins are known to participate in tissue modeling, antigen presentation, and in the case of cathepsin K, bone matrix destruction. Glycolipid storage in the cells of the reticuloendothelial tissue may induce an inflammatory response characterized by the recruitment of proinflammatory cytokines and other cells of the immune system. In turn, extensive tissue damage would ensue, particularly in the liver, spleen, and bones.

Brain involvement in Gaucher disease type 1. Type 1 Gaucher disease patients do not have clinical symptoms or signs referable to the nervous system. In this group, anatomic and biochemical examinations of the brain have been infrequent. Hippocampal gliosis has been found in brains of otherwise neurologically normal patients with Gaucher disease type 1 (185).

Pathologic features of neuronopathic phenotypes. In Gaucher disease types 1, 2, and 3, perivascular Gaucher cells have been observed within the Virchow-Robin spaces (46; 185).

In type 2 patients' brains, free Gaucher cells have been demonstrated within the parenchyma accompanied by gliosis and microglial nodules. These changes are present but much less frequently in type 3 patients' brains. Neuronal storage of lipid has been suggested in several reports, but this has not been confirmed ultrastructurally in any case of Gaucher disease. In type 2 disease, neuronophagia and neuronal cell death in the deeper layers of the cortex, thalamus, basal ganglia, brainstem nuclei, cerebellum, and spinal cord have been reported. Variable degrees of demyelination have been described in brains of type 2 patients. From the available information, one would have to conclude that the accumulation of acid beta-glucocerebrosidase in the brain produces dysfunction in surrounding cells long before discrete pathologic changes are seen (95). Extensive study of 14 brains from type 1 (including patients with parkinsonism), 2, and 3 Gaucher disease showed common neuropathologic findings in all forms of Gaucher disease (185). Unique patterns of gliosis and neuronal loss involving the hippocampal CA2-4 regions and layer 4b of the calcarine cortex were identified. Although these findings were common to all three Gaucher disease phenotypes, the extent of the changes varied depending on the severity of disease (185). Cerebral cortical layers 3 and 5, hippocampal CA2-4, and layer 4b were involved in all Gaucher disease patients. Neuronal loss predominated in both type 2 and type 3 patients with progressive myoclonic encephalopathy, whereas patients classified as having type 1 Gaucher disease had only astrogliosis. Adjacent regions and lamina, including hippocampal CA1 and calcarine lamina 4a and 4c were spared of pathology, highlighting the specificity of the vulnerability of selective neurons. Elevated acid beta-glucocerebrosidase expression by immunohistochemistry was found in CA2-4. Hippocampal 45Ca(2+) uptake autoradiography in rat brain was performed, demonstrating that hippocampal CA2-4 neurons, rather than CA1 neurons, were calcium-induced calcium-release-sensitive (CICR-sensitive). These findings match biochemical studies linking elevated glucosylceramide levels to sensitization of CA2-4 RyaR receptors and 300% potentiation of neuronal CICR sensitivity (132). In two patients with type 1 Gaucher disease and parkinsonism, numerous synuclein positive inclusions, similar to brainstem-type Lewy bodies found in Parkinson disease, were also found in hippocampal CA2-4 neurons. These findings argue for a common cytotoxic mechanism linking aberrant acid beta-glucocerebrosidase activity, neuronal cytotoxicity, and cytotoxic Lewy body formation in Gaucher disease (185). Progression of the neuropathology is very predictable in the mouse model including microglial activation and astrogliosis were spatially and temporally correlated with selective neuron loss (59). Glucocerebroside accumulates to a similar extent in most brain areas, and upon reaching a certain threshold of accumulation inflammation and neurodegeneration is initiated in susceptible brain areas (58). The same group found that receptor interacting protein (RIP) kinases-mediated necrosis plays a major role in inducing inflammation and neuronal death in neuronopathic Gaucher disease (173). Ripk3 deficiency (double knockout mice) dramatically improved the clinical course of Gaucher disease mice with increased survival and motor coordination. Therefore, Ripk3 is a new therapeutic target for neuronopathic Gaucher disease (173). Using the mouse model for the most severe form of neuronopathic Gaucher, it was shown that altered lysosomal localization and cytoskeleton disruption precede the neuroinflammatory pathways, axonal dystrophy, and neuronal loss previously characterized in neuronal forms of Gaucher disease (195).

Glucosylsphingosine has been implicated in toxic damage to neural cells. This molecule has been shown to accumulate in the cerebral and cerebellar cortex of patients with type 2 and 3 Gaucher disease (123; 123; 124) and in Gaucher mice. The highest levels of this metabolite were found in brain tissue from two fetuses with hydrops fetalis (129). Studies have found markedly elevated levels in type 1, type 2, and type 3 disease (49). Glycosphingolipids, including glucosylsphingosine, inhibit protein kinase C, mitochondrial cytochrome c oxidase, and CTP:phosphocholine cytidylyltransferase, which has been postulated to interfere with signal transduction and cellular differentiation (77).

Studies on the pathogenesis of neuropathic Gaucher disease suggest that defective calcium homeostasis is a mechanism responsible for neuropathophysiology in acute neuronopathic Gaucher disease (132). Agonist-induced calcium release via the ryanodine receptor was significantly enhanced in brain microsomes from the acute neuronopathic form of Gaucher disease (type 2) compared to the subacute (type 3) and the non-neuronopathic (type 1) forms and controls and correlated with levels of GlcCer accumulation (132). The precise mechanism by which glucocerebroside enhances calcium release via the ryanodine receptor is not known, but these findings suggest the use of certain calcium channel blockers in neuronopathic Gaucher disease. Altered expression and distribution of cathepsins in the brain of the neuronopathic Gaucher mouse suggests that inflammation plays a role in the disease and, therefore, may be a target for therapeutic intervention (172). Elevation of GBA2, a nonlysosomal glucocerebrosidase, may play a role in the disease (32).

Relationship between glucocerebrosidase and synucleinopathies. Normally, glucocerebrosidase interacts with its substrate glucosylceramide as well as monomers of α-synuclein in lysosomes, facilitating the breakdown of both at acidic pH (156).

A decrease in the activity of glucocerebrosidase (eg, due to mutations) causes a gradual buildup of glucosylceramide and also causes a slowdown of α-synuclein degradation with the resulting formation of α-synuclein oligomers and fibrils (42; 188; 122).

Glucosylceramide stabilizes the α-synuclein oligomers (113), which are able to bind to the mutated glucocerebrosidase molecules and inhibit the enzymatic activity of glucocerebrosidase, further decreasing enzyme activity (113; 191; 190). These impaired lysosomes show impaired chaperone-mediated autophagy and autophagosome fusion. This results in an increased accumulation of α-synuclein in the cytoplasm, forming insoluble aggregates that ultimately form Lewy bodies. These aggregates block trafficking of glucocerebrosidase from the endoplasmic reticulum (ER) to the Golgi (151). Mutant glucocerebrosidase is retained in the endoplasmic reticulum (ER), which causes ER stress and evokes the ER stress response ("unfolded protein response") (84). Saposin C can have a modulating effect on this by binding to glucocerebrosidase (189; 74).

Epidemiology

All types of Gaucher disease are inherited as an autosomal recessive trait. The collective subtypes of Gaucher disease constitute one of the most prevalent forms of the sphingolipidoses and the most common lysosomal storage disorder. Gaucher disease type 1, the non-neuronopathic form, occurs with an incidence of about 1 in 40,000 to 60,000 in the general population and 1 in 500 to 1000 among Ashkenazi Jews. Patients with Gaucher type 3 disease (chronic neuronopathic Gaucher disease) constitute about 5% of the population of Gaucher patients in the United States and in Europe, with an estimated incidence of about 1 out of 100,000. However, reports suggest the neuronopathic form of Gaucher disease predominates in countries like China, Japan, Korea, Taiwan, Egypt, and Syria (175; 38; 161; 91; 51; 05; 73). The reason is that the N370S mutation is common in European countries but almost absent in Northern Africa and Asia, whereas the L444P mutation is particularly common in the East (175; 38; 161; 91; 51; 05; 73).

Prevention

Although it is more frequent among Eastern European Jews, Gaucher disease type 1 is usually less severe in this group. It is more severe among blacks. This generalization is helpful in counseling, but there are exceptions. Therefore, it is important to know the types of complications of the disease that have occurred in a family. In general, the severity of the disorder tends to be similar among siblings. A single exceptional family has been reported in which one type 1 and one type 2 case occurred in full siblings.

Implications for genetic counseling. Testing and genetic counseling of immediate family members of affected patients are recommended. The differences in clinical subtypes and variability of some forms of the disease should be pointed out. Because of the high carrier frequency among the Ashkenazi, wide-scale carrier testing has been suggested. Because the disease among this group is not uniformly catastrophic (in fact, many cases do not come to medical attention until late in life), this kind of testing, if desired, should be done within a system providing careful counseling that includes adequate information about the disorder and an explanation of the limitations of carrier detection. More than 300 mutations have been described in the GBA1 gene, four account for the majority of the nucleotide alterations. In the Ashkenazi Jewish population, the p.Asn409Ser mutant allele accounts for about 75% of the abnormal genes (73). In this ethnic group, four alleles (p.Asn409Ser [formerly described as Asn370Ser], c.84insG [formerly described as 84GG], p.Leu483Pro [formerly described as Leu444Pro], and IVS2+1G> A) account for more than 95% of the chromosomes, thus, permitting population screening (12). In the general population, many "private" alleles or mutations confined to a single kindred are found. Testing for the four mutations listed above results in identification of fewer than 50% of the cases. Thus, except for the Eastern European Jewish group, population screening is not appropriate for this disease with currently available technology.

Prenatal diagnosis. Affected fetuses with any form of Gaucher disease can be diagnosed prenatally by enzymatic assay of cultured amniocytes or chorionic villi. This is particularly important in families in whom the neuronopathic subtypes have occurred. Because of the difficulty of the assay for carriers (45) and the lack of appropriate control data, prenatal carrier detection is unreliable by enzyme assay. However, molecular genetic methods permit this identification without difficulty in families where the mutations are known.

Differential diagnosis

Confusing conditions

Other childhood diseases with signs and symptoms overlapping with Gaucher disease include the following (181):

Painless splenomegaly is the most common initial presenting sign in Gaucher disease type 1. Lymphoproliferative malignancies are the chief concern in the diagnostic evaluation of patients presenting in this manner. The results of bone marrow biopsies may suggest the presence of Gaucher cells; however, the diagnosis should be made by enzymatic assay of acid beta-glucocerebrosidase in leukocytes or fibroblast cultures. If a patient is Jewish, Gaucher disease should be considered and an enzyme assay performed. Results can usually be obtained within several days. A disease virtually identical to Gaucher can rarely be caused by a deficiency in saposin c, the protein that activates glucocerebrosidase (170; 92). In this case, glucocerebrosidase activity is normal but the patient has clinical features resembling Gaucher disease. In infants with hepatosplenomegaly and a neurodegenerative course, Niemann-Pick disease type A and GM1 gangliosidosis are sometimes similar in presentation but usually can be distinguished by clinical presentation.

Diagnostic workup

The diagnosis of Gaucher disease should be considered in any case of unexplained splenomegaly with or without a bleeding diathesis or other manifestations of the disease in the skeleton or liver. The disorder should be considered likely in any infant with hepatosplenomegaly and a neurodegenerative course. Elevation of tartrate-resistant acid phosphatase in serum is suggestive of the disease. Demonstration of characteristic Gaucher cells in bone marrow biopsies narrows the diagnostic possibilities. The definitive diagnosis is made by assay of acid beta-glucocerebrosidase in leukocytes, fibroblasts, chorionic villi, or urine.

A diagnostic algorithm has been proposed for pediatric patients without a family history of Gaucher disease (97; 181).

In a cross-sectional study of 44 patients, dry blood spot samples were used to assess levels of the deacylated form of glucocerebroside, glucosylsphingosine (lyso-Gb1) for Gaucher disease diagnosis (49). Of 444 screened subjects, 99 (22%) were diagnosed with Gaucher disease at a median age of 21 years (range 1 to 78). Lyso-Gb levels for genetically confirmed Gaucher disease patients (median 252 ng/mL [range 9 to 1340]) were markedly higher than for patients without Gaucher disease (median 5.4 ng/mL [range 1.5 to 16]). Patients diagnosed with Gaucher disease type 1 and mild GBA1 variants had significantly lower lyso-Gb1 levels (median 194 ng/mL [range 9 to 1050]) compared to Gaucher disease type 1 and severe GBA1 variants (median 447 ng/mL [range 38 to 1340]), and neuronopathic Gaucher disease (median 325 ng/mL [range 116 to 1270]).

Subjects with heterozygous GBA1 variants (carrier) had significantly higher lyso-Gb1 levels (5.8 ng/mL [2.5 to 15.3]) compared to wild-type GBA1 (4.9 ng/mL [1.5 to 16]). Based on these results, the authors called for a change in the diagnostic approach to Gaucher disease, based on lyso-Gb1 measurements and confirmatory GBA1 mutation analyses in dry blood spot samples (49).

After a diagnosis of Gaucher disease has been established, the burden of the disease should be assessed. Evaluation of the characteristic features of the disease should include hematological, visceral, skeletal, and quality of life parameters, as suggested by the Gaucher Disease Registry.

Minimum assessment recommendations in children with Gaucher disease type 1 have been proposed (93; 181).

Additional recommended tests for children with Gaucher disease type 3 include neurologic examination, neuro-ophthalmological investigations, testing of hearing, brain imaging, electroencephalography, periodic pulmonary evaluation for interstitial lung disease, and neuropsychological evaluation. In addition, for those with Gaucher disease type 3c, characterized mainly by cardiovascular and neuro-ophthalmological findings (101), periodic chest x-ray, echocardiography, cardiac computed tomography, angiography, or coronary artery catheterization have been recommended (181), but with little detailed guidance of when and how such studies will be useful in impacting the course of the disease or quality of life.

Hematologic workup. Hemoglobin, platelet count, tartrate resistant acid phosphatase, angiotensin converting enzyme, and chitotriosidase should be measured when the diagnosis is established; follow up measurement is recommended every 3 months for patients on enzyme replacement therapy and at least every 12 months for patients who choose not to receive enzyme replacement therapy.

Visceral studies. Spleen and liver volume should also be assessed at the time of diagnosis. The recommended technique is volumetric CT scan or MRI. These tests are recommended every 12 months in patients receiving enzyme replacement therapy and every 24 months in those patients who achieve a stable state.

Enzymatic diagnosis. A variety of natural and artificial substrates provide accurate assays for homozygotes, but they all have the same limitations when used for heterozygote detection (183; 45). The best substrate employed for the assay is the 4-methylumbelliferyl-beta-D-glucopyranoside. Modifications and improvements in the original description of the assay involving the addition of taurocholate have made it useful as a diagnostic tool and equivalent to assays employing the natural substrate. The assay is easy to use, and the substrate is widely available, making it the method of choice for a diagnostic laboratory. Conduritol B-epoxide is a specific inhibitor of mammalian acid beta-glucocerebrosidase and permits confirmation of the enzyme deficiency in systems where nonspecific beta-glucosidase may be interfering. Leukocytes and fibroblasts may be prepared and shipped to laboratories for assay as whole cells or extracts. High throughput method for newborn screening has been developed (127).

From the time of the introduction of the assay of acid beta-glucocerebrosidase, it has been observed that the values of heterozygotes overlap those of control subjects for the activity of acid beta-glucocerebrosidase in leukocytes and leukocyte subpopulations. This subject has been reviewed extensively. A variety of methods have been developed, but all have the same problem in the detection of heterozygotes. In studies of known heterozygotes, approximately 20% of the carriers fall into the normal range. Fibroblasts have a higher specific activity of acid beta-glucocerebrosidase than leukocytes; however, a wide range of activity in control cells, which varies with time in culture and conditions, makes these cells no more useful in heterozygote detection. In those for whom the genotype is known, carrier detection by molecular methods is completely reliable.

Skeletal assessment. Computerized tomography, radionuclide scan, and magnetic resonance imaging have been useful in assessing the extent of bone abnormalities. The recommended method for routine assessment of bone disease is T1- and T2-weighted MRI of the entire femora (34).

Bone density should also be assessed in patients with Gaucher disease. DEXA studies are useful to determine and assess the risk of pathological fractures. Spine, femur, forearm, and whole-body densitometries can be performed in these patients. These studies should be performed at baseline and every 12 months in patients receiving enzyme replacement therapy.

Management

The special case Gaucher disease type 2. This severe form of Gaucher disease poses sometimes significant diagnostic, management and ethical challenges. These are well addressed by one review (182).

Cell and organ transplantation. Splenic and renal transplantations have had little or no effect on the disease process in patients with Gaucher disease. Bone marrow transplantation has been reported in both Gaucher disease type 1 and type 3 patients. Rapid resolution of the enzyme deficiency in circulating white cells was achieved, indicating successful engraftment.

The indications for hematopoietic cell transplantation vary according to the subtype of Gaucher disease. In type 1 disease, allogeneic bone marrow transplantation leads to resolution of organomegaly within several months to 2 years. The bone disease is also ameliorated, with disappearance of Gaucher cells from the marrow, regression of bone lesions, improvement in linear growth, and disappearance of bone pain (160; 136; 80). Bone marrow transplantation may, therefore, be curative for type 1 disease, even though this therapeutic option is limited by donor availability and high short-term risks, which include a high percentage of procedure associated morbidity and mortality of about 15%. In type 2 and 3 disease, bone marrow transplantation has no effect on the neurologic disease and should be avoided (171).

Enzyme replacement. The final development of enzyme replacement therapy was a fortunate intersection of scientific disciplines that provided key findings:

The mannose moieties in the oligosaccharides are the key to successful enzyme replacement therapy. Mannose, rather than mannose-6-phosphate, binds to the mannose receptor on Kupffer cells. There are also mannose receptors on hepatocytes, although some of these are non-endocytic. Acid beta-glucocerebrosidase can, therefore, gain entry into both hepatocytes and Kupffer cells, but the delivery to Kupffer cells is much better for mannose-terminated (modified) acid beta-glucocerebrosidase.

Early studies of enzyme replacement produced great hope and excitement that Gaucher disease might be amenable to therapy by infusions of enzyme (29). The initial encouraging biochemical and clinical reports could not be confirmed. Even though amounts of enzyme in subsequent trials were increased significantly, no changes could be measured in response to enzyme. These trials were unsuccessful because they failed to target the enzyme to the proper cell in sufficient quantity.

A method was devised to improve that aspect of the problem (63). Using enzymatic degradation of the oligosaccharide side chains of acid beta-glucocerebrosidase to expose mannose residues, the enzyme molecule was tailored to bind to the naturally occurring mannose receptors on macrophage plasma membranes. In animal model studies, this engineered acid beta-glucocerebrosidase was delivered to reticuloendothelial cells in quantities 10 times greater than the unmodified native enzyme (63). The function of the oligosaccharide side chains is not fully understood, but they are probably important in maintaining the tertiary structure of the protein during its synthesis and translocation to the lysosome. If addition of the sugar side chains is prevented completely, the protein is not properly folded and has no enzyme activity.

A clinical study in one patient demonstrated that this mannose-terminated acid beta-glucocerebrosidase was therapeutic (11). A formal clinical trial confirmed these results (14; 13). The commercial preparation is now widely used in patients and successfully reverses the visceral, hematologic, and skeletal manifestations of the disease.

After several clinical trials of placental derived acid beta-glucocerebrosidase (alglucerase) for the treatment of Gaucher disease, alglucerase was approved in 1991 (13). Production of the same protein using recombinant DNA technology was achieved, and the resultant drug, imiglucerase, received regulatory approval in 1994.

A review of the hepatic, splenic, and hematologic outcome from 175 patients was reported by Weinreb and colleagues (180). The authors concluded that the following four responses could be expected after 12 to 24 months of enzyme replacement therapy:

The skeletal complications of Gaucher disease respond well within the same time frame as the other signs and symptoms of the disease. Bone pain is relieved, and the incidence of bone infarct is reduced dramatically to near zero in patients treated with 60 U/kg of recombinant human acid beta-glucocerebrosidase every 2 weeks prior to the development of a significant structural abnormality. The radiographic assessment of the skeletal complications, however, remains inadequate as a tool to measure the baseline and improvements in the bone in response to therapy.

Enzyme replacement has the same salutary effect on patients with type 3 Gaucher disease as it has in type 1 patients (144; 07). Treatment is indicated in these patients at a dose of 60 IU/kg of body weight every 2 weeks and should be closely monitored clinically and with laboratory studies to establish the response to the therapy (171). When initiated early in the course of the disease, enzyme replacement normalizes growth, corrects the hematologic abnormalities, and prevents the skeletal complications that occur in patients with type 3 Gaucher disease. There is no doubt that this therapy has markedly ameliorated the course of the disease in this group of patients who used to die in their teens prior to the advent of enzyme replacement. However, in type 2 disease, enzyme replacement therapy may lengthen the course of the illness, but the neurologic deterioration progresses, and the poor prognosis of the disease is unaltered; therefore, enzyme replacement therapy should not be initiated in this group (135).

The adverse effects of enzyme replacement therapy are few and rarely a reason for discontinuing the therapy (120). Antibodies to enzyme replacement therapy have been reported in approximately 15% of patients. The incidence of serious complications is very low.

Effects of enzyme therapy cessation and withdrawal have been evaluated in patients who completely discontinued their therapies. These studies suggest that a complete cessation of therapy will inevitably endanger the patient by deteriorating hematological, biochemical, and skeletal parameters (174; 178). However, successful experiences of enzyme withdrawal for short periods of time have been reported (145).

Early initiation of enzyme replacement therapy is very important in children because it allows prevention of serious, irreversible skeletal complications in children. Children show an important increase in bone mineral density and growth rates (93; 08).

Mouse models of Gaucher disease. The transgenic acid beta-glucocerebrosidase-deficient mouse model obtained by targeted knockout of the acid beta-glucocerebrosidase gene exhibits a severe phenotype with prenatal or perinatal death (169). A number of other mouse models were of limited use and did not represent a genuine model for the neuronopathic form of the disease (186). Two true models of the acute neuronopathic form of Gaucher disease were produced. One of the models had normal enzyme activity in hematopoietic cells but no enzyme activity in neuronal and astrocytic cells. There was no significant mitigation of the neuronal disease in this model, confirming the observation in patients that bone marrow transplantation is not effective in neuronopathic Gaucher disease (54).

Other therapeutic approaches. The success of enzyme replacement therapy has probably led to a diminished motivation to develop a gene therapy approach for the nonneurologic aspects of Gaucher disease. Because there is an unmet need to reverse or prevent the primary neurologic complications of Gaucher disease, research is ongoing to develop approaches that will enable delivery of acid beta-glucocerebrosidase or the normal copy of the GBA1 gene across the blood-brain barrier and into brain cells (155).

Substrate deprivation. N-butyldeoxynojirimycin (OGT-918), an inhibitor of the glucocerebroside synthase, initiates the glycosphingolipid pathway and catalyzes the formation of glucocerebroside (41). The rationale of this therapy is to decrease the formation of glucocerebroside to rates at which the residual acid beta-glucocerebrosidase activity of the patients would be sufficient to catabolize the substrate. Most patients experienced diarrhea, the most frequent side-effect, and two patients withdrew from the study because of this complaint. Some patients developed peripheral neuropathy. These results do not compare favorably to those obtained with enzyme replacement therapy, which normalizes or greatly impacts the clinical parameters with negligible side-effects (10; 117).

In a randomized controlled trial, miglustat was found not to be effective for the treatment of type 3 Gaucher disease (143).

A much more specific inhibitor of ceramide glycosyltransferase, the ceramide analogue eliglustat, has shown marked efficacy that is similar to enzyme replacement therapy at the 60 IU/kg/2 weeks dose (40). It does not have the side effect of miglustat but its metabolism is susceptible to the genotype of cytochrome P450 CYP2D6. Eliglustat was approved by the FDA in 2014 (134). Eliglustat tartrate is a P-glycoprotein substrate, possibly accounting for its poor distribution into the brain (147). Newer compounds with similar function but with good brain penetration are being developed and are being tested in human patients (112). Venglustat is currently being tested in adults with Gaucher disease type 3 (ClinicalTrials.gov Identifier: NCT02843035).

Pharmacological chaperones. A proposed approach for the treatment of Gaucher disease and other lysosomal storage diseases relies on the administration of active site competitive inhibitors that promote the correct folding and trafficking of the mutant protein (and even wild-type molecules). This process leads to increased level of the mutant enzyme that can then function during drug washout period (192; 176). The only pharmacological chaperone that is currently being clinically tested is ambroxol in high dose (109; 96).

It shows some promise in myoclonic forms of Gaucher disease type 3 (96). However, no controlled studies have been performed to confirm the clinical benefit of ambroxol.

Gaucher disease type 2. None of the specific therapeutic approaches described above have been shown to be efficacious in patients with the most severe form of Gaucher disease. The overall management of patients with Gaucher disease type 2 is very difficult and has been reviewed (182).

Gene therapy. Gene therapy using an adeno-associated virus vector injected intrathecally is in the advanced planning stages.

Special considerations

Pregnancy

Patients with Gaucher disease can become pregnant and deliver normal healthy infants. Precautions should be taken for women who are anemic or thrombocytopenic during the pregnancy. Excessive bleeding is a frequent complication following delivery. The teratogenic potential of acid beta-glucocerebrosidase has not been evaluated, and patients contemplating pregnancy should be advised of this fact. Prudence suggests that pregnant women should not be treated with enzyme replacement during the first trimester. However, pregnant patients who were treated with at least two forms of enzyme replacement have delivered normal babies (52; 53).

Anesthesia

Patients with anemia, thrombocytopenia, or pulmonary involvement are at an increased risk for complications related to anesthesia and surgery. A particularly instructive case is a patient who was not diagnosed prior to cardiac surgery (116).