Carnitine palmitoyltransferase II deficiency
Jul. 15, 2022
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This article includes discussion of Niemann-Pick disease type C, Niemann-Pick C disease, NPC, NP-C, neonatal cholestatic rapidly fatal form of Niemann-Pick disease type C, early-infantile neurologic-onset form of Niemann-Pick disease type C, late-infantile neurologic-onset form of Niemann-Pick disease type C, juvenile neurologic-onset form of Niemann-Pick disease type C, adult neurologic-onset form of Niemann-Pick disease type C, NP-C1, and NP-C2. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
Niemann-Pick disease type C is an autosomal recessive neurodegenerative lysosomal storage disorder characterized by impaired cellular trafficking of cholesterol and sphingolipids and caused by mutations in either the NPC1 or NPC2 gene. In this article, the author emphasizes the wide spectrum of clinical phenotypes, highlights new developments and difficulties in diagnostic strategies, and discusses the current status in the management and treatment of patients.
• Niemann-Pick disease type C (Niemann-Pick C disease, NPC) has a very broad spectrum of clinical phenotypes and is most likely underdiagnosed in adults.
• For laboratory diagnosis, initial orientation tests in plasma measuring particular oxysterols (cholestane-3ß-5α−6ß-triol) or bile acid derivatives, lyso-SM-509 (now PPCS) and lyso-sphingomyelin, have been developed. These biomarkers are sensitive but not specific to Niemann-Pick disease type C.
• Confirmation of the clinical diagnosis of Niemann-Pick disease type C is primarily based on molecular study of the NPC1 and NPC2 genes. Whenever 2 pathogenic mutant alleles have not been identified, demonstration of an impaired trafficking of endocytosed cholesterol in living cells (filipin test) is very useful to confirm the diagnosis.
• Miglustat is the first disease-modifying pharmacological agent aiming to stabilize or slow progression of neurologic manifestations in Niemann-Pick disease type C. It is currently approved for this indication in 42 countries, but it is not approved for Niemann-Pick disease type C in the United States.
• Among experimental therapies, 2 compounds are under clinical trials: intrathecal administration of 2-hydroxypropyl-beta-cyclodextrin (HPBCD) (currently in extension of phase 2b/3), oral administration of arimoclomol (in extension of phase 2/3), and intravenous administration of HPBCD (phase 1/2).
Historically, the eponym "Niemann-Pick disease" encompassed a heterogeneous group of autosomal recessive lysosomal lipid storage disorders, with common features of hepatosplenomegaly and sphingomyelin storage. In 1958, Crocker and Farber showed a wide variability both in age of onset and clinical expression and in the level of sphingomyelin storage in tissues (20). This led Crocker to propose a classification of Niemann-Pick disease into 4 subgroups, A to D (19). Type A was characterized by severe, early CNS deterioration and massive visceral and cerebral sphingomyelin storage. Type B showed a chronic course with marked visceral involvement but a sparing of the nervous system. Types C and D were characterized by a subacute nervous system involvement with a moderate and slower course and a milder visceral storage. Type D patients were individualized essentially on their homogenous Nova Scotia Acadian origin. Patients with a retrospective diagnosis of Niemann-Pick C disease were published in the 1960s and 1970s under numerous names: juvenile Niemann-Pick disease, juvenile dystonic lipidosis, atypical cerebral lipidosis, neurovisceral storage disease with vertical supranuclear ophthalmoplegia, maladie de Neville, DAF syndrome, adult dystonic lipidosis, adult neurovisceral lipidosis, giant cell hepatitis, and lactosyl ceramidosis (107; 100). Type D was later shown to belong to the same disease entity as type C.
From 1966 to 1968, Brady and associates demonstrated a severe deficiency in sphingomyelinase activity in tissues from patients with type A and type B, but not in types C and D. From that time, with a turn following seminal observations in a Balb/c murine model of the disorder (104), the concept of Niemann-Pick type C disease evolved from that of a sphingomyelin storage disorder to that of a cholesterol storage disorder (107). This and later work led to the reclassification of the disease as a cellular lipid trafficking disorder.
Today, the denomination "Niemann-Pick C disease" designates disorders characterized by unique abnormalities of intracellular transport of endocytosed cholesterol (105; 106; 138; 70; 107; 100; 157; 103; 150; 151). Major advances have been the description of 2 genetic complementation groups and the isolation of the underlying genes. NPC1, located at chromosomal segment 18q11, is involved in 95% of the families or more, including those with type D. NPC2, located at chromosomal segment 14q24.3, is involved in rare families (142; 154; 12; 36; 91). Although the precise functions of the NPC1 and NPC2 proteins are still elusive, current knowledge supports the idea that these proteins function in a coordinated fashion and that they are involved in the cellular postlysosomal/late endosomal transport of cholesterol and other molecules (100; 157; 63; 103; 153; 150; 148; 109). Stricto sensu, one should, therefore, distinguish between Niemann-Pick disease type C1 (www.ncbi.nlm.nih.gov/omim/257220) and Niemann-Pick disease type C2 (www.ncbi.nlm.nih.gov/omim/607625). Because the clinical manifestations and biochemical abnormalities of types C1 and C2 are similar, the distinction is rarely made. The generic terms of "Niemann-Pick disease type C," “Niemann-Pick C disease,” or “NPC” remain widely used in practice and in literature.
On the other hand, it is essential to remember that a diagnosis of "Niemann-Pick disease" without specification of a subgroup is ambiguous and a potential source of error. One should clearly distinguish acid sphingomyelinase deficiencies (due to SMPD1 mutations and including types A, B, and intermediate forms) from Niemann-Pick type C, with alterations in trafficking of endocytosed cholesterol (due to NPC1 or NPC2 mutations). Type D as a distinct entity is no longer justified.
The wide clinical spectrum of Niemann-Pick C disease, already evident in Crocker and Farber's series, is now well recognized (162; 158; 28; 55; 135; 130; 160; 47; 100; 48; 30; 134; 139; 177; 97; 99; 140; 179; 49; 46).
The age of presentation is highly variable, ranging from the perinatal period to late adulthood. The initial manifestations may also vary, being hepatic, neurologic, or psychiatric in nature. Note that systemic disease (involving the liver, spleen, and lung) and neurologic manifestations follow independent courses and that systemic disease, if present, always precedes onset of neurologic symptoms.
Liver involvement of varying severity is often present in the first months of life or even prenatally. During the course of the disease, a moderate, sometimes transient (hepato)splenomegaly is a common finding, but this sign may be missing. In typical patients, the neurologic disorder consists mainly of cerebellar ataxia, dysarthria, dysphagia, and progressive dementia. The vast majority of cases show a characteristic vertical supranuclear gaze palsy. Cataplexy, seizures, and dystonia are other quite common features, and psychiatric disturbances are frequent in late-onset patients (162; 28; 130; 170; 30; 134; 177; 153; 81; 82; 99; 123). Mild to moderate hearing loss (56) and sleep problems may occur (122). Delayed motor development and hypotonia followed by pyramidal signs are the main features of a rarer, early-onset, rapidly progressive variant (162).
Perinatal presentation. Niemann-Pick C disease is now recognized as a relatively common cause of liver disease in early life. Fetal hydrops or fetal ascites can be observed (139). A prolonged neonatal cholestatic icterus associated with progressive hepatosplenomegaly is present in at least one third of patients (162; 55; 184; 99). In most cases, the jaundice resolves spontaneously by 2 to 4 months of age, and only hepatosplenomegaly remains for a highly variable period, preceding onset of neurologic symptoms at a variable age (see below).
In about 10% of those patients, however, the cholestasis quickly worsens and leads to liver failure. A number of children with this dramatic "acute" neonatal cholestatic rapidly fatal form die before the age of 6 months, before onset of neurologic symptoms. Some other infants, especially those having mutations in the NPC2 gene, present with a severe respiratory insufficiency (with hepatosplenomegaly or more severe liver disease) that may also be fatal. The name “perinatal form of Niemann-Pick C disease” should only be used to designate children who have died early from their systemic disease. This form may be associated in the same sibship with a neurologic form.
Systemic involvement in other presentations. Isolated hepatosplenomegaly or splenomegaly is often the presenting symptom in infants and children without a history of neonatal icterus. This may stay the only sign for many years or decades, until onset of neurologic symptoms (47; 177; 153). Splenomegaly is usually mild to moderate and tends to diminish with time. Importantly, it may not be observed in 10% to 15% of patients with a childhood neurologic form and in a larger proportion of patients with adolescent or adult onset of the neurologic symptoms. On the other hand, the finding of 3 patients aged 53 to 63 years with isolated splenomegaly and a biochemical and molecular diagnosis of Niemann-Pick C disease suggests the existence of a rare non-neuronopathic form of the disease (possibly corresponding to the ill-described historical "type E") (27; 87; 24). Such cases may remain undiagnosed (173). Nevertheless, apart from these exceptional cases and from infants with early death, all Niemann-Pick C disease patients develop neurologic symptoms.
Neurologic manifestations. Niemann-Pick C disease patients are best classified according to the age of onset of the neurologic symptoms because it correlates with the following course and life span (162; 28; 160; 100; 177; 153; 99; 140; 49; 46).
Severe early infantile neurologic form. In these infants, hepatosplenomegaly has been present since birth or the first months of life. Delay of developmental motor milestones and central hypotonia become evident between the ages of 9 months and 2 years. Subsequent clinical course includes a loss of acquired motor skills, proportionally less marked mental regression, followed by pronounced spasticity with pyramidal tract involvement. Many of these children never learn to walk. Intention tremor has frequently been noted; supranuclear gaze palsy is not always evident. Seizures are uncommon. Brain imaging shows signs of myelination delay and later of leukodystrophy and cerebral atrophy. Survival rarely exceeds 5 to 7 years. This form seems to be more frequent in Southern Europe (where it constitutes 20% to 25% of the cases) and the Middle East than in the United States (162; 48; 177).
Childhood neurologic form. “Classic Niemann-Pick C disease” (60% to 70% of the cases) includes patients with a late infantile or juvenile neurologic onset. In the late infantile form, hepatosplenomegaly or splenomegaly has usually, but not constantly, been present for a varying period. The child often presents with gait problems, frequent falls, and clumsiness between 3 and 5 years of age, due to ataxia. Language delay is frequent. The motor problems worsen, and cognitive dysfunction appears. In the juvenile form, a preexisting moderate splenomegaly is common, although absence of a detectable organomegaly (even by ultrasonography) has been reported to occur in at least 15% of cases. Clear neurologic symptoms appear between 6 and 12 to 15 years of age, but onset is insidious and variable. School problems with difficulty in writing and impaired attention are common and may lead to misdiagnosis. The child becomes clumsier, shows more learning disabilities, and ataxia becomes obvious (falls often, cannot run properly). Seizures or cataplexy may occasionally be the presenting symptom. In both forms, vertical supranuclear gaze palsy will develop (downward, upward, or both). It is often the initial sign, but it may require careful examination (initial slowing of voluntary vertical saccadic movements) (127; 01). Parents may notice compensatory head thrust when the child wants to look downward or upward. Cataplexy, with or without narcolepsy, typically laughter induced, is another common and specific sign (51; 93), observed in about one third of patients. As ataxia progresses, dysphagia, dysarthria, and dementia develop. The neurocognitive phenotype in children and adolescents has been studied (146). Action dystonia is also frequent, mostly in the juvenile form. Motor impairment is major and intellectual decline may be variable. About half of the patients with the classic form develop seizures, which may be partial, generalized, or both. They generally respond to standard treatment but refractory cases may occur, with some patients dying from status or complications of seizures. Severe epilepsy is of bad prognosis and significantly shortens the lifespan of the patients. At a later stage, dysarthria worsens and patients ultimately stop talking. The patients develop pyramidal signs and spasticity, and pronounced swallowing problems. Most require gastrostomy. Death usually occurs between 7 to 12 years in late infantile onset patients. The lifespan is quite variable in the juvenile form; some patients are still alive at the age of 30 years or later (177).
Adolescent and adult presentations. Patients with a late neurologic onset (often in the second or third decade, but as late as 50 years or older) have been described (135; 170; 58; 134; 90). With increasing awareness, more adult-onset patients have been identified in the past 5 years, but this diagnosis remains underestimated. Absence of clinically detectable splenomegaly has been reported in a significant proportion of patients but abdominal sonography may reveal a slightly enlarged spleen. Vertical supranuclear gaze palsy is nearly invariably present but not always detected because slow pursuit is maintained (60; 97). The usual symptomatology is that of an attenuated juvenile form with an insidious onset, with in at least one third of cases, a psychiatric presentation that is often isolated for several years before the onset of motor and cognitive signs. Psychiatric signs are most often consistent with psychosis, including paranoid delusions, auditory or visual hallucinations, and interpretative thoughts (97; 08). Onset may be acute or progressive, eventually with relapses. At this stage the neurologic examination may be normal. Other types of psychiatric disturbances are depressive syndrome, behavioral problems with aggressiveness, or social isolation. Cases have also been reported with bipolar disorders, obsessive-compulsive disorders, or transient visual hallucinations. Catatonia can also be seen. From compilation of the literature, the most common features are: cerebellar ataxia (76%), vertical supranuclear ophthalmoplegia (75%), dysarthria (63%), cognitive troubles (61%), movement disorders (58%), splenomegaly (54%), psychiatric disorders (45%), and dysphagia (37%) (134). Movement disorders (dystonia, parkinsonism, chorea) are more frequent than in the juvenile form. Some patients show severe ataxia, dystonia, and dysarthria with variable cognitive dysfunction, whereas psychiatric symptoms and dementia dominate in others. Epilepsy is rare in adult-onset Niemann-Pick C disease (15%). Later course is similar to that in the juvenile form.
Disease severity scoring. Two disease severity scoring scales have been proposed by the groups of M Pineda or of FD Porter (48; 182). The "Pineda scale," in its modified form, takes into consideration ambulation, manipulation, language, swallowing, and, in its latest modification, seizures and ocular movements (115; 113). The more complex NIH NPC severity score (NNSS) also evaluates cognition, memory, hearing, and a number of modifiers (182). A current clinical trial uses only the 5 following main domains: ambulation, manipulation, speech, swallowing, and cognition. In order to better take into consideration individual rates of progression, other authors have proposed to use an annual severity increment score (ASIS), in which the total score (based on main disability domains) is divided by the age of the patient (18).
The disease has been considered to invariably lead to death. However, the rate of progression and lifespan show considerable variation. Estimates of age at death in patients categorized from the age of onset of neurologic disease have been compiled for Spain, the United Kingdom, and France (177). Some patients with severe neonatal liver disease or respiratory distress may die before 3 to 6 months of age. Patients with the severe neurologic early infantile form rarely live longer than 6 to 7 years of age. Patients with a late-infantile and juvenile neurologic onset usually survive until their teenage years or later. A sizable proportion of patients reach the age of 30 years or more. Of note, because symptomatic management of patients (eg, gastrostomy) has considerably improved since 1990, current survival of patients is often longer now than found in early historical cohorts. In a large study, no significant change in survival was found for the past 20 years (06). Owing to improved diagnosis of patients with the adult-onset form in recent years, as of 2018 about half of the living patients are between 20 and 60 years of age in the United Kingdom, France, or Germany. Three proven NPC1 patients aged 53 years or more with splenomegaly, but no neurologic symptoms, have been documented (87; 24). In neurologic patients, 2 natural history surveys using different disease-specific disability scales concluded to a linear clinical progression over time (178; 182). The study including the broadest range of clinical phenotypes suggested varying slopes (178). This was confirmed in a survey from Germany (140).
The severity of the disease is primarily determined by the degree of nervous system involvement; however, liver failure may cause rapid death in 5% to 10% of neonates presenting with a severe cholestatic icterus, and a few patients (most of them with a severe NPC2 mutation) have died from severe pulmonary insufficiency. Neonatal cholestatic icterus is otherwise transient and usually resolves spontaneously by 4 months of age. Motor involvement is often more severe and more rapidly progressive than mental retardation. Progressive and severe dysphagia requiring gastrostomy is a common complication. Whenever epilepsy becomes very severe or intractable, the downhill course of the disease accelerates. Splenomegaly very rarely leads to hypersplenism. Recurrent pulmonary infections occur, and patients are recognized with severe pulmonary involvement. Psychiatric disturbances, in rare cases, may be prominent or even dramatic. Swallowing difficulty, due to bulbar palsy, is a common late complication. Niemann-Pick disease type C is also a rare cause of fetal hydrops or fetal ascites. Mild to moderate hearing loss involving the frequencies most important to speech understanding has been described in 75% of a cohort of 50 patients, and a recommendation was made to regularly monitor audiologically patients with Niemann-Pick disease type C, beginning at the time of diagnosis (56). Patients with NPC1 mutations seem to have an increased susceptibility to early-onset fistulising colitis with granuloma formation reminiscent of Crohn disease that has been linked to defective autophagosome function (132).
Importantly, patients with a purely systemic perinatal fatal form may have siblings with a neurologic form. Otherwise, affected siblings are usually categorized within the same global neurologic subtype of the disease. Fairly similar age at neurologic onset is generally observed within the early- or late-infantile forms, but more discordant courses between siblings are not rare in later onset forms.
Case 1. A female with an unremarkable neonatal period was noted to have hepatomegaly without splenomegaly at 3 years of age. From the age of 10 years, clumsiness and learning problems at school were noted. At the age of 13 years, moderate hepatosplenomegaly, cerebellar tract involvement, and vertical (upward) supranuclear gaze palsy were present. Slow waves were noted on EEG. At 15.5 years, diagnosis of Niemann-Pick disease type C was established; then status epilepticus and repeated generalized seizures developed. At 17 years of age, enlargement of liver or spleen was no longer present, but the patient lost the ability to walk. Pyramidal tract involvement developed later. At 22.5 years of age, she was bedridden, dystonic, but still understood simple sentences. She died at 28.5 years of age. In retrospect, this patient was found homoallelic for the p.I1061T NPC1 mutation.
Case 2. A male infant developed neonatal cholestatic icterus that resolved spontaneously at 5 months of age. At 3 years of age he was noticed to have hepatosplenomegaly. He had neurologic problems beginning at 5 years of age, with unsteady gait and loss of language. At 7 years, onset of laughter-induced cataplectic attacks occurred and a diagnosis of Niemann-Pick disease type C was established. At 8 years of age, ataxic gait, autistic behavior, no language, and vertical supranuclear gaze palsy were present. At 10 years of age, epilepsy, dystonic features, and pyramidal tract involvement developed; cataplexy (3 to 4 attacks per day) and narcolepsy worsened but improved with clomipramine and modafinil. At 16 years of age, he could no longer walk and swallowing problems were such that a gastrostomy became necessary. He is a compound heterozygote for the p.I1061T and p. S940L NPC1 mutations.
Case 3. A female, with a twin sister who died at 3 months of age from the cholestatic rapidly fatal form of Niemann-Pick disease type C (diagnosed from autopsy material), had neonatal cholestatic icterus that was resolving by 3 months of age. Hepatosplenomegaly, delayed motor milestones (she sat up at 10 months of age, and could not stand up at 12 months of age), and relatively good mental development were present in infancy. Motor regression and a progressive intention tremor began at 15 months of age. At the age of 28 months, she could no longer stay in sitting position or even hold her head up. She also developed swallowing problems. Severe pyramidal tract involvement occurred at the age of 33 months. The child died at 38 months. This patient was found homoallelic for a p.Q775P NPC1 mutation, located in the putative sterol-sensing domain of the protein.
Case 4. This female patient had no early medical history. She attended school without problems but was noticed to be mildly clumsy since the age of 14 years. Neurologic problems were first noticed at the age of 17 years, including dysarthric speech, gait and limb cerebellar ataxia, and mild dysphagia. At the age of 19 years, vertical ophthalmoplegia was also observed, together with brisk deep tendon reflexes and dystonia of right lower and upper limbs. She also experienced several sudden falls while standing, without loss of consciousness, that were described as a loss of tonus and were suggestive of cataplexia. General examination did not show splenomegaly, which was found with ultrasonography. At 20 years, her mental status was still well preserved. Her motor status continued to deteriorate, but she was still able to walk. She died at 31 years of age. She had been treated with miglustat from the age of 23. One of her NPC1 alleles carried a p.G992A mutation, the second one a p.G993 frameshift mutation.
Mutations in either of 2 separate genes, NPC1 and NPC2, may cause the disease (142; 154; 12; 91). Approximately 95% of patients have mutations in the NPC1 gene (18q11-q12), which encodes a large multipass membrane protein; the remainder have mutations in the NPC2 gene (14q24.3), which encodes a small soluble protein. Both proteins localize to the late endosomal/lysosomal compartment. Mutations in the 2 respective genes result in a similar cellular lesion, including a unique impairment in processing and utilization of endocytosed cholesterol. This can explain cholesterol storage and secondary alterations of sphingomyelin metabolism in extraneural tissues. Glycosphingolipid storage also occurs and is particularly prominent in neurons. Early studies in cells and tissues from NPC1 and NPC2 patients could not disclose any biochemical marker that was specific to any of the groups, suggesting that both proteins may function in tandem or sequentially (154). Comparison between double mutant mice deficient in both NPC1 and NPC2 and the single mutants demonstrated a nonredundant functional cooperativity of the 2 proteins in a common pathway for lipid cellular transport (137). Over 250 disease-causing mutations of the NPC1 gene are known, as well as over 250 polymorphisms. Approximately 60 families have been documented with mutations in the NPC2 gene. The full functions of the NPC1 and NPC2 proteins are still unclear, which greatly complicates understanding of the pathophysiology (100; 157; 143; 103; 153; 151; 150).
Lipid accumulation in tissues. The pattern of accumulating lipids is different in brain and in non-neural organs, but similar profiles have been observed in NP-C1 and NP-C2 disease (107; 160; 137; 153; 151). In liver and spleen a complex lipid storage pattern, with no predominating compound, is observed; accumulated lipids include unesterified cholesterol and sphingomyelin, bis (monoacylglycerol) phosphate (also named LBPA or BMP), glycolipids (essentially glucosylceramide, lactosylceramide, globotriaosylceramide and ganglioside GM3), free sphingosine and sphinganine. In human patients (at variance with the mouse and cat models), accumulation is more pronounced in spleen than in liver, where changes may be subtle. In brain tissue, neither cholesterol nor sphingomyelin overtly accumulate, but histochemical staining by filipin or BC-theta reveals a storage of unesterified cholesterol in the late endosomal/lysosomal compartment of neurons. The most conspicuous lipid abnormality in cerebral gray matter consists of a significant increase of GM2 and GM3 gangliosides, and of glucosyl- and lactosylceramide. In brain, free sphingosine is only very mildly elevated. Myelin lipids are markedly affected in the severe infantile neurologic form only.
Indication of oxidative stress participation in pathogenesis led to reappraisal on the status of cholesterol oxidation products in the Npc1 mouse model. In liver, high levels of multiple nonenzymatically formed species were observed early in life, whereas in brain, only cholestane-3ß,5a,6ß-triol was significantly elevated. Increased levels of these metabolites also occurred in plasma (120). Conversely, both in brain and plasma, there was a decrease of the enzymatically (CYP46A1) formed 24(S)-hydroxycholesterol, that plays a major role in brain cholesterol homeostasis. Findings in murine plasma were recapitulated in plasma of patients, suggesting that oxysterols could serve as biomarkers of disease (120; Tortelli et al. 2014).
Main features of neuropathology. Neuronal storage with meganeurite formation and extensive growth of ectopic dendrites, as well as formation of neurofibrillary tangles, are important neuropathological features together with neuroinflammation and neuroaxonal dystrophy. The paired helical filaments tau is indistinguishable from that in Alzheimer disease. Alterations of the amyloid metabolism have also been described (78). As the disease progresses, neuronal death becomes prominent, affecting more specifically certain regions, particularly Purkinje cells of the cerebellum; the basis of this selective neuronal vulnerability is unclear (100; 166). The primordial role of NPC1 expression in neurons has been emphasized by studies in conditional mutant mice, showing that neuron-specific expression of NPC1 in null mutant Npc1 mice rescued disease (73), whereas neuron-specific NPC1 deficiency was sufficient to cause disease (Yu et a 2011). However, NPC1 expression in both neurons and oligodendrocytes appeared important for CNS myelin formation and maintenance (186). On the other hand, astrocyte-specific deletion of NPC1 did not cause a phenotype.
Early work demonstrating impaired intracellular cholesterol transport as the cellular hallmark of NPC disease. Initial studies by Peter Pentchev and associates and further work from several laboratories demonstrated, in cultured skin fibroblasts of patients, a disruption in intracellular transport of endocytosed cholesterol (107; 100). Endocytosed low-density lipoproteins are delivered to the late endosomal/lysosomal compartment (LE/Lys), where they are hydrolyzed. In normal fibroblasts, the unesterified cholesterol released by acid lipase is transported rapidly out of LE/Lys to the plasma membrane and the endoplasmic reticulum. In Niemann-Pick type C disease cells (either NPC1 or NPC2), the cholesterol does not exit the endocytic pathway but accumulates within the LE/Lys compartment. This anomaly constitutes the cellular hallmark of the disease. Due to this sequestration, the subsequent induction of all low-density lipoprotein cholesterol-mediated homeostatic responses (more specially cholesteryl ester formation) is retarded. Normal responses are inducible by membrane-permeable oxysterols, showing that the ability of the cell to respond is maintained. Similar changes have been described in cells with NPC1 or NPC2 mutations (154). Lysosomal storage of unesterified cholesterol may show a variable intensity, and a “variant” biochemical phenotype with mild abnormalities was described (159). Later studies showed that this phenotype was underlined by specific NPC1 mutations (see below). Unexpectedly, a large proportion of obligate heterozygotes have been found to show mild but definite changes (159; 161).
The NPC1 and NPC2 proteins and their coordinated role in intracellular cholesterol transport. The mature native NPC1 is a large transmembrane glycoprotein (1252 amino acid for the mature protein), for which early topological work indicated 13 transmembrane domains, 3 large (and 4 small) luminal loops, 6 small cytoplasmic loops, and a cytoplasmic tail (22). NPC1 resides primarily in late endosomes and interacts transiently with lysosomes (40). Its C-terminal cytoplasmic tail ends with a dileucine motif (LLNF) necessary for proper targeting of the protein, apparently via its interaction with the cytosolic clathrin adaptor AP-1A. By contrast, the NPC2 protein is small (132 amino acids for the mature protein), soluble, transported to the lysosome via the mannose-6-phosphate receptor, secreted, and recaptured. NPC2 binds cholesterol with submicromolar affinity, and its cholesterol binding domain has been well identified (143).
Proposed model in systemic cells. There is ample evidence that both NPC1 and NPC2 are required for cholesterol egress from the endolysosomal system. Work from several laboratories, essentially from that of J Goldstein and M Brown, has led to a "hand-off" model for the coordinated role of the 2 proteins (63). In cells such as hepatocytes or fibroblasts, after endocytosis of LDL, "free" cholesterol, released from cholesteryl esters by the lysosomal acid lipase, first binds to NPC2 with its hydroxyl group exposed; NPC2 then hands it off to NPC1, reversing orientation of the cholesterol molecule so that its hydrophobic side chain can lead the way into the membrane or the glycocalix. A defect in either of the 2 proteins will lead to a progressive accumulation of unesterified cholesterol in the endolysosomal compartment, which is well in line with what is observed in systemic tissues of patients. Domains of NPC1 with specific functions have been identified, and structural studies on the NPC1 protein have shed further light in favor of this model (34; 67; 68). The first luminal loop (amino acids 25 to 264), also named N-terminal domain (NTD), possesses a cholesterol binding site (63). The second loop (or middle luminal domain, MLD) was shown to directly bind NPC2 (23; 66), and observation of a complex between NPC1-MLD and NPC2 has suggested a mechanism for cholesterol transfer from NPC2 to the N-terminal domain of NPC1 (66). Early on, a sterol-sensing domain (SSD) (amino acids 615 to 797, corresponding to transmembrane helices 3 to 7) had been defined because of homologies with the sterol-sensing domain of HMG-CoA reductase, SCAP, patched, and NPC1L (12). Its exact role is still unclear, but computational modeling suggests that the sterol-sensing domain forms a cavity that can accommodate a cholesterol molecule (67). The third large, cysteine-rich luminal loop (or CTD) (amino acid residues 855 to 1098) contains a ring-finger motif and harbors a large proportion of mutations described in patients, including most of those associated with a lesser impairment of cholesterol trafficking in fibroblasts (variant filipin phenotype, see below). Its 3-D conformation suggests a possible interaction with the N-terminal domain, keeping the latter in the proper orientation to receive cholesterol from NPC2 (68). Summarizing those data, after binding cholesterol, NPC2 undergoes a conformational change that enhances its binding to NPC1-MLD (66) and transfers cholesterol to NPC1(NTD), which inserts cholesterol into the lysosomal membrane. But the precise mechanism by which NPC1 mediates cholesterol egress from late endosomes or lysosomes to endoplasmic reticulum and plasma membrane remains unclear (74; 148; 109).
Adaptation of the model to central nervous system. At first, it appears difficult to apply this model to the brain cells because brain cholesterol is synthesized locally (mostly by oligodendroglial cells and, to a lesser extent, by astrocytes and neurons), due to the prevention of lipoprotein uptake from the circulation by the blood brain barrier. However, neurons also partly acquire cholesterol by glial delivery through endocytosis of an apo-E-cholesterol-phospholipid lipoprotein (149). Trafficking of this particular cholesterol source through the late endosome/lysosome compartment will not need hydrolysis by the acid lipase but will require functional NPC2 and NPC1 proteins (05). Quantitatively, this pathway involves quite small cholesterol amounts. This could explain why dissected cerebral gray matter from human patients does not show significant increase of cholesterol concentrations by chemical measurement (152), although neuronal bodies of single Npc1 or Npc2 mutant mice as well as those of the double mutant definitely store small amounts of unesterified cholesterol detectable by filipin staining (137). Studies in cultured sympathetic neurons from Npc1 mutant mice gave indication that cholesterol did accumulate in cell bodies but was decreased in distal axons, leading to a distribution imbalance (54; 53). Nevertheless, the exact participation of cellular cholesterol transport abnormalities in the pathophysiology of the neurodegenerative disease remains elusive.
Wider role of the NPC1 protein and some important features of the pathophysiological cascade. There are still many uncertainties regarding the complete functions of NPC1. Work reveals that NPC1 binds to the putative amino acid transporter SLC38A9 and inhibits cholesterol-mediated mTORC1 activation via its sterol transport activity and is, therefore, implicated in regulation of cellular growth (13). There are also data indicating a role for NPC1 in regulating vesicle budding or fission (33). On the other hand, a direct effect on sphingolipid transport has so far not been proven. The increase in ganglioside GM2 and other glycolipids appears due to the cholesterol-induced inhibition of their lysosomal degradative pathway (128; 04). Sphingosine can exert various cellular deleterious effects (167). Above all, it appears to be a likely cause of the reduced calcium release from lysosomes seen in the disease (72; 41). Elegant studies have demonstrated a defective lysosomal egress of this metabolite in NP-C cells (42), but there is currently no evidence for a direct role of NPC1 for its export. Although sphingosine certainly participates in the pathophysiology of the disease, whether it should be considered as the primary offending metabolite, remains controversial (72). Of note, NPC1 plays a role of receptor for filoviruses; the second luminal loop is the site for binding of a glycoprotein from Ebola virus, to which Npc1-/- mice are resistant (88; 37; 34). In order to better understand the cause of brain dysfunction in Niemann-Pick type C disease, attention is currently devoted to several aspects of the physiopathological cascade. One of them is autophagy. Regarding NPC1 deficiency, there is a consensus on an increase in the steady state number of autophagosomes and late endosomes, but there is some controversy regarding the global mechanism. An impairment of the autophagic flux, with defective amphisone formation leading to retarded autophagic cargo clearance has been reported (129). This should be kept in mind regarding therapeutic approaches, including dosing of 2-hydroxypropyl-ß-cyclodextrin (181). Because more than half of the NPC1 mutations (including the common p.I1061T and many others) are missense ones, which often induce reduced protein levels through increased degradation via ubiquitin-proteasome, endoplasmic reticulum quality control pathways are also important to study. Work is indeed in progress to investigate the rescuing effect of proteasome inhibitors, pharmacological chaperones, or molecular chaperones (31; 75; 131; 172).
Molecular genetics. Cell hybridization studies and linkage analysis have established the existence of 2 complementation groups, NP-C1 and NP-C2, in Niemann-Pick C disease (142; 154).
Approximately 95% of families belong to the NP-C1 group. The NPC1 gene, mapped to chromosome 18q11-q12, spans 56 kb and contains 25 exons (12; 100). About 700 NPC1 genetic variants have already been reported, among which about 450 are considered pathogenic, with only few common or recurrent (often in certain populations) mutations. The globally most frequent mutant allele is p.I1061T (86). It constitutes about 25% of alleles in the United Kingdom, 15% in France, but much fewer in Italy, Germany, or the Czech Republic. In the homoallelic state, it correlates with a juvenile or adolescent neurologic onset form. In the heteroallelic state, it has been found associated with all clinical forms although exceptionally with the most severe infantile neurologic onset form. Among the few additional most recurrent mutations, 3 are typical of the “variant” biochemical phenotype (less pronounced cholesterol trafficking abnormalities): p.P1007A (second most frequent mutation in Europe, also quite prevalent in Brazil), p.G992W, typical of Nova-Scotian patients, previously known as Niemann-Pick type D (35; 87; 144), and p.G992R. All types of mutations have been described, including large deletions (125) and deep intronic mutations, but more than half are missense mutations. A number of mutant alleles have remained unidentified. Many polymorphic genetic variants have been described.
Genotype-phenotype studies in homoallelic patients generally showed good correlation between nonsense or frameshift mutations and the most severe neurologic course. At the other end of the clinical spectrum, some NPC1 mutations (eg, p.R978C, p.G992R, p.D874V) appear to be associated with an adult-onset form. Some adult onset patients have been described with 1 severe or null allele combined to a mild allele. From observations in multiplex families, it can be concluded that mutations show correlations with the global subtype of neurologic disease but are not predictive of the systemic course: one sibling may have died from fetal hydrops and another one suffered from a juvenile neurologic onset form. Some missense mutations have underscored the functional significance of particular domains of the NPC1 protein: p.R518Q, and to a lesser extent p.R404W, were shown to alter NPC2 binding. Homozygous mutations in the sterol-sensing domain were generally found very deleterious (87). The cysteine-rich luminal loop, early described as a mutational hot spot, contains approximately one third of all described mutations, with a variable cellular and clinical impact (35; 87; 95; 84); interestingly, mutations leading to a minor impairment of cellular trafficking (“variant” phenotype) are typically located on this loop (87; 144). NPC2 was shown to correspond to a previously identified gene, HE1, mapped to chromosome 14q24.3 (91). The NPC2 mutational pattern (26 pathogenic mutations described to date) shows 1 more common nonsense mutation (p.E20X) and many mutations leading to a truncated protein associated with very severe phenotypes. Described missense mutations correspond to various phenotypes, including juvenile- and adult-onset patients; p.C99R appears frequent in Moroccan patients (85; 163); p.S120P (first observed in a patient with a juvenile neurologic onset and slowly progressive form), which has been instrumental to confirm the functional significance of the cholesterol binding site of the NPC2 protein, has been described in patients from Turkey and Iran. The frequency of families with NPC2 mutations varies greatly between countries. A significant number have been described from North Africa, Turkey, and Italy. Several spontaneous animal mutants involving the Npc1 gene are known (2 in the mouse and 1 in the cat). An Npc2 mouse mutant has been generated (137), 1 mouse harboring a p.D1005G Npc1 missense mutation has later been characterized (79), and an Npc1 p.I1061T mouse has also been made (121). These models are particularly useful to study brain dysfunction and to facilitate various types of experiments, including preclinical therapeutic trials.
The disease shows autosomal inheritance and is panethnic. In Western Europe, the minimal incidence has been estimated to be 1 in 120,000 live births (177), but data indicate that it might be closer to 1 in 100,000 (153; 49; 173). The prevalence is estimated to approximately 1 in 1.106. Two genetic NPC1 isolates have been described, French Acadians in Nova Scotia (initially described as type D Niemann-Pick disease) (p.G992W mutation) and individuals of Hispanic descent in southern Colorado and New Mexico (p.I1061T mutation) (20; 175; 35; 86).
Niemann-Pick C disease is genetically inherited following an autosomal recessive mode. Prenatal diagnosis is possible (153; 97) and is best achieved using chorionic villus sampling (CVS) at 10 to 12 weeks, as studies on amniotic fluid cells will lead to a later diagnosis. Today, prenatal diagnosis of NPC is performed by molecular genetic analysis, which can be applied to uncultured CVS. As a prerequisite, both mutated alleles must have been identified in the index case, and parents also studied. Few laboratories, if any, currently agree to perform prenatal diagnosis by cellular biology testing because the procedure is difficult, less reliable, and not applicable to “variant” families. This approach (in parallel with molecular testing) can still be helpful in systematic prenatal screening for lysosomal diseases in cases of nonimmune hydrops fetalis or fetal ascites (16). Preimplantation genetic testing should in principle be possible.
Although a proportion of heterozygotes show mild abnormalities by the filipin test (158; 159; 161), detection of heterozygotes is not reliable using cellular biology methods and must be done by genetic testing. Identification of mutations in every Niemann-Pick C disease case, followed by parental study, is important as it allows reliable prenatal diagnosis for the couple as well as precise genotyping in their relatives.
In the neonate and young infant, Niemann-Pick disease type C must be differentiated from idiopathic neonatal hepatitis and other causes of cholestatic icterus. Onset of cholestasis usually occurs in the early neonatal period. Associated splenomegaly is a useful orienting sign. In a young child with or without a history of transient neonatal cholestatic jaundice, differential diagnosis of various causes of splenomegaly or hepatosplenomegaly should include not only Niemann-Pick type B, but also Niemann-Pick type C, because the systemic manifestations in this disease may remain isolated for a long period. In children and adults, diagnosis may be difficult in patients without overt visceromegaly (at least 15% of the cases). Depending on the symptoms, other conditions with cerebellar ataxia, dystonia, psychosis, and vertical supranuclear gaze palsy need to be considered (60; 97; 61). Targeted new generation sequencing panels (eg, for ataxias) have already led to the diagnosis of Niemann-Pick disease type C. Demonstration of foam cells in bone marrow or, better, typical inclusions on electron-microscopic examination of a skin or conjunctival biopsy are useful discriminating signs. Among lipidoses, differential diagnosis before onset of the neurologic disease involves mostly Niemann-Pick disease type B, but also atypical Niemann-Pick disease type A, or possibly Gaucher disease or acid lipase deficiency. It is important to note that in leukocytes or lymphocytes, sphingomyelinase activity is not decreased below normal range in Niemann-Pick disease type C. Differential diagnosis has been discussed by Patterson and colleagues (97). The most difficult differential diagnosis occurs in patients with psychiatric manifestations and no systemic symptoms.
Brain imaging is not particularly useful for diagnosis of the patients (except for differential diagnosis), as findings are variable and too aspecific, but imaging is useful for follow up of patients and therapies.
Foam cells and sea-blue histiocytes are often present in bone marrow, but these findings are not specific for Niemann-Pick disease type C (they are also observed, among others, in Niemann-Pick disease types A and B), and storage cells may be missing.
In recent years, substantial progress has been achieved in the field of NP-C screening and diagnosis, and the filipin test is no longer considered the primary tool. Several plasma metabolites have emerged as sensitive biomarkers for NP-C, and their study, completed by genetic analyses, should now be considered a first-line testing once NP-C is suspected clinically. However, identification of certain mutations can be missed by sequencing, and further genetic tests may be necessary. Also, interpretation of new NPC1 genetic variants as disease-causing is often difficult. In such cases, the historical filipin test, although requiring a skin fibroblast culture and an experienced laboratory, is still useful (155; 96).
Orientation tests in plasma.
Chitotriosidase. Chitotriosidase activity is often, but not constantly, elevated, and this finding is very aspecific.
Oxysterols studies (cholestane-3ß-5α−6ß-triol and/or 7-ketocholesterol). Following the initial publication (120), several laboratories have implemented these assays, using a similar or modified method (07; 69; 59; 94; 124; 117; 126). Increased values were found for more than 90% of Niemann-Pick C patients tested (albeit close to cut off for some of them), but a mild elevation is also observed in 25% of Niemann-Pick disease type C heterozygotes. Furthermore, an increase of both metabolites is now well documented in acid sphingomyelinase deficiencies, acid lipase deficiencies, cerebrotendinous xanthomatosis, and some other causes of neonatal cholestasis; an isolated increase of the less specific 7-ketocholesterol further occurs in Smith-Lemli-Opitz syndrome and some peroxisomal diseases. Although technically more demanding than the 7-ketocholesterol assay, measuring the more specific cholestane-3ß-5α−6ß-triol alone has now been adopted by a number of laboratories.
Bile acid metabolites. Independently, the groups of DS Ory in the U.S. and of P Clayton in the UK proposed an alternative bile acid−based approach (50; 80). Measurement of 3β,5α,6β-trihydroxy-cholanoyl-glycine appears to offer many technical advantages over the cholestane-3ß-5α−6ß-triol assay (no in vitro "self-generation," no need for derivatization, application to dried blood spots and, therefore, to eventual neonatal screening). Discrimination between patients and heterozygotes also seems quite good. Of note, an increase in acid sphingomyelinase deficiencies has also been reported (50).
Lysosphingomyelin measurements. The simultaneous measurement of the so-called “lyso-SM-509” (with a recently elucidated structure of N-palmitoyl-O-phosphocholineserine [PPCS]) (136) and of lysosphingomyelin is a promising tool for initial screening of all Niemann-Pick diseases, types A or B (ASMD) and C (174; 32; 62). Striking elevation of lyso-SM-509 (PPCS) occurs in all types, whereas for lyso-sphingomyelin, a large increase occurs only in ASMD, with marginal or no elevation in type C. A combined assay, thus, provides good discrimination between NP-C and ASMD (62; 108; 118). Multiplex lysosphingolipid assays have been developed (108; 118; 165; 119), allowing, among others, simultaneous measurement of glucosylsphingosine, a biomarker for Gaucher disease, another potential differential diagnosis of Niemann-Pick disease type C, in the same assay. Several studies indicate that measurement of lyso-SM-509 (PPCS) is not fully reliable in dry-blood spots (62; 119; 136).
Thus, a combination of cholestane-3ß-5α−6ß-triol (or trihydroxy-cholanoyl-glycine) and lyso-SM-509 (PPCS)/lyso-sphingomyelin measurements appears today as a useful first screening test for Niemann-Pick disease type C (and also for ASMD). A nonexhaustive list of laboratories that were offering such tests is provided.
But the diagnosis needs, in all cases, to be confirmed by a more specific approach.
Molecular analysis of the NPC1 and NPC2 genes. Mutation analysis should be considered mandatory to confirm the diagnosis of Niemann-Pick disease type C. More than 95% of patients with Niemann-Pick disease type C have mutations in the NPC1 gene (25 exons), and the remaining ones in the NPC2 gene (5 exons). Besides Sanger sequencing, next generation sequencing technologies and several gene panels can now provide accurate and sensitive methods for genetic analysis. But mutations may be overlooked in gDNA sequencing (large deletions are not exceptional, and an increasing number of deep intronic mutations are being described). Complementary tests (quantitative methods to asses copy number, MLPA, cDNA sequencing) have been recommended. Whenever sequencing or a more refined study identify in the 2 alleles mutations known as surely pathogenic in either the NPC1 or NPC2 genes, the diagnosis is certain. To ensure segregation of alleles, it is, thus, important that at least 1 of the parents (or both, in case of an apparent homozygous mutation) has also been studied (97; 155; 96). Nevertheless, in practice, interpretation of the data remains uncertain in 10% to 15% of the cases, mostly due to the finding of variants of unknown significance and also because a small proportion of mutated NPC1 alleles have remained unidentified in proven patients. In such cases, the filipin test should be conducted. Some of the problems with exome sequencing as primary test have been discussed from an illustrative case (187).
The filipin test. This assay consists of the cytochemical demonstration of a lysosomal accumulation of unesterified cholesterol, as shown by intense perinuclear fluorescence after staining with filipin (a probe forming specific complexes with unesterified cholesterol). In practice, living cells (fibroblasts) are conditioned in a cholesterol-free medium to upregulate LDL receptors, then exposed to a pulse of LDL-enriched medium for 24 hours before fixation, staining, and examination by fluorescence microscopy (159; 151). Very clear-cut results are observed in 85% of patients, but a lesser cholesterol accumulation ("variant" phenotype) occurs in the remaining ones (159).
The "variant" phenotype is determined by certain NPC1 mutations (among which p.P1007A or p.G992W/R). It is more frequently observed in patients with late-onset neurologic disease (151). A mildly abnormal filipin pattern has also been observed in a number of heterozygotes (161), in acid sphingomyelinase deficiencies, and other pathological states (among reported ones MEGDEL syndrome, Smith-Lemli-Opitz syndrome, Tangier disease) (151). Because the threshold for cut off is difficult to precisely define, when filipin testing was the gold standard test, it was advised in such cases to sequence the NPC1 and NPC2 genes. Main drawbacks of the filipin test are the requirement of a skin biopsy for establishment of a fibroblast culture, the minimal 5- to 7-week delay between the skin biopsy and the results (most of it due to the establishment of the culture), and the need to have it conducted in a specialized center. Clear-cut results are obtained in a majority of cases, but interpretation requires rigorous technical conditions and experience in about 18% of the requests. As said above, filipin test and molecular analysis remain complementary, and the combined study considerably reduces the number of unclear cases.
Updated algorithm for testing. The algorithm summarizing laboratory diagnosis of Niemann-Pick disease type C published in 2012 has now been updated (97; 96), owing to validation of plasma biomarkers and progress in molecular genetics technology.
When clinical symptomatology (and eventual further orientation data) suggest Niemann-Pick disease type C, testing should first enlist plasma biomarkers (oxysterols and lysosphingomyelins), followed by genetic testing for definitive confirmation, with filipin test mandatory only in the some 15% of cases in which genetic testing remains inconclusive. Depending on local conditions (cost and insurance reimbursement rules, availability of tests), and if a cell culture is already available, filipin test could still be used in priority, but even in clear-cut cases diagnosed by the filipin test, undertaking gene testing is of utmost importance because molecular genetic study is required for prenatal diagnosis as well as identification of carriers in blood relatives.
The clinical diagnosis of Niemann-Pick disease type C is relatively easy in patients with the most typical symptoms, such as combined splenomegaly, ataxia, and vertical gaze palsy. However, strikingly different clinical presentations exist, especially in infants and neonates. Also, neurologic onset may be delayed until adolescence or adulthood, and isolated spleno- or hepatosplenomegaly can be the presenting symptom. The disease may not be suspected in cases lacking overt organomegaly. Finally, psychiatric illness may precede motor problems (60). To help clinicians, a suspicion index tool for patients more than 4 years old has been elaborated (176), and there is also a similar tool for young children (112). A posthoc evaluation was published (111). Clinical patient groups with an increased probability of suffering from Niemann-Pick disease type C have also been discussed (96).
Ultrastructural studies on conjunctival, skin, liver, or rectal biopsies can provide strong support to the diagnosis, but false-negative results often occur on liver biopsy studied by light microscopy only (55). Analysis of the lipids in a liver biopsy may also be inconclusive. Sphingomyelinase activity is normal in leucocytes, but it is often partially deficient in cultured skin fibroblasts. Routine laboratory tests give normal results, except in patients with cholestatic jaundice or hypersplenism. Low HDL cholesterol is a common finding, but it is not universal.
Conventional imaging and neurophysiologic studies are nonspecific. MRI and CT scans may be normal or show cerebellar or cortical atrophy, or, in the severe infantile form, white matter changes. Specific studies, including size and volume of corpus callosum, have been associated with disease severity and used for follow-up of therapy (169; 168; 171; 65; 09; 64; 77). Longitudinal MRS studies might be useful for follow-up of therapy (145; 133). Serial acoustic reflexes, brainstem auditory evoked responses, EEGs, and somatosensory evoked potentials studied in 36 patients showed early absence of the acoustic reflexes, later changes of the brainstem auditory evoked responses, EEG abnormalities in 57%, and abnormalities in median or peroneal nerve somatosensory evoked potentials in all patients (39). Several methods of analyzing movement abnormalities (29; 43) or neuropsychological profiles (58) have been evaluated.
To date, management remains largely symptomatic. Information and support to families can be obtained through organizations devoted specifically to Niemann-Pick diseases (in the United States, Canada, United Kingdom, Germany, Spain, Italy, Argentina, The Netherlands, etc.) or to lysosomal (France) or inherited metabolic diseases. Genetic counseling should be made available for family members.
Disease-modifying treatment. In murine and feline NPC1 mutants, bone marrow transplantation did not improve the neurologic disease; similarly, after bone marrow transplantation the neurologic status of a child continued to deteriorate, although there was a regression of hepatosplenomegaly and lung infiltration (44). Liver transplantation performed in a few NPC1 cases with cirrhosis did not influence the course of neurologic deterioration. Because the NPC2 protein is soluble, secreted, and recaptured, there is a rationale supporting early hematopoietic stem cell transplantation in NPC2 patients, but experience on the long-term outcome is limited (10). The primary trigger for brain pathology remains elusive. Treatment strategies based on the hypothesis that cholesterol is the offending metabolite were first proposed in the early 1990s. The combination of hypocholesterolemic drugs and a low-cholesterol diet partially reduced the cholesterol load in the liver, but no amelioration of the neurologic disease was seen in patients after 2 years of treatment (130). It is important to note that at variance with Niemann-Pick disease type B, Niemann-Pick disease type C patients usually do not have elevated cholesterol values in serum. Because glycolipid storage appears to contribute to at least some of the neuropathologic features, substrate reduction therapy using miglustat (N-butyldeoxynojirimycin,NB-DNJ, OGT 918), an iminosugar inhibitor of glucosylceramide synthase (later approved for oral treatment of mild to moderate type 1 Gaucher disease), was administered to NPC1 mouse and cat mutants. It resulted in delayed onset of neurologic symptoms in both species and a 20% longer survival of the mice. This led to a controlled clinical trial of miglustat in neurologically symptomatic adolescent and adult patients (> 12 years) (102) and a trial in children (4 to 12 years of age). Long-term data from open-label extension treatment (up to 66 months) have been reported (101; 180). The disease course was stabilized in 72% of patients treated for 1 year or more, based on a composite assessment of horizontal saccadic eye movement velocity, ambulation, swallowing, and cognition (102). A multicenter observational cohort study in 66 patients treated off-label with miglustat further showed a significant reduction in the annual rate of progression of the disease (115). Miglustat was approved for treatment of progressive neurologic manifestations in adult and pediatric patients with NP-C in 2009 in the European Union and later in many other countries, including Canada. Currently, miglustat is not U.S. FDA-approved for this indication. Studies in the cat model indicate that miglustat ameliorates several aspects of the brain pathology and may do so in large part through mechanisms different from substrate reduction (141).
In parallel, research toward new therapeutic approaches has been pursued in animal and cellular models (97), with some leading to clinical trials. This includes cell-signaling targets, curcumin, NSAIDs, beta-cyclodextrin (02; 21; 71; 05), inhibitors of histone deacetylases (HDAC) (89; 116; 03), and inducers of hsp70 (eg, arimoclomol) (57). Work is also ongoing toward gene therapy in the Npc1 mouse (in spite of NPC1 being a transmembrane protein) (14; 45) and also the Npc2 mouse (76). Promising results have been obtained by intrathecal administration of 2-hydroxypropyl-beta-cyclodextrin (HPßCD) in Niemann-Pick disease type C mouse and cat models (05; 164), leading to a phase 1/2a clinical trial using a VTS-270 (Kleptose-HPB) preparation (183), now called adrabetadex, for which encouraging results have been published (92). However, from press releases, preliminary analyses of the phase 2b/3 trial at 12 months showed puzzling results, with significant progression neither in the treated group (expected) nor the (sham) control one (very unexpected). (See clinicaltrials.gov/NCT02534844 for more information). In parallel, a phase 2/3 trial using oral administration of the hydroxylamine derivative arimoclomol, a heatshock 70/40 inducer in cells under stress, has been conducted (clinicaltrials.gov/NCT02612129). From press releases, significant differences were said to have been reached at 12 months between the treated and placebo groups for patients aged 4 to 18 years. Of note, in both trials patients previously treated with miglustat continued to receive that drug throughout the trial. Finally, a phase 1 clinical trial with intravenous infusion of TrappsolRCycloTM HPBCD is currently ongoing (clinicaltrials.gov/NCT02939547).
In human patients with Niemann-Pick disease type C, the lack of good disease markers and clinical endpoints, as well as the broad clinical spectrum, make follow-up of trials particularly difficult. Efforts have been made to identify new biomarkers (15; 17; 78; 92).
Symptomatic management. Symptomatic management has been reviewed by Patterson and coworkers (97). Guidelines for seizures generally respond at least partially to antiepileptic drugs until a fairly advanced stage of the disease. Cataplexy can usually be controlled by clomipramine, protriptyline, or modafinil. Anticholinergic agents have been reported to improve dystonia and tremor in some patients. In a pilot study, acetyl-dl-leucine showed some improvement of ataxia (11). A phase 2 clinical trial using N-acetyl-L-leucine has just started (IB1001-201; NCT03759639). Physiotherapy is useful in the management of spasticity and the prevention of contractures. Melatonin can be used to treat sleep inversion. Patients with significant hearing impairment benefit from hearing aids. Patients with significant hearing impairment benefit from hearing aids. Patients with a slow course of the disease may benefit from special schooling for handicapped children. Prominent splenomegaly and hypersplenism are rare. Spleen size tends to diminish during the course of the disease. Proper management of infections and of feeding difficulties (gastrostomy) is essential at an advanced stage of the disease.
Treatment with miglustat. Several case series and single reports have been published (113; 25; 26; 38; 52; 98; 90; 110). For review, see Pineda and colleagues (114). Late-onset forms generally appeared as the best responders. Overall, miglustat appeared to stabilize clinically assessed progression in a majority of these patients, at least for a certain period, and even led to improvement of some neurologic indicators. On the other hand, therapeutic benefit seems minimal among patients with early-infantile neurologic onset. Indication and monitoring of patients have been reviewed (177; 97). It has been recommended to treat patients as soon as they show neurologic manifestations of any type, but not patients with only systemic disease as miglustat is not expected to improve the systemic manifestations of Niemann-Pick disease type C, and it has known adverse effects, such as diarrhea, weight loss, and tremor.
Several women known to the author have undergone uneventful pregnancies prior to neurologic onset of their disease.
Perianesthetic morbidity in 1 retrospective study on 32 patients aged 1.8 to 33 years (median age 6.9 years) having undergone 64 anesthesias for diagnostic procedures disclosed need for tracheal reintubation as well as pneumonitis, hypothermia, and seizures (83).
Marie T Vanier MD PhD
Dr. Vanier, Director of Research Emeritus at Institut National de la Santé et de la Recherche Médicale has received honorariums from Orphazyme, Orchard Therapeutics, and Sanofi Genzyme and consulting fees from Orchard Therapeutics.See Profile
Raphael Schiffmann MD
Dr. Schiffmann of Baylor Scott & White Research Institute received research grants from Amicus Therapeutics, Takeda Pharmaceutical Company, Protalix Biotherapeutics, and Sanofi Genzyme.See Profile
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