Clinical manifestations

Presentation and course

Systemic manifestations.

Fetal and neonatal presentations. The ealiest (rare) presentation of Niemann-Pick disease type C is with fetal hydrops or, more frequently, fetal ascites (128; 15); many of these newborns develop liver or respiratory failure and die rapidly.

In the neonatal period, prolonged cholestatic icterus associated with progressive hepatosplenomegaly is present in at least one third of patients (149; 53; 165; 94). 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.

In a small subset (fewer than 10%) of patients with prolonged neonatal jaundice, the cholestasis quickly worsens and leads to liver failure and death, in most cases before the age of 6 months. 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 rapidly fatal. Infants with this dramatic "acute" perinatal rapidly fatal systemic form die before onset of neurologic symptoms (other than an aspecific general hypotonia). Many examples in multiplex families have shown that this presentation can be associated with a neurologic form in the same sibship. Only infants who have died early from their systemic disease should, therefore, be included in this category.

Systemic involvement in infants, children, or adults with Niemann-Pick disease type C. Isolated hepatosplenomegaly or splenomegaly is often the presenting symptom in infants and children without a history of prolonged neonatal cholestatic jaundice. This may stay the only sign for many years or decades, until onset of neurologic symptoms (45; 161; 139). 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 three 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 (24). Such cases may remain undiagnosed (158). Nevertheless, apart from these exceptional cases and from infants with early death, more than 90% of patients with Niemann-Pick disease type C develop neurologic symptoms.

Neurologic and psychiatric manifestations.

Patients with Niemann-Pick disease type C are best classified according to the age of onset of their neurologic symptoms because age correlates with the subsequent course and lifespan (149; 28; 147; 95; 161; 139; 94; 129; 47; 44).

Severe early infantile neurologic form. In most of these infants, prolonged neonatal jaundice has been observed, and hepatosplenomegaly has been present since birth or the first months of life (94). Delay of developmental motor milestones and central hypotonia become evident between the ages of 6 to 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 affected 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 (149; 46; 161; 119). This form seems more frequent in Southern Europe (where it constitutes about 20% of the cases) and the Middle East than in the United States.

Late infantile neurologic form. A history of prolonged neonatal jaundice is frequent; 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. Gelastic cataplexy is frequent and sometimes can be the initial symptom. More than half of patients develop epileptic seizures of all types (94). Vertical supranuclear gaze palsy, which should more appropriately be renamed vertical supranuclear saccade palsy is a nearly constant sign.

Juvenile neurologic 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 15 years of age, but onset is more variable. Vertical supranuclear gaze/saccade palsy also constitutes a typical sign. In younger children (some of them overlapping with the late infantile category), school problems with difficulty in writing, impaired attention, or apraxia are often the first signs and may lead to misdiagnosis. The child then develops clumsiness and shows more learning disabilities, and ataxia becomes obvious (falls often, cannot run properly). Seizures or gelastic cataplexy may occasionally be the presenting symptom and can become a major problem in more than half of these patients. In pre-adolescents, the onset is often more insidious, with slower progression of ataxia and other symptoms. Movement disorders more often include dystonia and, sometimes, myoclonic jerks. This “late juvenile” form shows overlaps with the adult-onset form, as shown from data in several large cohorts where siblings were classified in either category (47; 44).

The late infantile and juvenile forms, considered the “classic” Niemann-Pick type C, constitute in most countries about half of the patients. The disease progression is globally similar. As ataxia progresses, dysphagia, dysarthria, and dementia develop. Action dystonia is also frequent, mostly in the (late) juvenile form. Cataplexy, without or with narcolepsy (87; 67), typically laughter-induced, is another common and specific sign, observed in about one third of patients (51). Motor impairment is major, and intellectual decline may be variable. The neurodevelopmental and neurocognitive phenotypes in children and adolescents have been studied (135; 134). 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 will require gastrostomy. Death usually occurs between 7 and 12 years of age 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 (161).

Adolescent and adult form. With increasing awareness, more patients with a late neurologic onset (often in the second or third decade, but as late as 50 years or older) have been identified in the past 10 years, although this diagnosis likely remains underestimated (121; 155; 55; 120; 83). The global symptomatology is that of an attenuated juvenile form with an insidious onset, but patients are likely to present with fragmentary phenotypes. About one third of cases may have a psychiatric presentation that can be 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 (89; 06). 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. Some patients may also enter the disease with cognitive decline and mainly frontal syndrome. More in continuum with the juvenile form, many patients present with a gait disorder and cerebellar dysfunction, often associated with dystonia and, more rarely, with myoclonus. Movement disorders (dystonia, parkinsonism, chorea) are more frequent than in the juvenile form. The cognitive dysfunction can be very variable. Epilepsy is rare in adult-onset Niemann-Pick disease type C. 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 for a long period (56; 89; 11). The later disease progression is essentially similar to that in the juvenile form.

In all forms of Niemann-Pick disease type C, vertical supranuclear gaze palsy will develop. Parents may notice compensatory head thrust when the child wants to look downward or upward. The initial sign, which may require careful examination, is an increased latency and slowing of voluntary vertical saccadic movements, slowly evolving towards vertical supranuclear gaze palsy; horizontal supranuclear saccades palsy occurs later (114; 01). A prospective study of 72 subjects recently described ocular motor manifestations of Niemann-Pick disease type C in detail (11).

Prognosis and complications

Several clinical (neurologic) disability scales have been developed for clinical follow-up of patients and evaluation of therapies. A first scale was proposed by Pineda's group, and assessed four domains: ambulation, manipulation, language, and swallowing (46). It was later modified to add seizures and ocular movements (104). The 17-domain Niemann-Pick type C clinical severity scale (NPCCSS) proposed by Porter’s group at the National Institutes of Health provides the most complete panorama of the disease course, but it is time-consuming in regular clinical practice. It evaluates nine major domains (further including cognition, memory, and hearing) and eight modifiers (164). Its abbreviated five-domain version (ambulation, manipulation, speech, swallowing, and cognition) appears to be a valid alternative in regular clinical practice (75; 92). To evaluate and compare individual rates of progression, an annual severity increment score (ASIS) has been proposed, in which the total score is divided by the age of the patient (16).

The disease is considered to invariably lead to death. However, the rate of progression and lifespan show considerable variation. Estimates of age at death in patients have been compiled, either globally (05), or categorized from the age of onset of neurologic disease (161; 139). Some (5% to 10%) 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 might be longer than found in very early historical cohorts. In a large study, no significant change in survival was found over the past 20 years (05). Owing to improved diagnosis of patients with the adult-onset form in recent years, as of 2021, slightly more than half of the living patients are aged between 20 and 60 years 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 (81; 24). In neurologic patients, two natural history surveys using different disease-specific disability scales concluded a linear progression over time (162; 164); however, the slopes varied when a broader range of clinical phenotypes was studied (162; 129).

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 early from severe pulmonary insufficiency. Neonatal cholestatic jaundice is otherwise transient and usually resolves spontaneously by 4 months of age. Motor involvement is often more severe and more rapidly progressive than intellectual disability. 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. The commonest cause of death is aspiration pneumonia, related to a combination of brainstem failure and pulmonary infection.

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 perform regular audiological monitoring in patients with Niemann-Pick disease type C, beginning at the time of diagnosis (54). Patients with NPC1 mutations seem to have an increased susceptibility to early-onset fistulizing colitis with granuloma formation reminiscent of Crohn disease, which has been linked to defective autophagosome function (117).

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.

Clinical vignette

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, she showed clumsiness and learning problems at school. 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, the 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 (three to four 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, which were described as a loss of tonus and were suggestive of cataplexy. 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 years. One of her NPC1 alleles carried a p.G992E mutation, the second one a p.G993 frameshift mutation.

Biological basis

Etiology and pathogenesis

Years after the breakthrough discovery by P. Pentchev of abnormal cholesterol trafficking as the cellular hallmark of Niemann-Pick disease type C, the existence of two genes responsible for the disease was established by complementation studies using cell hybridization and linkage analysis (131; 141). This also showed, before identification of the NPC1 gene, that approximately 95% of families belonged to the main NP-C1 group, which still holds true. Early studies in cells and tissues from NP-C1 and NP-C2 patients could not disclose any biochemical marker that was specific to any of the groups. The NPC1 gene was identified in 1997 by positional cloning (13), the NPC2 gene in 2000 by proteomics (84). Clinical and biochemical comparison between double mutant mice deficient in both NPC1 and NPC2 and the single mutants demonstrated a nonredundant functional cooperativity of the two proteins (125) and established that mutations in either of the genes result in a similar cellular lesion.

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 filament tau is indistinguishable from that in Alzheimer disease (95). Alterations of the amyloid metabolism have also been described (71). 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 (95; 151). The primordial role of NPC1 expression in neurons was underscored by studies in conditional mutant mice, showing that neuron-specific expression of NPC1 in null mutant Npc1 mice rescued disease, whereas neuron-specific NPC1 deficiency was sufficient to cause disease). NPC1 expression in both neurons and oligodendrocytes appeared important for CNS myelin formation and maintenance (166), but astrocyte-specific deletion of NPC1 did not cause a phenotype.

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 NPC1 and NPC2 disease (patients and mouse models) (125; 139; 140). 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, and free sphingosine is only mildly elevated. But histochemical staining (by filipin or BC-theta) reveals a significant storage of unesterified cholesterol in the late endosomal/lysosomal compartment of neurons. By biochemical analysis of cerebral gray matter, a striking increase of GM2 and GM3 gangliosides is the most conspicuous lipid abnormality. Myelin lipids are markedly affected in the severe infantile neurologic form only (140).

Indication of oxidative stress participation in pathogenesis led to reappraisal on the status of cholesterol oxidation products in the Npc1 mouse model. In liver, elevated 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. Conversely, both in brain and plasma, there was a decrease of the enzymatically (CYP46A1) formed 24(S)-hydroxycholesterol, which 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 (108; 136).

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, three large (and four small) luminal loops, six small cytoplasmic loops, and a cytoplasmic tail (22). A sterol-sensing domain (SSD) (amino acids 615 to 797, corresponding to transmembrane helices 3 to 7) had previously been defined because of gene homologies with the SSD of HMG-CoA reductase, SCAP, patched, and NPCL1 (13). This structure was confirmed by cryo-electron microscopic or cristallographic studies (62). NPC1 resides primarily in late endosomes and interacts transiently with lysosomes. 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 (132).

Model in systemic cells. There is ample evidence that both NPC1 and NPC2 are required for cholesterol egress from the endolysosomal system. Following early studies by P. Pentchev and coworkers, work from several laboratories (essentially that of J. Goldstein and M. Brown) led to a "hand-off" model for the coordinated sequential role of the two proteins (59). In cells such as hepatocytes or fibroblasts, after endocytosis of LDL via LDL-receptor, unesterified cholesterol, released from cholesteryl esters by 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 two proteins will, thus, lead to a progressive accumulation of unesterified cholesterol in the endolysosomal compartment. Specific functions were assigned to certain domains of NPC1. The first luminal N-terminal loop possesses a cholesterol-binding site. The middle luminal loop was shown to directly bind NPC2. Next steps were conditioned by further structural observations (100), which led to a first, partly hypothetical model (137). Significant progress came from studies of orthologs of Niemann-Pick type C proteins in eukaryotes (160), soon shown to also apply to mammals. The current knowledge regarding the intimate mechanism of cholesterol transfer can be summarized as follows: facilitated by bis(monoacylglycero)phosphate, NPC2 binds cholesterol from intraluminal vesicles, docks into the NPC1 middle luminal domain, and transfers the sterol to the N-terminal loop of NPC1. The latter rotates to form a tunnel between the middle luminal loop and C-terminal domain loops, allowing cholesterol to pass through the glycocalyx and reach the sterol-sensing domain (100; 110). NPC1 is then involved in the efflux of cholesterol from the lysosome to redistribute it to the plasma membrane and organelles including the endoplasmic reticulum (74).

Adaptation of the model to the 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 (138). 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 (02). 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 (140), 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 (125). 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 (138). 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 prominent features of the pathophysiological cascade. There are still many uncertainties regarding the complete functions of NPC1. 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 the regulation of cellular growth. In NPC1-deficient cells, aberrant mTORC1 signaling drives lysosomal dysfunction downstream of cholesterol accumulation, and inhibition of mTORC1 restores organelle homeostasis (23). There are also data indicating a role for NPC1 in regulating vesicle budding or fission (34). On the other hand, a direct effect on sphingolipid transport has not been proven. The increase in ganglioside GM2 and other glycolipids appears due to the cholesterol-induced inhibition of their lysosomal degradative pathway (09). Sphingosine can exert various cellular deleterious effects (152). It appears as a likely cause of the reduced calcium release from lysosomes seen in the disease (65; 41). Elegant studies have demonstrated a defective lysosomal egress of this metabolite in NP-C cells (42). There is currently no evidence for a direct role of NPC1 in its export, but one recent study has correlated defective lysosomal egress of free sphingosine to reduced activity of sphingosine kinase 1 (85). Although sphingosine certainly participates in the Niemann-Pick disease type C pathogenic cascade, its exact role is still unclear. Of note, NPC1 plays a role as receptor for filoviruses; the second luminal loop (middle luminal domain) is the site for binding of a glycoprotein from Ebola virus to which Npc1-/- mice are resistant (82; 38; 35). To better understand the cause of brain dysfunction in Niemann-Pick disease type C, attention is also 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. An impairment of the autophagic flux, with defective amphisone formation leading to retarded autophagic cargo clearance has been reported (115). Because 60% to 70% of the NPC1 mutations (including the common p.I1061T and many others) are missense variants, which often induce reduced protein levels through increased degradation via ubiquitin-proteasome, endoplasmic reticulum quality control pathways are also important to study. Work is in progress to investigate the rescuing effect of proteasome inhibitors, pharmacological chaperones, or molecular chaperones (32; 116; 157).

Molecular genetics

The NPC1 gene maps to chromosome 18q11-q12, spans 56 kb, and contains 25 exons (13). About 700 NPC1 variants have already been reported, among which about 450 are considered pathogenic, with only a few that are common or recurrent, often showing geographical or ethnic distribution (19). All types of mutations have been described, including large deletions (113) and deep intronic mutations; approximately two thirds are missense variants. Missense mutations can be located in all domains of the protein, but a larger proportion affects the third cysteine-rich luminal loop, also called the C-terminal domain, including the two globally most frequent alleles, p.I1061T and p.P1007A. The p.I1061T variant constitutes about 25% of alleles in the United Kingdom and 10% in France but fewer in Germany, the Czech Republic, or Italy (4%) (80; 32). 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. The second most frequent variant in Europe is p.P1007A (also quite prevalent in Brazil and the United States). It appears more commonly associated to an adult or juvenile form than with a late infantile one. The p.G992W variant is typical of patients in Nova-Scotian (36). The mutational profile of representative national cohorts has been reported for Spain (27; 66), the United Kingdom (44), the Czech Republic (47), and Italy (19).

Genotype-phenotype studies in homoallelic patients have generally shown good correlation between nonsense or frameshift mutations and the most severe neurologic course. The frequency of null alleles is highest (about 60%) for patients with the early infantile neurologic form, compared with 10% in the juvenile or adult forms, in which missense variants (about 85%) largely predominate. At the other end of the clinical spectrum, some NPC1 variants (particularly p.G992R and p.D874V, but also some others) appear to be associated with an adult-onset form, even in combination with a null 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 (31). Some missense mutations have underscored the functional significance of specific 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 often found very deleterious (81). 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; interestingly, mutations leading to a minor impairment of cellular cholesterol trafficking (eg, p.P1007A, p.G992R); “biochemical variant” phenotype) are typically located on this loop (144).

NPC2 was shown to correspond to a previously identified gene, HE1, mapped to chromosome 14q24.3 (84). The NPC2 mutational pattern (26 pathogenic mutations described to date) shows one more common nonsense mutation (p.E20X) and many mutations leading to a truncated protein, associated with very severe phenotypes, including a large genomic deletion. Described missense mutations correspond to various phenotypes, including juvenile- and adult-onset patients; p.C99R appears frequent in Moroccan patients; p.S120P, which has been instrumental to confirm the functional significance of the cholesterol-binding site of the NPC2 protein, has been described in patients with the juvenile or adult form from Turkey and Iran.

Several spontaneous animal mutants involving the Npc1 gene are known (two in the mouse and one in the cat). An Npc2 mouse mutant has been generated (125), one mouse harboring a p.D1005G Npc1 missense mutation has been characterized (72), and an Npc1 p.I1061T mouse has also been made (109). These models are particularly useful to study brain dysfunction and facilitate various types of experiments, including preclinical therapeutic trials.

Epidemiology

In Western Europe, the minimal incidence, estimated earlier to be 1 in 120,000 live births, appears closer to at least 1 in 100,000 (158; 31). The population prevalence is estimated to be approximately 1 to 1.5 in 1.10⁶. The most common NPC1 variants differ according to the geographical or ethnic origin of the patients. One main genetic NPC1 isolate has been described, French Acadians in Nova Scotia (initially described as type D Niemann-Pick disease) (p.G992W variant). The p.G992W variant appears also predominant in Palestinian patients. The frequency of families with NPC2 mutations varies greatly between countries. A significant number have been described from North Africa, Turkey, and Italy.

Prevention

Prenatal diagnosis is best achieved using chorionic villus sampling at 10 to 12 weeks, as studies on amniotic fluid cells will lead to a later diagnosis (139; 89). It is performed by molecular genetic analysis, which can be applied to uncultured chorionic villus sampling. As a prerequisite, both mutated alleles must have been identified in the index case, and parents also must be studied. Preimplantation genetic testing is, in principle, possible. Prenatal diagnosis by cellular biology testing has been abandoned because the procedure is difficult, less reliable, and not applicable to “biochemically variant” families.

Detection of heterozygotes is not reliable using biochemical or cellular biology methods and must be done by genetic testing. Identification of mutations in an index case, followed by parental study, allows reliable prenatal diagnosis for the couple as well as precise genotyping in their blood relatives.

Differential diagnosis

Confusing conditions

Differential diagnosis has been discussed by Patterson and colleagues (89).

In the neonate and young infant, Niemann-Pick disease type C must be differentiated from idiopathic neonatal hepatitis and other causes of cholestatic jaundice. 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, the differential diagnosis of various causes of splenomegaly or hepatosplenomegaly should include, among lysosomal storage disorders, not only Gaucher disease and acid sphingomyelinase deficiency (Niemann-Pick disease type B) but also Niemann-Pick disease type C because the systemic manifestations in the latter may remain isolated for a long period.

In children and adults, the 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 should be considered (56; 89; 57). Targeted next-generation sequencing gene panels (eg, for ataxias) can be useful and are increasingly leading to the diagnosis of Niemann-Pick disease type C.

The most difficult differential diagnosis occurs in patients with psychiatric manifestations and no systemic symptoms.

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, but these examinations are no longer common practice. 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 dried blood spots, sphingomyelinase activity is not decreased below normal range in Niemann-Pick disease type C.

Diagnostic workup

General complementary tests (suspicion index tools). Routine laboratory tests give normal results, except in patients with cholestatic jaundice or hypersplenism. Low HDL cholesterol is a common but not universal finding.

Brain imaging is not particularly useful for diagnosis of the patients (except for differential diagnosis) as findings are unspecific, but it has proven useful for follow-up of patients and therapies. 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 the corpus callosum, have been associated with disease severity and used for follow-up therapy (154; 153; 156; 61; 07; 60; 70). Longitudinal MRS studies might be useful for follow-up of therapy (133; 118). 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 (40).

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 acid sphingomyelinase deficiency (Niemann-Pick disease types A and B), and storage cells may be missing.

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 (53).

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 (56). To help clinicians, a suspicion index tool for patients older than 4 years of age has been elaborated (159), and there is also a similar tool for young children (103). A post hoc evaluation was published (101). Clinical patient groups with an increased probability of suffering from Niemann-Pick disease type C have also been discussed (88).

Specific laboratory testing. Of note, in cases with isolated (hepato)splenomegaly, it is recommended to first exclude Gaucher disease and acid sphingomyelinase deficiency. In Niemann-Pick disease type C, chitotriosidase activity is often (but not constantly) moderately elevated, and this finding is very aspecific. Acid sphingomyelinase activity is normal in leucocytes or dried blood spots, but it is often partially deficient in cultured skin fibroblasts.

In recent years, substantial progress has been achieved for laboratory screening and diagnosis of Niemann-Pick disease type C. Several plasma metabolites have emerged as sensitive Niemann-Pick disease type C biomarkers, and their study, completed by sequencing of the NPC1 and NPC2 genes, constitutes the recommended first-line testing once Niemann-Pick disease type C is suspected clinically. Because identification or interpretation of certain genetic variants can be difficult, the historical filipin test, although requiring a skin fibroblast culture and an experienced laboratory, keeps its usefulness in uncertain cases as the best functional test (142; 88; 31). A proposed strategy for laboratory diagnosis of Niemann-Pick disease type C is outlined in an updated algorithm.

Orientation tests in plasma (or dried blood spots): biomarker studies.

Oxysterols (cholestane-3ß-5α−6ß-triol and/or 7-ketocholesterol). Following the initial publication, an increasing number of laboratories have implemented these assays (108; 142). Elevated values of cholestane triol (C-triol) or 7 ketocholesterol (7-KC) are observed in more than 90% of patients with Niemann-Pick disease type C (albeit close to cut-off for some of them), but a mild increase also occurs in 25% of heterozygotes. Importantly, an increase of both metabolites is not specific to Niemann-Pick disease type C but is also found in acid sphingomyelinase deficiency, acid lipase deficiency, cerebrotendinous xanthomathosis, and some other causes of neonatal cholestasis; an increase of the less specific 7-KC further occurs in Smith-Lemli-Opitz syndrome and some peroxisomal diseases.

Bile acid metabolites. The groups of DS Ory in the United States and of P Clayton in the United Kingdom proposed an alternative bile acid-based approach (49; 73; 123). Measurement of 3β,5α,6β-trihydroxy-cholanoyl-glycine (TCG) appears to offer many technical advantages over the cholestane-3ß-5α−6ß-triol (C-triol) assay (no in vitro "self-generation," no need for derivatization). Discrimination between patients and heterozygotes is also much better. At variance with C-triol, it can be applied to dried blood spots and, therefore, to neonatal screening (50); TCG is incorporated in a 14-disease pilot study that has just started. Of note, an increase has also been reported in acid sphingomyelinase deficiency (49). Few diagnostic laboratories worldwide, however, currently offer measurement of this metabolite.

"Lysosphingomyelin-509"/Palmitoyl-phosphocholine-serine (PPCS) and lysosphingomyelin measurements. The simultaneous measurement of the so-called “lyso-SM-509” (with a recently elucidated structure of N-palmitoyl-O-phosphocholineserine [PPCS]) (122; 68) and of lysosphingomyelin is a promising tool for initial screening of all types of Niemann-Pick disease, types A or B (ASMD) as well as C. Striking elevation of PPCS/"lyso-SM-509" occurs in type C but also types A and B (ASMD), whereas a large increase of lysosphingomyelin only occurs in ASMD, with marginal or no elevation in type C (33; 58; 122; 124). A combined assay, thus, provides good discrimination between Niemann-Pick disease type C and ASMD (58; 99; 106). Multiplex lysosphingolipid methods have been developed (99; 106; 107), allowing simultaneous measurement of glucosylsphingosine (lyso-Gb1), 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 PPCS/lyso-SM-509 is not fully reliable in dried blood spots (58; 107).

In short, measurements of C-triol or TCG or of PPCS/"lyso-SM-509" coupled to that of lysosphingomyelin, preferably in combination, constitute today a powerful primary screening test for Niemann-Pick disease type C (48). But in all cases, the diagnosis must be confirmed by genetic testing.

Definitive diagnosis by 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 have mutations in the NPC1 gene (25 exons), the remaining mutations in the NPC2 gene (5 exons). Next-generation sequencing technologies and Sanger sequencing provide accurate and sensitive methods for genetic analysis. But some variants can be overlooked: copy number assessment is mandatory, and an increasing number of deep intronic variants and deleterious effect of synonymous variants have been described. Complementary testing may, thus, be needed. Whenever sequencing or a more refined study identifies a pathogenic variant in the two alleles for either the NPC1 or the NPC2 gene, the diagnosis is certain. To ensure segregation of alleles, it is important that at least one of the parents (or both, in case of an apparent homozygous mutation) has also been studied (89; 88; 142). In practice, interpretation of the data remains uncertain in a few cases, mostly due to the finding of variants of unknown significance or because one presumably mutant allele has remained unidentified.

The "filipin test" and its current use. Historically and until recently, the "filipin test" (ie, demonstration in cultured cells of an accumulation of unesterified cholesterol in perinuclear vesicles, visualized by fluorescence microscopy after staining with filipin) was considered the gold standard diagnostic assay for Niemann-Pick disease type C (140). This requires a skin biopsy followed by a fibroblast culture prior to actual testing; hence, there was a lengthy turn-around time. Although cells from 85% of Niemann-Pick disease type C cases show intense fluorescent vesicles in a perinuclear distribution (classical profile), approximately 15% display a milder accumulation. This "biochemical variant" profile (defined by specific mutations, eg, p.P1007A, p.G992W/R) may overlap with that seen in some heterozygotes or in other diseases (especially ASMD). It is also more frequently observed in patients with a late-onset neurologic form of Niemann-Pick disease type C (140), who often have also less typical symptoms.

Management

Disease-directed therapies

Historically, treatment based on the hypothesis that cholesterol was the offending metabolite was 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. It is important to note that at variance with Niemann-Pick disease type B, type C patients usually do not have elevated cholesterol values in serum.

The few attempts to supplement the NPC1 protein by bone marrow or liver transplantation in animals and humans with NPC1 mutations have not had a proven effect on the neurologic disease, presumably due to the fact that NPC1 is a membrane protein, and, thus, does not allow cross-correction. Conversely, the NPC2 protein is mannose-6-phosphorylated and undergoes secretion-recapture; bone marrow transplantation has, therefore, been tried in a handful of children with NPC2 mutations, but the results have so far been discouraging (08; 119). There have been reports related to gene therapy in the Npc1 murine model with results better than expected (14; 43; 21), but no gene therapy trial is planned for the near future.

All clinical trials in humans have been focused on downstream intervention strategies using different pharmacological molecules. No Niemann-Pick disease type C-directed therapy is currently approved by the United States Food and Drug Administration.

Miglustat. Only one compound, miglustat, is registered for neurologic manifestations of Niemann-Pick disease type C; it has been registered in the European Union since 2009 and later in many other countries. Although not FDA-approved for this indication, miglustat is used off-label for treatment of many patients with Niemann-Pick disease type C in the United States. Miglustat is a small iminosugar partially able to cross the blood-brain barrier, administered orally, whose action includes inhibition of glucosylceramide synthase but appears more complex in Niemann-Pick disease type C. 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 (130). Its inhibitory effect on the nonlysosomal glucosylceramidase GBA2 may be involved (69). Indications of miglustat as well as its clinical utility and monitoring have been discussed (161; 89; 31). The numerous studies in patients published before 2018 have been comprehensively reviewed (Pineda Walterfang 2018). Outcomes of the treatment are discussed below.

Hydroxypropyl-beta-cyclodextrin. Promising results had been obtained in preclinical studies after intracerebroventricular administration of 2-hydroxypropyl-beta-cyclodextrin (HPBCD) in Npc1 mouse and cat models (02; 150). This led to a phase 1/2a clinical trial, using intrathecal administration of a VTS-270 (Kleptose-HPB) preparation now called adrabetadex, for which encouraging results were published (86). A blinded international, multicentric, sham-controlled phase 2b/3 trial (NCT02534844) was conducted in 54 patients (who also could be receiving miglustat) for 1 year. The results, however, failed to show a significant difference between the treated and the sham groups in the co-primary outcomes, with the puzzling observation that no significant progression occurred neither in the treated group (expected) nor the sham-control one (very unexpected); the open extension was recently stopped due to a too low benefit/risk balance (04). Over 100 patients with Niemann-Pick disease type C, however, have currently been treated for various lengths of time with intrathecal adrabetadex through expanded access or open-label extensions (04). The future of intrathecal adrabetadex therapy is still under discussion with the United States Food and Drug Administration.

Because intraperitoneal or subcutanous administration of high doses of HPßCD had shown a therapeutic effect in murine or to a lesser extent in feline models, respectively (20; 64; 150) a phase 1/2 clinical trial with intravenous long infusion of another HPBCD preparation (TrappsolRCycloTM) has been conducted. A placebo-controlled phase 3 study to evaluate intravenous Trapsol®Cyclo™ in pediatric (3 years and older) and adults with NPC1 (NCT04860960) is currently recruiting in the United States.

Arimoclomol. A randomized, blinded, placebo-controlled phase 2/3 trial using oral administration of the hydroxylamine derivative arimoclomol, a heatshock 70/40 inducer in cells under stress, has also been conducted in 50 patients aged 2-18 years, a majority of which also received miglustat (clinicaltrials.gov/NCT02612129), and is currently in a still ongoing open extension phase. Results of the 12-month trial have recently been published (76). Although the primary endpoint was reached, the product is not FDA-approved. Arimoclomol, however, is available in several countries through an early access program. The product appears well tolerated.

N-acetyl-L-leucine. This compound, which is administered orally, is said to exert a neuroprotective effect. An international open-label, rater-blinded phase 2 short 6-week study (NCT05163288) was conducted in 32 patients with Niemann-Pick disease type C aged 7 to 64 years, who served as their own control. The primary and secondary outcomes were reached (12).

In human patients with Niemann-Pick disease type C, the lack of good noninvasive biomarkers to monitor CNS response to therapy as well as the broad clinical spectrum, make follow-up of trials particularly difficult.

Symptomatic and supportive management

To date, management of patients remains largely symptomatic. The various aspects have been discussed in detail in several reviews and guidelines (89; 88; 31; 04). 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 short-time study, N-acetyl-DL-leucine showed some improvement of ataxia (10), and its L-enantiomer N-acetyl-L-leucine (IB1001-201; NCT03759639) is under evaluation (12). Besides pharmacological treatment, 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 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 swallowing or feeding difficulties (gastrostomy) is essential at an advanced stage of the disease.

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 (see International Niemann–Pick Disease Alliance). Genetic counseling should be made available for family members.

Outcomes

Treatment with miglustat. Several case series and single reports have been published (104; 26; 39; 52; 93; 83; 102). Pineda and colleagues provide a helpful review (105). Miglustat has no effect on the visceral manifestations of Niemann-Pick disease type C. Initial data indicating an effect on swallowing impairment have been confirmed (127), as has stabilization of patients for 1 year or more and a slower rate of progression of the disease after treatment. Patients with later onset forms, especially juvenile and adult forms, are the best responders (93; 90; 91; 102; 83). But patients with the most severe early infantile neurologic form respond poorly in the experience of one group (29), and some patients, even with the adult form, do not respond (83). A large study combining retrospective and prospective data supported global longer survival in patients treated with miglustat (Patterson and Garver 2020). The most common adverse effects of miglustat are gastrointestinal (diarrhea), for which management guidelines are available (03; 31), as well as weight loss and tremor. For optimal benefit, treating early in the course of the disease is important. However, there are patients diagnosed with Niemann-Pick disease type C on visceral manifestations early in life who did not show neurologic symptoms until years or several decades later. Therefore, patients with isolated systemic symptoms should be carefully followed, but it is recommended not to start miglustat treatment until some sign of neurologic involvement appears; treatment may, however, be started earlier in cases of an elder affected sibling, giving better prediction of the neurologic form (89; 31).

Special considerations

Pregnancy

Several women known to the author have undergone uneventful pregnancies prior to neurologic onset of their disease.

Anesthesia

Perianesthetic morbidity in one 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 (79).