Alternative titles; symbols
HGNC Approved Gene Symbol: NPC2
Cytogenetic location: 14q24.3 Genomic coordinates (GRCh38): 14:74,479,935-74,493,512 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 14q24.3 | Niemann-pick disease, type C2 | 607625 | Autosomal recessive | 3 |
Kirchhoff et al. (1990) originally cloned HE1 by differential screening of a human epididymis cDNA library. Krull et al. (1993) found HE1 to be highly expressed in all parts of the epididymis. Kirchhoff et al. (1996) cloned the full-length HE1 cDNA and found that it corresponds to a single copy gene which is well-conserved among mammals. The 151-amino acid HE1 glycoprotein has a molecular mass of 25 to 27 kD and contains a 19-amino acid signal that, along with homologs from numerous mammalian species, represents a major secretory component of the epididymal fluid. By Northern blot analysis, Naureckiene et al. (2000) detected a single transcript of 0.9 kb in all tissues examined, with highest levels in testis, kidney, and liver and lowest levels in lung and muscle. Subcellular fractionation of rat liver showed that HE1 is a lysosomal protein.
By radiation hybrid analysis and by inclusion within mapped clones, Naureckiene et al. (2000) mapped the HE1 gene to chromosome 14q24.3.
Given its lysosomal location and previous identification as a cholesterol-binding protein, Naureckiene et al. (2000) hypothesized that HE1 may be involved in Niemann-Pick disease type C2 (607625). HE1 was undetectable in fibroblasts from NPC2 patients but present in fibroblasts from unaffected controls and NPC1 (257220) patients. Treatment of NPC2 fibroblasts with exogenous recombinant HE1 protein ameliorated lysosomal accumulation of low density lipoprotein-derived cholesterol.
Friedland et al. (2003) reported on the crystal structure of the NPC2 protein and characterized its ligand binding properties. Human NPC2 binds the cholesterol analog dehydroergosterol with submicromolar affinity at both acidic and neutral pH. The structure of the bovine protein revealed a loosely packed region penetrating from the surface into the hydrophobic core that forms adjacent small cavities with a total volume of approximately 160 angstroms. Friedland et al. (2003) proposed that this region represents the incipient cholesterol-binding site that dilates to accommodate a cholesterol molecule of approximately 740 angstroms.
Mutations in the NPC2 gene associated with Niemann-Pick disease type C2 cause abnormally high cholesterol accumulation in cells. Ko et al. (2003) found that purified NPC2, a secreted soluble protein, binds cholesterol specifically with a much higher affinity than previously reported. Genetic and biochemical studies identified single amino acid changes that prevent both cholesterol binding and the restoration of normal cholesterol levels in mutant cells. The amino acids that affect cholesterol binding surround a hydrophobic pocket in the NPC2 protein structure, identifying a candidate sterol-binding location. On the basis of evolutionary analysis and mutagenesis, 3 other regions of the NPC2 protein emerged as important, including 1 required for efficient secretion.
In a patient with type C1 Niemann-Pick disease (257220) in whom mutations in the NPC1 gene were found (607623.0022-607623.0023), Blom et al. (2003) demonstrated that NPC2 was upregulated and accumulated in cholesterol-storing late endocytic organelles. The authors suggested that NPC1 may govern the endocytic transport of NPC2.
By yeast 2-hybrid screening of a human testis cDNA library, Kharel et al. (2004) identified NPC2 as a DHDDS (608172)-interacting protein. They confirmed the interaction by coimmunoprecipitation analysis.
By yeast 2-hybrid screening of a human heart cDNA expression library, Harrison et al. (2009) found that NGBR (NUS1; 610463) interacted with NPC2. The majority of NPC2 localized to lysosomes at steady state, but the C-terminal domain of NGBR interacted with nascent NPC2 in the endoplasmic reticulum lumen and stabilized NPC2 against proteasomal degradation. Knockdown of NGBR in human cell lines or knockout of Ngbr in mouse embryonic fibroblasts reduced NPC2 content, leading to increased intracellular cholesterol accumulation and loss of sterol sensing.
In 2 patients with Niemann-Pick disease type C (257220), Naureckiene et al. (2000) identified homozygous mutations in the HE1 gene (601015.0001-601015.0002).
Park et al. (2003) identified 7 different mutations in the NPC2 gene as the basis of Niemann-Pick disease type C.
In 6 unrelated patients with NPC2, Verot et al. (2007) identified 5 different mutations in the NPC2 gene (see, e.g., 601015.0008-601015.0010). The authors stated that a total of 15 disease-causing mutations had been identified in 22 unrelated families to date. E20X (601015.0001) was the most common mutation, accounting for 34% of mutant alleles.
Millat et al. (2001) reported the first comprehensive study of 8 unrelated families with NPC2, originating from France, Algeria, Italy, Germany, the Czech Republic, and Turkey. These cases represented essentially all patients with NPC2 who had been reported, as well as those known to the authors. All 16 mutant alleles were identified representing, however, only 5 different mutations, all with a severe impact on the protein: 2 nonsense mutations, glu20 to ter (E20X; 601015.0001) and glu118 to ter (E118X; 601015.0004), a 1-bp deletion (27delG; 601015.0005), a splice mutation (IVS2+5G-A; 601015.0003), and a missense mutation (S67P; 601015.0006) resulting in reduced amounts of abnormal HE1 protein. E20X, with an overall allele frequency of 56%, was established as the common mutant allele. In 7 families homozygosity for E20X, E118X, S67P, and compound heterozygosity for E20X and 27delG mutations resulted in severe and rapid disease course, with age at death being 6 months to 4 years. A remarkable feature was the pronounced lung involvement, leading, in 6 patients, to early death caused by respiratory failure. Two patients also developed a severe neurologic disease with onset during infancy. Conversely, the splice mutation corresponded to a very different clinical presentation, with juvenile onset of neurologic symptoms and prolonged survival. This mutation generated multiple transcripts, including a minute proportion of normally spliced RNA, which may explain the milder phenotype.
The NPC1 (607623) and NPC2 proteins are required for the egress of lipids from the lysosome. To gain insight into the normal function of NPC2 and to investigate its interactions, if any, with NPC1, Sleat et al. (2004) generated a murine Npc2 hypomorph that expressed 0 to 4% residual protein in different tissues and examined its phenotype in the presence and absence of Npc1. The phenotypes of Npc1 and Npc2 single mutants and an Npc1/Npc2 double mutant were similar or identical in terms of disease onset and progression, pathology, neuronal storage, and biochemistry of lipid accumulation. These findings provided genetic evidence that the NPC1 and NPC2 proteins function in concert to facilitate the intracellular transport of lipids from the lysosome to other cellular sites.
Naureckiene et al. (2000) identified a patient with NPC2 (607625) who was homozygous for a G-to-T transversion in exon 1 of the HE1 gene, resulting in a glu20-to-ter substitution.
Millat et al. (2001) stated that they found that the E20X mutation had an overall allele frequency of 56% in a study of 8 families with NPC2 from widely separated geographic areas. Homozygosity was associated with a severe rapid disease course. Pronounced lung involvement leading to death from respiratory failure occurred in some patients.
Verot et al. (2007) reported 2 unrelated patients with NPC2 who were homozygous for the E20X mutation. They had a severe phenotype, with early death at ages 15 months and 3.5 years, respectively.
In a patient with NCP2 (607625), Naureckiene et al. (2000) identified compound heterozygosity for a glu20-to-ter mutation (601015.0001) and a single nucleotide deletion (A) at position 111 of the HE1 gene, resulting in a frameshift in exon 2 that generated a stop codon 4 codons downstream of the frameshift.
In an Algerian patient with NPC2 (607625), alive at age 21 years, Millat et al. (2001) found a splice donor site mutation, IVS2+5G-A, in the HE1 gene. The female patient was found to have splenomegaly at age 6.5 years, school problems, epileptic fits beginning at age 11 years, mental retardation, and slow progression, but was still able to walk normally at age 21 years. Remarkably an affected sister with a similar course was still alive at age 27 years. The parents were consanguineous.
In a patient with NPC2 (607625), Millat et al. (2001) found homozygosity for a G-to-T transversion at nucleotide 352 of the HE1 gene, resulting in a glu118-to-ter (E118X) amino acid change. The patient, who died at 7 months of age, had severe interstitial pneumonia, hepatosplenomegaly, and respiratory failure at 3 months of age.
In a French NPC2 (607625) patient who died at 6 months of age, Millat et al. (2001) found compound heterozygosity for deletion of a G at nucleotide 27 of the HE1 gene (27delG) on one allele, and E20X (601015.0001) on the other. The parents were nonconsanguineous.
In a Turkish NPC2 (607625) patient, Millat et al. (2001) detected homozygosity for a T-to-C transition at nucleotide 199 of the HE1 gene, resulting in a ser67-to-pro (S67P) amino acid change.
In 2 sisters with adult-onset, slowly progressive NPC2 (607625) who were descendants of a group of 17th century French Canadians, Klunemann et al. (2002) identified a homozygous 115G-A transition in exon 2 of the NPC2 gene, resulting in a val39-to-met (V39M) substitution in exon 2. Both presented in their early forties with vertical supranuclear gaze paresis, dysarthria, and cognitive decline characterized by expressive aphasia, perseverative behavior, and impaired conceptualization and planning. The proband developed ataxia and athetoid movements, and her sister developed facial dyskinesias and bradykinesia. Cholesterol esterification of cultured fibroblasts from 1 patient was abnormally low at 26% of normal. Detailed histories revealed that both patients exhibited early cognitive symptoms in adolescence. Postmortem examination of the proband revealed frontal lobe atrophy and neuronal lysosomes with oligolamellar inclusions typical for NPC, but no visceromegaly. Klunemann et al. (2002) suggested that the cases represented a novel phenotypic variant of NPC2.
In a female infant from Sri Lanka with NPC2 (607625), Verot et al. (2007) identified a homozygous T-to-C transition in intron 1 of the NPC2 gene (IVS1+2T-C), resulting in 3 different abnormally spliced gene products. Although 1 of the gene products could have theoretically resulted in the synthesis of a mutated protein, Western blot analysis and immunofluorescent studies showed no detectable protein in patient cells. She died at age 12.5 months after complications from a bone marrow transplant.
In an Algerian male infant with NPC2 (607625), Verot et al. (2007) identified a homozygous mutation in the NPC2 gene, resulting in a gln146-to-ter (Q146X) substitution. Although this mutation could have theoretically resulted in a protein lacking only the last 6 residues, Western blot analysis and immunofluorescent studies showed no detectable protein. However, RT-PCR studies showed normal levels of mRNA, which excluded nonsense-mediated decay. Verot et al. (2007) concluded that the mutant protein must be unstable. The patient had neonatal cholestatic icterus and hepatosplenomegaly with ascites, from which he recovered, and was alive at 9 months of age with no neurologic signs.
In a Turkish patient with NPC2 (607625), Verot et al. (2007) identified a homozygous mutation in the NPC2 gene, resulting in a pro120-to-ser (P120S) substitution. Western blot analysis and immunofluorescent studies showed reduced levels of the protein that correctly localized to lysosomes. Computer modeling suggesting that the P120S mutation lies in a pocket important for cholesterol binding and that the mutant protein may be unable to bind cholesterol efficiently. The patient had a less severe phenotype, with onset of tremor and speech problems at age 12 years and onset of dystonia and mental retardation at age 17 years.
Blom, T. S., Linder, M. D., Snow, K., Pihko, H., Hess, M. W., Jokitalo, E., Veckman, V., Syvanen, A.-C., Ikonen, E. Defective endocytic trafficking of NPC1 and NPC2 underlying infantile Niemann-Pick type C disease. Hum. Molec. Genet. 12: 257-272, 2003. [PubMed: 12554680] [Full Text: https://doi.org/10.1093/hmg/ddg025]
Friedland, N., Liou, H.-L., Lobel, P., Stock, A. M. Structure of a cholesterol-binding protein deficient in Niemann-Pick type C2 disease. Proc. Nat. Acad. Sci. 100: 2512-2517, 2003. [PubMed: 12591954] [Full Text: https://doi.org/10.1073/pnas.0437840100]
Harrison, K. D., Miao, R. Q., Fernandez-Hernando, C., Suarez, Y., Davalos, A., Sessa, W. C. Nogo-B receptor stabilizes Niemann-Pick type C2 protein and regulates intracellular cholesterol trafficking. Cell Metab. 10: 208-218, 2009. [PubMed: 19723497] [Full Text: https://doi.org/10.1016/j.cmet.2009.07.003]
Kharel, Y., Takahashi, S., Yamashita, S., Koyama, T. In vivo interaction between the human dehydrodolichyl diphosphate synthase and the Niemann-Pick C2 protein revealed by a yeast two-hybrid system. Biochem. Biophys. Res. Commun. 318: 198-203, 2004. [PubMed: 15110773] [Full Text: https://doi.org/10.1016/j.bbrc.2004.04.007]
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Park, W. D., O'Brien, J. F., Lundquist, P. A., Kraft, D. L., Vockley, C. W., Karnes, P. S., Patterson, M. C., Snow, K. Identification of 58 novel mutations in Niemann-Pick disease type C: correlation with biochemical phenotype and importance of PTC1-like domains in NPC1. Hum. Mutat. 22: 313-325, 2003. [PubMed: 12955717] [Full Text: https://doi.org/10.1002/humu.10255]
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