Niemann-Pick type C disease: NPC1 mutations associated with severe and mild cellular cholesterol trafficking alterations

Niemann-Pick type C disease (NPC) is a rare neurodegenerative disorder char acterised by lysosomal/late endosomal accumulation of endocytosed unesterif ied cholesterol and delayed induction of cholesterol homeostatic reactions. The large majority of mutations in the NPC1 gene described thus far have b een associated with severe cellular cholesterol trafficking impairment (cla ssic biochemical phenotype, present in about 85% of NPC patients). In our p opulation of 13 unrelated NP-C1 patients, among which 12 were of Portuguese extraction, we observed an unusually large proportion of families presenti ng mild alterations of intracellular cholesterol transport (variant biochem ical phenotype), without strict correlation between the biochemical phenoty pe and the clinical expression of the disease. Mutational studies were carr ied out to compare molecular lesions associated with severe and mild choles terol traffic impairment. Levels of NPC1 protein were studied by Western bl ot in cultured fibroblasts of four patients with homozygous mutant alleles. Ten novel mutations were identified (Q92R, C177Y, R518W, W942C, R978C, A10 35V, 2129delA, 3662delT, IVS23+1 G>A and IVS16-82 G>A). The mutational prof ile appeared to be correlated with the biochemical phenotype. Splicing muta tions, 11061T and A1035V, corresponded to "classic" alleles, while three mi ssense mutations, C177Y, R978C and P1007A, could be defined as "variant" al leles. All "variant" mutations described so far appear to be clustered with in the cysteine-rich luminal loop between TM 8 and 9, with the remarkable e xception of C177Y The latter mutant allele, at variance with P1007A, was co rrelated to a decreased level of NPC1 protein and a severe course of the di sease, and disclosed a new location for "variant" mutations, the luminal lo op located at the N-terminal end of the protein.