Niemann-Pick C1 disease: Correlations between NPC1 mutations, levels of NPC1 protein, and phenotypes emphasize the functional significance of the putative sterol-sensing domain and of the cysteine-rich luminal loop

To obtain more information of the functional domains of the NPC1 protein, t he mutational spectrum and the level of immunoreactive protein were investi gated in skin fibroblasts from 30 unrelated patients with Niemann-Pick C1 d isease. Nine of them were characterized by mild alterations of cellular cho lesterol transport (the "variant" biochemical phenotype). The mutations sho wed a wide distribution to nearly all NPC1 domains, with a cluster (11/32) in a conserved NPC1 cysteine-rich luminal loop. Homozygous mutations in 14 patients and a phenotypically defined allele, combined with a new mutation, in a further 10 patients allowed genotype/phenotype correlations. Prematur e-termination-codon mutations, the three missense mutations in the sterol-s ensing domain (SSD), and A1054T in the cysteine-rich luminal loop all occur red in patients with infantile neurological onset and "classic" (severe) ch olesterol-trafficking alterations. By western blot, NPC1 protein was undete ctable in the SSD missense mutations studied (L724P and Q775P) and essentia lly was absent in the A1054T missense allele. Our results thus enhance the functional significance of the SSD and demonstrate a correlation between th e absence of NPC1 protein and the most severe neurological form. In the rem aining missense mutations studied, corresponding to other disease presentat ions (including two adults with nonneurological disease), NPC1 protein was present in significant amounts of normal size, without clear-cut correlatio n with either the clinical phenotype or the "classic"/"variant" biochemical phenotype. Missense mutations in the cysteine-rich luminal loop resulted i n a wide array of clinical and biochemical phenotypes. Remarkably, all five mutant alleles (I943M, V950M, G986S, G992R, and the recurrent P1007A) defi nitively correlated with the "variant" phenotype clustered within this loop , providing new insight on the functional complexity of the latter domain.