International Center for Types A and B Niemann-Pick Disease
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Physician/ScientistGeneticsThe Molecular Genetics of Niemann-Pick Disease (NPD)A total of 18 mutations causing the Types A or B forms of NPD had been published as of July, 1998. Of these, seven occured within exon 2, three within exon 3, one each in exons 4 and 5, five in exon 6, and one within an intronic splice site. There were 11 missense, 2 nonsense, and 3 frameshift mutations, one caused by an in-frame three base deletion that led to the removal of a single amino acid, while the other two were caused by single base alterations. Mutations Causing Type A NPDAshkenazic Jewish Type A NPD Mutations In an initial study of 27 unrelated Ashkenazic Jewish Type A NPD patients (54 NPD alleles), the R496L, L302P, and fsP330 mutations were found to represent 36, 24, and 32 percent of the mutant alleles, respectively. Thus, it was estimated that about 92 percent of the total ASM alleles causing Type A NPD in the Ashkenazic Jewish population could be detected by testing for the presence of these three mutations. Subsequently, 1224 Ashkenazi with no family history of NPD were screened for the three mutations, and fourteen carriers were identified (2pq = 1:80 based on 92% detection) (Caggana et al. 1997). In contrast, the R496L mutation was found in only one of twenty non-Jewish Type A NPD patients, and the L302P and fsP330 mutations have not been found in any of the 40 non-Jewish Type A NPD alleles. PCR amplification and dot-blot hybridization conditions have been established and mismatched PCR primers constructed to detect the mutations by restriction enzyme analyses. Non-Jewish Type A NPD Mutations Mutations Causing Type B NPDAshkenazic Jewish Type B NPD MutationsA mildly affected Ashkenazic Jewish Type B NPD patient was identified who was heteroallelic for the Type A R496L mutation and a three base deletion that predicted the removal of an arginine residue from position 608 of the ASM polypeptide (designated R608). A second Ashkenazic Jewish Type B patient was subsequently found to be heteroallelic for R608 and a new mutation, designated H575L. Although it is intriguing that two unrelated Ashkenazic Jewish Type B NPD patients carried the R608 mutation, the frequency of Type B NPD among Ashkenazic Jews is low and the sample size too small to allow an accurate estimate of the frequency of the R608 mutation in this population. No other mutations in Ashkenazic Jewish Type B NPD patients have been reported to date. Non-Jewish Type B NPD Mutations Expression of the ASM Mutations and Phenotype/Genotype CorrelationsAs summarized above, a total of 18 mutations have been identified causing Types A and B NPD. Of these, published expression data are available for eleven. As expected, the fsL178, L261X, and fsP330 mutations, each of which created premature stop codons in the ASM coding region, did not express catalytically active ASM in COS-1 cells. Thus, these mutations most likely result in truncated ASM polypeptides that are rapidly degraded. The R496L, L302P, M382I, N383S, and G577S mutations also did not express catalytically active enzyme in the transient expression system. These findings are consistent with the fact that individuals who were homoallelic for the R496L, L302P, and G577S mutations had the severe Type A phenotype. In addition, a Type A NPD patient has been identified who was heteroallelic for the fsL178 and M382I mutations. In contrast, the G242R, S436R, and R608 mutations expressed significant residual enzymatic activity in COS-1 cells. As noted above, the R608 mutation was originally identified in an Ashkenazic Jewish Type B NPD patient whose other ASM allele contained the Type A R496L mutation. R496L Mild Type B NPD patients also have been identified in France who were homoallelic for the R608 mutation. These observations suggest that one copy of the R608 mutation is "neuroprotective," that is, it will prevent the occurrence of the neuronopathic Type A phenotype. The S436R mutation was identified as both alleles in a mildly affected Type B NPD patient from Japan who had residual activity, and the G242R mutation was identified as one allele in a Type B NPD patient of European ancestry. The other allele in this patient was N383S, which did not express catalytically active ASM. These studies provide the initial basis for genotype/phenotype correlations in NPD. Within the Ashkenazic Jewish population, such genotype/phenotype correlations will be useful in counseling families with newly diagnosed offspring and in decisions concerning prenatally diagnosed fetuses. In contrast, each of the ASM mutations in most non-Jewish families is likely to be "private" (other than R608), precluding genotype/phenotype predictions, unless clinical information is available from other family members or other affected individuals with the same genotype. The Population Genetics of NPDIt is notable that two other sphingolipid storage diseases, Tay-Sachs disease and Gaucher disease, also occur more frequently among Ashkenazic Jewish individuals than in the general population. During the past three decades, the mechanism(s) underlying the higher gene frequencies for these diseases in the Ashkenazic Jewish population has been the subject of much interest and debate. It has been suggested that higher gene frequencies for these diseases resulted from a higher mutation rate, founder effect and genetic drift, and/or selection. The molecular evidence demonstrating that two or more mutations occur in each disease argues that heterozygotes for these alleles may, in fact, have had a selective advantage. Although a variety of hypotheses have been proposed to explain this selective advantage, a likely basis could have been increased resistance to certain infections that have been epidemic in European cities during the past two millennia. Furthermore, the fact that three sphingolipid storage diseases are more frequent among the Ashkenazim suggests a common mechanism for this selective advantage. It has been shown that certain membrane sphingolipids are involved in the recognition and binding of various bacteria and bacterial toxins, perhaps providing an explanation for the increased resistance to bacterial infections. It is tempting to speculate that the slight accumulation of these membrane lipids in heterozygous individuals and the slightly higher levels of these lipids in the circulation could lead to an increased ability to bind, endocytose, and degrade these toxic agents, thereby providing a selective advantage for carriers of these mutant alleles. Although this hypothesis is intriguing, the true nature of the selective advantage for these mutations remains unknown and is an area for future investigation.Along these lines, it has recently been shown that ASM mediates entry of the bacterium, Neisseria gonorrhoeae, into epithelial cell lines and cultured skin fibroblasts (Grassme et al. 1997). Furthermore, ASM deficient fibroblasts obtained from Type A NPD patients did not internalize this bacterium, as opposed to normal fibroblasts, suggesting that NPD heterozygotes also may have some resistence to gonorrhoeae infection, and possibly conferring to them an increased resistance to this and perhaps other infections. |