Familial Mediterranean fever (FMF) is a rheumatic disease characterized by intermittent bouts of fever with abdominal pain, pleurisy, and/or arthritis; some patients also develop systemic amyloidosis, leading eventually to renal failure. FMF is inherited as a single autosomal recessive gene (designated MEF), but the biochemical lesion is unknown. The purpose of this project has been to identify the FMF gene by the method of positional cloning. By the beginning of the current year, we had succeeded in mapping the gene causing FMF to the short arm of chromosome 16. During 1992-3 we have considerable refined the genetic mapping of MEF. After examining 12 genetic markers from distal chromosome 16p, we have narrowed the candidate region to the interval between D16S63 and D16S246, a genetic distance of 3.4 Cm (sex-averaged). Moreover, we have identified a highly informative new microsatellite polymorphism at D16S63 that allows rapid haplotyping of FMF families with small amounts of DNA. To define genetic distances in this region more precisely, we have also typed the CEPH reference families for this new microsatellite, as well as D16S80, D16S246, and D16S283. Among our 51 non-Ashkenazi Jewish FMF families there are at least 4 recombinants on the centromeric end of the D16S63-D16S246 interval, indicating the potential to narrow the region of interest even more. We found that FMF in Arabs maps to the same interval, thus increasing the potential pool of informative families. We have used two additional strategies aimed at localizing MEF: linkage disequilibrium mapping and homozygosity mapping. Previous studies of linkage disequilibrium at D16S94, D16S283, and D16S291 had indicated a probable founder effect among Moroccan Jewish FMF families, but no allelic associations in other subpopulations. More recent typings for D16S246, which is closer to MEF than the other three markers, does indicate strong allelic associations in all Jewish subpopulations studied. This suggests that MEF is closer to the telomeric side of the D16S63-D16S246 interval. Homozygosity mapping is a novel strategy for mapping recessively inherited disease genes in inbred families. With our large panel of consanguineous families, we did find high levels of homozygosity in affected offspring of inbred marriages for a number of loci on chromosome 16p. However, we found this strategy to have limited utility for refined mapping, because of the high gene frequency for FMF, and the relatively low heterozygosities of markers in the region.
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