The nail patella syndrome (NPS) represents a classic example of pleiotropy, exhibiting nail dysplasia, hypoplasia of the patella, radial head and scapula, iliac exostoses, club foot deformities and, occasionally, nephritis and ocular abnormalities. Although first recognized as an inherited disorder nearly 100 years ago, the basic defect remains unknown. Identification of the disease gene will illuminate the developmental process common to the affected tissues. In particular, a greater understanding of skeletogenesis will be gained. Previous work has placed the NPS locus near the adenylate kinase gene (AK1) on chromosome 9q34. Genetic linkage analysis using highly polymorphic dinucleotide repeats has reduced the candidate region to approximately 1cM between AK1 and the anonymous markers D9S60, D9S266 and D9S1113. Four polymorphic markers each give LOD scores greater than 10 with no recombination. Since no candidate gene can be placed in this interval with confidence a positional cloning approach is most likely to be successful in identification of the disease gene. The genetic interval will be refined using families not available for previous analyses to search for informative recombination events. The physical interval will be determined by establishment of a series of overlapping YAC clones covering the candidate region using STS fingerprinting. This contig will be assessed for candidate genes by testing ESTs previously placed in this region to determine whether they lie within the interval and exhibit a pattern of expression or other characteristics compatible with an NPS candidate gene. If necessary, novel genes will be identified within the interval by isolation of coding sequences (exon trapping) from a contig of cosmid or BAC clones covering the region. Proof of a gene's involvement in NPS will require demonstration of a de novo mutation in a sporadic case supported by identification of mutations segregating with the phenotype in multiplex families. Candidate genes will be tested for genomic rearrangements by Southern blotting, for aberrant gene expression by Northern blotting and RT-PCR, and for point mutations by a modified SSCP protocol. The range of mutations which cause NPS will be determined to facilitate recognition of functional domains within the gene product. Genotype-phenotype analyses will be performed to assess the involvement of different alleles in interfamilial phenotype variation. Identification of the NPS gene may reveal means by which serious aspects of the phenotype (nephritis, bone deformity) might be ameliorated not only in NPS patients but in persons affected with these conditions in isolation.
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