The group of autosomal dominant disorders associated with branchial anomalies are characterized by external, middle and inner ear malformations, branchial cleft sinuses, cervical fistulas, mixed hearing loss, renal anomalies and occasional other manifestations. The Branchio-oto-renal (BOR), gene on chromosome 8q, has been identified as EYA1, however, some BOR, Branchio-renal (BR) and Branchio-oto (BO) families have not shown any mutation in the EYA1 gene. Gene mapping data suggests that multiple genes are involved and that this may be partly responsible for the variable phenotypic expression seen between families. We identified one large family with branchial and hearing anomalies (BO) unlinked to the 8q region and recently mapped the gene (named BGS2) to chromosome 1q. Also, there are at least two large families found to be unlinked to both regions suggesting the presence of a third locus associated with branchiogenic disorders. More than 50 - 60 percent of our BOR families did not show a mutation in the EYA1 gene or genetic linkage to 1q or 8q. We have collected the world's largest series of BOR type families, a resource that will allow us to identify the various genes involved in BOR syndrome. We propose to determine the distribution of mutations associated with the EYA1 gene, and to analyze clinical differences between families to determine to what degree they are correlated with different linkage groups. The possibility of genetic heterogeneity will be continually explored and families unlinked to 8q and 1q will be put through another round of genome searching to determine the location of any new BOR-related genes. The critical region of BGS2 has been narrowed from 22 cM to 9 cM. New families will be ascertained and tested for linkage with the markers on chromosome 8q and 1q. Refining the region is a gradual process, however, based on the completion of human genome project it may not be necessary to decrease the resolution to a smaller interval. Strong candidate genes lying within the critical region will be tested. The BGS2 gene will be identified among the candidate genes on the basis of its genomic position, tissue specific expression and consistent disease causing mutations in affected individuals. The BOR syndrome results in craniofacial anomalies in affected individuals and poses serious health problems. Defining the spectrum of defects and mapping and cloning the BGS genes are the first and foremost steps to a more comprehensive understanding of the pathogenesis and etiology of this syndrome. Finding the genes will lay the foundation for further research concerning effective treatment and genetic counseling.
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