Discovery of the BMPR1A promoter and germline mutations that cause juvenile polyposis

Juvenile polyposis (JP) is an autosomal dominant hamartomatous polyposis syndrome where affected individuals are predisposed to colorectal and upper gastrointestinal cancer. Forty-five percent of JP patients have mutations or deletions involving the coding regions of SMAD4 and BMPR1A, but the genetic basis of other cases is unknown. We set out to identify the JP gene in a large kindred having 10 affected members without SMAD4 or BMPR1A coding sequence mutations or deletions. We found a germline deletion segregating in all affected members, mapping 119 kb upstream of the coding region of BMPR1A by multiplex ligation-dependent probe amplification and comparative genomic hybridization. To further understand the genomic structure of BMPR1A, we performed 5' RACE from lymphoblastoid cell lines and normal colon tissue, which revealed four non-coding (NC) exons and two putative promoters. Further analysis of this deletion showed that it encompassed 12 433 bp, including one promoter and NC exon. The activities of each promoter and deletion constructs were evaluated by luciferase assays, and the stronger promoter sequence analyzed for changes in JP patients without SMAD4 or BMPR1A alterations. A total of 6 of 65 JP probands were found to have mutations affecting this promoter. All probands examined had diminished BMPR1A protein by ELISA, and all promoter mutations but one led to significantly reduced luciferase activity relative to the wild-type promoter reporter. We conclude that we have identified the promoter for BMPR1A, in which mutations may be responsible for as many as 10% of JP cases with unknown mutations.