Regulation of SMAD4 and BMPR1A Expression in Juvenile Polyposis

点击翻译按钮获取中文摘要

Juvenile Polyposis (JP) is an autosomal dominant syndrome predisposing to the development of hamartomatous polyps of the colon, rectum, and stomach. Affected patients have an approximately 50% risk of developing gastrointestinal cancer. Work in our laboratory has identified 2 genes that cause JP, both of which are members of the transforming growth factor beta (TGF-β) super family. One of these genes, SMAD4, is the common intracellular mediator of signaling through the TGF-β, bone morphogenetic protein (BMP), and activin pathways. The other, BMPR1A, is a cell surface receptor which transduces BMP signals from the cell membrane into the cytoplasm. Mutations in the coding sequence of each gene have been found in the germline of approximately 20% of JP cases, respectively. The cause for the other 60% of JP cases is unknown, and could include other undiscovered genes predisposing to JP, larger deletions of the known genes not detectable by sequencing, and changes in non-coding regions of these genes, thereby altering gene expression. The focus of our JP studies over the past several years has been to discover new JP genes through a linkage-based genome screen of a large JP kindred, and direct sequencing of other genes in the TGF-β super family for mutations in a large number of JP families. We have also explored the prevalence of exonic deletions, which accounted for another 4% of JP cases. A recent provocative finding came from this genome screen and deletion studies. In a large JP family without coding mutations of SMAD4 or BMPR1A, it was discovered that a third JP gene was not the cause of JP, but rather, a deletion of the putative promoter region and first non-coding exon of BMPR1A was inherited through the germline. This has led us to focus our studies upon investigating how the known JP genes are regulated and lead to altered BMP signaling. Another JP patient has been found with a deletion of the putative promoter and first 2 non-coding exons of SMAD4, lending further support to this approach. Therefore, in this grant we propose to examine factors which affect the expression of JP genes and proteins, a largely unexplored and potentially very important mechanism underlying the genetic basis of autosomal dominant cancer syndromes. Our specific aims are: 1) to characterize the promoter region and non-coding exons of SMAD4 and BMPR1A and evaluate JP patients for germline changes; and 2) to determine the influence of mutations in coding and non-coding regions of JP genes on RNA and protein expression.