CDHR5 tumor suppressor function in the intestine - Cadherin related family member 5 (CDHR5) is a type 1 transmembrane adhesion molecule that is highly expressed in the gut epithelium, but is almost always lost in colorectal cancer (CRC). The promising CRC chemopreventive drug 5-ASA (mesalazine) potently upregulates CDHR5 expression as part of its chemopreventive activity, and direct overexpression of CDHR5 reduces the tumor formation potential of CRC cells that have been injected into mice. Together, these results implicate CDHR5 as a tumor suppressor in the gut. Despite this, the functional properties of CDHR5 that allow it to act as a tumor suppressor have not been defined. This represents a significant gap in knowledge of how CDHR5 acts as a barrier against CRC. This knowledge gap prevents the development of chemopreventive CRC therapies involving upregulation of CDHR5, as seen with the drug 5-ASA. The long-term goal of this study is to understand the role of the adhesion molecule CDHR5 in the intestine. The current objective of this proposal is to directly investigate the properties of CDHR5 that allow it to suppress CRC in the gut. Preliminary studies discovered that CDHR5 localizes to the distal tips of microvilli that cover the apical surface of intestinal epithelial cells. Here, CDHR5 forms a strong extracellular interaction with another cadherin, CDHR2, to physically link neighboring microvilli together, organizing them into an ordered array known as the intestinal brush border. Overexpression of CDHR5 results in a striking effect on CRC cells, with cells polarizing to form an increased density of apical microvilli that are longer in length. This shift towards a ‘hyper-polarized’ state is partially dependent upon the intracellular cytoplasmic domain of CDHR5. For this proposal, the central hypothesis is that CDHR5 contributes to the integrity of the intestinal epithelium to guard against CRC development, in a manner dependent on CDHR5 cytoplasmic binding partners and its extracellular adhesion activity. This hypothesis will be tested through three specific aims: 1) Determine the functional interplay between the two splice isoforms of CDHR5. This aim will explore the discovery that the two splice isoforms of CDHR5 interact to form a mature functional complex that targets to apical microvilli. 2) Identify factors associated with the cytoplasmic domain of CDHR5. This aim will employ an innovative protein purification approach to directly identify the in vivo binding partners for the cytoplasmic domain of CDHR5. The role of binding partners in CDHR5 function will be explored using knockdown/knockout studies an enterocyte model. 3) Determine the functional properties of CDHR5 required to reduce the tumor formation potential of CRC cells that have been implanted in a nude mouse model. This aim will assess whether the adhesion capacity and/or cytoplasmic binding partners are required for CDHR5 to reduce tumor formation of CRC cells. The approach is innovative, since it challenges the existing viewpoint that CDHR5 functions as a tumor suppressor in the gut by interacting with and sequestering β-catenin. The proposed research is significant because loss of CDHR5 is highly correlated with development of CRC and predicts poor survival of cancer patients.
