Microsatellite instability (MSI) is a hallmark for many tumors, especially colon, endometrial, gastric and bladder. Bax, a tumor suppressor... Show moreMicrosatellite instability (MSI) is a hallmark for many tumors, especially colon, endometrial, gastric and bladder. Bax, a tumor suppressor and pro-death Bcl-2 family member, is frequently mutated in MSI tumors. A microsatellite mutation produces a frameshift with premature termination, leading to “Bax-negative” tumors. Although low Bax expression in tumors is often associated with poor prognosis, several studies have correlated lack of Bax in MSI tumors with improved prognosis. However, the molecular explanation for this paradox is unknown. Here we show that “Bax-negative” tumors in fact generate a novel family of anti-tumor Bax-MSI isoforms through alternative splicing. The thesis includes two parts. In Chapter One, we fully characterize one Bax-MSI isoform, BaxΔ2. We show that BaxΔ2 is detrimental to cancer cells but through a non-conventional death pathway, with differential sensitivity to chemotherapeutics. In Chapter Two, we present an entire family of Bax-MSI isoforms, and illustrate a potential molecular mechanism behind its production. We show that the frequency of Bax alternative splicing is significantly higher in MSI than non-MSI tumors, and that BaxΔ2 trans splicing requirements are ubiquitous in human cell lines. The discovery of functional Bax isoforms in Bax-mutated tumors may help explain why the apparent loss of Baxα in tumors is sometimes associated with a better prognosis. In addition, the unique sequences of Bax-MSI isoforms can serve as biomarkers for diagnostic and treatment purposes. Importantly, identification of Bax-MSI isoforms will provide a great opportunity from a genetic approach or drug design for treatment of MSI cancer. Ph.D. in Biology, December 2011 Show less
