Differential Expression of Cohesin SA in Early Colorectal Carcinogenesis: Potential Biomarker for Racial Disparities

Abstract

Introduction: The ˜50% higher mortality in colorectal cancer (CRC) in blacks versus whites \led the ACG to make groundbreaking recommendation initiating CRC average-risk screening at age 45 years in blacks (Agarwal et al., Am J Gastro 2005). Unfortunately, there appears to be no progress in ameliorating this disparity over the last 20 years (Aizer et al., Cancer 2014) underscoring the need to focus on the mechanisms. From a genetic perspective, there are clear biological differences in CRCs (DNA mismatch repair, methylation). However, early molecular changes in carcinogenesis have not been elucidated despite clinical evidence of differences (altered location, incidence of adenomas; Lieberman et al., Gastro 2014). Our group has focused on understanding field carcinogenesis as a modality for risk assessment (Gastro 2011, Clin Gastro Hep 2012). We have recently become interested in the role of cohesins in early cancer as these proteins regulate gene expression via chromatin looping. We, therefore, focused on the cohesin SA1 as a putative mediator of racial disparities. Methods: To determine SA1 expression in colonic tumors, we used immunohistochemical (IHC) assessment of tissue arrays with a monoclonal antibody. Staining was scored by independent pathologists. For field carcinogenesis, we obtained biopsies from endoscopically normal rectal mucosa of patients undergoing colonoscopy and assessed SA1 level via real-time PCR. Finally, for biological relevance we developed a stable knockdown of SA1 in the human CRC cell line HT29 and assessed proliferation via cell proliferation assay. Results: IHC data showed a loss in SA1 starting at the adenoma stage and progressively decreasing based on stage of CRC (average loss 67%; p<0.00005). Rectal biopsies comparing patients harboring neoplasia versus those neoplasia-free showed a modest induction in whites whereas in blacks there was a marked downregulation in SA1. Biological relevance was noted by a 50% increase in HT29 proliferation (p<0.05) after achieving a 40% knockdown of SA1. Conclusion: We demonstrate that the cohesin SA1 was altered progressively in early into late stage CRC and appeared to be a tumor suppressor gene based on tissue array data. Strikingly, it appears that at the initiation stages (field carcinogenesis) SA1 was decreased only in blacks and not whites. This provides an important biological insight into racial CRC disparity and may serve as a potential biomarker for personalizing screening decisions in a generally underserved community in order to mitigate this critical racial disparity.