Abstract 6069: HOXA9 overexpression contributes to stem cell overpopulation that drives development and growth of colorectal cancer

Abstract

Abstract HOX genes encode master regulatory transcription factors that regulate stem cells (SCs), but their role in carcinogenesis is not fully understood. SC overpopulation drives colorectal cancer (CRC) development, which is also linked with the aberrant expression of specific HOX genes. One such gene is HOXA9. Our goal is to find out how HOXA9-regulatory mechanisms play a role in the SC origin of CRC. Previous research in our lab reported that HOXA9 as well as other HOX proteins (e.g. HOXA4 & HOXD10) are selectively expressed in human colonic cancer SCs (CSCs). The SC marker aldehyde dehydrogenase (ALDH) and other retinoic acid (RA) pathway components are also selectively expressed in CSCs. Moreover, HOXA9 expression is positively correlated with expression of ALDH genes such as ALDH1A1. These findings suggest that RA signaling regulates HOXA9 gene expression in colonic ALDH+ SCs. Hypothesis: Dysregulated HOXA9 expression leads to ALDH+ SC overpopulation, which contributes to CRC growth and development. We used immunofluorescent (IF) mapping to determine the histologic location of HOXA9+ cells in relation to colonic SCs and other differentiated colonic cell types in normal and neoplastic human colonic tissues. We also measured the effects of all-trans retinoic acid (ATRA) and the ALDH enzyme inhibitor diethylamino benzaldehyde (DEAB) on HOXA9 expression, cell proliferation and differentiation using CRC cell lines. Immunofluorescence mapping indicated that not only is HOXA9 selectively expressed in ALDH-positive SCs, but that HOXA9 expression is increased in colon carcinoma tissue compared to normal colon epithelium. Our results also showed that both ATRA and DEAB treatment decreased the proliferation of HT29 and SW480 CRC cell lines. Western blot analysis showed that HOXA9 has reduced expression in ATRA-treated HT29 CRC cells. Additionally, our independent bioinformatics analyses show that HOXA9 is predominantly expressed in both stem cells and neuroendocrine cells (NECs) in normal human colonic epithelium. Taken together, these findings indicate that regulation of HOXA9 gene expression by RA signaling controls differentiation of ALDH+ SCs along the NEC lineage in human colon tissue. Thus, our results show that HOXA9 is regulated by RA signaling and when dysregulated, contributes to CRC development and growth. These findings provide a mechanism that may be targeted in the design of novel treatments against SC overpopulation in CRC. Citation Format: Brian T. Osmond, Caroline O. Facey, Lynn M. Opdenaker, Bruce M. Boman. HOXA9 overexpression contributes to stem cell overpopulation that drives development and growth of colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6069.