ABSTRACT — Combination chemoradiation is utilized to treat multiple gastrointestinal (GI) cancers including rectal cancer. Rectal cancer affects 40,000 people per year in the US. Approximately 85% of patients have an incomplete or poor response to treatment increasing their risk of recurrence. We have found that poor responders harbor sub-clones that are more resistant to treatment, and that the enzyme ST6Gal-1 is enriched in these sub- clones. ST6Gal-1 is a Golgi glycosyltransferase that adds the negatively-charged sugar, sialic acid (SA), to specific proteins destined for the cell surface. SA can have profound effects on the structure and function of proteins. ST6Gal-1 is one of the most pervasively upregulated glycosyltransferases in cancer cells. ST6Gal-1 has been shown to specifically promote tumor cell survival and resistance via sialylation. In addition, ST6Gal-1 has been found in extracellular vesicles (ECVs) made by cancer cells. ECVs are particles with a lipid membrane that contains RNA and protein cargo; thus, they are potential mediators of transferable resistance between cancer sub-clones. The role of ST6Gal-1 and ECVs in resistance to chemoradiation has not been investigated. The overall objective of this application is to ascertain the role of ST6Gal-1 in innate and transferable resistance to chemoradiotherapy in rectal cancer. Based on our preliminary data, we hypothesize that ST6Gal-1 mediates resistance to chemoradiation in individual sub-clones in rectal cancer, that this resistance is transferred to other sub-clones via ECVs spreading resistance, and that this resistance is regulated by ST6Gal-1 cleavage by BACE1. We have found that ST6Gal-1 is increased in rectal cancer models after treatment with chemoradiation. We will investigate our hypothesis with 3 aims: AIM 1 — Determine the role of ST6Gal-1 in chemoradiation resistance in human rectal cancer. We hypothesize that ST6Gal-1 causes treatment resistance after chemoradiation by inhibiting apoptosis. We will employ cell sorting, sequencing, and shRNA approaches. We will also conduct studies to investigate its function in patient samples. AIM 2 — Determine if ECVs carrying ST6Gal-1 transfer resistance to chemoradiation between sub-clones in rectal cancer. We hypothesize that ECVs act as vectors that impart resistance to chemoradiotherapy from sub-clone to sub-clone by trafficking ST6Gal-1, and thus, glycoprotein sialylation, in rectal cancer causing decreased apoptosis in the recipient sub-clones. AIM 3 — Determine if BACE1 promotes chemoradiosensitivity in rectal cancer due, in part, to ST6Gal-1 cleavage. We show that BACE1 mRNA is increased in tumors from patients who completely respond to chemoradiotherapy. BACE1 is known to cleave ST6Gal-1, and we found through inhibitor studies that BACE1 appear to regulate SA due to cleavage of ST6Gal-1 by BACE1. This research will evaluate a previously unknown mechanism of resistance to chemoradiotherapy in rectal cancer, with future p...