Genomic characterization of therapy-associated polyposis reveals an alkylating mutational signature from prior treatment

Gastrointestinal (GI) polyposis is a major risk factor for colorectal cancer (CRC) and a defining feature of hereditary polyposis syndromes such as familial adenomatous polyposis (FAP). Therapy-associated polyposis (TAP), however, is a rare and incompletely characterized condition that develops decades after treatment for childhood or young adult cancers (CYAC), most often following abdominopelvic radiation or exposure to alkylating agents. As long-term CYAC survival improves, the burden of late GI toxicity, including markedly elevated risks of polyps, CRC, and secondary cancers, continues to rise, yet the molecular features of TAP remain poorly understood. Here, we present the largest clinicopathological and genomic study of TAP to date, comprising 29 patients diagnosed at a median age of 49 years and a median latency of 29 years after primary cancer therapy. Most patients (78%) had received alkylating agents and exhibited high rates of secondary malignancies. Histopathology revealed mixed polyp subtypes with a predominance of adenomas. Given these features and the presence of family history in a subset of patients, we investigated the possibility of Hereditary Mixed Polyposis Syndrome (HMPS). Whole-genome sequencing excluded HMPS by demonstrating absence of the canonical 40-kb GREM1 duplication and lack of consistent GREM1 overexpression. Comparative genomic analysis revealed that TAP adenomas exhibit more extensive genome fragmentation and a higher burden of large structural variants than FAP adenomas. Mutational signature profiling identified strong contributions from age-associated signatures (SBS1, SBS5) and a strong, pervasive contribution of the alkylating-agent signature SBS25, even in samples lacking matched normal tissue, whereas platinum-associated SBS31 was minimal. Patient-derived organoids from TAP adenomas showed impaired differentiation, suggesting persistent therapy-induced stem cell dysfunction. Together, these findings define TAP as a distinct polyposis syndrome marked by heterogeneous histology, long latency, profound structural genomic injury, and chemotherapy-specific mutational scars. This work supports early and tailored GI surveillance for CYAC survivors and provides mechanistic insight into the long-term consequences of cytotoxic therapy on intestinal epithelial homeostasis.