Dr. Yan Gao

Canada

FAM72A potentiates APOBEC3B-driven cytidine deamination and mutagenic repair to promote colorectal tumorigenesis

Yan Gao 1, Mohammad Khakpoor Koosheh1, Bhupesh Thakur 1, Maribel Berru1, Alberto Martin 1

1 Department of Immunology, University of Toronto, Toronto, Ontario M5S 1A8, Canada

Abstract

Background

APOBEC3B (A3B) promotes cytidine-to-uracil (C‚ÜíU) deamination and generates clustered mutations characteristic of many human cancers. A major barrier to A3B-induced mutagenesis is the rapid removal of uracil by UNG and subsequent high-fidelity repair. Recent evidence suggests that FAM72A antagonizes UNG activity in B cells by preserving uracil lesions during class-switch recombination (CSR). We hypothesize that FAM72A similarly stabilizes A3B-induced dU lesions in intestinal epithelial cells, shifting repair toward error-prone pathways and promoting colorectal cancer (CRC).

Methods

We established Apc^Min, Villin-Cre A3B transgenic, FAM72A transgenic, and A3B × FAM72A double transgenic mouse models to dissect in vivo interactions between A3B and FAM72A. Tumor burden was assessed at 12 weeks. In vitro, MC38 FAM72A knockout and overexpression lines were analyzed for mutation-associated phenotypes, including persistence of dU lesions, γH2AX accumulation, and altered UNG/MMR pathway engagement. CH12F3 CSR assays were used to probe dU preservation and mutagenic repair signatures.

Results

A3B or FAM72A alone produced mild increases in intestinal polyps, whereas A3B √ó FAM72A double transgenic mice exhibited a synergistic elevation in tumor number and size. Mechanistically, FAM72A overexpression prolonged the lifespan of A3B-generated dU lesions, reduced UNG-mediated excision, and increased engagement of mutagenic MMR and error-prone repair pathways. FAM72A knockout diminished A3B-associated DNA damage and reduced mutation-associated signatures.

Conclusions

FAM72A promotes CRC development by stabilizing A3B-induced uracil lesions and redirecting repair toward mutagenic pathways. This work identifies FAM72A as a mechanistic enhancer of APOBEC-driven genomic instability and a potential target for limiting mutagenesis in colorectal cancer.