S2516
Radiobiology – DNA damage repair
ESTRO 2026
University Cancer Research Institute, Seoul, Korea, Republic of. 5 Internal Medicine, Seoul National University Hospital, Seoul, Korea, Republic of. 6 Research Center, Dongnam Institute of Radiological and Medical Science, Busan, Korea, Republic of Purpose/Objective: Colorectal cancers in patients with Lynch syndrome (LS) are characterized by high microsatellite instability (MSI) due to mismatch repair (MMR) deficiency, most commonly caused by MLH1 mutations.1) Ionizing radiation (IR), which plays an important role in colorectal cancer treatment, induces DNA double- strand breaks but may also contribute to radiation- induced carcinogenesis in normal tissues.2) However, the mechanisms of tumorigenesis in the context of MLH1 mutations and cellular responses with IR exposure remain poorly understood.3) This study aimed to investigate the effects of MLH1 mutations and IR exposure on normal colorectal tissues using murine 3D organoid models and whole-genome
sequencing (WGS).4) Material/Methods:
Conclusion: Homozygous MLH1 loss induces distinct MMR- deficient mutational signatures and elevated MSI. In contrast, heterozygous MLH1 mutations, although known to predispose for MMR deficiency upon loss of the second allele, do not result in MMR-related mutational features or radiosensitization in normal colorectal tissues. IR exposure did not significantly exacerbate mutational burden in any genotype. Our findings provide critical insights into the mutational landscape of LS and suggest that MLH1 heterozygosity alone may not increase the risk of radiation-induced carcinogenesis in normal colorectal epithelium. This has implications for personalized therapeutic strategies and risk assessment in colorectal cancers for LS patients. References: 1) Sinicrope FA. Lynch Syndrome-Associated Colorectal Cancer. N Engl J Med. 2018;379(8):764-773. doi:10.1056/NEJMcp17145332) Rombouts AJM, Hugen N, van Beek JJP, et al. Does pelvic radiation increase rectal cancer incidence? - A systematic review and meta-analysis. Cancer Treat Rev. 2018;68:136-144. doi:10.1016/j.ctrv.2018.05.0083) Shrestha KS, Aska EM, Tuominen MM, et al. Tissue-specific reduction in MLH1 expression induces microsatellite instability in intestine of Mlh1+/- mice. DNA Repair (Amst). 2021;106:103178. doi:10.1016/j.dnarep.2021.1031784) Youk J, Kwon HW, Lim J, et al. Quantitative and qualitative mutational impact of ionizing radiation on normal cells. Cell Genom. 2024;4(2):100499. doi:10.1016/j.xgen.2024.100499 Keywords: MLH1, Mismatch repair, Lynch syndrome
Organoid cultures were clonally expanded from single cells derived from the lower gastrointestinal tracts of wild-type (Mlh1+/+; WT), heterozygous (Mlh1+/ − ; HE), and homozygous (Mlh1 − / − ; HM) knockout mice. WGS and mutational signature profiling were performed to analyze mutational burden and associated signatures. MSI testing was performed to evaluate MMR status. Organoids were exposed to gamma irradiation at doses of 0, 2, 4, and 8 Gy, and viability assays were conducted to assess radiation sensitivity and dose- response relationships. Results: HM mice exhibited markedly increased single- nucleotide variants (SNVs), insertion–deletion mutations (indels), and high levels of SBS15, a mutational signature associated with MMR deficiency (Figure 1). In contrast, neither WT nor HE mice showed MMR-deficient mutational patterns or elevated MSI rates (Figure 2). Age-related accumulation of mutations and MSI was evident only in HM mice. IR exposure did not significantly increase SNVs, MMR- related mutational signatures, or MSI across all genotypes. IR dose was negatively correlated with cell viability, but it did not correlate with mutational burden and patterns. HE mice showed no significant differences from WT in mutational patterns, MSI, or radiosensitivity, indicating that heterozygous MLH1 mutation alone maintains a wild-type phenotype under baseline and irradiated conditions.
Made with FlippingBook - Share PDF online