ESTRO 2021 Abstract Book

S297

ESTRO 2021

Conclusion Taken together, these studies indicate that the APOBEC mutation signature is often mutually exclusive with the HRD phenotype, but when found together these tumor cells are highly sensitive to, and patients therefore suitable for, treatment with irradiation. OC-0400 The mutational landscape of cancer’s sensitivity to ionizing radiation M. Abazeed 1 , T. Bera 2 , J. Castrillon 3 , A. Petty 3 , B. Yard 3 , P. Gopal 1 1 Northwestern University, Radiation Oncology, Chicago, USA; 2 Northwestern University , Radiation Oncology, Chicago, USA; 3 Cleveland Clinic, Translational Hematology Oncology Research, Cleveland, USA Purpose or Objective The impact of common or rare gene mutations on the sensitivity of cancers to ionizing radiation remains largely unknown. We conducted a systematic, arrayed (single variant per well) profiling effort to identify gene mutations that alter cellular sensitivity to radiation and validated some of our findings using a clinical cohort of patients who received thoracic radiotherapy alone. Materials and Methods Candidate mutations were prioritized on the basis of genotype-phenotype associations from our previously completed large-scale cancer cell line irradiation profiling study, location within conserved protein domains, and predicted functional impact. We used site-directed mutagenesis to generate mutant clones (2 clones per variant) and transferred the ORFs into lentiviral vectors for stable expression in SV40 lung primary immortalized cells (BEAS2B). Variants that modulated sensitivity in BEAS2B were also tested in TERT-HU1 or NCI-H520 cell lines. For variants predicted to confer loss of gene function, the endogenous loci were deleted using a novel intro-exon junctional CRISPR method (J-CRISPR). To validate putative clinical biomarkers, an IRB-approved study was used to identify patients treated with lung radiotherapy alone. 197 patients with primary (stage I–IV) or recurrent lung cancer and patients with other cancer types and solitary metastases or oligometastases to the lung were included. Fine and Gray regression modeling was used to examine potential predictors of local failure. Results Over 600 cancer variants comprising 91 genes were tested. We identified new resistant and sensitive tumor variants involved in several functional categories (i.e. signaling, cytoskeleton, cell cycle, apoptosis, DNA methylation, etc). Variants that conferred resistance in BEAS2B cells were significantly more likely to confer resistance in TERT-HU1 and NCI-H520 cells, suggesting that most functional variants are cellular context indifferent. Several infrequent cancer variants (<1% prevalence in cancer), including those in ERBB3, SMAD4, TGFBR1, VHL, CTNNB1, and MAP2K1, conferred radiation resistance. Some genes (e.g. KRAS ) demonstrated significant intragenic allelic variation in the magnitude of conferred resistance and other genes (e.g. CTNNB1) displayed both resistance and sensitivity in a protein domain-dependent manner; these results combine to demonstrate that biomarkers predictions are enhanced by detailed variant identity. In our clinical cohort, KRAS (HR 2.23; P =0.02) and CTNNB1 exon 3 (HR 0.3; P = 0.04) mutants conferred resistance and sensitivity, respectively. Conclusion Determining the impact of cancer mutations on the clinical responses to radiation remains a major obstacle in the implementation of personalized radiotherapy. We developed an integrated preclinical to clinical platform