Abstract A120: Adaptive mutability of colorectal cancers in response to targeted therapies

Success in eradicating human cancer with targeted therapies is limited by the emergence of secondary resistance. The prevalent view is that resistance is a fait accompli: when treatment is initiated, tumors already contain drug-resistant mutant cells. However, when cancer cells are challenged with targeted agents, the emergence of drug tolerant ‘persister’ cell population is often observed. Persisters survive exposure to targeted therapies through non-genetic, poorly understood mechanisms, and constitute a reservoir from which genetically divergent, drug-resistant derivatives eventually emerge. Analogously, when bacteria are exposed to stress, such as antibiotic treatment, persister cells can survive and, by switching from high-fidelity to low-fidelity DNA replication process, increase transiently their mutation rate (adaptive mutability), thus improving chances of survival. We used colorectal cancer (CRC) as model system to explore the hypothesis that, in addition to pre-existing drug resistant cells, resistance to targeted therapies could be fostered by a transient increase in genomic instability during treatment, leading to de novo genetic alterations. We found that, when exposed to EGFR and/or BRAF inhibition, mismatch repair proficient (MMRp) CRC cell lines exhibited a down-modulation of MMR and Homologous Recombination (HR) DNA repair genes, and a concomitant up-regulation of error-prone polymerases, resulting in reduced MMR and HR proficiency. Therapy-induced modulation of DNA repair genes was transient and returned to initial levels upon removal of the drugs, or when the cells developed permanent resistance to targeted agents. Notably, MLH1 and MSH2 MMR genes were down-regulated in patients-derived xenografts and tissue samples obtained at clinical response (minimal residual disease) compared to pre-treatment samples, confirming the clinical relevance of our findings. Activation of error-prone polymerases was associated with increased ROS production and accumulation of DNA damage marker γ-H2AX, in a time- a dose-dependent manner upon drug administration. The combination of DNA repair down-modulation and error-prone polymerases upregulation increases mutagenic ability and triggers microsatellite instability under drug-induced stress in CRC cells. Our results demonstrate that cancer cells, like unicellular organisms, evade therapeutic pressures by enhancing mutability. The notion that cancer cells exposed to targeted therapies activate a stress-induced adaptive mutability process may prompt the design of novel therapeutic strategies aimed at interfering with clonal evolution, reducing the generation of new variants during therapeutic treatment.