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Adaptive mutability of colorectal cancers in response to targeted therapies

A cross-kingdom tale of drug resistance Physicians who treat bacterial infections and those who treat cancer often face a common challenge: the development of drug resistance. It is well known that when bacteria are exposed to antibiotics, they temporarily increase their mutation rate, thus increasing the chance that a descendant antibiotic-resistant cell will arise. Russo et al. now provide evidence that cancer cells exploit a similar mechanism to ensure their survival after drug exposure (see the Perspective by Gerlinger). They found that human colorectal cancer cells treated with certain targeted therapies display a transient up-regulation of errorprone DNA polymerases and a reduction in their ability to repair DNA damage. Thus, like bacteria, cancer cells can adapt to therapeutic pressure by enhancing their mutability. Science, this issue p. 1473; see also p. 1458 Cancer cells can evade therapeutic pressure by increasing their mutation rate. The emergence of drug resistance limits the efficacy of targeted therapies in human tumors. The prevalent view is that resistance is a fait accompli: when treatment is initiated, cancers already contain drug-resistant mutant cells. Bacteria exposed to antibiotics transiently increase their mutation rates (adaptive mutability), thus improving the likelihood of survival. We investigated whether human colorectal cancer (CRC) cells likewise exploit adaptive mutability to evade therapeutic pressure. We found that epidermal growth factor receptor (EGFR)/BRAF inhibition down-regulates mismatch repair (MMR) and homologous recombination DNA-repair genes and concomitantly up-regulates error-prone polymerases in drug-tolerant (persister) cells. MMR proteins were also down-regulated in patient-derived xenografts and tumor specimens during therapy. EGFR/BRAF inhibition induced DNA damage, increased mutability, and triggered microsatellite instability. Thus, like unicellular organisms, tumor cells evade therapeutic pressures by enhancing mutability.