For cancer patients whose tumors resist immunotherapy, understanding why some tumors respond and others don't has been one of oncology's most pressing puzzles. A convergence of molecular biology and immunology is now reframing that question around a cellular system most people associate with cancer risk rather than cancer treatment: the DNA damage response.
When tumor cells carry defects in DNA damage response (DDR) machinery — or are exposed to DDR-targeting drugs — they generate two distinct immunogenic signals simultaneously. First, accumulated mutations produce neoantigens, novel peptide fragments displayed on cell surfaces that tumor-specific T cells can recognize as foreign. Second, DNA fragments spill into the cytoplasm, activating the cGAS-STING pathway, a molecular sensor that triggers type I interferon production and actively recruits immune effector cells into the tumor microenvironment. These two mechanisms act synergistically, not additively, substantially amplifying responsiveness to immune checkpoint blockade agents such as anti-PD-1 and anti-CTLA-4 therapies.
This mechanistic clarity has significant implications for how oncologists might sequence or combine therapies. Tumors with BRCA1/2 mutations, mismatch repair deficiency, or homologous recombination deficiency — all characterized by DDR dysfunction — may represent an especially actionable population for checkpoint combination strategies. However, several caveats warrant attention. Resistance mechanisms remain incompletely mapped, and not all DDR-deficient tumors respond uniformly, suggesting that additional tumor microenvironment factors modulate outcomes. Most supporting evidence comes from preclinical models and correlative clinical data rather than large randomized trials isolating DDR status as the sole variable. The field is advancing rapidly, but translation from mechanistic insight to validated treatment algorithms requires prospective cohort data stratified by specific DDR defect type. This review represents a timely synthesis of an emerging framework rather than established clinical protocol.