Implications of homologous recombination repair pathway mutations on the immune microenvironment in gastric cancer.

e16019 Background: Homologous recombination repair (HRR), a key pathway in DNA damage response (DDR), is linked to the efficacy of platinum-based chemotherapy, PARP inhibitors, and immunotherapy across cancers. However, its mutational landscape and impact on the immune microenvironment in gastric cancer remain unclear. Methods: Next-generation sequencing (NGS) and multiplex immunofluorescence were used to analyze the mutational profiles and immune microenvironment of 213 gastric cancer patients, focusing on the impact of HRR mutations. Results: Of the 213 patients, 67 (31.45%) harbored HRR mutations, including 46 with variants of uncertain significance (VOUS). The most frequently mutated HRR genes were RAD50 (4.69%), BLM (4.23%), and BRCA2(2.35%), with BLM/RAD50 co-mutations being the most common (2.35%). HRR mutations were significantly co-occurring with KMT2D mutations and mutually exclusive with TP53 mutations. We analyzed the association of HRR pathway mutations with immune biomarkers, stratifying patients by pathogenic/likely pathogenic and VOUS mutations. Pathogenic mutations were linked to high tumor mutational burden (TMB; p < 0.0001) and microsatellite instability-high (MSI-H) status but showed no association with PD-L1 expression (CPS = 1/5/10) or EBV infection. Similar trends were seen for VOUS mutations. Further analysis of HRR mutation burden (≥2 mutated HRR genes) demonstrated an association with high TMB and MSI-H status, as well as a tendency toward positive PD-L1 CPS expression (CPS ≥ 1). Immune microenvironment analysis revealed that HRR- patients had higher proportions of M2 macrophages, suggesting polarization toward an immunosuppressive phenotype. However, the M1/M2 macrophage ratio showed no significant difference, potentially due to compensatory effects from other immune cells. HRR- patients also displayed elevated CD56dim NK cells and a higher CD56dim/bright NK cell ratio, indicating a dominance of cytotoxic NK cells. In addition, HRR- patients had more CD4⁺ T cells (p = 0.0004) and a higher CD4/CD8 ratio, though not statistically significant. The observed infiltration of CD4⁺FOXP3⁻ T cells suggests that HRR mutations might influence immune cell composition and contribute to the formation of a non-inflammatory immune microenvironment in HRR-mutant gastric cancers. Conclusions: In gastric cancer, pathogenic/likely pathogenic HRR mutations are associated with higher TMB levels and MSI, while HRR variants of uncertain significance show a similar trend. Immune microenvironment analysis suggests that HRR pathway mutations may be linked to a non-inflammatory immune microenvironment. These findings indicate that HRR mutations could be relevant to the efficacy of immunotherapy in gastric cancer patients, warranting further investigation into their clinical value.