Comprehensive Analysis Reveals Adaptive DNA Repair and Replication Stress Networks in Genomically Unstable Breast Cancer

Genomic instability is a defining hallmark of breast cancer, yet the mechanisms by which tumors tolerate persistent DNA damage remain poorly understood. We performed a comprehensive, multi-cohort analysis of breast cancer datasets to define how DNA damage response (DDR) and replication stress tolerance (RST) networks are rewired in genomically unstable tumors. Using fraction of genome altered (FGA) as a chromosomal instability metric, we show that BRCA-mutant tumors exhibit elevated genomic instability coupled with increased expression of homologous recombination, Fanconi anemia, mismatch repair, base excision repair, and alternative end-joining pathways. Strikingly, heightened pathway activity correlates with increased genome alteration, supporting a model of damage tolerance rather than repair restoration. RST programs, including fork remodeling, protection, and single strand DNA gap suppression, further contribute to tumor fitness under replication stress. These adaptive states are enriched in aggressive subtypes, intensified with progression, and associate with pathway-specific mutational burden. Co-occurrence and mutual exclusivity mapping uncovered non-random subtype-relevant genetic interactions states among major drivers and DDR genes, nominating context-specific synthetic lethal opportunities. Our findings identify compensatory genome-maintenance programs as central drivers of tumor resilience and highlight pathway-specific vulnerabilities for targeted therapeutic intervention.