DNA damage response as a prognostic indicator in metastatic breast cancer via mutational analysis

Background High tumor heterogeneity contributes to breast cancer recurrence and metastasis. However, the lack of indicators to serve as precise and reliable means of predicting breast cancer prognosis has yet to be addressed. This study aims to reveal the prognostic relevance of mutations in metastatic breast cancer (MBC) by large-scale circulating tumor DNA (ctDNA) analysis in China. Methods We performed ctDNA panel-captured sequencing of 958 blood samples from MBC patients including 494 hormone receptor (HR)-positive cases, 130 human epidermal growth factor receptor 2-positive cases, and 177 triple-negative breast cancer (TNBC) cases. The somatic mutations and potential targets were assessed. Progression-free survival (PFS) was analyzed using the Kaplan-Meier method. Results In 801 of the 958 MBC blood samples, 663 mutated genes and 5,829 nonsynonymous alterations were identified. Mutated genes of the highest frequency were tumor protein p53 (TP53, 54%), phosphatidylinositol-4,5-bisphosphate 3-kinase, catalytic subunit alpha (PIK3CA, 41%), estrogen receptor 1 (ESR1, 12%), myeloid/lymphoid or mixed-lineage leukemia protein 3 (MLL3, 11%), DNA (cytosine-5)-methyltransferase 3A (DNMT3A, 10%), erb-b2 receptor tyrosine kinase 2 (ERBB2, 10%), GATA binding protein 3 (GATA3, 8%), FAT atypical cadherin 1 (FAT1, 7%), phosphatase and tensin homolog (PTEN, 6%), and mitogen-activated protein kinase kinase kinase 1 (MAP3K1, 6%). Enriched mutations and driver genes in MBC varied across stages and in multiple subtypes. Moreover, TP53, ERBB2, or coexisting TP53/PIK3CA mutations in MBC were remarkably related with shorter PFS. Mutated DNA damage response (DDR) genes were significantly associated with tumor mutation burden and mutant-allele tumor heterogeneity score, as well as with worse clinical outcome. Conclusions Our findings indicate that the mutations of TP53, PIK3CA, ERBB2, and in particular, DDR genes, in MBC might be relevant indicators of unfavorable prognosis in MBC.