The impact of BRCA1 insufficiency on TNBC-TME interactions in the context of chronic inflammation

Introduction: Breast cancer (BC) is the most common malignancy in women and a leading cause of cancer-related deaths. Among its four BC subtypes, triple-negative BC (TNBC) is characterized by its aggressive phenotype and poor prognosis. BRCA1 mutations are common in TNBC and are characterized by increased risk for BC. These mutations impair BRCA1 protein function, disrupting homologous recombination repair of dsDNA breaks and leading to elevated mutation load in the cells. Beyond mutations, down-regulated expression levels of BRCA1 are noted in sporadic TNBC patients, resulting from promoter hypermethylation that leads to an epigenetic inactivation of gene expression. This phenotype is associated with greater responsiveness to adjuvant chemotherapy and improved disease-free survival compared to patients without BRCA1 alterations. Given the significant roles of inflammatory cytokines in promoting TNBC progression, in this study we asked what is the impact of BRCA1 insufficiency on the pro-metastatic activities of human TNBC cells in the context of the pro-inflammatory TME.

Materials and methods: BRCA1 down-regulation was induced in human TNBC cells using shRNA, giving rise to a strong deficiency of the protein in MDA-MB-231 (MDA) cells and to marked down-regulation of BRCA1 expression BT-549 (BT) cells. Pro-metastatic activities were then assessed following inflammatory stimulation with TNFα or IL-1β, each alone or together.

Results and discussion: Our findings indicate that under pro-inflammatory stimulation, BRCA1 insufficiency plays a protective role in MDA cells. Specifically, down-regulation of BRCA1 expression has given rise to reduced TNFα-induced invasion; stimulation of MDA cells with TNFα ± IL-1β has given rise to lower production of pro-metastatic tumor-promoting inflammatory factors (IL-6, CCL2 and lipocalin-2), and to reduced levels of tumor cells expressing PD-L1. BRCA1 insufficiency in BT cells has also given rise to reduced production of pro-metastatic factors, although with a different profile of those affected in MDA cells.

Conclusions: Down-regulation of BRCA1, driven by low expression rather than BRCA1 mutation, may have protective effects against disease progression which is driven by the inflammatory TME in TNBC tumors.