Interdependent roles of BRCA1, CtBP, estrogen, and NADH/NAD+ ratio in error-prone repair, breast cancer initiation, and metastasis.

Genomic instability may result from a shift in the double-strand break (DSB) repair pathway from homologous recombination (HR) to error-prone non-homologous end joining (NHEJ). Normal BRCA1 expression is essential for high-fidelity HR, and its deficiency may promote error-prone NHEJ. Similarly, a low NADH/NAD+ ratio promotes low-fidelity HR, whereas a high NADH/NAD+ ratio promotes NHEJ. Further, although p53 inhibits HR, it is required for the high fidelity of this process. Furthermore, estrogen promotes NHEJ and nuclear export of p53, leading to low-fidelity HR or error-prone NHEJ. Thus, a shift in BRCA1 expression, NADH/NAD+ ratio, or a higher level of estrogen may cause genomic instability, which may initiate breast cancer. Furthermore, hypoxia may shift DSB repair from HR to NHEJ by repressing BRCA1 through dimeric CtBP, which forms under an elevated NADH/NAD+ ratio caused by hypoxia. Genomic instability caused by this shift in the DSB repair mechanism under hypoxia promotes EMT-induced breast cancer metastasis. This review discusses the roles of CtBP, BRCA1, estrogen, and metabolic shift linked to an altered NADH/NAD+ ratio in the initiation of breast cancer and EMT-mediated metastasis.