BRCA1 is a tumor suppressor protein that safeguards genomic stability, with its BRCT domain playing a critical role in the DNA damage response by recognizing phosphorylated binding partners such as BACH1 and CtIP, thereby coordinating repair signaling and cell cycle control. Missense variants in this domain are associated with increased breast cancer risk, but their clinical interpretation remains challenging due to variable effects on protein stability and binding, coupled with limitations of predictive algorithms and functional assays. Here, we present the first experimental characterization of the BRCA1 M1652I and F1662S variants, identified in Bahraini and Lebanese breast cancer cohorts, in comparison with the pathogenic R1699W variant. Methionine 1652 and phenylalanine 1662 reside within the β1 strand of the BRCT N-domain’s hydrophobic core, where M1652I represents a conservative substitution and F1662S a radical one. Biophysical analyses demonstrated that both M1652I and F1662S retain secondary structure and thermal stability (Tm = 49.1°C and 48.6°C, respectively) comparable to wild type (Tm = 47.2°C), whereas R1699W exhibited marked destabilization (Tm = 41.9°C). Binding assays further revealed that M1652I and F1662S preserve wild-type affinity for BACH1 and CtIP peptides, while R1699W completely abolishes binding. These findings suggest that M1652I and F1662S are likely benign in isolation but may act as sensitizing variants in combination with other destabilizing mutations—an important consideration in consanguineous populations of the MENA region with a high prevalence of hereditary breast cancer. Overall, our study highlights the value of integrating biophysical evidence with regional population genetics to improve BRCA1 variant classification and support precision oncology in underrepresented cancer cohorts.
Nomikos et al. (Sun,) studied this question.