Characterisation of the BRCT Domains of the Breast Cancer Susceptibility Gene Product BRCA1

The breast cancer susceptibility gene product BRCA1 is a tumour suppressor but the biochemical and biological functions that underlie its role in carcinogenesis remain to be determined. Here, we characterise the solution properties of the highly conserved C terminus of BRCA1, consisting of a tandem repeat of the BRCT domain (BRCT-tan), that plays a critical role in BRCA1-mediated tumour suppression. The overall free energy of unfolding of BRCT-tan is high (14.2 kcal mol−1 at 20 °C in water) but unfolding occurs via an aggregation-prone, partly folded intermediate. A representative set of cancer-associated sequence variants was constructed and the effects on protein stability were measured. All of the mutations were highly destabilising and they would be expected to cause loss of function for this reason. Over half could not be purified in a soluble form, indicating that these residues are critical for maintaining structural integrity. The remaining mutants exhibited much greater aggregation propensities than the wild-type, which is most likely a consequence of their reduced thermodynamic stability relative to the partly folded intermediate. The mutations characterised here are located at different sites in the BRCT-tan structure that do not explain fully their effects on the proteinʹs stability. Thus, the results indicate an important role for biophysical studies in assessing the significance of sequence variants and in determining how they cause disease.