Identification of RAD51 foci in cancer-associated circulating cells of patients with high-grade serous ovarian cancer: association with treatment outcomes

OBJECTIVE: Fifty percent of patients with high-grade serous ovarian cancer harbor defects in the homologous recombination repair pathway. RAD51 foci form where DNA is damaged, indicating its involvement in repairing double-stranded breaks. High levels of RAD51 in ovarian cancer tissue have been associated with a poorer prognosis. OBJECTIVE: To demonstrate RAD51 foci in circulating cancer-associated cells of patients with ovarian cancer and their association with clinical outcomes. METHODS: One hundred and twenty-four patients with high-grade serous ovarian cancer had blood samples taken at strategic points during treatment and follow-up. Cells were stained using WT1 and RAD51 antibodies with immunofluorescence and reviewed under Leica camera microscopy; RAD51 foci were counted. Correlations were made between numbers of RAD51 foci and treatment response, BRCA status, and progression-free survival. RESULTS: RAD51 foci were identified in all patients (n=42) with wild-type BRCA. BRCA mutant/homologous recombination deficiency-positive patients (n=8) had significantly lower numbers of RAD51 foci (p=0.009). Responders to treatment (n=32) had a reduction in circulating cells (p=0.02) and RAD51 foci (p=0.0007). Numbers of RAD51 foci were significantly higher in the platinum-resistant population throughout treatment: at the start of treatment, in 56 platinum-sensitive patients there was a mean of 3.6 RAD51 foci versus 6.2 in 15 platinum-resistant patients (p=0.02). Patients with a high number of RAD51 foci had worse median progression-free survival: in 39 patients with a mean of <3 RAD51 foci at treatment start, median progression-free survival had not been reached, compared with 32 patients with >3 RAD51 foci whose progression-free survival was 13 months (p=0.04). CONCLUSIONS: Levels of RAD51 foci in circulating cancer-associated cells of patients with high-grade serous ovarian cancer are associated with clinical outcomes and may be a more pragmatic method of determining a homologous repair-deficient population.