13P - Methylation of BRCA1 and RAD51C promoters determined by droplet digital PCR predicts homologous repair deficiency in patients with high-grade serous ovarian cancer: Results of the BOVARY-pilot study in paired FFPE and plasma samples and the ICL ovarian cohort

Background

Ovarian cancer (OC) stands as the ninth leading cause of cancer-related deaths in women and represents the most prevalent gynaecological malignancy. Patients with OC are more likely to benefit from iPARP if their tumours present deleterious mutations of BRCA1/2 genes, and if they have a homologous recombination deficiency (HRD). Somatic or germline deleterious mutations of BRCA1/2 occurs in 15 to 20 % of HRD OC. However, not all HRD cases are due to these mutations. Indeed, studies have shown that BRCA1 and RAD51C promoter methylation are responsible for 19% and 2% of HRD cases, respectively.

Methods

Two hundred and twenty-four patients with ovarian cancer were included in the ICL cohort, and 20 in the BOVARY cohort. All of them give their informed consent. DNA was extracted from FFPE tissue and cfDNA from plasma. Low-pass whole-genome sequencing was used to determine HRD status. Methylation of BRCA1 and RAD51C promoters was analysed using droplet digital PCR (ddPCR) after enzymatic conversion. A threshold of 10% was set for defining a sample as methylated.

Results

HRD status was determined for 193 of the 224 patients. A total of 77 patients were identified as HRD. Among the patients with HRD, 25 were found to have a deleterious BRCA1 or 2 mutation. Additionally, 3 patients exhibited a deleterious mutation in the RAD51C gene. After analysis of BRCA1 and RAD51C methylation in all the samples, 32 patients were identified with BRCA1 promoter methylation, while 1 patient had a RAD51C promoter methylation. All patients with promoter methylation were HRD. In the BOVARY cohort, BRCA1 promoter methylation was detected in only one HRD sample, with concordant methylation found in the paired cfDNA sample. No methylation of the RAD51C gene was found.

Conclusions

We demonstrate the feasibility of ddPCR for determining the methylation of BRCA1 and RAD51C promoters to identify patients with HRD. Furthermore, although the number of cfDNA samples analysed remains modest, our findings suggest that liquid biopsy could also be used to determine BRCA1 and RAD51C methylation statuses.

Editorial acknowledgement

Clinical trial identification

NCT03881683.

Legal entity responsible for the study

Institut de Cancérologie de Lorraine.

Funding

Tesaro.

Disclosure

All authors have declared no conflicts of interest.