In Canadian women, OC is the second most frequent gynaecological cancer and the fifth leading cause of cancer deaths.4 The high lethality is, in part, attributed to advanced stages of cancer at initial diagnosis and limited treatment options. The standard of care for advanced OC is surgical cytoreduction and platinum-based chemotherapy.5 Despite high overall response rates with primary therapies, 70% of women relapse within 3 years.6

The strongest risk factor for OC is family history of ovarian or breast cancer with an estimated 20%–30% of epithelial OC related to an inherited predisposition.1 2 Most hereditary OCs are caused by inherited (germline) disease-causing variants in either the BRCA1 or BRCA2 genes, which result in a 39%–63% and 16.5%–27% cumulative lifetime risk for BRCA1 and BRCA2 carriers, respectively.7 8 For OC, it is estimated that germline disease-causing variants in BRCA1/2 contribute to 15%–20% of cases whereas disease-causing variants in homologous recombination genes such as RAD51C, RAD51D and BRIP1 contribute to up to 3% of cases,1 and disease-causing variants in mismatch repair genes causative for Lynch syndrome (MLH1, MSH2, MSH6 and PMS2) contribute to 0.5% of cases. HGSC is the most common OC subtype and accounts for up to 70% of all epithelial OC, with the highest frequency of germline BRCA1/2 disease-causing variants. Women having other OC subtypes (low-grade serous carcinoma, endometrioid carcinoma, clear cell carcinoma) also have an appreciable risk of carrying germline BRCA1/2 disease-causing variants whereas women with mucinous OC are less likely to be carriers.9–11 Several guidelines recommend that all women diagnosed with epithelial OC be offered germline genetic testing for BRCA1/2, and other OC susceptibility genes, irrespective of their clinical features, age of diagnosis or family cancer history.2 12 13 In Canada, eligibility criteria for germline genetic testing in OC varies across provinces, with some provinces providing testing for all women with non-mucinous OC but limited in other provinces to women with HGSC.1

BRCA1/2 proteins mediate repair of double-stranded DNA breaks by homologous recombination repair while PARP mediates repair of single-stranded DNA breaks. The presence of a BRCA1/2 disease-causing variant in a tumour results in HRD. Inhibition of PARP, in combination with HRD, results in cell death due to the accumulation of double-stranded breaks, a phenomenon known as ‘synthetic lethality’.14 Patients with HRD in tumour tissue due to BRCA1/2 disease-causing variants are therefore sensitive to medications that inhibit the PARP pathway.15–17 Sequencing of DNA derived from HGSC tumours has estimated that 15%–20% of tumours carry germline BRCA1/2 disease-causing variants and approximately 8% of tumours have a somatic (acquired) disease-causing variant.18 19 Clinical trials have demonstrated that women with either germline or somatic BRCA1/2 disease-causing variants respond well to PARPi treatment.15 17 20

In May 2016, Health Canada approved the use of PARPi for treatment of platinum-sensitive, relapsed BRCA1/2 mutated (germline or somatic), high-grade serous epithelial ovarian, fallopian or primary peritoneal cancers.21 Due to the growing need across Canadian labs to provide BRCA1/2 tumour testing, this current guideline was initiated by a working group of the CCMG with representation from the Canadian Association of Pathologists to provide best practice recommendations for testing of BRCA1/2 in the context of HGSC.

Definitions and abbreviations related to the content of this guideline are shown in Box 1.

Bioinformatics: the application of computational and statistical sciences to the collection, organisation and analysis of biological data.

CNV: a region that contains gains or losses of genetic material. This may involve a single exon through to several thousands of kilobases of DNA and may be clinically benign, uncertain or pathogenic.

Disease-causing variant: a variant with sufficient evidence to classify as pathogenic or likely pathogenic variant according to the germline American College of Medical Genetics and Genomics/Association for Molecular Pathology variant interpretation guidelines.

FFPE: formalin-fixed, paraffin embedded.

Germline variant: genetic change originating from a gamete (a sperm or an egg), which is present in all (or the majority of) cells of the body; the germline variant could be passed to offspring.

HGVS: Human Genome Variation Society.

HRR (homologous recombination repair): cellular mechanism to repair double-stranded breaks.

HRD (homologous recombination deficiency): a deficiency in HRR.

LLOD (lower limit of detection): the lowest variant allele frequency which can be reliably distinguished from sequencing errors.

MLPA (multiplex ligation-dependent probe amplification): a molecular technique to detect exon-level CNVs.

NGS (next-generation sequencing also known as massively parallel sequencing): high-throughput technologies used to determine nucleotide sequences and genome dosage at numerous loci using a single test, including targeted variant, single gene, targeted gene panels, whole exomes and/or whole genome sequence determination.

OC (ovarian cancer including ovarian, fallopian tube cancer and primary peritoneal cancers): the majority of cases of OC are of epithelial origin (∼90%), with five main histological subtypes: high-grade serous carcinoma (70%), low-grade serous carcinoma (<5%), endometrioid carcionoma (10%), clear cell carcinoma (CCC) (10%) and mucinous carcinoma (3%).

PARPi (PARP inhibitor): poly (ADP-ribose) polymerase (PARP) inhibitor.

Read depth: the number of sequence reads at a particular base; each read preferably represents a unique molecule of genomic DNA, although this is dependent on assay design.

Somatic variant: a genetic change originating in a somatic cell (not a gamete), and therefore present in only a subset of cells of the body and not passed on to the offspring.

SNV: single nucleotide variant.

Tumour cellularity: fraction of tumour cells to total number of cells in the specimen.

VAF (variant allele frequency): proportion of reads with the variant.