Familial breast and ovarian cancer

Familial breast and ovarian cancer

What do I need to know?

Highly penetrant and penetrance Penetrance refers to the proportion of people with a particular genetic variant who will go on to develop the condition. For example, people carrying an autosomal dominant variant may not always develop the condition – this is called ‘incomplete penetrance’. If a condition is 100% penetrant, an individual will definitely develop the condition. If penetrance is 80%, most but not all individuals will develop the condition. Other genes and lifestyle factors, such as diet, exercise and smoker status, may affect the penetrance of some conditions. For more information, refer to MedlinePlus’ ‘What are reduced penetrance and variable expressivity?’ gene variants Gene variants are small DNA sequence changes (ie additions, duplications, deletions, substitutions). These variants can have a range of effects: some may cause disease (pathogenic variant), while others do not cause disease but may modify an individual’s risk of disease (i.e may increase risk or provide a protective effect). The vast majority of gene variants are benign and do not result in disease but rather contribute to the differences between people. in BRCA1 and BRCA2 genes are associated with increased risk of several cancers, particularly breast and ovarian. These show autosomal dominant inheritance When a condition follows an autosomal dominant pattern of inheritance, the family tree will usually reveal multiple affected members in multiple generations on the same side of the family. Dominant conditions or traits are expressed when only a single gene variant is inherited. Wide variability in clinical expression is common in many autosomal dominant conditions, even within the same family. Early onset of conditions, such as cancer, can be indicative of autosomal dominant inheritance within a family. Not all dominant conditions show 100% penetrance (eg BRCA1 gene mutations). pattern. Pathogenic variants A pathogenic variant is a genetic variant that increases an individual’s susceptibility or predisposition to certain diseases. Pathogenic variants are also known as mutations. in BRCA1 and BRCA2 are associated with increased risk of other cancers including prostate cancer and pancreatic cancer.

The lifetime risk of breast cancer in Australian women is approximately one in eight. Pathogenic variants of the BRCA1 and BRCA2 genes increase the chance of developing breast cancer to about 70% (cumulative risk to 80 years of age). Despite this, BRCA1 and BRCA2 variants account for only about 5% of all breast cancer cases, because these variants are relatively rare.

Several other genes (low-to-moderate penetrant variants) predisposing to breast and/or ovarian cancer can now also be tested. Features within a family that are suggestive of increased risk of having a pathogenic BRCA1 or BRCA2 variant include:

• multiple affected relatives on the same side of the family (maternal or paternal)
• breast and ovarian cancer in the same woman
• breast cancer diagnosed <40 years of age
• Ashkenazi Jewish ancestry (from central and eastern Europe)
• bilateral breast cancer
• male breast cancer

A three-generation family history is key to identifying high-risk families who are most likely to benefit from genetic testing. Such a history should include first-degree and second-degree relatives on both sides of the family, and ethnic background (eg Ashkenazi Jewish). Note that sex-specific cancer can be inherited through maternal or paternal sides of the family (eg BRCA variants can be passed from the paternal side). Type of cancer (including bilateral) and age of onset should be recorded where available.

Use existing risk criteria to identify families who are at increased risk of having a pathogenic BRCA1 or BRCA2 variant (high risk), or women who may require additional screening or chemoprevention (moderately increased risk).

Genetic testing

Family cancer clinics can assess individual risk to determine the utility of genetic testing for BRCA1 and BRCA2 variants. As new genes in which variants predisposing to breast cancer are identified, testing of these genes may be offered as part of a panel of genetic tests through the family cancer clinic. If a pathogenic variant is identified in an individual, testing will be offered to relatives.

A range of options are available to carriers of BRCA1 and BRCA2 variants to manage their increased cancer risks.

A rebate is available for BRCA1 and BRCA2 gene testing under the Medicare Benefits Schedule (MBS).

Validated polygenic risk scores (PRS) for breast cancer are commercially available. In combination with other known risk factors (e.g. family history, lifestyle and hormonal risks) PRS may be used to provide individualised information about breast cancer risk. Their precise role in risk-based breast cancer prevention and screening in Australia is yet to be determined.

When should I refer?

The recommended breast cancer screening strategy for women and individuals at increased risk of breast cancer is outlined in the Royal Australian College of General Practitioners (RACGP’s) 2016 Guidelines for preventive activities in general practice (Red Book).

Other considerations

A cancer antigen 125 (CA 125) blood test and transvaginal ultrasound are not recommended as screening tests for ovarian cancer, even in women who are at high risk as such screening has not been shown to improve prognosis.

Women at increased risk for breast and ovarian cancer should be encouraged to:

• discuss their family history with all first-degree relatives
• advise family members to discuss their risk with their general practitioner (GP).