BRCA Testing: What Does a Negative Result Mean?
By Soo Chin Lee, MD

Breast cancer is the most common female cancer in the United States, and about 5-15% of cases are hereditary. Two breast cancer causing genes, BRCA1 and BRCA2, have been identified in the last five years. Together, they account for about 30-60% of all inherited breast cancers. Clinical testing for BRCA1/2 has been available for several years, and it involves a simple blood draw. The three types of testing for BRCA1/2 are full gene sequencing, a panel for the founder mutations common in the Ashkenazi Jewish population, and a mutation-specific assay. The tests, when they are used, and how they are interpreted are illustrated in the following cases.

Case 1
Full gene sequencing
Joan is concerned about hereditary breast cancer because of her family history. Her sister, Mary, is 35 years old and was recently diagnosed with breast cancer. Both her mother and maternal aunt had breast cancer. Her family is not of Ashkenazi Jewish descent. Using the Couch* model, the estimated likelihood of the family carrying a BRCA1/2 mutation is about 5-20%. Joan is considering prophylactic mastectomy and feels that genetic testing may help in her decision-making.

pedigree
Diagram for Case 1: BrCa = breast cancer

Full gene sequencing is currently the gold standard for BRCA testing and would be the appropriate test in this case. Ideally, testing should start with an affected family member such as Mary, her sister. This is because if Joan were tested first, a negative test result could mean two different things: that there is a mutation in her family but Joan did not inherit it or that a mutation could be present in her family but was not detected using current technology. By sequencing the entire BRCA1/2 genes, the test is potentially able to identify mutations along the entire length of the gene. However, full sequencing is laborious and costly to perform, about $2,400.

Full gene sequencing was performed on Mary and the test was negative. Mary was informed that, although the test was negative, there is a 15-20% chance that she has a BRCA1/2 mutation that could not be identified by current technology. This is because only the coding sequences were examined and, as a result, mutations in promoter regions and large deletions were not detected. Alternatively, Mary may have a mutation in a gene other than BRCA1/2 that is yet to be discovered. Thus, the possibility of her breast cancer having a genetic component cannot be ruled out completely with a negative BRCA1/2 test. Joan was informed that she would not be tested since Mary has tested negative and further testing is unlikely to be informative. Joan's risk for breast cancer was estimated based on family history and breast cancer screening was recommended accordingly.

Case 2
Screening panel for the three common founder mutations and mutation-specific assay
Sharon is 40 years old and of Ashkenazi Jewish descent. Both her mother and sister, Judy, have breast cancer. Using the Couch model, the likelihood of her family carrying a BRCA1/2 mutation is estimated to be about 30-60%. In this case, the appropriate strategy would be to first screen for the three common founder mutations, which are present in approximately 3% of the general Ashkenazi Jewish population and account for about 90% of all identified BRCA mutations among Jewish women. This test is specific, technically simple to perform, and relatively inexpensive, about $350.

pedigree
Diagram for Case 2: BrCa = breast cancer

Sharon's sister, Judy, underwent the founder mutation screening test and was found to be negative. However, Judy may still carry a rare BRCA mutation that is not detected. Full gene sequencing has the potential to find a rare BRCA1/2 mutation.

Judy next underwent full gene sequencing, and a mutation was identified on BRCA1. Because BRCA1 is autosomal dominant, Sharon has a 50% chance of carrying the same mutation as Judy, who inherited it from their mother. She and other blood relatives can be tested for this single mutation using a mutation-specific assay, a simple test that only looks for a specific mutation unique to their family. This test is highly specific and inexpensive. Sharon was found not to have the mutation. She was reassured that despite her family history, she is not at greater risk than the general population for developing breast cancer, which is about 12%. Therefore, Sharon does not need to obtain more aggressive screening for breast cancer than the general population.

The three different types of genetic testing currently available for BRCA1/2 are appropriate in different clinical settings. Because of technical limitations and lack of full knowledge of all the genes that cause breast cancer, negative results have to be interpreted with great caution. Expertise in interpreting gene test results and providing post-test counseling remains a critical component of the cancer genetics service.

*The Couch model estimates the likelihood of detecting a BRCA1 mutation based on family and personal history of breast and/or ovarian cancer. Couch FJ et al. (1997). BRCA1 Mutations in women attending clinics that evaluate the risk of breast cancer. NEJM, 336(20): 1409-15.

Dr. Soo Chin Lee is a visiting oncologist from Singapore.