Sensitivity of BRCA1/2 mutation testing in 466 breast/ovarian cancer families.

A ffected individuals from 431 families gave blood for mutational analysis in BRCA1 and BRCA2 mainly to develop genetic tests for their family. Individuals were eligible if there was at least a 50% chance of a gene predisposing to breast cancer (not necessarily BRCA1/2) in their family. Assessment was made using the Cancer and Steroid Hormone (CASH) dataset and the Claus curves. 2 A minimal requirement was two close relatives with breast cancer before the age of 50 years, but combinations of male and female breast cancer, and breast and ovarian cancer were particularly identified. An exception to this were two research projects where population based cases of breast cancer before the age of 31 years and sporadic breast cancer before the age of 36 years were screened for both genes. Male breast cancer (MBC) families presenting to the clinic with at least one MBC before the age of 60 years or at any age if female breast cancer had occurred were screened for BRCA2. Initial screening for mutations involved a whole gene assessment using single strand conformational polymorphism (SSCP) analysis and protein truncation testing (PTT) of exon 11 in each gene. All mutations were confirmed, in both orientations, by direct fluorescent sequencing of the appropriate exon. We excluded one exon 13 duplication and two exonic deletions detected on screening 95 BRCA1 negative breast/ovarian families. A further two exon 13 duplications in five subsequent BRCA1 negative breast/ovary families originating from east of the Pennines were also excluded. Of the non large-scale rearrangement mutations, 26/78 (33%) were detected outside the commonly screened regions of BRCA1 (exons 2, 11, 20) in the UK (table 1). Similarly 15/50 (30%) BRCA2 mutations were detected outside exons 10 and 11. In an attempt to assess the sensitivity of our techniques, we studied the outcome of testing in families with two or more confirmed ovarian cancers, which also had a total of at least four breast/ovarian cancers (breast cancer before the age of 60 years) and male breast cancer families with a similar four or more breast/ovarian cancers in total. Of breast/ovarian families fulfilling the above criteria, 25/38 (66%) had pathogenic BRCA1 mutations (five had BRCA2 mutations and one had a BRCA1 deletion) and 9/14 (64%) male breast cancer families had pathogenic BRCA2 mutations. These results would suggest high sensitivity for the techniques, particularly for BRCA2. From Breast Cancer Linkage Consortium data, 90% of such breast-ovarian families were linked to BRCA1, and 76% of male breast cancer families were linked to BRCA2. This would suggest sensitivity of close to 85% for BRCA2 and 73% for BRCA1. However, previous attempts to validate SSCP have shown that only 65–72% of BRCA1 mutations are correctly ascribed by SSCP. 8 PTT detected all 15 deleterious mutations in one study, but SSCP only detected 10. Nevertheless sensitivity is not just dependent on the proportion of different mutations detected, but on the frequency of each in a particular population. Taking all these factors into account we have estimated that our whole gene testing technique (without testing for large deletions or duplications) would have a sensitivity of at least 66% (see table 2), and that about 6% (4/66) of our breast/ovarian families have deletions or large-scale rearrangements. The proportion of breast/ovarian cancers attributable to BRCA1 or BRCA2 depends on the ethnic origin of families. Many countries or ethnic groups have particular founder mutations that are not seen in other populations. In countries with a small founder population, very few mutations may account for the vast majority of breast cancer families. The Ashkenazi Jewish population have three founder mutations, 185delAG and 5382insC in BRCA1, and 6174delT in BRCA2, which are found in over 2% of the this population. At least two studies have shown that one of the three mutations is present in the majority (59%–80%) of high risk families , and all three account for nearly all of the involvement of these genes in families. Another country with a small number of mutations is Iceland, where one mutation, BRCA2 995del5, accounts for most familial breast cancer. Populations that are more outbred, such as the UK, have larger numbers of mutations, and founder mutations occur at lower frequencies. Even the exon 13 duplication, a known UK founder, is