Identification of personal risk of breast cancer: genetics

Eccles, Diana M. (2008) Identification of personal risk of breast cancer: genetics. Breast Cancer Research, 10 (Suppl 4), p.S12. (doi:10.1186/bcr2172).

Record type: Article

Abstract

Family history has long been recognized to be a potent risk factor for breast cancer [1]. Family histories often comprise only one or two affected relatives, who frequently were not particularly young at onset. Occasionally, a family contains a striking constellation of young-onset breast (and often other) cancers apparently transmitted as a dominant genetic trait. Over a century ago, the much referenced French neurologist Paul Broca wrote about the family of his wife, in which the occurrence of many cancers, particularly of the breast, strongly suggested the presence of a genetic predisposition [1]. It was to be more than a hundred years before the discovery of the structure of DNA and rapid advances in molecular technology would allow the first major breast cancer susceptibility gene, BRCA1, to be mapped to chromosome 17q [2]. In families linked to the BRCA1 locus, a much younger age at onset than average and the frequent occurrence of ovarian cancers were noted to be typical. At about the same time, the cause of a rare but devastating familial cancer syndrome was identified; mutations in the TP53 gene had also been found to account for the very rare Li Fraumeni syndrome associated with very young onset breast cancers and childhood malignancy, particularly soft tissue sarcomas [3,4].
A second major breast cancer susceptibility gene, BRCA2, was mapped to chromosome 13q in 1994 shortly before the sequence of the BRCA1 gene was clarified and family-specific disease-causing mutations started to be reported [5,6]. The full BRCA2 gene sequence was reported in 1995 [7]. Families with mutations in the BRCA2 gene were noted to be more likely to contain a male breast cancer case than might be expected by chance (and more than had been seen in families due to BRCA1 mutations); ovarian and fallopian tube cancers still occurred with increased frequency but overall not as frequently as in BRCA1-associated families. With the discovery of the correct gene sequences for BRCA1 and BRCA2 it became possible to offer predictive genetic testing to members of families in which the causative gene mutation had been identified. Predictive testing for breast cancer susceptibility was introduced as a clinical service from the mid-1990s in some centres in the UK [8]. Families in which mutations were identified through research studies were the first to be informed [9]. Many of these families had been identified because of their high incidence of cancer, and so inevitably the estimated lifetime chance that a carrier of one of these genes would develop cancer was high [10,11].
The BRCA1 and BRCA2 genes are very large, and mutation testing was either very expensive (in the USA) or very slow (in European countries) when it first became available. It soon became clear that mutations in these genes accounted for a relatively small proportion of all families with breast cancer clusters. Families with four or more cancer cases developing at young age, particularly those families in which ovarian cancers occurred, were more likely to yield pathogenic mutations when DNA from a cancer-affected family member was screened for mutations. Smaller clusters of later onset cancers, although clearly familial, were noted to have a much lower probability of harbouring a BRCA1 or BRCA2 mutation [12].
In the presence of a BRCA1 or BRCA2 mutation, the lifetime risk for developing breast cancer may be as high as 80% to 90%, although other genes and lifestyle factors may reduce the lifetime risk to levels as low as 26%. Ovarian cancer risk in BRCA1 gene carriers is estimated at about 50%, but again this risk is likely to be modified by both genetic and other risk factors. For BRCA2, ovarian cancer risk is about 15%, but all these average risks are likely to be modified by context [13].

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