A-T heterozygotes seem to have an increased risk but its size is unknown
Genetic predisposition accounts for 5-10% of breast cancer, and two genes--BRCA1 and BRCA2--have attracted most attention as high risk factors. However, these two genes probably account for only a small proportion of the genetic risk while other more common but less penetrant genes may explain the remainder of genetically predisposed breast cancers. One such candidate is the gene, ATM, mutated in the human genetic disorder ataxiatelangiectasia (A-T). A-T heterozygotes (estimated to be 1% of the population) do not show any of the major symptoms of the disease, though there is good evidence that they have an underlying cellular radiosensitivity, but to a lesser extent than observed in A-T homozygotes. These observations, together with earlier epidemiological studies, reveal a raised incidence of mortality from cancer among blood relations of patients with ataxia-telangiectasia, with the greatest relative risk for breast cancer (5.1) in female relatives of patients.
An association between the incidence of breast cancer and A-T heterozygosity was also revealed in two separate but smaller studies.[6 7] Based on an independent assessment of all these data the relative risk of breast cancer in A-T heterozygotes was estimated to be 3.9, with A-T carriers representing 3.8% of all cases,
With knowledge of the sequence of the ATM gene, Fitzgerald et al detected heterozygous mutations in 2/202 (1%) healthy women with no personal history of cancer.9 The frequency of 1% is consistent with that predicted from epidemiological studies. When patients with early onset breast cancer ([is less than] 40 years) were screened 2/410 (0.5%) showed mutations in the ATM gene. Fitzgerald et al therefore concluded that "heterozygous ATM mutations do not confer genetic predisposition to early onset breast cancer. On the other hand, a recent study by Athma et al using molecular genotyping suggested that A-T heterozygotes are predisposed to breast cancer, Among 33 women with breast cancer 25 were A-T heterozygotes compared with an expected 15. For patients with earlier onset disease ([is less than] 60 years) the odds ratio was 2.9 (21 cases), while for older patients it was 6.4 (12 cases) Based on these relative risks the authors calculated that 6.6% of all cases of breast cancer in America occur in A-T heterozygotes.
Clearly these two studies appear to be in conflict. In an analysis of these data Bishop and Hoppe pointed out that precise estimates were difficult since the study of Fitzgerald et al relied on a small number of mutations while that of Athma et al analysed only a small number of breast cancers. Larger scale studies are required with emphasis on age of onset of breast cancer to address conclusively the potential association between mutations in ATM and risk of developing breast cancer. In a workshop last November in Clermont-Ferrand results were presented from studies in several countries, but the connection between A-T heterozygosity and breast cancer remains unresolved.
If a link between breast cancer and A-T heterozygosity is established, what are the clinical implications? As for any gene that increases the risk of breast cancer, A-T carriers should ideally be identified, but given the size of the ATM cDNA (9.168 kb) and the known distribution of mutations over the entire length of the cDNA it would be difficult and expensive to conduct general population screening. Relying on identifying carriers in A-T families would narrow the scope and usefulness of such screening. A-T carriers would need to be identified by some other characteristic. One such feature does exist--cellular radiosensitivity--but it is not amenable to a widespread screening assay.
This intermediate radiosensitivity does, however, raise another issue which is pertinent to the development of breast cancer. Swift et al concluded that diagnostic or occupational exposure to ionising radiation probably increases the risk of breast cancer in women heterozygous for A-T. High doses of ionising radiation, particularly before puberty, are known to increase the risk of breast cancer. What has emerged as a contentious issue is whether mammography screening leads to an increased risk for A-T carriers. A well conducted mammographic examination involves an absorbed dose of about 0.3 cGy/breast, which if applied annually over 35 years (40-75 years) would give rise to a lifetime radiation dose of 10.5 cGy--approximately the same as background radiation. Exposures of this order, at the age of 40, are estimated to increase the number of deaths from breast cancer by about 1/2000 women, which is insignificant compared with the natural lifetime risk of 1/9 for breast cancer.
What then of carriers of the A-T gene? A-T heterozygotes are intermediate in cellular sensitivity to radiation between controls and A-T patients--that is, at best 1.5-fold to twofold more sensitive than controls. Thus a total dose of 10.5 cGy would not be expected to increase significantly the lifetime risk for breast cancer in A-T carriers.
For A-T carriers the picture that emerges is that while epidemiological studies point to a threefold to fourfold increased risk for breast cancer there remains uncertainty whether this is supported by mutation analysis of the ATM gene. Screening of increased numbers of patients with breast cancer is required to support a small moderate increased relative risk for A-T heterozygotes. It seems unlikely that the intermediate cellular radiosensitivity in A-T carriers increases the risk of breast cancer during mammographic screening, at least when this procedure is restricted to women aged over 40.
Martin Lavin Professor of molecular oncology Queensland Institute of Medical Research/Department of Surgery, University of Queensland, Brisbane, Queensland 4029, Australia
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