Carrying the ATM gene
People who carry a single copy of the mutated ATM gene are often known as ‘carriers’. They are sometimes also known by scientists as ‘heterozygotes’, as opposed to people who have A-T who are known as ‘homozygotes’.
There are many more carriers of the ATM gene than you might think, given how rare A-T itself is. Though different figures are sometimes quoted, it is likely that 1 person in 300 is affected. To put this in perspective, if you attended an average-sized secondary school in the UK, there are likely to have been 6 A-T carriers there during your time at the school.
However in communities where people often marry relatives, the percentage of carriers can increase. There is a relatively isolated population living in the south of Israel where the incidence for carriers has been estimated to be as high as 1 in 12.
Carriers do not get A-T nor suffer from any of the disorders that people with the condition do. It appears that the single copy of the gene is able to produce enough ATM protein to allow normal function.
Increased risk of breast cancer
However there is one area where carrying a single copy of the mutated ATM gene does have an effect. There is a moderately increased risk for women carriers of developing breast-cancer. Over the course of a life-time, the risk of developing breast cancer for a female A-T carrier appears to be slightly more than double that for a non-carrier. For more information on this, visit our A-T and breast cancer page.
This risk appears to be confined to breast cancer. There is no convincing evidence at the moment of an increased risk for carriers of the mutated ATM gene in developing any other form of cancer.
Carriers and radiation
It is well known that people who have A-T are more sensitive to the effects of radiation of the kind that is used in x-rays and radiotherapy and some heterozygote carriers have wondered whether they too may have increased sensitivity to radiation.
However, there is no good evidence to suggest that their cells will be more sensitive to x-rays than cells from anyone else. Radiotherapy is regularly given to carriers who have developed breast cancer and there is no increased clinical sensitivity of normal tissue to the x-rays. Similarly, there is no reason to think that radiation doses received through surveillance (mammography) will increase breast cancer rates over and above what is already to be expected.
There is currently no reliable method of screening ATM mutation carriers amongst the general population. However, if you are closely related to someone who has A-T and their mutations have been identified (this is normally the case in the UK), it is usually possible for you to be tested. For more information, see our Carrier testing page.