How does genetic testing for LS work?

A person referred to a genetics clinic will be asked to provide family history information so that a family tree can be developed. This helps us determine if the family history looks like sporadic CRC or LS.

Patients may be asked to sign medical release forms that would allow to access medical records pertaining to a cancer diagnosis. Similar forms may be provided for the patient to share with relatives (or their next of kin) who have had cancer.

If a family is suspected of having LS, genetic testing may be offered. The first step in testing for LS is to look at tumor samples. The majority of tumors that are associated with LS show particular characteristics called microsatellite instability. The tumors may also show the loss of the normal product of a LS gene (called immunodeficiency). If these characteristics are seen in a tumor then we move on to the second step.

The second step is genetic testing of a blood sample. There are four genes (MLH1, MSH2, MSH6 and PMS2) that are known to be associated with LS. Genetic testing looks for a mutation in one of these genes. If a mutation is identified in a family, other relatives in that family can be tested to see if they inherited the gene mutation.

Sometimes the testing process does not identify a mutation in a family. In these cases we may not be able to confirm or rule out a hereditary cancer syndrome.

The decision to have genetic testing involves a careful consideration of the potential impact of the results. If an individual is eligible for genetic testing, they must receive genetic counseling prior to testing to learn about the benefits, risks, and limitations of genetic testing.