What is Genetic testing?

Genetic testing is a process by which specific genes contained in an individual's DNA are examined for mutations.

Genetic testing and family histories of cancer

In the case of family histories of cancer, genetic testing is usually a two-step process as follows:

1. Mutation analysis

It is usually necessary for DNA (in a blood sample) to be examined from an affected family member before DNA analysis from other affected and unaffected family members can be undertaken (unless the patient is of Askenazi Jewish decent). This is because in a specific gene (such as a tumour suppressor gene) there could be individually different mutations at different points in the gene. As examination of each gene is a long and painstaking process, (rather like looking for a needle in a haystack) mutation analysis is more likely to be successful if the genetic fault is identified in an affected individual's DNA first and then compared with the DNA from other family members.

2. Predictive testing

If a mutation is identified in an affected family member, it is then possible to offer predictive genetic testing to unaffected relatives to determine if they have inherited the same faulty gene.

The Genetic Testing Process

Before genetic testing is undertaken

It is essential that the individual has had several opportunities to discuss the risks, benefits and limitations of the test.
This will usually involve one or more outpatient appointments with the Clinical Genetics team. The individual should be sure of their own personal reasons for undertaking the test and not be feeling pressurised to take the test by other family members. The individual should also be aware of the implications of undergoing genetic testing.

Identifying a mutation

Written consent for the mutation analysis is needed. A blood sample is needed in order to undertake mutation analysis. The blood sample is usually obtained by a member of the clinical genetics team, either in the clinic or a home setting. If a blood sample is not available, DNA can also be obtained from skin, bone marrow, fresh or frozen tumour samples and tumour blocks. Consent is obtained by a member of the clinical genetics team to examine other stored sources of DNA samples.
In the laboratory, DNA is extracted from the white cells in the blood sample. The DNA is amplified so that it can be analysed and the target gene is then directly sequenced, to compare the sample with a "normal" sequence in order to identify mutations.

Identifying a mutation - time scale

The initial analysis of the DNA sample may take several months or even years. Unfortunately, with current technology, in approximately half of the families with a strong family history mutations may not be found. This is because there may be genes that influence the development of cancer which have not yet been discovered, or because laboratory techniques are not sensitive enough to detect all gene mutations. However, as there are constantly new developments within the field of cancer genetics, future mutation detection rates may increase.

Having a predictive test

If a mutation is identified in an affected family member, a blood sample can be examined from unaffected family members to determine if they have inherited the same faulty gene. Testing of unaffected family members may be undertaken more quickly than mutation analysis as the specifically affected area of gene has been found.

The family member(s) can change their mind about having their DNA analysed at any time during the testing process