Genetic Testing

Some medical conditions, including cancer, can occur because of an inherited predisposition: that is, when a harmful gene alteration is passed down through the family from one generation to the next. A genetic test, which is usually done as a blood test, can sometimes be used tell a person whether they carry such an alteration, and whether they have an increased chance of developing certain health problems.

This section will focus on genetic testing for gene alterations thought to increase an individual’s chance of developing cancer.

How does genetic testing work?

Genetic tests look for alterations in a person’s genes. Genes are like sets of instructions that tell the body how to grow and develop. These instructions are arranged in a precise sequence and make up a specific gene code. Genes have a range of functions and control characteristics such as height, eye colour, and protection against disease.

As a rule genes are very similar from person to person, but usually there are small variations in the sequence of the gene codes between individuals. Some of this variation is harmless, but other variations mean that a gene can no longer function as it should. Sometimes this sort of variation occurs within specific genes which are known to protect the body against cancer. When the function of these genes becomes disrupted, a person becomes more susceptible to cancer; these genes are commonly referred to as cancer predisposition genes.

The sequence which makes up a cancer predisposition gene can be extremely long, and alterations may occur anywhere along it (although not all these alterations will be associated with an increased risk of cancer). It is often very difficult to differentiate between potentially harmful alterations in this code, and alterations which confer no increased cancer risk, particularly if an alteration is rare or has not been found in another individual before. It is impossible to know before testing, where in a gene an alteration will be found.

For most families with a strong history of cancer, genetic testing will first be carried out on an individual within that family who is already affected by, or has been affected by, cancer (a diagnostic genetic test). This is because the process involved in analysing long gene sequences can be extremely complicated, and the interpretation of such an analysis can be very difficult (especially in an individual who is unaffected by cancer); also, not all individuals within a family will carry the harmful gene alteration (if there is such an alteration present within that family) but a person affected by cancer is more likely to be a carrier. By testing that person you are more likely to successfully find the gene alteration causing cancer in that family. At the moment, on the NHS, this testing process generally takes at least six months.

Unfortunately, genetic testing for cancer predisposition is a newly emerging medical technique, and for most cancer predisposition genes it is not yet possible to test the entire gene sequence. So sometimes, even when there is a harmful gene alteration present, it is not always possible to identify that alteration. It should also be noted that sometimes it is not possible to identify a harmful inherited gene alteration because the cancers within a family have occurred by chance. An additional complication can arise because not all cancer predisposition genes have been identified - it is therefore possible that an individual does have a genuine family history of cancer, but doctors don’t know in which gene to look for it.

If, however, it does prove possible to identify a family specific gene alteration, testing can then be offered to unaffected family members – this is called predictive genetic testing, and will look for that family’s specific alteration. Individuals who carry the alteration will have an increased cancer risk; individuals who do not carry the alteration are believed to have the same risk as the general population.


Next Section >> Positive and negative aspects of genetic testing