Is cancer genetic? Separating facts from fiction

Your genes might be able to tell you if you are at increased risk. Photo / 123RF

Around 5 to 10% of cancers are caused by faulty genes. Here are five things you can do to help prevent it

As a doctor working in the United Kingdom’s health system, I see with my own eyes that cancer is on the rise. Today, figures show it affects one in two people in their lifetime.

In the UK, waiting times for cancer treatment are also at an all-time high, with more than 30% of people waiting beyond the 62-day target before receiving treatment following urgent referral to a specialist.

Unsurprisingly, we are more interested in what is driving the increase in cancer rates than ever before – and what we can do to prevent it.

Around 5-10% of cancers are caused by inherited, faulty genes

This can increase an individual’s risk of developing cancer. All cancers involve changes to a person’s genes which happen over their lifetime, but not all cancers are necessarily ‘genetic’.

Faulty genes can be passed down from parent to child but it doesn’t necessarily mean the child will develop cancer, just that they are at increased risk. This is known as genetic predisposition.

For a cancer to grow, further genetic changes need to occur, and the likelihood of this is stronger if you already have inherited one faulty gene from your parents.

Environmental factors also impact the development of cancer and can tip the balance towards getting a cancer or not. The way our environment influences our genes, and therefore our cancer risk, is known as epigenetics.

The genetic link is strongest for breast, bowel and gastric (stomach) cancers. Research suggests other cancers, such as lung and skin cancer, are driven more by environmental risk factors including smoking, diet, alcohol, chemicals, and some infections. Age is also a strong risk factor for developing most cancers.

If you have ‘cancer genes’ in the family, you may have:

• Multiple relatives on the same side of the family having cancer (all on paternal or maternal side);
• The relatives all having the same type of cancer or cancers caused by the same gene fault (such as breast and ovarian);
• Cancers developing when their relatives are young (such as less than 50 years for breast, bowel and womb cancers).

Genetic tests will show faulty genes but won’t tell you anything for certain

A genetic test is a blood test looking for changes to a person’s DNA. Some genetic changes show faulty genes which are associated with an increased risk of developing a cancer in a person’s lifetime. Knowing this information can help inform a person of their individual cancer risk and risk to their children.

It’s important to remember that picking up a genetic change doesn’t usually tell you for certain if a person will develop cancer. Things such as smoking, UV exposure, physical activity, and diet also play a large role.

We have two versions of each gene, one from the mother’s gene pool and one from the father’s. Usually for a cancer to develop, there needs to be two faulty genes.

Inheriting one faulty gene means you may only need one ‘further hit’ from an environmental trigger for the cancer to develop, while someone who didn’t inherit a faulty gene would need two ‘hits’ and may still develop the cancer, just at a later age.

A well-known example is the BRCA genes (breast cancer genes: BRCA1 and BRCA 2) which increase the risk of developing breast cancer. Women with a strong family history are screened for these genes.

However, having the gene does not say for certain if the person will go on to develop breast cancer. Statistics show around 70 in every 100 women with a faulty BRCA1 or BRCA2 gene will develop breast cancer by the age of 80.

This stat varies between genes associated with other cancers, including, but not limited to ovarian, prostate, bowel, pancreatic, thyroid and kidney cancers.

Australian National University researchers have now developed a new test which picks up previous Covid-19 infection in a patient's blood. Photo / 123RF

If you have a family history of cancer, you can be referred for genetic tests

If there is a family history of cancer, or if a person has a suspected genetic health condition, referral for genetic counselling may be offered by Genetic Health Service NZ (GHSNZ), part of New Zealand’s public health system.

You might be referred for a genetic test if:

• You present with health conditions or cancer that has high likelihood of being caused by a change to their genes;
• Someone in your family has a cancer that is linked to faulty genes;
• You have multiple family members with the same type of cancer, or cancers all caused by the same gene;
• You develop cancer at a young age;
• You have a family member who developed a cancer at a young age.

Manage your individual cancer risk, irrespective of family history

There are some cancers that have a genetic predisposition, meaning it’ll occur at a younger age with each successive generation. This depends on the gene involved and will be discussed by a genetic counsellor if this gene is detected.

Without one of the above clinical suspicions, someone wouldn’t be routinely referred for genetic testing. It’s important everyone manages their individual cancer risk and modifies cancer risk factors (through diet, exercise, and stopping smoking), irrespective of family history.

The presence of a gene does not directly correlate with developing cancer, it just informs a person if they are at increased risk.