Inherited cancer risk

If 1 or 2 family members have cancer, some people may think that they have a higher risk of cancer or have a cancer genegeneThe basic biological unit of heredity passed from parents to a child. Genes are pieces of DNA and determine a particular characteristic of an individual. in their family. Having a close family member with cancer may slightly or moderately increase your risk for that specific type of cancer. However, this does not mean that the family has one of the rare inherited or hereditary cancer syndromes. Only about 5%–10% of all cancers are hereditary.

Familial cancers

Familial cancers are cancers that occur in families more often than would be expected by chance. Familial cancers are likely due to a combination of factors shared by a family, including environmental and lifestyle factors, as well as some genetic factors. The pattern of familial cancer does not fit the features of a hereditary cancer syndrome, but may be associated with an increased cancer risk in family members.

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Hereditary cancers

A familial clustering of cancer (more occurrences of cancer within a family than usually expected) may be due to an inherited mutation in a cancer predisposition gene. Sometimes these inherited mutations can result in a rare hereditary cancer syndrome. Clues that a familial clustering of cancer may be present include:

• The cancer is seen in at least 2 generations.
• Several family members have the same type or a related type of cancer.
  • Generally, 2 or more close relatives (especially first-degree family members on the same side of the family) have the same cancer.
    • First-degree relatives are parents, brothers, sisters and children.
    • Second-degree relatives are grandparents, aunts, uncles, nieces, nephews and grandchildren.
• Two or more close relatives have types of cancer belonging to a known hereditary cancer syndrome.
• Family members were diagnosed with cancer at a younger age than usually expected.
• Family members have had more than one type of primary cancer (such as breast and ovarian cancer).
• Family members’ cancers appear in both organs of a pair (such as both breasts or both eyes) or there are many tumours in the same organ.
• One or more family members have had a rare type of cancer.

When trying to decide whether a family history suggests a hereditary link, it is important to note that the relatives with cancer only count if they are blood relatives of each other.

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Dominant and recessive inheritance

Mutations passed on from a parent to a child through the father’s sperm cell or the mother’s egg cell (germ cells) are inherited mutations (also called germline mutations). The chance of inheriting a gene mutation and the disorder associated with it depends on whether the gene is dominant or recessive.

• Dominant inheritance means that a person only needs to inherit one copy of the gene mutation (the abnormal gene is passed on from one parent) to inherit the cancer susceptibility syndrome. Most hereditary cancer syndromes are associated with dominant inheritance.
• Recessive inheritance means that a person needs to inherit 2 copies of a gene mutation (the abnormal gene is passed on from both parents) to inherit the cancer susceptibility syndrome.

A parent with a mutation has a 50% chance (or 1 in 2 chance) of passing on the mutation to each child. This also means there is a 50% chance that a child will not inherit the gene mutation.

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Types of genetic changes

Genes can be mutated in a number of different ways. The simplest type of mutation is a change in the deoxyribonucleic acid (DNA)deoxyribonucleic acid (DNA)The molecules inside the cell that program genetic information. DNA determines the structure, function and behaviour of a cell. that makes up the gene. DNA is made up of 4 different bases, which are arranged in a specific order. A mutation occurs when this order is changed. Other types of gene mutations include insertions, deletions and duplications. Sometimes genetic mutations involve changes to the structure of an entire chromosomechromosomeThe part of a cell that contains DNA (genetic information)., which changes the function of one or more genes. Examples of changes in the structure of a chromosome include translocations and inversions.

Some genetic changes are very rare. Others are common in the general population. Genetic changes that occur in more than 1% of the population are called polymorphisms. They are common enough to be considered a normal or harmless change in the DNA. Polymorphisms are responsible for many of the normal differences between people, such as eye or hair colour and blood type. Although many polymorphisms do not cause health problems, some of these variations can affect the risk of developing certain conditions.

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Genetic mutations and cancer risk

Mutations in cancer susceptibility genes can increase a person’s chances (susceptibility) of developing cancer. In many cases, a single mutation by itself is not enough to cause cancer. Other factors affect the chances that cancer will develop. Although people who inherit certain cancer susceptibility gene mutations (such as BRCA gene mutations) have a higher risk of developing cancer, it does not mean that cancer will definitely occur. Some people with the mutation will never develop cancer.

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Hereditary (inherited) cancer syndromes

Several hereditary conditions are associated with an increased risk of cancer because of an inherited gene mutation, but these are relatively rare. They are called hereditary (inherited) cancer syndromes.

People of a certain ancestry or ethnic group can have specific mutations in a gene that have been passed down. For this reason, a certain genetic disorder may occur more frequently in this ethnic group.

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Reducing your risk

At this time, mutated genes cannot be fixed. If you have been told you may have an increased risk of cancer because of your family history or genetic predisposition, you may be referred to a provincial or territorial program for a genetic risk assessment, genetic counselling and possibly genetic testing. Your doctor may talk to you about taking certain measures to help prevent or find cancer early, depending on your level of risk.

Early detection

Early detection means finding cancer early. Measures to detect cancer early could include a combination of:

• examinations done by a doctor
• self-examinations you do on your own
• tests (like mammography or colonoscopy)

People who are at higher than average risk of developing certain types of cancer may need to be tested more often or at an earlier age than people with average risk. If you think you may have a higher than average risk, talk to your doctor about a personal plan for testing.

Risk reduction strategies

Risk reduction strategies help lower your chance of developing cancer. Research into cancer risk reduction gives doctors information about how to lower the risk of developing cancer. Some measures that may be suggested for high-risk individuals include:

• lifestyle changes and making healthy choices
• drugs that may help lower risk (chemopreventionchemopreventionThe use of drugs, vitamins or other substances to reduce the risk, delay the development or prevent the recurrence of cancer.)
• preventive (prophylactic) surgeries

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