How do genes work?
A gene is a section of DNA (deoxyribonucleic acid) inside every cell in the body which carries a set of instructions for that cell, telling it what to do and when. This might be an instruction to make a specific protein, or to grow or divide. Every cell in the body has roughly 25,000 genes.
Most of these genes reside within chromosomes, which are long DNA chains formed around a specific protein (histone). There are 22 pairs of 'autosomes' (non-sex chromosomes) and one pair of sex chromosomes (XX in women and XY in men), making 46 chromosomes (23 pairs) in total. Germ cells - sperm cells and egg cells have only 23 chromosomes. When an egg cell is fertilised by a sperm cell, the child inherits one chromosome from each parent.
As all cells contain the same instruction set, they selectively turn on and off particular genes to create the specialised cell that is needed in that part of the body, for example to become a skin cell or a blood cell. Some genes are permanently activated (for example to make specific proteins) and some genes are switched off once they have played their role.
How are genes linked to cancer?
Cancer occurs when there are changes (mutations) in the way genes function which in turn affect how the cell works, particularly how the cell grows and divides.
There are two broad types of genetic mutations:
- They can be inherited from our parents, where these changes were present in the germ cells. Called 'germline changes' these are contained in every cell in the body. Cancers caused by hereditary factors account for 5-10% of all cancers*.
- They can occur in individual cells in the body caused by an external factor, such as UV radiation (from exposure to the sun) or other radiation, or exposure to cancer-causing (carcinogenic) substances such as tobacco smoke. These changes are called 'somatic' or 'acquired' changes. Cancers caused by acquired factors account for 90-95% of all cancers *.
There are also differences in these mutations - some may affect only one nucleotide (a 'unit' of DNA) and others may affect larger sections of the DNA. And in other cases, the changes are not in the DNA itself, but in chemical marks ('epigenetic modifications') that determine if the gene is 'expressed'.
Once a cancer cell has formed, it will often also mutate again, creating yet more changes within the collection of cancerous cells - the tumour.
*sometimes it may appear that specific cancers run in families, however this may not be due to hereditary factors, it may instead be due to common lifestyle factors, for example smoking.
Common gene mutations
There are many genes where a mutation may be associated with cancer. These mutations are sometimes hereditary, but most of the time mutations are only present within the tumours and are not passed on to other family members.
Examples of mutations in cancer include:
BRCA1 and BRCA2
These genes are often hereditary and are associated with a higher risk of developing breast cancer and ovarian cancer, and they may also be associated with pancreatic cancer, and prostate and breast cancer in men. Recently, targeted drugs (called PARP inhibitors) have been developed that can specifically target the abnormal DNA repair pathway that result from BRCA mutations, or from other genetic alterations similar to BRCA.
This is another gene that inhibits tumour growth, and a mutation in PTEN is associated with breast, thyroid and endometrial cancers.
This gene produces a protein that inhibits tumour growth, which can lead to a higher risk of developing certain cancer types. This mutation is the most common across all cancer types and is often a somatic (acquired) alteration.
Approximately half of melanomas have a mutation within a gene called BRAF. These mutations often enable the melanoma to grow and survive, and there are now drugs that can target this mutation (BRAF inhibitors) and can be very effective in shrinking advanced melanoma. These mutations are not hereditary, so there is no risk of ‘passing on’ the BRAF gene to your family. Testing for the mutation is often done on one of the biopsy samples from an area where the melanoma has spread.
BRAF mutations, and other genetic alterations in the same pathway, are sometimes also in a number of other cancers including ovarian and bowel cancers. Treatment with targeted drugs in these cases is still an experimental approach, and is an active area of research.
Genetic testing is considered where there is a strong possibility that a particular cancer may be hereditary, to determine whether the family member carries the specific gene mutation related to that cancer and to guide treatment options for those patients with identified, targetable mutations.