Genetic mutations account for an estimated 5% to 10% of bowel cancer cases

Non Modifiable Bowel Cancer Australia
Around 70% of people who develop bowel cancer have no family history of the disease.
However, for around 30% of all bowel cancer cases diagnosed there is a family history, hereditary contribution or a combination of both.
Generally speaking, the more members of the family affected by bowel cancer, and the younger they were at diagnosis, the greater the chance of a family link.
Genetic mutations have been identified as the cause of inherited cancer risk in some bowel cancer–prone families; these mutations are estimated to account for only 5% to 10% of bowel cancer cases overall.
The three most common inherited syndromes linked with bowel cancers are:

  • Hereditary Non-Polyposis Colorectal Cancer (HNPCC) (also known as Lynch Syndrome)
  • Familial adenomatous polyposis (FAP)
  • MYH-Associated Polyposis (MAP)

Hereditary Non-Polyposis Colorectal Cancer (HNPCC) - Lynch Syndrome
In HNPCC, mutation carriers' lifetime risk for bowel or other syndrome cancers is 70-90%.
If a person has a HNPCC mutation, each of their children has a 50% chance of inheriting the mutation.
Lynch Syndrome is caused by a change in a gene that normally functions to protect a person from getting cancer.
Each person inherits genes from both their parents and HNPCC is caused by a fault in one of the genes known as the 'mismatch repair' genes.
Someone who inherits HNPCC from their parents has a normal gene and a 'faulty' gene, which increases their risk of developing bowel cancer and other types of cancer.
HNPCC affects less than 5% of those who develop bowel cancer.  Although rare, this risk relates to a clear family history of bowel or specific, related cancers e.g. some gynaecological cancers, digestive tract, urinary tract, brain or bowel cancer.
The typical age of diagnosis of this kind of Hereditary Non-Polyposis Colon Cancer (HNPCC or Lynch Syndrome) is usually between 40-50 years (compared to 60-70 years amongst the general population).
In families where there is a clear history of HNPCC, screening with colonoscopy every 1 to 2 years usually commences from age 25 or 5 years earlier than the youngest relative diagnosed, whichever comes first.
Where HNPCC is suspected, your GP will refer you to a Family Cancer Clinic for support and on-going management of the condition.

Familial Adenomatous Polyposis (FAP)
In untreated FAP, mutation carriers have a lifetime risk for bowel cancer close to 100%.
An affected person has a 50% chance of passing the condition on to each of their children.
FAP is characterised by the presence of hundreds to thousands of adenomatous polyps in the large bowel of affected individuals, which often start in adolescence.
Cancerous polyps are very common in this condition, usually by age 40, without active management of the polyps and screening on a regular basis.
Diagnosis is usually made following colonoscopy to confirm the presence of polyposis.  Testing for mutation of the APC gene currently detects 95% of mutations present.
In families where there is a clear history of FAP, screening usually commences from age of 12-15 or from diagnosis with annual flexible sigmoidoscopy or colonoscopy.
Where FAP is suspected, your GP will refer you to a Family Cancer Clinic for support and on-going management of the condition, because it has been known to affect adolescents and teenagers.
The treatment for FAP is usually a planned operation to remove the affected part of the colon once polyposis has become established.  This normally occurs in the late teens or early twenties.
These are very rare conditions and you will need the specialist help and support of an experienced colorectal team to help make the right decisions for the individual affected.

MYH-Associated Polyposis (MAP)
Although considered rare, people diagnosed with MYH-Associated Polyposis (MAP) have close to 100% risk of developing bowel cancer by the age of 65, if they are not monitored closely with colonoscopy.
MAP is characterised by the development of 10 to a few hundred polyps, which can develop into cancer.
A person has a 25% chance of being affected with MAP when both parents pass on the gene mutation.
MAP is a genetic condition caused by a mutation in the MYH gene, also known as the MUTYH gene.
In order for a person to have an increased risk of MAP, each parent must pass on a copy of the altered MUTYH gene.
A person who has only one copy of the gene mutation (from one parent) is called a carrier, and they are not at increased risk themselves. However they may pass the gene on to their children.
Most individuals with MAP will develop dozens, sometimes hundreds, of polyps, in their colon over their lifetime. It is rare that people with MAP will have no polyps present at all.
The risk of bowel cancer is increased if these polyps are not removed.
In families where there is a history of MAP, your GP will refer you to a Family Cancer Clinic for support and ongoing management of the condition.
Where MAP is suspected, individuals and families should undergo DNA testing, and test for MAP in those who do not have a mutation in the APC gene (APC gene associated with FAP/AFAP).
Modifiable Bowel Cancer Ausralia Screening Surveillance

Genetic Counselling
Do you know if anyone in your family has had bowel or any other kind of cancer?  Talk to your family and make sure you all know your family history.
If you think that you have a strong family history of bowel cancer, you should make an appointment with your GP to talk about your concerns.
If your GP agrees with you, they will refer you to a Family Cancer Clinic.
A genetic specialist will go through your family history with you in great detail and ask you to provide accurate information about who has been affected, how old they were when they were diagnosed, and the site where their cancer developed.
You may also have to have blood tests as part of this investigation.
If the genetic specialist agrees you are at increased risk, you will be referred to a specialist to talk about what types of screening and/or surveillance they recommend, at what age you (and/or other family members) should commence screening and/or surveillance and how often.
Regular screening and/or surveillance will ensure that any signs of bowel cancer are spotted and treated quickly.
Helpline Bowel Cancer Australia