To explain what constitutional mismatch repair deficiency (CMMRD) is, we first need to talk about Lynch Syndrome. Lynch Syndrome increases a person’s risk of getting certain types of cancer in their lifetime, including bowel cancer, which is the most common cancer associated with this genetic condition.
What is Lynch Syndrome?
Lynch Syndrome affects genes called mismatch repair genes. They are genes responsible for correcting changes in genetic code when cells grow and divide. As the cells grow and make copies of their DNA, they can make mistakes, which mismatch repair genes rectify so that those errors don’t lead to cancer. So if these mismatch repair genes are abnormal or have a “mutation”, they may not repair those mistakes and cancer can occur.
What is CMMRD?
CMMRD, like Lynch Syndrome, is a genetic condition that makes it more likely for a person to get certain types of cancer, except this time it occurs when a child has inherited mutated genes from both parents, and the cancer risk is even higher. PMS2 is the most commonly affected gene in CMMRD.
To illustrate this: If mum has one “good” copy of the gene and one bad copy, I could inherit her good gene or her bad gene. And the same goes for my dad. My sister could inherit both of their good copies, and therefore doesn’t have Lynch Syndrome or CMMRD, and neither do her children. Whereas if I inherited both of their bad genes I therefore have CMMRD.
It is thought that… As many as 1 in 300 may have Lynch Syndrome, while the odds of having CMMRD are one in a million – or 0.0001%.
Lynch Syndrome Ireland 