Constitutional mismatch repair deficiency (CMMRD) syndrome is a rare disorder that greatly increases the risk of developing one or more types of cancer in children and young adults. The cancers that most commonly occur in CMMRD syndrome are cancers of the (large intestine) and rectum (collectively referred to as colorectal cancer), brain, and blood (leukemia or lymphoma).
Almost all people with CMMRD syndrome develop cancer before age 18, generally in late childhood. The age of diagnosis varies depending on the cancer type; brain cancers, leukemia, and lymphomas tend to occur at younger ages than colorectal cancer in people with CMMRD syndrome. It is estimated that 20 to 40 percent of people with CMMRD syndrome who develop cancer will develop another cancer later in life.
People with CMMRD syndrome may develop multiple noncancerous (benign) growths (adenomas) in the colon that are likely to become cancerous (malignant) over time. Brain cancers in CMMRD syndrome often involve certain cells called glial cells, causing gliomas or glioblastomas. The most common blood cancer in CMMRD syndrome is called which affects white blood cells. Other cancers that can occur in CMMRD syndrome include cancers of , , or uterine lining ().
Many people with CMMRD syndrome develop features similar to those that occur in a condition called neurofibromatosis type 1. These features include changes in skin coloring (pigmentation), which are characterized by one or more flat patches on the skin that are darker than the surrounding area (). Some affected individuals have freckling or patches of skin that are unusually light in color (hypopigmented). Rarely, people with CMMRD syndrome will develop a feature of neurofibromatosis type 1 called Lisch nodules, which are benign growths that often appear in the colored part of the eye (the iris). Lisch nodules do not interfere with vision. Some people with CMMRD syndrome are initially misdiagnosed with neurofibromatosis type 1.