Dr. David Johnson discusses the importance and current practices of genetic testing in colorectal cancer (CRC). Dr. Johnson highlights the evolution of genetic testing for CRC, beginning with the discovery of the APC gene in 1991, which is linked to familial adenomatous polyposis, and the identification of mismatch repair gene abnormalities (MLH1, MSH2, PMS2) related to Lynch syndrome in 1993.
Dr. Johnson emphasizes that around 1 million people in the United States have Lynch syndrome, an autosomal dominant disease, but most are unaware of it. He estimates that gastroenterologists may encounter one to two hereditary colon cancer patients per month, or 12-24 with Lynch syndrome per year. In a practice of 10 GI providers, this could mean 100-240 patients annually.
The article focuses on standard testing for Lynch syndrome, which involves looking for mismatch repair frequency deficiency, present in 15%-20% of sporadic cancers. This testing can be done through microsatellite instability (MSI) evaluation or immunohistochemistry (IHC) testing. Dr. Johnson explains that these tests are not the same: IHC measures the mismatch repair proteins expressed in the sample, while MSI measures the mismatch repair function by detecting changes in DNA that occur when there is a major mismatch repair function loss.
Dr. Johnson’s discussion underscores the significance of employing genetic testing correctly in the management and diagnosis of colorectal cancer, particularly in identifying Lynch syndrome.