According to the American Cancer Society¹, 149,500 people will be diagnosed with colorectal cancer in 2021 alone. If you are one of them, you may be asking yourself, "What is colon cancer MSI and MSS?" Read on to find out what MSI and MSS mean in relation to colon cancer and how they may affect your cancer treatment and prognosis.
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Colorectal cancer, also called colon cancer, is cancer that starts in the colon or the rectum. It’s caused by a combination of environmental and hereditary factors. Depending on where cancer began, your doctor may refer to your cancer as colon cancer or rectal cancer.
MSI stands for microsatellite instability. It's one of the molecular changes that happen to cells when they start to grow irregularly and become cancerous. Microsatellite instability occurs in genes that regulate DNA and make sure the DNA is working as it should.
These "genetic spell-checker" genes, as they are known, are mismatch repair genes or MMR. When they stop working correctly, errors occur in producing new cells because their ability to repair errors on the molecular level is impaired. When this happens, the cells are not only likely to become cancerous, but they also become unstable.
Cancerous cells with this abnormal, unstable behavior are called MSI-High (MSI-H) or MRR deficient (dMMR). The presence of MSI-H cancer cells indicates that there is a defect in the cells' MMR genes.
When your doctor diagnoses you with colorectal cancer, you need to be tested to learn the tumor’s microsatellite instability or MSI² status. The testing is often done when the cancer is surgically removed, although it can be done afterward. Your colon cancer's MSI-high status can make a difference in how your cancer is treated, as well as your prognosis. Here's why:
Because MSI-high tumors have unstable cells that behave abnormally, they are easier to detect. On the other hand, MSS tumors (tumors that are determined to be MSI-low or with no MSI instability) have cells that are stable and behave normally. These stable cancer cells are more difficult to find and diagnose.
Studies³ have shown that colon cancer with high MSI tends to have a better prognosis than MSS or MSI-low colon cancer. That's partly because colon cancers with MSI-high status are less likely to metastasize (spread) to lymph nodes and distant areas. If they do spread, they are easier to spot because of their cells' abnormal and unstable behavior and may respond to several targeted therapies.
About 15% of all colon cancers test as MSI-high. 3% of MSI-high colon cancers test positive for the Lynch Syndrome gene, a hereditary gene that increases the risk of developing colorectal cancer.
If you are tested for Lynch Syndrome, and the results are positive, you and your family will be referred for genetic counseling to better understand screening, prevention, and treatment options,
There are various treatment options for colon cancer, depending upon its stage and MSI status:
Surgery works best in the early stages before cancer has spread or even if it has spread to the liver and some lymph nodes. If caught early enough, sometimes surgery is all that is necessary. Other times, your doctor will combine treatments.
Chemotherapy works to shrink the tumor or keep cancer from spreading. It may be recommended in the later stages of colon cancer. Chemotherapy is not usually recommended for stage II MSI-high colon cancer, for instance, since this has a good prognosis⁴.
MSI-high colon cancers respond well to immunotherapies, which help to strengthen the patient's immune system to fight cancer.
Using targeted therapies in specific areas help keep tumors from forming new blood vessels, keeping cancer from spreading.
This method targets specific areas with high-energy rays to shrink the tumors.
Your doctor will biopsy your tumor when it is removed to determine your colon cancer's MSI status. This can also be done after surgery.
If you have been diagnosed with colorectal cancer, there are many treatment options, and in many cases, the outlook is good.
First, you should determine what stage your colon cancer is in and whether it is MSI-high. If your cancer is still localized (meaning it has not spread past the colorectal area) and is MSI-high, studies show that it has a high chance of responding well to surgery, is easier to detect, and has a high cure rate.
MSI-high colorectal cancers that have spread to other areas are more likely to be found. With surgery and a combination of different therapies, they may be treated successfully. Talk to your doctor about what treatments are right for you.
Cancer Statistics Center | American Cancer Society
What is MSI and MSS? | Fight Colorectal Cancer