Microsatellite Instability (MSI)
Description
Microsatellite Instability (MSI) is a form of genetic instability manifested by changes in the length of repeated single- to six-nucleotide sequences (known as DNA microsatellite sequences). High MSI, found in about 15% of colorectal carcinomas, is an adverse prognostic factor for colorectal carcinomas and predicts poor response to 5-FU chemotherapy (although the addition of oxaliplatin in FOLFOX regimens negates the adverse effects (page 266 AJCC manual)). High MSI is a hallmark of hereditary nonpolyposis colorectal carcinoma, also known as Lynch syndrome. Describes cancer cells that have a greater than normal number of genetic markers called microsatellites. Microsatellites are short, repeated, sequences of DNA. Cancer cells that have large numbers of microsatellites may have defects in the ability to correct mistakes that occur when DNA is copied in the cell. Microsatellite instability is found most often in colorectal cancer, other types of gastrointestinal cancer, and endometrial cancer. It may also be found in cancers of the breast, prostate, bladder, and thyroid. Knowing whether cancer is microsatellite instability high may help plan the best treatment. The microsatellite instability (MSI) test is a genetic test performed on tumor tissue to look for differences in length of certain non-functioning sections of DNA. The differences are caused by problems with the genes that encode proteins that normally repair certain types of DNA damage. A high proportion of colon cancers arising in patients with hereditary nonpolyposis colorectal cancer (HNPCC) (also known as Lynch syndrome) have high MSI and a smaller percentage of colon cancers not associated with Lynch syndrome have high MSI. Patients with colon cancers with high MSI may be further tested to determine if they have HNPCC. In addition, MSI is a useful prognostic marker in that patients with high MSI colon cancers have better response to surgery and survival.Notes
**Note 1:** **Physician Statement** * Physician statement of MSI can be used to code this data item when no other information is available. **Note 2:** **Applicable stages** * MSI may be recorded for all stages; however, it is primarily performed for invasive neoplasms * For non-invasive neoplasms (behavior /2), code to 9 if no information available. **Note 3:** **Results from nodal or metastatic tissue** * Results from nodal or metastatic tissue may be used for Microsatellite Instability.| Code | Description |
|---|---|
| 0 | Microsatellite instability (MSI) stable; microsatellite stable (MSS); negative, NOS AND/OR Mismatch repair (MMR) intact, no loss of nuclear expression of MMR proteins MMR proficient (pMMR or MMR-P) |
| 1 | MSI unstable low (MSI-L) |
| 2 | MSI unstable high (MSI-H) AND/OR MMR deficient (dMMR or MMR-D) loss of nuclear expression of one or more MMR proteins, MMR protein deficient) |
| 8 | Not applicable: Information not collected for this case (If this information is required by your standard setter, use of code 8 may result in an edit error.) |
| 9 | Not documented in medical record MSI-indeterminate MSI-equivocal Microsatellite instability not assessed or unknown if assessed |