Mutations in the MMR gene disrupt the function of the MMR protein, causing the production of repetitive DNA sequences and DNA repair errors, resulting in microsatellite instability (MSI) in the patient’s DNA, in addition to detecting and correcting small insertions and losses in microsatellite-like repetitive DNA sequences that occur during replication. Lynch syndrome is an autosomal dominant disorder caused by germline mutations in the mismatch repair (MMR) gene. MSI testing has important clinical significance: 1) Diagnosis of Lynch syndrome: MSI is an important biological marker for Lynch syndrome, and the diagnosis can be established when MSI-H is confirmed; 2) Judgment of prognosis and treatment outcome: Colorectal cancer patients with MSI-H have poor efficacy against the chemotherapeutic drug 5-fluorouracil; 3) Differentiation between hereditary germline origin and sporadic origin of MSI colon Colorectal cancer: The etiology of sporadic MSI-H colorectal cancer is usually hMLH1 gene promoter methylation, and only a small proportion is MMR gene silencing due to BRAF gene mutation and immunohistochemistry negative with MSI. if the tumor is positive for MMR gene promoter methylation or BRAF gene mutation, it indicates sporadic cancer; 4. Screening: Bethesda revised guidelines recommend routine MSI screening for patients younger than 50 years of age for routine MSI testing. According to the Bethesda revised guidelines, MSI testing is required for patients with the following indications: 1. patients younger than 50 years of age; 2. tumors located in the right hemicolectomy; 3. multiple tumors prone to occur in the intestine or other organs outside the intestine; 4. prone to recurrence of other primary malignancies after surgery; 5. tumors with lymphocytic infiltration; 6. multiple mucin secretion or presence of imprinted cells; 7. infiltrative growth pattern. MSI detection currently has the following two detection methods: 1. PCR technology detection method Paraffin sections are manually microdissected to extract DNA, and some microsatellite spots are used as markers to guide the synthesis of primers for PCR, and the MSI spectrum is derived by gel electrophoresis analysis for comparative analysis. The 2 most commonly used Promega analysis systems consist of 5 single nucleotide marker sites (BAT-25, -26, MONO-27, NR-21, -24); while the National Cancer Institute (NCI) reference system consists of 3 dinucleotides (D2S123, D5S346 and D17S250) and 2 single nucleotides (BAT-25 and -26) . The diagnosis of MSI can be established when the length of microsatellites in the germline profile is changed. There are 3 results based on the analysis of microsatellite data: high microsatellite instability (MSI-H), low microsatellite instability (MSI-L) and microsatellite stability (MSS). 2. Immunohistochemistry (ICH) assay The main DNA MMR genes include MLH1, MSH2, MSH6, PMS2 and PMS1. ICH method usually detects four genes, MLH1, MSH2, MSH6 and PMS2. Compared with PCR methods, ICH analysis is more convenient and cheaper. In addition, ICH analysis can directly detect genes that may be mutated. In the MMR protein heterodimer pairing, MLH1 and MSH2 are required dimers. NOTE: *MSH2 and MSH6 expression deficiencies are usually due to MSH2 germline mutations, although they can also be caused by transcriptional translation of the adjacent gene EPCAM capable of inactivating the MSH2 gene. #MLH1 can be inactivated by germline mutations or by high methylation of the MLH1 promoter.MLH1 promoter methylation is usually accompanied by somatic BRAF V600E mutations. Abbreviations: +, expression; -, deletion; Currently, MSI testing for colorectal cancer with clinicopathological features can reduce the clinical underdiagnosis of Lynch syndrome, and MSI testing is also important for definitive diagnosis, guiding clinical prognostic assessment, medication and follow-up, so we should make good use of MSI testing as a tool.
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