Research progress of microsatellite instability in colorectal cancer

張楚悅

Abstract： Colorectal malignant tumor is a highly heterogeneous disease.Its morbidity ranks second among all malignant tumors and fifth among all cancer-related mortality.It has the characteristics of not obvious early symptoms，high morbidity，and high mortality.Microsatellite instability is caused by the defect of DNA mismatch repair protein.It is related to the occurrence and development of colorectal cancer.It has become an important basis for clinical routine diagnosis and treatment of colorectal cancer，and is a prognostic evaluation index for immune check site treatment.In recent years，genes related to microsatellite instability have been discovered.The detection of these genes can quickly help clinicians diagnose and treat tumor patients and provide reliable evidence for individualized treatment of patients.

Keywords： colorectal cancer; DNA mismatch repair defect; microsatellite instability

【中圖分類號】R246.5 【文獻標識碼】A?【文章編號】1673-9026（2021）02-240-02

Colorectal cancer （CRC） is one of the common malignant tumors of the digestive tract，and its incidence and mortality have been increasing year by year.The survival rate of CRC has not improved significantly in the past ten years [1].CRC is a malignant tumor that occurs in the mucosal epithelium and glands of the large intestine.Most CRC is sporadic （non-hereditary），that is，sporadic colorectal cancer （SCRC）; and a few have a genetic background，among which are hereditary non-polyps HNPCC （hereditary nonpolyposis colorectal cancer，HNPCC），also known as Lynch syndrome （LS），has an incidence of 3%-5% of the total incidence of CRC，more than 90% of LS and 10%-15 % Of SCRC is related to microsatellite instability （MSI） [2].This article reviews the research on colorectal cancer microsatellite instability at home and abroad in recent years and the related literature on the progress of MSI-related genes in CRC.

1 Microsatellite

Microsatellite DNA （microsatellite DNA） is widely distributed in the genomes of prokaryotic and eukaryotic organisms.Because the length of its allele fragments is generally less than 350 base pairs，it is also called short tandem repeats.Microsatellite DNA is located in the exon，intron，and promoter or the junction of exon and intron of the gene.The most important feature of microsatellites is the low mutation rate.Since the spontaneous mutation rate is between 10-5 and 10-4，the number of bases in the core sequence is small and the mutation rate is low.It is precisely because of the highly stable characteristics of microsatellites that it can be used as a marker to measure the stability of the genome [3].

2 Microsatellite instability

According to the degree of expression，MSI is divided into high microsatellite instability （MSI-high，MSI-H），low microsatellite instability （MSI-low，MSI-L），and microsatellite stability （MSS）.When protein defects occur due to DNA mismatch repair （MMR） functional defects，the inability to perform the normal mismatch repair function results in gene instability，expressed as repair errors，and replication errors that cannot be corrected in time will continue to accumulate，leading to MSI Occurrence，making it unable to play a normal regulatory role.MSI can be retained in the cell genome through replication and cell division，causing abnormal cell proliferation and differentiation，leading to tumors [4-5].MSI phenotypes are currently known to exist in a variety of solid tumors，including CRC，gastric cancer，pancreatic cancer，cholangiocarcinoma，endometrial cancer and urothelial cancer [2].

3 MSI detection

MSI is mostly caused by the lack of MMR protein expression and the loss of MMR function.Therefore，the expression of 4 MMR proteins （MLH1，MSH2，MSH6 and PMS2） in tumor tissues can be detected by immunohistochemical methods.When all the proteins are positively expressed，the phenotype is MSS/MSI-L，and the absence of any MMR protein is the MSI-H phenotype.The current "gold standard" for MSI detection is multiple fluorescent PCR capillary electrophoresis.In the "CSCO Colorectal Cancer Diagnosis and Treatment Guidelines 2018"，it is recommended to use the five microsatellite sites recommended by NCI （BAT-25，BAT-26，D2S123，D5S346 and D17S250） perform MSI detection，when 5 sites are stable as MSS，1 site is unstable as MSI-L，and 2 or more sites are unstable as MSI-H[2].

4 The clinical significance of MSI

4.1 Initial screening of LS

LS is an autosomal dominant genetic disease.It is caused by heterozygous pathogenic germline mutations in one of the mismatch repair genes，or the absence of MSH2 due to the deletion of EpCAM gene [2，6]，which causes DNA duplication.Errors in sequence generation and DNA repair lead to MSI in the patients DNA.MSI induces the activation of tumor-promoting genes or the inactivation of tumor suppressor genes，which increases the gene mutation rate and speeds up the time required for adenoma to develop into adenocarcinoma.Therefore，MSI testing can be used as a preliminary screening method for clinical LS.

4.2 Medication guidance and prognosis prediction for patients with stage II CRC

At present，postoperative adjuvant chemotherapy is clinically recommended for patients with stage II CRC.However，for stage II CRC with poor histological differentiation and MSI-H phenotype，the prognosis is better and it is classified as low-grade adenocarcinoma [2].A meta-analysis showed that the death risk of stage II MSI-H patients is 35% lower than that of MSI-L/MSS patients，so MSI-H is an independent good prognostic indicator of stage II CRC [2，7].Postoperative 5-FU single-agent adjuvant chemotherapy is not recommended for stage II CRC patients with MSI-H phenotype.Studies have shown that 5-FU single-agent adjuvant chemotherapy for stage II MSI-H patients will not benefit from it，but survival Instead it shortens [8].However，treatment with immune checkpoint inhibitors （such as pembrolizumab，nivolumab） has a significant effect [9].Therefore，MSI testing can not only judge the prognosis of patients with stage II CRC，but also provide a basis for the choice of chemotherapy drugs.

5 Research progress of MSI-related genes in tumors

5.1 The relationship between Ras/Braf gene and MSI in colorectal cancer

Braf gene is located on human chromosome 7q34 and belongs to the Raf/Mil family of serine/threonine protein kinases.Braf blocks its binding with Bcl-2 by phosphorylation of Bim，thereby inhibiting cell apoptosis.Braf gene mutations can interfere with Bim to affect normal physiological functions and inhibit cell apoptosis，thereby promoting the occurrence and development of colorectal cancer.The T1799A mutation in the 15th exon activation region of the Braf gene can encode the B-RAFV600E mutant protein，and its serine/threonine protein kinase activity is 500 times that of the wild type，which can make Ras/MAPK/ERK signal The continuous abnormal activation of the pathway promotes the occurrence and development of colorectal cancer.It has been pointed out in the literature that the mutation rate of Ras gene is negatively correlated with MSI，while the mutation rate of Braf gene is positively correlated with MSI [10].Some scholars believe that the Ras/Braf gene mutation has a smaller impact on colorectal cancer than MSI.But in Chinese and Japanese CRC，the mutation frequency of KRAS is as high as 30%-44% [11].

5.2 The relationship between p53 gene and MSI in colorectal cancer

The p53 gene is located on human chromosome 17p13.1.It can actively identify damaged DNA and initiate repair mechanisms.The normal expression of P53 protein can block the G1/G0 phase，prevent the cells from entering the S phase，inhibit the abnormal proliferation of cells，and play a role in suppressing cancer.When the p53 gene is mutated，the effect of p53 in inducing cell apoptosis disappears and mediates the abnormal proliferation of heterogeneous cells.Studies have found that the mutation rate of p53 in colorectal cancer reaches 40-50%，and the mutation rate of p53 in advanced colorectal adenomas is 80%.Cheng Lei and others pointed out that P53 tumor suppressor gene has deletion or mutation in the process of CRC adenoma transforming into adenocarcinoma.Liu et al.pointed out that 84% of MSS tumors have mutations in P53 and P53 mutant tumors have a poor prognosis in MSS tumors [12].Zhao Xilians experimental results showed that the mutation rate of P53 in MSI was 42.6%，which was negatively correlated with MSI.MSI patients have a better prognosis and benefit from 5-FU chemotherapy drugs [13].

6 Conclusion

In summary，DNA mismatch repair proteins have important clinical significance in colorectal cancer，endometrial cancer and other solid tumors.With the development of various detection technologies，the general screening of the MSI status of malignant tumors has been promoted，and the diagnosis and treatment rate of the disease by clinicians has been improved.It has also provided a reliable basis for the individualized treatment of colorectal cancer patients.Genes such as Ras/Braf and P53 are closely related to MSI.Due to the different mechanisms and positions of genes，different MSI states result in different gene expressions.

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云南省昆明市?昆明醫科大學第二附屬醫院?650101