Difference analysis of somatic mutations between deficient mismatch repair and proficient mismatch repair gene related with colorectal cancer

Objective: To analyze the difference of somatic mutation of DNA mismatch repair (MMR) protein deletion (dMMR) /integrity (pMMR) in colorectal cancer (CRC). Methods: A total of 93 cases of paraffin pathological tissue derived from CRC patients underwent surgical treatment and postoperative routine immunohistochemical diagnosed as dMMR in the Second Affiliated Hospital of Zhejiang University Medical College from January 2015 to January 2017 were collected and conducted the second-generation sequencing test. The expressions of 4 MMR proteins (MLH1, MSH2, MSH6 and PMS2) in CRC tissue were detected by immunohistochemistry method, and the immunohistochemistry results were re-interpreted according to the American Association of Pathologists (CAP) standard. Second-generation sequencing technology was used to detect somatic mutations of 41 genes in 93 cases of paraffin pathological CRC tissue, and Fisher's exact test was used to analyze the gene mutation differences between groups. Results: After re-evaluation according to CAP standard, 31 cases were divided into pMMR group and 62 cases in dMMR group among the 93 CRC patients. The medium number of gene mutations in the dMMR group was 9.5, higher than 3.0 of the pMMR group (P<0.001). Somatic mutation differences were found in 17 genes between the dMMR and pMMR groups, including breast cancer susceptibility genes 1 (BRCA1), BRCA2, MLH1, PDGFRA, PIK3CA, APC, ATM, KIT, MET, PMS2, MSH6, POLE, MSH2, PTCH1, epidermal growth factor receptors (EGFR), TP53 and ERBB2 genes. The pathogenic somatic mutation rates of BRAF, MLH1, MSH2 and MSH6 in the dMMR group were higher than those in the pMMR group [21.0% (13/62) vs 9.7% (3/31), 9.7% (6/62) vs 0 (0/31), 21.0% (13/62) vs 0 (0/31), 22.6% (14/62) vs 0 (0/31), P<0.05]. The mutation rate differences of BLM N515fs, BRAF V600E, PTCH1 R1308fs and KRAS G13D sites were statistically different between the dMMR group and the pMMR group [22.6% (14/62) vs 0 (0/31), 19.4% (12/62) vs 3.2% (1/31), 11.3% (7/62) vs 0 (0/31), 16.1% (10/62) vs 3.2% (1/31), P<0.05]. The mutation rates of 3 uncommon sites including BLM N515fs, MSH6 F1088fs and PTCH1 R1308fs were 28.2% (11/39), 15.4% (6/39) and 15.4% (6/39) in patients with dMMR who were missing MLH1 and PMS2 together, statistically different from all of 0 (0/31) in pMMR patients (P<0.05). Conclusions: CRC Patients with dMMR have more related gene somatic mutations. The BRAF V600E mutation is closely related to dMMR. KRAS G13D, BLM N515fs and PTCH1 R1308fs mutation sites are also associated with the expression of MMR proteins.