Tumor Mutational Burden Predicts The Efficacy Of Immune Checkpoint Inhibitors And Screening Of Related Mutation Genes In Gastric Cancer

Purpose:Immune checkpoint inhibitors（ICIs）are only effective for part of tumor patients.Tumor mutational burden（TMB）is considered as a potential tumor marker（TM）for predicting the response and effect of immunotherapy.However,the relationship between specific gene mutations and TMB and the prognosis of patients remains unclear,nor is it clear whether they can be used as predictive biomarkers of ICIs.Herein,a systematic review and meta-analysis was conducted to explore whether TMB can be used as a potential prognostic TM for cancer patients treated with ICIs.Bioinformatics analysis was used to find mutational genes associated with TMB that can indeed predict the prognosis of patients and the efficacy of ICIs in gastric cancer.Methods:（1）Relevant literature published in the Pub Med,Embase,Web of Science,and Cochrane databases up to December 28,2020 were systematically retrieved.All cohort studies and clinical trials that reported hazard ratios（HRs）for overall（OS）and progression-free survival（PFS）,as well as the corresponding 95%confidence intervals（CIs）of high and low TMB patients were included.After the data were extracted,the effects combination,subgroup analysis,sensitivity analysis and publication bias analysis were carried out.All statistical analyses were performed using the R software.（2）The somatic mutation data of gastric cancer from The Cancer Genome Atlas（TCGA）and the International Cancer Genome Consortium（ICGC）dataset was downloaded.The frequently mutated genes in both TCGA gastric cancer cohort and ICGC Chinese gastric cancer cohort were obtained.The correlation between gene mutation and TMB and prognosis of patients was analyzed through R software.Gene set enrichment analysis（GSEA）was used to find immune signal pathways related to CUB and Sushi multiple domains 1（CSMD1）mutation（CSMD1-mut）.TIMER2.0 was used to analyze the correlation between CSMD1-mut and immune infiltration.CBioportal was used to analyze genomic alterations,co-expressed genes and related signal pathways associated with CSMD1-mut.Results:（1）Pooled results from a total of 32 studies with 6,131 participants showed significantly increased OS（HR:0.61,95%CI:0.53–0.71;P<0.01）and PFS（HR:0.51,95%CI:0.44–0.60;P<0.01）for the high TMB group receiving ICIs as compared to the low TMB group.Particularly,results were found to be more significant in studies with larger sample sizes（≥30）,Western patients,higher TMB cut-off values（≥20mut/Mb）,anti-PD-1 therapy,and when the sample source was tissue and tumor type was either melanoma,small cell lung cancer,or gastric cancer.According to the meta-regression results,the heterogeneity between studies whose outcome is OS may be ascribed to sample size,sample source,or TMB cut-off value.As for studies whose outcome is PFS,only sample size could explain some of the sources of heterogeneity.（2）A total of nine genes with frequent mutations in TCGA gastric cancer samples and ICGC Chinese gastric cancer samples were identified including TP53,TTN,CSMD1,SYNE1,CSMD3,OBSCN,ZFHX4,PLEC,and COL12A1,and only CSMD1-mut was associated with higher TMB and better prognosis in patients.Mutation site analysis showed multiple mutations in the PHA02927（secreted complement-binding protein）domain of CSMD1.The genetic map showed that the loss of chromosome 4,5q,8p and 9p decreased,and the status of microsatellite instability（MSI）increased in CSMD1-mut samples.The analysis results of the molecular subtype of gastric cancer showed that the proportion of genomically stable（GS）and chromosome Instability（CIN）subtypes decreased significantly.In contrast,the MSI subtype increased significantly in the CSMD1-mut group.GSEA analysis showed that immune-related pathways were enriched in CSMD1-mut samples,including C-type lectin receptors,downstream signaling events of B cell receptor,protein expression by JAK-STAT signaling after interleukin 12 stimulation,interleukin 12 signaling,interleukin 12family signaling,presentation of soluble exogenous antigens endosomes,antigen processing cross-presentation and HDAC in antigen presentation down.Immune infiltration analysis showed that anti-tumor immune cells including CD4⁺Th1 cells（P=0.009）,NK cells（P=0.033）,M1 macrophage cells（P=0.007）and plasmacytoid dendritic cells（P=0.010）had a higher proportion and tumor-promoting immune cells including regulatory T cells（P=0.007）,M2 macrophage cells（P=0.037）and endothelial cells（P=0.007）had a lower proportion in the CSMD1-mut group.Gene co-expression analysis showed that the expression of PD-L1 and P53 was higher and the expression of ALK was lower in CSMD1-mut samples.Signal pathway analyses showed ten related pathways,in which RTK-RAS,HIPPO and WNT pathways were the three most relevant signal pathways.Conclusion:（1）TMB is a promising independent prognostic biomarker for cancer patients receiving ICIs,which could provide a new potential therapeutic strategy for high TMB patients who have failed traditional therapy.（2）CSMD1-mut in gastric cancer was associated with higher TMB and better survival,and may have potential significance in predicting the efficacy of ICIs.