The Study On Clincopathologic Features And Molecular Pathology In Wild-type Gastrointestinal Stromal Tumor

Gastrointestinal stromal tumors(GIST)are the most common mesenchymal tumors of the gastrointestinal trac,the incidence of which is increased year by year.Approximately 80%of GIST harbors proto-oncogene tyrosine kinase KIT mutations,while 8-10%GIST contains platelet-derived growth factor receptor alpha(PDGFRA)mutations,both KIT and PDGFRA belong to the family of tyrosine kinase receptors characterized by sustained kinase activity and extensive downstream signaling in the absence of natural ligands.Targeted molecular therapy with tyrosine kinase inhibitors has been shown to be effective against GISTs with the KIT and PDGFRA gene mutations.Approximately 10-15%of GIST is free from KIT and PDGFRA mutations and designated as "wild type" GIST resistant to tyrosine kinase inhibitors(TKIs)such as Imatinib mesylate.Representing characteristic heterogeneity of the tumor,the wild type GIST clinical features,morphology,immunophenotype,and molecular genetics remain largely unclear.To provide a basis for the improvement of diagnosis,prognosis,and individualized treatment,this study describes clinical pathology and sub-classification of wild type GIST,and analyzes the molecular signature of the different GIST subsets,and explores the pathogenesis of these tumors.Materials and methods:1.The 69 GIST cases with wild type KIT and PDGFRA gene were selected.The selection criteria were based on Immunohistochemistry and Sequencing tests of KIT and PDGFRA genes.For all GISTs,the clinical pathology data,immunophenotype and molecular characteristics was collected and summarized.2.Further,wild type GISTs were classified by the Succinate dehydrogenase(SDH)immunophenotype into SDH-deficient and non-SDH-deficient GIST,and the clinical pathology data,immunophenotype and molecular characteristics was collected and summarized.To investigate the specific molecular mechanisms of SDH defects,the SDHB,SDHC,and SDHD gene statuses were detected by Sequencing and the SDHA and SDHB gene promoter methylation statues was analyzed using Methylation Sequencing.3.In wild-type GIST,the expression and gene status of HER-2 was detected by immunohistochemistry and FISH,while the mutation status of EGFR,K-RAS,B-RAF,and PIK3CA genes were detected by Sequencing.For all mutation GISTs,the clinical pathology data,immunophenotype and molecular characteristics was collected and summarized.Results:1.The clinicopathological features of SDH-deficient GISTsStatistical comparison of the clinical and pathological characteristics in the SDH-deficient and KIT mutant GISTs,detected the SDH-deficient GIST patients tended to be younger and were more likely to occur in the stomach.According to the detected histological differences,the SDH-deficient GISTs were divided into 3 sub-types:spindle cell type GIST,epithelioid cell type GIST,and mixed spindle-epithelioid cell type GIST,statistical analysis indicated that SDH-deficient GIST tumors showed mostly epithelioid morphology.The positive rate of SMA was higher in SDH-deficient GIST than in KIT mutant GIST,the estimated Ki-67 proliferation index tends to be lower.Kaplan-Meier analysis indicated that tumor size greater than 10cm was associated with poor prognosis in SDH-deficient GIST and positive S-100 expression was linked to poor prognosis in the routine immunohistochemistry(IHC).2.Analysis of molecular mechanisms of signaling in SDH-deficient GISTWe detected SDHB,SDHC,and SDHD gene mutation in 38.46%(10/26)of SDH-deficient GIST.Within this group,we also found 3 cases with double mutations.The SDHX mutations were defined as deletions that directly affect the related protein amino acid sequence.Using methylation specific PCR approach,we detected SDHX gene promoter methylation in 38.46%(10/26)of the SDH-deficient GIST.This group contained 1 case with both SDHA and SDHB gene.3.The Clinicopathological and Molecular characteristics of sub-groups of the wild-type GIST(1)We detected 3 cases with EGFR mutations in wild type GIST,2 cases tested positive for the common deletion mutations in exon 19,and one case had a rare point mutation(A859T)in exon 21.(2)BRAF mutations(V600E)were detected in 9 GIST cases with an incidence of 13.04%(9/69)for the wild type GIST.(3)We detected two cases with PIK3CA mutations and two cases KRAS synonymous GIST mutations.These GIST have typical GIST morphological characteristics,mainly containing spindle-like cells and no specific histological or immunophenotypical characteristics.Conclusions:1.Representing characteristic heterogeneity of the tumor,the wild type GIST clinical features,morphology,immunophenotype,and molecular genetics are different with KIT and PDGFRA mutations GIST.2.We define the SDHB negative GIST as SDH-deficient GIST that includes wild type tumor with KIT and PDGFRA gene mutations.Mostly younger patients are prone to this type of GIST,which often occurs in the stomach.As a GIST sub-type with unique clinicopathological and molecular characteristics,the tumor is marked by resistance to Imatinib,slow progress,and good prognosis.3.We suggest that the inactivation of SDHB is at least partly caused by the mutation of SDHX gene and/or the methylation of SDHX gene promoter in SDH-deficient GIST.4.EGFR,KRAS,BRAF and PIK3CA mutant GIST belong to a subclass of wild-type GIST.These mutations have specific biological functions and might be considered as markers for future GIST-targeting therapy.