Evidence of genetic progression in human gastric carcinomas with microsatellite instability

Abstract

Mutator phenotype tumors provide unique opportunities to unravel malignant progression because of various gene alterations acquired during clonal tumor evolution. Gastric carcinomas, which have been known to show frequent genetic instability, would be composed of initial gene alterations shared by most tumor areas and subsequent alterations restricted to particular tumor sites. To analyse the timing of genetic events, we examined separate sites of tumor tissue obtained from a given gastric carcinoma patient with microsatellite instability (MSI). Our study included 95 normal/tumor area pairs from 25 patients. Six of the 25 patients (24%) demonstrated various levels of MSI ranging from 7% (two of 30) to 97% (28 of 29) of markers tested in multiple tumor sites. Of the six patients, ®ve manifested frameshift mutations in a tract of ten deoxyadenosines within transforming growth factor b receptor type II and four demonstrated frameshift mutations in a tract of eight deoxyguanosines within BAX. These mutations were common to all tumor sites regardless of the various level of MSI phenotype, indicating initial events. Two of the six patients exhibited frameshift mutations in mononucleotide repeats of mismatch repair genes, hMSH3 and hMSH6, and the insulin-like growth factor II receptor in restricted tumor areas, indicating additional alterations. Insulin-like growth factor II receptor mutations appear to be caused by hMSH3 and hMSH6 mutations because the former mutations were con®ned to tumor portions with the latter two mismatch repair lesions. These results provide genetic progression evidence for gastric carcinomas of the mutator pathway. In this pathway, mismatch repair insuciency initially targets mononucleotide tracts of transforming growth factor b receptor type II and BAX. During tumorigenesis, primary mismatch repair failure may give rise to the secondary mismatch repair lesions, frameshift mutations of hMSH3 and hMSH6, which result in another tumorigenic mutation in the insulin-like growth factor II receptor.

Role of BAX mutations in mismatch repair-deficient colorectal carcinogenesis

“…In four of the six tumours bearing BAX mutations, identical alterations in the [(G)8] tract were found in all sites sampled within each cancer, supporting the hypothesis that BAX mutation was present in the founder malignant clone. This pattern is also consistent with that described for BAX mutations in gastric cancers (Chung et al, 1997). However, in two of six cases we demonstrated a second pattern in which BAX mutation is not shared by all the tumour sites of the same cancer.…”

“…These observations strongly suggest that microsatellite instability is acquired at the adenoma-carcinoma interface in the evolution of sporadic tumours, but can appear at an Cae=caecum, asc=ascending colon, sig=sigmoid colon, mucoid=mucoid carcinoma, ac md=moderately di erentiated adenocarcinoma, ac pd=poorly di erentiated adenocarcinoma, ac/mu=mixed adenocarcinoma glandular/mucoid pattern, +(loss)=deletion of one G from the [(G)8] tract, +(gain)=insertion of one G in the [(G)8] tract earlier stage in patients who carry germline mutations in MMR genes. Previous studies have shown clonal expansion of shifts of di erent amplitude at the same microsatellite locus sampled from di erent sites in the same tumour (Chung et al, 1997), as observed in the present work also. This indicates that a proportion of these microsatellite mutations are acquired as clonal variants throughout the process of tumour formation and re¯ect but do not cause the evolution of such tumours.…”

p53 mutation with frequent novel codons but not a mutator phenotype in BRCA1- and BRCA2-associated breast tumours

“…The presence of a mutator phenotype, similar to that observed in Hereditary Non-Polyposis Colon Cancer (HNPCC) and gastric carcinomas was investigated by searching for frameshift mutations in TGF-b type IIR and Bax. Mutations in polypurine tracts within these genes have been reported in human tumours, such as gastric carcinoma, with MSI Parsons et al, 1995;Chung et al, 1997). However, the absence of any such changes in the BRCA-associated tumours in the present series implies that a classical mutator phenotype is not present in BRCA-associated tumours.…”

“…The presence of sequence changes in a 268 bp fragment of b-globin was tested by SSCP using PCR conditions previously described (Bauer et al, 1991). Analysis for the presence of MSI-associated frameshift mutations in the coding regions of TGF bRII [poly(A) 10 ] tract and Bax [poly(G) 8 ] tract was performed as described previously by Chung et al (1997).…”

Genetic progression in microsatellite instability high (MSI-H) colon cancers correlates with clinico-pathological parameters: A study of theTGRβRII,BAX,hMSH3,hMSH6,IGFIIR andBLM genes

“…This genetic model is also supported by other indications. By analyzing separate tumor areas from a small number of MMP ϩ gastric carcinomas, Chung et al reported a progressive accumulation of mutations, beginning with frameshifts in TGF␤RII and BAX followed by lesions in hMSH3 and hMSH6 and, as a last step, by mutations in IGFIIR (Chung et al, 1997). TGF␤RII mutations were found during the transition from colon adenoma to carcinoma, earlier than any other reported CDRs MSI (Grady et al, 1998).…”