title: Exclusive KRAS mutation in microsatellite-unstable human colorectal carcinomas with sequence alterations in the DNA mismatch repair gene, MLH1.
abstract: Microsatellite instability (MSI) is regarded as reflecting defective DNA mismatch repair (MMR). MMR defects lead to an increase in point mutations, as well as repeat instability, on the genome. However, despite the highly unstable microsatellites, base substitutions in representative oncogenes or tumor suppressors are extremely infrequent in MSI-positive tumors. Recently, the heterogeneity in MSI-positive colorectal tumors is pointed out, and the 'hereditary' and 'sporadic settings' are proposed. Particularly in the former, base substitution mutations in KRAS are regarded as relatively frequent. We sequenced the KRAS gene in a panel of 76 human colorectal carcinomas in which the MSI status has been determined. KRAS mutations were detected in 22 tumors (28.9%). Intriguingly, all of the KRAS-mutant MSI-H (high) tumors harbored sequence alterations in an essential MMR gene, MLH1, which implies that KRAS mutation more frequently and almost exclusively occurs in MMR gene-mutant MSI-H tumors. Furthermore, in contrast with the prevailing viewpoint, some of these tumors are derived from sporadic colorectal cancer patients. The tight connection between MMR gene mutation and KRAS mutation may suggest previously unrecognized complexities in the relationship between MSI and the mutator phenotype derived from defective MMR.
authors.affiliation: Institute for Clinical Research, National Kyushu Cancer Center, 3-1-1, Notame, Fukuoka 811-1395, Japan.
authors.forename: Yan
authors.initials: Y
authors.lastname: Zhao
authors.forename: Kaname
authors.initials: K
authors.lastname: Miyashita
authors.forename: Takayuki
authors.initials: T
authors.lastname: Ando
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Kakeji
authors.forename: Takeharu
authors.initials: T
authors.lastname: Yamanaka
authors.forename: Kenichi
authors.initials: K
authors.lastname: Taguchi
authors.forename: Toshikazu
authors.initials: T
authors.lastname: Ushijima
authors.forename: Shinya
authors.initials: S
authors.lastname: Oda
authors.forename: Yoshihiko
authors.initials: Y
authors.lastname: Maehara
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics*; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001483:Base Sequence; D003062:Codon; D015179:Colorectal Neoplasms / Q000235:genetics*; D053843:DNA Mismatch Repair; D005260:Female; D006801:Humans; D008297:Male; D053842:Microsatellite Instability; D008875:Middle Aged; D000070957:MutL Protein Homolog 1; D009154:Mutation / Q000235:genetics*; D009687:Nuclear Proteins / Q000235:genetics*; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D016283:Proto-Oncogene Proteins p21(ras); D018631:ras Proteins / Q000235:genetics*
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D003062:Codon; C117307:KRAS protein, human; C501486:MLH1 protein, human; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D000070957:MutL Protein Homolog 1; D016283:Proto-Oncogene Proteins p21(ras); D018631:ras Proteins