Frameshift Mutations and a Length Polymorphism in the hMSH3 Gene and the Spectrum of Microsatellite Instability in Sporadic Colon Cancer

Mutations in the hMSH3 gene in sporadic colon cancer with microsatellite instability (MSI) were investigated, since several mismatch repair genes were known to be mutated in cancers with MSI, but only deletions in the (A)₈ region in the hMSH3 gene have been reported. We also analyzed the relationships between hMSH3 mutations and the spectrum of MSI. We screened MSI in 79 sporadic colon cancer samples using mono- and dinucleotide repeat markers and the samples with MSI were further analyzed for tri- and tetranucleotide repeat instability and mutations in the hMSH3 gene by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis. Five (6%) out of 79 tumors were MSI-H and 15 (19%) were MSI-L. Two MSI-H tumors showed insertion in the (C)₈ region in the hMSH6 gene and one tumor showed insertion and deletion in the (A)₈ region in the hMSH3 gene, and two of the three above tumors showed MSI in tri-and tetranucleotide repeats. One MSI-L tumor showed somatic alteration in a 9-bp repeat sequence in hMSH3. No frameshift mutations were found in the (A)₇ and (A)₆ regions in hMSH3. Thus, we confirmed that the (A)₈ region in hMSH3 is a hot spot and mutations in the (A)₇ and (A)₆ regions in hMSH3 are not common. The hMSH3 mutation may enhance genomic instability in some colorectal cancers.