Although CRCs with the MSI phenotype evolve because of loss of DNA MMR activity, there is little consensus on the sequence and timing of target gene inactivation in the evolution of tumors that occur through this pathway. It has been reported that the appearance of MSI, at least at noncoding microsatellites, is an early event in Lynch syndrome CRCs [
4,
17], and that frameshift alterations in the functionally critical target genes may occur at an early stage in these tumors [
18‐
20].
TGFβRII frameshift mutations are present in 80-90% of MSI CRCs, and it has been reported that these occur at the adenoma-carcinoma transition in MSI adenomas [
21,
22]. The precise timing of the onset of MSI, and the occurrence of mutations at other genetic targets, have not been confidently determined.
In this study, we examined the timing of frameshift mutations of
TGFβRII,
IGFIIR,
BAX,
hMSH3,
hMSH6 and
Fas antigen in 9 early stage and 4 late stage MSI CRCs. Interestingly, no mutations were identified in any early stage MSI-H CRC at
TGFβRII,
BAX, hMSH3 or
hMSH6 , but these were relatively frequent events in late stage MSI CRCs, and in the MSI CRC cell lines. There is a statistical association between mutation in any one gene and tumor stage. The frequency of frameshift mutations in mononucleotide repeat regions within exons might increase with stage in microsatellite unstable colorectal cancer. Our findings suggest that
TGFβRII,
BAX,
hMSH3 and
hMSH6 frameshift mutations are relatively later stage events in tumor progression for sporadic CRC with MSI. It has been reported that
TGFβRII mutations are infrequent in the early stages of sporadic gastric cancers with MSI [
23], but these occur more frequently in gastric tumors that are larger [
24]. It has been reported that the loss of
TGFβRII in intestinal epithelial cells promotes the invasion and malignant transformation of tumors initiated by
APC mutations [
25]. However, few studies have clarified how mutations of the target genes we have studied mediate tumor development in the MSI pathway. These findings suggested that the mutations of
TGFβRII,
BAX,
hMSH3, and
hMSH6 might play an important role in cancer progression from early to late stage tumors, rather than early in carcinogenesis. However, there still may be additional, currently unappreciated genes or DNA sequences involved in the process [
18,
26‐
28]. Interestingly, no microsatellite mutations in
IGFIIR and
Fas antigen were found in any cell line or in any CRC. It has also been reported that no mutations in the
IGFIIR genes were found in 27 MSI-H gastric tumors [
29]. These findings suggest that mononucleotide repeat of
IGFIIR and
Fas antigen might be unimportant as target genes in MMR-deficient CRCs. A limitation of our study is the small number of tumor samples, while a significant difference between the early stage cancer and late stage cancers was recognized. Large numbers of patients with MSI cancer might be necessary in the future to conclude the correlation between tumor stage and frameshift mutations in target gene.