Detection of Frameshift Mutations of the Transforming Growth Factor β Receptor II in Gastric Cancers with Microsatellite Instability

DISCUSSION

Approximately 10-15% of gastrointestinal tumors are caused by defective MMR, characterized by the presence of MSI and the absence of protein expression for any of the various genes involved in DNA MMR.^[11] In fact MMR gene defects do not provide mutated cells with a direct selective advantage, but rather increase the probability of mutations in other genes that other-wise would occur at low frequency. In our research, we found that MSI occurred in more than 50% of sporadic gastric carcinomas, accompanied by hMLHl méthylation at a high frequency (41.5%). In contrast, MSI and hMLHl méthylation were not detected in normal gastric tissue samples, and MSI-H was exclusively associated with hMLHl hyperméthylation which was rare in MSS and MSI-L samples. Furthermore, the majority of gastric carcinomas with hMLHl méthylation exhibited decreased hMLHl protein expression. Thse findings show that hMLHl promoter hyperméthylation is associated with hMLHl transcriptional inactivation and MMR deficiency, suggesting that hMLHl méthylation and MSI are very important mechanisms in gastric carcinogenesis.

HMLHl méthylation and MSI were noted in 1 case of carcinoma in situ and 2 cases of early gastric carcinoma, which strongly suggests that hyperméthylation of hMLHl and MSI are initial vital events in gastric carcinogenesis. The lesions with hMLHl hyperméthylation and MSI may be more prone to develop carcinoma. lino et al.^[12]have also reported on cases of serrated adenoma with hMLHl hyperméthylation leading to MSI-H cancer. Taken together, these findings suggest that epigenetic silencing of hMLHl may play an important role in initiation of gastric cancer, which, after subsequent inactivation of hMLHl, is required for entry into the pathway leading to microsatellite-unstable gastric cancer. HMLHl méthylation may represent an early event in gastric oncogenesis that culminates in disordered DNA MMR.

Tumor cells with an inactivated MMR system often exhibit marked MSI. However, because most microsatellites are noncoding, mutations of such sequences are thought to only reflect MMR defects rather than participate in tumor development. A direct oncogenetic effect of a high mutation rate in coding regions has recently been suggested for the TGFβR II gene, which is often observed in cancers exhibiting MSI (+). The new candidate tumor suppressor gene, TGFβR II, which is also targeted for MMR defects, was identified. The TGFβ-TGFβR II signaling system is important in growth regulation and cancer progression. [B1 The TGFβR family is divided into 3 groups: TGFβR I, TGFβR II and TGFβR III. TGFβR II phosphorylâtes TGFβR I, which activates TGFβR I kinase and initiates downstream signaling. So the normal function of the TGFβR II is the most important factor when the TGF beta signaling system fulfills its biological role, such as arresting the cell in the G1 phase, suppressing the growth of certain cancer cells, induction of apoptosis and inhibition of proliferation.

A lack of the expression of TGF and/or TGFβR II, as well as mutations of the related genes have been reported in human and animal malignancies^[14] These abnormalities were considered to be the cause of interruption of the growth signal from TGF to the cell nucleus, resulting in the uncontrolled growth of the involved cells. In gastric and ovarian cancers, the loss of TGFβ growth inhibition has been attributed to mutations of TGFβR II.^[15] Understanding the molecular mechanisms of gastric epithelial cell transformation and escape from normal growth regulation is critical to the discovery of effective methods for gastric cancer prevention, detection, and cure.

FMs in the TGFβR II gene were frequently detected in gastric cancers with hMLHl méthylation and MSI-H phenotype in our study. These mutations will likely yield truncated, non-functional proteins, resulting in the loss of both growth inhibition and apoptotic responses by TGFβ, which would favor tumor formation. Therefore, the inactivation of this receptor may be an important step in tumorigenesis. In our study, TGFβR II mutations in 5 (62.5%) of the 8 MSI-H tumors but in none of the MSI-L or MSS gastric cases, indicate that these mutations are specific for MSI-H tumors that exhibit a tendency to accumulate FMs. Our study has shown that gastric cancers with hMLHl méthylation accumulate MSI and FMs within the TGFβR II gene. There have been reports regarding TGF effects which support the concert that TGF is involved in cancer invasion and metastasis.^[15] We suggest that one of the molecular features of the malignant epithelium is an acquired resistance to the antiproliferative effects of TGF-β because mutated TGFβR II could neither bind to TGF nor play its role. Insensitivity to TGF-mediated growth arrest will lead to a cellular malignant phenotype.

On the basis of our findings, we can conclude that hMLHl hyperméthylation correlates well with its transcriptional silence and MMR defects, which lead to MSI and TGFβR II FMs. MSI and TGFβR II are the targets of MMR defects, so detection of hMLHl méthylation, MSI and TGFβRII FMs may be useful in early diagnosis of gastric cancer.