Discussion
It is currently believed that genetic and epigenetic events interact to help drive tumor progression [
13]. The combination of the two sides may provide a new viewpoint to better understand the pathogenesis of gastric carcinoma [
14]. Great attention has been aroused on the significance of DNA methylation for early diagnosis and prognosis prediction in malignant tumors [
15]. Gastric carcinoma (GC) is one of the most frequent malignancies and remains the second leading cause of cancer-related mortality in the world [
16]. Although the clinical outcome of gastric cancer has gradually improved, diagnosis of gastric carcinoma is still disappointing. Thus, early detection of gastric cancer is a key measure to reduce the mortality and improve the prognosis of gastric carcinoma [
17]. Increasing evidence now suggests that, in addition to genetic alterations, epigenetic changes, including DNA methylation, histone modification and RNA interference, also play important roles in the development and progression of gastric carcinoma [
18]-[
20]. As a well-defined epigenetic mechanism, DNA methylation plays a major role in cancer through transcriptional silencing of critical growth regulators such as tumor suppressor genes, along with gene mutation and deletions, ultimately leading to carcinogenesis [
21].
In addition to genetic causes, tumors can also be considered an epigenetic disease [
22]. DNA methylation is the most important area of epigenetics which plays an important role in genomic imprinting and in the silencing of retrotransposon. In normal cells, DNA methylation may modulate compartmentalization of DNA to assure that transcriptionally active chromatin regions replicate earlier than the bulk transcriptionally inactive chromatin. Regional hypermethylation observed during tumor progression may involve in inactivating one of the two X chromosomes. These changes in chromatin structure may, through direct transcriptional inactivation of genes and allelic deletions, mediate the decreased expression of tumor suppressor genes associated with tumor development [
23]. Hypermethylation of tumor suppressor genes has attracted much attention recently and DNA methylation inhibitors are being tested as potential anticancer agents [
24],[
25]. Drugs that inhibit DNA methylation may help patients live longer with fewer side effects than conventional cytotoxic therapy. Aberrant methylation of HOXA11 gene promoter has been found in various tumors [
26]-[
28], but the mechanism and roles involved in GC have not been elucidated. Therefore, further investigation into the roles of HOXA11 methylation in GC development and identification of its regulators are necessary.
Homeobox (HOX) genes were originally discovered in Drosophila melanogaster [
29]. Several investigations demonstrate that HOX genes act as transcriptional regulators involved in the process of cell to cell communication during normal morphogenesis, the alteration of which may contribute to the development of cancer [
30],[
31]. HOX gene homology domain is able to bind to specific DNA sequences, and to regulate gene transcription [
32]. However the mechanisms of HOX genes involved in tumorigenesis have not been elucidated. HOXA11 is a murine Abdominal-B-type homeobox gene that regulates lower abdominal development in Drosophila, and control differentiation of the müllerian ducts into the fallopian tubes, uterus and cervix. Studies show that low expression of HOXA11 gene has been found in various tumors, such as ovarian cancer and endometrial cancer, and it plays the role of tumor suppressor genes [
33]. Previous research has shown that high methylation and low expression of HOXA11 gene is prevalent in gynecologic cancer. Because HOXA11 gene is a developmental related gene, the majority of researches have focused on its effects on the mesoderm organ development and expression. The regulation mechanism involved in tumors has not been elucidated. Cary Miller
et al. [
34] observed that Wnt7a regulates the expression of HOXA11 gene in mice, suggesting HOXA11 gene may associate with Wnt involved in regulatory network of development. In this study, we find the methylation frequencies of HOXA11 in GC tissues and adjacent cancer tissues are higher than those in normal gastric mucosa (
P < 0.05). Moreover, expression of HOXA11 gene is down regulated when the promoter region is hypermethylated, suggesting that HOXA11 gene may play an important repressor role in GC tumorigenesis, and aberrant promoter methylation is the main reason causing loss or down-regulation of HOXA11 gene, which may be involved in the carcinogenesis of GC. Furthermore, we found that hypermethylation of HOXA11 was significantly associated with lymph node metastasis and TNM stage in gastric cancer (
P < 0.05), but not significantly associated with other clinicopathological factors such as sex, age, tumor size, differentiation degree and invasive depth. Our results suggest that hypermethylation of HOXA11 may therefore be an important indicator of molecular biology for evaluating the degree of malignancy and lymph node status of gastric cancer. In addition, results in vitra shown HOXA11 may control cell growth and its defect enhanced the cell proliferation which may involved in carcinogenesis of GC. However, repression of HOXA11 did not significantly promote the cell migration, indicating HOXA11 defect does not augment cancer cell metastasis.
In conclusion, aberrant promoter methylation of HOXA11 is a frequent event and may be one of the main phenotypes that induces HOXA11 gene aberrant expression in GC. As a transcription regulator, loss and down-regution of HOXA11 may lead silence of cancer suppressor genes and excessive growth such as tumors. Hypermethylation of CpG islands may appear early in carcinogenesis which facilitates malignant growths, suggesting that the detection of DNA methylation from gastric juice and blood samples could serve as a molecular marker for predicting tumor progression and prognosis. DNA methylation occurred in tumors are easier to correct than to correct DNA sequence mutations or genetic damage. How to restore DNA expression by developing gene-targeting therapies by reversing aberrant methylation is currently considered a promising new approach of gastric cancer treatment. Since aberrant methylation of HOXA11 genes is significantly correlated with pathobiological behaviors in GC, analysis of DNA methylation could be used in tumor diagnosis, evaluation of chemosensitivity and prognosis.