Research On The Regulation Of Beclin 1 Expression In Gastric, Breast And Colorectal Cancer

Beclin 1, was the first autophagic gene identified in mammarlian cells with roles in mediating autophagy and apoptosis. Earlier studies showed that beclin 1 expression was down-regulated with mono-allelic deletions in human breast cancer cells and the decreased expression of beclin 1 could contribute to tumorigenesis. But the status of its expression and regulatory mechanisms in other cancers were remained to be determined. In the present work, we investigated the effects of possible mechanisms on beclin 1 expression in breast, gastric and colorectal cancers for estabilishing the basis between autophagic genes and tumorigenesis of beclin 1 in these cancers.In our present work, we detected the expression status of beclin 1 in the tumor tissues and corresponding adjacent normal tissues from 60 pairs of breast, gastric and colorectal tumors. We found that beclin 1 expression was associated with breast and gastric cancers, in which 19 samples were obtained with significantly decreased expression of beclin 1. Association between beclin 1 mRNA expression and clinocopathologic parameters was found in none of the three different kinds of cancer. Furthermore, in breast tumors, we examined the expression of p53, BRCA1 and BRCA2, which play important roles in the development of breast cancer and found that beclin 1 mRNA level was significantly greater in the BRCA1 positive tumors than in the negative ones, suggesting beclin 1 expression may be parallel to cell growth in breast cancer.Then we explored the possible regulatory mechanisms of beclin 1 expression in three aspects, loss of heterozygosity (LOH), point mutation and DNA methylation. The results showed a LOH rate of 45% and 20% at the beclin 1 locus respectively in breast and gastric tumors. But LOH was not found in colorectal tumors. We also examined mutations throughout 12 exons of beclin 1 and no mutation was found, which showed that mutation might not be a common mechanism affecting beclin 1 expression. Inspection of beclin 1 genomic location, we found a large and dense CpG island ranging from the promoter to the intron 2 of the beclin 1 gene. Therefore, we detected the methylation status of the CpG island in breast, gastric and colotectal tumors. In the promoter from -528 to -65 (from transcription start site) and the intron 2 from 733 to 977, methylation was detected in parts of tumors with significant decreased beclin 1 expression and hardly detected in the corresponding normal tissues. Furthermore, beclin 1 mRNA expression was restored in an ovarian cancer cell line, SKOV3, with 5-aza-dC treatment. The results indicated that aberrant DNA methylation of beclin 1 was responsible for its decreased expression in these tumors.In summary, our work indicated that decreased expression of beclin 1 is frequently observed in breast, gastric and colorectal cancer. Aberrant DNA methylation and LOH were demonstrated to be responsible for the decrease of beclin 1 expression in breast and gastric cancer, and DNA methylation was also found to play a role in colorectal tumors. Aberrant DNA methylation and LOH of beclin 1 associated with decreased expression suggest that epigenetic and genetic alterations of this gene may be interesting targets for cancer therapy.