Epigenetic changes, such as DNA methylation, have been shown to promote breast cancer progression. However, the mechanism by which cancer cells acquire and maintain abnormal DNA methylation is not well understood. We have previously identified an aberrant splice form of a DNA methyltransferase, DNMT3B7, expressed in virtually all cancer cell lines but at very low levels in normal cells. Furthermore, aggressive MDA-MB-231 breast cancer cells have been shown to express increased levels of DNMT3B7 compared to poorly invasive MCF-7 cells, indicating that DNMT3B7 may have a role in promoting a more invasive phenotype. Using data gathered from The Cancer Genome Atlas, we show that DNMT3B7 expression is increased in breast cancer patient tissues compared to normal tissue. To determine the mechanism by which DNMT3B7 was functioning in breast cancer cells, two poorly invasive breast cancer cell lines, MCF-7 and T-47D, were stably transfected with a DNMT3B7 expression construct. Expression of DNMT3B7 led to hypermethylation and down-regulation of E-cadherin, altered localization of β-catenin, as well as increased adhesion turnover, cell proliferation, and anchorage-independent growth. The novel results presented in this study suggest a role for DNMT3B7 in the progression of breast cancer to a more aggressive state and the potential for future development of novel therapeutics.

Breast cancer is the second-leading cause of cancer death among women in the United States and the progression from non-invasive to invasive/metastatic disease correlates with an increased risk of death. Changes in DNA methylation have been shown to be important in breast cancers. However, the mechanism by which cancer cells acquire and maintain abnormal methylation is not well understood. DNA methyltransferases (DNMTs) are responsible for monitoring and altering DNA methylation patterns in cells. Interestingly, we have identified many aberrant DNMT3B transcripts in cancer cells compared to normal cells. Therefore, we hypothesize that aberrant DNMT3B isoforms alter gene and protein expression to promote tumor progression in breast cancer cells. Our studies indicate that the breast cancer cell lines MCF-7 and MDA-MB-231 express many aberrant DNMT3B isoforms including DNMT3B7 and ΔDNMT3B6. Interestingly, the invasive MDA-MB-231 cells express increased levels of DNMT3B7 and ΔDNMT3B6 compared to the poorly invasive MCF-7 cells. To further our studies, poorly invasive MCF-7 cells were stably transfected with a DNMT3B7 expression construct to determine the role of this aberrant transcript in tumor progression. Preliminary data indicate that the increased expression of DNMT3B7 in these cells correlates with a significant increase in growth. Specifically, cells expressing more DNMT3B7 grow faster than those with lower levels of the aberrant protein. Furthermore, E-cadherin expression is inversely correlated with DNMT3B7 expression indicating that the expression of this aberrant protein may lead to a more aggressive phenotype. Finally, soft agar assays have been performed to determine the functional significance of these findings. The novel results presented in this study suggest the need for additional studies to determine the role of aberrant DNMT3B transcripts in cancer. Ultimately, this work may lead to a better understanding of breast cancer progression and the development of novel therapeutics. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1053. doi:1538-7445.AM2012-1053