Chromatin remodeling agents for cancer therapy

Alterations in chromatin structure profoundly influence gene expression during normal cellular homeostasis and malignant transformation. Methylation of cytosines within CpG islands located in promoter and proximal coding regions facilitates recruitment of chromatin-remodeling proteins, which inhibits gene expression. Posttranslational modifications, such as acetylation, methylation, and phosphorylation, of core histone proteins 'mark' regions of chromatin for recognition by multiprotein complexes, which promote either chromatin relaxation and gene expression or chromatin compaction and repression of gene expression. Many genes become transcriptionally silenced during the development of cancer. Covalent epigenetic modifications such as DNA hypermethylation and histone post-translational modifications are an important early event during carcinogenesis and tumor development. Genes involved in key DNA damage responses pathways, apoptosis signaling and DNA repair, can frequently become methylated and epigenetically silenced in tumors. This may lead to differences in intrinsic sensitivity of tumors to chemotherapy, depending on the specific function of the gene inactivated. The fact that cancer can have an epigenetic etiology has encouraged the development of a new therapeutic option that might be termed "epigenetic therapy". The DNA methylation paradox, manifested as derepression of cancer-testis antigens and silencing of tumor suppressors during malignant transformation, provides rationale for the utilization of chromatin remodeling agents for cancer therapy. In this review, the recent advances in the understanding and clinical development of DNA methyltransferase and Histone deacetylase inhibitors, as well as their current role in cancer therapy, will be discussed.