Aberrant promoter hypomethylation in CLL: does it matter for disease development?

Mini Review published: 11 August 2016 doi: 10.3389/fonc.2016.00182 Aberrant Promoter Hypomethylation in CLL: Does it Matter for Disease Development? Garland Michael Upchurch1, Staci L. Haney2 and Rene Opavsky1,2,3* 1 Fred and Pamela Buffett Cancer Center, Eppley Institute for Research in Cancer and Allied Diseases, University of Nebraska Medical Center, Omaha, NE, USA, 2 Department of Genetics, Cell Biology, and Anatomy, University of Nebraska Medical Center, Omaha, NE, USA, 3 Center for Leukemia and Lymphoma Research, University of Nebraska Medical Center, Omaha, NE, USA Edited by: Pierluigi Porcu, Ohio State University, USA Reviewed by: Alessandro Isidori, Aormn Hospital, Italy Basem M. William, Ohio State University, USA *Correspondence: Rene Opavsky Specialty section: This article was submitted to Hematology Oncology, a section of the journal Frontiers in Oncology Received: 20 May 2016 Accepted: 27 July 2016 Published: 11 August 2016 Citation: Upchurch GM, Haney SL and Opavsky R (2016) Aberrant Promoter Hypomethylation in CLL: Does It Matter for Disease Development? Front. Oncol. 6:182. doi: 10.3389/fonc.2016.00182 Frontiers in Oncology | www.frontiersin.org Over the last 30 years, studies of aberrant DNA methylation in hematologic malignancies have been dominated by the primary focus of understanding promoter hypermethylation. These efforts not only resulted in a better understanding of the basis of epigenetic silencing of tumor suppressor genes but also resulted in approval of hypomethylating agents for the treatment of several malignancies, such as myelodysplastic syndrome and acute myeloid leukemia. Recent advances in global methylation profiling coupled with the use of mouse models suggest that aberrant promoter hypomethylation is also a frequent event in hematologic malignancies, particularly in chronic lymphocytic leukemia (CLL). Promoter hypomethylation affects gene expression and, therefore, may play an important role in disease pathogenesis. Here, we review recent findings and discuss the potential involvement of aberrant promoter hypomethylation in CLL. Keywords: mouse models of cancer, chronic lymphocytic leukemia, DNA methyltransferases, hypomethylation, hematologic neoplasms, DNA methylation, leukemia, promoter methylation INTRODUCTION Cytosine methylation of DNA is an epigenetic modification affecting gene transcription and the integrity of the mammalian genome. The three catalytically active DNA methyltransferases in mammalian cells are DNMT1, DNMT3A, and DNMT3B. These enzymes are responsible for establishment and maintenance of DNA methylation during normal development and during mitotic cell division. Promoter methylation typically results in transcriptional repression of genes and plays a role in various normal physiologic processes, such as differentiation and hematopoiesis (1, 2). One of the main observations that contributed to an interest in studying this phenomenon came from studies that discovered that in virtually all types of cancer aberrantly increased methylation in gene promoters was associated with transcriptional inhibition (3). As a result, aberrant promoter hypermethylation resulting in silencing of tumor suppressor genes in cancer has been a major topic of numerous studies over the past 30 years. Such efforts not only resulted in identification of a number of epigenetically repressed tumor suppressor genes in hematologic malignancies, such as VHL, p16, and MLH1 (3), but also provided a conceptual approach to the treatment of cancer. Therefore, use of hypomethylating agents can be a valuable approach in anticancer therapy as reversal of DNA methylation may lead to reactivation of tumor suppressor genes and antagonize aberrant tumor proliferation and survival. Epigenetic studies in hematologic malignancies have 1 August 2016 | Volume 6 | Article 182 Upchurch et al. Aberrant Promoter Hypomethylation in CLL consequently resulted in the approval of hypomethylating agents for the treatment of several malignancies, including myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Numerous efforts are undergoing to refine hypomethylating agents and to test their efficacy in combination with other drugs targeting epigenetic changes. The recent emergence of new high-resolution methylation profiling techniques, such as whole-genome bisulfite sequencing (WGBS), have revealed that the methylome of cancer cells frequently contains promoters that are hypomethylated relative to their normal cellular counterparts. Such aberrant hypomethylation is frequently accompanied by increased gene expression at differentially methylated loci. Whether or not such deregulated expression contributes to the initiation and progression of hematologic malignancies is currently unresolved and is actively under investigation. Here, we will first review data obtained in mouse studies focusing on DNA methyltransferase loss of function in various hematologic malignancies. We will then discuss recent findings that strongly support the idea that aberrant promoter hypomethylation accompanied by gene re-expression may contribute to the development of chronic lymphocytic leukemia (CLL) in a causative manner. that inactivation of Dnmt3a results in a differentiation block and accumulation of progenitor cells; however, this phenotype was mainly observed upon serial bone marrow transplantation (7). The first report demonstrating that Dnmt3a plays a tumor suppressive role in the prevention of hematologic malignancy was published by Peters et al. (8), which showed that conditional inactivation of Dnmt3a in HSPCs using EμSRα-tTA;TetoCre;Dnmt3afl/fl;Rosa26 LOXPEGFP/EGFP quadruple transgenic mice (designated as Dnmt3aΔ/Δ mice) results unexpectedly in the development of a CLL-like disease after 1 year’s time. This finding was surprising in view of a lack of genetic alterations in the DNMT3A locus in B-cell malignancies. Since this time, other groups have reported that Dnmt3a loss in HSPCs results in myriad types of malignancies, such as myeloproliferative disorders, AML, T-cell acute lymphoblastic leukemia (T-ALL), and B-cell acute lymphoblastic leukemia (B-ALL) (9, 10). Differences in phenotypes observed upon loss of Dnmt3a could stem from the different genetic background of mice used in these studies (Table 1) or the different properties of transgenes used to conditionally delete Dnmt3a alleles. In addition, Dnmt3a loss can collaborate with gain of function mutant c-kitD814V to induce B-ALL, T-ALL, and mastocytosis with myeloid blasts (17), and with KrasG12D/+ to promote progression of juvenile and chronic myelomonocytic leukemia (CMML) (19). However, under some circumstances Dnmt3a has acted as an oncogene by promoting the development of hematologic malignancies. For example, upregulation of Dnmt3a promoted AML/ETO-induced leukemia through de novo hypermethylation (23) and a methylation-independent repressor function of Dnmt3a enhanced T-cell lymphomagenesis (...truncated)