DNA methyltransferase 3B (Dnmt3b) belongs to a family of enzymes responsible for methylation of cytosine residues in mammals. DNA methylation contributes to the epigenetic control of gene transcription and is deregulated in virtually all human tumors. To better understand the generation of cancer-specific methylation patterns, we genetically inactivated Dnmt3b in a mouse model of MYC-induced lymphomagenesis. Ablation of Dnmt3b function using a conditional knockout in T cells accelerated lymphomagenesis by increasing cellular proliferation, which suggests that Dnmt3b functions as a tumor suppressor. Global methylation profiling revealed numerous gene promoters as potential targets of Dnmt3b activity, the majority of which were demethylated in Dnmt3b -/-lymphomas, but not in Dnmt3b -/-pretumor thymocytes, implicating Dnmt3b in maintenance of cytosine methylation in cancer. Functional analysis identified the gene Gm128 (which we termed herein methylated in normal thymocytes [Ment]) as a target of Dnmt3b activity. We found that Ment was gradually demethylated and overexpressed during tumor progression in Dnmt3b -/-lymphomas. Similarly, MENT was overexpressed in 67% of human lymphomas, and its transcription inversely correlated with methylation and levels of DNMT3B. Importantly, knockdown of Ment inhibited growth of mouse and human cells, whereas overexpression of Ment provided Dnmt3b +/+ cells with a proliferative advantage. Our findings identify Ment as an enhancer of lymphomagenesis that contributes to the tumor suppressor function of Dnmt3b and suggest it could be a potential target for anticancer therapies.

g DNA cytosine methylation is an epigenetic modification involved in the transcriptional repression of genes controlling a variety of physiological processes, including hematopoiesis. DNA methyltransferase 1 (Dnmt1) is a key enzyme involved in the somatic inheritance of DNA methylation and thus plays a critical role in epigenomic stability. Aberrant methylation contributes to the pathogenesis of human cancer and of hematologic malignancies in particular. To gain deeper insight into the function of Dnmt1 in lymphoid malignancies, we genetically inactivated Dnmt1 in a mouse model of MYC-induced T-cell lymphomagenesis. We show that loss of Dnmt1 delays lymphomagenesis by suppressing normal hematopoiesis and impairing tumor cell proliferation. Acute inactivation of Dnmt1 in primary lymphoma cells rapidly induced apoptosis, indicating that Dnmt1 is required to sustain T-cell lymphomas. Using high-resolution genome-wide profiling, we identified differentially methylated regions between control and Dnmt1-deficient lymphomas, demonstrating a locus-specific function for Dnmt1 in both maintenance and de novo promoter methylation. Dnmt1 activity is independent of the presence of Dnmt3a or Dnmt3b in de novo promoter methylation of the H2-Ab1 gene. Collectively, these data show for the first time that Dnmt1 is critical for the prevention and maintenance of T-cell lymphomas and contributes to aberrant methylation by both de novo and maintenance methylation.

The diversity of ZYMV isolates was analysed by the biological and molecular characterisation of 11 isolates sampled from cucumber, squash and zucchini between 2001 and 2006 in various localities of Slovakia and Czech Republic. Analysis of the molecular variability targeting three separate genomic regions of the ZYMV genome [P1, P3 and (Cter)NIb-(Nter)CP] revealed a remarkable low level of nucleotide variability between isolates, despite their temporal and spatial distinction. Phylogenetic analysis based on the 5'-terminal part of the CP gene highlighted the close relatedness of Slovak, Czech and other central European isolates. Low level of genetic diversity within central European ZYMV isolates is in contrast to the diversity observed for isolates from other geographical regions, in particular Asia. No evidence of recombination in the ZYMV genome was detected. Sequence comparison between aggressive and moderate ZYMV isolates revealed one amino acid difference in the N-terminal part of the P3 protein, potentially involved in the tolerance breaking.