Abstract
DNA methylation is an important mechanism of gene silencing in mammals catalyzed by a group of DNA methyltransferases including Dnmt1, Dnmt3a, and Dnmt3b which are required for the establishment of genomic methylation patterns during development and differentiation. In this report, we studied the role of DNA methyltransferases during retinoic acid induced neuronal differentiation of P19 cells. We observed an increase in the mRNA and protein level of Dnmt3b, whereas the expression of Dnmt1 and Dnmt3a was decreased after RA treatment of P19 cells which indicated that Dnmt3b is more important during neuronal differentiation of P19 cells. Dnmt3b enriched chromatin library from RA treated P19 cells identified dipeptidyl peptidase 6 (Dpp6) gene as a novel target of Dnmt3b. Further, quantitative ChIP analysis showed that the amount of Dnmt3b recruited on Dpp6 promoter was equal in both RA treated as well as untreated p19 cells. Bisulfite genomic sequencing, COBRA, and methylation specific PCR analysis revealed that Dpp6 promoter was heavily methylated in both RA treated and untreated P19 cells. Dnmt3b was responsible for transcriptional silencing of Dpp6 gene as depletion of Dnmt3b resulted in increased mRNA and protein expression of Dpp6. Consequently, the average methylation of Dpp6 gene promoter was reduced to half in Dnmt3b knockdown cells. In the absence of Dnmt3b, Dnmt3a was associated with Dpp6 gene promoter and regulated its expression and methylation in P19 cells. RA induced neuronal differentiation was inhibited upon ectopic expression of Dpp6 in P19 cells. Taken together, the present study described epigenetic silencing of Dpp6 expression by DNA methylation and established that its ectopic expression can act as negative signal during RA induced neuronal differentiation of P19 cells.
Highlights
Epigenetic organization of gene expression involves DNA methylation, histone modifications, chromatin remodeling, and RNA interference
As an initial step to study the role of DNA methyltransferases in retinoic acid (RA) induced neuronal differentiation of P19 cells, we examined the mRNA levels of Dnmt1, Dnmt3a, and Dnmt3b at different time periods during differentiation by real-time RT-PCR analysis
methylation specific PCR (MSP) results corroborated well with the results of bisulfite genomic sequence (BGS) and COBRA as PCR bands were only observed with the primer pair specific for methylated DNA (Fig. 2F). These results clearly demonstrated that the CpG Island present in the promoter region of dipeptidyl peptidase 6 (Dpp6) gene was heavily methylated in P19 cells which remained methylated after RA treatment
Summary
Epigenetic organization of gene expression involves DNA methylation, histone modifications, chromatin remodeling, and RNA interference. These mechanisms control many important cellular functions, including cell proliferation, differentiation, and development [1]. DNA methylation represents covalent modification of the cytosine residues at the CpG islands which are found in the proximal promoter regions of almost 50% of mammalian genes. DNA methylation is catalyzed by DNA methyltransferases (Dnmts) that consist of a family of enzymes including Dnmt, Dnmt3a, and Dnmt3b [4,5]. Dnmt3a and Dnmt3b are involved in the establishment of new methylation patterns during development, and they are the de novo methyltransferase enzymes [7]. Dnmt and Dnmt3b null mice die during gestation period, whereas Dnmt3a null mice die shortly after birth [10]
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