Food for thought: novel revelations in human DNA demethylation coupled mechanism discovered in fish

             A rapidly evolving focus in research is the understanding of epigenetic (the superficial inheritable mechanisms of DNA modification) factors contributing to development and malignancy. DNA methlylation is one of the key mechanisms by which epigenetics proceeds, through the prevention of specific gene expression. Explicitly, typical DNA methlylation occurs on the 5 position of cytosine nucleotides (5-meC) occupying (60-90% of) CpG spans of genomic sequence (in mammalian somatic cells). Resultant in altering cellular differentiation, phenotypic expression, and other fundamental cellular function, DNA methlylation is documented to be greatly affiliated with perturbations, such as neoplasia and congenital disorders (e.g. Retts syndrome). Although, much is understood regarding the contributing protein factors performing DNA methylation, little is known with regarding the converse demethlyation process. The zebrafish (Danio rerio) is robust vertebrate model for analysis of genetic cellular development in vivo, due to its small size, high fecundity, and simple husbandry. Amenable to transgenic strategies, the ex utero development of transparent embryos allow for ease of phenotypic analysis and genetic manipulation.

             In a revolutionary study conducted by Rai et al., zebrafish embryos, at the one cell stage were microinjected with a 736 bp hyper-methlyated strand of DNA, to monitor methlylation levels. Elucidation through knockdown and overexpression assays provided evidence of a coupled deamination/ glycosylation mechanism for DNA demethylation. Deamination by either activation induced deaminase (AID) or apolipoprotein B RNA-editing catalytic components (Apobec) alters 5-meC into thymine, resulting in a G:T mismatch. Excision of the thymine is promoted by glycosylation via Mbd4 in a proposed base excision repair-like manner. In fact, overexpression studies actually resulted in demethylation of the zebrafish intrinsic genome, as well as the injected methlyated DNA. Furthermore, the non-enzymatic protein, Gaad45, was found to enhance the interactions and efficacy of the two enzymatic groups, thereby promoting demethylation. As a stepping stone, clarifying a previously unknown, fundamental, natural genetic process regulating epigenetics, the findings of this study provide abundant opportunities for future novel therapeutic strategies to currently incurable DNA methlyation-related diseases.

Lauren Klein (Rygier)

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REFERENCES:

Berman JN, Kanki JP, Look AT. (2005) Zebrafish as a model for myelopoiesis during embryogenesis. Exp Hematol. 33: 997-1006.

Delgado S, Gomez M, Bird A, Antequera F. (2001) Initiation of DNA replication at CpG islands in mammalian chromosomes. EMBO J. 17(8): 2426-2435.

Rai K, Huggins IJ, James SR, Karof AR, Jones DA, Cairns BR. (2008) DNA demethylation in zebrafish involves the coupling of a demainase, a glycosylase, and Gadd45. Cell. 135(7): 1201-1212.