Effects of Tdg deficiency on overall transcription in mouse embryonic fibroblasts and during cell differentiation

Because it excises thymine from GT mismatches, TDG was proposed to counter mutagenesis by 5-methylcytosine deamination. Yet, TDG was also observed to attack 5-methycytosine itself, making it a candidate DNA demethylase, and interactions with transcription factors implicated additional functions in gene regulation. Unlike other DNA glycosylases, TDG is essential for embryonic development. Fibroblasts from Tdg null embryos show massively impaired gene regulation, and this correlates with imbalanced histone modification and CpG methylation. TDG associates with the promoters of affected genes in MEFs and in embryonic stem cells, but epigenetic aberrations appear only in differentiated cells. TDG also contributes to the maintenance of active and bivalent chromatin during cell differentiation, using its DNA glycosylase activity to counter aberrant de novo methylation. Thus, TDG dependent DNA repair stabilizes epigenetic states during cell differentiation.

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Cortázar D, Kunz C, Selfridge J, Lettieri T, Saito Y, MacDougall E, Wirz A, Schuermann D, Jacobs AL, Siegrist F, Steinacher R, Jiricny J, Bird A, Schär P