Epigenetic regulation of gene expression by Ten Eleven Translocation (TET) and 6mA in drosophila: myth or reality?

Le September 19 2023 at 10h30 Séminaire

Members of the Ten Eleven Translocation (TET) family are well known for their capacity to oxidize and demethylate 5-methylcytosine (5mC), a prevalent epigenetic mark in vertebrates. Yet, these epigenetic enzymes also seem to have other substrates and enzymatic-independent functions, which are less well characterized. The drosophila genome is largely devoid of 5mC and does not code for any cytosine DNA methyltransferase but presents a single and well-conserved Tet gene. This organism thus represents a well-suited model to decipher the non-canonical functions of TET enzymes. Recently, the presence of 6-methyladenine (6mA) in DNA has been reported in various eukaryotes and, in drosophila, this epigenetic modification was shown to be erased by TET. Yet, the existence and significance of 6mA in metazoans remain controversial and the role of TET in 6mA oxidation is unexpected.

Here, we re-evaluated 6mA presence as well as TET function and mode of action in drosophila. Using LC-MS/MS measurements and strict axenic breeding conditions, we detected only traces of 6mA DNA (corresponding to a few hundred adenines per genome) in different tissues and developmental stages and we found that these levels were not increased in the absence of TET. In addition, SMRT-seq analyses suggest that TET loss does not affect 6mA deposition on the genome and we found no evidence that TET can directly demethylate 6mA DNA in vitro. Actually, our results indicate that in contrast with TET expression, TET enzymatic activity is not required for adult fly emergence and survival. Focusing on the larval central nervous system, we further show that TET essentially acts in a catalytic-independent manner in the larval central nervous system and our results suggest that it acts together with the Polycomb complex to control gene expression.

Overall, we propose that TET is not involved in 6mA erasure and that it controls gene expression and fly development mostly via catalytic-independent mechanism.