Novel Function of Transcription Factor TTF1 in UV Mediated DNA Damage Repair in Mammalian Cells

Various DNA repair machineries have evolved in the cell to maintain the integrity of the genome for proper functioning of the same. Repair of ultraviolet (UV) irradiation mediated DNA damage occurs by nucleotide excision repair pathway through transcription coupled repair (TCR), a process in which the damage is repaired on transcriptionally active genic regions. This process requires various protein complexes including heterodimer DNA Damage Binding 1 (DDB1) protein. Defects in TCR have been found in patients with mutations in the Cockayne syndrome (CS) group A and group B genes and in the Xeroderma pigmentosum (XP) group G gene. Transcription factors (TFs) play a very important role in regulation of TCR system, especially those TFs which binds to DNA at specific loci. Several TFs have been shown to modulate the repair of photolesions either by inducing or inhibiting the TCR. However, the mechanism behind their action is not very clear. Mammalian TTF1 is an essential multifunctional transcription factor involved in transcription initiation, termination, DNA fork blockage, chromatin remodelling etc., and has been shown to interact with CSB protein. Hence, to discover its role in TCR and to identify its interaction partners, we purified this protein and did a pull-down assay with HEK293T cell lysate followed by LC-MS and discovered DDB1 as one of its major interactors. This established our confidence that TTF1 protein might be playing a critical role in TCR. Further, we discovered that, upon UV mediated DNA damage in HEK293T cells the expression of TTF1 is significantly induced and is co-localized with _γ H2AX protein. To our surprise, we found that after knockdown of TTF1, DDB1 level decreases in HEK293T cells while knockdown of DDB1, increases TTF1 level in the cells. Hence, our study opens up a new avenue towards exploring a noble function of the transcription factor TTF1, which in turn could establish the potential to develop therapeutics towards cancers and other diseases.