The regulatory functions of TIRR in DNA repair and p53-mediated cell fate

Abstract

The tumor suppressor protein, TP53BP1, is an important DNA double-strand break (DSB) repair protein that is extensively regulated by post-translational modifications and protein-protein interactions. Our lab and others established the role of the protein, Tudor Interacting Repair Regulator (TIRR), as a regulator of 53BP1 responses in DSB repair. TIRR binds the chromatin recognition motif of 53BP1 and restricts its access to DSBs. Consequently, TIRR overexpression inhibits 53BP1 recruitment to DSBs and impairs its DNA repair function. TIRR loss, however, does not have a similar impact on 53BP1 activities in DNA repair. This raises two fundamental questions: 1) What is the cellular role of endogenous TIRR? 2) Does TIRR impact 53BP1 functions that are not reliant upon its recruitment to DSBs? Recent evidence suggests that 53BP1 is an activator of the global p53 transcriptional response. Loss of 53BP1 impairs p53-mediated cell cycle arrest and cell survival. This function of 53BP1 is completely independent of its ability to localize to damaged DNA. In this study, we elucidate how TIRR regulates 53BP1’s role in the p53 transcriptional response. Here, we show that the loss of TIRR causes an aberrant increase in p53-mediated gene-transactivation. TIRR deficiency causes an abnormal induction in several key p53 pathway targets including CDKN1A (p21), MDM2, BBC3, and BAX, under different forms of cellular stress. Functionally, TIRR deficiency impacts several cell fate programs - decreased cell survival, higher senescence, and an increase in p21-mediated checkpoint arrest. From the mechanistic standpoint, TIRR modulates the stress-induced interaction of 53BP1 and p53. Our structural data demonstrates that TIRR specifically inhibits the complex formation between the Tudor domain of 53BP1 and a post-translationally modified form of p53 (dimethylated at K382) that is poised for transcriptional activation of its target genes. Analysis of cancer genomes reveals that high TIRR expression levels negatively correlate with expression of key p53 targets, thereby mimicking diminished p53 activity. Furthermore, amplifications of the TIRR genomic locus are mutually exclusive from TP53 mutation/deletions and amplifications of the MDM2/4 loci, suggesting that these are distinct mechanisms of suppressing p53 function in human cancer. Thus, we establish for the first time, the significance of TIRR as an upstream inhibitor of the 53BP1-p53 complex that regulates cell fate.