The NBS1–Treacle complex controls ribosomal RNA transcription in response to DNA damage

Abstract

Chromosome breakage elicits transient silencing of ribosomal RNA synthesis, but the mechanisms involved remained elusive. Here we discover an in trans signalling mechanism that triggers pan-nuclear silencing of rRNA transcription in response to DNA damage. This is associated with transient recruitment of the Nijmegen breakage syndrome protein 1 (NBS1), a central regulator of DNA damage responses, into the nucleoli. We further identify TCOF1 (also known as Treacle), a nucleolar factor implicated in ribosome biogenesis and mutated in Treacher Collins syndrome, as an interaction partner of NBS1, and demonstrate that NBS1 translocation and accumulation in the nucleoli is Treacle dependent. Finally, we provide evidence that Treacle-mediated NBS1 recruitment into the nucleoli regulates rRNA silencing in trans in the presence of distant chromosome breaks.

A localized nucleolar DNA damage response facilitates recruitment of the homology-directed repair machinery independent of cell cycle stage

“…While the use of γ-irradiation and laser microirradiation to induce DSBs has yielded conflicting results (Kruhlak et al 2007;Moore et al 2011;Larsen et al 2014), here we unequivocally demonstrated that DSBs within rDNA are sufficient to induce ATM-dependent inhibition of Pol I transcription. The introduction of the CRISPR/Cas9 allowed us to demonstrate that DSBs in both transcribed and nontranscribed regions of the rDNA repeat induce this response.…”

“…Other identified nucleolar targets include the Pol I transcription termination factor TTF1 and a component of the early pre-rRNA processing machinery, UTP14A. Finally, Treacle-a factor implicated in transcription and rRNA modification and an interaction partner of Nijmegen breakage syndrome protein 1 (NBS1)-is also an ATM target (Ciccia et al 2014;Larsen et al 2014).…”

“…It should be pointed out, however, that similar experiments performed in human cells failed to show any inhibition of Pol I transcription following γ-irradiation (Moore et al 2011). More surprisingly, it has been demonstrated that laser microirradiation-induced DSBs outside of nucleoli lead to an ATM-dependent pan-nuclear silencing of Pol I transcription (Ciccia et al 2014;Larsen et al 2014). The difficulty in interpreting such experiments is that we have no knowledge of the number or distribution of DSBs.…”

Nucleolar Reorganization Upon Site-Specific Double-Strand Break Induction

“…Next, we analyzed the recruitment of different DDR factors to upstream binding factor (UBF)-positive NCs. As recent articles reported the interaction of TCOF1 and NBS1, 33,34 we wanted to examine the role of TCOF1 in nucleolar DDR more closely. We show that TCOF1 interacts with chromatin modifiers such as CARM1 and HDAC1 following rRNA gene cleavage.…”

“…Next, it is plausible that nucleolus has important differences in its DDR when compared with non-nucleolar DNA repair. This is exemplified by the NBS1 TCOF1 interaction described recently, 33,34 but there are other examples. Nucleolin, an Editor's Highlight Figure 6.…”