Deregulated DNA ADP-Ribosylation impairs telomere replication

The recognition that DNA can be ADP-ribosylated has major consequences for our understanding of how ADP-ribosylation contributes to genome stability, epigenetics, and immunity. Yet, whether DNA ADP-ribosylation (DNA-ADPr) contributes to genome stability and how it is regulated remains unknown. Here, we show that telomeres are subject to DNA-ADPr catalyzed by PARP1 and removed by TARG1. Mechanistically, we show that DNA-ADPr is coupled to lagging telomere DNA strand synthesis, forming at single-stranded DNA present at unligated Okazaki fragments and on the 3’ single-stranded telomere overhang. Ultimately, persistent DNA-linked ADPr due to TARG1 deficiency leads to telomere shortening. Furthermore, utilizing the bacterial DNA-ADP-ribosyltransferase (DarT) toxin to directly modify DNA at telomeres, we demonstrate that unhydrolyzed DNA-linked ADP-ribose compromises telomere replication and telomere integrity. Thus, by identifying telomeres as chromosomal targets of PARP1 and TARG1-regulated DNA-ADPr, whose deregulation compromises telomere replication and integrity, our study provides fundamental knowledge of the critical importance of controlling DNA-ADPr turnover for sustained genome stability.