Pyrimidine Pool Disequilibrium Induced by a Cytidine Deaminase Deficiency Inhibits PARP-1 Activity,Leading to the Under Replication of DNA

Genome stability is jeopardized by imbalances of the dntp pool; such imbalances affect the rate of fork progression. for example,cytidine deaminase (cda) deficiency leads to an excess of dctp,slowing the replication fork. we describe here a novel mechanism by which pyrimidine pool disequilibrium compromises the completion of replication and chromosome segregation: the intracellular accumulation of dctp inhibits parp-1 activity. cda deficiency results in incomplete dna replication when cells enter mitosis,leading to the formation of ultrafine anaphase bridges between sister-chromatids at “difficult-to-replicate” sites such as centromeres and fragile sites. using molecular combing,electron microscopy and a sensitive assay involving cell imaging to quantify steady-state par levels,we found that dna replication was unsuccessful due to the partial inhibition of basal parp-1 activity,rather than slower fork speed. the stimulation of parp-1 activity in cda-deficient cells restores replication and,thus,chromosome segregation. moreover,increasing intracellular dctp levels generates under-replication-induced sister-chromatid bridges as efficiently as parp-1 knockdown. these results have direct implications for bloom syndrome (bs),a rare genetic disease combining susceptibility to cancer and genomic instability. bs results from mutation of the blm gene,encoding blm,a recq 3’-5’ dna helicase,a deficiency of which leads to cda downregulation. bs cells thus have a cda defect,resulting in a high frequency of ultrafine anaphase bridges due entirely to dctp-dependent parp-1 inhibition and independent of blm status. our study describes previously unknown pathological consequences of the distortion of dntp pools and reveals an unexpected role for parp-1 in preventing dna under-replication and chromosome segregation defects. © 2015 gemble et al.