DNA polymerase zeta contributes to heterochromatin replication to prevent genome instability

The DNA polymerase zeta (Polζ) plays a critical role in bypassing DNA damage. REV3L, the catalytic subunit of Polζ is also essential in mouse embryonic development and cell proliferation for reasons that remain incompletely understood. In this study, we reveal that REV3L protein interacts with heterochromatin components including repressive histone marks, and localizes in pericentromeric regions through direct interaction with HP1 dimer. We demonstrate that Polζ/REV3L ensures progression of replication forks through difficult-to-replicate pericentromeric heterochromatin, thereby preventing spontaneous DNA and chromosome break formation. We also find that Rev3l-deficient cells are compromised in the repair of heterochromatin-associated DSBs, eliciting deletions in late replicating regions. Lack of REV3L leads to further consequences that may be ascribed to heterochromatin replication-associated functions of Polζ, with a disruption of the temporal replication program at specific loci. This is correlated with changes in epigenetic landscape and transcriptional control of developmentally regulated genes. These results reveal a new function of Polζ in preventing chromosome instability during replication of hetero­chromatic regions. Nascent-DNA from early S-phase versus nascent-DNA from late S-phase