Anaphase Onset Before Complete DNA Replication with Intact Checkpoint Responses

Abstract

Cellular checkpoints prevent mitosis in the presence of stalled replication forks. Whether checkpoints also ensure the completion of DNA replication before mitosis is unknown. Here, we show that in yeast smc5-smc6 mutants, which are related to cohesin and condensin, replication is delayed, most significantly at natural replication-impeding loci like the ribosomal DNA gene cluster. In the absence of Smc5-Smc6, chromosome nondisjunction occurs as a consequence of mitotic entry with unfinished replication despite intact checkpoint responses. Eliminating processes that obstruct replication fork progression restores the temporal uncoupling between replication and segregation in smc5-smc6 mutants. We propose that the completion of replication is not under the surveillance of known checkpoints.

Smc5–Smc6 complex suppresses gross chromosomal rearrangements mediated by break-induced replications

“…Because the Smc5-Smc6 complex has many roles during DNA replication and chromosome segregation [35,41], it is difficult to ascertain which defects by the smc6-9 mutation cause DNA damage to translocations. For example, the cohesion defect impairs sister chromatid recombination [31,66], increasing intra-chromatid recombination in the tandem array of ribosomal repeats [36].…”

“…Defects in Smc5-Smc6 cause delayed DNA replication of repetitive sequences, such as rDNA and lead to mitosis before the completion of replication in these regions [41]. It would cause an increase in DNA DSBs during mitosis [35], which could provide potential substrates for GCR.…”

“…The inactivation of the Smc5-Smc6 complex does not activate the DNA damage checkpoint [41]. Nevertheless, we questioned whether lack of checkpoint activity in smc6-9 mutants affects GCR formation.…”

“…The impaired chromosome segregation of repetitive DNA sequences such as rDNA or telomeres was observed by defects in the Smc5-Smc6 complex [35,41]. Some of these defects due to incorrect repair during the G2/M phases likely cause an increase in spontaneous double strand breaks (DSBs) [36].…”

The (elusive) role of the SMC5/6 complex

“…Supporting this view, Smc5/6 mutants enter anaphase before completing DNA replication. 5 Whereas overall DNA replication proceeds normally in NSMCE2-deficient mouse cells, 2 replication might not be fully completed at certain sites, which would generate JMs and hamper segregation. But why would SMC5/6 be needed for only some breaks or replication forks?…”