Spontaneous slow replication fork progression elicits mitosis alterations in homologous recombination-deficient mammalian cells

Abstract

Homologous recombination deficient (HR~) mammalian cells spontaneously display reduced replication fork (RF) movement and mi-totic extra centrosomes. We show here that these cells presenta complex mitotic phenotype, including prolonged metaphase arrest, anaphase bridges, and multipolar segregations. We thenasked whether the replication and the mitotic phenotypes are interdependent. First, we determined low doses of hydroxyureathat did not affect the cell cycle distribution or activate CHK1phosphorylation but did slow the replication fork movement ofwild-type cells to the same level than in HR- cells. Remarkably,these low hydroxyurea doses generated the same mitotic defects(and to the same extent) in wild-type cells as observed in unchallenged HR- cells. Reciprocally, supplying nucleotide precursors toHR- cells suppressed both their replication deceleration and mi-totic extra centrosome phenotypes. Therefore, subtle replicationstress that escapes to surveillance pathways and, thus, fails toprevent cells from entering mitosis alters metaphase progression and centrosome number, resulting in multipolar mitosis.Importantly, multipolar mitosis results in global unbalancedchromosome segregation involving the whole genome, evenfully replicated chromosomes. These data highlight the crosstalk between chromosome replication and segregation, andthe importance of HR at the interface of these two processesfor protection against general genome instability.