Aberrant chromosome morphology in human cells defective for Holliday junction resolution

Abstract

Exchange of sister chromatids to form four-stranded Holliday junctions occurs naturally during meiosis, to hold sister chromatids together, and during various repair events. In eukaryotes, double Holliday junctions that escape dissolution by a helicase–topoisomerase (BTR) complex are instead processed by one of several nucleases, known as resolvases. In this study, Stephen West and colleagues define the activities of the GEN1, MUS81–EME1 and SLX1–SLX4 resolvases in the absence of BLM, the helicase component of BTR that is mutated in Bloom's syndrome. The use of these alternatives may come at a price, however, because Bloom's syndrome cells exhibit genomic instability and patients experience a broad spectrum of early-onset cancers. Exchange of sister chromatids to form four-stranded Holliday junctions occurs naturally during meiosis, to hold sister chromatids together, and during various repair events. In eukaryotes, double Holliday junctions that escape dissolution by a helicase/topoisomerase (BTR) complex are instead processed by one of several nucleases known as resolvases. This study defines the activities of the GEN1, MUS81-EME1 and SLX1-SLX4 resolvases in the absence of BLM, the helicase component of BTR that is mutated in Bloom's syndrome. In somatic cells, Holliday junctions can be formed between sister chromatids during the recombinational repair of DNA breaks or after replication fork demise. A variety of processes act upon Holliday junctions to remove them from DNA, in events that are critical for proper chromosome segregation. In human cells, the BLM protein, inactivated in individuals with Bloom’s syndrome, acts in combination with topoisomerase IIIα, RMI1 and RMI2 (BTR complex) to promote the dissolution of double Holliday junctions1,2. Cells defective for BLM exhibit elevated levels of sister chromatid exchanges (SCEs) and patients with Bloom’s syndrome develop a broad spectrum of early-onset cancers caused by chromosome instability3. MUS81–EME1 (refs 4–7), SLX1–SLX4 (refs 8–11) and GEN1 (refs 12, 13) also process Holliday junctions but, in contrast to the BTR complex, do so by endonucleolytic cleavage. Here we deplete these nucleases from Bloom’s syndrome cells to analyse human cells compromised for the known Holliday junction dissolution/resolution pathways. We show that depletion of MUS81 and GEN1, or SLX4 and GEN1, from Bloom’s syndrome cells results in severe chromosome abnormalities, such that sister chromatids remain interlinked in a side-by-side arrangement and the chromosomes are elongated and segmented. Our results indicate that normally replicating human cells require Holliday junction processing activities to prevent sister chromatid entanglements and thereby ensure accurate chromosome condensation. This phenotype was not apparent when both MUS81 and SLX4 were depleted from Bloom’s syndrome cells, suggesting that GEN1 can compensate for their absence. Additionally, we show that depletion of MUS81 or SLX4 reduces the high frequency of SCEs in Bloom’s syndrome cells, indicating that MUS81 and SLX4 promote SCE formation, in events that may ultimately drive the chromosome instabilities that underpin early-onset cancers associated with Bloom’s syndrome.