A sliding scale: The many faces of Ctf7/Eco1 cohesion establishment factor in DNA repair

A baker desired to open a second factory from which to prepare her wares. The issue of course was the revered but single recipe book. She painstakingly reproduced each page and, for safekeeping, clipped together each original and page-copy pair. When the process was complete, each pair was unclipped and then separated to form two recipe books. So it is with chromosomes—the revered instructions of life. Each chromosome is faithfully replicated during S phase to produce two identical sister chromatids which are later segregated during mitosis into the newly-forming daughter cells. To identify chromatids as sisters between S phase and mitosis, replication products are “clipped” together (Fig. 1). In budding yeast, sister chromatid clips (termed cohesins) are composed of Mcd1/Scc1, Irr1/Scc3, Smc1 and Smc3. Higher eukaryotes also require Sororin to maintain pairing, highlighting the complexity of cohesin structure.¹ Further obscuring interpretations are findings that cohesins function in chromosome condensation, DNA replication fork progression, DNA repair and transcription regulation. How separable are these roles? At the heart of cohesion is Establishment—a process by which cohesins are converted to a pairing competent state. Establishment requires the acetyltransferase Ctf7/Eco1 (ESCO1 and ESCO2 in humans) that modifies Smc3 specifically during S phase in unperturbed cells and Mcd1/Scc1 in response to DNA damage during G₂/M.² Ctf7/Eco1 is critical for chromosome condensation, DNA replication fork progression, DNA repair and transcription regulation—providing a target from which to test whether these functions are indeed separable.

Figure 1.

Sister chromatid pairing (cohesion) can occur through Ctf7/Eco1-dependent acetylation of Smc3 during DNA replication (top portion, green) or through Ctf7/Eco1-dependent acetylation of Mcd1/Scc1 in response to DNA damage repair pathways (middle portion, blue). New findings raise an interesting possibility that Reciprocal Crossover (RCO)-based repair through mitotic recombination between sister chromatids (homologous chromosomes not shown) might occur through Ctf7/Eco1-dependent acetylation of an as yet unidentified factor (lower portion, purple).

In a previous issue of Cell Cycle, lead authors Lu and Goering head a collaborative effort from the laboratories of Jennifer Gerton and Sue Jaspersen (Stowers Institute of Medical Research) regarding just this issue.³ These authors explored the effects of several ctf7/eco1 alleles in budding yeast - focusing on a ctf7/eco1^W216G mutation, which they report phenocopies the acteyltransferase-deficient human ESCO2^W539G allele responsible for the developmental malady Roberts Syndrome (RS).⁴ Lu, Goering and colleagues performed in vivo cohesion assays at discrete loci along the chromosome length and report that ctf7/eco1^W216G cells are predominantly sister chromatid-pairing competent but exhibit extreme sensitivity to double-strand breaks induced by X-ray. In part, this study extends a report on Roberts Syndrome patient cells that contain paired sisters but exhibit sensitivity to a subset of genotoxic agents (sister chromatids in RS patient cells exhibit a railroad configuration in which only heterochromatic regions appear unpaired).⁵ A comparison of these cell context sensitivities will be immensely informative.

Two additional findings attest to this separation of Ctf7/Eco1 function. The first concerns RAD61 (WAPL in humans). Rad61 is an antiestablishment factor in that RAD61 deletion bypasses the need for Ctf7/Eco1 in cohesion establishment.² However, RAD61 deletion fails to suppress X-ray sensitivity in ctf7/eco1^W216G cells—highlight diverse roles for Rad61 and Ctf7/Eco1 in both cohesion and DNA damage.³ The second finding concerns targets: neither Smc3 nor Mcd1/Scc1 over-expression rescued ctf7/eco1^W216G cell sensitivity to double strand breaks, despite numerous findings attesting to the identity of these Ctf7/Eco1 targets.⁶ While several alternate explanations are certainly plausible, these findings raise the possibility that neither is the target of interest under the conditions tested.

A molecular explanation may lie in dosage. In a wonderful convergence of studies, Heidinger-Pauli and colleagues found a continuum of function depending on cohesin dosage. Adapting a clever tRNA nonsense suppressor system to eukarotic cells, these researchers reported that DNA repair (and chromosome condensation) appear exquisitely sensitive to even moderate decreases in cohesin levels while sister chromatid pairing appears unperturbed—the latter requiring a dramatic reduction in cohesin levels.⁷ This dosage dependency for cohesins likely parallels ever-decreasing Ctf7/Eco1 function in which DNA damage capabilities are the first to go.^3,7

Double-strand breaks can be resolved by homologous recombination (HR): in mitotic cells the template being the sister chromatid. In this fashion, heterozygosity is maintained and haplo-insufficiency averted. Lu, Goering and colleagues found reduced levels of reciprocal crossovers in ctf7/eco1^W216G cells, suggesting that this mutation reduces sister exchanges, which might then lead to repair mechanisms that promote loss of heterozygosity. A similar outcome was reported by Covo and colleagues, except in that study the role of Mcd1/Scc1 was found to be a critical factor in promoting sister chromatid exchange and maintaining heterozygosity.⁸ In combination, these studies provide important insights into mechanisms of cancer progression, genome instability and even developmental maladies.