The TFIIH complex is required to establish and maintain mitotic chromosome structure

RNAP II antagonizes mitotic chromatin folding and chromosome segregation by condensin

Condensin shapes mitotic chromosomes by folding chromatin into loops, but whether it does so by DNA-loop extrusion remains speculative. Although loop-extruding cohesin is stalled by transcription, the impact of transcription on condensin, which is enriched at highly expressed genes in many species, remains unclear. Using degrons of Rpb1 or the torpedo nuclease Dhp1XRN2 to either deplete or displace RNAPII on chromatin in fission yeast metaphase cells, we show that RNAPII does not load condensin on DNA. Instead, RNAPII retains condensin in cis and hinders its ability to fold mitotic chromatin and to support chromosome segregation, consistent with the stalling of a loop extruder. Transcription termination by Dhp1 limits such a hindrance. Our results shed light on the integrated functioning of condensin, and we argue that a tight control of transcription underlies mitotic chromosome assembly by loop-extruding condensin.

Release of Histone H3K4-reading transcription factors from chromosomes in mitosis is independent of adjacent H3 phosphorylation

Histone modifications influence the recruitment of reader proteins to chromosomes to regulate events including transcription and cell division. The idea of a histone code, where combinations of modifications specify unique downstream functions, is widely accepted and can be demonstrated in vitro. For example, on synthetic peptides, phosphorylation of Histone H3 at threonine-3 (H3T3ph) prevents the binding of reader proteins that recognize trimethylation of the adjacent lysine-4 (H3K4me3), including the TAF3 component of TFIID. To study these combinatorial effects in cells, we analyzed the genome-wide distribution of H3T3ph and H3K4me2/3 during mitosis. We find that H3T3ph anti-correlates with adjacent H3K4me2/3 in cells, and that the PHD domain of TAF3 can bind H3K4me2/3 in isolated mitotic chromatin despite the presence of H3T3ph. Unlike in vitro, H3K4 readers are still displaced from chromosomes in mitosis in Haspin-depleted cells lacking H3T3ph. H3T3ph is therefore unlikely to be responsible for transcriptional downregulation during cell division.

Condensin dysfunction is a reproductive isolating barrier in mice

Reproductive isolation occurs when the genomes of two populations accumulate genetic incompatibilities that prevent interbreeding1,2. Understanding of hybrid incompatibility at the cell biology level is limited, particularly in the case of hybrid female sterility3. Here we find that species divergence in condensin regulation and centromere organization between two mouse species, Mus musculus domesticus and Mus spretus, drives chromosome decondensation and mis-segregation in their F1 hybrid oocytes, reducing female fertility. The decondensation in hybrid oocytes was especially prominent at pericentromeric major satellites, which are highly abundant at M. m. domesticus centromeres4-6, leading to species-specific chromosome mis-segregation and egg aneuploidy. Consistent with the condensation defects, a chromosome structure protein complex, condensin II7,8, was reduced on hybrid oocyte chromosomes. We find that the condensin II subunit NCAPG2 was specifically reduced in the nucleus in prophase and that overexpressing NCAPG2 rescued both the decondensation and egg aneuploidy phenotypes. In addition to the overall reduction in condensin II on chromosomes, major satellites further reduced condensin II levels locally, explaining why this region is particularly prone to decondensation. Together, this study provides cell biological insights into hybrid incompatibility in female meiosis and demonstrates that condensin misregulation and pericentromeric satellite expansion can establish a reproductive isolating barrier in mammals.

Regulation of the mitotic chromosome folding machines

Over the last several years enormous progress has been made in identifying the molecular machines, including condensins and topoisomerases that fold mitotic chromosomes. The discovery that condensins generate chromatin loops through loop extrusion has revolutionized, and energized, the field of chromosome folding. To understand how these machines fold chromosomes with the appropriate dimensions, while disentangling sister chromatids, it needs to be determined how they are regulated and deployed. Here, we outline the current understanding of how these machines and factors are regulated through cell cycle dependent expression, chromatin localization, activation and inactivation through post-translational modifications, and through associations with each other, with other factors and with the chromatin template itself. There are still many open questions about how condensins and topoisomerases are regulated but given the pace of progress in the chromosome folding field, it seems likely that many of these will be answered in the years ahead.

TRF1 uses a noncanonical function of TFIIH to promote telomere replication

Telomeric DNA challenges the replisome and requires TRF1 for efficient duplication. TRF1 recruits the BLM helicase, but BLM loss does not explain the extensive telomere fragility, ATR signaling, and sister telomere associations (STAs) induced by TRF1 deletion. Here, we document that Helix2 of the TRFH domain and Helix1 of the Myb domain of TRF1 are required for efficient telomere replication. Mutation of both helices generated a TRF1 separation-of-function mutant (TRF1-E83K/LW-TI) that induced severe telomere replication defects but no ATR signaling or STAs. We identified the transcription and nucleotide excision repair (NER) factor TFIIH as a critical effector of TRF1. Loss of TFIIH subunits, but no other NER factors, caused the same telomere replication phenotypes as the TRF1-E83K/LW-TI mutant independent of the effects on TRF1 expression. TFIIH subunits coimmunoprecipitated with wild-type TRF1 but not with TRF1-E83K/LW-TI. These results establish that the major mechanism by which TRF1 ensures telomere replication involves a noncanonical function of TFIIH.

Phosphorylation of XPD drives its mitotic role independently of its DNA repair and transcription functions

The helicase XPD is known as a key subunit of the DNA repair/transcription factor TFIIH. However, here, we report that XPD, independently to other TFIIH subunits, can localize with the motor kinesin Eg5 to mitotic spindles and the midbodies of human cells. The XPD/Eg5 partnership is promoted upon phosphorylation of Eg5/T926 by the kinase CDK1, and conversely, it is reduced once Eg5/S1033 is phosphorylated by NEK6, a mitotic kinase that also targets XPD at T425. The phosphorylation of XPD does not affect its DNA repair and transcription functions, but it is required for Eg5 localization, checkpoint activation, and chromosome segregation in mitosis. In XPD-mutated cells derived from a patient with xeroderma pigmentosum, the phosphomimetic form XPD/T425D or even the nonphosphorylatable form Eg5/S1033A specifically restores mitotic chromosome segregation errors. These results thus highlight the phospho-dependent mitotic function of XPD and reveal how mitotic defects might contribute to XPD-related disorders.

Transcription factor Sp1 regulates mitotic chromosome assembly and segregation

Aneuploidy is a pervasive feature of cancer cells that results from chromosome missegregation. Several transcription factors have been associated with aneuploidy; however, no studies to date have demonstrated that mammalian transcription factors directly regulate chromosome segregation during mitosis. Here, we demonstrate that the ubiquitously expressed transcription factor specificity protein 1 (Sp1), which we have previously linked to aneuploidy, has a mitosis-specific role regulating chromosome segregation. We find that Sp1 localizes to mitotic centromeres and auxin-induced rapid Sp1 degradation at mitotic onset results in chromosome segregation errors and aberrant mitotic progression. Furthermore, rapid Sp1 degradation results in anomalous mitotic chromosome assembly characterized by loss of condensin complex I localization to mitotic chromosomes and chromosome condensation defects. Consistent with these defects, Sp1 degradation results in reduced chromosome passenger complex activity and histone H3 serine 10 phosphorylation during mitosis, which is essential for condensin complex I recruitment and chromosome condensation. Together, these data provide the first evidence of a mammalian transcription factor acting specifically during mitosis to regulate chromosome segregation.

Making Mitotic Chromosomes in a Test Tube

Mitotic chromosome assembly is an essential preparatory step for accurate transmission of the genome during cell division. During the past decades, biochemical approaches have uncovered the molecular basis of mitotic chromosomes. For example, by using cell-free assays of frog egg extracts, the condensin I complex central for the chromosome assembly process was first identified, and its functions have been intensively studied. A list of chromosome-associated proteins has been almost completed, and it is now possible to reconstitute structures resembling mitotic chromosomes with a limited number of purified factors. In this review, I introduce how far we have come in understanding the mechanism of chromosome assembly using cell-free assays and reconstitution assays, and I discuss their potential applications to solve open questions.