Hop1 and the meiotic DNA-damage response

Checkpoint control in meiotic prophase: Idiosyncratic demands require unique characteristics

Chromosomal transactions such as replication, recombination and segregation are monitored by cell cycle checkpoint cascades. These checkpoints ensure the proper execution of processes that are needed for faithful genome inheritance from one cell to the next, and across generations. In meiotic prophase, a specialized checkpoint monitors defining events of meiosis: programmed DNA break formation, followed by dedicated repair through recombination based on interhomolog (IH) crossovers. This checkpoint shares molecular characteristics with canonical DNA damage checkpoints active during somatic cell cycles. However, idiosyncratic requirements of meiotic prophase have introduced unique features in this signaling cascade. In this review, we discuss the unique features of the meiotic prophase checkpoint. While being related to canonical DNA damage checkpoint cascades, the meiotic prophase checkpoint also shows similarities with the spindle assembly checkpoint (SAC) that guards chromosome segregation. We highlight these emerging similarities in the signaling logic of the checkpoints that govern meiotic prophase and chromosome segregation, and how thinking of these similarities can help us better understand meiotic prophase control. We also discuss work showing that, when aberrantly expressed, components of the meiotic prophase checkpoint might alter DNA repair fidelity and chromosome segregation in cancer cells. Considering checkpoint function in light of demands imposed by the special characteristics of meiotic prophase helps us understand checkpoint integration into the meiotic cell cycle machinery.

A chromatin-associated protein required for inducing and limiting meiotic DNA double-strand break formation

Programmed DNA double-strand breaks (DSBs) are required for meiotic recombination, but the number is strictly controlled because they are potentially harmful. Here we report a novel protein, Pars11, which is required for Spo11-dependent DSB formation in the protist Tetrahymena. Pars11 localizes to chromatin early in meiotic prophase in a Spo11-independent manner and is removed before the end of prophase. Pars11 removal depends on DSB formation and ATR-dependent phosphorylation. In the absence of the DNA damage sensor kinase ATR, Pars11 is retained on chromatin and excess DSBs are generated. Similar levels of Pars11 persistence and DSB overproduction occur in a non-phosphorylatable pars11 mutant. We conclude that Pars11 supports DSB formation by Spo11 until enough DSBs are formed; thereafter, DSB production stops in response to ATR-dependent degradation of Pars11 or its removal from chromatin. A similar DSB control mechanism involving a Rec114-Tel1/ATM-dependent negative feedback loop regulates DSB formation in budding yeast. However, there is no detectable sequence homology between Pars11 and Rec114, and DSB numbers are more tightly controlled by Pars11 than by Rec114. The discovery of this mechanism for DSB regulation in the evolutionarily distant protist and fungal lineages suggests that it is conserved across eukaryotes.

The meiotic checkpoint network: step-by-step through meiotic prophase

The generation of haploid gametes by meiosis is a highly conserved process for sexually reproducing organisms that, in almost all cases, involves the extensive breakage of chromosomes. These chromosome breaks occur during meiotic prophase and are essential for meiotic recombination as well as the subsequent segregation of homologous chromosomes. However, their formation and repair must be carefully monitored and choreographed with nuclear dynamics and the cell division program to avoid the creation of aberrant chromosomes and defective gametes. It is becoming increasingly clear that an intricate checkpoint-signaling network related to the canonical DNA damage response is deeply interwoven with the meiotic program and preserves order during meiotic prophase. This meiotic checkpoint network (MCN) creates a wide range of dependent relationships controlling chromosome movement, chromosome pairing, chromatin structure, and double-strand break (DSB) repair. In this review, we summarize our current understanding of the MCN. We discuss commonalities and differences in different experimental systems, with a particular emphasis on the emerging design principles that control and limit cross talk between signals to ultimately ensure the faithful inheritance of chromosomes by the next generation.

Homeostatic regulation of meiotic DSB formation by ATM/ATR

Ataxia-telangiectasia mutated (ATM) and RAD3-related (ATR) are widely known as being central players in the mitotic DNA damage response (DDR), mounting responses to DNA double-strand breaks (DSBs) and single-stranded DNA (ssDNA) respectively. The DDR signalling cascade couples cell cycle control to damage-sensing and repair processes in order to prevent untimely cell cycle progression while damage still persists [1]. Both ATM/ATR are, however, also emerging as essential factors in the process of meiosis; a specialised cell cycle programme responsible for the formation of haploid gametes via two sequential nuclear divisions. Central to achieving accurate meiotic chromosome segregation is the introduction of numerous DSBs spread across the genome by the evolutionarily conserved enzyme, Spo11. This review seeks to explore and address how cells utilise ATM/ATR pathways to regulate Spo11-DSB formation, establish DSB homeostasis and ensure meiosis is completed unperturbed.

Dot1-dependent histone H3K79 methylation promotes activation of the Mek1 meiotic checkpoint effector kinase by regulating the Hop1 adaptor

During meiosis, accurate chromosome segregation relies on the proper interaction between homologous chromosomes, including synapsis and recombination. The meiotic recombination checkpoint is a quality control mechanism that monitors those crucial events. In response to defects in synapsis and/or recombination, this checkpoint blocks or delays progression of meiosis, preventing the formation of aberrant gametes. Meiotic recombination occurs in the context of chromatin and histone modifications, which play crucial roles in the maintenance of genomic integrity. Here, we unveil the role of Dot1-dependent histone H3 methylation at lysine 79 (H3K79me) in this meiotic surveillance mechanism. We demonstrate that the meiotic checkpoint function of Dot1 relies on H3K79me because, like the dot1 deletion, H3-K79A or H3-K79R mutations suppress the checkpoint-imposed meiotic delay of a synapsis-defective zip1 mutant. Moreover, by genetically manipulating Dot1 catalytic activity, we find that the status of H3K79me modulates the meiotic checkpoint response. We also define the phosphorylation events involving activation of the meiotic checkpoint effector Mek1 kinase. Dot1 is required for Mek1 autophosphorylation, but not for its Mec1/Tel1-dependent phosphorylation. Dot1-dependent H3K79me also promotes Hop1 activation and its proper distribution along zip1 meiotic chromosomes, at least in part, by regulating Pch2 localization. Furthermore, HOP1 overexpression bypasses the Dot1 requirement for checkpoint activation. We propose that chromatin remodeling resulting from unrepaired meiotic DSBs and/or faulty interhomolog interactions allows Dot1-mediated H3K79-me to exclude Pch2 from the chromosomes, thus driving localization of Hop1 along chromosome axes and enabling Mek1 full activation to trigger downstream responses, such as meiotic arrest.

In sexually reproducing organisms, meiosis divides the number of chromosomes by half to generate gametes. Meiosis involves a series of interactions between maternal and paternal chromosomes leading to the exchange of genetic material by recombination. Completion of these processes is required for accurate distribution of chromosomes to the gametes. Meiotic cells possess quality-control mechanisms (checkpoints) to monitor those critical events. When failures occur, the checkpoint blocks meiotic progression to prevent the formation of aneuploid gametes. Genetic information is packaged into chromatin; histone modifications regulate multiple aspects of DNA metabolism to maintain genomic integrity. Dot1 is a conserved methyltransferase, responsible for histone H3 methylation at lysine 79, that is required for the meiotic recombination checkpoint. Here we decipher the molecular mechanism underlying Dot1 meiotic checkpoint function. We show that Dot1 catalytic activity correlates with the strength of the checkpoint response. By regulating Pch2 chromatin distribution, Dot1 controls localization of the chromosome axial component Hop1, which, in turn, contributes to activation of Mek1, the major effector kinase of the checkpoint. Our findings suggest that, in response to meiotic defects, the chromatin environment created by a constitutive histone mark orchestrates distribution of structural components of the chromosomes supporting activation of the meiotic checkpoint.

MU2 and HP1a regulate the recognition of double strand breaks in Drosophila melanogaster

Chromatin structure regulates the dynamics of the recognition and repair of DNA double strand breaks; open chromatin enhances the recruitment of DNA damage response factors, while compact chromatin is refractory to the assembly of radiation-induced repair foci. MU2, an orthologue of human MDC1, a scaffold for ionizing radiation-induced repair foci, is a widely distributed chromosomal protein in Drosophila melanogaster that moves to DNA repair foci after irradiation. Here we show using yeast 2 hybrid screens and co-immunoprecipitation that MU2 binds the chromoshadow domain of the heterochromatin protein HP1 in untreated cells. We asked what role HP1 plays in the formation of repair foci and cell cycle control in response to DNA damage. After irradiation repair foci form in heterochromatin but are shunted to the edge of heterochromatic regions an HP1-dependent manner, suggesting compartmentalized repair. Hydroxyurea-induced repair foci that form at collapsed replication forks, however, remain in the heterochromatic compartment. HP1a depletion in irradiated imaginal disc cells increases apoptosis and disrupts G2/M arrest. Further, cells irradiated in mitosis produced more and brighter repair foci than to cells irradiated during interphase. Thus, the interplay between MU2 and HP1a is dynamic and may be different in euchromatin and heterochromatin during DNA break recognition and repair.

Tetrahymena meiotic nuclear reorganization is induced by a checkpoint kinase-dependent response to DNA damage

In the ciliate Tetrahymena, meiotic micronuclei (MICs) undergo extreme elongation, and meiotic pairing and recombination take place within these elongated nuclei (the "crescents"). We have previously shown that elongation does not occur in the absence of Spo11p-induced DNA double-strand breaks (DSBs). Here we show that elongation is restored in spo11Delta mutants by various DNA-damaging agents including ones that may not cause DSBs to a notable extent. MIC elongation following Spo11p-induced DSBs or artificially induced DNA lesions is probably a DNA-damage response mediated by a phosphokinase signal transduction pathway, since it is suppressed by the ATM/ATR kinase inhibitors caffeine and wortmannin and by knocking out Tetrahymena's ATR orthologue. MIC elongation occurs concomitantly with the movement of centromeres away from the telomeric pole of the MIC. This DNA damage-dependent reorganization of the MIC helps to arrange homologous chromosomes alongside each other but is not sufficient for exact pairing. Thus, Spo11p contributes to bivalent formation in two ways: by creating a favorable spatial disposition of homologues and by stabilizing pairing by crossovers. The polarized chromosome orientation inside the crescent resembles the conserved meiotic bouquet, and crescent and bouquet also share the putative function of aiding meiotic pairing. However, they are regulated differently because in Tetrahymena, DSBs are required for entering rather than exiting this stage.