Luspatercept stimulates erythropoiesis, increases iron utilization, and redistributes body iron in transfusion-dependent thalassemia.

Luspatercept, a ligand-trapping fusion protein, binds select TGF-ß superfamily ligands implicated in thalassemic erythropoiesis, promoting late-stage erythroid maturation. Luspatercept reduced transfusion burden in the BELIEVE trial (NCT02604433) of 336 adults with transfusion-dependent thalassemia (TDT). Analysis of biomarkers in BELIEVE offers novel physiological and clinical insights into benefits offered by luspatercept. Transfusion iron loading rates decreased 20% by 1.4 g (~7 blood units; median iron loading rate difference: -0.05 ± 0.07 mg Fe/kg/day, p< .0001) and serum ferritin (s-ferritin) decreased 19.2% by 269.3 ± 963.7 µg/L (p < .0001), indicating reduced macrophage iron. However, liver iron content (LIC) did not decrease but showed statistically nonsignificant increases from 5.3 to 6.7 mg/g dw. Erythropoietin, growth differentiation factor 15, soluble transferrin receptor 1 (sTfR1), and reticulocytes rose by 93%, 59%, 66%, and 112%, respectively; accordingly, erythroferrone increased by 51% and hepcidin decreased by 53% (all p < .0001). Decreased transfusion with luspatercept in patients with TDT was associated with increased erythropoietic markers and decreasing hepcidin. Furthermore, s-ferritin reduction associated with increased erythroid iron incorporation (marked by sTfR1) allowed increased erythrocyte marrow output, consequently reducing transfusion needs and enhancing rerouting of hemolysis (heme) iron and non-transferrin-bound iron to the liver. LIC increased in patients with intact spleens, consistent with iron redistribution given the hepcidin reduction. Thus, erythropoietic and hepcidin changes with luspatercept in TDT lower transfusion dependency and may redistribute iron from macrophages to hepatocytes, necessitating the use of concomitant chelator cover for effective iron management.

Cabozantinib in combination with atezolizumab in patients with metastatic castration-resistant prostate cancer: results from an expansion cohort of a multicentre, open-label, phase 1b trial (COSMIC-021).

BACKGROUND: Patients with metastatic castration-resistant prostate cancer have few treatment options after novel hormonal therapy (eg, abiraterone or enzalutamide). We aimed to evaluate cabozantinib, a tyrosine kinase inhibitor with immunomodulatory properties, in combination with the PD-L1 inhibitor atezolizumab in metastatic castration-resistant prostate cancer. METHODS: COSMIC-021 is an ongoing, multicentre, open-label, phase 1b study with a dose-escalation stage followed by tumour-specific expansion stages. Expansion cohort 6 in metastatic castration-resistant prostate cancer was enrolled at 42 cancer research centres in France, Italy, the Netherlands, Spain, and the USA. Eligible patients were aged 18 years or older and had metastatic castration-resistant prostate cancer with radiographic soft tissue progression following treatment with either enzalutamide or abiraterone, or both; measurable soft tissue disease per Response Evaluation Criteria In Solid Tumours (RECIST) version 1.1; and an Eastern Cooperative Oncology Group performance status of 0 or 1. Patients received oral cabozantinib 40 mg per day and intravenous atezolizumab 1200 mg once every 3 weeks. Study treatment continued until progressive disease or unacceptable toxicity. All enrolled patients were assessed for efficacy and safety. The primary endpoint was objective response rate per RECIST version 1.1 as assessed by the investigator. This study is registered with ClinicalTrials.gov, NCT03170960. FINDINGS: Between April 24, 2018, and Aug 31, 2020, 132 patients were enrolled and received at least one dose of study treatment. At data cutoff (Feb 19, 2021), median duration of follow-up was 15·2 months (IQR 9·6-21·7). Objective response rate was 23% (95% CI 17-32; 31 of 132 patients), with three (2%) confirmed complete responses and 28 (21%) confirmed partial responses. 72 (55%) of 132 patients had grade 3-4 treatment-related adverse events, with the most common being pulmonary embolism (11 [8%] patients), diarrhoea (nine [7%]), fatigue (nine [7%]), and hypertension (nine [7%]). There was one grade 5 treatment-related adverse event (dehydration). 74 (56%) of 132 patients had serious adverse events of any causality. 28 (21%) of 132 patients had treatment-related adverse events leading to discontinuation of either study drug. INTERPRETATION: Cabozantinib plus atezolizumab showed promising antitumour activity in patients with metastatic castration-resistant prostate cancer after novel hormonal therapy with an acceptable safety profile, supporting further evaluation of this combination. FUNDING: Exelixis.

Deubiquitinase USP9X stabilizes MCL1 and promotes tumour cell survival.

MCL1 is essential for the survival of stem and progenitor cells of multiple lineages, and is unique among pro-survival BCL2 family members in that it is rapidly turned over through the action of ubiquitin ligases. B- and mantle-cell lymphomas, chronic myeloid leukaemia, and multiple myeloma, however, express abnormally high levels of MCL1, contributing to chemoresistance and disease relapse. The mechanism of MCL1 overexpression in cancer is not well understood. Here we show that the deubiquitinase USP9X stabilizes MCL1 and thereby promotes cell survival. USP9X binds MCL1 and removes the Lys 48-linked polyubiquitin chains that normally mark MCL1 for proteasomal degradation. Increased USP9X expression correlates with increased MCL1 protein in human follicular lymphomas and diffuse large B-cell lymphomas. Moreover, patients with multiple myeloma overexpressing USP9X have a poor prognosis. Knockdown of USP9X increases MCL1 polyubiquitination, which enhances MCL1 turnover and cell killing by the BH3 mimetic ABT-737. These results identify USP9X as a prognostic and therapeutic target, and they show that deubiquitinases may stabilize labile oncoproteins in human malignancies.

A whole-genome RNAi screen identifies an 8q22 gene cluster that inhibits death receptor-mediated apoptosis.

Deregulation of apoptosis is a common occurrence in cancer, for which emerging oncology therapeutic agents designed to engage this pathway are undergoing clinical trials. With the aim of uncovering strategies to activate apoptosis in cancer cells, we used a pooled shRNA screen to interrogate death receptor signaling. This screening approach identified 16 genes that modulate the sensitivity to ligand induced apoptosis, with several genes exhibiting frequent overexpression and/or copy number gain in cancer. Interestingly, two of the top hits, EDD1 and GRHL2, are found 50 kb apart on chromosome 8q22, a region that is frequently amplified in many cancers. By using a series of silencing and overexpression studies, we show that EDD1 and GRHL2 suppress death-receptor expression, and that EDD1 expression is elevated in breast, pancreas, and lung cancer cell lines resistant to death receptor-mediated apoptosis. Supporting the relevance of EDD1 and GRHL2 as therapeutic candidates to engage apoptosis in cancer cells, silencing the expression of either gene sensitizes 8q22-amplified breast cancer cell lines to death receptor induced apoptosis. Our findings highlight a mechanism by which cancer cells may evade apoptosis, and therefore provide insight in the search for new targets and functional biomarkers for this pathway.

Cabozantinib Plus Atezolizumab or Cabozantinib Alone in Patients With Advanced NSCLC Previously Treated With an Immune Checkpoint Inhibitor: Results From the Phase 1b COSMIC-021 Study.

Introduction: We evaluated efficacy and safety of cabozantinib plus atezolizumab or cabozantinib alone in advanced NSCLC previously treated with an immune checkpoint inhibitor (ICI). Methods: COSMIC-021 (NCT03170960) is a phase 1b, multicenter study in advanced solid tumors. This analysis included patients with stage IV non-squamous NSCLC without actionable genomic aberrations in EGFR, ALK, ROS1, or BRAF-V600E who progressed on one prior ICI and less than or equal to two prior lines of systemic anticancer therapy. Patients received cabozantinib 40 mg orally/day plus atezolizumab 1200 mg intravenously every three weeks (combination cohort) or cabozantinib 60 mg orally/day (single-agent cabozantinib cohort). Primary end point of the combination cohort was objective response rate per Response Evaluation Criteria in Solid Tumors v1.1 by investigator. Outcomes in the single-agent cabozantinib cohort were exploratory. Results: Eighty-one patients assigned to combination therapy and 31 assigned to single-agent cabozantinib received greater than or equal to one dose of study treatment. Median (range) follow-up was 26.1 months (12.1-44.2) and 22.4 months (1.5-29.0), respectively. Objective response rate was 20% (95% confidence interval: 11.7%-30.1%) in combination cohort and 6% (95% confidence interval: 0.8%-21.4%) in single-agent cabozantinib cohort. Treatment-related adverse events (TRAEs) occurred in 86% of patients in the combination cohort and 90% in the single-agent cabozantinib cohort; grade 3/4 TRAEs were 44% and 48%, respectively. There were two grade 5 TRAEs: pneumonitis (n = 1, combination) and gastric ulcer hemorrhage (n = 1, single-agent). Neither PD-L1 expression in tumor cells nor tumor mutation burden correlated with outcomes. Conclusions: Cabozantinib plus atezolizumab demonstrated modest clinical activity and manageable toxicity in advanced NSCLC after progression on prior ICI.

Proteasome subunit Rpn1 binds ubiquitin-like protein domains.

The yeast protein Rad23 belongs to a diverse family of proteins that contain an amino-terminal ubiquitin-like (UBL) domain. This domain mediates the binding of Rad23 to proteasomes, which in turn promotes DNA repair and modulates protein degradation, possibly by delivering ubiquitinylated cargo to proteasomes. Here we show that Rad23 binds proteasomes by directly interacting with the base subcomplex of the regulatory particle of the proteasome. A component of the base, Rpn1, specifically recognizes the UBL domain of Rad23 through its leucine-rich-repeat-like (LRR-like) domain. A second UBL protein, Dsk2, competes with Rad23 for proteasome binding, which suggests that the LRR-like domain of Rpn1 may participate in the recognition of several ligands of the proteasome. We propose that the LRR domain of Rpn1 may be positioned in the base to allow the cargo proteins carried by Rad23 to be presented to the proteasomal ATPases for unfolding. We also report that, contrary to expectation, the base subunit Rpn10 does not mediate the binding of UBL proteins to the proteasome in yeast, although it can apparently contribute to the binding of ubiquitin chains by intact proteasomes.

Combination of azacitidine and enasidenib enhances leukemic cell differentiation and cooperatively hypomethylates DNA.

Azacitidine and enasidenib are two therapies available for treatment of acute myelogenous leukemia (AML), and the mechanisms of action of these drugs involve alteration of aberrant DNA methylation. We hypothesized that a combination of these agents could have interactive effects on DNA methylation and enhance differentiation in mIDH2 cells. Combination treatment enhanced cellular differentiation in TF-1 cells overexpressing IDJ2R140Q through increased hemoglobinization and increased hemoglobin γ RNA expression compared with the effects of single agents. Furthermore, in primary AML samples (IDH2R140Q or R172K), combination treatment reduced CD34+ cells and increased CD15+ cells to a greater extent than attained with single agents. To explore the mechanism of enhanced differentiation with combination treatment, the TF-1 epigenome was analyzed by profiling 5-hydroxymethylcytosine (5hmC) and 5-methylcytosine (5mC) DNA methylation changes. Enasidenib treatment alone increased 5hmC, consistent with reactivation of ten-eleven-translocation (TET) enzyme activity. Compared with treatment with azacitidine alone, combination treatment reduced 5mC levels at greater numbers of sites and these loci were significantly enriched in regions with increased 5hMC (25.8% vs. 7.4%). Results are consistent with a model in which enasidenib-mediated reactivation of ten-eleven-translocation enzymes cooperates with azacitidine-mediated inhibition of DNA methyltransferase enzymes, leading to greater reductions in DNA methylation and enhanced erythroid differentiation.

Swm1/Apc13 is an evolutionarily conserved subunit of the anaphase-promoting complex stabilizing the association of Cdc16 and Cdc27.

The anaphase-promoting complex (APC/C) is a large ubiquitin-protein ligase which controls progression through anaphase by triggering the degradation of cell cycle regulators such as securin and B-type cyclins. The APC/C is an unusually complex ligase containing at least 10 different, evolutionarily conserved components. In contrast to APC/C's role in cell cycle regulation little is known about the functions of individual subunits and how they might interact with each other. Here, we have analyzed Swm1/Apc13, a small subunit recently identified in the budding yeast complex. Database searches revealed proteins related to Swm1/Apc13 in various organisms including humans. Both the human and the fission yeast homologues are associated with APC/C subunits, and they complement the phenotype of an SWM1 deletion mutant of budding yeast. Swm1/Apc13 promotes the stable association with the APC/C of the essential subunits Cdc16 and Cdc27. Accordingly, Swm1/Apc13 is required for ubiquitin ligase activity in vitro and for the timely execution of APC/C-dependent cell cycle events in vivo.

Current Trends in Ligand Binding Real-Time Measurement Technologies.

Numerous advances in ligand binding assay (LBA) real-time measurement technologies have been made within the last several years, ranging from the development of novel platforms to drive technology expansion to the adaptation of existing platforms to optimize performance and throughput. In this review, we have chosen to focus on technologies that provide increased value to two distinct segments of the LBA community. First, experimentally, by measuring real-time binding events, these technologies provide data that can be used to interrogate receptor/ligand binding interactions. While overall the platforms are not new, they have made significant advances in throughput, multiplexing, and/or sensitivity. Second, clinically, these point-of-care (POC) technologies provide instantaneous information which facilitates rapid treatment decisions.

MEDI1873, a potent, stabilized hexameric agonist of human GITR with regulatory T-cell targeting potential.

Glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) is part of a system of signals involved in controlling T-cell activation. Targeting and agonizing GITR in mice promotes antitumor immunity by enhancing the function of effector T cells and inhibiting regulatory T cells. Here, we describe MEDI1873, a novel hexameric human GITR agonist comprising an IgG1 Fc domain, a coronin 1A trimerization domain and the human GITRL extracellular domain (ECD). MEDI1873 was optimized through systematic testing of different trimerization domains, aglycosylation of the GITRL ECD and comparison of different Fc isotypes. MEDI1873 exhibits oligomeric heterogeneity and superiority to an anti-GITR antibody with respect to evoking robust GITR agonism, T-cell activation and clustering of Fc gamma receptors. Further, it recapitulates, in vitro, several aspects of GITR targeting described in mice, including modulation of regulatory T-cell suppression and the ability to increase the CD8+:CD4+ T-cell ratio via antibody-dependent T-cell cytotoxicity. To support translation into a therapeutic setting, we demonstrate that MEDI1873 is a potent T-cell agonist in vivo in non-human primates, inducing marked enhancement of humoral and T-cell proliferative responses against protein antigen, and demonstrate the presence of GITR- and FoxP3-expressing infiltrating lymphocytes in a range of human tumors. Overall our data provide compelling evidence that MEDI1873 is a novel, potent GITR agonist with the ability to modulate T-cell responses, and suggest that previously described GITR biology in mice may translate to the human setting, reinforcing the potential of targeting the GITR pathway as a therapeutic approach to cancer.

Pharmacokinetics, Pharmacodynamics, and Safety of MEDI4212, an Anti-IgE Monoclonal Antibody, in Subjects with Atopy: A Phase I Study.

INTRODUCTION: The anti-IgE therapy omalizumab is currently licensed for the treatment of moderate to severe allergic asthma and chronic idiopathic urticaria. Owing to limitations in the use of omalizumab, a need exists for optimized anti-IgE therapies to broaden clinical indications and patient populations, and to improve dosing schedules. The objective of this phase I, randomized, placebo/omalizumab-controlled, first-in-human, dose-escalation study was to evaluate the pharmacokinetics, pharmacodynamics, and safety of the high-affinity, anti-IgE therapy MEDI4212 in non-Japanese and Japanese subjects with atopy and/or diagnostic IgE ≥ 30 IU/mL. METHODS: Subjects with atopy and/or baseline IgE ≥ 30 IU/mL were randomized to a single dose of subcutaneous (5, 15, 60, 150, or 300 mg) or intravenous (300 mg) MEDI4212, subcutaneous omalizumab, or placebo. Following administration, pharmacokinetic, pharmacodynamic [IgE (free and total), and cellular FcεRI expression], and safety assessments were made. RESULTS: MEDI4212 rapidly suppressed free serum IgE to a greater extent than omalizumab; however, recovery of free IgE to baseline in MEDI4212-dosed subjects was rapid when compared with the slow and gradual recovery seen in omalizumab-dosed individuals. The loss of IgE suppression corresponded with a rapid decrease of serum MEDI4212. FcεRI expression on dendritic cells and basophils was reduced following MEDI4212 dosing. MEDI4212 was well tolerated by subjects; adverse events were generally of low severity and no subjects discontinued due to adverse events. CONCLUSIONS: The increased potency of MEDI4212 may be of clinical interest for individuals with high-diagnostic IgE levels where more extensive IgE suppression is required for clinical response. However, the modest duration of free IgE suppression below the target concentration noted with MEDI4212 in this study suggests limited potential for dosing schedule advantages over omalizumab. FUNDING: MedImmune. TRIAL REGISTRATION: ClinicalTrials.gov identifier, NCT01544348.

Receptor occupancy assessment by flow cytometry as a pharmacodynamic biomarker in biopharmaceutical development.

Receptor occupancy (RO) assays are designed to quantify the binding of therapeutics to their targets on the cell surface and are frequently used to generate pharmacodynamic (PD) biomarker data in nonclinical and clinical studies of biopharmaceuticals. When combined with the pharmacokinetic (PK) profile, RO data can establish PKPD relationships, which are crucial for informing dose decisions. RO is commonly measured by flow cytometry on fresh blood specimens and is subject to numerous technical and logistical challenges. To ensure that reliable and high quality results are generated from RO assays, careful assay design, key reagent characterization, data normalization/reporting, and thorough planning for implementation are of critical importance during development. In this article, the authors share their experiences and perspectives in these areas and discuss challenges and potential solutions when developing and implementing a flow cytometry-based RO method in support of biopharmaceutical drug development.

Multiplexing of receptor occupancy measurements for pharmacodynamic biomarker assessment of biopharmaceuticals.

BACKGROUND: Receptor occupancy (RO) assays measure drug target engagement, and are used as pharmacodynamic (PD) biomarkers. RO assays are commonly performed by flow cytometry and often require multiplexing for assessment of multiple PD biomarkers when specimen volumes are limited. We present multiplexed RO assays for an IGF1R-EGFR bispecific antibody (Bs-Ab) and a CTLA4-Ig recombinant fusion protein to demonstrate key considerations for accurate RO assessment. METHODS: RO in cynomolgus monkeys was determined in whole blood using flow cytometry. Free and total receptors were measured using anti-receptor fluorescence-labeled detection reagents, competitive and noncompetitive to drug, respectively. RESULTS: RO of IGF1R was examined as PD for Bs-Ab, since IGF1R was expressed on blood cells. Multiplexed measurements of free and total IGF1R showed that IGF1R expression measured by total receptor was highly variable, impacting interpretation of free-IGF1R. Normalization of free-over-total IGF1R measurements compensated for variability of receptor expression allowing for accurate RO assessment. RO of CTLA4-Ig, a recombinant fusion protein targeting CD80 and CD86 receptors, was multiplexed to simultaneously measure target engagements for both receptors. Both RO methods demonstrated specificity of receptor measurements without cross-reactivity to each other in multiplexed formats. RO methods were used for evaluation of PD activity of Bs-Ab and CTLA4-Ig in cynomolgus monkeys. In both cases, RO results showed dose-dependent target engagement, corresponding well to the pharmacokinetics. CONCLUSIONS: Multiplexed RO methods allowed accurate assessment of PD activity for Bs-Ab and CTLA4-Ig, facilitating development of these biopharmaceuticals from preclinical to clinical stages.

Evaluation of assay interference and interpretation of CXCR4 receptor occupancy results in a preclinical study with MEDI3185, a fully human antibody to CXCR4.

BACKGROUND: Receptor occupancy (RO) assays provide a means to measure the direct interaction of therapeutics with their cell surface targets. Free receptor assays quantify cell-surface receptors not bound by a therapeutic while total receptor assays quantify the amount of target on the cell surface. METHODS: We developed both a flow cytometry-based free RO assay to detect free surface CXCR4, and a total surface CXCR4 assay. In an effort to evaluate potential displacement interference, we performed in vitro experiments to compare on-cell affinity with the IC50 values from in vitro and in vivo from the free CXCR4 assay. We determined free and total surface CXCR4 on circulating blood cells in cynomolgus monkeys dosed with MEDI3185, a fully human monoclonal antibody to CXCR4. RESULTS: We devised an approach to evaluate displacement interference during assay development and showed that our free assay demonstrated little to no displacement interference. After dosing cynomolgus monkeys with MEDI3185, we observed dose-dependence in the magnitude and duration of receptor occupancy and found CXCR4 to increase on lymphocytes, monocytes, and granulocytes. In a multiple dose study, we observed time points where surface CXCR4 appeared fully occupied but MEDI3185 was not detectable in serum. These paradoxical results represented a type of assay interference, and by comparing pharmacokinetic, ADA and total CXCR4 results, the most likely reason for the free CXCR4 results was the emergence of neutralizing anti-drug antibodies (ADA). The total CXCR4 assay was unaffected by ADA and provided a reliable marker of target modulation in both in vivo studies.

Interference in immunoassays to support therapeutic antibody development in preclinical and clinical studies.

During preclinical and clinical studies, immunoassays are used to measure the concentration of the therapeutic antibody, anti-drug antibodies and soluble protein biomarkers. The reliability of these assays is crucial since the results are routinely used for safety assessment and dose selection. Furthermore, soluble protein biomarkers can provide information about target engagement, proof of mechanism, proof of principle and prediction of response. Study samples mostly consist of complex matrices that can exhibit considerable interference, resulting in inaccurate measurements. This perspective discusses the source of interference and strategies to mitigate or eliminate interference in immunoassays used during preclinical and clinical drug development of drugs with a focus on the development of therapeutic antibodies.

Next generation ligand binding assays-review of emerging real-time measurement technologies.

Over the last few years, numerous ligand binding assay technologies that utilize real-time measurement have been introduced; however, an assemblage and evaluation of these technologies has not previously been published. Herein, we describe six emerging real-time measurement technologies: Maverick™, MX96 SPR™, NanoDLSay™, AMMP®/ViBE®, SoPrano™, and two Lab-on-a-Chip (LoC) microfluidic devices. The development stage gate of these technologies ranges from pre-commercial to commercially available. Due to the novelty, the application and utility of some of the technologies regarding bioanalysis are likely to evolve but it is our hope that this review will provide insight into the direction the development of real-time measurement technologies is moving and the vision of those that are taking us there. Following the technology discussions, a comprehensive summary table is presented.

Identification and elimination of target-related matrix interference in a neutralizing anti-drug antibody assay.

Biopharmaceuticals administered to the human body have the potential to trigger the production of anti-drug (also called anti-therapeutic) antibodies (ADA) that can neutralize the therapeutic activity. For antibody therapeutics, cell-based neutralizing ADA assays are frequently used to evaluate ADA in clinical studies. We developed a method to detect neutralizing antibodies against MEDI-575, a fully human IgG2κ antagonistic antibody against PDGFR-α. We evaluated three assay formats, two of which measured late responses, cell proliferation and apoptosis, whereas the third assay detected an early signaling event, phosphorylation of PDGFR-α. Measuring phosphorylation provided a superior assay window and therefore was developed as a neutralizing ADA (NAb) assay. Matrix interference, however, was significant, and could be identified to be caused by PDGF-AA and PDGF-AB, apparently the two most abundant ligands of PDGFR-α present in human serum samples. A simple pre-treatment step, addition of an inhibitory antibody to PDGF-A, a subunit present in PDGF-AA and PDGF-AB, was found to eliminate matrix interference, increasing assay reliability and sensitivity. We integrated the pre-treatment step into assay development and qualified a robust NAb assay.

PK-PD modeling of protein drugs: implications in assay development.

Pharmacokinetic-pharmacodynamic (PK-PD) modeling is an integral part of the preclinical and clinical development of protein drugs. Bioanalytical data from appropriately selected and well-characterized PK and PD biomarker assays can be incorporated into mechanistic PK-PD models and allow a quantitative relationship between protein drug exposure, target modulation, and biochemical, physiological and pathophysiological effects to be established. The selection of PD biomarkers that assess target engagement and modulation in the extracellular milieu and downstream cellular effects can provide proof-of-mechanism and define the magnitude and duration of target modulation following drug administration. The PK-PD data can provide an important link between magnitude of target modulation and clinical efficacy and safety outcomes, and guide the selection of doses and dosing schedules for clinical trials. In this article, approaches to the selection and development of fit-for-purpose, PK and PD assays for protein drugs are reviewed, and the applications of the assay results in PK-PD models are discussed.

Monopolar attachment of sister kinetochores at meiosis I requires casein kinase 1.

In meiosis, a single round of DNA replication is followed by two consecutive rounds of chromosome segregation, called meiosis I and II. Disjunction of maternal from paternal centromeres during meiosis I depends on the attachment of sister kinetochores to microtubules emanating from the same pole. In budding yeast, monopolar attachment requires recruitment to kinetochores of the monopolin complex. How monopolin promotes monopolar attachment was unclear, as its subunits are poorly conserved and lack similarities to proteins with known functions. We show here that the monopolin subunit Mam1 binds tightly to Hrr25, a highly conserved casein kinase 1 delta/epsilon (CK1delta/epsilon), and recruits it to meiosis I centromeres. Hrr25 kinase activity and Mam1 binding are both essential for monopolar attachment. Since CK1delta/epsilon activity is important for accurate chromosome segregation during meiosis I also in fission yeast, phosphorylation of kinetochore proteins by CK1delta/epsilon might be an evolutionary conserved process required for monopolar attachment.

The yeast APC/C subunit Mnd2 prevents premature sister chromatid separation triggered by the meiosis-specific APC/C-Ama1.

Cohesion established between sister chromatids during pre-meiotic DNA replication mediates two rounds of chromosome segregation. The first division is preceded by an extended prophase wherein homologous chromosomes undergo recombination. The persistence of cohesion during prophase is essential for recombination and both meiotic divisions. Here we show that Mnd2, a subunit of the anaphase-promoting complex (APC/C) from budding yeast, is essential to prevent premature destruction of cohesion in meiosis. During S- and prophase, Mnd2 prevents activation of the APC/C by a meiosis-specific activator called Ama1. In cells lacking Mnd2 the APC/C-Ama1 enzyme triggers degradation of Pds1, which causes premature sister chromatid separation due to unrestrained separase activity. In vitro, Mnd2 inhibits ubiquitination of Pds1 by APC/C-Ama1 but not by other APC/C holo-enzymes. We conclude that chromosome segregation in meiosis depends on the selective inhibition of a meiosis-specific form of the APC/C.