Segregation of homologous maternal and paternal centromeres to opposite poles during meiosis I depends on post-replicative crossing over between homologous non-sister chromatids, which creates chiasmata and therefore bivalent chromosomes. Destruction of sister chromatid cohesion along chromosome arms due to proteolytic cleavage of cohesin's Rec8 subunit by separase resolves chiasmata and thereby triggers the first meiotic division. This produces univalent chromosomes, the chromatids of which are held together by centromeric cohesin that has been protected from separase by shugoshin (Sgo1/MEI-S332) proteins. Here we show in both fission and budding yeast that Sgo1 recruits to centromeres a specific form of protein phosphatase 2A (PP2A). Its inactivation causes loss of centromeric cohesin at anaphase I and random segregation of sister centromeres at the second meiotic division. Artificial recruitment of PP2A to chromosome arms prevents Rec8 phosphorylation and hinders resolution of chiasmata. Our data are consistent with the notion that efficient cleavage of Rec8 requires phosphorylation of cohesin and that this is blocked by PP2A at meiosis I centromeres.

SUMMARY Accurate chromosome segregation during mitosis and meiosis depends on shugoshin proteins that prevent precocious dissociation of cohesin from centromeres. Shugoshins associate with PP2A, which is thought to de-phosphorylate cohesin and thereby prevent cleavage by separase during meiosis I. A crystal structure of a complex between a fragment of human Sgo1 and an AB’C PP2A holoenzyme reveals that Sgo1 forms a homodimeric parallel coiled-coil that docks simultaneously onto PP2A’s C and B’ subunits. Sgo1 homo-dimerization is a pre-requisite for PP2A binding. While hSgo1 interacts only with the AB’C holoenzymes, its relative Sgo2 interacts with all PP2A forms and may thus lead to dephosphorylation of distinct substrates. Mutant shugoshin proteins defective in the binding of PP2A cannot protect centromeric cohesin from separase during meiosis I or support the spindle assembly checkpoint in yeast. Finally, we provide evidence that PP2A’s recruitment to chromosomes may be sufficient to protect cohesin from separase in mammalian oocytes.

SUMMARY The mitotic checkpoint acts to maintain chromosome content by generation of a diffusible anaphase inhibitor. Unattached kinetochores catalyze a conformational shift in Mad2, converting an inactive open form into a closed one that can capture Cdc20, the mitotic activator of the APC/C ubiquitin ligase. Mad2 binding is now shown to promote a functional switch in Cdc20, exposing a previously inaccessible site for binding to BubR1’s conserved Mad3 homology domain. BubR1, but not Mad2, binding to APC/CCdc20 is demonstrated to inhibit ubiquitination of cyclin B. Closed Mad2 is further shown to catalytically amplify production of BubR1-Cdc20 without necessarily being part of the complex. Thus, the mitotic checkpoint is produced by a cascade of two catalytic steps, an initial one acting at unattached kinetochores to produce a diffusible Mad2-Cdc20 intermediate and a diffusible one in which that intermediate amplifies production of BubR1-Cdc20, the inhibitor of cyclin B ubiquitination by APC/CCdc20.

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The identification of prognostic factors in patients with glioblastoma multiforme (GBM) represents an area of increasing interest. Carbonic anhydrase IX (CA-IX), a hypoxia marker, correlates with tumor progression in a variety of human cancers. However, the role of CA-IX in GBM remains largely unknown. In the present study, we evaluated the prognostic role of CA-IX in GBM patients. In total, 66 consecutive patients with GBM who received concomitant chemoradiotherapy and adjuvant chemotherapy with temozolomide were retrospectively reviewed, and all patients received temozolomide chemotherapy for at least 3 months. Kaplan-Meier curves and log-rank tests were used for analysis of progression-free survival (PFS) and overall survival (OS), and a multivariate Cox proportional hazard model was employed to identify factors with an independent effect on survival. The median OS was longer in patients with low levels of CA-IX expression (18 months) compared to patients overexpressing CA-IX (9 months) (P = 0.004). There was not a statistically significant difference in median PFS (3.5 vs. 8 months, P = 0.054) between patients with high or low levels of CA-IX expression. In multivariate analysis, the variables that were identified as significant prognostic factors for OS were preoperative Karnofsky performance scale score (KPS) (hazard ratio (HR), 3.703; P = 0.001), CA-IX overexpression (HR, 1.967; P = 0.019), and incomplete adjuvant temozolomide treatment (HR, 2.241; P = 0.003) and gross-total resection (HR, 1.956; P = 0.034). Our findings indicated that CA-IX may be a potential prognostic biomarker in the treatment of GBM.

This study utilizing chemotherapy with bevacizumab did not result in an increased rate of morbidity related to the unresected primary tumor. Survival is not compromised by leaving the primary colon tumor intact.