The mouse Dmc1 gene is an E. coli RecA homolog that is specifically expressed in meiosis. The DMC1 protein was detected in leptotene-to-zygotene spermatocytes, when homolog pairing likely initiates. Targeted gene disruption in the male mouse showed an arrest of meiosis of germ cells at the early zygotene stage, followed by apoptosis. In female mice lacking the Dmc1 gene, normal differentiation of oogenesis was aborted in embryos, and germ cells disappeared in the adult ovary. Meiotic chromosome analysis of Dmc1-deficient mouse spermatocytes revealed random spread of univalent axial elements without correct pairing between homologs. In rare cases, however, we observed complex pairing among nonhomologs. Thus, the mouse Dmc1 gene is required for homologous synapsis of chromosomes in meiosis.

Autosomal-dominant spinocerebellar ataxias (SCAs) are a heterogeneous group of neurodegenerative disorders. In this study, we performed genetic analysis of a unique form of SCA (SCA36) that is accompanied by motor neuron involvement. Genome-wide linkage analysis and subsequent fine mapping for three unrelated Japanese families in a cohort of SCA cases, in whom molecular diagnosis had never been performed, mapped the disease locus to the region of a 1.8 Mb stretch (LOD score of 4.60) on 20p13 (D20S906-D20S193) harboring 37 genes with definitive open reading frames. We sequenced 33 of these and observed a large expansion of an intronic GGCCTG hexanucleotide repeat in NOP56 and an unregistered missense variant (Phe265Leu) in C20orf194, but we found no mutations in PDYN and TGM6. The expansion showed complete segregation with the SCA phenotype in family studies, whereas Phe265Leu in C20orf194 did not. Screening of the expansions in the SCA cohort cases revealed four additional occurrences, but none were revealed in the cohort of 27 Alzheimer disease cases, 154 amyotrophic lateral sclerosis cases, or 300 controls. In total, nine unrelated cases were found in 251 cohort SCA patients (3.6%). A founder haplotype was confirmed in these cases. RNA foci formation was detected in lymphoblastoid cells from affected subjects by fluorescence in situ hybridization. Double staining and gel-shift assay showed that (GGCCUG)n binds the RNA-binding protein SRSF2 but that (CUG)(6) does not. In addition, transcription of MIR1292, a neighboring miRNA, was significantly decreased in lymphoblastoid cells of SCA patients. Our finding suggests that SCA36 is caused by hexanucleotide repeat expansions through RNA gain of function.

The spindle checkpoint ensures accurate chromosome segregation by delaying anaphase in response to misaligned sister chromatids during mitosis. Upon checkpoint activation, Mad2 binds directly to Cdc20 and inhibits the anaphase-promoting complex or cyclosome (APC/C). Cdc20 binding triggers a dramatic conformational change of Mad2. Consistent with an earlier report, we show herein that depletion of p31comet (formerly known as Cmt2) by RNA interference in HeLa cells causes a delay in mitotic exit following the removal of nocodazole. Purified recombinant p31comet protein antagonizes the ability of Mad2 to inhibit APC/CCdc20 in vitro and in Xenopus egg extracts. Interestingly, p31comet binds selectively to the Cdc20-bound conformation of Mad2. Binding of p31comet to Mad2 does not prevent the interaction between Mad2 and Cdc20 in vitro. During checkpoint inactivation in HeLa cells, p31comet forms a transient complex with APC/CCdc20-bound Mad2. Purified p31comet enhances the activity of APC/C isolated from nocodazole-arrested HeLa cells without disrupting the Mad2–Cdc20 interaction. Therefore, our results suggest that p31comet counteracts the function of Mad2 and is required for the silencing of the spindle checkpoint

MAD2 is a key component of the spindle checkpoint that delays the onset of anaphase until all the kinetochores are attached to the spindle. It binds to human p55CDC and prevents it from promoting destruction of an anaphase inhibitor, securin. Here we report the characterization of a novel MAD2-binding protein, CMT2. Upon the completion of spindle attachment, formation of the CMT2-MAD2 complex coincides with dissociation of the p55CDC-MAD2 complex. Overexpression of CMT2 in cells arrested by the spindle checkpoint causes premature destruction of securin and allows exit from mitosis without chromosome segregation. Depletion of CMT2 induces cell death following a transient delay in the onset of anaphase. These results indicate that CMT2 interacts with the spindle checkpoint and coordinates cell cycle events in late mitosis.