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Summary Meiotic recombination is essential for fertility and allelic shuffling. Canonical recombination models fail to capture the observed complexity of meiotic recombinants. Here, by combining genome-wide meiotic heteroduplex DNA patterns with meiotic DNA double-strand break (DSB) sites, we show that part of this complexity results from frequent template switching during synthesis-dependent strand annealing that yields noncrossovers and from branch migration of double Holliday junction (dHJ)-containing intermediates that mainly yield crossovers. This complexity also results from asymmetric positioning of crossover intermediates relative to the initiating DSB and Msh2-independent conversions promoted by the suspected dHJ resolvase Mlh1-3 as well as Exo1 and Sgs1. Finally, we show that dHJ resolution is biased toward cleavage of the pair of strands containing newly synthesized DNA near the junctions and that this bias can be decoupled from the crossover-biased dHJ resolution. These properties are likely conserved in eukaryotes containing ZMM proteins, which includes mammals. [ABSTRACT FROM AUTHOR]

MicroRNAs of the miR-16 and miR-34 families have been reported to inhibit cell cycle progression, and their loss has been linked to oncogenic transformation. Utilizing a high-throughput, genome-wide screen for miRNAs and mRNAs that are differentially regulated in osteosarcoma (OS) cell lines, we report that miR-449a and miR-424, belonging to the miR-34 and miR-16 families, respectively, target the major S/G2 phase cyclin, cyclin A2 (CCNA2), in a bipartite manner. Wefound that the 3-UTR of CCNA2 is recognized by miR-449a, whereas the CCNA2 coding region is targeted by miR-424. Of note, we observed loss of both miR-449a and miR-424 in OS, resulting in derepression of CCNA2 and appearance of aggressive cancer phenotypes. Ectopic expression of miR-449a and miR-424 significantly decreased cyclin A2 levels and inhibited proliferation rate, migratory potential, and colony-forming ability ofOScells. To further probe the roles of miR-449a and miR-424 in OS, we developed an OS mouse model by intraosseous injection of U2OS cells into the tibia bone of NOD-scid mice, which indicated that miR-449a and miR-424 co-expression suppresses tumor growth. On the basis of this discovery, we analyzed the gene expression of human OS biopsy samples, revealing that miR-449a and miR-424 are both down-regulated, whereas cyclin A2 is significantly up-regulated in these OS samples. In summary, the findings in our study highlight that cyclin A2 repression by miRNAs of the miR-16 and miR-34 families is lost in aggressive OS. [ABSTRACT FROM AUTHOR]

The migration of chromosomes during mitosis is mediated primarily by kinesins that bind to the chromosomes and move along the microtubules, exerting pulling and pushing forces on the centrosomes. We report that a DNA replication protein, Sld5, localizes to the centrosomes, resisting the microtubular pulling forces experienced during chromosome congression. In the absence of Sld5, centriolar satellites, which normally cluster around the centrosomes, are dissipated throughout the cytoplasm, resulting in the loss of their known function of recruiting the centrosomal protein, pericentrin. We observed that Sld5-deficient centrosomes lacking pericentrin were unable to endure the CENP-E- and Kid-mediated microtubular forces that converge on the centrosomes during chromosome congression, resulting in monocentriolar and acentriolar spindle poles. The minus-end-directed kinesin-14 motor protein, HSET, sustains the traction forces that mediate centrosomal fragmentation in Sld5-depleted cells. Thus, we report that a DNA replication protein has an as yet unknown function of ensuring spindle pole resistance to traction forces exerted during chromosome congression. [ABSTRACT FROM AUTHOR]

The tumor microenvironment is characterized by nutrientdeprived conditions in which the cancer cells have to adapt for survival. Serum starvation resembles the growth factor deprivation characteristic of the poorly vascularized tumor microenvironment and has aided in the discovery of key growth regulatory genes and microRNAs (miRNAs) that have a role in the oncogenic transformation. We report here that miR-874 down-regulates the major G1/S phase cyclin, cyclin E1 (CCNE1), during serum starvation. Because the adaptation of cancer cells to the tumor microenvironment is vital for subsequent oncogenesis, we tested for miR-874 andCCNE1interdependence in osteosarcoma cells. We observed that miR-874 inhibits CCNE1 expression in primary osteoblasts, but in aggressive osteosarcomas, miR-874 is down-regulated, leading to elevated CCNE1 expression and appearance of cancer-associated phenotypes. We established that loss of miR-874--mediated control of cyclin E1 is a general feature of osteosarcomas. The down-regulation of CCNE1 by miR-874 is independent of E2F transcription factors. Restoration of miR-874 expression impeded S phase progression, suppressing aggressive growth phenotypes, such as cell invasion, migration and xenograft tumors, in nude mice. In summary, we report that miR-874 inhibits CCNE1 expression during growth factor deprivation and that miR-874 down-regulation in osteosarcomas leads to CCNE1 up-regulation and more aggressive growth phenotypes. [ABSTRACT FROM AUTHOR]