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Structural maintenance of chromosomes (SMC) proteins promote homolog-independent recombination repair in meiosis crucial for germ cell genomic stability.

Bickel JS, Chen L, Hayward J, Yeap SL, Alkers AE, Chan RC - PLoS Genet. (2010)

Bottom Line: Chromosome fragments associated with HR defects have only been reported in mutants, which have disrupted inter-homolog crossover.Surprisingly, the smc-5 and smc-6 mutations did not disrupt the formation of chiasmata, the cytologically visible linkages between homologous chromosomes formed from meiotic inter-homolog crossovers.Together, these results demonstrate that the successful completion of homolog-independent recombination is crucial for germ cell genomic stability.

View Article: PubMed Central - PubMed

Affiliation: Department of Human Genetics, University of Michigan Medical School, Ann Arbor, Michigan, United States of America.

ABSTRACT
In meiosis, programmed DNA breaks repaired by homologous recombination (HR) can be processed into inter-homolog crossovers that promote the accurate segregation of chromosomes. In general, more programmed DNA double-strand breaks (DSBs) are formed than the number of inter-homolog crossovers, and the excess DSBs must be repaired to maintain genomic stability. Sister-chromatid (inter-sister) recombination is postulated to be important for the completion of meiotic DSB repair. However, this hypothesis is difficult to test because of limited experimental means to disrupt inter-sister and not inter-homolog HR in meiosis. We find that the conserved Structural Maintenance of Chromosomes (SMC) 5 and 6 proteins in Caenorhabditis elegans are required for the successful completion of meiotic homologous recombination repair, yet they appeared to be dispensable for accurate chromosome segregation in meiosis. Mutations in the smc-5 and smc-6 genes induced chromosome fragments and dismorphology. Chromosome fragments associated with HR defects have only been reported in mutants, which have disrupted inter-homolog crossover. Surprisingly, the smc-5 and smc-6 mutations did not disrupt the formation of chiasmata, the cytologically visible linkages between homologous chromosomes formed from meiotic inter-homolog crossovers. The mutant fragmentation defect appeared to be preferentially enhanced by the disruptions of inter-homolog recombination but not by the disruptions of inter-sister recombination. Based on these findings, we propose that the C. elegans SMC-5/6 proteins are required in meiosis for the processing of homolog-independent, presumably sister-chromatid-mediated, recombination repair. Together, these results demonstrate that the successful completion of homolog-independent recombination is crucial for germ cell genomic stability.

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Possible outcomes for the combined genetic disruptions of smc-5 or smc-6 and him-3.(A) Schematic diagram depicts the biased repair of SPO-11 DSBs via inter-homolog recombination repair in meiosis. (B) In the absence of HIM-3, SPO-11 DSBs are repaired via homolog-independent (i.e. sister-chromatid) recombination [48]. (C–D) Two predicted repair outcomes for the double mutants. (C) If the SMC-5/6 proteins function mainly in inter-homolog repair which is already disrupted by the him-3 mutation, then the overall repair efficiency as monitored by the removal of RAD-51 staining should be the same in the double mutants as compared to the smc-5 and smc-6 single mutants. (D) If the SMC-5/6 proteins function mainly in inter-sister repair, then the double mutant should exacerbate the repair defects and the RAD-51 intermediates should persist at late pachytene.
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pgen-1001028-g006: Possible outcomes for the combined genetic disruptions of smc-5 or smc-6 and him-3.(A) Schematic diagram depicts the biased repair of SPO-11 DSBs via inter-homolog recombination repair in meiosis. (B) In the absence of HIM-3, SPO-11 DSBs are repaired via homolog-independent (i.e. sister-chromatid) recombination [48]. (C–D) Two predicted repair outcomes for the double mutants. (C) If the SMC-5/6 proteins function mainly in inter-homolog repair which is already disrupted by the him-3 mutation, then the overall repair efficiency as monitored by the removal of RAD-51 staining should be the same in the double mutants as compared to the smc-5 and smc-6 single mutants. (D) If the SMC-5/6 proteins function mainly in inter-sister repair, then the double mutant should exacerbate the repair defects and the RAD-51 intermediates should persist at late pachytene.

Mentions: Based on our findings, the SMC-5/6 proteins appear to be required for homologous recombination that does not affect inter-homolog crossover formation, which possibly involves inter-homolog non-crossover repair or homolog-independent sister-chromatid recombination. To address the two possibilities, we tested whether the smc-5(ok2421) and the smc-6(ok3294) mutations will confer the RAD-51 defect in the homolog synapsis mutant him-3(gk149). HIM-3 is an axial element of the synaptonemal complex that promotes inter-homolog recombination by stabilizing the close association of homologous chromosomes in wild-type (Figure 6A; [47]). In the absence of HIM-3 (Figure 6B), inter-homolog non-crossover repair should be disrupted. If the smc-5 or the smc-6 mutation also disrupted inter-homolog non-crossover repair, then the him-3 double mutant with smc-5 or smc-6 should be similar to the him-3 single mutant (Figure 6C). Conversely, if the SMC-5/6 proteins promote homolog-independent recombination, which should be functional in the him-3 mutant [48], then the RAD-51 staining in the him-3 double mutant should be more persistent than the him-3 single mutant (Figure 6D).


Structural maintenance of chromosomes (SMC) proteins promote homolog-independent recombination repair in meiosis crucial for germ cell genomic stability.

Bickel JS, Chen L, Hayward J, Yeap SL, Alkers AE, Chan RC - PLoS Genet. (2010)

Possible outcomes for the combined genetic disruptions of smc-5 or smc-6 and him-3.(A) Schematic diagram depicts the biased repair of SPO-11 DSBs via inter-homolog recombination repair in meiosis. (B) In the absence of HIM-3, SPO-11 DSBs are repaired via homolog-independent (i.e. sister-chromatid) recombination [48]. (C–D) Two predicted repair outcomes for the double mutants. (C) If the SMC-5/6 proteins function mainly in inter-homolog repair which is already disrupted by the him-3 mutation, then the overall repair efficiency as monitored by the removal of RAD-51 staining should be the same in the double mutants as compared to the smc-5 and smc-6 single mutants. (D) If the SMC-5/6 proteins function mainly in inter-sister repair, then the double mutant should exacerbate the repair defects and the RAD-51 intermediates should persist at late pachytene.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2908675&req=5

pgen-1001028-g006: Possible outcomes for the combined genetic disruptions of smc-5 or smc-6 and him-3.(A) Schematic diagram depicts the biased repair of SPO-11 DSBs via inter-homolog recombination repair in meiosis. (B) In the absence of HIM-3, SPO-11 DSBs are repaired via homolog-independent (i.e. sister-chromatid) recombination [48]. (C–D) Two predicted repair outcomes for the double mutants. (C) If the SMC-5/6 proteins function mainly in inter-homolog repair which is already disrupted by the him-3 mutation, then the overall repair efficiency as monitored by the removal of RAD-51 staining should be the same in the double mutants as compared to the smc-5 and smc-6 single mutants. (D) If the SMC-5/6 proteins function mainly in inter-sister repair, then the double mutant should exacerbate the repair defects and the RAD-51 intermediates should persist at late pachytene.
Mentions: Based on our findings, the SMC-5/6 proteins appear to be required for homologous recombination that does not affect inter-homolog crossover formation, which possibly involves inter-homolog non-crossover repair or homolog-independent sister-chromatid recombination. To address the two possibilities, we tested whether the smc-5(ok2421) and the smc-6(ok3294) mutations will confer the RAD-51 defect in the homolog synapsis mutant him-3(gk149). HIM-3 is an axial element of the synaptonemal complex that promotes inter-homolog recombination by stabilizing the close association of homologous chromosomes in wild-type (Figure 6A; [47]). In the absence of HIM-3 (Figure 6B), inter-homolog non-crossover repair should be disrupted. If the smc-5 or the smc-6 mutation also disrupted inter-homolog non-crossover repair, then the him-3 double mutant with smc-5 or smc-6 should be similar to the him-3 single mutant (Figure 6C). Conversely, if the SMC-5/6 proteins promote homolog-independent recombination, which should be functional in the him-3 mutant [48], then the RAD-51 staining in the him-3 double mutant should be more persistent than the him-3 single mutant (Figure 6D).

Bottom Line: Chromosome fragments associated with HR defects have only been reported in mutants, which have disrupted inter-homolog crossover.Surprisingly, the smc-5 and smc-6 mutations did not disrupt the formation of chiasmata, the cytologically visible linkages between homologous chromosomes formed from meiotic inter-homolog crossovers.Together, these results demonstrate that the successful completion of homolog-independent recombination is crucial for germ cell genomic stability.

View Article: PubMed Central - PubMed

Affiliation: Department of Human Genetics, University of Michigan Medical School, Ann Arbor, Michigan, United States of America.

ABSTRACT
In meiosis, programmed DNA breaks repaired by homologous recombination (HR) can be processed into inter-homolog crossovers that promote the accurate segregation of chromosomes. In general, more programmed DNA double-strand breaks (DSBs) are formed than the number of inter-homolog crossovers, and the excess DSBs must be repaired to maintain genomic stability. Sister-chromatid (inter-sister) recombination is postulated to be important for the completion of meiotic DSB repair. However, this hypothesis is difficult to test because of limited experimental means to disrupt inter-sister and not inter-homolog HR in meiosis. We find that the conserved Structural Maintenance of Chromosomes (SMC) 5 and 6 proteins in Caenorhabditis elegans are required for the successful completion of meiotic homologous recombination repair, yet they appeared to be dispensable for accurate chromosome segregation in meiosis. Mutations in the smc-5 and smc-6 genes induced chromosome fragments and dismorphology. Chromosome fragments associated with HR defects have only been reported in mutants, which have disrupted inter-homolog crossover. Surprisingly, the smc-5 and smc-6 mutations did not disrupt the formation of chiasmata, the cytologically visible linkages between homologous chromosomes formed from meiotic inter-homolog crossovers. The mutant fragmentation defect appeared to be preferentially enhanced by the disruptions of inter-homolog recombination but not by the disruptions of inter-sister recombination. Based on these findings, we propose that the C. elegans SMC-5/6 proteins are required in meiosis for the processing of homolog-independent, presumably sister-chromatid-mediated, recombination repair. Together, these results demonstrate that the successful completion of homolog-independent recombination is crucial for germ cell genomic stability.

Show MeSH
Related in: MedlinePlus