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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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Chromosome fragmentation defects linked to homolog-independent repair of meiotic DSB.(A–F) Micrographs of DAPI-stained diakinesis chromosomes are shown at the same magnification. Chromosomal fragments are indicated by white arrowheads, and scale bars = 5 µm. (G) A graph of the percentages of diakinesis oocytes containing chromosome fragments. The smc-5(ok2421) and the smc-6(ok3294) mutant oocytes showed significantly higher frequency of chromosome fragmentation over wild-type (*p<0.001, Fisher's Exact Test). The smc-5(ok2421) fragmentation defect was drastically suppressed by the spo-11(ok79) mutation (**p = 0.006). The disruption of inter-homolog recombination by destabilizing homolog-synapsis consistently enhanced the fragmentation defects in all three smc-5 and smc-6 mutant strains (***). By contrast, the brc-1 mutation affecting sister-chromatid recombination [7] failed to enhance the fragmentation defect of the smc-5(ok2421) mutation (†). The numerical values, sample sizes, and statistical comparisons are summarized in Table S5.
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pgen-1001028-g007: Chromosome fragmentation defects linked to homolog-independent repair of meiotic DSB.(A–F) Micrographs of DAPI-stained diakinesis chromosomes are shown at the same magnification. Chromosomal fragments are indicated by white arrowheads, and scale bars = 5 µm. (G) A graph of the percentages of diakinesis oocytes containing chromosome fragments. The smc-5(ok2421) and the smc-6(ok3294) mutant oocytes showed significantly higher frequency of chromosome fragmentation over wild-type (*p<0.001, Fisher's Exact Test). The smc-5(ok2421) fragmentation defect was drastically suppressed by the spo-11(ok79) mutation (**p = 0.006). The disruption of inter-homolog recombination by destabilizing homolog-synapsis consistently enhanced the fragmentation defects in all three smc-5 and smc-6 mutant strains (***). By contrast, the brc-1 mutation affecting sister-chromatid recombination [7] failed to enhance the fragmentation defect of the smc-5(ok2421) mutation (†). The numerical values, sample sizes, and statistical comparisons are summarized in Table S5.

Mentions: Approximately 20% of the diakinesis oocytes from the smc-5(ok2421) and smc-6(ok3294) mutants contained chromosome fragments (blue arrows in Figure 5, and white arrowheads in Figure 7). These fragments were disproportionately smaller than the linked homologs at diakinesis, and also showed staining for the cohesin SMC-3 protein indicating they were derived from chromosomes (Figure 5E). By contrast, no fragments were observed in wild-type oocytes (Figures 7A and 7G). Intriguingly, the fragmentation defect was rarely seen in the smc-5(tm2868) mutant oocytes and appeared to correlate with the severity of SMC-5 and SMC-6 protein disruption because the smc-5(tm2868) mutants still produced a truncated SMC-5 protein (Figure 1B). The fragmentation defect also correlated with the more severe RAD-51 staining defect at late pachytene that was found in the smc-5(ok2421) and smc-6(ok3294) mutants (Figure 4A). Similar to the RAD-51 defect, the spo-11(ok79) mutation potently reduced fragmentation from 20.5% in smc-5(ok2421) mutant oocytes to 2.4% in the smc-5;spo-11 double mutant oocytes (Figure 7G).


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)

Chromosome fragmentation defects linked to homolog-independent repair of meiotic DSB.(A–F) Micrographs of DAPI-stained diakinesis chromosomes are shown at the same magnification. Chromosomal fragments are indicated by white arrowheads, and scale bars = 5 µm. (G) A graph of the percentages of diakinesis oocytes containing chromosome fragments. The smc-5(ok2421) and the smc-6(ok3294) mutant oocytes showed significantly higher frequency of chromosome fragmentation over wild-type (*p<0.001, Fisher's Exact Test). The smc-5(ok2421) fragmentation defect was drastically suppressed by the spo-11(ok79) mutation (**p = 0.006). The disruption of inter-homolog recombination by destabilizing homolog-synapsis consistently enhanced the fragmentation defects in all three smc-5 and smc-6 mutant strains (***). By contrast, the brc-1 mutation affecting sister-chromatid recombination [7] failed to enhance the fragmentation defect of the smc-5(ok2421) mutation (†). The numerical values, sample sizes, and statistical comparisons are summarized in Table S5.
© Copyright Policy
Related In: Results  -  Collection

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

pgen-1001028-g007: Chromosome fragmentation defects linked to homolog-independent repair of meiotic DSB.(A–F) Micrographs of DAPI-stained diakinesis chromosomes are shown at the same magnification. Chromosomal fragments are indicated by white arrowheads, and scale bars = 5 µm. (G) A graph of the percentages of diakinesis oocytes containing chromosome fragments. The smc-5(ok2421) and the smc-6(ok3294) mutant oocytes showed significantly higher frequency of chromosome fragmentation over wild-type (*p<0.001, Fisher's Exact Test). The smc-5(ok2421) fragmentation defect was drastically suppressed by the spo-11(ok79) mutation (**p = 0.006). The disruption of inter-homolog recombination by destabilizing homolog-synapsis consistently enhanced the fragmentation defects in all three smc-5 and smc-6 mutant strains (***). By contrast, the brc-1 mutation affecting sister-chromatid recombination [7] failed to enhance the fragmentation defect of the smc-5(ok2421) mutation (†). The numerical values, sample sizes, and statistical comparisons are summarized in Table S5.
Mentions: Approximately 20% of the diakinesis oocytes from the smc-5(ok2421) and smc-6(ok3294) mutants contained chromosome fragments (blue arrows in Figure 5, and white arrowheads in Figure 7). These fragments were disproportionately smaller than the linked homologs at diakinesis, and also showed staining for the cohesin SMC-3 protein indicating they were derived from chromosomes (Figure 5E). By contrast, no fragments were observed in wild-type oocytes (Figures 7A and 7G). Intriguingly, the fragmentation defect was rarely seen in the smc-5(tm2868) mutant oocytes and appeared to correlate with the severity of SMC-5 and SMC-6 protein disruption because the smc-5(tm2868) mutants still produced a truncated SMC-5 protein (Figure 1B). The fragmentation defect also correlated with the more severe RAD-51 staining defect at late pachytene that was found in the smc-5(ok2421) and smc-6(ok3294) mutants (Figure 4A). Similar to the RAD-51 defect, the spo-11(ok79) mutation potently reduced fragmentation from 20.5% in smc-5(ok2421) mutant oocytes to 2.4% in the smc-5;spo-11 double mutant oocytes (Figure 7G).

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