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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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Aberrant RAD-51 accumulation in meiotic germ cells from the smc-5 and smc-6 mutants.(A) The average numbers of RAD-51 foci per nucleus in the transition zone and pachytene regions of the gonad arms are represented in the bar graph; the error bars represent standard errors. The pachytene region was divided into three equal length areas (early, mid and late) and each quantified separately. The smc-5 and smc-6 mutants showed significant increases in RAD-51 foci in comparison to wild-type at the mid- to late-pachytene regions (p≤0.01; two-tailed t-Test). The numerical values and sample sizes are summarized in Table S3. (B–K) Micrographs of RAD-51 immunofluorescence and DAPI-DNA fluorescence in late pachytene nuclei from the wild-type and the mutant germlines. Scale bars = 5 µm.
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pgen-1001028-g004: Aberrant RAD-51 accumulation in meiotic germ cells from the smc-5 and smc-6 mutants.(A) The average numbers of RAD-51 foci per nucleus in the transition zone and pachytene regions of the gonad arms are represented in the bar graph; the error bars represent standard errors. The pachytene region was divided into three equal length areas (early, mid and late) and each quantified separately. The smc-5 and smc-6 mutants showed significant increases in RAD-51 foci in comparison to wild-type at the mid- to late-pachytene regions (p≤0.01; two-tailed t-Test). The numerical values and sample sizes are summarized in Table S3. (B–K) Micrographs of RAD-51 immunofluorescence and DAPI-DNA fluorescence in late pachytene nuclei from the wild-type and the mutant germlines. Scale bars = 5 µm.

Mentions: In the transition zone and at early pachytene, the meiotic germ cells exhibited slightly more RAD-51 foci in the smc-5 and smc-6 mutants than in wild-type (Figure 4A). The difference in the number of RAD-51 foci between the mutants and the wild-type was significantly more pronounced at mid- and late-pachytene (Figures 4A–4E). In contrast to the pre-meiotic RAD-51 staining, the aberrant pachytene RAD-51 staining in the smc-5(tm2868) and the smc-5(ok2421) mutants was significantly reduced by the spo-11(ok79) mutation (Figures 4F and 4G), which suggests that the RAD-51 staining defect at pachytene resulted primarily from a defect in meiotic DSB repair rather than from prior DNA damage in the pre-meiotic region.


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)

Aberrant RAD-51 accumulation in meiotic germ cells from the smc-5 and smc-6 mutants.(A) The average numbers of RAD-51 foci per nucleus in the transition zone and pachytene regions of the gonad arms are represented in the bar graph; the error bars represent standard errors. The pachytene region was divided into three equal length areas (early, mid and late) and each quantified separately. The smc-5 and smc-6 mutants showed significant increases in RAD-51 foci in comparison to wild-type at the mid- to late-pachytene regions (p≤0.01; two-tailed t-Test). The numerical values and sample sizes are summarized in Table S3. (B–K) Micrographs of RAD-51 immunofluorescence and DAPI-DNA fluorescence in late pachytene nuclei from the wild-type and the mutant germlines. Scale bars = 5 µm.
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Related In: Results  -  Collection

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

pgen-1001028-g004: Aberrant RAD-51 accumulation in meiotic germ cells from the smc-5 and smc-6 mutants.(A) The average numbers of RAD-51 foci per nucleus in the transition zone and pachytene regions of the gonad arms are represented in the bar graph; the error bars represent standard errors. The pachytene region was divided into three equal length areas (early, mid and late) and each quantified separately. The smc-5 and smc-6 mutants showed significant increases in RAD-51 foci in comparison to wild-type at the mid- to late-pachytene regions (p≤0.01; two-tailed t-Test). The numerical values and sample sizes are summarized in Table S3. (B–K) Micrographs of RAD-51 immunofluorescence and DAPI-DNA fluorescence in late pachytene nuclei from the wild-type and the mutant germlines. Scale bars = 5 µm.
Mentions: In the transition zone and at early pachytene, the meiotic germ cells exhibited slightly more RAD-51 foci in the smc-5 and smc-6 mutants than in wild-type (Figure 4A). The difference in the number of RAD-51 foci between the mutants and the wild-type was significantly more pronounced at mid- and late-pachytene (Figures 4A–4E). In contrast to the pre-meiotic RAD-51 staining, the aberrant pachytene RAD-51 staining in the smc-5(tm2868) and the smc-5(ok2421) mutants was significantly reduced by the spo-11(ok79) mutation (Figures 4F and 4G), which suggests that the RAD-51 staining defect at pachytene resulted primarily from a defect in meiotic DSB repair rather than from prior DNA damage in the pre-meiotic region.

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