Limits...
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.

Show MeSH

Related in: MedlinePlus

SMC-5 and SMC-6 accumulate in germ cells and on meiotic chromosomes.(A) A drawing representation of an adult hermaphrodite gonad arm. The progression of germ cell proliferation and meiosis are indicated by the arrows starting from the distal tip region of the gonad arm. (B) SMC-6 staining was detected in the nucleus of germ cells in the distal gonad arm. Mitotic germ nuclei are located in the pre-meiotic region proximal to the distal tip. Pre-meiotic S phase nuclei should be located proximal to the transition zone nuclei [66]. The green bracket marks the transition zone region. (C) SMC-6 staining in pachytene nuclei appeared to accumulate on chromosomes, which differed from the more diffuse nucleoplasmic staining in the distal gonad arm in (B). In the diplotene (D) and diakinesis (E) stages of meiotic prophase I, SMC-5 and SMC-6 staining were detected exclusively on chromosomes. Scale bars = 5 µm.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2908675&req=5

pgen-1001028-g002: SMC-5 and SMC-6 accumulate in germ cells and on meiotic chromosomes.(A) A drawing representation of an adult hermaphrodite gonad arm. The progression of germ cell proliferation and meiosis are indicated by the arrows starting from the distal tip region of the gonad arm. (B) SMC-6 staining was detected in the nucleus of germ cells in the distal gonad arm. Mitotic germ nuclei are located in the pre-meiotic region proximal to the distal tip. Pre-meiotic S phase nuclei should be located proximal to the transition zone nuclei [66]. The green bracket marks the transition zone region. (C) SMC-6 staining in pachytene nuclei appeared to accumulate on chromosomes, which differed from the more diffuse nucleoplasmic staining in the distal gonad arm in (B). In the diplotene (D) and diakinesis (E) stages of meiotic prophase I, SMC-5 and SMC-6 staining were detected exclusively on chromosomes. Scale bars = 5 µm.

Mentions: The SMC-6 protein was detected by indirect immunofluorescence microscopy in the nuclei of germ cells throughout the adult hermaphrodite gonad (Figure 2). Beginning at the distal tip region of the gonad (Figure 2A), SMC-6 staining is detected in the nucleus of germ cells in mitotic proliferation, pre-meiotic S phase and in the early stages of meiosis (the transition zone), which are equivalent to leptotene and zygotene (Figure 2B). Interestingly, SMC-6 staining became more enriched on chromosomes at pachytene (Figure 2C), which coincided with the timing of meiotic DSB repair [14]. The SMC-6 immunofluorescence became more intense as the germ cells exited pachytene and progressed through the diplotene and diakinesis stages of prophase (Figures 2D and 2E). The pachytene and diakinesis staining of SMC-6 was specifically disrupted by the smc-6(ok3294) mutation (Figures S1C and S1E). Even though we could not detect SMC-5 immunostaining on pachytene chromosomes, we found that the smc-5(tm2868) and smc-5(ok2421) mutations reduced SMC-6 staining on pachytene chromosomes (Figures S1A and S1B), indicating a possible dependency on SMC-5 for the localization of SMC-6 at pachytene. The immunostaining for SMC-5 was detected on diplotene and diakinesis chromosomes specifically in wild-type (Figures 2D and 2E), but not in the smc-5(tm2868) and smc-5(ok2421) mutant oocytes (Figures S1D and S1H). The SMC-5 and SMC-6 chromosomal staining in diplotene (data not shown) and diakinesis oocytes (Figures S1F, S1G and S1I) also appeared to be interdependent. In addition to germ cell staining, SMC-5 and SMC-6 immunostaining were detected in somatic cells during early embryogenesis (data not shown). The three smc-5 and smc-6 mutations caused frequent chromatin-bridges to appear in the intestine, even though immunostaining was significantly weaker in the intestine than the germline (Bickel and Chan, unpublished observations). These results suggest that the SMC-5/6 proteins accumulate in the soma and the germline, with greater enrichment seen in the germ cells.


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)

SMC-5 and SMC-6 accumulate in germ cells and on meiotic chromosomes.(A) A drawing representation of an adult hermaphrodite gonad arm. The progression of germ cell proliferation and meiosis are indicated by the arrows starting from the distal tip region of the gonad arm. (B) SMC-6 staining was detected in the nucleus of germ cells in the distal gonad arm. Mitotic germ nuclei are located in the pre-meiotic region proximal to the distal tip. Pre-meiotic S phase nuclei should be located proximal to the transition zone nuclei [66]. The green bracket marks the transition zone region. (C) SMC-6 staining in pachytene nuclei appeared to accumulate on chromosomes, which differed from the more diffuse nucleoplasmic staining in the distal gonad arm in (B). In the diplotene (D) and diakinesis (E) stages of meiotic prophase I, SMC-5 and SMC-6 staining were detected exclusively on chromosomes. Scale bars = 5 µm.
© Copyright Policy
Related In: Results  -  Collection

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

pgen-1001028-g002: SMC-5 and SMC-6 accumulate in germ cells and on meiotic chromosomes.(A) A drawing representation of an adult hermaphrodite gonad arm. The progression of germ cell proliferation and meiosis are indicated by the arrows starting from the distal tip region of the gonad arm. (B) SMC-6 staining was detected in the nucleus of germ cells in the distal gonad arm. Mitotic germ nuclei are located in the pre-meiotic region proximal to the distal tip. Pre-meiotic S phase nuclei should be located proximal to the transition zone nuclei [66]. The green bracket marks the transition zone region. (C) SMC-6 staining in pachytene nuclei appeared to accumulate on chromosomes, which differed from the more diffuse nucleoplasmic staining in the distal gonad arm in (B). In the diplotene (D) and diakinesis (E) stages of meiotic prophase I, SMC-5 and SMC-6 staining were detected exclusively on chromosomes. Scale bars = 5 µm.
Mentions: The SMC-6 protein was detected by indirect immunofluorescence microscopy in the nuclei of germ cells throughout the adult hermaphrodite gonad (Figure 2). Beginning at the distal tip region of the gonad (Figure 2A), SMC-6 staining is detected in the nucleus of germ cells in mitotic proliferation, pre-meiotic S phase and in the early stages of meiosis (the transition zone), which are equivalent to leptotene and zygotene (Figure 2B). Interestingly, SMC-6 staining became more enriched on chromosomes at pachytene (Figure 2C), which coincided with the timing of meiotic DSB repair [14]. The SMC-6 immunofluorescence became more intense as the germ cells exited pachytene and progressed through the diplotene and diakinesis stages of prophase (Figures 2D and 2E). The pachytene and diakinesis staining of SMC-6 was specifically disrupted by the smc-6(ok3294) mutation (Figures S1C and S1E). Even though we could not detect SMC-5 immunostaining on pachytene chromosomes, we found that the smc-5(tm2868) and smc-5(ok2421) mutations reduced SMC-6 staining on pachytene chromosomes (Figures S1A and S1B), indicating a possible dependency on SMC-5 for the localization of SMC-6 at pachytene. The immunostaining for SMC-5 was detected on diplotene and diakinesis chromosomes specifically in wild-type (Figures 2D and 2E), but not in the smc-5(tm2868) and smc-5(ok2421) mutant oocytes (Figures S1D and S1H). The SMC-5 and SMC-6 chromosomal staining in diplotene (data not shown) and diakinesis oocytes (Figures S1F, S1G and S1I) also appeared to be interdependent. In addition to germ cell staining, SMC-5 and SMC-6 immunostaining were detected in somatic cells during early embryogenesis (data not shown). The three smc-5 and smc-6 mutations caused frequent chromatin-bridges to appear in the intestine, even though immunostaining was significantly weaker in the intestine than the germline (Bickel and Chan, unpublished observations). These results suggest that the SMC-5/6 proteins accumulate in the soma and the germline, with greater enrichment seen in the germ cells.

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