ArticleMeiotic recombination, synapsis, meiotic inactivation and sperm aneuploidy in a chromosome 1 inversion carrier
Introduction
In order to pair during meiosis, inversions theoretically adopt an inversion loop to allow the homologous partner to synapse. Despite improved synapsis between the inverted and normal chromosome, asynapsis does occur. Homosynapsis of the distal regions with either asynapsis or heterosynapsis of the inverted region has been observed (Gabriel-Robez and Rumpler, 1994).
Disrupted meiotic behaviour of inversion carriers may be responsible for abnormal sperm parameters in these carriers. The formation of the inversion loop could disrupt other meiotic machinery and impede progression through meiosis (Forejt et al., 1981). In addition, reduced recombination has been observed within the inversion loop, which in general has been shown to have a detrimental effect on meiosis (Brown et al., 1998).
Even in cases of heterosynapsis of the inverted region, inversion carriers are at risk for producing chromosomally abnormal spermatozoa. If an odd number of crossovers occurs within the inverted sequence, then resulting spermatozoa will have an unbalanced chromosome complement. The frequency of unbalanced spermatozoa in inversion carriers has been studied in numerous carriers (reviewed in Anton et al., 2005) and has been shown to be as low as 0%, when the segment is very short (Balkan et al., 1983), and as high as 30% when the segment is long (Navarro et al., 1993). Variation in aneuploidy levels likely reflects the variation in the chromosome involved, the proportion of the chromosome involved in the inversion and the likelihood of a recombination event occurring within the inverted region. Furthermore, as the frequency of unbalanced spermatozoa reflects the proportion of unbalanced embryos, analysis of inversion segregation would allow for a more personalized risk assessment for unbalanced offspring in inversion carriers.
During the first meiotic division, chromosome synapsis is facilitated by the synaptonemal complex. Immunofluorescence techniques have allowed the detailed study of pairing and recombination and have provided insight into the role of meiotic errors in infertility. Infertile men display reduced recombination and increased synaptic errors (Ferguson et al., 2007, Gonsalves et al., 2004, Judis et al., 2004, Ma et al., 2006, Sun et al., 2004, Sun et al., 2008). Meiotic defects may be caught by meiotic checkpoints, leading to spermatogenic arrest and reduced sperm concentration (Baker et al., 1995, Edelmann et al., 1996). Furthermore, errors in both the frequency (Hassold et al., 1991, Reish et al., 2004, Shi et al., 2001) and distribution (Ferguson et al., 2009, Hassold et al., 1995, Lamb et al., 1996) of crossovers have been linked with non-disjunction and the production of aneuploid spermatozoa.
Immunofluorescence studies of carriers of chromosome abnormalities such as inversions have been limited. Six meiotic studies in human males have been carried out in translocations carriers (Ferguson et al., 2008, Leng et al., 2009, Oliver-Bonet et al., 2005, Pigozzi et al., 2005, Sciurano et al., 2007, Sun et al., 2005) in which quadrivalents displayed a high frequency of asynapsis and a tendency to associate with the sex chromosomes. Electron microscopy studies have also indicated a preferential association of chromosome abnormalities with the sex chromosomes in reciprocal translocations (Chandley et al., 1986, Luciani et al., 1987), Robertsonian translocations (Luciani et al., 1984, Rosenmann et al., 1985) and numerical abnormalities (Johannisson et al., 1983).
During meiosis, the X and Y chromosomes pair along two small pseudoautosomal regions and the XY body undergoes transcriptional silencing, known as meiotic sex-chromosome inactivation (MSCI; Handel, 2004, Solari, 1974). MSCI involves localization of phosphorylated H2AX (Celeste et al., 2002) and BRCA1 (Turner et al., 2004, Xu and Stern, 2003). XY body association has been noted in chromosome abnormalities with a high degree of asynapsis (Chandley et al., 1986, Ferguson et al., 2008) and may allow silencing of the unsynapsed regions of the rearrangement in a similar manner to MSCI. Chromosome abnormalities displaying XY body association have shown reduced transcription in the chromosome abnormalities and increased transcriptional activity on the X chromosome (Homolka et al., 2007). This has led to concern that XY body association can result in disruption of XY body inactivation leading to activation of meiotic checkpoints, meiotic arrest and reduced sperm parameters.
As far as is known, this is the first immunofluorescent examination of recombination and synapsis in an inversion (inv(1)) carrier. This study combined fluorescent in-situ hybridization (FISH) with immunofluorescent techniques to study synapsis and recombination, on the inverted chromosome as well as chromosomes 13, 18, 21 and the sex chromosomes. This also allowed the presence of XY body association to be determined. FISH on spermatozoa was used to determine the relationship between recombination and aneuploidy in specific chromosomes. Finally, using antibodies for γH2AX and BRCA1, this study assessed the transcriptional inactivation of the unsynapsed regions of the inversion.
Section snippets
Patient ascertainment
A 45-year-old male was presented with a 3-year history of infertility. He displayed azoospermia, normal hormonal profiles and absence of any physical obstruction in the reproductive tract. However, a karyotype showed that he is a heterozygous carrier of inv(1) (p21q31; Figure 1). Subsequently, he underwent a testicular biopsy to extract spermatozoa for intracytoplasmic sperm injection (ICSI); however, insufficient spermatozoa were retrieved to perform ICSI. A histological examination found
Results
Spermatocytes were classified into stages of meiotic prophase using the criteria described by Ma et al. (2006). A total of 317 cells were examined in the inv(1) carrier and 1553 cells in five control men (Table 1). Germ-cell maturation arrest was noted with increased frequency at zygotene stage (12.7%) and at the zygotene/pachytene transition stage (24.2%) compared with controls (5.3% and 13.8% respectively; P < 0.05).
A total of 53 pachytene nuclei from the inv(1) carrier and 475 control nuclei
Discussion
As far as is known, this is the first systematic examination of recombination, synapsis, meiotic inactivation and sperm aneuploidy in an inv(1) carrier. Similarly to previous studies of chromosome rearrangements carriers, we found a genome-wide frequency of recombination that was within the range observed in the control men. Some studies have displayed decreased recombination on the translocated chromosomes (Ferguson et al., 2008, Pigozzi et al., 2005) while others have not (Leng et al., 2009).
Acknowledgements
The authors thank the patients for their participation and Dr P Moens for providing the antibodies. We also thank Edgar Chan Wong for assisting in the revision of this manuscript. This work was funded by the Canadian Institute of Health Research (MOP53067 to SM). GK is a recipient of a graduate studentship from the Natural Sciences and Engineering Research Council of Canada.
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Gordon Kirkpatrick is currently a second-year medical student at the University of British Columbia (UBC). He graduated from UBC with an MSc. in Reproductive and Developmental Sciences. Under the supervision of Dr. Sai Ma, Gordon examined meiotic behavior in germ cells form infertile men with and without chromosome abnormalities.