The effect of cold storage on recombination frequencies in human male testicular cells

Effect of rapid cryopreservation on meiotic recombination in human spermatocytes

OBJECTIVE

To evaluate the safety of rapid cryopreservation for human testicular tissues by comparing the meiotic recombination in the fresh and thawed testis after rapid freezing.

METHODS

Twelve male patients with prostate cancer (PC) who had given birth to healthy children at youth and need to receive surgical removal of testicular tissue at present were selected in this study. Testicular tissues from 4 cases of PC patients were randomly divided into two parts, one part as fresh tissue and the other to receive rapid freezing treatment. Fidelity analysis for homologous genetic recombination and synapsis were performed by immunofluorescence after prepared by a micro-spreading technique.

RESULTS

The average number of MLH1 foci per cell, autosomal synaptonemal complex (SC) containing 0∼5 MLH1 foci and percent of cells with one MLH1 focus on XY chromosome showed no difference between the fresh and frozen thawed testicular tissues from the same case (P >0.05). And, no significant difference in the frequency of gaps and splits on SCs was observed in fresh and thawed spermatocytes (P > 0.05).

CONCLUSION

Rapid cryopreservation showed little effect on the frequency of meiotic recombination and fidelity of synapsis in human spermatocytes from PC patients, and acted as an effective method for preserving male fertility.

Immunofluorescence analysis of human spermatocytes

New immunofluorescence techniques allow visual identification of human cells in various stages of meiotic prophase. Antibodies to the synaptonemal complex, the centromere and sites of recombination allow these stages of meiotic prophase to be identified. The progress of chromosome synapsis, recombination and associated phenomena such as interference can be studied in normal men, translocation heterozygotes and men with infertility problems. This has greatly stimulated research in human meiosis, leading to many exciting studies on the mechanisms underlying recombination and the generation of chromosome abnormalities.

Cryopreservation has no effect on meiotic recombination and synapsis in testicular tissues

The effects of cryopreservation on meiotic progression, synapsis, recombination, and structure of synaptonemal complexes (SCs) in testicular tissues were evaluated by comparing the above-mentioned parameters in frozen and fresh testicular tissues from the same men. No differences in meiotic progression, the mean number of MLH1 foci per cell, the mean number of autosomal SCs with different numbers of MLH1 foci, or the fidelity of the synapsis were observed between fresh and frozen testicular tissues.

Reduced meiotic recombination on the XY bivalent is correlated with an increased incidence of sex chromosome aneuploidy in men with non-obstructive azoospermia

Both aberrant meiotic recombination and an increased frequency of sperm aneuploidy have been observed in infertile men. However, this association has not been demonstrated within individual men. The purpose of this study was to determine the association between the frequency of recombination observed in pachytene spermatocytes and the frequency of aneuploidy in sperm from the same infertile men. Testicular tissue from seven men with non-obstructive azoospermia (NOA) and six men undergoing vasectomy reversal (controls) underwent meiotic analysis. Recombination sites were recorded for individual chromosomes. Testicular and ejaculated sperm from NOA patients and controls, respectively, were tested for aneuploidy frequencies for chromosomes 9, 21, X and Y. There was a significant increase in the frequency of pachytene cells with at least one achiasmate bivalent in infertile men (12.4%) compared with controls (4.2%, P = 0.02). Infertile men also had a significantly higher frequency of sperm disomy than controls for chromosomes 21 (1.0% versus 0.24%, P = 0.001), XX (0.16% versus 0.03%, P = 0.004) and YY (0.12% versus 0.03%, P = 0.04). There was a significant correlation between meiotic cells with zero MLH1 foci in the sex body and total sex chromosome disomy (XX + YY + XY) in sperm from men with NOA (r = 0.79, P = 0.036).

Variation in crossover interference levels on individual chromosomes from human males

Crossovers (COs) generated by homologous recombination ensure the proper segregation of chromosomes during meiosis. COs exhibit interference, which leads to widely spaced COs along chromosomes. Strong positive CO interference has been found in humans. However, little is known about the extent of human CO interference. In this study, variations in CO interference over the entire human genome and among individuals were analyzed by immunofluorescence combined with fluorescence in situ hybridization of testicular biopsies from 10 control men. These methods allow for direct identification of the frequency and location of COs in specific chromosomes of pachytene cells. The strength of CO interference was estimated by fitting the frequency distribution of inter-CO distances to the gamma model. Positive interference among CO on chromosomes was observed in these men, and the strength of inter-arm interference was significantly stronger than that for intra-arm CO. In addition, interference was observed to act across the centromere. Significant inter-individual and inter-chromosomal variations in the levels of interference were found, with smaller chromosomes exhibiting stronger interference. Discontinuous chromosome regions (gaps) and unsynapsed chromosome regions (splits) in chromosome 9 had both cis and trans effects on CO interference levels. This is the first report that the interference level varies significantly across the whole genome and that, at least in the human male, anomalies in chromosome synapsis play an important role in altering CO interference levels.

Discontinuities and unsynapsed regions in meiotic chromosomes have a trans effect on meiotic recombination of some chromosomes in human males

During meiosis, homologous chromosome pairing and synapsis are essential for subsequent meiotic recombination (crossing-over). Discontinuous regions (gaps) and unsynapsed regions (splits) were most frequently observed in the heterochromatic regions of bivalent synaptonemal complex (SC) 9, and we have previously demonstrated that gaps and splits significantly altered the distribution of MLH1 recombination foci on SC 9. Here, immunofluorescence techniques (using antibodies against SC proteins and the crossover-associated MLH1 protein) were combined with a centromere-specific fluorescence in situ hybridization technique that allows identification of every individual chromosome. The effect of gaps/splits on meiotic recombination patterns in autosomes other than chromosome 9 during the pachytene stage of meiotic prophase was then examined in 6,026 bivalents from 262 pachytene cells from three human males. In 64 analyzed cells with a gapped SC 9, the frequency of MLH1 foci in SCs 5 and 10 and in SC arms 10q, 11p and 16q was decreased compared to 168 analyzed cells with a normally-synapsed SC 9 (controls). In 24 analyzed cells with splits in SC 9, there was a significant reduction in MLH1 focus frequency for SC 5q and the whole SC5 bivalent. The positioning of MLH1 foci on other SCs in cells with gapped/split SC 9 was not altered. These studies suggest that gaps and splits not only have a cis effect, but may also have a trans effect on meiotic recombination in humans.

Abnormal progression through meiosis in men with nonobstructive azoospermia

OBJECTIVE

To study meiotic abnormalities in men with nonobstructive azoospermia.

DESIGN

Analysis of synaptonemal complex and recombination in testicular tissue.

SETTING

Research laboratory.

PATIENT(S)

Twenty-nine men with nonobstructive azoospermia and 12 men with normal spermatogenesis.

INTERVENTION(S)

Testicular tissues were processed with immunofluorescent staining using antibodies against proteins associated with synaptonemal complex and recombination events.

MAIN OUTCOME MEASURE(S)

Synaptonemal complex configuration and recombination in meiosis I.

RESULT(S)

In patients with nonobstructive azoospermia, a marked heterogeneity in spermatogenesis was found: nearly half of them had a complete absence of meiotic cells, one case had germ cells arrested at the zygotene stage of meiotic prophase, and, in general, the rest had impaired fidelity of chromosome synapsis and recombination in pachytene cells. Compared with controls, these patients had significantly more cells in leptotene/zygotene and higher frequencies of unpaired chromosome regions in pachytene. Significantly reduced recombination, an increased frequency of achiasmate autosome bivalents, and sex univalents in pachytene were also observed in these patients with nonobstructive azoospermia.

CONCLUSION(S)

Defects in chromosome synapsis and decreased recombination during meiotic prophase may have led to spermatogenesis arrest and contributed in part to the unexplained infertility in these patients.

Crossover analysis using immunofluorescent detection of MLH1 foci in frozen–thawed testicular tissue

To date, the effects of freezing on spermatogenesis have not yet been fully investigated at a molecular level. Antibody localization studies have identified the MutL homolog 1 (MLH1) protein, a mis-match repair protein, at the prophase I stage of meiosis, which allows the detection of recombination foci during pachytene. This study investigated the effect of long-term testicular tissue cryopreservation on meiotic prophase I, identified by recombination foci frequency and synaptonemal complex (SC) integrity. Frozen-thawed testicular tissues from 12 males who had each fathered a child were used. Because vasectomy or reverse vasectomy procedures are rare in the locale of the investigation, it was not possible to obtain fresh testicular tissue and use the males as their own controls. Immunocytogenetic analysis of 612 spermatocytes at the pachytene stage was performed. The results indicated a mean number of MLH1 foci of 49.2 (SD +/- 5.9), and no correlation was found between the freezing period, the MLH1 frequency and the SC integrity. The results suggest that freezing of testicular tissue taken post-puberty does not appear to be detrimental to the crossover process as identified by occurrence of MLH1 loci.

Immunofluorescent synaptonemal complex analysis in azoospermic men

The molecular cause of germ cell meiotic defects in azoospermic men is rarely known. During meiotic prophase I, a proteinaceous structure called the synaptonemal complex (SC) appears along the pairing axis of homologous chromosomes and meiotic recombination takes place. Newly-developed immunofluorescence techniques for SC proteins (SCP1 and SCP3) and for a DNA mismatch repair protein (MLH1) present in late recombination nodules allow simultaneous analysis of synapsis, and of meiotic recombination, during the first meiotic prophase in spermatocytes. This immunofluorescent SC analysis enables accurate meiotic prophase substaging and the identification of asynaptic pachytene spermatocytes. Spermatogenic defects were examined in azoospermic men using immunofluorescent SC and MLH1 analysis. Five males with obstructive azoospermia, 18 males with nonobstructive azoospermia and 11 control males with normal spermatogenesis were recruited for the study. In males with obstructive azoospermia, the fidelity of chromosome pairing (determined by the percentage of cells with gaps [discontinuities]/splits [unpaired chromosome regions] in the SCs, and nonexchange SCs [bivalents with 0 MLH1 foci]) was similar to those in normal males. The recombination frequencies (determined by the mean number of MLH1 foci per cell at the pachytene stage) were significantly reduced in obstructive azoospermia compared to that in controls. In men with nonobstructive azoospermia, a marked heterogeneity in spermatogenesis was found: 45% had a complete absence of meiotic cells; 5% had germ cells arrested at the zygotene stage of meiotic prophase; the rest had impaired fidelity of chromosome synapsis and significantly reduced recombination in pachytene. In addition, significantly more cells were in the leptotene and zygotene meiotic prophase stages in nonobstructive azoospermic patients, compared to controls. Defects in chromosome pairing and decreased recombination during meiotic prophase may have led to spermatogenesis arrest and contributed in part to this unexplained infertility.

Temporal progression of recombination in human males

To date, immunocytology has been used in humans to detect a limited number of meiotic proteins: components of the synaptonemal complex (SCP1 and SCP3) and some proteins known to participate in recombination events, such as MLH1 or RAD51. However, the colocalization or coexistence of proteins known to participate during the different stages of human meiosis remains largely unstudied, and these studies could provide important clues about the mechanics of recombination. This work reports the relative timing and localization of five different meiotic proteins that have previously been implicated in human homologous recombination [RAD51, replication protein A (RPA), MSH4, MLH1 and MLH3]. MSH4 foci appear concurrently with synapsis initiation at zygotene, shortly after the first RAD51 foci are detected. The presence of RPA in MSH4 foci was noted, suggesting that these two proteins may act co-operatively. Both RPA and MSH4 foci reach maximal numbers at the end of zygotene, when synapsis is concluding. From this point, RPA foci all but disappear by the end of pachytene, whereas MSH4 foci decline to a stable number at mid-pachytene, where they localize with MLH1/MLH3 recombination sites. We discuss a possible role for MSH4 in synapsis initiation and/or maintenance.

Discontinuities and unsynapsed regions in meiotic chromosomes have a cis effect on meiotic recombination patterns in normal human males

During meiosis, homologous chromosome pairing is essential for subsequent meiotic recombination (crossover). Discontinuous chromosome regions (gaps) or unsynapsed chromosome regions (splits) in the synaptonemal complex (SC) indicate anomalies in chromosome synapsis. Recently developed immunofluorescence techniques (using antibodies against SC proteins and the crossover-associated MLH1 protein) were combined with fluorescence in situ hybridization (using centromere-specific DNA probes) to identify bivalents with gaps/splits and to examine the effect of gaps/splits on meiotic recombination patterns during the pachytene stage of meiotic prophase from three normal human males. Gaps were observed only in the heterochromatic regions of chromosomes 9 and 1, with 9q gaps accounting for 90% of these events. Most splits were also found in chromosomes 9 and 1, with 58% of splits occurring on 9q. Gaps and splits significantly altered the distribution of MLH1 foci on the SC. On gapped SC 9q, the frequency of MLH1 foci was decreased compared with controls, and single 9q crossovers tended toward a more distal distribution. Furthermore, the larger the gap the more distal the location of the MLH1 focus closest to the q arm's telomere. MLH1 foci on split SC 9 had distributions similar to those of gapped SC 9; however, splits did not change the frequencies of MLH1 foci on SC 9. This is the first demonstration that gaps and splits have an effect on meiotic recombination in humans.

Meiotic studies in an azoospermic human translocation (Y;1) carrier

A reciprocal translocation between the long arm of the Y chromosome and the long arm of chromosome 1 was observed in an infertile man with non-obstructive azoospermia. The study was performed using a combination of techniques: immunocytogenetic analysis, which allows the detection of synaptonemal complexes (SCs) and recombination sites (MLH1) simultaneously, and fluorescence in-situ hybridization analysis. Meiotic pairing analysis on 100 pachytene spreads showed the presence of a quadrivalent containing chromosomes 1 and Y. There were many abnormalities in chromosome pairing and recombination. These abnormalities included a great reduction of recombination events (as many as one fifth of the SCs had no MLH1 foci), and high proportions of unpaired regions and discontinuities in the SCs. We discuss the possibility that infertility in this patient may be due to transcriptional repression of part of chromosome 1 involved in the translocation, silencing some genes necessary for the progression of meiosis and causing defective meiotic pairing and recombination.