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Human Molecular Genetics Advance Access originally published online on June 27, 2006
Human Molecular Genetics 2006 15(15):2376-2391; doi:10.1093/hmg/ddl162
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Variation in MLH1 distribution in recombination maps for individual chromosomes from human males

Fei Sun1,2, Maria Oliver-Bonet1,2, Thomas Liehr3, Heike Starke3, Paul Turek4,5, Evelyn Ko2, Alfred Rademaker6 and Renée H. Martin1,2,*

1 Department of Medical Genetics, University of Calgary, Calgary, Canada T2N 4N1, 2 Department of Genetics, Alberta Children's Hospital, 1820 Richmond Road S.W., Calgary, Alberta, Canada T2T 5C7, 3 Institute of Human Genetics and Anthropology, 07743 Jena, Germany, 4 Department of Urology, University of California San Francisco, San Francisco, CA 94143-1695, USA, 5 Department of Obstetrics and Gynecology and Reproductive Sciences, University of California San Francisco, San Francisco, CA 94143-1695, USA and 6 Department of Preventive Medicine, Northwestern University Medical School, Chicago, IL 60611-4402, USA

* To whom correspondence should be addressed. Tel: +1 4039437369; Fax: +1 4035439100; Email: rhmartin{at}ucalgary.ca

Received April 18, 2006; Revised May 22, 2006; Accepted June 22, 2006


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 RESULTS AND DISCUSSION
 MATERIALS AND METHODS
 REFERENCES
 
Meiotic recombination is essential for the segregation of homologous chromosomes and the formation of normal haploid gametes. Little is known about patterns of meiotic recombination in human germ cells or the mechanisms that control these patterns. Documentation of the normal range of variability of recombination distribution over the genome among individuals is an essential prerequisite for understanding abnormal recombination patterns, which may be associated with non-disjunction and chromosome rearrangements. In this article, variation in recombination maps for individual chromosomes among 10 normal human males is examined for the first time. An immunocytogenetic approach allowed analysis of pachytene cells, using antibodies to detect the mature synaptonemal complex (SCP1/SCP3), the centromere (CREST) and sites of crossing over (MLH1). Individual bivalents were identified with centromere-specific multicolor fluorescence in situ hybridization. Significant heterogeneity in MLH1 focus frequency across donors was observed for larger chromosome arms (P<0.05, one-way ANOVA). Significant inter-donor variation in the overall crossover frequency per cell was also found (P<0.0001, one-way ANOVA). Furthermore, several chromosome arms showed significant differences in crossover distribution along the SCs among donors. Inter-individual variation in interference distances was observed for all chromosomes. The significance of altered recombination patterns among individuals and the role of interference are discussed.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
RESULTS AND DISCUSSION
 MATERIALS AND METHODS
 REFERENCES
 
Meiotic crossing over, the exchange of genetic materials between homologous chromosomes, generates not only genetic variation but is also vital for the correct segregation (disjunction) of homologous chromosomes during meiosis I (1). Historically, meiotic recombination in humans has been investigated by cytological and genetical methods and, more recently, by immunocytogenetic approaches. The cytological method is based on recording the numbers and locations of chiasmata in bivalents at late prophase I or metaphase I of meiosis (2–5). Genetic measures of recombination in the human are derived from the identification of genotype data in families (6). Immunocytogenetic assays of the mismatch repair protein MLH1 allow the precise identification of recombination foci along the synaptonemal complexes (SCs; the proteinaceous structure linking homologous chromosomes in prophase of meiosis I) (7–12). This assay, combined with centromere-specific multicolor fluorescence in situ hybridization (cenM-FISH) (12–14), permits the analysis of crossover frequencies and distributions in individual chromosomes in human germ cells in a precise manner (12,13,15). Thus, immunocytogenetic methods provide a means to directly determine the pattern of recombination across the whole genome, and at the chromosomal level—information that cannot be readily obtained by other means.

Variation in recombination frequency is revealed by these methods, among species and individuals, and between the sexes [for reviews see (16–19); for papers see (5,6,20–27)]. However, this variability has only been demonstrated in the overall number of recombinations per cell or in recombination frequencies in different genomic regions. Little is known about the recombination frequency and distribution for individual chromosomes in human males, and variation in the distribution of recombination among individuals has not been reported to date.

There have been very few studies on human recombination patterns. Hultén and colleagues have carried out cytological studies on chiasmata frequency in individual chromosomes from seven infertile men (3,28) and chiasmata locations in two men (one fertile, one infertile) (2,28). Using immunocytogenetic assays, recombination frequencies and distributions have been reported for only four chromosomes (1, 16, 21 and 22) in the male (10) and four chromosomes (X, 18, 21 and 22) in the female (11). Recently, we reported the first recombination maps for every autosome in a control donor (12). This report detailed the frequency and distribution of every MLH1 focus on each autosomal SC, and demonstrated that chromosomes at pachytene in the human male have a characteristic pattern of recombination distribution, i.e. a non-random distribution of crossovers. However, the variation in recombination frequency and distribution in individual chromosomes among different men has not been reported to date.

It is crucial to understand the extent of variation in the distribution of recombination events over the genome among individuals, as this provides information about the regulation of recombination and has important implications for the genesis of aneuploidy and identification of disease genes. For example, the total number of recombination events on a specific chromosome depends on a number of factors, such as size (in general, larger chromosomes undergo more meiotic exchanges) (10). This trend has also been observed in recombination maps for individual chromosomes in a human male (12). However, size does not appear to be the sole factor mediating recombination frequencies, as some smaller chromosomes have higher than expected frequencies of recombination (12). Variations in the distribution of recombination also has implications for the genesis of aneuploidy, as reduced recombination frequency and alterations in the positioning of crossovers are found to be risk factors for human non-disjunction (29). Thus, documentation of the normal range of variability is an essential prerequisite for the understanding of changes that are observed in abnormal situations such as non-disjunction, or when a chromosome rearrangement is present. In addition to its value for mapping and identifying diseases, this knowledge can help us explore the chromosomal effects on recombination patterns and provide clues to the source of variation in recombination.

In this study, we extend the analysis of the distribution of crossing-over patterns for all 22 autosomes to 10 ‘control’ males using immunocytogenetic and cenM-FISH assays, in order to identify inter-individual variation in crossover distribution. The ‘control’ population was composed of seven men seeking vasectomy reversal and three cancer patients. The testicular samples demonstrated normal spermatogenesis and we were gratified that there was no significant difference in results from cancer patients when compared with the vasectomy reversals. An ideal population for study would be normal fertile men, but they seem reluctant to donate testicular tissue. To our knowledge, this is the first whole-genome crossover distribution analysis in a population of men.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS AND DISCUSSION
MATERIALS AND METHODS
 REFERENCES
 
An example of a pachytene SC spread, with identification of individual bivalents, and cenM-FISH signals in the same cell, is shown in Figure 1. In all, 1000 pachytene-stage spermatocytes (100 cells per male) were analyzed to determine the mean MLH1 focus frequency for autosomal cells in these 10 men; overall, there was a mean of 49.7 MLH1 foci (range 21–67) per cell. This frequency was similar to that in previous reports (12,25,30). A total of 824 pachytene stage spreads (18 128 autosomal bivalents) with unambiguous cenM-FISH signals for every individual chromosome was analyzed to determine the MLH1 focus distribution in each bivalent for the 10 men.


Figure 1621
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  		Figure 1. (Upper) Human pachytene spermatocyte with SCs shown in red, centromeres in blue and MLH1 foci in yellow. (Lower) Subsequent cenM-FISH analysis allows identification of individual chromosomes so that recombination (MLH1) foci can be analyzed for each SC.

 
Inter-individual variation in recombination frequency
Donor age versus the mean number (±SD) of MLH1 foci per cell in each individual male is shown in Figure 2. Significant heterogeneity for the total mean MLH1 focus frequency for all chromosomes was observed among the males (P<0.0001) (Fig. 2), an inter-individual difference of nearly 13%, with a range of 46.4±5.5–53.2±4.9. In addition, the lower the mean number of autosomal MLH1 foci per cell, the more SCs with a single focus and the fewer SCs with multiple foci were observed (Table 1). Conversely, men with higher mean numbers of autosomal MLH1 foci had more multiple-focus SCs and fewer single-focus SCs (Table 1). Other cytogenetic studies (3) on diakinesis stage spermatocytes and MLH1 foci in pachytene spermatocytes (10,30) have also demonstrated significant heterogeneity in the frequency of crossing-over among men. Lynn et al. (10) found a difference of almost 15% in the mean frequency of MLH1 foci among 14 men. Codina-Pascual (30) reported two men with widely different mean MLH1 frequencies. However, none of these studies have systematically analyzed variation in MLH1 focus frequencies and the distribution of crossing-over for all bivalents in normal men.


Figure 1622
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  		Figure 2. The relationship between the mean number of MLH1 foci per cell and donor age among 10 individuals. Highly significant inter-individual variation was observed among the 10 individuals, with the mean number of foci ranging from 46.4 to 53.2. Age contributes significantly to this inter-individual difference.

 


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  		Table 1. MLH1 focus frequency in autosomal SCs per cell and frequency of non-crossover bivalents among 10 individual men. One hundred pachytene stage cells were analyzed per donor

 
The possibility that age influences the mammalian recombination frequency has been a contentious subject. Several groups have observed an age-related decline in recombination in oocytes from older mice (31–33) and hamsters (34). A small decrease in recombination, especially at the telomeres, with increasing maternal age has been reported for chromosome 21 in humans (35). However, there is little evidence for paternal age-related effects on recombination in mammals from linkage analysis (24), single-sperm genotyping (36) or MLH1 assays (10,25). In this study, it was noted that older men tended to have lower mean autosomal chromosome MLH1 focus frequencies (Fig. 2). Furthermore, the overall correlation between donor age and the mean MLH1 focus frequency per cell was significant (P<0.01) (Fig. 2). These results are different from previous observations (10,25) using the same assays. Whether donor age contributes to variation in the recombination frequency among individual males will need further investigation.

Non-crossover chromosome pairs (SC with 0 MLH1 foci) were also observed in this study. Sex chromosomes and G-group chromosomes had the highest frequencies of SCs displaying no MLH1 foci. The proportion of cells containing XY univalents, and non-crossover SCs 21 and 22 varied from 9–44%, 0–6% and 0–9%, respectively, in the 10 males (Table 1). The relationship between the mean frequency of MLH1 foci per autosomal SC, and the percentage of cells with 0 MLH1 foci in the XY body, in SC21 and in SC22 was also explored (Table 1). Unlike Codina-Pascual et al. (30,37), no significant correlation between lower recombination frequencies and a higher incidence of SCs without an MLH1 focus in the sex body or in the smallest SCs was observed. Thus, more studies are needed in order to determine whether or not a lower mean MLH1 frequency predisposes to non-crossover bivalents (possible future aneuploidy).

The mean length of each arm and of the whole chromosome at pachytene from the 10 men is provided in Table 2. In general, the relative lengths of meiotic chromosomes were closely correlated with the corresponding values from their mitotic counterparts (P<0.001) (38). Bivalent SCs 1–7 rank the first seven in length according to their meiotic lengths. However, unlike their mitotic counterparts (38), meiotic chromosome 5 was found to be slightly longer than 4, 12 was slightly longer than 11 and 15 was slightly longer than 13 and 14. All of these results were in close agreement with our previous data on one man (12). Recently, Kong et al. (6) reported that the intensity of G-band staining is inversely related to recombination frequency in humans. As SC lengths co-vary with the mean crossover frequency, this would predict that chromosomes with a higher percentage of G-bands would have shorter SCs and lower levels of recombination than that would be expected from their relative mitotic chromosome length. Indeed, we observed this pattern in the recombination map from one man (12), and this trend continued in the current study (Table 2). For example, chromosome 13 has a very high percentage of G-bands, and its relative SC length is much shorter than the corresponding mitotic length. SC 13 length ranks 18th (mean absolute length 11.02 µM), shorter than SCs 14–17 (lengths 11.62–12.78 µM). In results from 10 men, the mean MLH1 focus frequency in SC 13 is lower than that in SCs 15–17 (Table 3), corroborating results from a recombination map based on family genotyping (6). G-band chromatin regions are generally gene-poor. Thus, differences in G-band chromatin content may account for some of the variability in overall recombination frequency as well as the specific recombination patterns in different bivalents.


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  		Table 2. Length of 22 autosomal SCs. All autosomal bivalents were analyzed in 824 pachytene cells

 


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  		Table 3. MLH1 focus frequencies for individual autosomal SCs in 10 males

 
The mean number (±SD) of MLH1 foci in the p-arm, q-arm and entire bivalent for each chromosome is provided in Table 3. Significant heterogeneity in MLH1 focus frequency across individuals was observed on the p-arms of chromosomes 1–4 (P<0.0001 for chromosomes 1–3, P=0.0088 for chromosome 4), 6 (P=0.0077), 8 (P=0.0122), 10 (P=0.0189), 12 (P=0.0031), 13 (P=0.0396), 18 (P=0.0007), 20 (P=0.0104); q-arms of all chromosomes except 15–18, 21 (P<0.0001 for chromosomes 1–7, 9–12, P=0.0356 for chromosome 8, P=0.0396 for chromosome 13, P=0.005 for chromosome 14, P=0.0124 for chromosome 19, P=0.0064 for chromosome 20, P=0.0138 for chromosome 22) and whole bivalents of all chromosomes except 16 and 21 (P<0.0001 for chromosomes 1–7, 10–12, 18, P=0.0004 for chromosome 8, P=0.0002 for chromosome 9, P=0.0309 for chromosome 13, P=0.0024 for chromosome 14, P=0.0152 for chromosome 15, P=0.0063 for chromosome 17, P=0.0027 for chromosome 19, P=0.0009 for chromosome 20, P=0.0043 for chromosome 22) (Table 3). The degree of heterogeneity in bivalent MLH1 focus frequency in larger chromosomes (A–C groups) (all P<0.0001 or P<0.001) was greater than in smaller chromosomes (D–G groups) (all P<0.01). The results indicate that all of the larger chromosome arms, with the exception of 15q, have a significantly variable MLH1 focus frequency. That variation in MLH1 focus frequency occurs mostly in larger chromosome arms may be partly explained by the fact that larger chromosomes usually have more than two recombination sites and thus have more opportunity for variation. This, in turn, may contribute to the variation in mean cell MLH1 focus frequency, as changes in the recombination frequencies of all or many of the larger chromosomes (i.e. the significantly varied chromosomes) were reflected in the overall cell MLH1 focus frequencies (Table 3). For example, among all the donors, donor 4 had both the highest mean autosomal MLH1 focus frequency and the highest mean number of MLH1 foci in SCs 1, 3, 4, 7, 9–11 of the donors (Table 3). In contrast, inter-individual variation in MLH1 focus frequency in the smaller chromosome arms (i.e. the p-arms of the B–G groups and the q-arms of the E–G groups) was not significant. It is likely that this trend will remain so even if more men are examined as the smaller chromosome arms are almost always constrained to a single recombination focus and thus have less opportunity for variation.

Conventional cytogenetic approaches have been used to study genome-wide variation in chiasma distribution in human spermatocytes at the diakinesis stage (2,3,28). The overall frequency of chiasmata in autosomal cells was examined in seven infertile men, of which, results on the variation in individual chromosomes were obtained from two to four men, depending on the chromosome (3). In individual chromosomes, significant inter-individual variation was found in the large chromosome arms (p-arm of chromosomes 1 and 2; q-arm of chromosomes 1, 9, 12 and 14) (3). However, these data were derived from a small number of men with idiopathic infertility and thus they are of limited applicability.

On the basis of these results, the range of variability in autosomal cell recombination frequency within the control population of men is from 46.4 to 53.2. It is not possible to relate recombination frequencies observed in this study to sperm aneuploidy frequencies, as we do not have corresponding data on sperm chromosome abnormalities from these 10 men. However, low mean MLH1 focus frequencies have been associated with spermatogenic arrest in infertile human males (39,40), indicating that males with MLH1 focus frequencies that fall below the minimum needed for proper segregation are susceptible to infertility. The existence of a pachytene checkpoint for detecting abnormalities in meiotic recombination has been proposed (41,42). Mice with knockout mutations for MLH1 and MLH3 were observed to have meiotic arrest and apoptosis (43–45), which were attributed to the activation of a recombination checkpoint that arrests meiosis I at pachytene.

Significant reductions in recombination have been observed in paternally derived cases of trisomy 21 (Down syndrome) (46). Direct PCR analysis of human sperm (single sperm typing) indicated that lack of recombination in the pseudoautosomal region was a significant cause of XY non-disjunction (36). Also, liveborn paternally derived cases of Klinefelter Syndrome (47,XXY) were shown to be associated with a lack of recombination between the X and Y chromosomes (47,48). Thus, reduced or absent recombination is a molecular risk factor for chromosome non-disjunction in human males, which in turn predisposes to the formation of aneuploid (chromosomally imbalanced) gametes. This association is also evidenced in human females (49). Genetic maps based on maternal meiosis I non-disjunctional errors were generated for human trisomies of chromosomes 15, 16, 18 and 21 and the sex chromosomes, and in each instance the mapped genetic length was significantly shorter in trisomies than in controls [reviewed in (19)].

Inter-individual variation in recombination position and interference
The distribution of MLH1 foci within each chromosome for each male is graphed in Figure 3. Overall distributions of MLH1 foci are presented for each chromosome. Similar to an earlier investigation on one donor (12), MLH1 focus distribution on each chromosome in all 10 donors was generally found to be predominantly near the medial and terminal areas of the chromosome, with repression of foci near the centromeres. No MLH1 foci were observed in heterochromatic regions of the chromosomes.


Figure 1623
Figure 1623
Figure 1623
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  		Figure 3. Comparisons of the distribution of MLH1 foci for individual SCs among 10 men. The X-axis represents the position of the foci from the p telomere (left) to the q telomere (right). The centromere is labeled ‘c’. Each chromosome arm is divided into 10% intervals. The Y-axis shows the frequency of MLH1 foci in each interval. Data for p-arms of acrocentric chromosomes are not shown as there was no significant difference in the number of MLH1 foci in each interval among donors.

 
There were significant differences in the distribution of MLH1 foci among these 10 males in the p-arms of chromosomes 1–3 (P=0.0226 for chromosome 1, P=0.004 for chromosome 2, P=0.0013 for chromosome 3), 5 (P=0.0226), 6 (P<0.0001), 8–12 (P<0.0001 for chromosome 8, P=0.0042 for chromosome 9, P=0.034 for chromosome 10, P=0.0005 for chromosome 11, P=0.033 for chromosome 12), 16 (P=0.042), 17 (P=0.019); and in the q-arms of chromosomes 4 (P=0.0049), 7–12 (P=0.029 for chromosomes 7, 10, P=0.013 for chromosome 8, P=0.0017 for chromosome 9, P=0.037 for chromosome 11, P=0.0016 for chromosome 12), 15–17 (P=0.0026 for chromosome 15, P=0.0017 for chromosome 16, P=0.036 for chromosome 17) and 19–22 (P=0.01 for chromosome 19, P=0.0052 for chromosome 20, P=0.0041 for chromosome 21, P=0.0049 for chromosome 22) (Fig. 3). These significant differences in MLH1 focus position across donors occurred predominantly in distal regions of the arms. Of the chromosomes that had significant variability in MLH1 distribution, the maximum variability of MLH1 focus frequency per interval was found in the distal 10% of the arm (8 out of the 12 significant results for p-arm, 7 of 14 for q-arm) or the adjacent 10% (3/12 for p-arm, 6/14 for q-arm) of the arm (Fig. 3).

It appears that certain chromosome arms have altered recombination distribution, whereas other chromosomes of similar morphology do not. For example, most of the q-arms of D group chromosomes had two MLH1 foci and similar positioning patterns, only chromosome 15q showed inter-donor variability in MLH1 focus distribution. These findings suggest that recombination distribution is not altered in a simple fashion, and that more than one mechanism may alter recombination throughout the karyotype.

Interference (i.e. when the presence of a crossover in a specific chromosomal site reduces the likelihood of a nearby recombination event) affects the relative position of multiple exchanges. Positive interference is an important factor influencing recombination distribution on SCs. Random placement of two exchanges on an SC is expected to provide an average separation distance of ~33% (50). The mean relative separation distance between two MLH1 foci on all SCs was 68%, more than twice that distance, indicating that MLH1 foci demonstrate positive interference. Other studies have also found similar evidence of interference (10,30,51). The mean interference distance for every chromosome with two MLH1 foci in each male is provided in Table 4. There were significant differences in mean inter-focus distances in all chromosomes for the 10 men (P<0.0001 for chromosomes 1–4, 6–11, 13, 16–18, 20–22; P≤0.01 for chromosomes 5, 12, 14, 19; P=0.029 for chromosome 15).


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  		Table 4. Comparisons of interference distances on the SCs with two foci among different males

 
Interference is important in meiosis, as it results in a more even spacing of the limited number of chiasmata per meiosis along the SC arms, and therefore in homologues that are held together more securely by the crossovers. In this study, variation in interference distances among human males implies that the lessening of interference between two adjacent recombination foci can cause them to shift together on the SC, leaving more distance for the formation of crossovers. This in turn allows the SC to accommodate a larger number of MLH1 foci, and leads to the enrichment of foci in certain hotspot regions on the SCs. In addition, a decrease in the recombination frequency, accompanied by an increase in the degree of crossover interference was observed in this study (Tables 1 and 4). Thus, differences in the strength of crossover interference may play a role in the alteration of recombination maps. In addition, variation differences in interference distance are lower in smaller chromosomes than in larger chromosomes (Table 4). For example, the maximal difference of interference distance in SC1 was 26.2%, whereas SC20 was 7.5%. In other words, large chromosomes appear to have higher levels of interference than do smaller chromosomes. These results suggest that the strength of interference may not be the same in all regions of all chromosomes.

Hulten and colleagues (2,28) have also determined chiasma location in individual chromosomes in two men (one of normal fertility, and one infertile patient), using conventional cytogenetic approaches (2,3,28). Variation in chiasma distribution and mean inter-chiasma distances were also observed in some chromosomes (significant positional differences were found in the p-arms of chromosomes 1, 4, 8 and 9 and in the q-arms of 1, 2, 16 and 17; significant differences in interference distance were found in chromosome 1p, and in the q-arms of chromosomes 9 and 10) (2). However, these studies were limited in the small number of diakinesis cells that could be examined in only two men.

In this study, the first detailed information on the number and distribution of recombination foci on each chromosome in a population of men is presented. The existence of significant inter-individual variation in recombination maps implies that human males differ in genetic elements that may affect recombination patterns. These may be differences in genes that modify or regulate effects on recombination distribution, or differences in chromosome structure. Further research is needed to identify these elements to allow a better understanding of how recombination is regulated, and to explore potential clinical implications of this variation among men.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS AND DISCUSSION
 MATERIALS AND METHODS
REFERENCES
 
Sample collecting
Testicular samples were obtained from patients undergoing orchiectomy for testicular cancer (n=3; Calgary, Canada) and vasectomy reversal (n=7; University of California San Francisco, USA). These donors (47–81 years) were ascertained for reasons unrelated to meiotic defects and had testicular biopsies that showed normal spermatogenesis. Testicular tissues were kept in phosphate-buffered saline (PBS, pH 7.4) until use, and transferred on ice to Calgary by air courier, where appropriate. We have previously demonstrated that cold storage of testicular tissue for 2 days does not affect recombination frequencies (52). Patients gave informed consent and this study received ethical approval from the institutional review boards at the University of Calgary and at the University of California San Francisco.

Fluorescence immunostaining and CenM-FISH
Slides with meiotic cells were subjected to immunofluorescence staining as described previously (12). Primary antibodies against the following proteins were used: SCP1 (transverse filament proteins of the SC, 1:1000 dilution, a gift from P. Moens, York University), SCP3 (lateral element proteins of the SC, 1:250 dilution, a gift from T. Ashley, Yale University), MLH1 (marks the site of recombination foci, 1:100 dilution, Oncogene, San Diego, CA, USA), and CREST (Calcinosis, Raynaud's phenomenon, Esophageal dysfunction, Sclerodactyly, Telangiectasia, marks the centromere; 1:100 dilution, a gift from M. Fritzler, University of Calgary). These primary antibodies were detected using a cocktail of secondary antibodies (donkey antisera) conjugated with different fluorochromes: 1-amino-4-methylcoumarin-3 acetic acid and Cy3 (1:100 dilution, Jackson Immunoresearch, West Grove, PA, USA), Alexa 488 and Alexa 555 (1:125 dilution, Molecular Probes, Eugene, OR, USA). Primary and secondary antibodies were incubated at 37°C, overnight and for 90 min, respectively. Slides were examined on a Zeiss Axiophot epifluorescence microscope equipped with propidium iodide, fluorescein isothiocynate and 4',6-diamidino-2-phenylindole (DAPI) filters and a cooled charged coupled device camera. Three fluorescent images (red, green, blue) of the SC, MLH1 sites and CREST locations, respectively, were captured using Applied Imaging Cytovision 3.1 software (Applied Imaging Corporation, Santa Clara, CA, USA). Spreads were localized using a gridded finder slide.

Each pachytene-stage nucleus used for analysis met the following criteria (1): the correct number of bivalents (i.e. 22 autosomes and one sex body) were present (2); SCs were not broken or obscured by overlaps, allowing all foci to be scored. MLH1 signals were scored if they were distinct and localized on an SC. SCs were classified as normally synapsed if they were without irregularities such as visible bubbles, forks or separated loops. One hundred pachytene-stage cells were analyzed from each donor, and the number of MLH1 foci per autosomal bivalent and the total number of foci per autosomal complement were scored.

After analysis of the captured immunofluorescence images, cenM-FISH was carried out on the same spermatocytes, allowing simultaneous identification of each autosome. Techniques developed by Nietzel et al. (14) and Oliver-Bonet et al. (13) were modified to make use of the microwave-decondensed/codenatured FISH technique (53). Cells were decondensed for 5 s in dithiothreitol (DTT) followed by 30 s in 3,5-diiodosalicylic acid, lithium salt/DTT at medium power (550 W). Hybridization buffer [10% dextran sulfate, 2x standard sodium citrate (SSC), 55% formamide] was prewarmed to 50°C, added to the cenM-FISH probes and warmed at 50°C until all probe was dissolved. Probes were applied to the slide, a glass cover-slip was sealed in place with rubber cement, the probes and cells were microwave codenatured for 80 s at 1100 W and the slide was incubated in a humid chamber at 37°C for 24–48 h. A post-hybridization wash (0.4x SSC/1% NP-40, 70°C) was carried out, streptavidin-Alexa 647 (1:58 dilution, Molecular Probes) solution was applied under a glass cover-slip and the slide was incubated at 37°C for 40 min in a humid chamber. The slide was washed, with constant agitation, for 10 min in 4x SSC, air dried and mounted in DAPI. Cells previously analyzed by antibody immunostaining were relocated, and six fluorescent images (blue, aqua, green, gold, red and far red) were captured for each cell, using Applied Imaging Cytovision 3.1 software (Applied Imaging Corporation, Santa Clara, CA, USA).

Analysis of MLH1 focus distribution on SCs
After cenM-FISH identification of each pachytene bivalent, images of the corresponding SC spreads were analyzed for MLH1 focus distribution and measurement of the length of the SCs using MicroMeasure 3.3 (available from the MicroMeasure Website, http://www.colostate.edu/Depts/Biology/MicroMeasure). MicroMeasure is an image analysis application that allows collection of data for a wide variety of chromosome parameters from electronically captured images (54).

As there is no significant difference in the mean MLH1 focus frequency between the two groups (cancer patients and vasectomy patients; P=0.68, nested ANOVA, with the person nested within the group), data on the average absolute length for each autosomal SC were pooled for the 10 men. The numbers of MLH1 foci per bivalent and per SC spread were scored in all 10 males. The position of each MLH1 focus on each SC was recorded as a relative position using distance (percentage of total length of respective arms) from the centromere. Each SC arm was divided into 10% length intervals, and MLH1 focus distributions were obtained by summing the number of foci in each interval for SCs in the same length class. The overall MLH1 focus distribution for each SC in each male was displayed as a smooth line. In order to examine the variation in distribution, the distribution of MLH1 foci for each SC was accumulated in a graph from 10 males.

Inter-crossover distance was used to calculate interference in each chromosome, by measuring the intervals between MLH1 foci along the SCs, in order to determine if there are inter-individual differences among males. As the mean separation distance between foci on SCs with two foci was found to be more than twice the expected distance between two randomly placed foci on the SC (12), this distance is a significant indicator of positive interference. In this study, interference in each chromosome was expressed as the relative inter-focal distance along SCs with two MLH1 foci: first, absolute MLH1 focus-to-centromere distances were measured using MicroMeasure. Depending on whether both foci were located on one arm, or if one focus was located on each arm, the absolute distances for the two foci were subtracted or added, respectively, to determine the inter-focal distance. Finally, the inter-focal distance was expressed as a percentage of the total SC length to give the interference.

Data analysis
One-way ANOVA was used to assess inter-individual variation in mean MLH1 focus frequency per chromosome and per cell, and in interference distance. The relationships between the mean number of MLH1 foci per cell and (1) donor age, and (2) the percentage of cells without an MLH1 focus in XY body, SC21 or SC22 were examined by Spearman correlation analysis. Variation in MLH1 focus distribution among 10 men was compared by a {chi}2 test with 81 degrees of freedom.


   ACKNOWLEDGEMENTS
 
We thank T. Ashley, M. Fritzler and P. Moens for the generous gift of antibodies, M. Mikhaail-Philips for technical help and the patients for their participation in the study. R.H.M. holds the Canada Research Chair in Genetics, and the research was funded by the Canadian Institutes of Health Research (CIHR) grant MA7961. F.S. and M.O.B. are the recipients of a CIHR Strategic Training Fellowship in Genetics, Child Development and Health. T.L. is supported in part by the EU (ICA2-CT-2000-10012).

Conflict of Interest statement. None declared.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS AND DISCUSSION
 MATERIALS AND METHODS
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