Meiotic studies in two human reciprocal translocations and their association with spermatogenic failure

Detection of complex chromosome rearrangements using optical genome mapping

Chromosomal structural variations (SVs) are a main cause of human genetic disease. Currently, karyotype, chromosomal microarray analysis (CMA), and fluorescent in situ hybridization (FISH) form the backbone of current routine diagnostics (CRD). These methods have their own limitations. CRD cannot identify cryptic balanced SVs and complex SVs even if these techniques were performed either simultaneously or in a sequential manner. Optical genome mapping (OGM) is a novel technology that can identify several classes of SVs with higher resolution, but studies on the applicability of OGM and its comparison with CRD are inadequate for difficult and complicated chromosomal SVs are lacking. Herein, seven patients with definite complicated SVs involving at least two breakpoints (BPs) were recruited for this study. The results of BPs and SVs from OGM were compared with those from CRD. The results showed that all BPs of five samples and partial BPs of two samples were detected by OGM. The undetected BPs were all close to the repeat-rich gap region. Besides, OGM also detected additional SVs including a cryptic balanced translocation, two additional complex chromosomal rearrangement (CCR). OGM yielded the additional information, such as the orientation of acentric fragments, BP positions, and genes mapped in the BP region for all the cases. The accuracy of additional SVs and BPs detected by OGM was verified by FISH panel and next-generation sequencing and Sanger sequencing. Taken together, OGM exhibit a better performance in detecting chromosomal SVs compared to the CRD. We suggested that OGM method should be utilized in the clinical examination to improve the efficiency and accuracy of genetic disease diagnosis, supplemented by FISH or karyotyping to compensate for the SVs in the repeat-rich gap region if necessary.

Does reciprocal translocation affect the meiotic segregation products of non-translocation chromosomes?

This retrospective cohort study aimed to assess the effect of chromosomal reciprocal translocation on meiotic segregation products of non-translocation chromosomes. A total of 744 reciprocal translocation carriers and 875 non-carriers were included in this study. A total of 6,832 blastocysts were biopsied and tested by next-generation sequencing. Blastocysts from the carrier group were classified into five subgroups according to the theoretical segregation pattern of quadrivalent structure. For carrier patients, normal meiotic segregation products of the non-translocation chromosome were classified after excluding the segregation modes of the quadrivalent structure. The proportion of normal non-translocation chromosome meiotic segregation products was similar between the carrier and noncarrier groups (p = 0.69). The generalized Estimation Equation revealed that there was no correlation between reciprocal translocation and meiotic segregation products of non-translocation chromosomes. Moreover, subgroup analyses showed that the segregation modes of quadrivalent structure (p = 0.00) and carrier's gender (p = 0.00) may affect the meiotic segregation products of non-translocation chromosomes. In conclusion, reciprocal translocation does not directly reduce the proportion of normal segregation products of non-translocation chromosomes. The difference among subgroups of different quadrivalent segregation patterns implied that interchromosomal effect may exist but the high incidence of chromosomal abnormalities for reciprocal translocation carriers should not be attributed to interchromosomal effect.

Kinetic Activity of Chromosomes and Expression of Recombination Genes in Achiasmatic Meiosis of Tityus (Archaeotityus) Scorpions

Several species of Tityus (Scorpiones, Buthidae) present multi-chromosomal meiotic associations and failures in the synaptic process, originated from reciprocal translocations. Holocentric chromosomes and achiasmatic meiosis in males are present in all members of this genus. In the present study, we investigated synapse dynamics, transcriptional silencing by γH2AX, and meiotic microtubule association in bivalents and a quadrivalent of the scorpion Tityus maranhensis. Additionally, we performed RT-PCR to verify the expression of mismatch repair enzymes involved in crossing-over formation in Tityus silvestris gonads. The quadrivalent association in T. maranhensis showed delay in the synaptic process and long asynaptic regions during pachytene. In this species, γH2AX was recorded only at the chromosome ends during early stages of prophase I; in metaphase I, bivalents and quadrivalents of T. maranhensis exhibited binding to microtubules along their entire length, while in metaphase II/anaphase II transition, spindle fibers interacted only with telomeric regions. Regarding T. silvestris, genes involved in the recombination process were transcribed in ovaries, testes and embryos, without significant difference between these tissues. The expression of these genes during T. silvestris achiasmatic meiosis is discussed in the present study. The absence of meiotic inactivation by γH2AX and holo/telokinetic behavior of the chromosomes are important factors for the maintenance of the quadrivalent in T. maranhensis and the normal continuation of the meiotic cycle in this species.

Chromosome Changes in Soma and Germ Line: Heritability and Evolutionary Outcome

The origin and inheritance of chromosome changes provide the essential foundation for natural selection and evolution. The evolutionary fate of chromosome changes depends on the place and time of their emergence and is controlled by checkpoints in mitosis and meiosis. Estimating whether the altered genome can be passed to subsequent generations should be central when we consider a particular genome rearrangement. Through comparative analysis of chromosome rearrangements in soma and germ line, the potential impact of macromutations such as chromothripsis or chromoplexy appears to be fascinating. What happens with chromosomes during the early development, and which alterations lead to mosaicism are other poorly studied but undoubtedly essential issues. The evolutionary impact can be gained most effectively through chromosome rearrangements arising in male meiosis I and in female meiosis II, which are the last divisions following fertilization. The diversity of genome organization has unique features in distinct animals; the chromosome changes, their internal relations, and some factors safeguarding genome maintenance in generations under natural selection were considered for mammals.

Maternal germline factors associated with aneuploid pregnancy loss: a systematic review

BACKGROUND

Miscarriage describes the spontaneous loss of pregnancy before the threshold of viability; the vast majority occur before 12 weeks of gestation. Miscarriage affects one in four couples and is the most common complication of pregnancy. Chromosomal abnormalities of the embryo are identified in ∼50% of first trimester miscarriages; aneuploidy accounts for 86% of these cases. The majority of trisomic miscarriages are of maternal origin with errors occurring during meiotic division of the oocytes. Chromosome segregation errors in oocytes may be sporadic events secondary to advancing maternal age; however, there is increasing evidence to suggest possible maternal germline contributions to this.

OBJECTIVE AND RATIONALE

The objective of this review was to appraise critically the existing evidence relating to maternal germline factors associated with pregnancy loss secondary to embryo aneuploidy, identify limitations in the current evidence base and establish areas requiring further research.

SEARCH METHODS

The initial literature search was performed in September 2019 and updated in January 2021 using the electronic databases OVID MEDLINE, EMBASE and the Cochrane Library. No time or language restrictions were applied to the searches and only primary research was included. Participants were women who had suffered pregnancy loss secondary to numerical chromosomal abnormalities of the embryo. Study identification and subsequent data extraction were performed by two authors independently. The Newcastle-Ottawa Scale was used to judge the quality of the included studies. The results were synthesized narratively.

OUTCOMES

The literature search identified 2198 titles once duplicates were removed, of which 21 were eligible for inclusion in this systematic review. They reported on maternal germline factors having variable degrees of association with pregnancy loss of aneuploid origin. The Online Mendelian Inheritance in Man (OMIM) gene ontology database was used as a reference to establish the functional role currently attributed to the genes reported. The majority of the cases reported and included were secondary to the inheritance of maternal structural factors such as Robertsonian translocations, deletions and insertions. Germline factors with a plausible role in aneuploid pregnancy loss of maternal origin included skewed X-inactivation and CGG repeats in the fragile X mental retardation (FMR1) gene. Studies that reported the association of single gene mutations with aneuploid pregnancy loss were conflicting. Single gene mutations with an uncertain or no role in aneuploid pregnancy loss included mutations in synaptonemal complex protein 3 (SYCP3), mitotic polo-like kinase 4 (PLK4) and meiotic stromal antigen 3 (STAG3) spindle integrity variants and 5,10-methylenetetrahydrofolate reductase (MTHFR).

WIDER IMPLICATIONS

Identifying maternal genetic factors associated with an increased risk of aneuploidy will expand our understanding of cell division, non-disjunction and miscarriage secondary to embryo aneuploidy. The candidate germline factors identified may be incorporated in a screening panel for women suffering miscarriage of aneuploidy aetiology to facilitate counselling for subsequent pregnancies.

Familial Infertility (Azoospermia and Cryptozoospermia) in Two Brothers-Carriers of t(1;7) Complex Chromosomal Rearrangement (CCR): Molecular Cytogenetic Analysis

Structural aberrations involving more than two breakpoints on two or more chromosomes are known as complex chromosomal rearrangements (CCRs). They can reduce fertility through gametogenesis arrest developed due to disrupted chromosomal pairing in the pachytene stage. We present a familial case of two infertile brothers (with azoospermia and cryptozoospermia) and their mother, carriers of an exceptional type of CCR involving chromosomes 1 and 7 and three breakpoints. The aim was to identify whether meiotic disruption was caused by CCR and/or genomic mutations. Additionally, we performed a literature survey for male CCR carriers with reproductive failures. The characterization of the CCR chromosomes and potential genomic aberrations was performed using: G-banding using trypsin and Giemsa staining (GTG banding), fluorescent in situ hybridization (FISH) (including multicolor FISH (mFISH) and bacterial artificial chromosome (BAC)-FISH), and genome-wide array comparative genomic hybridization (aCGH). The CCR description was established as: der(1)(1qter->1q42.3::1p21->1q42.3::7p14.3->7pter), der(7)(1pter->1p2 1::7p14.3->7qter). aCGH revealed three rare genes variants: ASMT, GARNL3, and SESTD1, which were ruled out due to unlikely biological functions. The aCGH analysis of three breakpoint CCR regions did not reveal copy number variations (CNVs) with biologically plausible genes. Synaptonemal complex evaluation (brother-1; spermatocytes II/oligobiopsy; the silver staining technique) showed incomplete conjugation of the chromosomes. Associations between CCR and the sex chromosomes (by FISH) were not found. A meiotic segregation pattern (brother-2; ejaculated spermatozoa; FISH) revealed 29.21% genetically normal/balanced spermatozoa. The aCGH analysis could not detect smaller intergenic CNVs of few kb or smaller (indels of single exons or few nucleotides). Since chromosomal aberrations frequently do not affect the phenotype of the carrier, in contrast to the negative influence on spermatogenesis, there is an obvious need for genomic sequencing to investigate the point mutations that may be responsible for the differences between the azoospermic and cryptozoospermic phenotypes observed in a family. Progeny from the same parents provide a unique opportunity to discover a novel genomic background of male infertility.

BRUCE preserves genomic stability in the male germline of mice

BRUCE is a DNA damage response protein that promotes the activation of ATM and ATR for homologous recombination (HR) repair in somatic cells, making BRUCE a key protector of genomic stability. Preservation of genomic stability in the germline is essential for the maintenance of species. Here, we show that BRUCE is required for the preservation of genomic stability in the male germline of mice, specifically in spermatogonia and spermatocytes. Conditional knockout of Bruce in the male germline leads to profound defects in spermatogenesis, including impaired maintenance of spermatogonia and increased chromosomal anomalies during meiosis. Bruce-deficient pachytene spermatocytes frequently displayed persistent DNA breaks. Homologous synapsis was impaired, and nonhomologous associations and rearrangements were apparent in up to 10% of Bruce-deficient spermatocytes. Genomic instability was apparent in the form of chromosomal fragmentation, translocations, and synapsed quadrivalents and hexavalents. In addition, unsynapsed regions of rearranged autosomes were devoid of ATM and ATR signaling, suggesting an impairment in the ATM- and ATR-dependent DNA damage response of meiotic HR. Taken together, our study unveils crucial functions for BRUCE in the maintenance of spermatogonia and in the regulation of meiotic HR-functions that preserve the genomic stability of the male germline.

Detection of a balanced translocation carrier through trophectoderm biopsy analysis: a case report

BACKGROUND

Balanced translocation carriers are burdened with fertility issues due to improper chromosome segregation in gametes, resulting in either implantation failure, miscarriage or birth of a child with chromosomal disorders. At the same time, these individuals are typically healthy with no signs of developmental problems, hence they often are unaware of their condition. Yet, because of difficulties in conceiving, balanced translocation carriers often turn to assisted reproduction, some of whom may also undergo preimplantation genetic testing for aneuploidy (PGT-A) to improve the likelihood of achieving a successful pregnancy.

CASE REPORT

We describe a female patient, who pursued in vitro fertilization (IVF) treatment coupled with PGT-A following two consecutive miscarriages, unaware of her genetic condition. PGT-A was performed on blastocyst-stage embryos and the results of comprehensive chromosome screening from a first IVF cycle demonstrated reciprocal segmental aberrations on chromosome 7 and chromosome 10 in two out of four embryos. Due to distinct embryo profiles, the couple was then referred for genetic counselling and subsequent parental karyotyping revealed the presence of a previously undetected balanced translocation in the mother.

CONCLUSIONS

These results confirm previous reports that genome-wide PGT-A can facilitate the identification of balanced translocation carriers in IVF patients, providing explanation for poor reproductive outcome and allowing adjustments in treatment strategies.

Reproductive success of assisted reproductive technology in couples with chromosomal abnormalities

PURPOSE

Infertility is estimated to affect 15% of couples, having chromosome abnormalities an important role in its etiology. The main objective of this work was to access the reproductive success of ART in infertile couples with chromosomal abnormalities comparing to a control group with normal karyotype.

METHODS

A 7-year retrospective karyotype analysis of infertile couples was done. Data regarding type of infertility, couples' ages, ART performed, and their reproductive success were obtained. Adjusted odds ratio (OR) were used to estimate magnitude of association between the reproductive success and the different groups.

RESULTS

We found a prevalence of 7.83% of chromosome abnormalities in our population (233 couples out of 2989). Chromosomal anomalies were found in 82 men (34.75%) and 154 women (65.25%), with low-grade mosaicism being the most prevalent (50.85%), followed by autosomal translocations (17.37%) and sex chromosomes abnormalities (13.56%). Only 2359 couples were treated with ART. There was a non-significant lower reproductive success rate in the cases (OR = 0.899, p = 0.530) with IVF providing the higher success rate. In general, female carriers of chromosome anomalies had a higher success rate, although not significant.

CONCLUSION

Although the differences regarding success rate between groups were not found statistically significant, we still advocate that cytogenetic analysis should be performed routinely in all infertile couples namely before ART. This might help deciding the best treatment options including Preimplantation Genetic Testing for aneuploidies or structural rearrangements and minimize the risk of transmission of anomalies to the offspring.

Unbalanced X;9 translocation in an infertile male with de novo duplication Xp22.31p22.33

PURPOSE

Male carriers of an X-autosome translocation are generally infertile, regardless of the position of the breakpoint on the X chromosome while the pathogenicity of Xp22.3 subtelomeric duplications is under debate. To shed light into this controversy, we present a rare case, of an azoospermic male with no other significant clinical findings, in whom classical cytogenetics revealed additional unbalanced chromosomal material, at the telomere of the long arm of one homolog of chromosome 9.

METHODS

In peripheral blood specimens of the index case and his parents, we performed GBanding, Inverted-DAPI Banding, AgNOR staining, Telomere specific Fluorescence in Situ Hybridization (FISH), Molecular karyotyping by Multi-color FISH, whole genome SNP microarrays, sub-telomeric MLPA, and transcription analysis of the expression of KAL1 gene by RT-PCR.

RESULTS

Multi-color FISH revealed an unbalanced translocation involving the short arm of chromosome X. SNP microarray analysis combined to classical cytogenetics and MLPA demonstrated a de novo 8.796 Mb duplication of Xp22.31-p22.33. Compared to three control specimens, the patient presented significantly elevated expression levels of KAL1 mRNA in peripheral blood, suggesting transcriptional functionality of the duplicated segment.

CONCLUSIONS

The duplicated segment contains the pseudo-autosomal region PAR1 and more than 30 genes including SHOX, ARSE, STS, KAL1, and FAM9A and is not listed as polymorphic. Our data advocate that duplications of the Xp22.3 region may not be associated with a clinical consequence.

Analysis of segregation patterns of quadrivalent structures and the effect on genome stability during meiosis in reciprocal translocation carriers

STUDY QUESTION

Do specific factors affect the segregation patterns of a quadrivalent structure and can the quadrivalent affect genome stability during meiosis?

SUMMARY ANSWER

Meiotic segregation patterns can be affected by the carrier's gender and age, location of breakpoints and chromosome type, and the quadrivalent structure can increase genome instability during meiosis.

WHAT IS KNOWN ALREADY

Carriers of reciprocal translocations have an increased genetic reproductive risk owing to the complex segregation patterns of a quadrivalent structure. However, the results of previous studies on the factors that affect segregation patterns seem to be contradictory, and the effect of a quadrivalent on genome stability during meiosis is unknown.

STUDY DESIGN, SIZE, DURATION

We designed a retrospective study to analyze the segregation patterns of 24 chromosomes from reciprocal translocation and non-translocation patients. Data for 356 reciprocal translocation carriers and 53 patients with the risk to transmit monogenic inherited disorders (RTMIDs) undergoing PGD-single nucleotide polymorphism array analysis were collected. The study was performed between March 2014 and July 2017.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Segregation patterns of a quadrivalent in 1842 blastocysts from 466 assisted reproduction cycles of reciprocal translocation carriers were analyzed according to the location of chromosome breakpoints, the carrier's gender and age, and chromosome type. In addition, to analyze the effect of quadrivalent structure on genome stability, segregation products of chromosomes which are not involved in the translocation from translocation carriers were compared with those of 23 pairs of chromosomes in 318 blastocysts from 72 assisted reproduction cycles of patients with RTMIDs.

MAIN RESULTS AND THE ROLE OF CHANCE

The percentage of adjacent-2 products with severe asymmetric quadrivalent was significantly higher than those with mild asymmetric quadrivalent (P = 0.020) while, in contrast, the incidence of 4:0/others was lower (P = 0.030). The frequencies of adjacent-1, adjacent-2 and 3:1 products differed between male and female carriers (P < 0.001, P = 0.015 and P = 0.001, respectively), and also for adjacent-1 and 4:0/others products in young versus older carriers (P = 0.04 and P = 0.002, respectively). In addition, adjacent-1 products of a quadrivalent with an acrocentric chromosome were significantly higher than those of a quadrivalent without an acrocentric chromosome (P = 0.001). Moreover, a quadrivalent could significantly increase the frequencies of abnormal chromosomes compared to patients with RTMIDs (P = 0.048, odds ratio (OR) = 1.43, 95% CI = 1.01-2.43), especially for the male carriers (P = 0.018, OR = 1.58, 95% CI = 1.08-2.25). In contrast, for older carriers, no difference was found in both aneuploidy and segmental anomalies compared to patients with RTMIDs.

LIMITATIONS, REASONS FOR CAUTION

The study contained appropriate controls, yet the analysis was limited by a small number of control patients and embryos.

WIDER IMPLICATIONS OF THE FINDINGS

Until now, there had been no definite report about the effect of quadrivalents on genome stability in reciprocal translocation carriers compared with control samples, and in the present study the large sample size ensured a detailed analysis of factors with a possible impact on segregation patterns. These data provide a better insight into the meiotic mechanisms involved in non-disjunction events in gametes from reciprocal translocation carriers. In addition, our results will help to provide each reciprocal translocation carrier couple undergoing PGD with more appropriate genetic counseling and a better understanding of the large numbers of abnormal embryos with chromosome aneuploidy.

STUDY FUNDING/COMPETING INTEREST(S)

The research was supported by the Research Funding of Shanghai Ji Ai Genetics & IVF Institute and the authors declare a lack of competing interests in this study.

Time-lapse imaging reveals delayed development of embryos carrying unbalanced chromosomal translocations

PURPOSE

The purpose of the study was to compare the morphokinetic parameters of embryos carrying balanced chromosomal translocations with those carrying unbalanced chromosomal translocations using time-lapse microscopy.

METHODS

The study group included 270 embryos that underwent biopsies on day 3 for preimplantation genetic diagnosis (PGD) for chromosomal translocations in our unit between 2013 and 2015. All embryos were incubated under time-lapse microscopy and evaluated for timing of developmental events up to day 5. The timing of these events was compared between balanced and unbalanced embryos, potentially viable and nonviable variants, and maternal versus paternal inheritance of the translocation.

RESULTS

The PGD analysis found that 209 (77%) of the 270 biopsied embryos carried an unbalanced translocation. Embryos carrying unbalanced translocations, which are expected to lead to implantation failure or miscarriage, cleaved less synchronously and were delayed in time of cleavage to the 4-cell stage (t4) and in time of start of blastulation (tSB) compared with balanced embryos (P < 0.05). Furthermore, embryos carrying nonviable translocations demonstrated a significant delay at the time of pronuclei fading (tPNf) compared with those carrying potentially viable translocations (P < 0.05). Embryos whose unbalanced translocations were of maternal origin were significantly delayed in most of the morphokinetic parameters (including tPNf, t2, t3, t4, t6, t7, t8, cc2, s2, and tSB) compared with embryos carrying balanced translocations (P < 0.05).

CONCLUSIONS

Embryos carrying unbalanced chromosomal translocations mainly of maternal origin undergo delayed development and asynchronous cleavage that may lead to implantation failure or miscarriage.

Mechanisms of karyotype evolution in the Brazilian scorpions of the subfamily Centruroidinae (Buthidae)

The recently-revised subfamily Centruroidinae is part of the New World clade of buthid scorpions. In this study, we analyzed the cytogenetic characteristics of nine of the 10 Brazilian centruroidines, and one undescribed species of the genus Ischnotelson, using a phylogenetic approach to determine the chromosomal rearrangements responsible for the differentiation of karyotypes among the species. The cytogenetic data recorded in the present study supported the new taxonomic arrangement of the Centruroidinae, with all the species of the same genus sharing the same or similar diploid numbers, i.e., 2n = 20 or 22 in Troglorhopalurus lacrau and T. translucidus, 2n = 25 or 26 in Ischnotelson sp., I. guanambiensis and I. peruassu, and 2n = 28 in Jaguajir agamemnon, J. pintoi and J. rochae. The karyotype modelling in the ChromEvol software indicated 2n = 18 as the ancestral diploid number of the Centruroidinae. The differentiation of karyotypes among the centruroidine genera was based on increasing chromosome numbers resulting from progressive fission events. These changes probably occurred prior to the diversification of the genera Ischnotelson, Jaguajir, Physoctonus and Rhopalurus, and appear to have played a more important role in karyotype evolution at the intergeneric level than the interspecific one. However, the observed increase in diploid numbers was not accompanied by changes in the number or location of ribosomal genes or telomeric sequences. The identification of meiotic cells in female specimens also allowed us to discuss the mechanisms of achiasmatic meiosis in scorpions.

Sperm fluorescent in situ hybridisation study of interchromosomal effect in six Tunisian carriers of reciprocal and Robertsonian translocations

Carriers of structural chromosomal anomalies, translocations and inversions are at increased risk of aneuploid gametes production. Besides the direct effect on the involved chromosomes, these rearrangements might disturb the segregation of other structurally normal chromosomes during meiosis. Such event is known as interchromosomal effect. In this study, six male carriers of translocations, four reciprocals and two Robertsonians, were investigated. In addition, seven fertile men with normal 46,XY karyotypes and normal sperm characteristics were enrolled as a control group. Spermatic fluorescent in situ hybridisation specific for chromosomes X, Y, 18, 21 and 22 was carried out. The Mann-Whitney U-test was used to compare the aneuploidy rates between patients and controls. All translocation carriers showed significantly increased frequencies of disomy of all investigated chromosomes, and diploid gametes compared with the control group (p < .05). However, disomy XY was not significantly different between controls and patients (p > .05). We have also observed a considerable interindividual variability in disomy and diploidy rates. These results confirm that the interchromosomal effect seems to exist and could contribute to higher rates of abnormal prenatal aneuploidy, resulting in a small increase in the risk of miscarriage and birth of children with congenital abnormalities and a potential reduction in fertility.

Karyotype diversity and chromosomal organization of repetitive DNA in Tityus obscurus (Scorpiones, Buthidae)

Background

Holocentric chromosomes occur in approximately 750 species of eukaryotes. Among them, the genus Tityus (Scorpiones, Buthidae) has a labile karyotype that shows complex multivalent associations during male meiosis. Thus, taking advantage of the excellent model provided by the Buthidae scorpions, here we analyzed the chromosomal distribution of several repetitive DNA classes on the holocentric chromosomes of different populations of the species Tityus obscurus Gervais, 1843, highlighting their involvement in the karyotypic differences found among them.

Results

This species shows inter- and intrapopulational karyotype variation, with seven distinct cytotypes: A (2n = 16), B (2n = 14), C (2n = 13), D (2n = 13), E (2n = 12), F (2n = 12) and G (2n = 11). Furthermore, exhibits achiasmatic male meiosis and lacks heteromorphic sex chromosomes. Trivalent and quadrivalent meiotic associations were found in some cytotypes. In them, 45S rDNAs were found in the terminal portions of two pairs, while TTAGG repeats were found only at the end of the chromosomes. In the cytotype A (2n = 16), the U2 snRNA gene mapped to pair 1, while the H3 histone cluster and C₀t-1 DNA fraction was terminally distributed on all pairs. Mariner transposons were found throughout the chromosomes, with the exception of one individual of cytotype A (2n = 16), in which it was concentrated in heterochromatic regions.

Conclusions

Chromosomal variability found in T. obscurus are due to rearrangements of the type fusion/fission and reciprocal translocations in heterozygous. These karyotype differences follow a geographical pattern and may be contributing to reproductive isolation between populations analyzed. Our results also demonstrate high mobility of histone H3 genes. In contrast, other multigene families (45S rDNA and U2 snRNA) have conserved distribution among individuals. The accumulation of repetitive sequences in distal regions of T. obscurus chromosomes, suggests that end of chromosome are not covered by the kinetochore.

Meiotic outcome in two carriers of Y autosome reciprocal translocations: selective elimination of certain segregants

Background

Reciprocal Y autosome translocations are rare but frequently associated with male infertility. We report on the meiotic outcome in embryos fathered by two males with the karyotypes 46,X,t(Y;4)(q12;p15.32) and 46,X,t(Y;16)(q12;q13). The two couples underwent preimplantation genetic diagnosis (PGD) enabling determination of the segregation types that were compatible with fertilization and preimplantation embryo development. Both PGD and follow up analysis were carried out via fluorescence in situ hybridization (FISH) or array comparative genomic hybridization (aCGH) allowing the meiotic segregation types to be determined in a total of 27 embryos.

Results

Interestingly, it was seen that the number of female embryos resulting from alternate segregation with the chromosome combination of X and the autosome from the carrier gamete differed from the corresponding balanced males with derivative Y and the derivative autosome by a ratio of 7:1 in each case (P = 0.003) while from the adjacent-1 mode of segregation, the unbalanced male embryos with the combination of der Y and the autosome were seen in all embryos from couple A and in couple B with the exception of one embryo only that had the other chromosome combination of X and derivative autosome (P = 0.011). In both cases the deficit groups have in common the der autosome chromosome that includes the segment Yq12 to qter.

Conclusion

The most likely explanation may be that this chromosome is associated with the X chromosome at PAR2 (pseudoautosomal region 2) in the sex-body leading to inactivation of genes on the autosomal segment that are required for the meiotic process and that this has led to degeneration of this class of spermatocytes during meiosis.

Meiotic pairing and gene expression disturbance in germ cells from an infertile boar with a balanced reciprocal autosome-autosome translocation

Individuals carrying balanced constitutional reciprocal translocations generally have a normal phenotype, but often present reproductive disorders. The aim of our research was to analyze the meiotic process in an oligoasthenoteratospermic boar carrying an asymmetric reciprocal translocation involving chromosomes 1 and 14. Different multivalent structures (quadrivalent and trivalent plus univalent) were identified during chromosome pairing analysis. Some of these multivalents were characterized by the presence of unpaired autosomal segments with histone γH2AX accumulation sometimes associated with the XY body. Gene expression in spermatocytes was studied by RNA-DNA-FISH and microarray-based testis transcriptome analysis. Our results revealed a decrease in gene expression for chromosomes 1 and 14 and an up-regulated expression of X-chromosome genes for the translocated boar compared with normal individuals. We hypothesized that the observed meiotic arrest and reproductive failure in this boar might be due to silencing of crucial autosomal genes (MSUC) and disturbance of meiotic sex chromosome inactivation (MSCI). Further analysis revealed abnormal meiotic recombination (frequency and distribution) and the production of a high rate of unbalanced spermatozoa.

Molecular Dissection Using Array Comparative Genomic Hybridization and Clinical Evaluation of An Infertile Male Carrier of An Unbalanced Y;21 Translocation: A Case Report and Review of The Literature

Chromosomal defects are relatively frequent in infertile men however, translocations between the Y chromosome and autosomes are rare and less than 40 cases of Y-autosome translocation have been reported. In particular, only three individuals has been described with a Y;21 translocation, up to now. We report on an additional case of an infertile man in whom a Y;21 translocation was associated with the deletion of a large part of the Y chromosome long arm. Applying various techniques, including conventional cytogenetic procedures, fluorescence in situ hybridisation (FISH) analysis and array comparative genomic hybridization (array-CGH) studies, we identified a derivative chromosome originating from a fragment of the short arm of the chromosome Y translocated on the short arm of the 21 chromosome. The Y chromosome structural rearrangement resulted in the intactness of the entire short arm, including the sex-determining region Y (SRY) and the short stature homeobox (SHOX) loci, although translocated on the 21 chromosome, and the loss of a large part of the long arm of the Y chromosome, including azoospermia factor-a (AZFa), AZFb, AZFc and Yq heterochromatin regions. This is the first case in which a (Yp;21p) translocation has been ascertained using an array-CGH approach, thus reporting details of such a rearrangement at higher resolution.

Cytogenetic and molecular analyses of de novo translocation dic(9;13)(p11.2;p12) in an infertile male

BACKGROUND

Whole arm t(9;13)(p11;p12) translocations are rare and have been described only a few times; all of the previously reported cases were familial.

RESULTS

We present here an infertile male carrier with a whole-arm reciprocal translocation dic(9;13)(p11.2;p12) revealed by GTG-, C-, and NOR-banding karyotypes with no mature sperm cells in his ejaculate. FISH and genome-wide 400 K CGH microarray (Agilent) analyses demonstrated a balanced chromosome complement and further characterised the abnormality as a dicentric chromosome (9;13): dic(9;13)(pter→p11.2::p12→qter),neo(9)(pter→p12→neo→p11.2). An analysis of the patient's ejaculated cells identified immature germ cells at different phases of spermatogenesis but no mature spermatozoa. Most (82.5%) of the germ cells were recognised as spermatocytes at stage I, and the cell nuclei were most frequently found in pachytene I (41.8%). We have also undertaken FISH analysis and documented an increased rate of aneuploidy of chromosomes 15, 18, X and Y in the peripheral blood leukocytes of our patient. To study the aneuploidy risk in leukocytes, we have additionally included 9 patients with non-obstructive azoospermia with normal karyotypes.

CONCLUSIONS

We propose that the azoospermia observed in the patient with the dic(9;13)(p11.2;p12) translocation was most likely a consequence of a very high proportion (90%) of association between XY bivalents and quadrivalent formations in prophase I.

Impact of chromosomal translocations on male infertility, semen quality, testicular volume and reproductive hormone levels

OBJECTIVE

To analyse the relationship between male infertility and chromosomal translocations, and the influence of different types of chromosomal translocations on semen quality, testicular volume and hormone levels.

METHODS

A retrospective cohort of infertile men was recruited for chromosomal analysis using standard Giemsa stain banding. Physical examinations, semen analysis, hormonal analysis and the detection of azoospermia factor (AZF) microdeletions were carried out. Men with normal fertility were used as controls.

RESULTS

Among the 1056 infertile men, 22 had chromosomal translocations (2.1%), including seven with Robertsonian translocations (0.7%), 11 with autosome-autosome reciprocal translocations (1.0%) and four with gonosome-autosome reciprocal translocations (0.4%). Left and right testicular volumes of patients with chromosomal translocations were significantly smaller than those in the fertile control group. There were no significant differences in hormone levels between patients with chromosomal translocations and fertile controls, except for significantly lower testosterone levels in patients with Robertsonian and gonosome-autosome reciprocal translocations compared with the controls. All AZF microdeletion analyses showed normal results.

CONCLUSIONS

Chromosomal translocations may cause reductions in testicular volume and testosterone level, which may impact spermatogenesis, resulting in azoospermia or oligozoospermia and male infertility.

Non-Robertsonian translocation t (2;11) is associated with infertility in an oligospermic man

Infertility is a major health problem which affects approximately 22% of married couples in reproductive age. Chromosomal defects are the most common genetic abnormalities in infertile men, with an incidence of cytogenetic abnormalities ranging from 2.1% to 15.5%. We describe here the clinical and cytogenetic studies carried out in a couple with repeated abortions. Cytogenetic analysis of the couple showed a de novo chromosomal translocation t (2;11)(p14;q21) in the male partner and a normal 46, XX karyotype in the female counterpart. Such an autosomal translocation may lead to the disruption of genes responsible for spermatogenesis or impaired synaptic complex pairing during meiosis resulting in reproductive failure.

Balanced chromosomal translocations in men: relationships among semen parameters, chromatin integrity, sperm meiotic segregation and aneuploidy

PURPOSE

To analyse relationships between semen parameters, sperm chromatin integrity and frequencies of chromosomally unbalanced, disomic and diploid sperm in 13 Robertsonian and 37 reciprocal translocation carriers and to compare the results with data from 10 control donors.

METHODS

Conventional semen analysis, Sperm Chromatin Structure Assay and FISH with probes for chromosomes involved in the individual translocations and for chromosomes X, Y, 7, 8, 13, 18 and 21.

RESULTS

Normal semen parameters were found in 30.8 % of Robertsonian and 59.5 % of reciprocal translocation carriers. The rates of unbalanced sperm were 12.0 % in Robertsonian and 55.1 % in reciprocal translocation carriers with no difference between normospermic patients and those showing altered semen parameters. Significantly increased frequencies of spermatozoa showing defects in chromatin integrity and condensation, aneuploidy for chromosomes not involved in a translocation and diploidy were detected in translocation carriers with abnormal semen parameters. Normospermic reciprocal translocation carriers showed an increase in chromosome 13 disomy compared to the control group. There was no relationship between gametic and somatic aneuploidy in 12 translocation carriers studied by FISH on sperm and lymphocytes. The frequency of motile sperm was negatively correlated with the frequency of sperm showing disomy, diploidy and defective chromatin condensation.

CONCLUSIONS

Abnormal semen parameters can serve as indicators of an additional risk of forming spermatozoa with defective chromatin and aneuploidy in translocation carriers.

Accumulation of numerical and structural chromosome imbalances in spermatozoa from reciprocal translocation carriers

STUDY QUESTION

Is there a relationship between the occurrence of specific segregation modes and the production of additional numerical abnormalities in spermatozoa from reciprocal translocation carriers?

STUDY ANSWER

The production of aneuploid and diploid spermatozoa tends to be associated with an unbalanced segregation outcome of the rearranged chromosomes.

WHAT IS KNOWN ALREADY

Carriers of reciprocal translocations have an increased genetic reproductive risk as a consequence of producing higher numbers of unbalanced spermatozoa. These imbalances can originate during the segregation of the rearranged chromosomes and also from the occurrence of interchromosomal effects (ICEs). Usually, the outcome of both events is studied independently by means of sperm fluorescent in situ hybridization (FISH).

STUDY DESIGN, SIZE, DURATION

We designed a sequential FISH protocol based on two successive hybridization rounds to study the segregation outcome of the rearranged chromosomes and the presence of additional numerical abnormalities in the same sperm nuclei. The study was performed between February 2010 and February 2012.

MATERIALS, SETTING, METHODS

Sperm samples from eight reciprocal translocation carriers were processed for FISH analysis. Numerical abnormalities for chromosomes X, Y, 13, 18 and 21 were evaluated in the first hybridization round. The aneuploid and diploid nuclei were relocated and analysed for the segregation outcome of the rearranged chromosomes in the second hybridization round. In every carrier, another population of non-selected spermatozoa was also analysed with the aim of defining the general segregation outcome of each reorganization event.

MAIN RESULTS AND THE ROLE OF CHANCE

Overall, the selected population of aneuploid and diploid spermatozoa showed significant increased frequencies of unbalanced segregation modes of the rearranged chromosomes (3:1, 4:0 and 'other') when compared with the non-selected population of spermatozoa. A P-value of <0.05 was chosen to determine if differences observed were statistically significant.

LIMITATIONS, REASONS FOR CAUTION

FISH only allows the analysis of a limited number of chromosomes. Information about the content of additional chromosomes would have been useful in order to broaden the number of aneuploid spermatozoa population, and to infer a more accurate possible mechanism for generating chromosomal imbalances.

WIDER IMPLICATIONS OF THE FINDINGS

There was no previous data about a relationship between chromosomal numerical abnormalities and segregation of rearranged chromosomes. Our findings are consistent with a possible gathering of chromosomal abnormalities in a given nucleus. This information can be used towards a better understanding of the meiotic mechanisms involved in non-disjunction events in gametes from reciprocal translocation carriers. Also, it would help to provide a better reproductive genetic risk assessment in these patients.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by funding of projects SAF2010-2241 (Ministerio de Ciencia e Innovación, Spain), SGR2009-282 (Generalitat de Catalunya, Spain) and UAB CF-180034 (Universitat Autònoma de Barcelona, Spain). The authors declare the lack of competing interests in this study.

Failure of homologous synapsis and sex-specific reproduction problems

The prophase of meiosis I ensures the correct segregation of chromosomes to each daughter cell. This includes the pairing, synapsis, and recombination of homologous chromosomes. A subset of chromosomal abnormalities, including translocation and inversion, disturbs these processes, resulting in the failure to complete synapsis. This activates the meiotic pachytene checkpoint, and the gametes are fated to undergo cell cycle arrest and subsequent apoptosis. Spermatogenic cells appear to be more vulnerable to the pachytene checkpoint, and male carriers of chromosomal abnormalities are more susceptible to infertility. In contrast, oocytes tend to bypass the checkpoint and instead generate other problems, such as chromosome imbalance that often leads to recurrent pregnancy loss in female carriers. Recent advances in genetic manipulation technologies have increased our knowledge about the pachytene checkpoint and surveillance systems that detect chromosomal synapsis. This review focuses on the consequences of synapsis failure in humans and provides an overview of the mechanisms involved. We also discuss the sexual dimorphism of the involved pathways that leads to the differences in reproductive outcomes between males and females.

Interchromosomal effect analyses by sperm FISH: incidence and distribution among reorganization carriers

Structural reorganization carriers usually present compromised fertility accompanied by an increased risk of producing gametes with chromosomal abnormalities that can be transmitted to the offspring. In part these imbalances are ascribed to result from the occurrence of meiotic disturbances produced by the rearrangements in the proper segregation of other chromosome pairs. This phenomenon of interference has been called interchromosomal effect (ICE). Several studies have been performed to assess the occurrence of ICE in structural reorganization carriers by analyzing the frequencies of numerical abnormalities in the gametes. Nevertheless, the occurrence and distribution of these disturbing events still is a controversial issue. In this work we present compiled data from 130 sperm fluorescent in situ hybridization (FISH) studies performed in carriers of the most frequent structural rearrangements in humans: 44 Robertsonian translocations, 66 reciprocal translocations and 13 inversions. Data from 7 complex/multiple rearrangements will be considered in a separate group. Significant increases of gametes with numerical abnormalities have been detected in all types of reorganization carriers. Among the groups of non-complex/multiple rearrangements, Robertsonian translocations appear to be the most prone to produce such interference (54.5%) closely followed by reciprocal translocations (43.9%). In contrast, ICE's were only detected in 7.7% of the inversion carriers analyzed. The presence of complex/multiple rearrangements seems to be an important factor for promoting ICE, as 71.4% of these carriers presented increased rates of gametes with numerical abnormalities. Altogether, almost half of the structural reorganization carriers (45.4%) present a higher reproductive risk of producing aneuploid/diploid spermatozoa compared to the general population. This high incidence has been obtained by analyzing a small set of chromosomes, suggesting that underlying meiotic disorders could be present in these individuals. Further ICE studies in structural reorganization carriers will help to clarify the still unknown predisposing cytogenetic features that promote this phenomenon.

Balanced reciprocal translocation t(5;13)(q33;q12) and 9q31.1 microduplication in a man suffering from infertility and pollinosis

We describe the first case of two chromosomal abnormalities, balanced reciprocal translocation t(5;13)(q33;q12.1) and a microduplication in the region 9q31.1, in a man suffering from infertility and pollinosis. In the region 13q12.1 is located the TUBA3C (tubulin, alpha 3c) gene, which plays an important dynamic role in the motility of flagella. This case might support the opinion that haploinsufficiency of the TUBA3C gene could be the cause of sperm immotility and abnormal sperm morphology, resulting in infertility in the patient. Single-nucleotide polymorphism (SNP) array analysis revealed a novel 9q31.1 microduplication inherited from both parents, which contributes to the genomic instability.

Genetically enhanced asynapsis of autosomal chromatin promotes transcriptional dysregulation and meiotic failure

During meiosis, pairing of homologous chromosomes and their synapsis are essential prerequisites for normal male gametogenesis. Even limited autosomal asynapsis often leads to spermatogenic impairment, the mechanism of which is not fully understood. The present study was aimed at deliberately increasing the size of partial autosomal asynapsis and analysis of its impact on male meiosis. For this purpose, we studied the effect of t(12) haplotype encompassing four inversions on chromosome 17 on mouse autosomal translocation T(16;17)43H (abbreviated T43H). The T43H/T43H homozygotes were fully fertile in both sexes, while +/T43H heterozygous males, but not females, were sterile with meiotic arrest at late pachynema. Inclusion of the t(12) haplotype in trans to the T43H translocation resulted in enhanced asynapsis of the translocated autosome, ectopic phosphorylation of histone H2AX, persistence of RAD51 foci, and increased gene silencing around the translocation break. Increase was also on colocalization of unsynapsed chromatin with sex body. Remarkably, we found that transcriptional silencing of the unsynapsed autosomal chromatin precedes silencing of sex chromosomes. Based on the present knowledge, we conclude that interference of meiotic silencing of unsynapsed autosomes with meiotic sex chromosome inactivation is the most likely cause of asynapsis-related male sterility.

The role of asynapsis in human spermatocyte failure

The basic molecular mechanisms by which chromosomal rearrangements in heterozygous state produce spermatogenic disturbances are poorly understood. Testicular biopsies from five patients - one carrier of a Robertsonian translocation rob t(13;14), two carriers of two different Y-autosome translocations, a t(Y;6) and a t(Y;11), one carrier of a reciprocal translocation t(3;13) and one carrier of a heterochromatin duplication in chromosome 9 - were processed for histopathological analysis, electron microscopy and fluorescent immunolocalization of meiotic proteins. In all the patients, the asynaptic regions during pachytene are labelled by BRCA1 and retained RAD51 foci. The variant histone γ-H2AX is located on the chromatin domains of the asynaptic regions and the XY body. In contrast, these meiotic proteins are absent in those chromosomal segments that are non-homologously synapsed. The present observations on five new cases and a review of recent studies show that the common features shared by all these cases are the abnormal location of some meiotic proteins and the presence of transcriptionally silenced chromatin domains on asynaptic regions. The frequent association of these silenced regions with the XY body and the rescue of spermatocyte viability through non-homologous synapsis are also shared by all these carriers. A passive, random mechanism of clustering of asynaptic regions with the XY body is suggested.

De novo chromosomal translocation t(3;5)(q13;q35) in an infertile man

Chromosomal rearrangements are rare structural abnormalities that are usually associated with male infertility or sterility. We describe here the clinical and cytogenetic studies carried out in a couple with repeated abortions. Cytogenetic analysis of the male partner showed a de novo chromosomal translocation t(3;5)(q13;q35) which could be involved in the meiotic errors resulting in reproductive failure.

On the origin of crossover interference: A chromosome oscillatory movement (COM) model

BACKGROUND

It is now nearly a century since it was first discovered that crossovers between homologous parental chromosomes, originating at the Prophase stage of Meiosis I, are not randomly placed. In fact, the number and distribution of crossovers are strictly regulated with crossovers/chiasmata formed in optimal positions along the length of individual chromosomes, facilitating regular chromosome segregation at the first meiotic division. In spite of much research addressing this question, the underlying mechanism(s) for the phenomenon called crossover/chiasma interference is/are still unknown; and this constitutes an outstanding biological enigma.

RESULTS

The Chromosome Oscillatory Movement (COM) model for crossover/chiasma interference implies that, during Prophase of Meiosis I, oscillatory movements of the telomeres (attached to the nuclear membrane) and the kinetochores (within the centromeres) create waves along the length of chromosome pairs (bivalents) so that crossing-over and chiasma formation is facilitated by the proximity of parental homologs induced at the nodal regions of the waves thus created. This model adequately explains the salient features of crossover/chiasma interference, where (1) there is normally at least one crossover/chiasma per bivalent, (2) the number is correlated to bivalent length, (3) the positions are dependent on the number per bivalent, (4) interference distances are on average longer over the centromere than along chromosome arms, and (5) there are significant changes in carriers of structural chromosome rearrangements.

CONCLUSIONS

The crossover/chiasma frequency distribution in humans and mice with normal karyotypes as well as in carriers of structural chromosome rearrangements are those expected on the COM model. Further studies are underway to analyze mechanical/mathematical aspects of this model for the origin of crossover/chiasma interference, using string replicas of the homologous chromosomes at the Prophase stage of Meiosis I. The parameters to vary in this type of experiment will include: (1) the mitotic karyotype, i.e. ranked length and centromere index of the chromosomes involved, (2) the specific bivalent/multivalent length and flexibility, dependent on the way this structure is positioned within the nucleus and the size of the respective meiocyte nuclei, (3) the frequency characteristics of the oscillatory movements at respectively the telomeres and the kinetochores.

Small supernumerary marker chromosome (sSMC) derived from chromosome 22 in an infertile man with hypogonadotropic hypogonadism

We describe the first case of a supernumerary inv dup(22)(q11.1) in an infertile male with hypogonadotropic hypogonadism. This case supports the opinion that supernumerary inv dup(22)(q11.1) could play a role in male infertility. We suggest that the breakpoint in the region 22q11.1 and/or fourfold dosage of centromeric/pericentromeric sequences of the chromosome 22 may be the cause of hypogonadotropic hypogonadism resulting in impaired spermatogenesis and infertility in our patient.

Reciprocal balanced translocation: infertility and recurrent spontaneous abortions in a family

In this case report we present a family with infertile, azoospermic but otherwise apparently healthy males with history of recurrent spontaneous abortions (RSA) in females. Karyotype of the infertile man revealed a reciprocal balanced translocation t(8; 13) with breakpoints at 8q22 and 13p11.2. The reported reciprocal balanced translocation is associated with azoospermia. The same translocation is probably the cause of RSA in females of the family.

Synapsis and recombination in inversion heterozygotes

Inversion heterozygotes are expected to suffer from reduced fertility and a high incidence of chromosomally unbalanced gametes due to recombination within the inverted region. Non-homologous synapsis of the inverted regions can prevent recombination there and diminish the deleterious effects of inversion heterozygosity. The choice between non-homologous and homologous synapsis depends on the size of inversion, its genetic content, its location in relation to the centromere and telomere, and genetic background. In addition, there is a class of inversions in which homologous synapsis is gradually replaced by non-homologous synapsis during meiotic progression. This process is called synaptic adjustment. The degree of synaptic adjustment depends critically on the presence and location of the COs (crossovers) within the inversion loop. Only bivalents without COs within the loop and those with COs in the middle of the inversion can be completely adjusted and became linear.

Preimplantation genetic diagnosis (PGD) improves pregnancy outcome for translocation carriers with a history of recurrent losses

OBJECTIVE

To determine if preimplantation genetic diagnosis (PGD) for translocation carriers with three or more pregnancy losses reduces loss rates.

DESIGN

Retrospective review of data.

SETTING

Preimplantation genetic diagnosis laboratory servicing IVF groups.

PATIENT(S)

Patients (n = 192) undergoing PGD for either a reciprocal translocation or Robertsonian translocation who had three or more previous pregnancy losses.

INTERVENTION(S)

Preimplantation genetic diagnosis for translocations.

MAIN OUTCOME MEASURE(S)

Pregnancy loss rate, pregnancy success rate defined as delivery of at least one child or an ongoing pregnancy in the third trimester, and length of time to success.

RESULT(S)

Pregnancy loss rate was significantly reduced to 13% post-PGD compared with 88.5% in previous non-PGD pregnancies and to 35% to 64% from naturally conceived pregnancies as reported in the literature. Pregnancy success rate was 87%. Conception occurred after an average of 1.4 cycles or <4 months.

CONCLUSION(S)

Individuals with translocations who have experienced three or more losses benefit from PGD by realizing a significant reduction in loss rate and improvement in rate of success of pregnancy. Length of time to conceive is also dramatically reduced compared with data in the literature for similar populations not undergoing PGD.

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.

Cytogenetic determinants of male fertility

BACKGROUND

Cytogenetic abnormalities have been known to be important causes of male infertility for decades.

METHODS

Research publications from 1978 to 2008, from PubMed, have been reviewed.

RESULTS

These studies have greatly improved our information on somatic chromosomal abnormalities such as translocations, inversions and sex chromosomal anomalies, and their consequences to the cytogenetic make-up of human sperm. Also, we have learned that infertile men with a normal somatic karyotype have an increased risk of chromosomally abnormal sperm and children. New techniques such as single sperm typing and synaptonemal complex analysis have provided valuable insight into the association between meiotic recombination and the production of aneuploid sperm. These meiotic studies have also unveiled errors of chromosome pairing and synapsis, which are more common in infertile men.

CONCLUSIONS

These studies allow us to provide more precise information to infertile patients, and further our basic knowledge in the causes of male infertility.

Meiotic studies in an azoospermic boar carrying a Y;14 translocation

A reciprocal translocation between the q arm of the Y chromosome and the q arm of chromosome 14 was identified in a young, phenotypically normal boar presenting azoospermia. Testicular biopsies were analyzed by classical histological and immunolocalization techniques, and by fluorescence in situ hybridization. Meiotic pairing analysis of 85 pachytene spreads showed the presence of an open structure corresponding to a quadrivalent formed by chromosomes 14, X, and the derivative chromosomes 14 and Y in 84.7% of the cases. In the remaining cases (15.3%), a 'trivalent plus univalent' configuration was observed. Immunolocalization of gammaH2AX revealed the presence of this modified histone in the chromatin domains of unsynapsed segments (centromeric region of chromosome 14) and spreading of the gammaH2AX signal from the XY body throughout chromosome 14 in 7.05% of the cells analyzed. The potential causes of the observed infertility, i.e. activation of meiotic checkpoints and/or silencing of genes necessary for the progression of meiosis, are discussed.

Chromosomal abnormalities, meiotic behavior and fertility in domestic animals

Since the advent of the surface microspreading technique for synaptonemal complex analysis, increasing interest in describing the synapsis patterns of chromosome abnormalities associated with fertility of domestic animals has been noticed during the past three decades. In spite of the number of scientific reports describing the occurrence of structural chromosome abnormalities, their meiotic behavior and gametic products, little is known in domestic animal species about the functional effects of such chromosome aberrations in the germ cell line of carriers. However, some interesting facts gained from recent and previous studies on the meiotic behavior of chromosome abnormalities of domestic animals permit us to discuss, in the frame of recent knowledge emerging from mouse and human investigations, the possible mechanism implicated in the well known association between meiotic disruption and chromosome pairing failure. New cytogenetic techniques, based on molecular and immunofluorescent analyses, are allowing a better description of meiotic processes, including gamete production. The present communication reviews the knowledge of the meiotic consequences of chromosome abnormalities in domestic animals.

The mystery of chromosomal translocations in cancer

Chromosomal translocations in human cancer may result in products that can be suppressed by targeting drugs. An example is bcr-abl tyrosine kinase in chronic myelogenous leukemia that can be treated with imatinib mesylate. However, the mechanisms of translocations or exchanges of chromosomal segments are virtually unknown. In this summary, chromosomal translocations in human cancer are compared with 'crossing over' of chromosomal segments occurring during the first meiotic division. Several proposed mechanisms of the exchange of DNA between and among chromosomes are discussed. The conditions that appear essential for these events to occur are listed. Among them are proximity of the involved DNA segments, mechanisms of excising the target DNA, its transport to the new location, and integration into the pre-existing chromosome. The conclusion based on extensive review of the literature is that practically nothing is known about the mechanism of 'crossing over' or translocation. Based on prior work on normal human cells, it is suggested that only one of the two autosomes participates in these events that may include loss of heterozygozity, another common abnormality in human cancer.

Chromosomal rearrangement interferes with meiotic X chromosome inactivation

Heterozygosity for certain mouse and human chromosomal rearrangements is characterized by the incomplete meiotic synapsis of rearranged chromosomes, by their colocalization with the XY body in primary spermatocytes, and by male-limited sterility. Previously, we argued that such X-autosomal associations could interfere with meiotic sex chromosome inactivation. Recently, supporting evidence has reported modifications of histones in rearranged chromosomes by a process called the meiotic silencing of unsynapsed chromatin (MSUC). Here, we report on the transcriptional down-regulation of genes within the unsynapsed region of the rearranged mouse chromosome 17, and on the subsequent disturbance of X chromosome inactivation. The partial transcriptional suppression of genes in the unsynapsed chromatin was most prominent prior to the mid-pachytene stage of primary spermatocytes. Later, during the mid-late pachytene, the rearranged autosomes colocalized with the XY body, and the X chromosome failed to undergo proper transcriptional silencing. Our findings provide direct evidence on the MSUC acting at the mRNA level, and implicate that autosomal asynapsis in meiosis may cause male sterility by interfering with meiotic sex chromosome inactivation.