Analysis of chromosome segregation in sperm from a chromosome 2 inversion heterozygote and assessment of an interchromosomal effect

Genetic variations as molecular diagnostic factors for idiopathic male infertility: current knowledge and future perspectives

INTRODUCTION

Infertility is a major health problem, worldwide, which affects 10-15% of couples. About half a percent of infertility cases are related to male-related factors. Male infertility is a complex disease that is the result of various insults as lifestyle issues, genetics, and epigenetic factors. Idiopathic infertility is responsible for 30% of total cases. The genetic factors responsible for male infertility include chromosomal abnormalities, deletions of chromosome Y, and mutations and genetic variations of key genes.

AREAS COVERED

In this review article, we aim to narrate performed studies on polymorphisms of essential genes involved in male infertility including folate metabolizing genes, oxidative stress-related genes, inflammation, and cellular pathways related to spermatogenesis. Moreover, possible pathophysiologic mechanisms responsible for genetic polymorphisms are discussed.

EXPERT OPINION

Analysis and assessment of these genetic variations could help in screening, diagnosis, and treatment of idiopathic male infertility.

A mathematical model for predicting the number of transferable blastocysts in next-generation sequencing-based preimplantation genetic testing

PURPOSE

To investigate the clinical factors that could be used predict the number of transferable blastocysts in preimplantation genetic testing (PGT) cycles based on next-generation sequencing (NGS) and formed form a mathematical model to predict the chance likelihood of obtaining one transferable blastocyst, which is helpful for genetic counseling.

METHODS

This retrospective study enrolled couples undergoing PGT cycles for chromosomal structural rearrangement (PGT-SR, n = 363, 202 with reciprocal translocation carriers, 131 with Robertsonian translocation carriers, 30 with inversion carriers), monogenic diseases (PGT-M, n = 47), and for Aneuploidies (PGT-A, n = 132) from January 2015 to October 2018. Stepwise multiple linear regression analysis was used to identify the factors relevant for obtaining at least one transferable blastocyst. The factors that predict the number of biopsied blastocysts were further analyzed.

RESULTS

The transferable blastocyst rates were 29.94, 41.99, 49.09, 41.42, and 44.37% in the reciprocal translocation carrier, Robertsonian translocation carrier, inversion carrier, PGT-M, and PGT-A cycles, respectively. The number of transferable blastocysts in these cycles were 0.3004 × the number of biopsied blastocysts (NBB) - 0.0031, 0.4063 × NBB + 0.0460, 0.5762 × NBB - 0.3128, 0.3611 × NBB + 0.1910, and 0.4831 × NBB - 0.0970, respectively. Furthermore, the number of MII oocytes and female age were clinical predictors of NBB in reciprocal translocation and PGT-A couples, while the number of MII oocytes was the only clinical predictor in Robertsonian translocation carriers, inversion carriers, and PGT-M couples.

CONCLUSIONS

The number of biopsied blastocysts was the only clinical predictor of the ability to obtain a transferable blastocyst in PGT cycles; therefore, for clinical practice, theoretically the minimum numbers of biopsied blastocysts is 4 in reciprocal translocation carrier and 3 in couples undergoing PGT for other reasons. The number of MII oocytes and female age were clinical predictors of the number of biopsied blastocysts. With the mathematical models in our study as a reference, in clinical practice, clinicians will be able to conduct a more targeted genetic consultation for different kinds of PGT patients.

Interchromosomal effect: Report of a father and son, bearing different translocations of the same chromosome, and a review of the current literature

Interchromosomal effect is a controversial phenomenon postulating that during gametogenesis of translocation carriers, aside from the unbalanced segregation of chromosomes involved in the translocation, other, structurally normal chromosomes might also be affected and segregated abnormally. Here, we present a balanced reciprocal translocation carrier t(15;20)(q11;p13), and his son, bearing a different translocation of chromosome 15, t(15;Y)(q11;q12). To further elucidate the so-far-controversial interchromosomal effect phenomenon, published original articles and case reports about interchromosomal effect were reviewed. The father was a carrier of t(15;20)(q11;p13). His wife's karyotype was normal. During a pregnancy occurred without any preceding procedure, amniocentesis was recommended to the family and performed. Result of the amniocentesis revealed a different translocation of chromosome 15; t(15;Y)(q11;q12). To our knowledge, this is the first report of two generations within a family, bearing different translocations of a chromosome. On top of all previous studies investigating ICE, our case adds an important finding, showing not only the rate of aneuploidies of structurally normal chromosomes, but also the rate of this 'alternating translocations' might be increased in translocation carriers, and this could be an important clue about interchromosomal effects.

Infertility patients with chromosome inversions are not susceptible to an inter-chromosomal effect

PURPOSE

The aim of this study was to evaluate the incidence of an inter-chromosomal effect (ICE) in blastocyst-stage embryos from carriers of balanced chromosome inversions.

METHODS

Infertility patients (n = 52) with balanced inversions (n = 66 cycles), and maternal age-matched controls that concurrently cycled (n = 66), consented to an IVF cycle with preimplantation genetic testing for aneuploidy (PGT-A). Blastocyst-stage embryos underwent trophectoderm biopsy for PGT-A with only euploid blastocysts transferred in a subsequent frozen embryo transfer. Subtypes of inversions were included in aggregate: paracentric/pericentric, polymorphic/non-polymorphic, male/female carriers, and varying inversion sizes.

RESULTS

The incidence of aneuploidy was not significantly higher for the inversion patients compared to the controls (inversion = 48.8% vs. control = 47.2% ns). Following euploid blastocyst transfer, there were excellent live birth outcomes.

CONCLUSIONS

Carriers of balanced chromosome inversions did not exhibit higher aneuploidy rates for chromosomes that were not involved in the inversion compared to maternal age-matched controls, signifying the absence of an inter-chromosomal effect for this data set. These results provide the largest investigation of blastocyst embryos regarding the debated existence of an ICE resulting from the presence of an inversion during meiosis. However, further studies are warranted to investigate an ICE among inversions subtypes that were outside the scope of this study.

Different segregation patterns in five carriers due to a pericentric inversion of chromosome 1

Pericentric inversion can produce recombinant gametes; however, meiotic segregation studies on the relationship between the frequency of recombinants and the inverted segment size are rare. Triple-color fluorescence in situ hybridization (FISH) was performed to analyze the meiotic behavior in five inv(1) carriers with different breakpoints. Recombination gametes were absent in Patient 1, whereas the percentages of the recombinants in Patients 2, 3, 4, and 5 were of 9.2%, 15.3%, 17.3%, and 40.9%, respectively. A significant difference was present for the frequencies of the recombinant spermatozoa among the five patients (p < 0.001). For each patient, the frequency of the two types of recombinant gametes (dup(1p)/del(1q) or del(1p)/dup(1q)) did not exhibit a significant difference in comparison with the expected 1:1 ratio (p > 0.05). The meiotic segregation of nine inv(1) carriers (including those presented in this paper) is now available. A significant correlation was discovered between the rate of recombination and the proportion of the chromosome implicated in the inversion (R = 0.9435, p < 0.001). The frequency of the recombinant gametes was directly related to the proportion of the chromosome that was inverted. Sperm-FISH allowed an additional comprehension of the patterns of meiotic segregation and provided accurate genetic counseling.

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.

Chromosomal disorders and male infertility

Infertility in humans is surprisingly common occurring in approximately 15% of the population wishing to start a family. Despite this, the molecular and genetic factors underlying the cause of infertility remain largely undiscovered. Nevertheless, more and more genetic factors associated with infertility are being identified. This review will focus on our current understanding of the chromosomal basis of male infertility specifically: chromosomal aneuploidy, structural and numerical karyotype abnormalities and Y chromosomal microdeletions. Chromosomal aneuploidy is the leading cause of pregnancy loss and developmental disabilities in humans. Aneuploidy is predominantly maternal in origin, but concerns have been raised regarding the safety of intracytoplasmic sperm injection as infertile men have significantly higher levels of sperm aneuploidy compared to their fertile counterparts. Males with numerical or structural karyotype abnormalities are also at an increased risk of producing aneuploid sperm. Our current understanding of how sperm aneuploidy translates to embryo aneuploidy will be reviewed, as well as the application of preimplantation genetic diagnosis (PGD) in such cases. Clinical recommendations where possible will be made, as well as discussion of the use of emerging array technology in PGD and its potential applications in male infertility.

Meiotic recombination errors, the origin of sperm aneuploidy and clinical recommendations

Since the early 1990s male infertility has successfully been treated by intracytoplasmic sperm injection (ICSI), nevertheless concerns have been raised regarding the genetic risk of ICSI. Chromosome aneuploidy (the presence of extra or missing chromosomes) is the leading cause of pregnancy loss and mental retardation in humans. While the majority of chromosome aneuploidies are maternal in origin, the paternal contribution to aneuploidy is clinically relevant particularly for the sex chromosomes. Given that it is difficult to study female gametes investigations are predominantly conducted in male meiotic recombination and sperm aneuploidy. Research suggests that infertile men have increased levels of sperm aneuploidy and that this is likely due to increased errors in meiotic recombination and chromosome synapsis within these individuals. It is perhaps counterintuitive but there appears to be no selection against chromosomally aneuploid sperm at fertilization. In fact the frequency of aneuploidy in sperm appears to be mirrored in conceptions. Given this information this review will cover our current understanding of errors in meiotic recombination and chromosome synapsis and how these may contribute to increased sperm aneuploidy. Frequencies of sperm aneuploidy in infertile men and individuals with constitutional karyotypic abnormalities are reviewed, and based on these findings, indications for clinical testing of sperm aneuploidy are discussed. In addition, the application of single nucleotide arrays for the analysis of meiotic recombination and identification of parental origin of aneuploidy are considered.

Sperm-FISH analysis in a pericentric chromosome 1 inversion, 46,XY,inv(1)(p22q42), associated with infertility

No phenotypic effect is observed in most inversion heterozygotes. However, reproductive risks may occur in the form of infertility, spontaneous abortions or chromosomally unbalanced children as a consequence of meiotic recombination between inverted and non-inverted chromosomes. An odd number of crossovers within the inverted segment results in gametes bearing recombinant chromosomes with a duplication of the region outside of the inversion segment of one arm and a deletion of the terminal segment of the other arm [dup(p)/del(q) and del(p)/dup(q)]. Using fluorescence in-situ hybridization (FISH), the chromosome segregation of a pericentric inversion of chromosome 1 was studied in spermatozoa of a inv(1)(p22q42) heterozygous carrier. Three-colour FISH was performed on sperm samples using a probe mixture consisting of chromosome 1p telomere-specific probe, chromosome 1q telomere-specific probe and chromosome 18 centromere-specific alpha satellite DNA probe. The frequency of the non-recombinant product was 80.1%. The frequencies of the two types of recombinants carrying a duplication of the short arm and a deletion of the long arm, and vice versa, were respectively 7.6 and 7.2%, and these frequencies were not statistically significant from the expected ratio of 1:1. Sperm-FISH allows the further understanding of segregation patterns and their effect on reproductive failure and allows an accurate genetic counselling.

Meiotic segregation analysis in spermatozoa of pericentric inversion carriers using fluorescence in-situ hybridization

BACKGROUND

Pericentric inversions are structural chromosomal abnormalities resulting from two breaks, one on either side of the centromere, within the same chromosome, followed by 180 degrees rotation and reunion of the inverted segment. They can perturb spermatogenesis and lead to the production of unbalanced gametes through the formation of an inversion loop.

METHODS

We report here the analysis of the meiotic segregation in spermatozoa from six pericentric inversion carriers by multicolour fluorescence in-situ hybridization (FISH) and review the literature.

RESULTS

The frequencies of the non-recombinant products (inversion or normal chromosomes) were 80% for the inv(20), 91.41% for the inv(12), 99.43% for the inv(2), 68.12% for the inv(1), 97% for the inv(8)(p12q21) and 60.94% for the inv(8)(p12q24.1). The meiotic segregation of 20 pericentric inversions (including ours) is now available. The frequency of unbalanced spermatozoa varies from 0 to 37.85%. The probability of a crossover within the inverted segment is affected by the chromosome and region involved, the length of the inverted segment and the location of the breakpoints.

CONCLUSIONS

No recombinant chromosomes were produced when the inverted segment involved <30% of the chromosome length (independent of the size of the inverted segment). Between 30 and 50%, few recombinant chromosomes were produced, inducing a slightly increased risk of aneusomy of recombination in the offspring. The risk of aneusomy became very important when the inverted segment was >50% of the chromosome length. Studies on spermatozoa from inversion carriers help in the comprehension of the mechanisms of meiotic segregation. They should be integrated in the genetic exploration of the infertile men to give them a personalized risk assessment of unbalanced spermatozoa.

Chromosome segregation in an infertile man carrying a unique pericentric inversion, inv(21)(p12q22.3), analysed using fluorescence in situ hybridization on sperm nuclei: significance for clinical genetics. A case report

We report the case of a 40-year-old patient referred to our centre after 3 years of infertility. Karyotyping with the aid of fluorescence in situ hybridization (FISH) analysis showed a unique pericentric inversion of chromosome 21:46,XY,inv(21)(p12q22.3). This type of intrachromosomal structural rearrangement can lead to chromosome imbalance in offspring by producing unbalanced gametes if an odd number of crossover events occur within the inverted segment. Therefore, partial trisomy/monosomy with clinical consequences can be observed in the progeny of carriers. Semen samples from the inversion carrier were analysed by FISH using a combination of probes [a subtelomeric 21q probe and a locus-specific Down's syndrome critical region (DSCR) probe] to evaluate the proportion of recombinant chromosomes. Sperm-FISH analysis of 3400 spermatozoa revealed a 67.4% rate of balanced chromosomes (normal or inverted). The frequencies of recombinant chromosomes with duplication of the long arm and deletion of the short arm, and vice versa, were 11.2 and 21.4%, respectively. The risk for the couple of conceiving a child with an unbalanced chromosome 21 is estimated to be around 32%. This case study shows the utility of sperm-FISH analysis in the genetic counselling of a pericentric inversion in a male carrier to assess the frequency of recombinant chromosomes and therefore evaluate the probability of having a normal conception.

Genetic reproductive risk in inversion carriers

OBJECTIVE

To evaluate the risk of four inversion carriers for producing unbalanced gametes.

DESIGN

Prospective analysis of sperm nuclei by fluorescence in situ hybridization (FISH).

SETTING

Universitat Autònoma de Barcelona.

PATIENT(S)

Four inversion carriers.

INTERVENTION(S)

A semen sample from each patient was collected and prepared for FISH.

MAIN OUTCOME MEASURE(S)

The segregation outcome of each inversion was analyzed. The presence of interchromosomal effects (ICE) on chromosomes 13, 18, 21, X, and Y was also evaluated.

RESULT(S)

A variable production of unbalanced gametes, which implies a heterogeneous behavior of the inversions, was detected. This variability seems to be directly related to the size of the inversion, indicating that the production of recombinant gametes in inversion carriers would not be relevant when the inverted segment is smaller than 100 Mbp.

CONCLUSION(S)

Inversions have a well-defined reproductive effect on carriers. Carriers of inversions up to 100 Mbp have a low [corrected] reproductive risk and would not usually benefit from preimplantation genetic diagnosis.

FISH studies of chromosome abnormalities in germ cells and its relevance in reproductive counseling

Chromosome abnormalities are one of the major causes of human infertility. In infertile males, abnormal karyotypes are more frequent than in the general population. Furthermore, meiotic disorders affecting the germ cell-line have been observed in men with normal somatic karyotypes consulting for infertility. In both cases, the production of unbalanced spermatozoa has been demonstrated. Basically addressed to establish reproductive risks, fluorescence in situ hybridization (FISH) on decondensed sperm heads has become the most frequently used method to evaluate the chromosomal constitution of spermatozoa in carriers of numerical sex chromosome abnormalities, carriers of structural chromosome reorganizations and infertile males with normal karyotype. The aim of this review is to present updated figures of the information obtained through sperm FISH studies with an emphasis on its clinical significance. Furthermore, the incorporation of novel FISH-based techniques (Multiplex-FISH; Multi-FISH) in male infertility studies is also discussed.

Sperm studies in heterozygote inversion carriers: a review

The risk of producing unbalanced gametes in heterozygous inversion carriers mostly depends on the occurrence of recombination events within the inverted segment. Recombination determines the possibility of producing chromosomes with duplications/deficiencies (pericentric inversions) or with duplications/deficiencies which furthermore appear as dicentric and acentric fragments (paracentric inversions). In this work, a general description of the close relationship between the occurrence of crossovers in pericentric and paracentric inversions and the final segregation outcome is presented. After this introduction, a compilation of inversion segregation data and interchromosomal effect results from previously published sperm studies have been reviewed. Segregation results indicate a great heterogeneity in the percentage of unbalanced gametes, from 0 to 37.38%. The size of the inverted segments and their proportion in the chromosome are two parameters closely related with the incidence of recombination (P < 0.0001; using a quadratic model and Pearson's correlation test). These results suggest that the production of a significant level of unbalanced gametes would require a minimum inversion size of 100 Mbp and the inversion of at least 50% of the chromosome. Interchromosomal effects are seldom observed in chromosomal inversions. Finally, implications of the meiotic behavior of the inversions in the progeny of the carriers and the incorporation of sperm FISH segregation analysis for reproductive genetic counseling are discussed.

Unusual segregation products in sperm from a pericentric inversion 17 heterozygote

Chromosome segregation and interchromosomal effect were studied in spermatozoa from a carrier of a pericentric chromosome 17 inversion, 46,XY,inv(17)(p13.1q25.3). Sperm chromosome segregation, lymphocytes of the inversion carrier, and cells from his offspring were analysed by multicolour fluorescence in situ hybridization. The frequency of balanced sperm was 73%. An unusual segregation of recombinants was observed, viz. deletion of the p arm (14.6%) or duplication of the p arm with the presence of one q arm (8.4%), instead of the expected recombinants, viz. duplication of one arm with deletion of the other and vice versa. These unusual recombinants were explained by the position of the 17q breakpoint, which was between the q arm telomere-associated repeats and the unique q subtelomere region. The offspring of the donor were found to have a 17p deletion including the Miller-Dieker critical region, similar to the most frequent recombinant sperm class. The disomy frequency was significantly increased for chromosome 17 compared with other autosomes, suggesting that pairing and recombination of the inversion may predispose to non-disjunction. There was no significant difference between the frequencies of aneuploidy for chromosomes 13, 21, X and Y in the chromosome inversion heterozygote compared with controls. Thus, this unique pericentric inversion of chromosome 17 produces unusual recombinant products; no evidence was apparent of an interchromosomal effect in any of the tested chromosomes.