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

Abstract

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.

Keywords:

• chromosome aberrations
• chromosome non-disjunction
• infertility
• meiotic recombination
• sperm aneuploidy
• synaptonemal complex

Introduction

Concerns have been raised in the scientific community regarding the possible transmission of genetic disorders and increased risk of aneuploid offspring with the use of sperm from infertile men in intracytoplasmic sperm injection (ICSI) [Griffin et al. 2003]. This is of particular concern as chromosome aneuploidy is the leading cause of pregnancy loss and mental retardation in humans, occurring in at least 4% of all clinically recognized pregnancies [Hassold et al. 1993]. Most aneuploidy is maternal in origin, nevertheless paternally derived aneuploidy is also extraordinarily common accounting for 5–10% of autosomal aneuploidies and 50–100% of sex chromosome aneuploidies [Hassold et al. 1993]. This review will cover our current understanding of errors in meiotic recombination and how these contribute to sperm aneuploidy. In addition, the clinical recommendations for sperm aneuploidy screening will be discussed based on the sperm aneuploidy data in the literature of infertile men and men with karyotypic aberrations.

Meiotic recombination methodology

The advent of immunofluoresence methods has been crucial to unraveling the molecular events that take place during meiotic recombination and chromosome synapsis. This has been made possible through the identification and development of antibodies against key proteins in mammalian chromosome synapsis and recombination, specifically: the synaptonemal complex (SC; SCP1 and SCP3), sites of genetic recombination (MLH1- MutL homologue 1), and to the centromere (CREST– calcinosis, Raynaud's phenomenon, oesophageal dysfunction, sclerodactyly, telangiectasia) (Fig. 1A). The SC is a proteinaceous complex, composed of transverse and axial elements (SCP1 and SCP3, respectively) that join up homologous chromosomes during prophase in meiosis I [Topping et al. 2007]. Sites of recombination are visualized using antibodies against a DNA mismatch repair protein, identified in a male knock-out mouse cancer model with almost complete absence of meiotic recombination [Baker et al. 1996]. Antibodies to centromeres are frequently utilized to assist in the analysis of pachytene spermatocyte preparations. Identification of individual chromosome bivalents is made possible when immunofluoresence methods are combined with fluorescent in-situ hybridization (FISH) for chromosomes of interest or for all 24 chromosomes using more complex centromeric multi-color FISH (cen-MFISH). Cen-MFISH allows the simultaneous identification of all 24 human chromosomes using complex combinatorial fluorochrome labeling. To study meiotic recombination and chromosome synapsis it is essential to obtain a testicular biopsy containing meiotic cells in the pachytene stage of meiotic prophase I. The requirement of testicular material has restricted recombination studies to a relatively small subset of individuals; control groups usually consist of vasectomy reversal patients or cancer patients, with infertile patients predominantly consisting of non obstructive azoospermic (NOA) and obstructive azoospermic (OA) individuals.

Figure 1.  Human pachytene spermatocyte. (A) Synaptonemal complex is labeled in red, centromeres in blue, and sites of recombination (MLH1) foci in yellow. (B–E) Show individual synaptonemal complexes taken from the pachytene spermatocyte (A). These illustrate the preferential localization of recombination (indicated by arrows) and how the presence of two recombination foci results in positive interference. B and C show the presence of recombination foci on both the p-arm and the q-arm of submetacentric chromosomes, whereas acrocentric chromosomes shown in D and E only possess recombination foci on the q-arm.

Investigations into errors in chromosome synapsis

Timing and progression of SC formation is strictly organized and maintained (reviewed by: [Sun et al. 2007a; Tempest and Martin 2010a; Topping et al. 2007]). Aberrations in chromosome synapsis indicating improper chromosome pairing during prophase have been reported using various techniques (e.g., immunofluoresence, silver staining, and light microscopy) suggesting that this phenomenon is not an artifact. Specifically, chromosome synapsis errors include the formation of splits (unpaired regions) and gaps (discontinuities). Aberrations in chromosome synapsis frequently occur in the heterochromatic regions of chromosomes 1 and 9 and have been reported in all investigated groups (controls, NOA, and OA) [Sun et al. 2004a; 2007a]. However, an increase in the percentage of cells containing unsynapsed regions has been observed in NOA and OA men compared to controls (20%, 10%, and 8%, respectively) [Martin 2006].

Synaptonemal complex length and localization of sites of meiotic recombination

In addition to global per cell data, detailed recombination maps have been published for individual chromosomes [Sun et al. 2004b; 2006c]. This data provides evidence that there is a strong correlation between SC length and the number of recombination events. For example, chromosome 1 has a mean of 3.9 foci (range 3–5) whereas chromosomes X, Y, 21, and 22 essentially exhibit a single recombination event. Emerging research suggests that SC length is determined by genetic length and gene density rather than physical length. For example, chromosome 18 and 19 are of similar size in metaphase but are gene poor and gene rich, respectively [Cremer et al. 2003]. In this case, the SC for chromosome 19 is longer (10.6 μm vs. 7.8 μm) and has a higher frequency of recombinatorial events (2.03 vs. 1.83) [Sun et al. 2004b]. Recombination maps for individual chromosomes demonstrate preferential localization of sites of genetic exchange around the telomeric/subtelomeric regions with suppression around centromeric regions. Metacentric and submetacentric chromosomes almost always have the presence of at least one recombination foci on the p-arm and the q-arm (Fig. 1B and C), whereas acrocentric chromosomes usually only have recombination foci on the q-arm (Fig. 1D and E). However, the presence of two or more recombination events per SC results in positive interference (i.e., suppression of another recombination event close by), with a mean separation distance of 33–67% between recombination foci [Sun et al. 2004b]. There is little evidence to suggest any alteration in the localization of recombination events between controls and infertile men [Ferguson et al. 2009].

Frequency of meiotic recombination

As with SC formation the process of meiotic recombination is strictly controlled, both in number and localization of recombination events. The pachytene stage of meiosis is the longest phase, with approximately 80–90% of cells found to be at this stage in control males [Ferguson et al. 2007; Gonsalves et al. 2004; Sun et al. 2005a; 2007b]. Interestingly, in NOA males a large percentage of individuals have been identified as completely lacking meiotic cells (45–53%) [Ferguson et al. 2007; Sun et al. 2005a; 2007b; Topping et al. 2006], or exhibiting severely reduced frequencies (approximately 10%) due to a partial or complete block at the zygotene stage [Gonsalves et al. 2004]. Immunofluoresence data in control males report the average number of recombinatorial events per cell to be approximately 50 [Barlow and Hultén 1998; Hassold et al. 2004; Lynn et al. 2002; Sun et al. 2005a; 2006b; 2005b]. However, there is a significant degree of variability between controls (13–25%), but also between cells from the same individual (range 42.5–55.3 recombination foci per cell) [Hassold et al. 2004; Lynn et al. 2002; Sun et al. 2006b; 2005b]. The majority of published studies provide evidence of a significant reduction in recombination events per cell in NOA individuals compared to controls (40–42 vs. 46–49) [Gonsalves et al. 2004; Sun et al. 2005a; 2007b]. Two studies reported no such difference in overall recombination between NOA and controls [Codina-Pascual et al. 2005; Ferguson et al. 2007].

Absence of meiotic recombination

Achiasmate bivalents (bivalents lacking recombination foci) are of particular clinical significance given that, absence of recombination foci may lead to aneuploid sperm or infertility due to spermatogenic arrest resulting from aberrant segregation of chromosomes during meiosis. Achiasmate bivalents in controls have only been reported in a small proportion of cells (0.1–5%) [Ferguson et al. 2007; Sun et al. 2006b; 2005b; 2007b]. However, in NOA males significantly higher frequencies of achiasmate bivalents have been reported (up to 29%) [Martin 2006; Sun et al. 2007b]. Studies that have karyotyped SCs reveal that for the most part bivalents lacking recombination foci involve chromosomes 21, 22, X, and Y. This is not surprising given that the bivalents in question almost always only have the presence of a single recombination event. When considering the sex chromosomes alone (distinguishable without FISH analysis) there are mixed reports of increased frequencies of achiasmate sex bodies in NOA individuals, including several individuals with the complete absence of recombination [Ferguson et al. 2007; Ma et al. 2006]. Several studies report no significant difference in achiasmate bivalents in NOA males when considered as a group but do note increases in individuals compared to controls [Ferguson et al. 2007].

Do errors in meiotic recombination result in increased sperm aneuploidy frequencies?

Terry Hassold's group have published a large series of parental and meiotic origin of trisomy from trisomic fetuses or liveborns using molecular methods [Hassold and Sherman 1993]. These studies have demonstrated that the vast majority of 47,XXY of paternal origin result from meioses in which an association between failure of pseudoautosomal region recombination and XY chromosome non-disjunction was identified. To date only a few studies have directly addressed the following question: do errors in meiotic recombination result in increased sperm aneuploidy frequencies? In such studies, sperm aneuploidy screening has been performed in addition to meiotic recombination methods in the same individuals. One study investigated chromosomes 9, 21, X, and Y, reporting a significant increase in achiasmate bivalents in NOA patients compared to controls (12.4% vs. 4.2%) [Sun et al. 2008]. A significant correlation between achiasmate bivalents and increased sperm disomy for chromosomes 21, X, and Y was observed [Sun et al. 2008]. A similar study observed the same inverse correlation for XY disomy but did not observe such a relationship for chromosomes 13, 18, and 21 [Ferguson et al. 2007].

Sperm aneuploidy

Initially sperm aneuploidy was studied using the human sperm-hamster egg fusion assay, which is notoriously difficult to perform, technically demanding, costly, and provides data on relatively low numbers of cells and only those capable of fertilizing hamster eggs [Marchetti and Wyrobek 2005]. The advent of FISH on decondensed human spermatozoa quickly replaced this assay for sperm aneuploidy assessment (Fig. 2). FISH is technically simpler, rapid, provides data on thousands of cells but only allows analysis of up to five chromosomes per cell and does not allow the detection of structural chromosomal abnormalities (without the inclusion of break point specific probes). The majority of studies have utilized FISH based approaches, hence these will only be considered further. To date, over 50 studies have been published investigating sperm aneuploidy frequencies in different male populations including: normal fertile, infertile (varying degrees of infertility, based on semen parameter assessment), structural chromosome abnormalities (e.g., inversions, Robertsonian and reciprocal translocations), and environmental or lifestyle exposures (e.g., smoking, air pollution, and chemotherapy) (reviewed by [Robbins et al. 2005; Shi and Martin 2000; Tempest and Griffin 2004; Tempest et al. 2008a]).

Figure 2.  Fluorescent in-situ hybridization (FISH) on decondensed human spermatozoa for chromosomes 21 (red), X (yellow), and Y (green). In this field of view there are five spermatozoa, three are normal for the chromosomes tested (two Y bearing sperm and one X bearing sperm). The remaining two spermatozoa (indicated by arrows) are XY disomic sperm; if one of these spermatozoa had successfully fertilized an oocyte it would have resulted in a Klinefelter conceptus (47,XXY).

Sperm aneuploidy frequencies in fertile control populations

It is important to note that all fertile men have been found to have a proportion of aneuploid sperm. The level of aneuploidy is essentially equally distributed for all chromosomes. However, it is well established that chromosomes 21, 22, X, and Y are more prone to non-disjunction than autosomes 1–20 [Shi and Martin 2000; Tempest and Griffin 2004]. Reports suggest that approximately 3–4% of sperm in fertile men are aneuploid with a frequency of approximately 0.1% per chromosome, except for chromosomes 21, 22, X, and Y with levels of 0.3% per chromosome reported [Pang et al. 1999; Tempest and Griffin 2004]. This increased level of non-disjunction observed for chromosomes 21, 22, X, and Y correlates with meiotic recombination, in that, these chromosomes are frequently achiasmate [Ferguson et al. 2007; Sun et al. 2006a]. The observation that higher levels of non-disjunction are observed for these particular chromosomes suggests that this is indeed the case and that these errors persist through to the formation of spermatozoa and hence are not eliminated during spermatogenesis. The question of whether variability in sperm aneuploidy frequencies arises has been addressed previously [Amiel et al. 2002; Tempest et al. 2009]. These studies have demonstrated that aneuploidy levels from the same individuals over time are not significantly different. However, in control groups, there is evidence to suggest that some individuals are stable variants that consistently produce higher levels of sperm aneuploidy. In addition, significant increases or decreases in sperm aneuploidy have been observed in individuals at a single time point suggesting that a life event may impact sperm aneuploidy frequencies (e.g., changes in diet, infections, stress) [Tempest et al. 2009].

Sperm aneuploidy frequencies in infertile men

The majority of men presenting with infertility are karyotypically normal but have reduced semen parameters as defined by the World Health Organization [WHO 1993]. There are over 35 published studies reporting sperm aneuploidy frequencies in infertile men with wide varying semen parameters. These have previously been reviewed in detail elsewhere [Shi and Martin 2000; Tempest and Griffin 2004]. In virtually all cases, compared to their fertile counterparts, significantly increased levels of sperm aneuploidy have been reported in infertile men for all abnormal semen profiles (oligozoospermia, asthenozoospermia, teratozoospermia, and azoospermia). Increased aneuploidy frequencies have been reported for all investigated chromosomes, but in particular for those most prone to non-disjunction (chromosomes 21, 22, X, and Y). The majority of studies report an approximately threefold increase in sperm aneuploidy levels compared to controls, with levels of up to tenfold being reported predominantly for severe infertility (severe oligoasthenoteratozoospermia and azoospermia) [Bernardini et al. 2000; Palermo et al. 2002]. Of note, are a few reports of extraordinarily high levels of aneuploid and polyploid sperm (50–100%) in individuals with a high proportion of macrocephalic, multinucleated, and multiflagellate sperm [Benzacken et al. 2001; Devillard et al. 2002; In't Veld et al. 1997; Lewis-Jones et al. 2003]. To date there is a paucity of data investigating whether it is possible to reduce sperm aneuploidy frequencies. We have previously investigated the effect of traditional Chinese herbal medicine on sperm aneuploidy levels and the relevant biological activity, specifically endocrine and anti-oxidant properties of the herbs [Tempest et al. 2005; 2008b]. Our results have demonstrated a significant reduction in sperm aneuploidy frequencies coincident with treatment [Tempest et al. 2005]. Although this data is based on a small sample size, nonetheless it supports the hypothesis that changes in diet and lifestyle may be able to exert a positive effect on sperm aneuploidy levels.

Karyotypic abnormalities and their relationship with infertility and sperm aneuploidy

One major cause of infertility in humans is constitutional chromosome aberrations. The frequency of chromosome aberrations in the general human population is 0.6% [Berger 1975]. However, the level of chromosome abnormalities observed in the infertile male population is significantly higher (2–14%) dependent on the type of infertility. Karyotype abnormalities are observed in 2% of males presenting with fertility problems, 6% in oligozoospermia, and 14% in NOA [Shi and Martin 2000]. It is important to note that 6% of patients presenting to infertility clinics with a normal somatic karyotype have been found to have meiotic alterations in their spermatogenic cells [Egozcue et al. 2000]. The chromosome aberrations that will be considered further are sex chromosome aneuploidies and structural chromosome abnormalities (inversions, reciprocal and Robertsonian translocations).

Carriers of sex chromosome aneuploidies

The incidence of Klinefelter syndrome and 47,XYY syndrome is relatively common, each occurring in approximately 1:1,000 live births. Over ten studies have been published reporting the frequency of sperm aneuploidy in non-mosaic/mosaic Klinefelter syndrome individuals, (47,XXY and 46,XY/47,XXY, respectively) (reviewed by [Ferlin et al. 2005; Martin 2007; 2008; Sarrate et al. 2005]). A significant increase in sex chromosome disomy has been reported for almost all patients studied. In non-mosaic 47,XXY individuals an average of 6% (range 1–25%) disomic sperm has been reported, with mosaic 46,XY/47,XXY individuals having a lower frequency of 3% (range 0–7%). Additionally, evidence has been provided of an increase in autosome disomy levels compared to fertile controls [Morel et al. 2003]. Approximately ten studies have also reported a significant increase in sex chromosome disomy levels in 47,XYY individuals with an average of around 4% (range 0.1–14%). However, it should be noted that small numbers of patients have been enrolled in studies investigating sperm aneuploidy frequencies in individuals with sex chromosome aneuploidies.

These results suggest that the additional sex chromosome is not always eliminated during spermatogenesis and that some aneuploid cells are capable of initiating and completing meiosis, resulting in aneuploid gametes [Sarrate et al. 2005]. It is reassuring that the vast majority of studies reporting on the outcomes of pregnancies following ICSI in such individuals report healthy karyotypically normal offspring. However, two 47,XXY conceptuses have been reported (accounting for around 10% of published cases) [Friedler et al. 2001; Ron-el et al. 2000]. The increase in sperm aneuploidy has been mirrored by an equivalent increase in aneuploidies in embryos observed after preimplantation genetic diagnosis (PGD) [Staessen et al. 2003].

Structural chromosome abnormalities

FISH assessment of structural chromosome segregation patterns in sperm is made possible through the use of locus specific subtelomeric and centromeric specific probes. It is clear that structural rearrangements can give rise to unbalanced gametes. The extent of unbalanced sperm is clearly associated with the chromosomes involved, size of the involved segment, presence of heterochromatin, tendency for recombination events, and break points at G-positive or G-negative bands [Egozcue et al. 2003]. The presence of unbalanced gametes has relevant clinical ramifications as these can lead to pregnancy loss or offspring affected with chromosomal abnormalities dependent on the chromosomes and segments involved. Detailed reviews of structural aberrations in sperm have been published previously [Ferlin et al. 2005; Martin 2007; 2008; Sarrate et al. 2005].

Only a handful of studies have investigated the segregation of pericentric inversions [Anton et al. 2002; Jaarola et al. 1998; Mikhaail-Philips et al. 2004; 2005; Yakut et al. 2003]. The percentage of unbalanced sperm identified in these studies ranges from 1–54%. Over 20 cases of Robertsonian translocations (translocations involving only acrocentric chromosomes) have been investigated and a range of 3–36% of unbalanced spermatozoa has been reported. Not surprisingly the vast majority of cases involve the most common Robertsonian translocation t(13q;14q). To date, over 30 reciprocal translocations have been studied; in these cases the percentage of unbalanced spermatozoa is much higher than that found for inversions and Robertsonian translocations, with a range of 29–81%.

In addition, there is some evidence suggesting the presence of an interchromosomal effect (ICE) for certain chromosomes (reviewed by [Martin 2008]); that is abnormal behavior of one or more chromosomes not involved in the structural rearrangement. Thus, these individuals may be at an increased risk of chromosome non-disjunction for additional chromosomes not involved in the structural rearrangement.

Retrospective studies

A handful of studies have retrospectively assessed sperm aneuploidy levels in the fathers of paternally derived aneuploid offspring. For the most part these studies report significantly increased frequencies of aneuploidy for the chromosome involved for Down syndrome [Blanco et al. 1998], Turner syndrome [Martinez-Pasarell et al. 1999a; 1999b; Tang et al. 2004] and Klinefelter syndrome [Eskenazi et al. 2002] and in many instances for other chromosomes not involved suggesting a generalized tendency to non-disjunction [Blanco et al. 1998].

Prospective studies

Prospective studies investigating the relationship between increased sperm aneuploidy and reproductive outcome have revealed an association with increased sperm aneuploidy and lower pregnancy rates and live births [Nagvenkar et al. 2005], recurrent ICSI failure [Nicopoullos et al. 2008; Petit et al. 2005], and increased chromosome abnormalities in early embryos as determined by PGD [Gianaroli et al. 2005]. Increased sperm aneuploidy is also correlated with the severity of male infertility [Shi and Martin 2000; Tempest and Griffin 2004]. While the number of such prospective studies is small they do provide evidence that sperm aneuploidy levels correlate with and may even be predictive of ICSI outcome.

It is perhaps counterintuitive but there is little or no evidence to suggest that there is a preferential selection of a chromosomally normal sperm for fertilization. In fact there is compelling evidence to suggest that no such selection takes place. The latest European Society of Human Reproduction and Embryology (ESHRE) PGD consortium data [Goossens et al. 2008] for structural chromosome abnormalities, analyzed 3,130 embryos (considering only those with successful fertilization, biopsy, and diagnostic result). Of these, only 24% were suitable for transfer (normal/balanced embryos); therefore in 76% of the cases embryos were chromosomally abnormal which is somewhat higher than we might expect from the sperm aneuploidy data. However, this may be in part due to the small number of sperm aneuploidy studies. One PGD study has observed the same frequency of aneuploidy (for structural aberrations) in sperm and early embryos [Escudero et al. 2003].

It is worthy to note that the threefold increase in sperm aneuploidy observed in chromosomally normal infertile men, is mirrored by the number of de-novo chromosome abnormalities observed in conceptuses after ICSI [Van Steirteghem et al. 2002]. Prenatal diagnosis of ICSI pregnancies and newborns suggest around a 2–3% risk of a paternally derived de-novo chromosome abnormality [Van Opstal et al. 1997]. This is approximately threefold higher than the general population. Increased sperm aneuploidy also correlates with increased aneuploidy in early embryos [Gianaroli et al. 2005; Martin 1986]. Additionally, the first publication of increased sperm aneuploidy in infertile men reported an individual with XY disomic sperm that was ninefold higher than that of controls [Martin 1986]. This individual fathered a 47,XXY conceptus [Moosani et al. 1999].

Clinical implementation and recommendation of sperm aneuploidy testing

Sperm aneuploidy testing by FISH is relatively easy to perform and cost effective in terms of reagents since many infertility clinics routinely perform FISH for PGD. However, sperm aneuploidy testing suffers from numerous disadvantages: i) it is laborious and consuming of technician time, due to the large number of sperm required to be analyzed (5–10,000); ii) frequently only 3–5 chromosomes are investigated, the required assessment of a large number of sperm precludes 24 chromosome analysis; iii) it is not possible to screen the sperm to be used in ICSI, thus it can only be used as a tool to provide an assessment of risk; iv) there is a requirement to identify individuals who would benefit from screening; and v) the clinical significance of when an increased frequency of sperm aneuploidy translates to an increased risk of an aneuploid conception after ICSI remains unclear.

Routine clinical application is clearly not feasible unless it is possible to reduce the number of sperm to be scored (reducing cost) or through the implementation of automated scoring. Several recent studies have demonstrated that automation of sperm aneuploidy scoring is possible [Carrell and Emery 2008; Tempest et al. 2010b], however, the current high cost of automated systems precludes implementation in individual clinics. At present, it is not clear when an increased level of sperm aneuploidy represents a clinically relevant finding. However, the fact that de-novo aneuploidy levels in ICSI conceptuses echo those levels found in sperm suggests that any increase compared to normal fertile men is clinically relevant, and couples should be counseled appropriately. Perhaps the most critical issue is to identify those individuals at risk of possessing a high frequency of sperm aneuploidy. This would enable targeted screening of individuals in which sperm aneuploidy screening may be clinically useful. The data from the literature suggests that there are several groups who would benefit from screening given that a large variability of sperm aneuploidy frequencies or very high levels of sperm aneuploidy have been observed. These include:

1. Patients with structural chromosome aberrations, including chromosome translocations and inversions.

2. Patients with Klinefelter syndrome.

3. Patients with very high levels of macrocephalic, multinucleated, and multiflagellate sperm.

4. Patients with non-obstructive azoospermia, should a sufficient number of sperm from testicular biopsies be available for infertility treatment and aneuploidy screening.

5. Patients with severe oligoasthenoteratozoospermia, particularly if there is a history of ICSI failure.

Future clinical technologies

Application of high resolution molecular cytogenetics may soon shed further light on the paternal contribution to aneuploidy. This was highlighted in the recently published report of single cell PGD utilizing genome wide single nucleotide polymorphism (SNP) arrays [Handyside et al. 2009]. The advantage of techniques such as ‘karyomapping’ is that it allows Mendelian analysis of the genotypes, through analysis of inheritance of the four parental haplotypes and the position of any recombination event. If such technologies are routinely adopted in IVF clinics they would provide the field of aneuploidy with detailed information for all chromosomes. Karyomapping or other similar approaches will allow the detection of chromosome aneuploidy and structural chromosome aberrations, recombination data, and parent of origin information for IVF and ICSI conceptions.

Conclusions

Some would argue that investigation into paternal recombination and aneuploidy is of little relevance, given that the vast majority of aneuploidies are maternal in origin. It should be noted that the focus on the male system reflects the scarcity and difficulties in obtaining female gametes, which precludes widespread research. Despite the difficulties highlighted above regarding clinical implementation, sperm aneuploidy studies have proven to be a valuable research tool enhancing our understanding of the paternal contribution of aneuploidy and may be applicable to females. Research thus far has clearly shown that infertile men (in particular severe OAT and azoospermic individuals) compared to their fertile counterparts have significantly increased errors in chromosome synapsis and meiotic recombination leading to increased chromosome non-disjunction. Importantly increased sperm aneuploidy levels are mirrored in ICSI conceptions. Only through continued research into meiotic recombination errors and prospective sperm aneuploidy studies will we gain a better understanding of the clinical significance of increased sperm aneuploidy. The introduction of SNP array technology for PGD will provide a wealth of data to further our understanding of Mendelian inheritance, recombination, and parental origin. This will allow more precise and detailed investigations of chromosome and recombination errors than previously possible in infertile couples. In many situations, a wide variation has been observed in subsets of patients as listed (1–5) above. Performing routine sperm aneuploidy screening in these patients will enable a more individualized risk assessment of aneuploid offspring. This will impact care. For example, the genetic counseling of a translocation carrier with 10% aneuploid sperm will likely be very different than the counseling of an individual with 80% aneuploid sperm. This will ultimately allow couples to make better informed choices about their reproductive future based on their individualized risks, which has to be our ultimate end goal.

Declaration of Interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

Abbreviations

cen-MFISH:	=	centromeric multi-color fluorescent in-situ hybridization
ESHRE:	=	European Society of Human Reproduction and Embryology
FISH:	=	fluorescent in-situ hybridization
ICSI:	=	intracytoplasmic sperm injection
IVF:	=	in-vitro fertilization
MLH1:	=	mutL homologue 1
NOA:	=	non-obstructive azoospermia
OA:	=	obstructive azoospermia
PGD:	=	preimplantation genetic diagnosis
SNP:	=	single nucleotide polymorphisms
SC:	=	synaptonemal complex.