A comparison of mitochondrial and nuclear DNA status in testicular sperm from fertile men and those with obstructive azoospermia

Mitochondria Quality Control and Male Fertility

Mitochondria are pivotal to cellular homeostasis, performing vital functions such as bioenergetics, biosynthesis, and cell signalling. Proper maintenance of these processes is crucial to prevent disease development and ensure optimal cell function. Mitochondrial dynamics, including fission, fusion, biogenesis, mitophagy, and apoptosis, maintain mitochondrial quality control, which is essential for overall cell health. In male reproduction, mitochondria play a pivotal role in germ cell development and any defects in mitochondrial quality can have serious consequences on male fertility. Reactive oxygen species (ROS) also play a crucial role in sperm capacitation, but excessive ROS levels can trigger oxidative damage. Any imbalance between ROS and sperm quality control, caused by non-communicable diseases or environmental factors, can lead to an increase in oxidative stress, cell damage, and apoptosis, which in turn affect sperm concentration, quality, and motility. Therefore, assessing mitochondrial functionality and quality control is essential to gain valuable insights into male infertility. In sum, proper mitochondrial functionality is essential for overall health, and particularly important for male fertility. The assessment of mitochondrial functionality and quality control can provide crucial information for the study and management of male infertility and may lead to the development of new strategies for its management.

Analysis of the Growth and Development of Children Born with ICSI of Epididymal and Testicular Spermatozoa: A Propensity Matching Study

OBJECTIVE

The study aimed to evaluate whether singleton live births (at 0, 1, 6, 12, and 24 months) following intracytoplasmic sperm injection (ICSI) using sperm of different origins (ejaculated or non-ejaculated sperm) are associated with the growth and development of children born.

METHODS

This was a retrospective cohort study conducted at a single center from January 2016 to December 2019. Follow-up data of the children were obtained from the Jiangsu Province Maternal and Child database. A total of 350 singleton live births after fresh embryo transfer (ET) with ICSI were included. Based on the origin of the sperm, the patients were divided into two groups: the ejaculated group (n = 310) and the non-ejaculated group (n = 40). Propensity score matching was used to control for multiple baseline covariates, resulting in 80 singleton live births (ejaculated sperm) matched to 40 singleton live births (non-ejaculated). The non-ejaculated group was further divided into two subgroups: the PESA group (n = 23) and the TESA group (n = 17). The primary outcome of the study was the growth and development of children. Secondary outcomes included the 2PN rate, high-cleavage embryo rate, blastocyst formation rate, and others.

RESULTS

After matching parental age, BMI, occupation, and maternal serum AMH level, there was no significant difference found in the growth and development of children between the non-ejaculated and ejaculated group or the PESA group and TESA group, respectively. However, the 2PN rate and the blastocyst formation rate were higher in the ejaculated group compared to the non-ejaculated group (91.02 and 85.45, P = 0.002) and (67.37 and 56.06, P = 0.019), respectively. The high-quality cleavage embryo rate was also higher in the TESA group compared to the PESA group (85.06 and 65.63, P = 0.001).

CONCLUSION

This study suggests that there are no significant differences in the growth and development of children born following ICSI using sperm of different origins (ejaculated or non-ejaculated). For nonobstructive azoospermia (OA) patients, sperm derived from the testis may be more effective than derived from the epididymis. However, due to the limited sample size of the non-ejaculated group in this study, further investigations with larger sample sizes are needed to validate these findings.

A Systematic Review of the Impact of Mitochondrial Variations on Male Infertility

According to current estimates, infertility affects one in four couples trying to conceive. Primary or secondary infertility can be due either to both partners or only to the man or the woman. Up to 15% of infertility cases in men can be attributed to genetic factors that can lead to irreversible partial or complete spermatogenic arrest. The increased use of assisted reproductive technology (ART) has provided not only insights into the causes of male infertility but also afforded a diagnostic tool to detect and manage this condition among couples. Genes control a variety of physiological attributes, such as the hypothalamic–pituitary–gonadal axis, development, and germ cell differentiation. In the era of ART, it is important to understand the genetic basis of infertility so as to provide the most tailored therapy and counseling to couples. Genetic factors involved in male infertility can be chromosome abnormalities or single-gene disorders, mitochondrial DNA (mtDNA) mutations, Y-chromosome deletions, multifactorial disorders, imprinting disorders, or endocrine disorders of genetic origin. In this review, we discuss the role of mitochondria and the mitochondrial genome as an indicator of sperm quality and fertility.

Exploring the ovine sperm transcriptome by RNAseq techniques. I Effect of seasonal conditions on transcripts abundance

Understanding the cell molecular changes occurring as a results of climatic circumstances is crucial in the current days in which climate change and global warming are one of the most serious challenges that living organisms have to face. Sperm are one of the mammals’ cells most sensitive to heat, therefore evaluating the impact of seasonal changes in terms of its transcriptional activity can contribute to elucidate how these cells cope with heat stress events. We sequenced the total sperm RNA from 64 ejaculates, 28 collected in summer and 36 collected in autumn, from 40 Manchega rams. A highly rich transcriptome (11,896 different transcripts) with 90 protein coding genes that exceed an average number of 5000 counts were found. Comparing transcriptome in the summer and autumn ejaculates, 236 significant differential abundance genes were assessed, most of them (228) downregulated. The main functions that these genes are related to sexual reproduction and negative regulation of protein metabolic processes and kinase activity. Sperm response to heat stress supposes a drastic decrease of the transcriptional activity, and the upregulation of only a few genes related with the basic functions to maintain the organisms’ homeostasis and surviving. Rams’ spermatozoids carry remnant mRNAs which are retrospectively indicators of events occurring along the spermatogenesis process, including abiotic factors such as environmental temperature.

Impact of Mitochondrial Genetic Variants in ND1, ND2, ND5, and ND6 Genes on Sperm Motility and Intracytoplasmic Sperm Injection (ICSI) Outcomes

Sperm mitochondrial dysfunction causes the generation of an insufficient amount of energy needed for sperm motility. This will affect sperm fertilization capacity, and thus, most asthenozoospermic men usually require assisted reproductive techniques. The etiology of asthenozoospermia remains largely unknown. The current study aimed to investigate the effect of mitochondrial genetic variants on sperm motility and intracytoplasmic sperm injection (ICSI) outcomes. A total of 150 couples from the ICSI cycle were enrolled in this study. One hundred five of the male partners were asthenozoospermic patients, and they were subdivided into three groups according to their percentage of sperm motility, while forty-five of the male partners were normozoospermic. Genetic variants were screened using direct Sanger’s sequencing in four mitochondrial genes (nicotinamide adenine dinucleotide hydrogen (NADH) dehydrogenase 1 (ND1), NADH dehydrogenase 2 (ND2), NADH dehydrogenase 5 (ND5), and NADH dehydrogenase 6 (ND6)). We identified three significant variants: 13708G>A (rs28359178) in ND5, 4216T>C (rs1599988) in ND1, and a novel 12506T>A in ND5 with P values 0.006, 0.036, and 0.013, respectively. The medians of sperm motility, fertilization rate, embryo cleavage score, and embryo quality score were significantly different between men showing 4216T>C, 12506T>A, 13708G>A and wild type, Mann-Whitney P values for the differences in the medians were < 0.05 in all of them. The results from this study suggest that 13708G>A, 12506T>A, and 4216 T>C variants in sperm mitochondrial DNA negatively affect sperm motility and ICSI outcomes.

Causes and consequences of sperm mitochondrial dysfunction

Mitochondria have multiple functions, including synthesis of adenine triphosphate, production of reactive oxygen species, calcium signalling, thermogenesis and apoptosis. Mitochondria have a significant contribution in regulating the various physiological aspects of reproductive function, from spermatogenesis up to fertilisation. Mitochondrial functionality and intact mitochondrial membrane potential are a pre-requisite for sperm motility, hyperactivation, capacitation, acrosin activity, acrosome reaction and DNA integrity. Optimal mitochondrial activity is therefore crucial for human sperm function and semen quality. However, the precise role of mitochondria in spermatozoa remains to be fully explored. Defects in sperm mitochondrial function severely impair the maintenance of energy production required for sperm motility and may be an underlying cause of asthenozoospermia. Sperm mtDNA is susceptible to oxidative damage and mutations that could compromise sperm function leading to infertility. Males with abnormal semen parameters have increased mtDNA copy number and reduced mtDNA integrity. This review discusses the role of mitochondria in sperm function, along with the causes and impact of its dysfunction on male fertility. Greater understanding of sperm mitochondrial function and its correlation with sperm quality could provide further insights into their contribution in the assessment of the infertile male.

Mitochondrial replacement therapy - a new remedy for defects in reproduction

Mitochondria is an important subcellular organelle with the prime function being energy metabolism and supply of energy to the body cells for carrying out the vital functions. Energy is the primary requisite for the reproductive organs of both male and female for carrying out the normal functions. In the present article, we have described how mutation in mitochondrial DNA lead to defects in male and female reproduction. Mitochondria is an integral part of the mid-piece of sperm and also has role in other parts of male reproductive system. Similarly, mitochondrial DNA has role in female reproductive system including ovulation, zygote activation, fertilization, oocyte maturation and embryo development. Mitochondrial defect are collectively named as "mystondria" (mysterious diseases of mitochondria) and may be corrected through mitochondrial replacement therapy, popularly known as three parent baby concept, since there are no other scope for cure or treatment. Two approaches for mitochondrial replacement therapy are pronuclear transfer and spindle transfer. The first three parent baby was developed in April 2016 through mitochondrial replacement therapy. The present review is aimed at functional relevance of three-parent baby concept in animal reproduction.

ICSI outcome in patients with high DNA fragmentation: Testicular versus ejaculated spermatozoa

Sperm DNA fragmentation (SDF) has emerged as an important biomarker in the assessment of male fertility potential with contradictory results regarding its effect on ICSI. The aim of this study was to evaluate intracytoplasmic sperm injection (ICSI) outcomes in male patients with high SDF using testicular versus ejaculated spermatozoa. This is a prospective study on 36 men with high-SDF levels who had a previous ICSI cycle from their ejaculates. A subsequent ICSI cycle was performed using spermatozoa retrieved through testicular sperm aspiration. Results of the prior ejaculate ICSI were compared with those of the TESA-ICSI. The mean (SD) SDF level was 56.36% (15.3%). Overall, there was no difference in the fertilization rate and embryo grading using ejaculate and testicular spermatozoa (46.4% vs. 47.8%, 50.2% vs. 53.4% respectively). However, clinical pregnancy was significantly higher in TESA group compared to ejaculated group (38.89% [14 of 36] vs. 13.8% [five of 36]). Moreover, 17 live births were documented in TESA group, and only three live births were documented in ejaculate group (p < .0001). We concluded that the use of testicular spermatozoa for ICSI significantly increases clinical pregnancy rate as well as live-birth rate in patients with high SDF.

T26248G-transversion mutation in exon7 of the putative methyltransferase Nsun7 gene causes a change in protein folding associated with reduced sperm motility in asthenospermic men

The NOP2/Sun domain family, member 7 (Nsun7) gene, which encodes putative methyltransferase Nsun7, has a role in sperm motility in mice. In humans, this gene is located on chromosome 4 with 12 exons. The aim of the present study was to investigate mutations of exon 7 in the normospermic and asthenospermic men. Semen samples were collected from the Fatemezahra IVF centre (Babol, Iran) and analysed on the basis of World Health Organization (WHO) guidelines using general phenol-chloroform DNA extraction methods. Exon 7 was amplified using Sun7-F and Sun7-R primers. Bands on samples from asthenospermic men that exhibited different patterns of movement on single-strand conformation polymorphism gels compared with normal samples were identified and subjected to sequencing for further identification of possible mutations. Direct sequencing of polymerase chain reaction (PCR) products, along with their analysis, confirmed C26232T-transition and T26248G-transversion mutations in asthenospermic men. Comparison of normal and mutant protein structures of Nsun7 indicated that the amino acid serine was converted to alanine, the structure of the helix, coil and strand was changed, and the protein folding and ligand binding sites were changed in samples from asthenospermic men with a transversion mutation in exon 7, indicating impairment of protein function. Because Nsun7 gene products have a role in sperm motility, if an impairment occurs in exon 7 of this gene, it may lead to infertility. The transversion mutation in exon 7 of the Nsun7 gene can be used as an infertility marker in asthenospermic men.

Mitochondrial variations in the MT-ND4 and MT-TL1 genes are associated with male infertility

Mitochondrial gene mutations have been reported to be associated with sperm motility and the quality of semen. The aim of this study was to investigate whether the two mitochondrial genes (MT-ND4 and MT-TL1) are involved in Chinese male infertility. A total of 97 asthenospermia patients and 80 fertile controls were recruited in this case-control study. Genomic DNA were extracted from the sperm of all participants. Two mitochondrial DNA genes (MT-ND4 and MT-TL1) were amplified by using polymerase chain reaction (PCR) with the gene-specific primers and sequenced on an ABI 3730XL DNA sequencer. For the MT-ND4 gene, we found a total of 64 and 54 nucleotide substitutions in patients and controls, respectively, with no discrepancy in the mutation rates (66.0% vs. 67.5%, p>0.05). However, one mutation (g.11084A>G, p.T109A) leading to an amino acid substitution in a highly conserved residue and predicted to be deleterious was detected only in the cases. For another gene MT-TL1, a novel mutation (g.3263C>T) near the anticodon TAA was identified in an asthenospermia patient and was absent from normal controls. However, the mutation positions in the cases varied from the controls and one highly conserved mutation (g.11084A>G, p.T109A) which was not found in the controls and probably caused damage to the protein structure might contribute to asthenospermia. For another gene MT-TL1, a highly conservative novel mutation which is located closely next to the anticodon also might contribute to asthenospermia. Our result suggests that the MT-ND4 and MT-TL1 genes might be associated with Chinese male infertility.

ABBREVIATIONS

MT-ND4: mitochondrially encoded NADH dehydrogenase 4; MT-TL1: mitochondrially encoded tRNA leucine 1 (UUA/G); PCR: polymerase chain reaction; OXPHOS: mitochondrial oxidative phosphorylation; ATP: adenosine triphosphate; mtDNA: mitochondrial DNA; SNPs: single nucleotide substitutions; AD: alzheimer's disease; PD: parkinson's disease; MELAS: mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes; ROS: reactive oxygen species.

Association of large scale 4977-bp "common" deletions in sperm mitochondrial DNA with asthenozoospermia and oligoasthenoteratozoospermia

OBJECTIVE:

To determine the association of large-scale mitochondrial DNA (mtDNA) deletions with abnormal sperm or abnormal flagellar movement of human spermatozoa in asthenozoospermia and oligoasthenoteratozoospermia (OAT) subjects using percoll gradients fractionation and long-range polymerase chain reaction (PCR).

DESIGN:

We investigated sixty infertile men and thirty normal healthy fertile controls. Of sixty infertile men, 39 were asthenozoospermia and 21 were OAT.

MATERIALS AND METHODS:

Percoll gradients discontinuous technique was used for separation of spermatozoa on the basis of their motility. Long-range PCR was used for detection of “common” 4977-bp deletions, and primer shift technique was used for confirmation of deletions.

RESULTS:

Overall fourteen subjects (14/60; 23.3%) of which eight (8/39; 20.5%) asthenozoospermia and six (6/21; 28.6%) OAT had shown deletions of 4977-bp. Deletions were more common (23.3%) in 40% fraction than 60% (11.6%) and 80% (5%) fractions. Sequencing results had shown deleted region of mtDNA.

CONCLUSION:

Abnormal spermatozoa had more number of mtDNA deletions than normal sperm, and abnormal spermatozoa had lost genes for the oxidative phosphorylation. Our findings suggest that large-scale 4977-bp mtDNA deletions in the spermatozoa from the infertile subjects cause the asthenozoospermic and OAT pathophysiological conditions in infertile males.

The paternal genome and the health of the assisted reproductive technology child

As a number of children born by assisted reproductive technology (ART) are increasing each year across the developed world, the health of such offspring is a matter of public concern. Does the integrity of the paternal genome impact on offspring health? In societal terms, as birth rates fall, and the Western population become unsustainable, do the benefits outweigh the costs of creating and providing for this ART conceived subpopulation? There are little data to date to answer these questions. The long-term health of such children has largely been ignored, and success measured only by early (prebirth) outcomes such as embryo quality or pregnancy. However, there are powerful paradigms such as ageing and smoking that give vital clues as to the potential impact of unhealthy spermatozoa on disease risk, mental and physical health, fertility and mortality of these offspring.

Genetic evaluation of male infertility

Men with severe oligospermia (<5 million sperm/mL ejaculate fluid) or azoospermia should receive genetic testing to clarify etiology of male infertility prior to treatment. Categorization by obstructive azoospermia (OA) or non-obstructive azoospermia (NOA) is critical since genetic testing differs for the former with normal testicular function, testicular volume (~20 mL), and follicle-stimulating hormone (FSH) (1-8 IU/mL) when compared to the latter with small, soft testes and increased FSH. History and physician examination along with laboratory testing (following appropriate genetic counseling) is critical to accurate selection of genetic testing appropriate for azoospermia due to primary testicular failure as compared with congenital hypogonadotropic hypogonadism (HH). Genetic testing options include cystic fibrosis transmembrane conductance regulator (CFTR) testing for men with congenital absence of the vas, while karyotype, Y chromosome microdeletions (YCMD), and other specific genetic tests may be warranted depending on the clinical context of severe oligospermia or NOA. The results of genetic testing guide management options. The most recent techniques for genetic analysis, including sperm microRNA (miRNA) and epigenetics, are forming the foundation for future genetic diagnosis and therapeutic targets in male infertility.

Relationship of spermatozoal DNA fragmentation with semen quality in varicocele-positive men

The aim of the study was to assess the semen quality and levels of spermatozoal nuclear DNA fragmentation in subfertile subjects clinically diagnosed with varicocele, subfertile subjects without varicocele and healthy fertile controls. Semen samples were obtained from 302 subjects. Of them, 115 were healthy fertile controls having normal semen characteristics, 121 subfertile men diagnosed with varicocele, both, clinically and on ultrasonography, while 66 subjects were subfertile with no varicocele. Spermatozoal concentration, percentage motility, morphology and DNA fragmentation were measured. In the study population, deterioration in semen quality-decreased spermatozoal concentration, percentage motility and normal morphology was seen in subfertile subjects, especially with varicocele. Highest spermatozoal DNA fragmentation was observed in varicocele-positive subjects as compared with varicocele-negative subjects and healthy fertile controls. Significant negative correlation was seen between spermatozoal DNA fragmentation and concentration (r = -0.310), motility (r = -0.328) normal morphology, WHO method (r = -0.221) and Tygerberg strict criteria (r = -0.180) in the varicocele-positive subfertile subjects. In conclusion, this study suggests existence of a negative relationship between spermatozoal DNA fragmentation and semen quality in varicocele-positive subfertile subjects.

A multi-faceted approach to understanding male infertility: gene mutations, molecular defects and assisted reproductive techniques (ART)

BACKGROUND

The assisted reproductive techniques aimed to assist infertile couples have their own offspring carry significant risks of passing on molecular defects to next generations.

RESULTS

Novel breakthroughs in gene and protein interactions have been achieved in the field of male infertility using genome-wide proteomics and transcriptomics technologies.

CONCLUSION

Male Infertility is a complex and multifactorial disorder.

SIGNIFICANCE

This review provides a comprehensive, up-to-date evaluation of the multifactorial factors involved in male infertility. These factors need to be first assessed and understood before we can successfully treat male infertility.

Two novel mutations in COII and tRNA(His) mitochondrial genes in asthenozoospermic infertiles men

In this study we performed a systematic sequence analysis of 7 mitochondrial genes (cytochrome oxidase I, cytochrome oxidase II, cytochrome oxidase III, adenosine triphosphate synthase6, ATP synthase8, cytochrome b and tRNA(His)) in 64 infertile men suffering from asthenospermia (n=31) in comparison to normospermic infertile men (n=33) from Tunisian population. A total of 92 nucleotide substitutions in sperm mitochondrial DNA were found; 88 of them were previously identified and reported in the human mitochondrial DNA database (www.mitomap.org) and 4 were novel. We also detected in 4 asthenospermic patients a double novels mutations, the first was found in COXII gene (m.8021 G/A) that was absent in normospermic infertile men. This mutation substituting the Isoleucine at position 146 to Valine in a conserved amino acid in the transmembrane functional domain of the protein. And the second was detected in the tRNA(His) gene (m.12187C>A) this mutation was found in homoplasmic state and was absent in normospermic patients. It was conserved throughout evolution and affects a wobble adenine in the T-loop region at the 54 codon of mitochondrial tRNA(His) .

UVB irradiation as a tool to assess ROS-induced damage in human spermatozoa

One of the consequences of oxygen metabolism is the production of reactive oxygen species (ROS) which in a situation of imbalance with antioxidants can damage several biomolecules, compromise cell function and even lead to cellular death. The particularities of the sperm cell make it particularly vulnerable to ROS attack compromising its functionality, mirrored in terms of fertility outcome and making the study of the origin of sperm ROS, as well as the alterations they cause very important. In the present work, we used UVB irradiation, an easy experimental approach known as a potent inducer of ROS formation, to better understand the origin of ROS damage without any confounding effects that usually exist in disease models in which ROS are reported to play a role. To address these issues we evaluated sperm mitochondrial ROS production using the Mitosox Red Probe, mitochondrial membrane potential using the JC-1 probe, lipid peroxidation through BODIPY probe and vitality using PI. We observed that UVB irradiation leads to an increase in sperm mitochondrial ROS production and lipid peroxidation that occur previously to an observable mitochondrial dysfunction. We concluded that sperm UVB irradiation appears to be a good and easily manipulated in vitro model system to study mitochondria-induced oxidative stress in spermatozoa and its consequences, which may be relevant in terms of dissecting the action pathways of many other pathologies, drugs and contaminants, including endocrine disruptors.

A novel m.6307A>G mutation in the mitochondrial COXI gene in asthenozoospermic infertile men

Infertility affects 10-15% of the population, of which approximately 40% is due to male etiology and consists primarily of low sperm count (oligozoospermia) and/or abnormal sperm motility (asthenozoospermia). Recently, it has been demonstrated that mtDNA substitutions can influence semen quality. In this study, we performed a sequence analysis of the mitochondrial cytochrome oxidase I (COXI) gene in 31 infertile men suffering from asthenozoospermia in comparison to 33 normozoospermic infertile men and 100 fertile men from the Tunisian population. A novel m.6307A>G mutation was found in sperm mitochondrial DNA (mtDNA). This mutation was found in six asthenozoospermic patients, and was absent in normozoospermic and fertile men. We also detected 21 known substitutions previously reported in the Human Mitochondrial Database. The m.6307A>G mutation substitutes a highly conserved asparagine at position 135 to serine. In addition, PolyPhen-2 analysis predicted that this variant is "probably damaging.

Mitochondrial DNA Mutations in etiopathogenesis of male infertility

Objective

To understand role of mitochondrial (mt) mutations in genes regulating oxidative phosphorylation (OXPHOS) in pathogenesis of male infertility. Infertility affects approximately 15% of couples trying to conceive. Infertility is frequently attributed to defects of sperm motility and number. Mitochondrion and mitochondrial DNA (mtDNA) play an important role in variety of physiological process. They control the oxidative energy supply and thus are central to growth, development and differentiation. Mitochondrial function is controlled by a fine-tuned crosstalk between mtDNA and nuclear DNA (nDNA). As mitochondria supply energy by OXPHOS, any mutation in mtDNA disrupts adenosine triphosphate (ATP) production and thus result in an impaired spermatogenesis and impaired flagellar movement. As sperm midpiece has few mtDNA copies, thus enhanced number of mutant mtDNA results in early phenotypic defect which manifest as spermatogenic arrest or asthenozoospermia. Oxidative stress and mtDNA mutations are positively correlated and mutations in mitochondrial genome (mt genome) are implicated in the lowered fertilising capacity of the sperm and affects the reproductive potential of an individual.

Materials and Methods

A thorough review of articles in the last 15 years was cited with reference to the below-mentioned keywords. The articles considered discuss the role of mt genome in the normal functioning of sperm and the factors associated with mt mutations and impact of these mutations on the reproductive potential.

Results

Sperm motility is a very important factor for the fertilisation of ova. The energy requirements of sperm are therefore very critical for sperm. Mutations in the mitochondrial genes as COX II, ATPase 6 and 8 play an important role and disrupts ATP production affecting the spermatogenesis and sperm motility. Therefore, the aberrations in mt genome are an important etiopatholgy of male infertility.

Conclusion

In the context of male infertility, mt mutations, generation of reactive oxygen species and lowered antioxidant capacity are interlinked and constitute a unified pathogenic molecular mechanism. In the era of assisted reproduction technique (ART), it is very important to distinguish between mutations in nuclear and mitochondrial genomes in sperm, as mtDNA mutations are better diagnostic and prognostic markers in infertile men opting for ART.

Role of genetics in azoospermia

OBJECTIVE

To review established genetic causes of azoospermia, the most severe form of male infertility, and help clinicians, scientists, and infertile couples considering assisted reproductive technologies (ART) to understand the complexity of the disorder and to maximize the chances of having a healthy infant through proper counseling and treatment.

METHOD

An initial literature search was performed on PubMed using the key words "azoospermia" "oligospermia," and "genetics." The results were limited to the studies on humans and written in English, which were written within last 10 years. Although preliminary query results showed more than 900 articles, further queries using key words, such as "Y chromosome," "monogenics," "aneuploidy," "mitochondrial DNA," and "epigenetics," along with "azoospermia," narrowed the results to 30 papers, which were included in the present study.

RESULTS

Genetic defects causing azoospermia were categorized into two large categories: chromosomal and nonchromosomal. Chromosomal defects were further categorized into (1) structural abnormalities, such as Y chromosome micro/macrodeletions, chromosomal inversions, and translocations; and (2) numerical abnormalities, also known as aneuploidy. Nonchromosomal defects included sperm mitochondrial genome defects and epigenetic alterations of genome.

CONCLUSIONS

As a result of advancements in ART, understanding the potential implications of genetic disorders for infertile couples is critical. Analysis of a potential genetic role in azoospermia holds promise to expand our knowledge to evaluate male infertility and to guide treatments.

Correlation of the 4977 bp mitochondrial DNA deletion with human sperm dysfunction

BACKGROUND

Several studies have examined the association between mitochondrial DNA (mtDNA) deletions, in particular the "common" 4977-bp deletion, and human sperm dysfunction, but have produced contradictory results.

FINDINGS

Here we show that PCR slippage and primer miss-match to nuclear DNA may lead to overestimates in the frequency of deletions. Our investigation resolves this issue and gives strong negative correlation between the proportion of the "common" deletion and sperm motility. Furthermore, for the first time, we present data which reinforce the hypothesis for a negative correlation between the mtDNA "common" deletion and fertilization efficiency of spermatozoa.

CONCLUSION

The present analysis resolves several literature inconsistencies and opens the way for diagnostic use of the "common" deletion as a molecular indicator of sperm fertility potential.

Testicular spermatozoa have statistically significantly lower DNA damage compared with ejaculated spermatozoa in patients with unsuccessful oral antioxidant treatment

OBJECTIVE

To compare DNA damage in ejaculated and testicular spermatozoa in patients with previously unsuccessful oral antioxidant treatment.

DESIGN

Prospective clinical study.

SETTING

University-affiliated teaching hospital.

PATIENT(S)

Twelve men with persistently high sperm DNA damage.

INTERVENTION(S)

Evaluation of DNA damage of ejaculated and testicular spermatozoa by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL) assay.

MAIN OUTCOME MEASURE(S)

The DNA damage of ejaculated spermatozoa compared with that of testicular spermatozoa, both samples collected on the day of intracytoplasmic sperm injection.

RESULT(S)

Ejaculated spermatozoa showed a threefold higher DNA damage when compared with testicular samples (39.7% +/- 14.8 vs. 13.3% +/- 7.3).

CONCLUSION(S)

Our results indicated that in patients with previously unsuccessful oral antioxidant treatment the retrieved testicular spermatozoa had a lower degree of DNA damage compared with ejaculated sperm collected on the same day.

The comet assay in male reproductive toxicology

Due to our lifestyle and the environment we live in, we are constantly confronted with genotoxic or potentially genotoxic compounds. These toxins can cause DNA damage to our cells, leading to an increase in mutations. Sometimes such mutations could give rise to cancer in somatic cells. However, when germ cells are affected, then the damage could also have an effect on the next and successive generations. A rapid, sensitive and reliable method to detect DNA damage and assess the integrity of the genome within single cells is that of the comet or single-cell gel electrophoresis assay. The present communication gives an overview of the use of the comet assay utilising sperm or testicular cells in reproductive toxicology. This includes consideration of damage assessed by protocol modification, cryopreservation vs the use of fresh sperm, viability and statistics. It further focuses on in vivo and in vitro comet assay studies with sperm and a comparison of this assay with other assays measuring germ cell genotoxicity. As most of the de novo structural aberrations occur in sperm and spermatogenesis is functional from puberty to old age, whereas female germ cells are more complicated to obtain, the examination of male germ cells seems to be an easier and logical choice for research and testing in reproductive toxicology. In addition, the importance of such an assay for the paternal impact of genetic damage in offspring is undisputed. As there is a growing interest in the evaluation of genotoxins in male germ cells, the comet assay allows in vitro and in vivo assessments of various environmental and lifestyle genotoxins to be reliably determined.

Outbreeding selects for spiteful cytoplasmic elements

In addition to their nuclear genome, the vast majority of eukaryotes harbour cytoplasmic genomes, e.g. in mitochondria or chloroplasts. In the majority of cases, these cytoplasmic genomes are transmitted maternally only, leading to selective pressures divergent from those that act on nuclear genes. In particular, cytoplasmic genes, which reduce the fitness of males that carry them, but have no fitness effect in females, are believed to be selectively neutral. Here, we go a step further and argue that in outbreeding populations (i.e. populations with inbreeding avoidance), 'spiteful' cytoplasmic elements that reduce the number of offspring produced by males are in fact selected for. We study this process by means of a stochastic model, analysing both the probability of spread and the impact that such a spiteful cytotype can have on population dynamics. Our results demonstrate that the probability of spread of the spiteful cytotype can be several times higher in outbreeding than in panmictic populations. Spread and fixation of the spiteful cytotype can lead to different qualitative effects on the population dynamics, including extinction, decreased or increased stable population size. We discuss our results in respect to cytoplasmically induced male infertility and cytoplasmic incompatibility.

Paternal mitochondrial DNA transmission during nonhuman primate nuclear transfer

Offspring produced by nuclear transfer (NT) have identical nuclear DNA (nDNA). However, mitochondrial DNA (mtDNA) inheritance could vary considerably. In sheep, homoplasmy is maintained since mtDNA is transmitted from the oocyte (recipient) only. In contrast, cattle are heteroplasmic, harboring a predominance of recipient mtDNA along with varying levels of donor mtDNA. We show that the two nonhuman primate Macaca mulatta offspring born by NT have mtDNA from three sources: (1) maternal mtDNA from the recipient egg, (2) maternal mtDNA from the egg contributing to the donor blastomere, and (3) paternal mtDNA from the sperm that fertilized the egg from which the donor blastomere was isolated. The introduction of foreign mtDNA into reconstructed recipient eggs has also been demonstrated in mice through pronuclear injection and in humans through cytoplasmic transfer. The mitochondrial triplasmy following M. mulatta NT reported here forces concerns regarding the parental origins of mtDNA in clinically reconstructed eggs. In addition, mtDNA heteroplasmy might result in the embryonic stem cell lines generated for experimental and therapeutic purposes ("therapeutic cloning").

The impact of mitochondrial genetics on male infertility

Summary Human mitochondrial DNA (mtDNA) encodes 13 of the polypeptides associated with the process of oxidative phosphorylation (OXPHOS), the cells most important ATP generating pathway. Until recently, the effects of mtDNA rearrangements on male fertility have been largely ignored. However, it is becoming increasingly evident that both point mutations and large-scale deletions may have an impact on sperm motility and morphology. We discuss the implications of these rearrangements in the context of the clinical setting. We further discuss the possible consequences resulting from the transmission of sperm mtDNA deletions to the offspring. The role of nucleo-cytoplasmic interaction is investigated in the context of nuclear transcription and replication factors that regulate mtDNA transcription and replication.

The potential risks of abnormal transmission of mtDNA through assisted reproductive technologies

The recent introduction of more invasive assisted reproductive techniques offers the possibility to provide a wider treatment profile to patients. However, some of these technologies are of considerable concern as they are fraught with the possible transmission of genetic abnormalities to the offspring they create. To date, much analysis of these technologies has been conducted at the chromosomal DNA level. While some analysis has been conducted on the extranuclear, mitochondrial genome (mtDNA), this has been mainly descriptive. In the vast majority of cases, it appears that mtDNA is maternally inherited. The impact that leakage of sperm mtDNA transmission might have for the offspring is discussed in the light of the recent identification of sperm mtDNA presence in a patient with mtDNA disease. The implications of introducing donor mtDNA into a recipient oocyte through both cytoplasmic and nuclear transfer are also discussed. Again, the implications for offspring survival are discussed and suggestions made as to why the techniques might provide valuable insights into mtDNA transmission, replication and transcription. In order to be confident that patients and their offspring are being offered safe treatment, it is argued that potentially some of these treatments may be of considerable benefit in the future but significant scientific research is required before these treatments can be effectively employed in the clinic.

Degradation of paternal mitochondria after fertilization: implications for heteroplasmy, assisted reproductive technologies and mtDNA inheritance

Maternal inheritance of mitochondrial DNA has long been regarded as a major paradox in developmental biology. While some confusion may still persist in popular science, research data clearly document that the paternal sperm-borne mitochondria of most mammalian species enter the ooplasm at fertilization and are specifically targeted for degradation by the resident ubiquitin system. Ubiquitin is a proteolytic chaperone that forms covalently linked polyubiquitin chains on the targeted proteinaceous substrates. The polyubiquitin tag redirects the substrate proteins to a 26-S proteasome, a multi-subunit proteolytic organelle. Thus, specific proteasomal inhibitors reversibly block sperm mitochondrial degradation in ooplasm. Lysosomal degradation and the activity of membrane-lipoperoxidating enzyme 15-lipoxygenase (15-LOX) may also contribute to sperm mitochondrial degradation in the ooplasm, but probably is not crucial. Prohibitin, the major protein of the inner mitochondrial membrane, appears to be ubiquitinated in the sperm mitochondria. Occasional occurrence of paternal inheritance of mtDNA has been suggested in mammals including humans. While most such evidence has been widely disputed, it warrants further examination. Of particular concern is the documented heteroplasmy, i.e. mixed mtDNA inheritance after ooplasmic transplantation. Intracytoplasmic sperm injection (ICSI) has inherent potential for delaying the degradation of sperm mitochondria. However, paternal mtDNA inheritance after ICSI has not been documented so far.

Effects of age on DNA double-strand breaks and apoptosis in human sperm

OBJECTIVE

This study was designed to explore the relationship between men's age and DNA damage and apoptosis in human spermatozoa.

DESIGN

Semen samples were collected from men between the ages of 20 and 57 years. Sperm DNA double-strand breaks were assessed using the neutral microgel electrophoresis (comet) assay, and apoptosis was estimated using the DNA diffusion assay.

SETTING

Academic medical center.

PATIENT(S)

Sixty-six men aged 20 to 57 years were recruited from infertility laboratory and general populations and consented to donate a semen sample. Recruitment was determined by time and day of analysis; the only exclusions were for azoospermia, prostatitis, or prior cancer therapy.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

DNA damage and apoptosis in human sperm.

RESULT(S)

Age correlated with an increasing percentage of sperm with highly damaged DNA (range: 0-83%) and tended to inversely correlate with percentage of apoptotic sperm (range: 0.3%-23%). For example, percentage of sperm with highly damaged DNA, comet extent, DNA break number, and other comet measures was statistically significantly higher in men aged 36-57 years than in those aged 20-35 years, but percentage apoptosis was statistically significantly lower in the older group. Semen analysis showed percentage motility to be significantly higher in younger age groups.

CONCLUSION(S)

This study clearly demonstrates an increase in sperm double-stranded DNA breaks with age. Our findings also suggest for the first time an age-related decrease in human sperm apoptosis. These novel findings may indicate deterioration of healthy sperm cell selection process with age.

Differences in mitochondrial and nuclear DNA status of high-density and low-density sperm fractions after density centrifugation preparation

OBJECTIVE

To determine the mitochondrial DNA and nuclear DNA fragmentation of sperm populations separated by using discontinuous density gradient.

DESIGN

Analysis of mitochondrial and nuclear DNA status of sperm from high and low density layers.

SETTING

Regional fertility center.

PATIENT(S)

Twenty-eight men who presented for an initial infertility investigation.

MAIN OUTCOME MEASURE(S)

Semen was prepared by using discontinuous density gradient (90.0%:45.0%) and subjected to a modified long polymerase chain reaction to assess mitochondrial DNA deletions and to a modified single-cell alkaline gel electrophoresis assay to determine nuclear DNA fragmentation.

RESULT(S)

The high-density fraction displayed significantly more wild-type mitochondrial DNA (75% of samples) than did the low-density fraction (25% of samples). In the high-density fraction, the incidence of single deletions was higher than that of double or multiple deletions, and the deletions were predominantly small. A strong correlation was observed between nuclear DNA fragmentation and the number and size of mitochondrial DNA deletions.

CONCLUSION(S)

Density centrifugation isolates a population of sperm with high-quality mitochondrial DNA and nuclear DNA.

Importance of mitochondrial and nuclear sperm DNA in sperm quality assessment and assisted reproduction outcome

Intracytoplasmic sperm injection (ICSI) has made irrelevant the conventional criteria of concentration, motility and morphology for assessment of sperm quality and so we urgently need new assays by which to gauge sperm 'health'. ICSI may be facilitating the transfer of genetic disorders to future generations by bypassing all the natural hurdles for sperm selection without imposing more pertinent criteria of selection. Sperm DNA quality is vital to the future offspring irrespective of whether the child is conceived naturally, by in vitro fertilization (IVF) or by ICSI. The DNA integrity of sperm can be determined quickly and accurately using a range of techniques that also have strong prognostic power in predicting successful IVF and ICSI outcomes with ejaculated sperm. Moreover, there is a close correlation between testicular nuclear DNA integrity and pregnancy rates in ICSI. Mitochondrial DNA can be measured using long PCR in ejaculated and testicular sperm and is also useful for predicting success in assisted conception. This review discusses how the integrity of both nuclear and mitochondrial affect the choice of sperm for assisted conception.