Large Scale 7436-bp Deletions in Human Sperm Mitochondrial DNA with Spermatozoa Dysfunction and Male Infertility

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.

Association between the single nucleotide variants of the mitochondrial cytochrome B gene (MT-CYB) and the male infertility

BACKGROUND

Idiopathic male infertility can be attributed to genetic predispositions that affect sperm performance and function. Genetic alterations in the mitochondrial DNA (mtDNA) have been linked to certain types of male infertility and abnormal sperm function. Mutations in the mitochondrial cytochrome B (MT-CYB) gene might lead to some deficiencies in mitochondrial function. Thus, in the current study, we aimed to investigate the effect of mutations in the MT-CYB gene on sperm motility and male infertility.

METHODS AND RESULTS

Semen specimens were collected from 111 men where 67 men were subfertile and 44 were fertile. QIAamp DNA Mini Kit and REPLI-g Mitochondrial DNA Kit from QIAGEN were used to isolate and amplify the mitochondrial DNA. Followed by PCR and Sanger sequencing for the target sequence in the MT-CYP gene. Sequencing of the MT-CYB gene revealed a total of thirteen single nucleotide polymorphisms (SNPs). Eight SNPs were non-synonymous variant (missense variant) including: rs2853508, rs28357685, rs41518645, rs2853507, rs28357376, rs35070048, rs2853506, and rs28660155. While five SNPs were Synonymous variant: rs527236194, rs28357373, rs28357369, rs41504845, and rs2854124. Among these SNPs, three variants showed a significant difference in the frequency of the genotypes between subfertile and fertile groups: rs527236194 (T15784C) (P = 0.0005), rs28357373 (T15629C) (P = 0.0439), and rs41504845 (C15833T) (P = 0.0038). Moreover, two SNPs showed a significant association between allelic frequencies of rs527236194 (T15784C) (P = 0.0014) and rs41504845 (C15833T) (P = 0.0147) and male subfertility.

CONCLUSION

The current study showed a significant association between the MT-CYB gene polymorphisms and the development of male infertility. In particular, rs527236194, rs28357373 and rs41504845 variants were found to be the most related to the subfertility group. Further studies on larger and other populations are required to reveal the exact role of this gene in the development of male infertility. In addition, functional studies will be helpful to elucidate the molecular impact of the MT-CYP polymorphisms on mitochondrial function.

Kearns–Sayre Syndrome Minus: Two Cases of Identical Large-Scale Mitochondrial DNA Deletions with Presentations outside the Classical Triad

A curious triad of retinitis pigmentosa, external ophthalmoplegia, and complete heart block was presented by Sayre et al. in 1958. Since then, the disorder named Kearns–Sayre syndrome (KSS) has come to represent patients with mitochondrial DNA deletions presenting before adulthood, primarily with chronic progressive external ophthalmoplegia (CPEO) and pigmentary retinopathy. However, it is increasingly noted that the presentations can well be variable despite similar genetic deletions. Here, we present two cases with identical large-scale mitochondrial DNA deletions but very dissimilar outlook.

Correlation of Sperm Mitochondrial DNA 7345 bp and 7599 bp Deletions with Asthenozoospermia in Jordanian Population

BACKGROUND

Alterations in sperm mitochondrial DNA (mtDNA) affect the functions of some OXPHOS proteins which will affect sperm motility and may be associated with asthenozoospermia. The purpose of this study was to investigate the correlation between 7599-bp and 7345-bp sperm mtDNA deletions and asthenozoospermia in Jordan.

METHODS

Semen specimens from 200 men including 121 infertile and 79 healthy individuals were collected at the Royal Jordanian Medical Services In-vitro fertilization (IVF) units. The mtDNA was extracted followed by mtDNA amplification. Polymerase chain reaction (PCR) was conducted for the target sequences, then DNA sequencing was performed for the PCR products. Chi-square, Fisher's and Spearman's tests were used to calculate the correlation.

RESULTS

The results showed a significant correlation between the presence of 7599-bp mtDNA deletion and infertility where the frequency of the 7599-bp deletion was 63.6% in the infertile group compared to the fertile 34.2% (p<0.001, (OR=3.37, 95% CI=1.860 to 6.108)). Additionally, the sperm motility showed a significant association with the frequency of the 7599-bp deletion (p=0.001, r=-0.887). The 7345-bp mtDNA deletion showed no assoctiation with the infertility (p=0.65, (OR=0.837, 95% CI= 0.464-1.51)) or asthenozoospermia (p=0.98, r=0.008).

CONCLUSION

We demonstrated a significant correlation between asthenozoospermia and the 7599-bp mtDNA deletion but not the 7345-bp mtDNA deletion in the infertile men in Jordan. Screening for deletions in sperm mtDNA can be used as a pre-diagnostic molecular marker for male infertility.

Genetic aspects of idiopathic asthenozoospermia as a cause of male infertility

Infertility is a worldwide problem affecting about 15% of couples trying to conceive. Asthenozoospermia (AZS) is one of the major causes of male infertility, diagnosed by reduced sperm motility, and has no effective therapeutic treatment. To date, a few genes have been found to be associated with AZS in humans and mice, but in most of cases its molecular aetiology remains unknown. Genetic causes of AZS may include chromosomal abnormalities, specific mutations of nuclear and mitochondrial genes. However recently, epigenetic factors, altered microRNAs expression signature, and proteomics have shed light on the pathophysiological basis of AZS. This review article summarises the reported genetic causes of AZS.

Molecular Changes Induced by Oxidative Stress that Impair Human Sperm Motility

A state of oxidative stress (OS) and the presence of reactive oxygen species (ROS) in the male reproductive tract are strongly correlated with infertility. While physiological levels of ROS are necessary for normal sperm functioning, elevated ROS production can overwhelm the cell's limited antioxidant defenses leading to dysfunction and loss of fertilizing potential. Among the deleterious pleiotropic impacts arising from OS, sperm motility appears to be particularly vulnerable. Here, we present a mechanistic account for how OS contributes to altered sperm motility profiles. In our model, it is suggested that the abundant polyunsaturated fatty acids (PUFAs) residing in the sperm membrane serve to sensitize the male germ cell to ROS attack by virtue of their ability to act as substrates for lipid peroxidation (LPO) cascades. Upon initiation, LPO leads to dramatic remodeling of the composition and biophysical properties of sperm membranes and, in the case of the mitochondria, this manifests in a dissipation of membrane potential, electron leakage, increased ROS production and reduced capacity for energy production. This situation is exacerbated by the production of cytotoxic LPO byproducts such as 4-hydroxynonenal, which dysregulate molecules associated with sperm bioenergetic pathways as well as the structural and signaling components of the motility apparatus. The impact of ROS also extends to lesions in the paternal genome, as is commonly seen in the defective spermatozoa of asthenozoospermic males. Concluding, the presence of OS in the male reproductive tract is strongly and positively correlated with reduced sperm motility and fertilizing potential, thus providing a rational target for the development of new therapeutic interventions.

Associations of sperm mitochondrial DNA copy number and deletion rate with fertilization and embryo development in a clinical setting

STUDY QUESTION

Are sperm mitochondrial DNA copy number (mtDNAcn) and deletion rate (mtDNAdel) associated with odds of fertilization and high embryo quality at Days 3 and 5?

SUMMARY ANSWER

Higher sperm mtDNAcn and mtDNAdel were associated with lower odds of high quality Day 3 embryos and transfer quality Day 5 embryos, both of which were primarily driven by lowered odds of fertilization.

WHAT IS KNOWN ALREADY

Sperm mtDNAcn and mtDNAdel have been previously associated with poor semen parameters and clinical male infertility. One prior study has shown that mtDNAdel is associated with lower fertilization rates. However, it is unknown whether these characteristics are linked with ART outcomes.

STUDY DESIGN, SIZE, DURATION

This prospective observational study included 119 sperm samples collected from men undergoing ART in Western Massachusetts. ART outcomes were observed through to Day 5 post-insemination.

PARTICIPANTS/MATERIALS, SETTINGS, METHODS

As part of the Sperm Environmental Epigenetics and Development Study (SEEDS), 119 sperm samples were collected from men undergoing ART in Western Massachusetts. Sperm mtDNAcn and mtDNAdel were measured via triplex probe-based qPCR. Fertilization, Day 3 embryo quality and Day 5 embryo quality measures were fitted with mtDNAcn and mtDNAdel using generalized estimating equations.

MAIN RESULTS AND THE ROLE OF CHANCE

After adjusting for male age and measurement batches, higher sperm mtDNAcn and mtDNAdel were associated with lower odds of fertilization (P = 0.01 and P < 0.01), high quality Day 3 embryos (P = 0.02 for both) and transfer quality Day 5 embryos (P = 0.01 and P = 0.09). However, the associations of mtDNAcn and mtDNAdel with Day 3 high quality status and Day 5 transfer quality status were attenuated in models restricted to fertilized oocytes. Sperm mtDNAcn and mtDNAdel remained statistically significant in models adjusted for both male age and semen parameters, although models including both mtDNA markers generally favoured mtDNAdel.

LIMITATIONS, REASONS FOR CAUTION

Our sample only included oocytes and embryos from 119 couples and thus large diverse cohorts are necessary to confirm the association of sperm mtDNA biomarkers with embryo development.

WIDER IMPLICATIONS OF THE FINDINGS

To our knowledge, our study is the first to assess the associations of sperm mtDNAcn and mtDNAdel with fertilization and embryo quality. The biological mechanism(s) underlying these associations are unknown. Multivariable models suggest that sperm mtDNAcn and mtDNAdel provide discrimination independent of age and semen parameters; therefore, future investigation of the utility of sperm mtDNA as a biomarker for ART outcomes is warranted.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by Grant (K22-ES023085) from the National Institute of Environmental Health Sciences. The authors declare no competing interests.

TRIAL REGISTRATION NUMBER

N/A.

Sperm mitochondrial DNA measures and semen parameters among men undergoing fertility treatment

RESEARCH QUESTION

To examine associations between sperm mitochondrial DNA copy number (mtDNAcn), sperm mitochondrial DNA deletions (mtDNAdel), semen parameters and clinical infertility in an IVF setting.

DESIGN

A total of 125 sperm samples were collected from men undergoing assisted reproductive procedures in an IVF clinic in Western Massachusetts, USA. Sperm mtDNAcn and mtDNAdel were measured by probe-based quantitative polymerase chain reaction. Semen parameters, clinical diagnoses of infertility, and infertility based on consecutive semen parameters, were fitted with mtDNAcn and mtDNAdel in linear models. The utility of sperm mtDNAcn and mtDNAdel to predict infertility was assessed by receiver operating characteristic curves.

RESULTS

Adjusting for relevant covariates, both sperm mtDNAcn and mtDNAdel were associated with lower sperm concentration, count, motility and morphology (P ≤ 0.03). Sperm mtDNAcn and mtDNAdel were also associated with increased risks of clinical infertility based on current and consecutive semen samples. Sperm mtDNAcn had high predictive accuracy for consecutive diagnoses of clinical infertility (C-statistic: 0.91), whereas sperm mtDNAdel had moderate predictive accuracy (C-statistic: 0.75).

CONCLUSIONS

Sperm mtDNAcn is a measure of consecutive abnormal semen parameters and has promise as a diagnostic test.