The Impact of Single- and Double-Strand DNA Breaks in Human Spermatozoa on Assisted Reproduction

Abstract

Several cellular insults can result in sperm DNA fragmentation either on one or both DNA strands. Oxidative damage, premature interruption of the apoptotic process and defects in DNA compaction during spermatogenesis are the main mechanisms that cause DNA breaks in sperm. The two-tailed Comet assay is the only technique that can differentiate single- (SSBs) from double- (DSBs) strand DNA breaks. Increased levels of the phosphorylated isoform of the H2AX histone are directly correlated with DSBs and proposed as a molecular biomarker of DSBs. We have carried out a narrative review on the etiologies associated with SSBs and DSBs in sperm DNA, their association with reproductive outcomes and the mechanisms involved in their repair. Evidence suggests a stronger negative impact of DSBs on reproductive outcomes (fertilization, implantation, miscarriage, pregnancy, and live birth rates) than SSBs, which can be partially overcome by using intracytoplasmic sperm injection (ICSI). In sperm, SSBs are irreversible, whereas DSBs can be repaired by homologous recombination, non-homologous end joining (NHEJ) and alternative NHEJ pathways. Although few studies have been published, further research is warranted to provide a better understanding of the differential effects of sperm SSBs and DSBs on reproductive outcomes as well as the prognostic relevance of DNA breaks discrimination in clinical practice.

Magnetic-Activated Cell Sorting (MACS): A Useful Sperm-Selection Technique in Cases of High Levels of Sperm DNA Fragmentation

“…However, this variability in results could be partly due to the different origin and types of DNA strand breaks in sperm. In this sense, abortive apoptosis mainly originates from DNA double-strand breaks in sperm [ 11 ], and this type of DNA fragmentation is associated with an increase in miscarriage risk [ 35 ]. As the MACS procedure is based on the selective elimination of apoptotic cells, the sperm population isolated after this sperm-selection technique presumably had a significant reduction in sperm containing double-strand breaks and thereby reduced the chance that one of these spermatozoa would be selected for ICSI cycles.…”

“…Apoptotic mechanisms induce the activation of nuclear endonucleases that lead to the generation of double-strand breaks (DSBs) on sperm DNA. These are closely related with implantation failures and miscarriage [ 11 ]. Sertoli cells are able to distinguish and eliminate those sperm cells with apoptotic markers on the outer plasma membrane, such as phosphatidylserine (PS) residues.…”

Increased DNA strand breaks and neoplastic transformation in human bladder cells treated with pioglitazone

“…As possible explanations for this situation, we suggest the following hypotheses: (1) PIO may increase the formation of DNA DSBs while it cannot induce SSBs. Some agents such as oxidative stress can damage both DNA SSBs and DSBs, whereas defective chromatin condensation and the induction of abortive apoptosis can result in the generation of DNA DSBs (Agarwal et al ., 2020). (2) DNA SSBs may convert into DNA DSBs in bladder cells treated with PIO.…”

Proteomic Analysis in Seminal Plasma of Fertile Donors and Infertile Patients with Sperm DNA Fragmentation

“…In this sense, recent studies using Comet assay provide information about different types of DNA breaks (single- and double-stranded). This distinction is important since our previous studies demonstrated that single stranded DNA damage is caused by oxidative stress and to the capacity of achieving a natural pregnancy, while double stranded DNA breaks are enzymatic and related to a higher risk of recurrent miscarriage [ 8 , 39 ]. In this sense, the DNA damage data obtained for the donors and patients included in the present study fit the specific profiles of single- and double-stranded DNA breaks defined previously [ 40 ].…”