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.

Keywords: DNA breaks; DNA damage; double-stranded DNA breaks; single-stranded DNA breaks.

Figure 1

Types of DNA damage. These can include mitochondrial DNA damage, telomere attrition, fragmentation and Y-microdeletions of sperm DNA, and epigenetic abnormalities.

Figure 2

Mechanisms of DNA fragmentation. Defective chromatin condensation and the induction of abortive apoptosis can result in the generation of double-strand DNA breaks (DSBs), while oxidative stress can damage DNA on both single (SSBs) or double-strands (DSBs). Abbreviations: 8-OHG—8-hydroxyguanosine; 8-OHdG—8-hydroxy-2′-deoxyguanosine.

Figure 3

DNA Repair Mechanisms. (A) Global overview of the DNA repair mechanisms available in spermatozoa; (B) Molecular description of the mechanisms involved in the DNA double-strand breaks repair. Abbreviations: HR homologous recombination; NHEJ – non-homologous end joining; Alt-EJ – alternative NHEJ.