Sperm DNA Fragmentation and Its Role in Wildlife Conservation

Abstract

Until about 20 years ago, sperm assessment in the laboratory was focused on motility, morphology and acrosomal integrity. Then came the gradual realisation that, because the main objective of a spermatozoon is to deliver an intact genetic payload of DNA to the egg, being able to check DNA quality of spermatozoa would be equally important, if not more so. Research over the last two decades has therefore led to the development of several techniques for reliably detecting DNA strand breaks, and the more recent focus has been directed towards understanding the fertility implications of DNA damage. It is now clear that evolutionary history has played an important role in determining the stability of sperm DNA under stressful conditions, and that the nature of the DNA-protein interactions also influence the extent to which fertility is affected by both technical procedures involved in sperm preservation and the basic biology of the species concerned. Here we present an overview of the principles involved in DNA assessment and also provide some cases studies that illustrate the influences of species diversity.

Effects of freezing and activation on membrane quality and DNA damage in Xenopus tropicalis and Xenopus laevis spermatozoa

“…Although the exact mechanisms that underlie the process of cryoinduced DNA damage are not fully understood, reactive oxygen species (ROS) formed during cryopreservation have been widely implicated in inducing DNA damage, both in fish (Li et al 2010) and in mammalian spermatozoa (Lopes et al 1998;Bennetts and Aitken 2005;Gosálvez et al 2014). An alternative explanation to the oxidative stress mechanism is the enhancement of pre-existing DNA damage, defects in DNA repair enzymes (Zribi et al 2010) or activation of caspase and apoptosis (Paasch et al 2004;Said et al 2010).…”

Reduced sperm DNA longevity is associated with an increased incidence of still born; evidence from a multi-ovulating sequential artificial insemination animal model

“…In clinical practice, the evaluation of SDF can be either conducted at the time of ejaculation, immediately after sperm liquefaction or when the sperm has been selected for insemination purposes. As we now have clear evidence from a range of species that DNA integrity declines following in vitro incubation [8,9,[14][15][16], a single SDF assessment determined at random time points makes direct comparison of these data, without some form of standardization, virtually meaningless. Consequently, we advocate determination of an rSDF over a defined time period to obtain a more informed measure of the quality and longevity of the sperm chromatin and so as to allow comparison between laboratories.…”

“…Hence, SDF measured immediately after ejaculation may not be as informative of DNA integrity as to when the sperm is Bchallenged^over time by means of incubation. We suggest that it is possible to partially mimic the conditions in the female reproductive tract by conducting a dynamic assessment of DNA damage, overtime, and representing it as a rate of DNA fragmentation (rSDF); the loss of DNA integrity during incubation is not only time, temperature and species dependent, it is also associated with the sperm protamine composition of the individual or species in question [7][8][9].…”

DNA fragmentation index (DFI) as a measure of sperm quality and fertility in mice

“…Our results clearly show the importance of testing DNA quality to ensure delivery of an intact genetic payload to oocytes by natural fertilization, intrauterine insemination (IUI), IVF or ICSI. This is particularly emphasized in the case of human reproduction where ICSI is now a routine procedure 55,56 , since ICSI requires only one sperm for the procedure and sperm from infertile men usually have high DFI 57 . Given its relevance to fertility, we believe that DFI used as an assessment of DNA fragmentation should be included as part of sperm quality analysis in humans as well as in animals in order to diagnose infertility, improve outcomes using ARTs, predict fertility, select animals for ART, guide selection of an ART procedure, and increase reproductive success.…”

“…The results presented in this report indicate that our TUNEL assay protocol is a simple, reliable, and reproducible method to assess sperm DNA damage in mice. It is important to point out, however, that intraspecific differences between sperm structure and chromatin chemistry should guide the selection of the most appropriate technique for assessment of DNA fragmentation in animals, including humans 55 . Thorough validation and development of species-specific protocols are essential when developing such assays to ensure safety and reproducibility of analytical results.…”