|INVITED RESEARCH HIGHLIGHT
|Ahead of print publication
Chemically induced DNA damage and sperm and oocyte repair machinery: the story gets more interesting
Department of Environmental and Occupational Health, Milken Institute School of Public Health, George Washington University, Washington, DC, USA
|Date of Web Publication||19-May-2015|
Department of Environmental and Occupational Health, Milken Institute School of Public Health, George Washington University, Washington, DC
Source of Support: None, Conflict of Interest: None
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Assisted reproductive technologies have allowed us to bypass elusive questions of how spermatogenesis truly works so that end results, namely conception, can be achieved quickly and efficiently. Yet, reproductive biology, andrology, and molecular mutagenesis have not completely abandoned unanswered questions about how spermatogenesis unfolds, where things can go wrong in the process, and what are the impacts on the conceptus when sperm are somehow defective. Additional questions stem from the recognition that model systems are only approximations of how human reproduction actually happens in a world replete with extraneous exposures, from micronutrients to everyday chemicals. New evidence is adding to our understanding of how exceptionally stage-dependent spermatogenesis is, and how genotoxic exposures at exact stages differentially affect sperm cells. Recently reported new mouse model evidence demonstrates that postfertilization, oocytes misrepair sperm DNA defects that allow chromosomal structural aberrations to persist in the conceptus. These findings give new perspectives on both intrinsic actions of DNA repair checkpoints and on critical windows of sperm vulnerability to mutagenicity and toxicity.
In 2011, over 60 000 infants were born in the US due to assisted reproductive technologies (ART).  Due to rapid advances in treatments such as intrauterine insemination (IUI) and intracytoplasmic sperm injection (ICSI), intended to bypass problems with sperm production and action, fundamental questions as to why and how sperm fail to form or function properly have received less scrutiny in recent years. Yet, we are reminded that basic questions about how germ lines are somehow disrupted still need answers. Recent evidence from Icelandic parent-child trios shows us that given a father's average age of 29.7 years, the average de novo mutation rate is 1.20 × 10−8 per nucleotide per generation.  The strongest single predictor of SNP mutation rate was paternal age at the time of conception. The effect was an increase of two mutations per year; exponentially, the model estimated paternal mutations doubling every 16.5 years. We are left with further fundamental questions. Are there evolutionary pressures that result in de novo mutations and why is the male germ line differentially more affected by point mutations and chromosomal structural aberrations? Are there factors beyond chance and numeric probability (e.g., numbers of meiotic divisions) that cause these de novo mutations in the male germ line? Is it possible that mutations and other types of DNA damage in the male germ line are more prevalent than currently understood but are self-repaired or other-repaired during fertilization by oocyte action? What happens when they are not repaired at all or somehow misrepaired by fertilization machinery?
A recent report by Marchetti et al.  gives us new insight into stage-specific sperm DNA repair mechanisms using a mouse model. Their study demonstrates that spermatogenic phase-specific chemical dosing induces DNA damage that remains seemingly undetected by sperm cell repair machinery. This damage persists in mature sperm, which successfully achieve fertilization, and are then inadequately repaired by oocyte DNA repair machinery. The result is chromosomal structural aberrations (CSA) that persist in the offspring.
Using melphalan (MLP), a bifunctional alkylating agent used in chemotherapy and an established male germ cell mutagen, sperm cell DNA damage was induced during meiosis. DNA lesions persisted through active DNA repair phases of spermatogenesis and remained unchanged throughout the spermatogenic phases. The DNA-damaged sperm then successfully fertilized eggs, after which oocyte DNA repair machinery apparently attempted to remedy the lesions, albeit ineffectively. MLP-induced DNA lesions in diplotene cells were not identified as having CSA at the subsequent metaphase I or metaphase II stages of meiosis. However, CSA were seen in the first zygotic metaphase after fertilization. Damage in diplotene spermatocytes persisted through both phases of meiosis as well as through 3 weeks of development postmeiosis, from exposure to fertilization. From stem cells, to spermatogonia, to spermatocytes, to spermatids, each of these spermatogenesis stages are known to involve ongoing DNA repair action, however at no point were the lesions induced by MLP effectively repaired. Only during the first round of DNA replication after fertilization did the egg attempt DNA repair, as the sperm underwent remodeling into the male pronucleus.
In addition to delineating what appear to be inefficiencies in both sperm and oocyte DNA repair functions, extrapolating from mouse to human, these findings have broader human health implications. Male patients receiving alkylating agent chemotherapies such as MLP remain at risk for mutagenic impacts on their sperm production for over 3 months following treatment. Agents such as MLP demonstrate that exogenous chemicals have discernable effects on sperm genomic integrity and that sperm DNA repair machinery do not appear to overcome these mutagenic impacts which do not impair the sperm's capacity for fertilization but do have enduring impacts on sperm and offspring. The exquisite vulnerability of both sperm and eggs to chemical induced perturbations clearly needs deeper attention. In addition, the fact that sperm DNA lesions could persist silently until the first DNA replication in the zygotic cycle suggests that defining a suitable male sperm donor or even defining competent sperm selection for ART may likely remain elusive until we can clearly identify precipitants to sperm CSAs and de novo mutations, and why they persist in otherwise viable sperm. The report by Marchetti et al.  takes an important step forward in answering such questions.
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