Researchers have turned their attention to the genetic contents of sperm, as embryo development depends in part on the inherent integrity of sperm DNA. DNA integrity testing is relatively new to the armamentarium for fertility specialists. While originally described in 1993, it failed to gain traction as a clinical test due to lack of availability and standardization. Sperm DNA is highly compacted by binding tightly to protamine. A certain degree of sperm DNA damage can often be repaired by the oocyte’s antioxidant enzymes. When damage exceeds the repair capability of the oocyte, deleterious effects of sperm DNA fragmentation (SDF) may result, such as miscarriage and poor embryo development (Hum Reprod 1999; 14:1039-49).
Because of this, more attention has turned to testing for SDF. This test does not evaluate the actual genetic code of the DNA within the sperm, but rather the overall superstructure of the DNA strands. There are many different ways to test for SDF, each with pros and cons. We will focus our discussion on the most commonly used tests as well as what is considered the gold standard, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) (Asian J Androl 2016; 18:205-12).
The acridine orange (AO) test uses a fluorescent dye that emits a different wavelength of light based on whether it is bound to double-strand DNA (normal) or single-strand DNA (abnormal). This test is fast, simple, and inexpensive but limited by inter-laboratory variations and lack of reproducibility. Another relatively simple test, the “halo” test, has similar pros and cons as the AO test, but evaluates the characteristic halo of dispersed DNA loops around sperm with non-fragmented DNA.
The latest technology using TUNEL detects “nicks” or free ends of DNA by utilizing fluorescent nucleotides with a flow cytometer (or fluorescence microscopy) to quantify the incorporation of dUTP into DNA breaks. This test is sensitive, reliable, and has minimal inter-observer variability. It can also be performed on samples with severe oligozoospermia. Many now consider this the gold standard for DNA fragmentation testing. More importantly, SDF can provide valuable information for both the infertile couple and fertility specialists in guiding treatment.
In men with infertility and varicoceles, some will not see any improvement in semen parameters after varicocelectomy, and SDF testing may help to identify those in whom varicocele ligation would be of most benefit. SDF can also be used in couples with recurrent pregnancy loss or who have failed IUI as a predictive tool to identify and effectively stratify patients based on elevated SDF and potentially offer IVF or ICSI sooner rather than later. In patients who have already failed IVF or ICSI with ejaculated sperm, SDF testing can help determine the next steps in treatment.
Because DNA fragmentation increases during sperm transit through the epididymis, these patients may benefit from IVF/ICSI with sperm extracted from the testis instead of repeated IVF cycles with ejaculated or epididymal sperm. SDF may also help identify environmental exposures and lifestyle choices that affect male fertility. Factors like smoking, obesity, and occupational exposures can all worsen SDF. In these patients, SDF testing can help predict their fertility potential and monitor response to lifestyle modification.
Both oxidative stress testing and DNA fragmentation testing are relatively new technologies that will help clinicians choose the most appropriate therapies for their patients. While no large-scale randomized studies have been performed yet, emerging evidence on both are promising. We believe they will eventually become commonly used in clinical practice for the management of male infertility.
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