What is DNA fragmentation in sperm?

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Deoxyribonucleic acid (DNA) is a self-replicating material present in all cells and is the main component of chromosomes. DNA carries genetic material and instructs cells to make different proteins and other substances required for growth and development, among other things.  

As with all other human cells, sperm cells contain DNA. The role of the sperm cell is to transmit the sperm DNA into an egg (oocyte), thus creating the fertilized zygote, with half of the zygote DNA coming from the oocyte and half from the sperm. Sperm DNA integrity is critical to the normal development of the embryo and for the achievement of a healthy pregnancy.ⁱⁱ‍

DNA fragmentation describes a type of DNA damage where nicks or breaks appear in the DNA molecules. This type of damage occurs within sperm DNA as well as within DNA in other cells in our bodies. Sometimes, fragmentation can be repaired by specific DNA repair mechanisms. Developing sperm have the ability to repair errors in their DNA, but once the sperm reach maturity and are stored in the epididymis, they are no longer able to make repairs. Thus, damage that occurs to sperm DNA after storage or post-ejaculation is carried with the sperm if it fertilizes the egg. In some cases, the oocyte and embryo can repair the damaged sperm DNA after fertilization.

The proportion of sperm with DNA fragmentation is related to male fertility, with a high proportion of SDF corresponding to a decrease in fertility.ⁱⁱⁱ The proportion of SDF is measured by the DNA fragmentation index.

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What causes sperm DNA fragmentation?

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Mutiple different mechanisms can induce SDF. On a molecular level, a few of the most common hypothesized causes are:

1. DNA breaks during sperm development: In order for sperm DNA to be stored inside the tiny sperm head, it needs to be compactly coiled. During sperm development, the DNA is coiled and re-coiled multiple times, and it is thought that small nicks may be made in the DNA to allow the DNA to bend and be compactly packaged. It is possible that these nicks can accumulate as DNA fragmentation if they are not repaired by the time the sperm matures.^iv  
2. Reactive oxygen species (ROS): The presence of high amounts of damaging molecules known as reactive oxygen species (ROS) within the male reproductive tract may also contribute to SDF. ROS are unstable oxygen-containing molecules that easily react with other molecules. When they react with neighboring molecules – including DNA, RNA and proteins – they can cause damage. Interestingly, immature sperm produce ROS, and it is thought that mature sperm may be exposed to these ROS when they are housed next to immature sperm in the seminiferous tubules or part of the epididymis.^v  
3. Apoptosis: Apoptosis is the process by which cells undergo programmed cell death. This is a normal process triggered by cellular damage, such as a too-low or too-high level of oxygen or nutrient deficiency. A full 50-60 percent of germ cells starting the process to become sperm will be marked for apoptosis before maturity because they are defective. Normally, these cells marked for apoptosis will break down and not become sperm. However, it is thought that some of these early defective DNA-containing germ cells may not be broken down appropriately and may ultimately develop into fully mature sperm. As the genetic content of the sperm may not be associated with any morphologic or motility issues, the sperm may appear normal, but contain defective – or fragmented – DNA.^vi  

Lifestyle factors may also influence SDF. A 2018 study of Saudi men undergoing intracytoplasmic sperm injection cycles (ICSI) found that 14 percent had high SDF, defined by the researchers as a DNA fragmentation index of greater than 30 percent. They showed that increasing age and higher body mass index (BMI) were associated with high SDF.^vii‍

Gonadotoxins (substances that damage the testicles and ovaries) such as chemotherapy or radiation used for cancer treatment can lead to DNA sperm fragmentation through the molecular mechanisms described above.^viii Similarly, varicoceles can lead to an increase in ROS, which has been shown to increase DNA sperm fragmentation.^ix Lifestyle and patient-related factors such as age,^x smoking,^xi and obesity^xii have been associated with high DNA sperm fragmentation in some studies, but not others; more research is needed in this area.

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How does high sperm DNA fragmentation impact fertility?

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Multiple studies reveal an association between high SDF and lower male fertility.  A large prospective study by Buck Louis et al (2013) followed 501 couples that discontinued contraception and were trying to conceive. They measured SDF in human semen samples and showed that a high DNA fragmentation index was associated with a longer time to pregnancy.^xiii  

Spanò et al (2000) followed 231 couples trying to conceive and found that male fertility may begin to decline once the SDF index exceeds 20 percent and decreases significantly once the SDF index exceeds 40 percent.^xiv  Furthermore, a large meta-analysis from 2019 demonstrated that higher rate of DNA sperm fragmentation in the male partner was associated with recurrent pregnancy loss.^xv Studies also reveal an association between SDF and assisted reproductive technology outcomes.  

A cohort study by Oleszczuk et al (2016) showed that semen samples with an SDF index of greater than 20 percent had a significantly decreased live birth rate with in-vitro fertilization (IVF) treatment, compared to the control group where the SDF index was less than 10 percent. Interestingly, there was no difference between the groups if ICSI was used, leading the researchers to conclude that infertility due to high SDF index may be overcome by using ICSI instead of conventional IVF.^xvi

Intrauterine insemination (IUI) has also been found to be less effective in men with a high rate of SDF. Bungun et al (2007) found that in patients with SDF index less than or equal to 30 percent, the per cycle rate of deliveries following IUI was 19 percent, but in patients with SDF greater than 30 the delivery rate dropped to 1.5 percent.^xvii A recent meta-analysis pooling 10 studies confirmed this association, finding that high SDF was associated with lower pregnancy rate and live birth rate with IUI.^xviii

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How is sperm DNA fragmentation tested?

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SDF test methods rely on standardized protocols with stringent quality control. These methods include the sperm chromatin structure assay (SCSA), terminal deoxynucleotidyl transferase-mediated dUTP  end labeling assay (TUNEL), sperm chromatin dispersion (SCD), and single cell electrophoresis assay (Comet). These are genetic tests that measure the level of various types of sperm DNA damage.^xix‍

All these methods involve laboratory testing of sperm from a semen sample. A 2-to-3-day period of abstinence is generally recommended prior to the semen analysis test.^xx

Indications for SDF testing, summarized in a 2017 study, include inability to conceive as well as:^xxi

• Varicocele (an abnormal enlargement of the veins within the scrotum) with normal or borderline semen analysis parameters
• Unexplained infertility  
• Recurrent pregnancy loss
• Recurrent (IUI) failure
• Recurrent IVF and/or ICSI failure
• Modifiable lifestyle risk factors, such as smoking and obesity, with borderline abnormal or normal semen analysis parameters

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How can sperm DNA fragmentation be improved?

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Men with high SDF and other sperm defects may be able to improve sperm genetic integrity by adopting healthier lifestyle habits. These include oral antioxidant intake, weight loss, and smoking cessation.

Showell et al (2019) completed a systematic review of studies that focused on subfertile males referred to fertility clinics and found an association between oral antioxidant intake and lower SDF. The group estimated that the baseline chance of live birth was 12 percent following placebo or no treatment, and 14 to 26 percent following the use of antioxidants. However, this was based on a small sample size of 124 live births in a total of 750 couples.^xxii The antioxidants used in the studies included arginine, carnitines, carotenoids, Coenzyme Q10, cysteine, micronutrients (folate, selenium, zinc), vitamin E and vitamin C.

Weight loss may also improve one’s SDF index. In one study from 2018, 105 Indian men were enrolled in a weight loss program and SDF index was studied. On average, the SDF index improved from 20.2 percent to 17.5 percent after the weight loss program.^xxiii

Because there is an established association between smoking and increased DNA fragmentation, smoking cessation may improve SDF and subsequent fertility.^xxiv

There is a small body of evidence that a shorter time between ejaculations may reduce SDF, which is particularly relevant to those providing semen for IUI, IVF or ICSI. In one study by Gosalvez et al (2011), there was a 48 percent reduction in SDF in patients following a 3-hour abstinence period compared to a 24-hour abstinence period. Additional studies are required on this subject and currently there is no recommendation that several semen samples be obtained within 24 hours in the case of high SDF index.^xxv

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What fertility treatments can support people with high DNA sperm fragmentation?

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Patients with high SDF may have an increased risk of infertility and have several options for assisted reproduction treatment to improve reproductive outcomes if they are unable to conceive at home. Success rates vary due to factors such as severity of SDF, fertility treatment type, and other concurrent health conditions or infertility diagnoses.

IVF is a fairly common fertility treatment. IVF may be less successful when the sperm used have a high SDF index. A study by Simon et al (2013) of 203 couples undergoing IVF found that couples with less than 25 percent SDF had a live birth rate of 33.3 percent. In comparison, couples with SDF of up to 50 percent had a live birth rate of just 13.1 percent.^xxvi

Given that IVF may be less successful in the case of a high SDF index, ICSI may give couples with this issue a higher chance of success. In ICSI, a single sperm cell is selected and injected directly into the egg. A meta-analysis of 20 articles by Zhang et al (2015) found that the clinical pregnancy rate was similar between patients with high SDF and low SDF when ICSI was used, indicating that ICSI can improve fertility in patients with high SDF.^xxvii

Finally, high DNA sperm fragmentation has been associated with poor success with intrauterine insemination (IUI), with one study demonstrating a live birth rate of just 1.5 percent when the DNA fragmentation index is above 30 percent.^xxviii

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Conclusion

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Sperm DNA fragmentation is one possible contributor to male factor infertility and an important area of current research. SDF testing may be recommended if a person is struggling with infertility and has certain other health conditions or factors including a varicocele, unexplained infertility, or recurrent pregnancy losses with normal or nearly-normal semen analysis parameters. If high SDF is diagnosed, interventions such as weight loss, smoking cessation, and taking antioxidant supplements may help improve male fertility potential. However, in some cases, physicians may recommend advanced reproductive techniques such as ICSI, which has had proven success in patients with high SDF.  

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Individuals experiencing subfertility are encouraged to talk to their health care providers with further questions.