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Guide to Sperm DNA Fragmentation

What is sperm DNA fragmentation, and how does it affect fertility? How can we identify and treat DNA damage within sperm? Read on for our in-depth guide to sperm DNA fragmentation.

 

Table of contents:
  1. Why the genetic health of sperm is important
  2. What is sperm DNA fragmentation?
  3. Sperm DNA fragmentation and other semen parameters
  4. How sperm DNA fragmentation affects male fertility
  5. How sperm DNA fragmentation affects the health of the child
  6. What causes sperm DNA fragmentation?
  7. How to test for sperm DNA fragmentation
  8. References
Why is the genetic health of sperm important?

The driving purpose of sperm is a single task: to carry genetic material (DNA) from one biological parent and combine with the genetic material contained within the egg of the other, creating a unique genetic code for the offspring. Think about the structure of sperm. The majority of real estate is dedicated to its head, where chromosomes—molecules of DNA—are coiled tightly within a nucleus. The function of the neck and tail are to allow the sperm to transport its precious “cargo” most efficiently.

Since DNA carries all of the instructions for the development of an embryo, it makes sense that the genetic integrity of sperm is a key consideration in fertility. Any damage to the DNA within sperm may interfere with the sperm’s ability to properly fertilize an egg or develop into a healthy, thriving embryo. That’s exactly the issue with sperm DNA fragmentation.

Sperm DNA fragmentation is less often tested—and generally less well understood—than other semen parameters and sperm quality measures, such as sperm count, motility, and morphology. But as research continues to clarify the relationship between sperm DNA fragmentation and birth rates, there’s an emerging belief among experts that sperm genetic integrity may be one of the most important factors for male fertility.

 

What is sperm DNA fragmentation?

In order to understand sperm DNA fragmentation, we first need to understand the proper structure of DNA. DNA is organized in a long, spiral staircase-like structure called a “double helix.” The steps of the staircase are made up of pairs of chemicals known as nucleotides; the different combinations of these base pairs are what create each organism’s unique genetic sequence. The nucleotides are bonded to each other, and to the long strands of sugars and phosphates that make up the “railings” of the staircase.

DNA is packaged into structures called chromosomes. While most cells within the human body contain 46 chromosomes, sex cells or “gametes” (eggs and sperm) contain just 23. When they combine, they create a fully realized organism with the proper number of chromosomes.

Sperm DNA fragmentation refers to a change or deletion of bases, or a break or separation in one or both strands of DNA contained within sperm. This damage can occur at several different points in the sperm’s life cycle: while it’s being produced within the testicles by a series of cell divisions called spermatogenesis, while it’s in “storage” in the epididymis prior to ejaculation, or even possibly post-ejaculation.

Sperm DNA fragmentation: natural repair?

DNA damage is actually quite common in the human body, but most cells have the ability to identify and correct damage to their own genetic code. While immature sperm that are still in progress may have the ability to repair their own DNA to some extent, mature sperm do not.

Eggs can actually somewhat compensate for this, and upon fertilization, have their own processes to repair errors in sperm DNA. But if the damage is too extensive—or if the egg is less able to perform this repair, possibly due to age—the damage can endure to the embryo stage. The result may be the embryo’s failure to develop or implant (infertility), miscarriage (possibly recurring), or genetic abnormalities or illness within the offspring.

 

Understanding DNA fragmentation index

The term “DNA fragmentation index,” or DFI, refers to the percentage of sperm in a particular semen sample with fragmented DNA. A higher DFI means that a larger percentage of a man’s sperm contains genetic damage. While experts disagree on specific cut-off values, generally speaking, a DFI over 30–50% is considered high, and may impact fertility.

In some cases, subfertility may be experienced even among those with a DFI of 15–30%, especially if other abnormal semen parameters are present (such as low sperm count or poor motility or morphology), or the female partner is older, as older eggs are less able to correct the damaged DNA within sperm.

 

Sperm DNA fragmentation and other semen parameters

Research demonstrates that men who have high levels of sperm DNA fragmentation are also more likely to have other abnormal semen parameters, such as poor sperm motility or morphology.

In one 2002 study of 88 infertile men, researchers found that the average level of DNA damage was significantly higher among those with other abnormal semen parameters. This was confirmed in a 2009 study, in which higher sperm DNA damage was correlated with poorer semen parameters (sperm concentration, motility, morphology). Patients with abnormal results in all three categories had higher levels of DNA fragmentation and were more likely to have high DFI (over 30%) than patients with normal semen parameters.

This can be seen even in studies of those with just one problem with their sperm. In a 2014 study of 196 semen samples, it was found that men with teratozoospermia—a high level of abnormally shaped sperm—had higher DFI than men with normal sperm morphology. That same year, another study of over a thousand infertile men with isolated sperm defects (meaning just poor motility, poor morphology, or low sperm count, and not a combination of the above) found that levels of sperm DNA fragmentation were significantly higher among men with poor sperm motility. Approximately 31% of those men had high sperm DNA fragmentation, defined in this study as over 30% DFI.

Why does this happen? It’s probable that the same underlying factors that contribute to abnormal semen parameters, such as advanced paternal age, smoking, or toxin exposure, also cause high levels of sperm DNA fragmentation. There may be other connections or mechanisms connecting these measures, as well.

This is important to understand because, as stated above, abnormal semen parameters can compound sperm DNA fragmentation issues. However, as we’ll explore below, high sperm DNA fragmentation can impact fertility even when not paired with other factors.

 

Sperm DNA fragmentation and male fertility

First, a note: conception is absolutely possible for men with sperm DNA fragmentation. Even in studies that report lower pregnancy rates, longer time to conception, or higher miscarriage rates among men with a high DNA fragmentation index, researchers note that full-term pregnancies have been achieved.

That being said, studies demonstrate that men with high rates of sperm DNA fragmentation are at an increased risk for infertility. In one study, men presenting for infertility evaluation—who had not been able to achieve a pregnancy for a year or more—had, on average, over twice as many sperm with DNA fragmentation than fertile men (27.6% vs. 13.3%). In another, couples experiencing infertility or miscarriage were more likely to have a male partner with moderate or high levels of DNA fragmentation; researchers concluded that “spermatozoa with denatured DNA… were the best predictor for whether a couple would not achieve pregnancy.”

 

Sperm DNA fragmentation and fertility treatment

This trend—higher levels of DNA fragmentation within sperm correlating with lower live birth rates—holds true even for those being treated for infertility with in vitro fertilization (IVF). In one study of 360 couples undergoing IVF, higher sperm DNA fragmentation levels were associated with lower fertilization rates, embryo quality, and pregnancy rates. The couples who were not able to achieve pregnancy had an average of 51.7% sperm with DNA fragmentation, as opposed to 39.5% in the pregnant couples. The researchers concluded that DNA fragmentation “can predict ART [assisted reproductive technology] outcome.”

This study was especially valuable because, other than DNA fragmentation results, there were few significant differences between the group that achieved clinical pregnancy and the group that didn’t. The sperm count, motility, semen volume, and other parameters were all basically equivalent between the two study groups. This allows us to surmise that DNA fragmentation impacts fertility independently of other semen parameters.

In another study of IVF cycles, researchers found a significant negative correlation between DFI and live birth rate. Specifically, implantation rates (the percentage of embryos that implant into the uterine lining for pregnancy) were lower for couples in which the male partner had a DFI of 30% or above. Those couples had an implantation rate of just 11.7%, compared with the 22.6% for couples in which the male partner had a DFI of less than 30%.

The trend appears to hold across different types of fertility treatment, as well. In a study of IUI (intrauterine insemination), IVF, and ICSI (intracytoplasmic sperm injection, a treatment in which a single sperm is injected directly into the egg), it was found that high DFI—defined as over 27%—had the power to predict whether or not a couple would achieve pregnancy. In this study, pregnancy rates for couples with high DFI were less than half that of low-DFI couples (14% vs. 29%).

 

Sperm DNA fragmentation and embryo quality

Embryo quality is a term used in fertility medicine; it’s a way of “grading” the health and development of an embryo created in the lab. There are several different methods used to score embryo quality, but most examine the number of cells, cell size, and rate and pattern of cell division, in order to predict the likelihood that the embryo will implant and become a healthy baby. Embryo quality may also refer to embryo genetic health (having the proper number of chromosomes).

In many cases of high sperm DNA fragmentation, embryos fail early, during the four- to eight-cell stage; researchers note that sperm DNA integrity can cause “difficulty reaching the blastocyst stage.” Past that point, there’s some evidence that high sperm DNA fragmentation is correlated with lower embryo quality scores, but research results are mixed. In a study of IVF patients, increasing DNA fragmentation indexes were associated with lower embryo scores. Those with DFI above 60% scored approximately half as well as those with DFI of less than 20%.

In another study of 286 embryos created with IVF, there were several important correlations: higher sperm DNA fragmentation was associated with higher embryo aneuploidy (genetically abnormal embryos), and increasing paternal age was associated with both. For men over 50, sperm DNA fragmentation rates were around 40%, and less than 30% of embryos created from their sperm were genetically normal.

But in a review of 28 studies analyzing the relationship between sperm DNA fragmentation and embryo quality, only 11 studies found a clear correlation between DNA damage in sperm and the genetic health or development of the embryo. In the remaining 17 studies, no relationship could be identified.

 

Sperm DNA fragmentation and miscarriage

Research indicates that sperm DNA damage is associated with pregnancy loss, especially early pregnancy loss, and may be implicated in recurrent miscarriage as well. In one study of IVF patients, couples in which the male partner had high levels of sperm DNA fragmentation were significantly more likely to experience miscarriage compared to couples in which the male partners had low DNA fragmentation rates (50% miscarriage rate vs. 16%). The same study also indicated that pregnancy rates were lower for high DFI patients. Researchers concluded that “high sperm DNA fragmentation can compromise ‘embryo viability,’ resulting in pregnancy loss.”

In another study, high DFI (defined in this case as over 27%) was again correlated with higher miscarriage rates. In this study, miscarriage rates for high-DFI couples was approximately 27%, as opposed to 10% for low-DFI couples. And again, in a third study of IUI patients, the early miscarriage rate for high-DFI couples (over 30%) was 27%, compared to 8% for the low-DFI couples.

While it’s not entirely clear that sperm DNA fragmentation causes recurrent pregnancy loss (defined as two or more consecutive miscarriages), research demonstrates that the two issues are correlated. In one study, 45% of male partners in cases of recurrent pregnancy loss had high sperm DNA fragmentation levels, compared to just 15% of the control group. In another, couples experiencing recurrent miscarriage were more likely to have male partners with high rates of sperm DNA damage. Researchers concluded that the genetic integrity of sperm was a “significant predictor for future [miscarriage] and infertility.”

 

Sperm DNA fragmentation and the health of the child

Because of the many factors that contribute to the development of disease, it’s complicated to conduct research into specific causes. Some experts believe that “loss of sperm DNA integrity not only impacts reproductive and psychological health of the infertile couple but also increases childhood disease burden,” and that healthy sperm DNA is essential for “the health and well being of the next generation.” But the links are difficult to make conclusively.

A strong correlation has been found between the development of retinoblastoma, a childhood eye cancer, and DNA damage in paternal sperm. Researchers found that DFI levels were significantly higher in the sperm of fathers of children with retinoblastoma. It’s also been shown that advanced paternal age has been associated with many conditions in offspring—including cleft palate, heart defects, schizophrenia, autism, and epilepsy—possibly due to increased sperm DNA fragmentation with age.

 

What causes sperm DNA fragmentation?

There are a few possible ways that sperms’ genetic material becomes damaged, and it can happen at several points in the process of producing and storing sperm.

 

How sperm gets damaged

 

Errors or damage during “condensation”

During spermatogenesis (the sperm production process), the genetic material within sperm condenses. The DNA wraps around a few specific proteins to organize itself snugly in the sperm’s head. This allows large amounts of DNA to occupy a very small space, and—it’s theorized—may protect the DNA from damage.

But this process also has the potential to introduce damage. Firstly, it’s quite a large amount of genetic material that needs to be compacted tightly within the nucleus of the sperm; it’s possible that this twisting could cause physical breaks in the DNA. Additionally, if there are problems with the mechanisms that package this genetic material, such as a lack of the proteins required to facilitate the condensation process, DNA will be more susceptible to damage from outside forces.

 

Oxidative stress

Those “outside forces” include free radicals. Free radicals, also known as reactive oxygen species (ROS), are unstable molecules produced as natural byproducts of daily life. At low levels, ROS are not typically damaging and may even be useful to cells. But if left unchecked, free radicals can cause damage to other molecules inside our cells, such as DNA. This damage is known as “oxidative stress.”

Our bodies typically use compounds called antioxidants—produced naturally by our body and absorbed from food or supplements—to neutralize these molecules and prevent damage. In fact, semen is known to contain relatively high quantities of antioxidants such as vitamin E, vitamin C, and glutathione, to protect sperm from oxidative stress. Antioxidants are also produced in other parts of the male reproductive system, including the epididymis, where sperm is stored.

There are certain experiences or behaviors that can cause levels of free radicals to rise beyond what our body can typically handle: exposure to environmental toxins such as pesticides, heavy metals, or pollution; radiation; infections; smoking tobacco; and drinking alcohol are all known to increase the presence of ROS. On the other hand, if levels of antioxidants are too low, possibly due to a poor diet, they won’t be able to adequately counteract free radicals.

Any disruption of the delicate balance of free radicals and antioxidants can result in oxidative stress. Researchers believe that oxidative stress is the major cause of sperm DNA fragmentation. In research, it’s been shown that as oxidative stress increases, sperm exhibit elevated levels of DNA damage; at the highest levels of oxidative stress, high DFI is observed alongside a loss of sperm motility.

 

Abortive apoptosis

Apoptosis is programmed cell death that’s part of normal growth and development. Think of it as a “self-cleaning” mechanism that eliminates unnecessary, damaged, or infected cells from the body. Though apoptosis involves the death of cells, it’s beneficial and important for the health of the overall organism. A cell that’s been marked for “deletion” can be identified by the presence of a specific protein, Fas, that induces apoptosis.

If the body is constantly cleaning out faulty cells, how do sperm cells with DNA damage make it into the semen? It seems that, sometimes, the body can recognize sperm with DNA damage and “earmark” them with Fas, but the process gets interrupted somewhere along the way, and damaged sperm are able to escape. This is known as abortive apoptosis. Why this happens is not entirely clear, but we know it has an impact on fertility: in one study, men with abnormal sperm parameters display higher levels of Fas on their sperm.

 

These mechanisms work together

These mechanisms are interrelated. It’s likely that, as opposed to a single one of these processes causing sperm DNA fragmentation, it’s a series of failures that results in high levels of DNA damage. For example, if the DNA within sperm is not packaged correctly, it’s more vulnerable to oxidative stress. Too-low levels of antioxidants allow that oxidative stress to damage sperm, and finally, the body fails to completely eliminate these damaged cells.

 

Risk factors for sperm DNA fragmentation

Paternal age

Research indicates that increasing age is associated with increased sperm DNA fragmentation, and the increase becomes especially significant for men in their late 40s and early 50s. One study of over 2,500 subjects found that while men under age 30 had an average DNA fragmentation index of 35.6%, men over 50 had an average DFI of 47.7, a level that is likely to come with impacts on fertility.

Another study of 2,681 male patients in Chile found a similar result: men over the age of 50 were more than four times more likely to present sperm DNA fragmentation than men under 30.

Why does this happen? There are several hypotheses. With age, cells are less able to repair their own DNA damage, possibly resulting in the production of sperm with increased damage. Additionally, as men age, they accumulate DNA damage in their testes, as well (thanks to oxidative stress), which is believed to impact the sperm cells created. The increased damage could also be due to the fact that there’s a lower number of cells that are integral to the production of sperm in the testicles with age, or that the mitochondria that power sperm cells are less effective with age.

Smoking

Cigarette smoking is a known cause of exposure to many damaging chemicals (such as cadmium and nicotine) as well as increased oxidative stress. Several studies have found that smokers have increased levels of sperm DNA fragmentation, and that the level of DNA damage is correlated with how long the patient has been a smoker and how many cigarettes they smoke.

In one study of infertility patients, researchers found that the percentage of sperm with abnormal DNA condensation was significantly higher among smokers than non-smokers, and that the abnormalities were proportional to both the number of cigarettes smoked per day and the duration of smoking.

In another study, researchers found that of four patient groups—a control group, an alcohol-drinking group, a cigarette-smoking group, and a drinking/smoking group—the two groups with smoking patients exhibited the highest average levels of sperm DNA fragmentation. Drinking did appear to have a small impact on DNA damage to sperm, but smoking appeared to be much more impactful with regards to sperm DNA fragmentation.

 

Illness or infection

Possibly because they increase oxidative stress, illness or infection—such as influenza, cancer, or sexually transmitted infections—are associated with increased sperm DNA fragmentation. In a study of men with infections from chlamydia or mycoplasma (a bacteria that causes inflammation in the urinary tract), patients had increased DFI compared to controls (an average of 35% compared to 11%).

In a case study of one patient with a fever illness, sperm DNA fragmentation testing revealed that DFI increased significantly in the month following the fever—by 24% in the 15 days following the fever and 36% in the 37 days following— reflecting the infection and fever’s impact on the sperm that were in production at the time of the illness. The DFI did not decrease to normal levels until approximately 2–3 months after the fever resolved, once again reflecting the duration of spermatogenesis (it takes approximately 70–90 days to produce sperm).

And, while cancer treatment is well known for its impact on fertility (more on that below), research is revealing that cancer itself may have a damaging effect on sperm DNA. In a study of pre-treatment cancer patients that compared them to fertile and infertile patients, cancer patients had higher levels of sperm DNA fragmentation than even infertile men, before even receiving any chemotherapy or radiation treatments.

 

 

Cancer treatment

Cancer treatments such as chemotherapy and radiation are life-saving, but may have the side effect of impacting fertility.

Chemotherapy works by using medications to identify fast-dividing cancer cells and kill them or prevent them from dividing. But because the medication is administered through the bloodstream to the entire body, it can also attack other cells, including sperm cells. Research in male rats demonstrates that the chemotherapy medications most often used for testicular cancer are associated with an increase in sperm DNA damage. A case study of a human cancer patient presented a similar impact, revealing a substantial increase in sperm DNA damage after 8 weeks of chemotherapy treatment for leukemia, and persisted nearly a year later.

Radiation therapy involves directing high-energy rays at the cancer, in hopes of killing those cells. But by extension, this treatment can also damage parts of the body surrounding the cancer as well, potentially slowing down or stopping sperm cell production. Some men who undergo radiation will experience a temporary drop in fertility that recovers in the years following treatment; for others who undergo higher doses of radiation, sperm production may stop permanently. It’s been demonstrated in lab experiments that exposure to radiation creates a dose-dependent increase in DNA damage in sperm.

Up to 75% of reproductive-age men who are diagnosed with cancer may face infertility as a result of treatment, according to Livestrong. That’s why it’s highly recommended that men diagnosed with cancer freeze their sperm prior to treatment. Learn more about sperm freezing.

 

Chemical, toxin, or radiation exposure

According to the CDC, exposure to a number of chemicals or heavy metals are known to cause DNA damage within sperm. Examples include phthalates and styrene (chemicals used in plastic production); pesticides/insecticides such as organophosphate, carbaryl, or fenvalerate; lead; or benzene, a chemical that’s widely used in manufacturing and also found in gasoline and tobacco smoke.

Radiation is also implicated in sperm DNA damage, with one study of healthcare workers exposed to ionizing radiation (used in X-rays) concluding that sperm DNA fragmentation levels were significantly higher among exposed men than among non-exposed men.

The most damaging long-term, repeated exposure is typically thanks to a man’s occupation. Healthcare workers, for example, have repeated long-term exposure to radiation, as illustrated above. Other occupations with exposure risk include jobs in manufacturing, agriculture, and the military.

 

How can sperm DNA fragmentation be tested?

Sperm DNA fragmentation is not tested as part of a typical semen analysis. There are several different tests that use different technologies to identify and quantify DNA damage within sperm:

  • Sperm chromatin dispersion test (SCD)
  • Transferase-mediated terminal uridine nick-end labeling (TUNEL)
  • Comet assay
  • Sperm chromatin structure assay (SCSA)

Each of these tests has its own pros and cons with regard to accuracy/sensitivity, cost, and speed of results. SCSA is the most commonly used sperm DNA fragmentation and could be considered the industry standard.

 

References
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