Many of our new male fertility patients are perplexed about their inability to conceive. They have been told that it is not a female factor and that their semen quality is excellent. Their sperm have perfectly shaped oval heads, normal-sized mid-pieces, and straight well aligned tails, and they demonstrate great motility (i.e. sperm movement) and forward progression. On visual inspection, one would assume that their sperm could fertilize an egg with ease producing a normal embryo that would develop into a beautiful baby. Unfortunately, in spite of their good looks, their sperm may never be baby-makers. The integrity of their DNA might be impaired. Even if a sperm is able to fertilize an egg, DNA fragmentation will most likely prevent normal development of an embryo, resulting in a failed pregnancy or miscarriage.
It’s a growing concern amongst fertility specialists that sperm DNA fragmentation might be the cause of many couples’ inability to achieve a pregnancy naturally or through an assisted reproductive technology protocol using IUI, IVF, or ICSI. Many medical practices specializing in fertility have added sperm DNA fragmentation testing to the battery of tests they typically perform to evaluate a man’s fertility potential. Our practice recently started screening male fertility patients with known association for higher levels of DNA fragmentation such as advanced male age, presence of large varicoceles, history of miscarriages. We have found greater than 40% of these men have abnormal levels of fragmentation. This has resulted in many questions from patients anxious to know what it means, how it happens, and what options are available to enable them to achieve a pregnancy. We hope to answer many of these questions here. We have provided links for those wanting additional information.
The Nobel Prize in Physiology or Medicine 1962 was awarded jointly to Francis Harry Compton Crick, James Dewey Watson and Maurice Hugh Frederick Wilkins for their discovery of the molecular structure of DNA – the double helix shown here.
DNA is an organic molecule that consists of two strands (Double Helix) of repeating building blocks called nucleotides. Each nucleotide consists of a sugar, a phosphate group, and a nitrogen base. The nucleotides in each strand are tightly bonded. The two strands however are loosely held together by weaker bonds. DNA fragmentation occurs when separations and breaks occur in these bonds. Our chromosomes are composed primarily of DNA, and segments of that DNA form genes, whose specific sequence of nitrogen bases constitutes genetic code. DNA fragmentation can impair the transmission of this genetic code, disrupting a cell’s ability to function. For a sperm cell, DNA fragmentation can prevent fertilization or proper development of an embryo if fertilization occurs.
Scientists have identified several factors that contribute to an increase in sperm DNA fragmentation. Oxidative stress is a major cause of DNA damage. Oxidative stress is the damage caused by free radicals reacting with molecules like DNA and thus disrupting their bonds. Physical and environmental changes can increase the level of free radicals to which sperm DNA is exposed. Recent studies have linked environmental toxins in certain chemical air pollutants, pesticides, and plastics to elevated oxidative stress and sperm DNA fragmentation. Obesity, alcohol consumption, and tobacco have also been associated with DNA damage from increased oxidative stress.
Scrotal varicoceles have been shown to exacerbate sperm DNA fragmentation. Varicoceles elevate the temperature inside the testes to levels that disrupt the bonds of DNA molecules, resulting in fragmentation. Excess fat around the scrotum from being overweight will also elevate testicular temperatures to unfavorable levels that damage the DNA bonds.
Chemotherapy and radiation therapy have been linked to sperm DNA fragmentation, and further investigation is needed to determine if this is a long-term or even permanent side-effect of some cancer treatments. Reproductive aged men who have been diagnosed with cancer should be encouraged to cryopreserve sperm prior to starting treatment as a means of preserving their fertility. Post treatment fertility evaluations should include an assessment of sperm DNA fragmentation, especially since men with normal semen analyses can have high levels of sperm DNA fragmentation.
Glitches in the process of sperm maturation can cause sperm DNA fragmentation. The structure of the human sperm cell makes it more susceptible to DNA fragmentation than any other cell in the body. Its nucleus has 40% less space compared to most other cells, so sperm DNA has to be highly compacted and coiled to enable it to fit in the nucleus. This highly compacted and structured arrangement protects the sperm cell’s genetic material (DNA) as it travels through the male and female reproductive tracts, and it enables the proper fusion of DNA from both sperm and egg at fertilization. Sperm maturation is a process that transforms a round germ cell into an elongated mature sperm cell with a head, acrosome, midpiece, and a tail. During this process DNA is uncoiled, transcribed (which is the process of reading the DNA’s genetic code) and recoiled a couple of times. Intentional nicks and breaks are made in sperm DNA to enable it to be tightly compacted and recoiled. These nicks are supposed to be repaired when the DNA is being transcribed, but external factors, such as oxidative stress and advanced paternal age, often prevent this and hence result in sperm DNA fragmentation.
Sertoli cells which function to support and nourish developing sperm cells in the testes also screen sperm cells and mark defective ones for elimination. Sometimes external factors disrupt this process and marked sperm cells are not eliminated, or apoptosis (cell death) is initiated but not completed. These sperm cells have a much higher incidence of DNA fragmentation.
Sperm with normal DNA fluoresces green and can be quantified with a flow cytometer which can identify these sperm and count the number of them present
Sperm with fragmented DNA fluoresces orange and can be counted with a flow cytometer which can identify these sperm and count the number of them present
Sperm DNA fragmentation can occur during the movement of sperm from the seminiferous tubules through the vas deferens, an event that allows immature and mature sperm to be tightly packed together. Immature sperm produce a high level of reactive oxygen species (a type of free radical), which often react with and damage the DNA of mature sperm. Reactive oxygen species also activate enzymes called caspases and endonucleases that are part of the process of apoptosis, further damaging sperm DNA. This is the primary reason that sperm retrieved from the testes has less DNA damage than sperm from ejaculate.
If you are reading this blog because your doctor has recommended you be tested for sperm DNA fragmentation or you recently had testing that indicated a high level of DNA damage, your situation is not hopeless. For many men, simple lifestyle changes can improve sperm DNA fragmentation. Weight loss, healthy diet, cessation of smoking, reduced consumption of alcohol, and antioxidants have all been shown to improve DNA integrity. For older men, who typically experience DNA damage because the mechanisms of sperm maturation function less efficiently, and men who have undergone treatment for cancer, it may not be possible to reduce sperm DNA damage. However, sperm can be surgically retrieved from the testis and used in IVF/ICSI, as the incidence of DNA fragmentation is usually much lower in sperm from the testes than from ejaculate.
Sakkas D, Alvarez J. Sperm DNA fragmentation: mechanisms of origin, impact on reproductive outcome, and analysis. Fertil Steril, Mar 2010; 90(4): 1027-1036.
Ribas-Maynou J, Garcia-Peiro A, et al. Comprehensive analysis of sperm DNA fragmentation by five different assays: TUNEL assay, SCSA, SCD test and alkaline and neutral Comet assay. Andrology, 2013; 1: 715-722.
Erenpreiss J, Spano M, et al. Sperm chromatin structure and male fertility: biological and clinical aspects. Asian J Androl, 2006; 8(1): 11-29.
Agarwal A, Said T. Sperm chromatin assessment. From http://www.clevelandclinic.org/reproductiveresearchcenter/docs/agrach016.pdf