"Epigenetics and male fertility – not all sperm are created equal"
What is epigenetics and why is it important for reproduction?
Epigenetics relates to changes in gene expression which are not caused by changes in gene sequence. Epigenetic regulation can involve several mechanisms, the most studied being modification of histone residues and methylation of cytosine nucleotides. DNA methylation is extremely important for a process called genetic imprinting which takes place in placental mammals. Imprinted genes are biallelic genes which, due to the epigenetic inactivation of one of the copies either in the sperm or the oocytes, are expressed only through the maternal or the paternal allele. After fertilization, the embryo undergoes a genome wide wave of demethylation which erases most of the epigenetic marks inherited from the gametes, but does not affect imprinted genes. These marks are usually maintained in the somatic lineages of the offspring. In the germline, however, these imprints must be reprogrammed, therefore primordial germ cells undergo a second genome wide wave of demethylation (now including imprinted genes), followed by de novo methylation.
In recent years, several studies have indicated a possible link between epigenetic factors in sperm and male infertility. Some studies have suggested a higher prevalence of imprinting disorders in children born after assisted reproductive techniques (ART). Aberrant imprinting in sperm DNA has been associated with abnormal sperm parameters (including reduced sperm count and morphological abnormalities), increased rates of stillbirths and spontaneous abortions, and the levels of DNA methylation in sperm have been suggested to predict ART outcome.
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