Levels of DNA accessibility in human sperm vary across individuals with differing reproductive parameters

Abstract

Mammalian sperm DNA is packaged in a much denser form than in somatic cells, protecting the DNA from damage and reducing the size of the nucleus to improve passage towards the oocyte. While defective sperm DNA packaging correlates with reduced fertilization rates and impaired pre-implantation embryonic development, these defects are not currently examined in standard semen analysis. Here, we adapted NicE-view, an assay that directly labels accessible DNA, for use in human sperm and applied this method including extensive image quantification to examine spermatozoa from individuals with normal conventional semen parameters but variable reproductive outcomes. We found that two sub-populations of cells differing greatly in their DNA accessibility exist within both total and motile sperm. The frequencies of these two sub-populations vary between individuals, and selection of motile sperm by swim-up generally enriches for sperm with high DNA accessibility. Individuals with high frequencies of sperm with high DNA accessibility possess decreased sperm concentrations and increased DNA nicking levels and a subset of these individuals have a history of post-fertilization embryogenic failure. NicE-view shows much clearer separation of staining levels than alternative DNA labeling approaches, and represents a valuable tool for the assessment of human sperm.