Evaluation of nuclear DNA damage in human spermatozoa in men opting for assisted reproduction

Abstract

Diagnosis of sperm DNA integrity of semen sample is important for consistently high reproductive efficiency. The conventional parameters of semen analysis take into account morphology, motility, and concentration of spermatozoa in the sample, which are insufficient for evaluation of reproductive potential. Current studies have implicated abnormal organization of genomic material in sperms as a probable cause in 20 per cent cases of male infertility. This is especially important in the era of assisted reproduction technique (ART) when a majority of infertile couples opt for assisted reproduction and in where cases DNA integrity is a better diagnostic and prognostic marker as compared to routine semen analysis. This article reviews and discusses some of the current techniques employed for evaluating chromatin structure or DNA damage in spermatozoa. These different techniques include single cell gel electrophoresis (COMET assay), Terminal tranferase dUTP Nick End Labelling (TUNEL), sperm chromatin structure assay (SCSA), In situ nick translation (ISNT) and acridine orange test. These techniques are independent measure of sperm quality and assist in semen quality assessment by detecting defects in DNA integrity or chromatin structure. The discussed techniques vary in their level of accuracy, cost input, sophistication of analysis and their application depends upon the sensitivity required for analysis. The article also briefly outlines the DNA packaging and the causes of DNA damage in spermatozoa. During chromatin packing 85 per cent of the histones are replaced by protamine while the residual histones act as marker of genes which are expressed in early embryonic development. Among the different aetiological factors observed to be responsible for DNA damage in human spermatozoa increased reactive oxygen species (ROS), oxidative stress is highly correlated with greater DNA fragmentation index (DFI). Oxidative stress leads to single and double strand breaks in sperm DNA. Apoptosis and abnormal chromatin packing also contribute to DNA damage. The significance of chromatin structure studies is more stressed owing to the greater awareness to transmission of genetic diseases because of higher incidence of gene imprinting defects, increased cancer frequency and other congenital and non-congenital defects in children conceived through assisted reproduction techniques.