The relationship between sperm nuclear DNA fragmentation, mitochondrial DNA fragmentation, and copy number in normal and abnormal human ejaculates

Abstract

BackgroundWhile it is common to clinically evaluate sperm nuclear DNA fragmentation, less attention has been given to sperm mitochondrial DNA. Recently, a digital PCR assay has allowed accurate estimation of the proportion of fragmented mtDNA molecules and relative copy number.ObjectivesTo determine the correlation of classical sperm parameters, average mtDNA copies per spermatozoon and the level of mtDNA fragmentation (SDF‐mtDNA) to that of nuclear DNA fragmentation (SDF‐nDNA), measured as the proportion of global, single‐strand DNA (SDF‐SSBs) and double‐strand DNA breaks (SDF‐DSBs). To determine whether the level of nuclear and mitochondrial DNA fragmentation and/or copy number can differentiate normozoospermic from non‐normozoospermic samples.Materials and methodsEjaculates from 29 normozoospermic and 43 non‐normozoospermic were evaluated. SDF was determined using the sperm chromatin dispersion assay. mtDNA copy number and SDF‐mtDNA were analyzed using digital PCR assays.ResultsRelative mtDNA copy increased as sperm concentration or motility decreased, or abnormal morphology increased. Unlike SDF‐mtDNA, mtDNA copy number was not correlated with SDF‐nDNA. SDF‐mtDNA increased as the concentration or proportion of non‐vital sperm increased; the higher the mtDNA copy number, the lower the level of fragmentation. Non‐normozoospermic samples showed double the level of SDF‐nDNA compared to normozoospermic (median 25.00 vs. 13.67). mtDNA copy number per spermatozoon was 3× higher in non‐normozoospermic ejaculates (median 16.06 vs. 4.99). Although logistic regression revealed SDF‐Global and mtDNA copy number as independent risk factors for non‐normozoospermia, when SDF‐Global and mtDNA copy number were combined, ROC curve analysis resulted in an even stronger discriminatory ability for predicting the probability of non‐normozoospermia (AUC = 0.85, 95% CI 0.76–0.94, p < 0.001).ConclusionHigh‐quality ejaculates show lower nuclear SDF and retain less mtDNA copies, with approximately half of them fragmented, so that the absolute number of non‐fragmented mtDNA molecules per spermatozoon is extremely low.

Addendum to: The relationship between sperm nuclear DNA fragmentation, mitochondrial DNA fragmentation and copy number in normal and abnormal human ejaculates

“…Aliquots of fresh neat‐ejaculated semen samples were incubated at 37°C for 48 h in a water bath, resembling the temperature of the female genital tract. Aliquots of spermatozoa for analysis were obtained at 0, 6, 24 and 48 h, to estimate the values of SDF‐global (standard SCD test) and of SDF‐DSBs (DSBs‐SCD assay) 9 . The values of the subpopulation whose DNA fragmentation was exclusively SSBs (SDF‐SSBs) were calculated by the subtraction of sperm containing DNA fragmentation by DSBs (SDF‐DSBs) from the SDF‐Global value 9 .…”

“…Aliquots of spermatozoa for analysis were obtained at 0, 6, 24 and 48 h, to estimate the values of SDF‐global (standard SCD test) and of SDF‐DSBs (DSBs‐SCD assay) 9 . The values of the subpopulation whose DNA fragmentation was exclusively SSBs (SDF‐SSBs) were calculated by the subtraction of sperm containing DNA fragmentation by DSBs (SDF‐DSBs) from the SDF‐Global value 9 . Values of mtDNAcn and SDF‐mtDNA were determined by the correspondent dPCR assays, as described 9 .…”

“…9 The values of the subpopulation whose DNA fragmentation was exclusively SSBs (SDF-SSBs) were calculated by the subtraction of sperm containing DNA fragmentation by DSBs (SDF-DSBs) from the SDF-Global value. 9 Values of mtDNAcn and SDF-mtDNA were determined by the correspondent dPCR assays, as described. 9 The rate of SDF (r-SDF), defined as the variation in SDF per time unit (h), was calculated as (SDF time b − SDF time a)/hours.…”

“…9 Values of mtDNAcn and SDF-mtDNA were determined by the correspondent dPCR assays, as described. 9 The rate of SDF (r-SDF), defined as the variation in SDF per time unit (h), was calculated as (SDF time b − SDF time a)/hours. The same was applied to the rates of mtDNAcn and of SDF-mtDNA.…”

“…In a recent report, SDF‐global and by distinguishing due to single‐strand DNA breaks (SSBs) and to double‐strand DNA breaks (DSBs), and mitochondrial DNA copy number (mtDNAcn) and its fragmentation level (SDF‐mtDNA), were investigated at baseline, immediately after ejaculation, in normozoospermic and non‐normozoospermic samples 9 . In this addendum, we extend the previous work to gain further insight into the dynamic evolution of SDF post‐ejaculation.…”