Sperm rDNA Copy Number and Methylation Are Associated with Male-Factor Infertility.

Both absolute and presumably active rDNA (with a hypomethylated promoter region) copy number (CN) in the haploid human sperm genome are highly variable among individuals. Using a combination of droplet digital PCR and deep bisulfite sequencing, we have quantified absolute and presumably active rDNA CN in sperm samples (N = 190) with normal (NSPs) vs. abnormal semen parameters (ASPs), as well as in samples leading or not leading to a clinical pregnancy. ASP samples had a significantly lower presumably active CN (104 ± 31) than normozoospermic samples (115 ± 31). The loss of presumably active rDNA copies is explained by an increased promoter methylation (13.9% in ASP vs. 12.1% in NSP). When correcting for confounding factors, most importantly semen quality, samples not leading to a clinical pregnancy after IVF/ICSI displayed a significantly lower absolute (225 ± 51) and presumably active CN (103 ± 30) than samples with pregnancy (249 ± 62 and 115 ± 31, respectively). This between-group difference was most noticeable in normozoospermic males: absolute CN 220 ± 54; presumably active CN 107 ± 32 in samples without pregnancy and absolute CN 246 ± 63; presumably active CN 120 ± 28 in samples with pregnancy. We propose that absolute/active rDNA CN in sperm is a modulating factor contributing to idiopathic male infertility. In NSP samples, presumably active CN increases with absolute CN, which may have a positive impact on fertility and ART outcome. Our results suggest that approximately 60 active sperm rDNA copies are sufficient to establish a pregnancy.