A common haplotype of protamine 1 and 2 genes is associated with higher sperm counts

Sperm chromatin compaction in the sperm head is achieved by histones being replaced by transition proteins and then protamines during spermatogenesis. Haploinsufficiency of the protamine 1 (PRM1) or PRM2 gene causes infertility in mice and sequence variants in PRM1 were associated with increased abnormal sperm morphology in infertile men. However, the published data remain inconclusive about a role of PRM1/2 variants in male infertility and their association with semen parameters. By full sequence analysis we assessed the frequency of sequence variations in PRM1 and PRM2 in groups of idiopathic patients with teratozoospermia and normal or reduced sperm concentration (N = 88 and 83, respectively) and in men with high percentage of normal sperm morphology and normal concentration (N = 77). Two rare (c.54G>A and c.102G>T) and one common SNP (c.230A>C) were identified in PRM1. In PRM2, some rare heterozygous mutations and the two common intronic SNPs 298G>C and 373C>A were detected. None of the PRM1/2 variants were associated with teratozoospermia or other semen parameters individually. However, significant linkage disequilibrium was detected between the common SNPs of PRM1 and PRM2 which formed haplotypes. The analysis of the pooled cohort revealed that homozygous carriers of the common haplotype ACC had a two-fold higher sperm concentration and count than men lacking this haplotype with sperm counts of heterozygotes for ACC being midway between the homozygotes. This markedly decreased sperm output might either be caused by spermatozoa lacking the ACC haplotype not being viable or these being negatively selected against. In addition, a significant deviation from Hardy-Weinberg-Equilibrium of these SNPs might indicate natural selection in favour of the ACC allele which lead to a higher sperm output and therefore better fertility. In conclusion, we describe for the first time an association of a common haplotype formed by PRM1 and PRM2 with sperm output in a large cohort of men.