Loss of Prm1 leads to defective chromatin protamination, impaired PRM2 processing, reduced sperm motility and subfertility in male mice

One of the key events during spermiogenesis is the hypercondensation of chromatin by substitution of the majority of histones by protamines. In humans and mice, protamine 1 (PRM1/Prm1) and protamine 2 (PRM2/Prm2) are expressed in a species-specific ratio. Using CRISPR-Cas9-mediated gene editing, we generated Prm1-deficient mice and demonstrated that Prm1+/- mice were subfertile, whereas Prm1-/- mice were infertile. Prm1-/- and Prm2-/- sperm showed high levels of reactive oxygen species-mediated DNA damage and increased histone retention. In contrast, Prm1+/- sperm displayed only moderate DNA damage. The majority of Prm1+/- sperm were CMA3 positive, indicating protamine-deficient chromatin, although this was not the result of increased histone retention in Prm1+/- sperm. However, sperm from Prm1+/- and Prm1-/- mice contained high levels of incompletely processed PRM2. Furthermore, the PRM1:PRM2 ratio was skewed from 1:2 in wild type to 1:5 in Prm1+/- animals. Our results reveal that PRM1 is required for proper PRM2 processing to produce mature PRM2, which, together with PRM1, is able to hypercondense DNA. Thus, the species-specific PRM1:PRM2 ratio has to be precisely controlled in order to retain full fertility.