Histone H3.3 Hira chaperone complex contributes to zygote formation in mice and humans

Elucidating the underlining mechanisms underpinning successful fertilisation is imperative in optimising IVF treatments, and may lead to a specific diagnosis and therefore potential treatment for some infertile couples. One of the critical steps involves paternal chromatin reprogramming, in which compacted sperm chromatin packed by protamines is removed by oocyte factors and new histones, including histone H3.3, are incorporated. This step is critical for the formation of the male pronucleus, without which the zygote contains only 1 pronucleus (1PN), in contrast to normally fertilised zygotes with two-pronuclei (2PN). 1PN zygotes are a frequently observed phenomenon in IVF treatments, therefore aberrant mechanism of action controlling paternal chromatin repackaging may be an important cause of abnormal fertilisation. Hira is the main H3.3 chaperone that governs this protamine-to-histone exchange. In this study, we investigated the maternal functions of two other molecules of the Hira complex, Cabin1 and Ubn1 in the mouse. Loss-of-function Cabin1 and Ubn1 mouse models were developed: their zygotes displayed abnormal 1PN zygote phenotypes, similar to the phenotype of Hira mutants. We then studied human 1PN zygotes, and found that the Hira complex was absent in 1PN zygotes which were lacking the male pronucleus. This result confirms that the role of the Hira complex in male pronucleus formation has coherence from mice to humans. Furthermore, rescue experiments showed that the abnormal 1PN phenotype derived from Hira mutants could be resolved by overexpression of Hira in the mouse oocytes. In summary, we have provided evidence of the role of Hira complex in regulating male pronucleus formation in both mice and humans, that both Cabin1 and Ubn1 components of the Hira complex are equally essential for male pronucleus formation, and that this can be rescued. We present a proof-of-concept experiment that could potentially lead to a personalised IVF therapy for oocyte defects.