The evolutionarily conserved Piwi proteins belong to the Piwi clade of the Argonaute family, each of which is specifically expressed during the development of animal germline. Piwi proteins are known to enlist germline-specific Piwi-interacting RNAs (piRNAs) to suppress transposable elements and protect the integrity of the genome in germ cells.

Extensive genetic studies in worms, flies, fish and mice indicate that Piwi proteins are essential for gametogenesis in animals. The human genome encodes four Piwi family members (Hiwi, Hili, Hiwi2 and Piwil3) which are mainly expressed in testis, but their potential function in human germline development has remained elusive.

Despite the identification of several single nucleotide polymorphisms (SNPs) in human Piwi genes, whether Piwi is an actual disease gene in human infertility remains unknown.

A team of scientists led by Dr. LIU Mofang at Shanghai Institute of Biochemistry and Cell Biology of Chinese Academy of Sciences collaborating with Dr. SHI Huijuan at Fudan University found that ubiquitination-deficient mutations in human Piwi (Hiwi) gene cause male infertility. The study was published in cell.

Researchers identified the ubiquination-resistant mutations in Hiwi gene in patients with azoospermia. To determine whether Hiwi D-box mutations play a causative role in male infertility, these mutations were constructed in the Miwi gene in a conditional knockin mouse.

They demonstrated that D-box mutations impeded MIWI degradation and contributed to male infertility. The knockin D-box mutant males could produce a small number of sperm, but the resulting sperm showed histone retention, abnormal morphology and severely compromised activity.

The mechanistic studies showed that MIWI binds the histone ubiquitin ligase RNF8 in a piRNA-independent manner and that MIWI stabilization sequesters RNF8 in the cytoplasm of late spermatids. More importantly, the defective spermiogenesis, as well as the activity of sperm, can be functionally rescued via blocking RNF8-MIWI interaction in spermatids with an RNF8-N peptide.

This work identified a role for Piwi function in human genetics and reveals mechanism for regulated packaging of genomic DNA into functional sperm.

This study was supported by the grants from the Chinese Academy of Sciences, National Natural Science Foundation of China, Ministry of Science and Technology, and Science and Technology Commission of Shanghai Municipality.