# Functional Investigation of a Novel PIWIL4 Mutation in Nonobstructive Azoospermia During the First Wave of Spermatogenesis

## TL;DR

A new mutation in the PIWIL4 gene disrupts piRNA binding and causes infertility by affecting early sperm development in mice.

## Contribution

Discovery of a novel PIWIL4 mutation linked to nonobstructive azoospermia through impaired piRNA binding and transposon regulation.

## Key findings

- The PIWIL4 mutation (c.805 C>T p.R269W) impairs piRNA binding and transposon silencing in mice.
- Homozygous mutant mice show abnormal gene expression during early spermatogenesis despite normal sperm morphology.
- The mutation leads to derepression of LINE-1 elements, suggesting a role in infertility pathogenesis.

## Abstract

PIWI-interacting RNAs (piRNAs) are small noncoding RNAs that are almost exclusively expressed in germ cells to silence harmful transposons to maintain genome stability. PIWIL4 is guided by its associated piRNAs to transposable elements, where it recruits the DNA methylation apparatus and instructs de novo DNA methylation. Herein, we identified a missense variant of PIWIL4 (c.805 C>T p.R269W) in two infertile males. Homozygous male mice carrying the orthologous knock-in variant displayed elevated transposable element expression and aberrant gene expression during the first wave of spermatogenesis, despite exhibiting normal sperm counts and morphology. Mechanistically, the mutated site altered the piRNA-binding ability of PIWIL4 and led to the derepression of endogenous LINE-1 elements. In summary, we identified a piRNA binding mutation in PIWIL4 that may be involved in human nonobstructive azoospermia.