CRISPR Based Transposon Silencing Leads to Infertility in Male Mice

Piwi interacting RNA’s (PiRNA’s) are small noncoding RNA’s expressed within the germline. They have been shown to be responsible for multiple factors which ultimately lead to male fertility. However, previously reported experiments were carried out in non-mammalian species, leading researchers at Michigan State University to question their role in mouse germline development. Specifically, they were interested in understanding the role the PIWI associated protein, PNLDC1, played in spermatogenesis.

Mature PiRNA’s are responsible for retrotransposon silencing within the germline. In order for a pre-PiRNA to become mature it must be trimmed on its 3 and 5 prime ends by various poly(A) specific ribonucleases, such as PNLDC1. In order to analyze the affect PNLDC1 played on PiRNA activity, researchers induced different mutations in the PNLDC1 coding region of male mice using CRISPR technology. The resulting mice, despite having different mutations, were all phenotypically normal besides having significantly reduced testes size. These mice also presented with elongated spermatids during late stage spermatogenesis, ultimately leaving the sperm inactive. In order to identify the mechanism behind this defect, researchers analyzed various parts of the PiRNA pathway and discovered that the PNLDC1 mutant mice spermatocytes had a significantly high expression of the retrotransposon LINE-1. These results indicated that PNLDC1 had a direct affect on retrotransposon silencing within the testes.

Cumulatively, the results of the paper identify the following mechanism of spermatogenesis regulation: Pre-PiRNA 3 prime trimming occurs through the poly(A) specific ribonuclease PNLDC1 in order to generate mature PiRNA. If PNLDC1 is inactive, then immature pre-PiRNA will elongate rather than reduce in size, ultimately leaving them unable to silence germline specific retrotransposons such as LINE-1. If LINE-1 is left active, it will cause spermatid elongation during late stage spermatogenesis and lead to the incompletion of spermatogenesis and therefore infertility. The identification of this mechanism, although in the early early stages, makes the authors hopeful of finding a novel means of male contraception completely separate from common hormonal therapy.

Reference

• Ding, Deqiang, et al. "PNLDC1 is essential for piRNA 3′ end trimming and transposon silencing during spermatogenesis in mice."  Nature Communications 8 (2017).

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