Yusuke Shiromoto

Germ cells can give rise to teratomas and express pluripotency-associated genes, yet they remain intrinsically resistant to reprogramming by Yamanaka factors. The mechanisms governing this resistance are poorly understood. Here we identify PIWIL2 as a key suppressor of pluripotency in the male germline. While spermatogonial stem cells (SSCs) normally lack SOX2 expression, Piwil2 - but not Piwil4 -deficient SSCs aberrantly express SOX2 and readily acquire pluripotency upon concurrent loss of Dmrt1 and Trp53 . piRNAs associated with the Sox2 locus are detected in embryonic gonocytes but not in SSCs. Piwil2 deficiency reduces DNA methylation at the Sox2 promoter and increases DMRT1 and the RNA-binding protein DAZL, which jointly restrict Sox2 expression. These findings reveal a previously unrecognized role of PIWIL2 in repressing germline pluripotency and underscore the importance of coordinated transcriptional and translational regulation in maintaining germ cell identity.

In vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) are 2 major assisted reproductive techniques (ARTs) used widely to treat infertility. Recently, spermatogonial transplantation emerged as a new ART to restore fertility to young patients with cancer after cancer therapy. To examine the influence of germ cell manipulation on behavior of offspring, we produced F₁ offspring by a combination of two ARTs, spermatogonial transplantation and ICSI. When these animals were compared with F₁ offspring produced by ICSI using fresh wild-type sperm, not only spermatogonial transplantation–ICSI mice but also ICSI-only control mice exhibited behavioral abnormalities, which persisted in the F₂ generation. Furthermore, although these F₁ offspring appeared normal, F₂ offspring produced by IVF using F₁ sperm and wild-type oocytes showed various types of congenital abnormalities, including anophthalmia, hydrocephalus, and missing limbs. Therefore, ARTs can induce morphological and functional defects in mice, some of which become evident only after germline transmission.

Cellular senescence plays a causal role in ageing and, in mice, depletion of p16^INK4a-expressing senescent cells delays ageing-associated disorders^1,2. Adenosine deaminases acting on RNA (ADARs) are RNA-editing enzymes that are also implicated as important regulators of human ageing, and ADAR inactivation causes age-associated pathologies such as neurodegeneration in model organisms^3,4. However, the role, if any, of ADARs in cellular senescence is unknown. Here we show that ADAR1 is post-transcriptionally downregulated by autophagic degradation to promote senescence through p16^INK4a upregulation. The ADAR1 downregulation is sufficient to drive senescence in both in vitro and in vivo models. Senescence induced by ADAR1 downregulation is p16^INK4a-dependent and independent of its RNA-editing function. Mechanistically, ADAR1 promotes SIRT1 expression by affecting its RNA stability through HuR, an RNA-binding protein that increases the half-life and steady-state levels of its target mRNAs. SIRT1 in turn antagonizes translation of mRNA encoding p16^INK4a. Hence, downregulation of ADAR1 and SIRT1 mediates p16^INK4a upregulation by enhancing its mRNA translation. Finally, Adar1 is downregulated during ageing of mouse tissues such as brain, ovary and intestine, and Adar1 expression correlates with Sirt1 expression in these tissues in mice. Together, our study reveals an RNA-editing-independent role for ADAR1 in the regulation of senescence by post-transcriptionally controlling p16^INK4a expression.

ADAR1 is involved in adenosine-to-inosine RNA editing. The cytoplasmic ADAR1p150 edits 3’UTR double-stranded RNAs and thereby suppresses induction of interferons. Loss of this ADAR1p150 function underlies the embryonic lethality of Adar1 null mice, pathogenesis of the severe autoimmune disease Aicardi-Goutières syndrome, and the resistance developed in cancers to immune checkpoint blockade. In contrast, the biological functions of the nuclear-localized ADAR1p110 remain largely unknown. Here, we report that ADAR1p110 regulates R-loop formation and genome stability at telomeres in cancer cells carrying non-canonical variants of telomeric repeats. ADAR1p110 edits the A-C mismatches within RNA:DNA hybrids formed between canonical and non-canonical variant repeats. Editing of A-C mismatches to I:C matched pairs facilitates resolution of telomeric R-loops by RNase H2. This ADAR1p110-dependent control of telomeric R-loops is required for continued proliferation of telomerase-reactivated cancer cells, revealing the pro-oncogenic nature of ADAR1p110 and identifying ADAR1 as a promising therapeutic target of telomerase positive cancers.