ADAR1 forms a complex with Dicer to promote microRNA processing and RNA-induced gene silencing.

Adenosine deaminases acting on RNA (ADARs) are involved in RNA editing that converts adenosine residues to inosine specifically in double-stranded RNAs. In this study, we investigated the interaction of the RNA editing mechanism with the RNA interference (RNAi) machinery and found that ADAR1 forms a complex with Dicer through direct protein-protein interaction. Most importantly, ADAR1 increases the maximum rate (Vmax) of pre-microRNA (miRNA) cleavage by Dicer and facilitates loading of miRNA onto RNA-induced silencing complexes, identifying a new role of ADAR1 in miRNA processing and RNAi mechanisms. ADAR1 differentiates its functions in RNA editing and RNAi by the formation of either ADAR1/ADAR1 homodimer or Dicer/ADAR1 heterodimer complexes, respectively. As expected, the expression of miRNAs is globally inhibited in ADAR1(-/-) mouse embryos, which, in turn, alters the expression of their target genes and might contribute to their embryonic lethal phenotype.

Antagonistic and stimulative roles of ADAR1 in RNA silencing.

Adenosine deaminases acting on RNA (ADARs) are involved in RNA editing that converts adenosine residues to inosine specifically in double-stranded RNAs (dsRNA). This A-to-I RNA editing pathway and the RNA interference (RNAi) pathway seem to interact antagonistically by competing for their common dsRNA substrates. For instance, A-to-I editing of certain microRNA (miRNA) precursors by ADAR1 and ADAR2 inhibits their processing to mature miRNAs. Recent studies unexpectedly revealed the presence of a completely different type of interaction between the RNA editing mechanism and the RNAi machinery. ADAR1 forms a complex via direct protein-protein interaction with Dicer, an RNase III gene family member involved in the RNAi mechanism. ADAR1 in the Dicer complex promotes pre-miRNA cleavage by Dicer and facilitates loading of miRNA onto RNA-induced silencing complexes, giving rise to an unsuspected stimulative function of ADAR1 on miRNA processing and RNAi mechanisms. ADAR1 differentiates its functions in RNA editing and RNAi by formation of either ADAR1-ADAR1 homodimer or Dicer-ADAR1 heterodimer complexes. Expression of miRNAs is globally inhibited in ADAR1-null mouse embryos, which, in turn, alters expression of their target genes and may contribute to their embryonic lethal phenotype.

Identification of the X-linked germ cell specific miRNAs (XmiRs) and their functions.

MicroRNAs (miRNAs) play a critical role in multiple aspects of biology. Dicer, an RNase III endonuclease, is essential for the biogenesis of miRNAs, and the germ cell-specific Dicer1 knockout mouse shows severe defects in gametogenesis. How miRNAs regulate germ cell development is still not fully understood. In this study, we identified germ cell-specific miRNAs (miR-741-3p, miR-871-3p, miR-880-3p) by analyzing published RNA-seq data of mouse. These miRNA genes are contiguously located on the X chromosome near other miRNA genes. We named them X chromosome-linked miRNAs (XmiRs). To elucidate the functions of XmiRs, we generated knockout mice of these miRNA genes using the CRISPR/Cas9-mediated genome editing system. Although no histological abnormalities were observed in testes of F0 mice in which each miRNA gene was disrupted, a deletion covering miR-871 and miR-880 or covering all XmiRs (ΔXmiRs) resulted in arrested spermatogenesis in meiosis in a few seminiferous tubules, indicating their redundant functions in spermatogenesis. Among candidate targets of XmiRs, we found increased expression of a gene encoding a WNT receptor, FZD4, in ΔXmiRs testis compared with that in wildtype testis. miR-871-3p and miR-880-3p repressed the expression of Fzd4 via the 3'-untranslated region of its mRNA. In addition, downstream genes of the WNT/ß-catenin pathway were upregulated in ΔXmiRs testis. We also found that miR-871, miR-880, and Fzd4 were expressed in spermatogonia, spermatocytes and spermatids, and overexpression of miR-871 and miR-880 in germ stem cells in culture repressed their increase in number and Fzd4 expression. Previous studies indicated that the WNT/ß-catenin pathway enhances and represses proliferation and differentiation of spermatogonia, respectively, and our results consistently showed that stable ß-catenin enhanced GSC number. In addition, stable ß-catenin partially rescued reduced GSC number by overexpression of miR-871 and miR-880. The results together suggest that miR-871 and miR-880 cooperatively regulate the WNT/ß-catenin pathway during testicular germ cell development.

Advanced Lung Adenocarcinoma with Nivolumab-associated Dermatomyositis.

We herein report a 42-year-old man with advanced lung adenocarcinoma and nivolumab-associated dermatomyositis. Nivolumab, an anticancer drug that is classified as an immune checkpoint inhibitor, often induces immune-related adverse events (irAEs). However, there have so far been no reports regarding nivolumab-associated dermatomyositis. This patient was diagnosed with dermatomyositis due to the presence of proximal muscle weakness with abnormal electromyography and magnetic resonance imaging findings; skin lesions, such as heliotrope rash, shawl sign, and periungual erythema; and an elevated serum aldolase level after nivolumab administration. It is important to consider drug-associated dermatomyositis in the differential diagnosis of patients presenting with skin lesions and muscle weakness after nivolumab treatment.

ADAR1 controls apoptosis of stressed cells by inhibiting Staufen1-mediated mRNA decay.

Both p150 and p110 isoforms of ADAR1 convert adenosine to inosine in double-stranded RNA (dsRNA). ADAR1p150 suppresses the dsRNA-sensing mechanism that activates MDA5-MAVS-IFN signaling in the cytoplasm. In contrast, the biological function of the ADAR1p110 isoform, which is usually located in the nucleus, is largely unknown. Here, we show that stress-activated phosphorylation of ADAR1p110 by MKK6-p38-MSK MAP kinases promotes its binding to Exportin-5 and its export from the nucleus. After translocating to the cytoplasm, ADAR1p110 suppresses apoptosis in stressed cells by protecting many antiapoptotic gene transcripts that contain 3'-untranslated-region dsRNA structures primarily comprising inverted Alu repeats. ADAR1p110 competitively inhibits binding of Staufen1 to the 3'-untranslated-region dsRNAs and antagonizes Staufen1-mediated mRNA decay. Our study reveals a new stress-response mechanism in which human ADAR1p110 and Staufen1 regulate surveillance of a set of mRNAs required for survival of stressed cells.