Our consortium combines the expertise of 3 partners in developmental and cellular biology, mouse and human genetics. We aim to decipher the role of the ADAR1 enzyme and Adenosine to Inosine (A-to-I) RNA editing in the development of Schwann cells and in the maintenance of myelin of the peripheral nervous system (PNS), both in normal and pathological conditions. Catalyzed by enzymes of the ADAR family, A-to-I deamination is one of the most prevalent form of RNA base modification in mammals. Because Inosines are subsequently recognized as Guanosines, this modification is a well-known contributor to transcriptomic and, to a lesser extent, proteomic diversity. To date, ADAR1 dysfunction was shown to induce recognition of unedited RNAs by cytosolic sensors, Mda5 in particular, subsequent upregulation of hundreds of interferon (IFN)-stimulated genes (ISG) expression, and cell death. On line with this model, Partner 3 discovered ADAR1 mutations as a cause of Aicardi-Goutières syndrome (a genetically determined inflammatory encephalopathy, ascribed to the group of type 1 interferonopathies). Heterozygous mutations in this gene also lead to dyschromatosis symmetrica hereditaria (characterized by hyper- and hypo-pigmented macules). These pigmentation defects suggested an important role for ADAR1 during the development of melanin-producing neural crest (NC)-derived melanocytes, but the role of this enzyme in other NC derivatives, including myelinating-Schwann cells, remained to be defined. To explore this possibility, partner 1 (P1) generated a mouse model that undergo NC-restricted deletion of Adar1. Consistent with the phenotype observed in humans, these mutants exhibit overall depigmentation due to apoptosis of melanocytes. Mutants also present with total absence of myelin along peripheral nerves, resulting from altered differentiation of Schwann cells. As in melanocytes, this defect occurs shortly after ISG upregulation. Supported by a recent clinical description indicating that a demyelinating sensorimotor neuropathy constitutes part of the clinical spectrum related to ADAR1 mutations in humans, these results demonstrate an essential role of ADAR1 and A-to-I RNA editing during the myelination of the PNS in humans and mice. To identify the deregulated pathways that contribute to these alterations, P1 performed RNA-seq on the sciatic nerves from Adar1cKO versus control animals, as well as a first serie of in vitro rescue experiments. Published and unpublished data collected by the consortium lead to hypothesize that the myelination defects observed in Adar1cK0 animals might result from i) deregulation of Mda5/Mavs signaling pathway; ii) the maintenance/up-regulation of a small number of transcription factors (known as repressors during myelination); and iii) formation of stress granules (cytosolic aggregates of proteins and RNA). Over the four years of the project, we propose to combine the expertise of the consortium to: 1) Determine the implication of the Mda5/Mavs signaling pathway, and the contribution of 3 differentially-expressed transcription factors, in the myelination defects observed in Adar1cKO animals (rescue experiments in vitro and in vivo, Task performed by partners 1 and 3), 2) Decipher the role of stress granules, along with their RNA and protein content, in phenotype genesis (their mechanism of formation in vitro and in vivo and OMICS, Partners 2 and 1), 3) Define the requirement for ADAR1 and RNA editing in the maintenance of myelin in adult mice (conditional knockout in adulthood, Task performed by partner 1, proposed on the basis of “post-natal onset” of clinical manifestations observed in some AGS patients), 4) Interrogate the mechanisms identified in mice in the human context (use of induced pluripotent stem cells from patients with ADAR1 mutations, Partners 1 and 3). Hence, our proposal will address questions of both fundamental research and of translational importance in clinics.