The role of ADAR2-associated RNA editing in pathogenesis of ALS

肌萎缩性侧索硬化症（ALS）是一种毁灭性的、致命的神经退行性疾病

Amyotrophic Lateral Sclerosis (ALS) is a devastating and fatal neurodegenerative disease that is characterized by progressive muscle weakness and eventual paralysis. The known risk factors for ALS are increasing age and male sex; recent data also implicate military service, thus putting an additional burden on the VA healthcare system. Approximately 90% of all ALS cases are classified as sporadic ALS (sALS) with no family history of the disease. The remaining cases are inherited in a dominant fashion, and mutations in ~30 genes (most commonly in C9orf72, SOD1, TARDBP, FUS) are currently known to cause ALS. Hyperexcitability that stems from pathophysiological disturbance of glutamatergic transmission has been reported in all types of ALS cases, and may be a common disease pathway that predisposes motoneurons to degeneration. It has been suggested that the glutamatergic (Glu) deficit in ALS may result from altered function of the AMPA Glu receptors due to inefficient RNA editing of the GluA2 AMPA subunit. The GluA2 editing is catalyzed by the editing enzyme ADAR2. Our data, as well as data published by others, showed the downregulation of ADAR2 and inefficient editing of the GluA2 in the motor cortex of patients with C9orf72-associated ALS (C9 ALS) as well as in spinal motoneurons of patients with sALS. We also reported the downregulation of ADAR2 and the consequent decrease of editing in several of its targets in animals subjected to spinal cord injury (SCI). Moreover, in SCI these events are triggered by inflammatory response and result in changes in gene expression that are likely to contribute to post-SCI motoneuron hyperexcitability. Based on these data and the fact that, as in SCI, neuroinflammation is present in ALS, we hypothesize that: (1) ADAR2 is downregulated in both C9 ALS and sALS, leading to alterations in RNA editing in its targets. In particular, in addition to GluA2, ADAR2 editing of other molecules which are important for neuronal excitability [such as, GluA3-4, kainate receptor subunits GluK1-2, serotonin 2C receptor (5-HT2CR), as well as potassium (Kv1.1) and calcium (Cav1.3) channels] is also altered in C9 ALS and sALS; (2) The downregulation of ADAR2 in ALS results from several different pathological features associated with the disease. Whereas the decrease of ADAR2 in sALS is triggered by neuroinflammation, the availability of the functional ADAR2 in C9 ALS is also influenced by its cytoplasmic retention, which is caused by a disruption of nucleocytoplasmic transport. We will test these hypotheses by characterizing ALS-associated alterations (1) in the ADAR2 expression and localization and (2) in editing of the transcripts that are targeted by ADAR2. These studies will be performed in laser microdissected neurons obtained from autopsy tissues of patients with C9 and sporadic ALS (Aim1) as well as in motoneurons differentiated from human induced pluripotent stem cells (hiPSC) derived from ALS patients (Aim2). In addition, previously published studies of ALS-associated transcriptional alterations employed whole (cellular heterogeneous) tissue specimens. However, because of the heterogeneity of cell types in the CNS, accurate assessment of the ALS transcriptome cannot be inferred from the data obtained from these specimens that combine signals from all cell types. Therefore, we will study cell- type-specific transcriptomes in C9 ALS and sALS (Aim3). These studies are enabled by the novel methods recently developed in our lab, and should enhance the likelihood of elucidating disease relevant variations, including those related to ADAR2 and editing. Overall, this is an innovative proposal that builds on the interdisciplinary nature of our collaborative team to investigate RNA editing changes that characterize ALS and to define unique changes occurring only in C9 or sporadic ALS. Our goal is to advance the current knowledge and to discover new treatments.