FXTAS Key Molecular Pathways Converge with Other Neurodegenerative Disorders - FXTAS KEY MOLECULAR PATHWAYS CONVERGE WITH NEURODEGENERATIVE DISORDERS A shared feature of neurodegenerative diseases (NDDs) is brain region-specific protein aggregates that commonly include ubiquitin, which regulates proteostasis and inflammatory signaling[1]. The ubiquitin-protein system is required for degradation of about 80% of intracellular proteins in eukaryotes. Furthermore, it regulates inflammatory signals which, when chronically sustained, incite microglia to assume a reactive neurotoxic phenotype[2]. While most NDDs are idiopathic, Fragile X-associated tremor/ataxia syndrome (FXTAS) is monogenic and therefore offers a great opportunity to gain insights into the pathological mechanisms for an NDD that stems from a defined genetic etiology. FXTAS is caused by a 55-200 CGG repeat expansion (55–200) in the fragile X mental retardation 1 gene (FMR1). FXTAS has features typical of other NDDs including Alzheimer’s disease (AD), Parkinson disease (PD), and C9orf72-mediated diseases. These include behavioral disinhibition, cognitive decline, dementia, ataxia, tremor and parkinsonism. FXTAS brains harbor neuronal ubiquitin-positive intranuclear protein aggregates, the most abundant of which are ubiquitin (RS27A), small ubiquitin-related modifier 2 (SUMO2), and sequestosome-1 (p62/SQSTM1)[3]. Cortex proteome analysis also demonstrates abundance of SUMO2, inflammatory cytokines[4], and activated microglia, the resident macrophages [5]. In FXTAS, decreased levels of FMRP are associated with augmentation of the metabotropic glutamate receptor 5 (mGLUR5) pathway, which when amplified leads to excitotoxicity[6]. The mechanism by which the FXTAS CGG expansion leads to aggregation of proteins and neurodegeneration is not known. I hypothesize that polyglycine peptides and mRNA-FMR1, produced from the CGG expansion, exacerbate protein clearance deficits over time. This results in neuronal response-driven increases in ubiquitin proteins that are deposited in inclusions that further disturb cellular function. The excitatory neurons, in particular, are most sensitive to damage due to increased mGlur5 signaling. In addition, the increased ubiquitin may in turn increase inflammatory signaling leading to neurotoxic reactive microglia. To compare the gene expression of protein degradation and inflammation pathways in FXTAS with controls and other NDDs, I will use bulk and single nuclei RNA sequencing (RNA-seq) to qualitatively and quantitatively examine the overall and individual cell transcriptome in postmortem human tissue. Analysis of gene expression from individual cells allows for the optimal annotation of cellular phenotypes, such as inflammatory states of microglia. Since NDDs involve complex and intertwined biological mechanisms, I will apply an unbiased and hypothesis-free approach for pathway discovery in FXTAS. This approach has been successfully employed to characterize molecular systems in metabolic[7, 8] and inflammatory diseases[9], depression[10], AD[11], and cancer[12]. I will also integrate novel FXTAS datasets with available AD, PD and C9orf72-mediated diseases datasets to cross-examine converging pathological processes. This interdisciplinary approach has been utilized to determine pathways most central to disease and to explore common therapeutic targets[13].
