Human Cerebral Cortex Proteome of Fragile X-Associated Tremor/Ataxia Syndrome

Abstract

Background: Fragile X-associated tremor/ataxia syndrome (FXTAS) is an adult-onset neurodegenerative disorder associated with premutation CGG-repeat expansions (55–200 repeats) in the 5′ non-coding portion of the fragile X mental retardation 1 (FMR1) gene. Core features of FXTAS include progressive tremor/ataxia, cognitive decline, variable brain volume loss, and white matter disease. The principal histopathological feature of FXTAS is the presence of central nervous system (CNS) and non-CNS intranuclear inclusions.Objective: To further elucidate the molecular underpinnings of FXTAS through the proteomic characterization of human FXTAS cortexes.Results: Proteomic analysis of FXTAS brain cortical tissue (n = 8) identified minor differences in protein abundance compared to control brains (n = 6). Significant differences in FXTAS relative to control brain predominantly involved decreased abundance of proteins, with the greatest decreases observed for tenascin-C (TNC), cluster of differentiation 38 (CD38), and phosphoserine aminotransferase 1 (PSAT1); proteins typically increased in other neurodegenerative diseases. Proteins with the greatest increased abundance include potentially novel neurodegeneration-related proteins and small ubiquitin-like modifier 1/2 (SUMO1/2). The FMRpolyG peptide, proposed in models of FXTAS pathogenesis but only identified in trace amounts in the earlier study of FXTAS inclusions, was not identified in any of the FXTAS or control brains in the current study.Discussion: The observed proteomic shifts, while generally relatively modest, do show a bias toward decreased protein abundance with FXTAS. Such shifts in protein abundance also suggest altered RNA binding as well as loss of cell–cell adhesion/structural integrity. Unlike other neurodegenerative diseases, the proteome of end-stage FXTAS does not suggest a strong inflammation-mediated degenerative response.

(Dys)function Follows Form: Nucleic Acid Structure, Repeat Expansion, and Disease Pathology in FMR1 Disorders

“…However, overexpression of FMRpolyG is not always associated with FXTAS pathology in mice [116]. Furthermore, FMRpolyG is not detected by mass spectroscopy of brain extracts of FXTAS patients [117] and is only present at very low levels in inclusions isolated from such patients [17]. This raises the possibility that despite the immunological detection of these proteins in patient samples, their concentration may be too low to account for the pathology observed in PM carriers.…”

Trinucleotide CGG Repeat Diseases: An Expanding Field of Polyglycine Proteins?

“…Of technical interest, these antibodies are directed against the N- or C-terminal parts of FMRpolyG, but none are directly targeting the polyglycine stretch. However, recent mass-spectrometry analyses of FXTAS brain samples revealed only trace amount of FMRpolyG, casting important questions on the expression level of this protein in individuals with FXTAS ( Ma et al, 2019 ; Holm et al, 2021 ). Such low levels of FMRpolyG found in proteomic analyses may originate from the high propensity of this protein to aggregates ( Sellier et al, 2017 ), so that little FMRpolyG is present in brain extract or in fluids of individuals with FXTAS.…”

“…Such low levels of FMRpolyG found in proteomic analyses may originate from the high propensity of this protein to aggregates ( Sellier et al, 2017 ), so that little FMRpolyG is present in brain extract or in fluids of individuals with FXTAS. Overall, these proteomic studies have important clinical consequences as they temper relevance of FMRpolyG as an easily quantifiable biomarker for FXTAS ( Ma et al, 2019 ; Holm et al, 2021 ). Finally, expression of FMRpolyG in cell and animal models is toxic, and Drosophila or mice expressing FMRpolyG show locomotor alterations, neuronal cell loss and ultimately, premature death of these animals ( Todd et al, 2013 ; Sellier et al, 2017 ).…”

Artificial neural network applied to fragile X-associated tremor/ataxia syndrome stage diagnosis based on peripheral mitochondrial bioenergetics and brain imaging outcomes

“…This is well exemplified by our study of sulforaphane-treated fibroblasts from carriers at late stages whose bioenergetics were recovered by controlling their unfolded protein response and antioxidant capacities 39 . Although in vitro overexpression of the repeat-associated non-AUG (RAN)-mediated FMRpolyG product resulted in mitochondrial dysfunction 40 , no significant FMRpolyG levels in biospecimens from carriers have been found 41 , 42 even with evident mitochondrial dysfunction.…”