Evidence for the toxicity of bidirectional transcripts and mitochondrial dysfunction in blood associated with small CGG expansions in the FMR1 gene in patients with parkinsonism

Abstract

Purpose Our previous results showed that both gray zone and lower end premutation range (40–85 repeats) fragile X mental retardation 1 (FMR1) alleles were more common among males with parkinsonism than in the general population. This study aimed to determine whether these alleles have a significant role in the manifestations and pathogenesis of parkinsonian disorders. Methods Detailed clinical assessment and genetic testing were performed in 14 male carriers of premutation and gray zone FMR1 alleles and in 24 noncarriers identified in a sample of males with parkinsonism. Results The premutation + gray zone carriers presented with more severe symptoms than disease controls matched for age, diagnosis, disease duration, and treatment. The Parkinson disease (Unified Parkinson’s Disease Rating Scale) motor score and the measures of cognitive decline (Mini-Mental State Examination and/or Addenbrooke’s Cognitive Examination Final Revised Version A scores) were significantly correlated with the size of the CGG repeat and the (elevated) levels of antisense FMR1 and Cytochrome C1 mRNAs in blood leukocytes. In addition, the carriers showed a significant depletion of the nicotinamide adenine dinucleotide, reduced dehydrogenase subunit 1 mitochondrial gene in whole blood. Conclusion Small CGG expansion FMR1 alleles (gray zone and lower end premutation) play a significant role in the development of the parkinsonian phenotype, possibly through the cytotoxic effect of elevated sense and/or antisense FMR1 transcripts involving mitochondrial dysfunction and leading to progressive neurodegeneration.

Novel methylation markers of the dysexecutive-psychiatric phenotype in FMR1 premutation women

“…This suggests a phenotypic influence from other factors previously described to contribute to PM-related disorders including deficient FMR1 protein (FMRP) translation, 5 abnormal expression of long noncoding RNA genes, ASFMR1/FMR4, 13 FMR5, and FMR6, 34 as well as repeat associated non-ATG translation leading to neuronal toxicity 35 and mitochondrial dysfunction. 13 Environmental factors are also likely to contribute, as in other settings these have been shown to have a direct effect upon epigenetic status, gene expression, and the expressed phenotype. 36,37 It is also important to note that all assays utilized in this study examined total methylation, which is converted to 5-hydroxymethylacytosine (5-hmC), and subsequently 5-formylcytosine and 5-carboxylcytosine.…”

“…The relative standard curve method was utilized for FMR1 59 and 39 mRNA quantification normalized to mRNA of 3 internal control genes, as previously described. 13 AR was determined using methylation-sensitive Southern blot targeting NruI restriction site in the FMR1 CpG island, as previously described.…”

Identification of Males with Cryptic Fragile X Alleles by Methylation-Specific Quantitative Melt Analysis

“…PM and GZ alleles have also been associated with increased FMR1 gene expression, related to the increased size of the CGG expansion and RNA gain-of-function toxicity observed in FMR1-related late-onset disorders (11,12 ). Normal size (NS) alleles are Ͻ45 CGG repeats, with 31 CGG repeats being the most common allele size in the general population (13 ).…”

Fragile X–related element 2 methylation analysis may provide a suitable option for inclusion of fragile X syndrome and/or sex chromosome aneuploidy into newborn screening: a technical validation study

“…[17][18][19] The more common small-expansion alleles, termed premutation (PM: 55-199 CGG repeats in 1 in ~300 individuals), 20 and gray-zone alleles (45-54 CGG repeats 1 in ~ 30 individuals) 21 do not cause the epigenetic silencing of FMR1 gene associated with FXS, but have been linked to late-onset neurological and reproductive disorders through an RNA toxicity mechanism. 22,23 The FREE2 blood test identifies cognitively impaired FM males and females with specificity and sensitivity approaching 100%, but cannot distinguish between small-expansion carriers and healthy controls or high-functioning males with unmethylated FM alleles. [11][12][13] This is highly advantageous for newborn, infant, or early childhood population screening because the test potentially detects only those who would directly benefit from early diagnosis.…”