The Spectrum of Neurological and White Matter Changes and Premutation Status Categories of Older Male Carriers of the FMR1 Alleles Are Linked to Genetic (CGG and FMR1 mRNA) and Cellular Stress (AMPK) Markers

Abstract

The fragile X premutation (PM) allele contains a CGG expansion of 55–200 repeats in the FMR1 gene’s promoter. Male PM carriers have an elevated risk of developing neurological and psychiatric changes, including an approximately 50% risk of the fragile X-associated tremor/ataxia syndrome (FXTAS). The aim of this study was to assess the relationships of regional white matter hyperintensities (wmhs) semi-quantitative scores, clinical status, motor (UPDRS, ICARS, Tremor) scales, and cognitive impairments, with FMR1-specific genetic changes, in a sample of 32 unselected male PM carriers aged 39–81 years. Half of these individuals were affected with FXTAS, while the non-FXTAS group comprised subcategories of non-affected individuals and individuals affected with non-syndromic changes. The dynamics of pathological processes at the cellular level relevant to the clinical status of PM carriers was investigated using the enzyme AMP-activated protein kinase (AMPK), which is a highly sensitive cellular stress-sensing alarm protein. This enzyme, as well as genetic markers – CGG repeat number and the levels of the FMR1 mRNA – were assessed in blood lymphoblasts. The results showed that the repeat distribution for FXTAS individuals peaked at 85–90 CGGs; non-FXTAS carriers were distributed within the lowest end of the PM repeat range, and non-syndromic carriers assumed an intermediate position. The size of the CGG expansion was significantly correlated, across all three categories, with infratentorial and total wmhs and with all motor scores, and the FMR1 mRNA levels with all the wmh scores, whilst AMPK activity showed considerable elevation in the non-FXTAS combined group, decreasing in the FXTAS group, proportionally to increasing severity of the wmhs and tremor/ataxia. We conclude that the size of the CGG expansion relates to the risk for FXTAS, to severity of infratentorial wmhs lesions, and to all three motor scale scores. FMR1 mRNA shows a strong association with the extent of wmhs, which is the most sensitive marker of the pathological process. However, the AMPK activity findings – suggestive of a role of this enzyme in the risk of FXTAS – need to be verified and expanded in future studies using larger samples and longitudinal assessment.

Total and Regional White Matter Lesions Are Correlated With Motor and Cognitive Impairments in Carriers of the FMR1 Premutation

“…Notably, when taken together with our previous study (19), the current findings imply a continuity in the clinical and white matter changes extending from non-affected older male carriers, to non-syndromic forms separate from FXTAS, and eventually to the full manifestation of FXTAS. Moreover, the finding of significant correlations between the wmhs in infratentorial regions and the motor and cognitive scores in the total sample of PM carriers suggests that the underlying primary pathology might be initially confined to this region.…”

“…This suggests that white matter alterations may serve as sensitive markers of incipient decline to FXTAS, or to asyndromic neurological manifestations not meeting the diagnostic criteria for FXTAS. This aspect was discussed in our earlier publication (19), where we reported an example of the presence of the MCP sign in some non-FXTAS PM carriers, consistent with similar observations in five such individuals most recently reported by Famula et al (20).…”

“…We have shown that whms within the infratentorial region is associated with impairments across the categories of clinical status for all the three motor rating scores, and a range of neuropsychological cognitive measures. This is supportive of the notion that the scope of the spectrum of premutation-related clinical and brain changes, especially those in infratentorial regions specifically affected in FXTAS, appear to extend beyond the classical syndromic forms of FXTAS, and across the categories of non-affected and non-syndromic carriers, comprising asymptomatic, asyndromic and FXTAS groups (19). In addition, in the FXTAS group we have demonstrated a highly significant relationship between supratentorial (periventricular) lesions and parkinsonism, and between both periventricular and supratentorial deep white matter lesions and the ICARS ataxia score.…”

“…However, this approach clearly served the purpose of our study, which aimed to establish whether white matter lesions in supratentorial, as well as infratentorial regions, contribute to motor and cognitive changes characteristic of the spectrum of premutation involvement. The validity of the wmh scores applied in this study was further attested by demonstration of the highly significant associations of all the wmhs scores used here with the levels of “toxic” FMR1 mRNA in a previous study (19). Finally, our study design did not include examination of the gray matter changes that have been reported by others in FXTAS (13, 14, 39, 40).…”

Differential Progression of Motor Dysfunction Between Male and Female Fragile X Premutation Carriers Reveals Novel Aspects of Sex-Specific Neural Involvement

“…Our data on progression of the premutation-associated changes from sub-symptomatic to clinical forms, especially in females, may have an important impact on future research, as well as on clinical approaches, because it introduces the concept of continuity. Combined with earlier evidence for the existence of subclinical neural or metabolic changes in both male and female carriers over a broad age range (Tassone et al, 2012 ; Wang et al, 2012 , 2013 , 2017 ; Battistella et al, 2013 ; Shelton et al, 2016 ; Loesch et al, 2017 , 2018 ; Hocking et al, 2019 ; O'Keeffe et al, 2019 ), these data have shown that the focus of such research should be on trajectories rather than on the final outcome of the premutation-associated process. The most recent study, based on a wide range of metabolic and proteomic biomarkers, showed a decline of mitochondrial activities in pre-symptomatic female PM carriers, leading the authors to the conclusion that the development of neurodegeneration or other clinical symptoms in older carriers could be linked to a lifetime accumulation of cellular damage, aggravated by the aging process (Napoli et al, 2020 ).…”