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.

Keywords: AMPK kinase; CGG repeats; FMR1 mRNA; FMR1 premutation; cellular stress; cognitive status; motor scores; white matter hyperintensities.

FIGURE 1

Plot of medians for the major wmhs scores in individual brain regions in the three groups: FXTAS, OTHERS, and Non-affected male carriers of premutation alleles. Infra-DWMH, infratentorial wmhs; Supra-DWMH, sum of frontal, parieto-occipital and temporal wmhs; PV-WMH, periventricular wmhs; Total-WMH, sum of wmhs in all five regions. ^∗p-value (two-sided) <0.05 for comparison between FXTAS and combined non-FXTAS, and between FXTAS and OTHERS for total infra DWMH. #p-value (two-sided) <0.05 for comparison between FXTAS and Non-affected for Total WMH.

FIGURE 2

Kernel density distribution for FXTAS and combined Non-FXTAS group, and for OTHERS and Non-affected groups against CGG repeat size.

FIGURE 3

Plot of distribution of AMPK activity for FXTAS, Non-FXTAS PM carriers, and Controls without CGG expansions.

FIGURE 4

Scatterplot of the log-transformed ICARS scores versus AMPK activity in the total sample of PM carriers.