Fragile X premutation carriers: A systematic review of neuroimaging findings

Highlights

• We systematically review neuroimaging findings in fragile X premutation carriers.

• We examine how MRI techniques have progressed our knowledge of the premutation.

• Developmental and degenerative processes in premutation carriers may be separate.

Abstract

Background

Expansion of the CGG repeat region of the FMR1 gene from less than 45 repeats to between 55 and 200 repeats is known as the fragile X premutation. Carriers of the fragile X premutation may develop a neurodegenerative disease called fragile X-associated tremor/ataxia syndrome (FXTAS). Recent evidence suggests that premutation carriers experience other psychiatric difficulties throughout their lifespan.

Methods

Medline, EMBASE and PsychINFO were searched for all appropriate English language studies published between January 1990 and December 2013. 419 potentially relevant articles were identified and screened. 19 articles were included in the analysis.

Results

We discuss key structural magnetic resonance imaging (MRI) findings such as the MCP sign and white matter atrophy. Additionally, we discuss how functional MRI results have progressed our knowledge of how FXTAS may manifest, including reduced brain activation during social and memory tasks in multiple regions.

Limitations

This systematic review may have been limited by the search for articles on just 3 scientific databases. Differing techniques and methods of analyses between research groups and primary research articles may have caused differences in results between studies.

Conclusion

Current MRI studies into the fragile X premutation have been important in the diagnosis of FXTAS and identifying potential pathophysiological mechanisms. Associations with blood based measures have also demonstrated that neurodevelopmental and neurodegenerative aspects of the fragile X premutation could be functionally and pathologically separate. Larger longitudinal studies will be required to investigate these conclusions.

Introduction

The fragile X-associated tremor/ataxia syndrome (FXTAS) is one of the most prevalent movement disorders with a known single gene causation [1]. FXTAS is a neurodegenerative disease which affects approximately 40% of males and 8–16% of females who carry the premutation allele of the FMR1 gene [2], [3]. At present, there is no evidence based treatment for FXTAS, although symptomatic treatments of associated cognitive, psychiatric and movement disorders have proven useful in a percentage of cases [4].

Premutation status is conferred by an expansion of the non-translated 5′ CGG repeat region of FMR1 from the normal range, which is less than 45 repeats, to between 55 and 200 repeats. Typically, an expansion of over 200 repeats is associated with DNA methylation and subsequent silencing of FMR1 leading to a lack of production of a protein called fragile X mental retardation protein (FMRP). This lack of FMRP manifests clinically as the severe neurodevelopmental disorder fragile X syndrome [1]. The premutation allele is unstable and the CGG repeat region is liable to expand through maternal transmission. Thus, a mother with the fragile X premutation is very likely to have a child with fragile X syndrome [5].

The classical clinical presentation of FXTAS is late-onset, usually male and over 50 years of age, with progressive symptoms of tremor, ataxia and cognitive decline. Gait ataxia, kinetic tremor and mild Parkinsonism typically are the first symptoms to appear in FXTAS [6]. Patients begin to experience frequent falls, and eventually become bed bound in the later stages of the disease. Peripheral neuropathy, dysfunction of the autonomic system and endocrine changes also form part of the FXTAS phenotype, however these occur less frequently [7]. Onset of cognitive decline is initially subtle and typically precedes appearance of motor symptoms. Cognitive decline in FXTAS mainly involves deficits in executive function, working memory, inhibition and visuospatial learning and progresses to full dementia in approximately 50% of patients [8], [9]. In patients with established FXTAS gross changes to white matter structure can be seen in almost all individuals using magnetic resonance imaging, suggesting that disturbances to brain connectivity underpin the disorder [10]. It is of note that FXTAS symptomatology is both broad and heterogeneous, with similarities to multiple other diseases, likely resulting in under- and misdiagnoses. Psychiatric problems (including, anxiety, irritability and obsessive–compulsive behaviours) and autistic traits have been identified in premutation carriers throughout their lifespan [11]. Such traits are also known to be associated with disturbances to executive function and changes to brain connectivity.

Unlike in fragile X syndrome, where the expansion exceeds 200 CGG repeats, the premutation allele remains unmethylated, and as such encodes a functional transcript of FMRP. FMRP is expressed at highest concentrations in the brain and is a transcriptional regulator with a diversity of functions. Most importantly it is heavily involved in the regulation of synaptic maturation and plasticity [1], [12]. In carriers of the premutation, production of FMR1 mRNA increases up to 8-fold the normal level, likely due to changes in expansion size altering chromatin structure and giving increased access to transcriptional modulators of the FMRP gene [13]. In addition, FMRP levels have been observed to be slightly lower in some individuals with the premutation, especially at the high end of the CGG repeat range [13], [14], [15]. The causation for this is debated, but it has been suggested that a fall in FMRP could arise from deficits in the mRNA translational efficiency [13]. It is possible that this small decrease in FMRP may contribute to increased rates of neurodevelopmental abnormalities in premutation carriers, including autistic behaviours. However, it is widely accepted that the high level of FMR1 mRNA in premutation carriers is the major causative factor in the molecular pathology of FXTAS [16]. Indeed, studies have shown that intranuclear inclusions in neurones and astrocytes, which are a pathological hallmark of FXTAS, are still formed without the FMRP coding region of the gene, and do not form without the CGG repeat expansion [17]. It seems that the mRNA has a toxic gain-of-function effect, which proceeds to disrupt numerous cellular pathways to cause neuronal damage or death. In particular, intranuclear inclusions containing FMR1 mRNA are present throughout the brain and brainstem. The exact mechanism of their formation is not fully understood, however the favourable theory is that an excess of FMR1 mRNA begins to sequester mRNA binding proteins such as histones, heat shock proteins and cytoskeletal proteins. In particular, neurofilament isoforms lamin A/C have been shown to often be involved in inclusion formation, which is likely to initiate neurofilament dysregulation and may be a major cause of peripheral neuropathy in FXTAS patients. These intranuclear inclusions likely not only cause physical blockages to cellular functions, but have knock-on effects through the sequestering and therefore inhibition of mRNA binding proteins [1], [18]. Repeat Associated Non-AUG initiated (RAN) translation has also been implicated in the pathogenesis of FXTAS. The CGG repeat region of the FMR1 gene has been shown to trigger translation of the polyglycine-containing protein FMRpolyG, despite being outside of the open reading frame. This protein has been demonstrated to be toxic in human cell lines, and to accumulate in intranuclear inclusions in cell culture, mouse models and human FXTAS patients. Given that intranuclear inclusions in FXTAS are ubiquitin-positive, it seems likely that the FMRpolyG protein may significantly contribute to neurodegeneration and it is suggested that in FXTAS, RNA and protein toxicity be additive or synergistic. Similar cases of RAN translation have also been implicated in multiple neurodegenerative diseases, such as ALS and frontotemporal dementia [19]. The antisense transcript ASFMR1, which overlaps the CGG repeat region of the FMR1 gene and is transcribed in an antisense orientation, has also been suggested to contribute to phenotypic variations associated with FMR1 gene repeat expansions. In a similar way to FMR1 expression, ASFMR1 mRNA is upregulated by the premutation allele and silenced by the full mutation. In the premutation, the gene is also alternatively spliced, which also indicates its possible association with FXTAS [20]. Despite the exact mechanisms of FMR1 mRNA gain-of-function toxicity, pathogenic RAN translation and antisense transcripts being unclear, it is probable that combined down-stream effects cause oxidative stress in neurones and consequent cell damage and apoptosis. Fig. 1 summarises the processes by which the FMR1 premutation may lead to the clinical features with which it is associated.

Several studies have examined whether CCG repeat length and FMRP levels correlate with the physiological, physical and psychiatric manifestations of the fragile X premutation. It has been identified that in patients with FXTAS, increased CGG repeat sizes are seen to correlate with increased severity of FXTAS symptoms [21]. This has prognostic value as identification of larger CGG repeat size may serve as a risk factor for a more severe form of FXTAS. The relationship between FMRP levels and FMR1 mRNA levels or the CGG repeat expansion remains unclear, although it is recognised that the FMR1 protein is modestly reduced by the premutation [13]. This association infers that premutation carriers could also suffer from more neurodevelopmental deficits throughout their life-span due to lower levels of FMRP [22]. In terms of motor symptoms, studies have shown that age of onset, and severity of tremor, ataxia and parkinsonism are positively correlated with CGG repeat size. However, some apparently contradictory results indicate that FMR1 mRNA levels and symptom severity in FXTAS showed no relationship [6], [21]. Severity of neuropathies and speed of nerve conduction scores have also been found to show a positive correlation with CGG repeat size [23]. Test scores into cognition have been shown to correlate with CGG repeat size, with processing speed, executive functioning and perceptual organisation scores decreasing at larger CGG repeat sizes [24], [25]. Again, these correlations concerning cognition indicate that individuals who carry larger CGG repeat sizes are at a greater prognostic risk for cognitive decline and eventual dementia. Regarding psychiatric symptomatology, genotype/phenotype relationships are less clear, however higher levels of FMR1 mRNA have been found in some cases to correlate with increased levels of obsessive–compulsive behaviours, depression, anxiety, hostility and psychoticism [26]. More research is required, however given the important role of FMRP in neurodevelopment, one would expect that FMRP levels may be negatively correlated with psychiatric symptoms.

Neuroimaging has been a cornerstone in the advancement of our insight into FXTAS and premutation status. Structural and functional MRI techniques have allowed researchers to pinpoint diagnostic criteria and begin to unravel the complex pathology of FXTAS. For example, the high incidence of increased T2 signal intensity at the middle cerebellar peduncles, known as the MCP sign, has become an integral part of the diagnosis for FXTAS [1]. It is hoped that current and future imaging research into the fragile X premutation and FXTAS will reveal more sophisticated measurements of subtle alterations in the brain and help clinicians move towards a more prognostic diagnosis. Here, we systematically review the literature concerning magnetic resonance imaging and the fragile X premutation, with aims to identify strengths, weaknesses and future directions in the research.