Fragile X Syndrome and FXTAS – Different clinical disorders and different disease mechanisms from a single gene
The fragile X mental retardation 1 (FMR1) gene (OMIM *309550), is responsible for the leading inherited form of intellectual disability, fragile X syndrome (reviews: Bagni and Greenough, 2005; Hagerman, 2006; Loesch et al., 2007; Loesch et al., 2004). This neurodevelopmental disorder occurs when a CGG element within the 5' untranslated portion of the gene expands to greater than ~200 CGG repeats (full mutation range); individuals within the general population normally have between 6 and 45 CGG repeats. Full mutation forms of the gene are generally methylated and silenced, resulting in the absence of FMR1 protein (FMRP) and consequent abnormal brain development (Figure 1 ).
By contrast, smaller CGG expansions (~55 to 200 CGG repeats) are associated with abnormally high gene activity, with levels of FMR1 mRNA elevated by as much as 5-10-fold. Elevated levels of this abnormal (expanded CGG repeat) mRNA is now believed to be directly toxic, leading to the late-onset, neurodegenerative disorder, FXTAS.
Clinical features of FXTAS
FXTAS appears to be restricted to premutation carriers, generally male, and represents the most severe form of clinical involvement associated with premutation FMR1 alleles. The core features of FXTAS are progressive intention tremor and/or ataxia, with lower extremity neuropathy, autonomic dysfunction (problems with bladder and/or bowel control, impotence), and gradual cognitive decline beginning with memory and executive function deficits. Psychiatric features such as anxiety, dysinhibition, depression, and apathy are also common problems (reviews: Bacalman et al., 2006; Berry-Kravis et al., 2007; Hagerman and Hagerman, 2007; Jacquemont et al., 2007). Although females also present with FXTAS, far fewer females than males develop the disorder, presumably because of the protective effect of the second X chromosome (Adams et al., 2007; Berry-Kravis et al., 2005; Hagerman et al., 2004; Jacquemont et al., 2004; Zuhlke et al., 2004).
The penetrance of tremor and ataxia in premutation carriers is incomplete but generally has an onset after the late 40's or 50's. At the time individuals present with motor symptoms, they usually already have mild cognitive features including memory problems and executive function deficits, which often progress to dementia.
MRI features of FXTAS include global brain atrophy with dilated ventricles, and white matter disease in the sub-cortical, middle cerebellar peduncle (MCP), and periventricular regions, (Brunberg et al., 2002; Cohen et al., 2006; Jacquemont et al., 2003). Only the MCP sign is a relatively distinct radiological feature of FXTAS (Table 1) although it only occurs in 60% of males and 13% of females with FXTAS (Adams et al., 2007). In a study of 36 male premutation carriers, the CGG repeat within the premutation range correlated with reductions in IQ and cerebellar volume, and with increased ventricular volume and whole-brain white matter disease (Cohen et al., 2006).
Neuropathology of FXTAS includes spongioform changes in cerebral and cerebellar white matter, Purkinje cell dropout, and the presence of ubiquitin-positive intranuclear inclusions in both neurons and astrocytes throughout the brain (Greco et al., 2006; Greco et al., 2002). Analysis of the protein composition of the inclusions reveals the presence of several proteins that could be related to disease pathogenesis (Arocena et al., 2005; Iwahashi et al., 2006).
Table 1. Diagnostic criteria for FXTAS(1)
| Definite FXTAS | Probable FXTAX | Possible FXTAS |
| Intention tremor or gait ataxia | Intention tremor and gait ataxia | Intention tremor or gait ataxia |
|
| ||
It should also be noted that the premutation is the most common known cause of premature ovarian failure (POF) in females in the general population, with approximately 2 to 14% of females with POF demonstrating the premutation (reviewed in Sullivan et al., 2005). In women with the premutation, approximately 20% will develop ovarian failure before age 40, with an additional 20% before age 45 (Sullivan et al., 2005). Even female carriers who are cycling have elevations of their follicle stimulating hormone (FSH) compared to controls (Welt et al., 2004). It has been hypothesized that the ovarian dysfunction in female carriers may also be related to RNA toxicity in the ovum (Hagerman and Hagerman, 2004; Sullivan et al., 2005).
The RNA “toxicity” model for the pathogenesis of FXTAS
There are several lines of evidence that support an RNA “toxic” gain-of-function model for FXTAS (Figure 2; reviewed in Hagerman and Hagerman, 2004).
- First, the disorder appears to be confined to carriers of active premutation alleles of the FMR1 gene; FXTAS has not been reported among older adults with fragile X syndrome, for which the gene is generally silent. This latter observation argues against a pathogenic mechanism involving lowered FMR1 protein levels. It also argues against DNA level effects, since full mutation alleles are generally many times larger than alleles in the premutation range. Thus, the gene must be transcriptionally active.
- Second, FMR1 gene expression is abnormal in at least three respects for alleles in the premutation range: (i ) for premutation alleles, FMR1 mRNA levels are elevated by as much as eight-fold over the levels found for normal alleles; (ii) the mRNA itself is altered due the presence of the expanded CGG repeat; (iii ) the start site for transcription is altered by the presence of the expanded repeat, such that the 5 end of the message is extended by about 50 nucleotides.
- Third, both mouse (Brouwer et al., 2007; Willemsen et al., 2003) and fly (Jin et al., 2007; Jin et al., 2003) models that harbor the CGG repeat expansions in the premutation range (~90 to 100 CGG repeats) manifest features of the neuropathology of FXTAS. In the case of the fly model, neuropathic features are present even when the expanded CGG repeat is transcribed upstream of an unrelated reporter gene. Therefore, the expanded repeat, as RNA, is capable of inducing several of the human disease features.
- Fourth, adding further direct support for an RNA-based pathogenesis for FXTAS is the presence of FMR1 mRNA within the inclusions (Tassone et al., 2004). This last observation provides another parallel with the foci of myotonic dystrophy, which also contain the expanded (CUG or CCUG) repeat RNAs.
Figure 2. Features of the RNA “toxicity” model, as envisioned for myotonic dystrophy (DM1, DMPK gene; DM2, ZNF9 gene) and FXTAS (Reviews: Hagerman and Hagerman, 2004; Ranum and Cooper, 2006). (A) Expanded repeats, represented as pairs of vertical bars, located in non-coding portions of the RNA transcripts for DM1 (CUG repeat, 3UTR), DM2 (CCUG repeat, intron 1), FXTAS (CGG repeat, 5UTR). (B) Excess recruitment of one or more RNA-binding proteins to the expanded repeat causes depletion (loss of function) of those proteins from other RNAs. Protein-RNA aggregates lead to formation of intranuclear inclusions in neurons and astrocytes (neuronal, intranuclear inclusion in post-mortem cerebral tissue from patient with FXTAS; DAPI nuclear counterstain). In myotonic dystrophy, loss of function of the sequestered proteins results in altered splicing of several other RNAs, leading to specific features of the disease phenotype.
