Introduction

The fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disease with a progressive cerebellar movement disorder and a pattern of cognitive deficits that is consistent with predominantly cerebellar and white matter pathology. FXTAS was unknown until 1999, when it was identified in the course of a study of the extended families of individuals with fragile X syndrome (Hagerman et al., 2001).

Fragile X Syndrome (FXS) is the most common inherited cognitive developmental disability, well-known as a cause of significant neuropsychological and behavioral pathology, including features of autistic spectrum disorder (ASD). Both FXS and FXTAS are caused by mutations of the fragile X mental retardation 1 gene (FMR1), but they are distinctly different disorders, with onset at either end of the life span. FXS is present in earliest childhood, whereas the first signs of FXTAS typically appear around the age of 60 (Jacquemont, Hagerman, Leehey, et al., 2003). Although they have much in common, the differences between FXS and FXTAS are clear-cut. The primary similarity is the type of genetic mutation that leads in some cases to the early-onset developmental syndrome, and in other cases to the late-onset movement disorder. In a very small percentage of cases, both may be observed in the same individual (Santa Maria et al., 2013).

The focus of this review is on the phenotypes, genetics, neuropathology, and epidemiolaogy of FXTAS, which, despite a relatively high prevalence, is unfamiliar to neuropsychologists, and to many neurologists who do not specialize in movement disorders. As a consequence, even 17 years after its discovery, the diagnosis of FXTAS is often missed. Therefore, the purpose of this paper is to clarify the different fragile X-related disorders, and to address the epidemiologic, genetic, and other factors that can facilitate diagnosis, treatment, management, and appropriate referral for genetic counseling. The paper begins with a brief discussion of FXS, as it was the first disorder associated with the relevant gene.

Fragile X Syndrome (FXS)

Fragile X syndrome, which has its onset in earliest development, tends to involve moderate to severe intellectual deficits, especially among males, although a small percentage of boys and men with FXS shows mild impairment of general intellect, often in association with certain specific cognitive deficits such as dyscalculia (e.g., Grigsby et al., 1987). On the other hand, a pattern of relatively mild cognitive impairment is characteristic of affected females, a majority of whom have a low average or borderline IQ (Cronister et al., 1991; de Vries et al., 1996) and specific cognitive deficits, including executive function disorders, dyscalculia, dysgraphia, and constructional dyspraxia (Grigsby et al., 1990). Psychiatric disorders are prevalent in FXS, including autistic spectrum behaviors (affecting about 50% to 65% of males, and about 20% of females) and attention deficit hyperactivity disorder (ADHD in up to 80% of males and about 30% of females)(Garber et al., 2008; Hagerman & Harris, 2008; Hatton et al., 2006; Hessl et al., 2005; Rogers et al., 2001). Shyness, anxiety, and depression are common (Cordeiro, Ballinger, Hagerman, Hessl, 2011; Hessl, Dyer-Friedman, Glaser, 2001; Hessl, Glaser, Dyer-Friedman, 2006; Yu & Berry-Kravis, 2014). Genetic Mutation Responsible for Fragile X-associated Mental Retardation.

FXS was probably first identified in 1943 by J.P. Martin and J. Bell (Opitz et al., 1984; Richards et al., 1981). Their paper discussed two generations of a family in which 11 males had significant cognitive impairment. Because of the differences observed between males and females with respect to penetrance and expression, Martin and Bell concluded that the disorder was associated with “a sex-linked recessive gene.” However, they didn’t understand the sex-linkage, and hypothesized that “some controlling factor caused suppression of the disease” in the two brothers who were thought to have been the source of the disorder, and suggested that two females were affected because, in their case, “the causal gene was incompletely recessive.” A somewhat similar pattern of inheritance was subsequently reported by other authors regarding apparently sex-linked developmental disorders (e.g., Renpenning et al, 1962), who considered FXS a “sex-linked recessive” condition (p. 956). Until the 1990s, precise genetic diagnoses were not yet possible, and hence it was necessary to rely on the clinical features of syndromes that might show considerable variability, often with little more to go on than the presence of an ill-defined cognitive developmental disability.

The Responsible Gene: FMR1

The discovery by Lubs (1969) of the Fragile X mental retardation 1 gene (FMR1) brought some clarity to the genetics of the fragile X. Lubs obtained data on three generations of a family in which four males had significant intellectual disability. Examining the chromosomes using karyotyping, Lubs found what he described as a “secondary constriction” on the long (q) arm of the X chromosome, which was subsequently localized to a site identified as Xq27.3. This constriction of the X was referred to as a “fragile site” because of its appearance under the microscope, and hence the name fragile X.

FMR1 was first sequenced by Verkerk et al. (1991), and related work was published by Yu et al. (1991). Even before the sequencing of FMR1, it was thought that the type of mutation causing FXS might be unusual, because, according to Verkerk and associates (1991, p. 912), it had “long been speculated that the fragile X site is a repeat of variable length” (e.g., Warren et al., 1987). This expectation proved to be correct, and while a very small percentage of people with FXS may have a point deletion, FXS turned out to be the first genetic disorder discovered that was caused by a newly-identified class of mutations, known as trinucleotide repeat expansions. Another member of this class of mutations is the better-known, but rarer, Huntington disease (Pringsheim, Wiltshire, Day, et al. 2012).

FMR1 Expansion and the FMR1 Protein (FMRP)

DNA consists of a string of nucleotides, each of which consists of a sugar (deoxyribose), a phosphate group (PO₄⁻), and one of four bases—adenine (A), cytosine (C), guanine (G), and thymine (T). In most cases, each sequential set of three consecutive nucleotides (i.e., a trinucleotide or triplet) contains the code needed to synthesize an amino acid, and the resulting amino acids are linked together in order to form proteins. Some triplets, instead of carrying the code for protein synthesis, are involved in initiating or terminating the process of replication. The sequence of nucleotides in DNA is copied into messenger RNA (mRNA) in a process called transcription, following which the mRNA is actively transported from inside the cell’s nucleus to the cytoplasm outside, where it has an affinity for ribosomes. Ribosomes read the mRNA sequence, and they produce and string together the amino acids required to make a protein. This process is called translation.

Thus, when FMR1 is activated, the strand of mRNA that is produced is transported from the nucleus to cytoplasm, where it associates with ribosomes and provides the instructions for synthesizing FMRP.Genetic Mutation Responsible for Fragile X-associated Mental Retardation.  If FMR1 is missing or sufficiently abnormal, neither transcription nor translation occurs, and the cell is unable to produce FMRP. The result is FXS (Tassone et al., 1999). This is what happens to most people who have a full mutation (FM) of FMR1. The FM usually causes FXS by disturbing a number of developmental processes requiring FMRP. FMR1 also may be silenced by methylation, which in this case is an epigenetic inactivation of the FMR1 gene. Methylation occurs commonly in individuals with an FM, preventing transcription. In some cases, however, there may be an unmethylated full mutation, and transcription can take place. Because these individuals are able to produce some FMRP, they are typically less severely affected. An FMR1 knock-out mouse, which lacks a functional copy of FMR1, makes no FMRP and hence is a good model for understanding the action of the protein (Berman et al., 2014; Dutch-Belgian Fragile X Consortium, 1994).

FXS is a trinucleotide repeat expansion disorder. That is, the FMR1 gene contains too many triplet repeats composed of the bases cytosine, guanine, and guanine (CGG), and this may interfere with normal functioning of the gene. The data suggest that a certain number of CGG repeats is necessary for the normal functioning of FMR1; the modal number of CGG repeats in FMR1 in the general population is 29 or 30, and the normal range is considered to be from 6 (CGG CGG CGG CGG CGG CGG) to 44 repeats.

But under certain conditions that are not yet fully understood, there may be an increase in the number of consecutive CGG triplet repeats in FMR1, sometimes punctuated by one or more AGG triplet interruptions (Yrigollen et al., 2014). Such expansions are generally stable when the number of CGG repeats is low, but with increased repeat size, the gene becomes increasingly unstable, and FMR1 is likely to undergo further expansion in subsequent generations (Oberlé et al., 1991). This is associated with the phenomenon of anticipation, or the Sherman paradox (Fu et al., 1991; Sherman et al., 1984, 1985), in which the manifestations of the disorder become more marked with successive generations. In general, trinucleotide repeat disorders are characterized by a relationship between the number of repeats and both age of onset and severity of illness.

Some triplet repeat disorders affect a part of a gene that codes for a protein. Huntington disease (HD), for example, involves a CAG repeat expansion; when present, a defective form of the protein huntingtin is produced. In contrast to HD, FMR1 does not affect the code for production of FMRP. Instead, the expansion occurs in the 5′ (five prime) end of FMR1, in the untranslated promoter region (UTR), just downstream of which is found a triplet (or codon) that initiates transcription. Hence, any FMRP that is produced is normal; other basic cellular processes, however, are adversely affected.

CGG Repeat Expansion Sizes and Associated Phenotypes

An FMR1 expansion of 200 or more CGG repeats—a full mutation—is likely to produce fragile X syndrome, and the number of repeats may range anywhere from 200 to several thousand. In this case, it is likely that no FMRP will be produced. However, if FMR1 has a somewhat smaller expansion—between about 55 and 200 CGG repeats—the gene is functional, but transcribes mRNA that reflects the mutant DNA’s pathological blueprint (i.e., too many CGG repeats). If FMR1 has a repeat expansion in this range, it is referred to as a premutation (PM), and is associated with a risk of developing the fragile X-associated tremor/ataxia syndrome (FXTAS). People with 45 to 54 repeats are referred to as being in the intermediate or gray zone, which is of uncertain clinical significance. There have been reports suggesting a modest association between alleles in the intermediate range and fragile X primary ovarian insufficiency (FXPOI), Parkinson disease, ataxia, multiple system atrophy, and even a few cases of a mild form of FXTAS (Hall, 2014a).Genetic Mutation Responsible for Fragile X-associated Mental Retardation.  For individuals with the PM, the number of CGG repeats is fairly consistently inversely related to age of onset of FXTAS, as well as the integrity of motor, cognitive, and psychiatric functioning (Grigsby, Brega, Jacquemont, et al., 2006; Hessl et al., 2007; Leehey et al., 2008).

Toxic Gain-of-Function Mechanism of FXTAS

Among carriers of the PM, the efficiency of FMR1 transcription is increased, as opposed to the transcriptional silencing observed with the FM and FXS. One result is significantly greater than normal FMR1 mRNA levels, and much of this excess RNA binds to proteins in the nucleus. Other proteins may in turn bind to these aggregates. Because proteins necessary for the cell’s functioning are thus encumbered, translation becomes less efficient, and although there may be a 2- to 10-fold increase in the level of mRNA in the cell, FMRP levels in tissue are typically somewhat decreased. However, it is thought that the cause of FXTAS is not the decreased level of FMRP itself, but rather a toxic gain of function, which means that the excess FMR1 mRNA itself is toxic to the cell. In particular, research is focused on the hypothesis that the mRNA binds to intracellular proteins, especially RNA-binding proteins, with the resultant sequestration of those proteins and their unavailability for normal cellular functioning (Greco et al., 2006; Sellier et al., 2014; Tassone, Beilina, Carosi, et al., 2007; Tasone, Iwahashi, Hagerman, 2004). These aggregations of mRNA and protein appear to become the inclusion bodies that are found in neurons and astrocytes throughout much of the brain and spinal cord, and in the thyroid, heart, testes, and possibly other organs (Greco et al., 2002, 2006; Iwahashi et al., 2006).Genetic Mutation Responsible for Fragile X-associated Mental Retardation.  These inclusion bodies, a neuropathologic hallmark of FXTAS, are eosinophilic, ubiquitin-positive, tau-negative, alpha-synuclein-negative, intranuclear aggregations containing 20 to 30 different proteins, as well as FMR1 messenger RNA (Greco et al., 2002; Iwahashi, Yasui, Greco, et al., 2006; Tassone, Iwahashi, Hagerman, 2004).

A gain-of-function mechanism has not yet been definitively established, and other variables (e.g., secondary genes, mitochondrial disorders, environmental factors) also may play a role in the development and progression of FXTAS. One possible contributor is inflammation, as neuropathological studies have found activated microglia in postmortem tissue (Greco 2002, 2006). Nevertheless, it is unclear whether inflammation might be a primary cause of neurodegeneration, or a secondary phenomenon in reaction to intracellular debris caused perhaps by the hypothesized gain of function mechanism. Mitochondrial dysfunction also has been implicated in FXTAS (Hagerman, 2013; Kaplan et al., 2012; Napoli et al., 2011).

Fragile X-Associate Tremor/Ataxia Syndrome: The Basic Phenotype