Olfactory dysfunction in fragile X tremor ataxia syndrome

Olfactory loss is a predisposing factor for depression, while olfactory enrichment is an effective treatment for depression

The loss of olfactory stimulation correlates well with at least 68 widely differing neurological disorders, including depression, and we raise the possibility that this relationship may be causal. That is, it seems possible that olfactory loss makes the brain vulnerable to expressing the symptoms of these neurological disorders, while daily olfactory enrichment may decrease the risk of expressing these symptoms. This situation resembles the cognitive reserve that is thought to protect people with Alzheimer’s neuropathology from expressing the functional deficit in memory through the cumulative effect of intellectual stimulation. These relationships also resemble the functional response of animal models of human neurological disorders to environmental enrichment, wherein the animals continue to have the induced neuropathology, but do not express the symptoms as they do in a standard environment with restricted sensorimotor stimulation.

A systematic-review of olfactory deficits in neurodevelopmental disorders: From mouse to human

Olfactory impairment is a common clinical motif across neurodevelopmental disorders, suggesting olfactory circuits are particularly vulnerable to disease processes and can provide insight into underlying disease mechanisms. The mouse olfactory bulb is an ideal model system to study mechanisms of neurodevelopmental disease due to its anatomical accessibility, behavioral relevance, ease of measuring circuit input and output, and the feature of adult neurogenesis. Despite the clinical relevance and experimental benefits, olfactory testing across animal models of neurodevelopmental disease has been inconsistent and non-standardized. Here we performed a systematic literature review of olfactory function testing in mouse models of neurodevelopmental disorders, and identified intriguing inconsistencies that include evidence for both increased and decreased acuity in odor detection in various mouse models of Autism Spectrum Disorder (ASD). Based on our identified gaps in the literature, we recommend direct comparison of different mouse models of ASD using standardized tests for odor detection and discrimination. This review provides a framework to guide future olfactory function testing in mouse models of neurodevelopmental diseases.

Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS): Pathophysiology and Clinical Implications

The fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder seen in older premutation (55-200 CGG repeats) carriers of FMR1. The premutation has excessive levels of FMR1 mRNA that lead to toxicity and mitochondrial dysfunction. The clinical features usually begin in the 60 s with an action or intention tremor followed by cerebellar ataxia, although 20% have only ataxia. MRI features include brain atrophy and white matter disease, especially in the middle cerebellar peduncles, periventricular areas, and splenium of the corpus callosum. Neurocognitive problems include memory and executive function deficits, although 50% of males can develop dementia. Females can be less affected by FXTAS because of a second X chromosome that does not carry the premutation. Approximately 40% of males and 16% of female carriers develop FXTAS. Since the premutation can occur in less than 1 in 200 women and 1 in 400 men, the FXTAS diagnosis should be considered in patients that present with tremor, ataxia, parkinsonian symptoms, neuropathy, and psychiatric problems. If a family history of a fragile X mutation is known, then FMR1 DNA testing is essential in patients with these symptoms.

Animal models of olfactory dysfunction in neurodegenerative diseases

Olfactory dysfunction seems to occur earlier than classic motor and cognitive symptoms in many neurodegenerative diseases, including Parkinson's disease (PD) and Alzheimer's disease (AD). Thus, the use of the olfactory system as a clinical marker for neurodegenerative diseases is helpful in the characterization of prodromal stages of these diseases, early diagnostic strategies, differential diagnosis, and, potentially, prediction of treatment success. The use of genetic and neurotoxin animal models has contributed to the understanding of the mechanisms underlying olfactory dysfunction in a number of neurodegenerative diseases. In this chapter, we provide an overview of behavioral and neurochemical alterations observed in animal models of different neurodegenerative diseases (such as genetic and Aβ infusion models for AD and neurotoxins and genetic models of PD), in which olfactory dysfunction has been described.

Chemosensory dysfunction in neurodegenerative diseases

A number of neurodegenerative diseases are accompanied by disordered smell function. The degree of dysfunction can vary among different diseases, such that olfactory testing can aid in differentiating, for example, Alzheimer's disease (AD) from major affective disorder and Parkinson's disease (PD) from progressive supranuclear palsy. Unfortunately, altered smell function often goes unrecognized by patients and physicians alike until formal testing is undertaken. Such testing uniquely probes brain regions not commonly examined in physical examinations and can identify, in some cases, patients who are already in the "preclinical" stage of disease. Awareness of this fact is one reason why the Quality Standards Committee of the American Academy of Neurology has designated smell dysfunction as one of the key diagnostic criteria for PD. The same recommendation has been made by the Movement Disorder Society for both the diagnosis of PD and identification of prodromal PD. Similar suggestions are proposed to include olfactory dysfunction as an additional research criterion for the diagnosis of AD. Although taste impairment, i.e., altered sweet, sour, bitter, salty, and umami perception, has also been demonstrated in some disorders, taste has received much less scientific attention than smell. In this review, we assess what is known about the smell and taste disorders of a wide range of neurodegenerative diseases and describe studies seeking to understand their pathologic underpinnings.

Fragile X tremor ataxia syndrome and white matter dementia

OBJECTIVE

Fragile X tremor ataxia syndrome (FXTAS) is an inherited neurodegenerative disease in which dementia is common and disabling. The pathogenesis of dementia in FXTAS is poorly understood, but the salience of executive dysfunction and slowed processing speed, the frequent presence of the middle cerebellar peduncle sign on magnetic resonance imaging (MRI), and striking neuropathological alterations of white matter all suggest that myelinated tracts are significantly involved. This paper considers the role of white matter disease in FXTAS dementia, particularly with regard to the concept of white matter dementia (WMD).

METHOD

A focused review of FXTAS in relation to known white matter disorders is provided to propose that the concept of WMD may illuminate the basis of dementia in FXTAS. The putative pathogenetic contribution of white matter involvement in other neurodegenerative diseases is also considered.

RESULTS

Considerable evidence supports the importance of white matter disease in the pathogenesis of dementia in FXTAS. Whereas, gray matter regions are also involved, white matter degeneration is prominent, even early in the disease, and correlates with executive dysfunction and slowed processing speed. Evidence for white matter involvement in other neurodegenerative diseases lends additional support to the relevance of white matter in FXTAS.

CONCLUSION

The dementia of FXTAS is closely related to the profile of WMD, and white matter involvement is also supported by MRI and neuropathological observations. White matter pathology is also relevant to the pathogenesis of other neurodegenerative diseases. Further study of white matter promises to clarify the origin of dementia in FXTAS.

Fragile X-associated tremor/ataxia syndrome - features, mechanisms and management

Many physicians are unaware of the many phenotypes associated with the fragile X premutation, an expansion in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene that consists of 55-200 CGG repeats. The most severe of these phenotypes is fragile X-associated tremor/ataxia syndrome (FXTAS), which occurs in the majority of ageing male premutation carriers but in fewer than 20% of ageing women with the premutation. The prevalence of the premutation is 1 in 150-300 females, and 1 in 400-850 males, so physicians are likely to see people affected by FXTAS. Fragile X DNA testing is broadly available in the Western world. The clinical phenotype of FXTAS at presentation can vary and includes intention tremor, cerebellar ataxia, neuropathic pain, memory and/or executive function deficits, parkinsonian features, and psychological disorders, such as depression, anxiety and/or apathy. FXTAS causes brain atrophy and white matter disease, usually in the middle cerebellar peduncles, the periventricular area, and the splenium and/or genu of the corpus callosum. Here, we review the complexities involved in the clinical management of FXTAS and consider how targeted treatment for these clinical features of FXTAS will result from advances in our understanding of the molecular mechanisms that underlie this neurodegenerative disorder. Such targeted approaches should also be more broadly applicable to earlier forms of clinical involvement among premutation carriers.

Fragile X mental retardation protein regulates olfactory sensitivity but not odorant discrimination

Fragile X syndrome (FXS) is the most common cause of inherited intellectual disability and is characterized by cognitive impairments and altered sensory function. It is caused by absence of fragile X mental retardation protein (FMRP), an RNA-binding protein essential for normal synaptic plasticity and function. Animal models have provided important insights into mechanisms through which loss of FMRP impacts cognitive and sensory development and function. While FMRP is highly enriched in the developing and adult olfactory bulb (OB), its role in olfactory sensory function remains poorly understood. Here, we used a mouse model of FXS, the fmr1 (-/y) mouse, to test whether loss of FMRP impacts olfactory discrimination, habituation, or sensitivity using a spontaneous olfactory cross-habituation task at a range of odorant concentrations. We demonstrated that fmr1 (-/y) mice have a significant decrease in olfactory sensitivity compared with wild type controls. When we controlled for differences in sensitivity, we found no effect of loss of FMRP on the ability to habituate to or spontaneously discriminate between odorants. These data indicate that loss of FMRP significantly alters olfactory sensitivity, but not other facets of basal olfactory function. These findings have important implications for future studies aimed at understanding the role of FMRP on sensory functioning.

Fragile X-associated tremor/ataxia syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder that affects some but not all carriers of small, noncoding CGG-repeat expansions (55-200 repeats; premutation) within the fragile X gene (FMR1). Principal features of FXTAS include intention tremor, cerebellar ataxia, Parkinsonism, memory and executive function deficits, autonomic dysfunction, brain atrophy with white matter disease, and cognitive decline. Although FXTAS was originally considered to be confined to the premutation range, rare individuals with a gray zone (45-54 repeats) or an unmethylated full mutation (>200 repeats) allele have now been described, the constant feature of the disorder remaining the requirement for FMR1 expression, in contradistinction to the gene silencing mechanism of fragile X syndrome. Although transcriptional activity is required for FXTAS pathogenesis, the specific trigger(s) for FXTAS pathogenesis remains elusive, highlighting the need for more research in this area. This need is underscored by recent neuroimaging findings of changes in the central nervous system that consistently appear well before the onset of clinical symptoms, thus creating an opportunity to delay or prevent the appearance of FXTAS.

Molecular Advances Leading to Treatment Implications for Fragile X Premutation Carriers

Fragile X syndrome (FXS) is the most common single gene cause of intellectual disability and it is characterized by a CGG expansion of more than 200 repeats in the FMR1 gene, leading to methylation of the promoter and gene silencing. The fragile X premutation, characterized by a 55 to 200 CGG repeat expansion, causes health problems and developmental difficulties in some, but not all, carriers. The premutation causes primary ovarian insufficiency in approximately 20% of females, psychiatric problems (including depression and/or anxiety) in approximately 50% of carriers and a neurodegenerative disorder, the fragile X-associated tremor ataxia syndrome (FXTAS), in approximately 40% of males and 16% of females later in life. Recent clinical studies in premutation carriers have expanded the health problems that may be seen. Advances in the molecular pathogenesis of the premutation have shown significant mitochondrial dysfunction and oxidative stress in neurons which may be amenable to treatment. Here we review the clinical problems of carriers and treatment recommendations.

Advances in clinical and molecular understanding of the FMR1 premutation and fragile X-associated tremor/ataxia syndrome

Fragile X syndrome, the most common heritable form of cognitive impairment, is caused by epigenetic silencing of the fragile X (FMR1) gene owing to large expansions (>200 repeats) of a non-coding CGG-repeat element. Smaller, so-called premutation expansions (55-200 repeats) can cause a family of neurodevelopmental phenotypes (attention deficit hyperactivity disorder, autism spectrum disorder, seizure disorder) and neurodegenerative (fragile X-associated tremor/ataxia syndrome [FXTAS]) phenotypes through an entirely distinct molecular mechanism involving increased FMR1 mRNA production and toxicity. Results of basic cellular, animal, and human studies have helped to elucidate the underlying RNA toxicity mechanism, while clinical research is providing a more nuanced picture of the range of clinical manifestations. Advances of knowledge on both mechanistic and clinical fronts are driving new approaches to targeted treatment, but two important necessities are emerging: to define the extent to which the mechanisms contributing to FXTAS also contribute to other neurodegenerative and medical disorders, and to redefine FXTAS in view of its differing presentations and associated features.