Peripheral nerve findings in Rett syndrome

Peripheral Somatosensory Neuron Dysfunction: Emerging Roles in Autism Spectrum Disorders

Alterations in somatosensory (touch and pain) behaviors are highly prevalent among people with autism spectrum disorders (ASDs). However, the neural mechanisms underlying abnormal touch and pain-related behaviors in ASDs and how altered somatosensory reactivity might contribute to ASD pathogenesis has not been well studied. Here, we provide a brief review of somatosensory alterations observed in people with ASDs and recent evidence from animal models that implicates peripheral neurons as a locus of dysfunction for somatosensory abnormalities in ASDs. Lastly, we describe current efforts to understand how altered peripheral sensory neuron dysfunction may impact brain development and complex behaviors in ASD models, and whether targeting peripheral somatosensory neurons to improve their function might also improve related ASD phenotypes.

Reduced axonal diameter of peripheral nerve fibers in a mouse model of Rett syndrome

Rett syndrome (RTT) is a neurological disorder characterized by motor and cognitive impairment, autonomic dysfunction and a loss of purposeful hand skills. In the majority of cases, typical RTT is caused by de novo mutations in the X-linked gene, MECP2. Alterations in the structure and function of neurons within the central nervous system of RTT patients and Mecp2-null mouse models are well established. In contrast, few studies have investigated the effects of MeCP2-deficiency on peripheral nerves. In this study, we conducted detailed morphometric as well as functional analysis of the sciatic nerves of symptomatic adult female Mecp2^+/- mice. We observed a significant reduction in the mean diameter of myelinated nerve fibers in Mecp2^+/- mice. In myelinated fibers, mitochondrial densities per unit area of axoplasm were significantly altered in Mecp2^+/- mice. However, conduction properties of the sciatic nerve of Mecp2 knockout mice were not different from control. These subtle changes in myelinated peripheral nerve fibers in heterozygous Mecp2 knockout mice could potentially explain some RTT phenotypes.

Neuropathology of Rett syndrome

Rett syndrome is a sporadic disorder (except for a few familial cases) occurring in 1 in 10,000 to 1 in 23,000 girls worldwide. It is associated with profound mental and motor handicap. About 90% of cases involve a mutation in the methyl-CpG binding protein 2 gene (MECP2). The role of this gene in the pathogenesis of this enigmatic disorder is being extensively investigated in animal models. Rett syndrome is associated with a complex phenotype that is unique in every aspect of its presentation, clinical physiology, chemistry, and pathology. Years of concentrated observations have defined the clinical presentation of classic Rett syndrome and its variants and related features (eg, neurophysiologic, radiologic, chemical, metabolic, and anatomic). This article reviews the neuropathology of Rett syndrome, which involves individual neurons, perhaps selected neurons, of decreased size, dendritic branching, and numbers of spines. This article also summarizes the studies in the human and mouse brain with Rett syndrome that are beginning to reveal the disorder's pathoetiology.

Rett syndrome: review of biological abnormalities

The Rett syndrome (RS) is a peculiar, sporadic, atrophic disorder, almost entirely confined to females. After the first six months of life there is developmental slowing with reduced communication and head growth for about one year. This is followed by a rapid destructive stage with severe dementia and loss of hand skills (with frequent hand wringing), apraxia and ataxia, autistic features and irregular breathing with hyperventilation. Seizures often supervene. Subsequently there is some stabilization in a pseudo-stationary stage during the preschool to school years, associated with more emotional contact but also abnormalities of the autonomic and skeletal systems. After the age of 15-20 years, a late motor deterioration occurs with dystonia and frequent spasticity but seizures become milder. RS has generally been considered an X-linked disorder in which affected females represent a new mutation, with male lethality. Linkage studies suggested a critical region at Xq28. In 1999, mutations in the gene MECP2 encoding X-linked methyl cytosine-binding protein 2 (MeCP2) were found in a proportion of Rett girls. This protein can bind methylated DNA. Analyses are leading to much further investigation of mutants and their effects on genes. Neuropathological and electrophysiological studies of RS are described. Description of neurometabolic factors includes reduced levels of dopamine, serotonin, noradrenaline and choline acetyltransferase (ChAT) in brain, also estimation of nerve growth factors, endorphin, substance P, glutamate and other amino acids and their receptor levels. The results of neuroimaging are surveyed, including volumetric magnetic resonance imaging (MRI) and positron emission tomography (PET).

Decreased cerebrospinal fluid levels of substance P in patients with Rett syndrome

To clarify the mechanism of brain and spinal cord impairment in Rett syndrome (RS), we measured the cerebrospinal fluid (CSF) levels of substance P in 20 patients with RS including 16 childhood patients and 4 adult patients. Findings were compared with those obtained in age-matched controls and diseased controls. The CSF level of substance P was significantly lower in patients with RS compared with controls. The alteration in the CSF level of substance P may be related to the neurological impairment, especially autonomic dysfunction, and neuropathological involvement of dorsal root ganglia and peripheral nerve observed in RS.

Rett syndrome

Rett syndrome (RS) is a neurological disorder that mainly, and possibly exclusively, affects girls. After its description in 1966 by Andreas Rett in the Wiener Klinische Wochenschrift, awareness and interest in RS were enhanced by the 1983 report of Hagberg et al. in the Annals of Neurology. Diagnosis, and indeed the hypothesis that it exists, continue to be based upon a consistent constellation of clinical features observed in thousands of female patients world-wide. A diagnostic marker has not been identified. Notwithstanding this serious limitation, it is generally agreed that RS is a distinct entity and that it is genetically determined. Although it is associated with devastating loss of function between infancy and the fifth year of life, its course becomes relatively static thereafter, setting it apart from most of the genetic neurodegenerative disorders of childhood. Neuropathological and neurochemical studies call attention to RS as a neurodevelopmental disorder. Clarification of its pathogenesis may provide new insight into normal brain development. This report summarizes existing information and concepts about RS, and presents recent advances.

Neuropathology of Rett syndrome: case report with neuronal and mitochondrial abnormalities in the brain

Neuronal changes in the brain of a Rett syndrome patient were examined in a frontal lobe biopsy performed at age 3 years and in the postmortem brain at age 15 years. In the brain biopsy, frontal cortex contained numerous scattered pyramidal neurons with cytoplasmic vacuolation and increased cytoplasmic density, with no neuronophagia or inflammation detected; electron microscopy showed these neurons to have large, lucent-appearing mitochondria, very abundant ribosomal content, and some lipofuscin granules. Postmortem brain 12 years later showed scattered neurons in frontal cortex, substantia nigra, and cerebellar folia, with increased electron density of the cytoplasm, stacks of ribosomal endoplasmic reticulum, and large amounts of disorganized membranous material, including autophagic-type organelles. Mitochondria of these neurons contained electron-dense, finely granular matrix inclusions; in the substantia nigra, some spherical mitochondrial inclusions completely filled the matrix space. Golgi preparations of (autopsy) frontal cortex and cerebellar folia showed truncation and thickening of dendrites and a degenerate appearance of cortical pyramidal neurons, similar to changes found in aged brain. Synaptophysin immunohistochemistry indicated that the density of synapses was not greatly altered compared to controls in frontal cortex and cerebellum. The patient also had a second genetic defect, severe combined immunodeficiency with thymic aplasia, which may be X-linked.

A prevalence study of Rett syndrome in an institutionalized population

An institution for the mentally retarded was surveyed to determine the prevalence rate of Rett syndrome (RS). Four patients with definite RS and one with probable RS were identified in a population of 350, yielding a prevalence rate of 1 in 87, about 1% of institutionalized male and female patients with mental retardation. In this population of patients with severe and profound mental retardation (N = 297), 138 females were surveyed, suggesting a prevalence rate among females of 1 in 34 in an institutional population of persons with mental retardation. Surveys of institutions for persons with mental retardation may be an effective method to identify adults with RS.

Polyneuropathies in paediatrics

Non-acute polyneuropathies (PNPs) encountered in paediatrics are reviewed. Emphasis is placed on three main groups of conditions: the relatively rare but treatable dysimmune PNP (chronic relapsing dysimmune polyneuropathies, CRDP); the more common hereditary motor/sensory neuropathies (HMSN and HSN); and the often missed symptomatic neuropathies of some heredodegenerative and neurometabolic disorders. Diagnostic procedures are discussed. One conclusion drawn is that so far metabolic screening procedures do not give any diagnostic or aetiological information in HMSN or in HSN, nor in heredoataxias or heredoparaplegias. When a specific neurometabolic disease is suspected from the clinical symptomatology, individually structured investigations are necessary.