Brain stem and spinal cord impairment in Rett syndrome: somatosensory and auditory evoked responses investigations

Probing a neural unreliability account of auditory sensory processing atypicalities in Rett Syndrome

BACKGROUND

In the search for objective tools to quantify neural function in Rett Syndrome (RTT), which are crucial in the evaluation of therapeutic efficacy in clinical trials, recordings of sensory-perceptual functioning using event-related potential (ERP) approaches have emerged as potentially powerful tools. Considerable work points to highly anomalous auditory evoked potentials (AEPs) in RTT. However, an assumption of the typical signal-averaging method used to derive these measures is "stationarity" of the underlying responses - i.e. neural responses to each input are highly stereotyped. An alternate possibility is that responses to repeated stimuli are highly variable in RTT. If so, this will significantly impact the validity of assumptions about underlying neural dysfunction, and likely lead to overestimation of underlying neuropathology. To assess this possibility, analyses at the single-trial level assessing signal-to-noise ratios (SNR), inter-trial variability (ITV) and inter-trial phase coherence (ITPC) are necessary.

METHODS

AEPs were recorded to simple 100 Hz tones from 18 RTT and 27 age-matched controls (Ages: 6-22 years). We applied standard AEP averaging, as well as measures of neuronal reliability at the single-trial level (i.e. SNR, ITV, ITPC). To separate signal-carrying components from non-neural noise sources, we also applied a denoising source separation (DSS) algorithm and then repeated the reliability measures.

RESULTS

Substantially increased ITV, lower SNRs, and reduced ITPC were observed in auditory responses of RTT participants, supporting a "neural unreliability" account. Application of the DSS technique made it clear that non-neural noise sources contribute to overestimation of the extent of processing deficits in RTT. Post-DSS, ITV measures were substantially reduced, so much so that pre-DSS ITV differences between RTT and TD populations were no longer detected. In the case of SNR and ITPC, DSS substantially improved these estimates in the RTT population, but robust differences between RTT and TD were still fully evident.

CONCLUSIONS

To accurately represent the degree of neural dysfunction in RTT using the ERP technique, a consideration of response reliability at the single-trial level is highly advised. Non-neural sources of noise lead to overestimation of the degree of pathological processing in RTT, and denoising source separation techniques during signal processing substantially ameliorate this issue.

Probing a neural unreliability account of auditory sensory processing atypicalities in Rett Syndrome

Background In the search for objective tools to quantify neural function in Rett Syndrome (RTT), which are crucial in the evaluation of therapeutic efficacy in clinical trials, recordings of sensory-perceptual functioning using event-related potential (ERP) approaches have emerged as potentially powerful tools. Considerable work points to highly anomalous auditory evoked potentials (AEPs) in RTT. However, an assumption of the typical signal-averaging method used to derive these measures is "stationarity" of the underlying responses - i.e. neural responses to each input are highly stereotyped. An alternate possibility is that responses to repeated stimuli are highly variable in RTT. If so, this will significantly impact the validity of assumptions about underlying neural dysfunction, and likely lead to overestimation of underlying neuropathology. To assess this possibility, analyses at the single-trial level assessing signal-to-noise ratios (SNR), inter-trial variability (ITV) and inter-trial phase coherence (ITPC) are necessary. Methods AEPs were recorded to simple 100Hz tones from 18 RTT and 27 age-matched controls (Ages: 6-22 years). We applied standard AEP averaging, as well as measures of neuronal reliability at the single-trial level (i.e. SNR, ITV, ITPC). To separate signal-carrying components from non-neural noise sources, we also applied a denoising source separation (DSS) algorithm and then repeated the reliability measures. Results Substantially increased ITV, lower SNRs, and reduced ITPC were observed in auditory responses of RTT participants, supporting a "neural unreliability" account. Application of the DSS technique made it clear that non-neural noise sources contribute to overestimation of the extent of processing deficits in RTT. Post-DSS, ITV measures were substantially reduced, so much so that pre-DSS ITV differences between RTT and TD populations were no longer detected. In the case of SNR and ITPC, DSS substantially improved these estimates in the RTT population, but robust differences between RTT and TD were still fully evident. Conclusions To accurately represent the degree of neural dysfunction in RTT using the ERP technique, a consideration of response reliability at the single-trial level is highly advised. Non-neural sources of noise lead to overestimation of the degree of pathological processing in RTT, and denoising source separation techniques during signal processing substantially ameliorate this issue.

Probing a neural unreliability account of auditory sensory processing atypicalities in Rett Syndrome

Background

In the search for objective tools to quantify neural function in Rett Syndrome (RTT), which are crucial in the evaluation of therapeutic efficacy in clinical trials, recordings of sensory-perceptual functioning using event-related potential (ERP) approaches have emerged as potentially powerful tools. Considerable work points to highly anomalous auditory evoked potentials (AEPs) in RTT. However, an assumption of the typical signal-averaging method used to derive these measures is "stationarity" of the underlying responses - i.e. neural responses to each input are highly stereotyped. An alternate possibility is that responses to repeated stimuli are highly variable in RTT. If so, this will significantly impact the validity of assumptions about underlying neural dysfunction, and likely lead to overestimation of underlying neuropathology. To assess this possibility, analyses at the single-trial level assessing signal-to-noise ratios (SNR), inter-trial variability (ITV) and inter-trial phase coherence (ITPC) are necessary.

Methods

AEPs were recorded to simple 100Hz tones from 18 RTT and 27 age-matched controls (Ages: 6-22 years). We applied standard AEP averaging, as well as measures of neuronal reliability at the single-trial level (i.e. SNR, ITV, ITPC). To separate signal-carrying components from non-neural noise sources, we also applied a denoising source separation (DSS) algorithm and then repeated the reliability measures.

Results

Substantially increased ITV, lower SNRs, and reduced ITPC were observed in auditory responses of RTT participants, supporting a "neural unreliability" account. Application of the DSS technique made it clear that non-neural noise sources contribute to overestimation of the extent of processing deficits in RTT. Post-DSS, ITV measures were substantially reduced, so much so that pre-DSS ITV differences between RTT and TD populations were no longer detected. In the case of SNR and ITPC, DSS substantially improved these estimates in the RTT population, but robust differences between RTT and TD were still fully evident.

Conclusions

To accurately represent the degree of neural dysfunction in RTT using the ERP technique, a consideration of response reliability at the single-trial level is highly advised. Non-neural sources of noise lead to overestimation of the degree of pathological processing in RTT, and denoising source separation techniques during signal processing substantially ameliorate this issue.

Gene Expression Profiling in Trigeminal Ganglia from Cntnap2-/- and Shank3b-/- Mouse Models of Autism Spectrum Disorder

Sensory difficulties represent a crucial issue in the life of autistic individuals. The diagnostic and statistical manual of mental disorders describes both hyper- and hypo-responsiveness to sensory stimulation as a criterion for the diagnosis autism spectrum disorders (ASD). Among the sensory domain affected in ASD, altered responses to tactile stimulation represent the most commonly reported sensory deficits. Although tactile abnormalities have been reported in monogenic cohorts of patients and genetic mouse models of ASD, the underlying mechanisms are still unknown. Traditionally, autism research has focused on the central nervous system as the target to infer the neurobiological bases of such tactile abnormalities. Nonetheless, the peripheral nervous system represents the initial site of processing of sensory information and a potential site of dysfunction in the sensory cascade. Here we investigated the gene expression deregulation in the trigeminal ganglion (which directly receives tactile information from whiskers) in two genetic models of syndromic autism (Shank3b and Cntnap2 mutant mice) at both adult and juvenile ages. We found several neuronal and non-neuronal markers involved in inhibitory, excitatory, neuroinflammatory and sensory neurotransmission to be differentially regulated within the trigeminal ganglia of both adult and juvenile Shank3b and Cntnap2 mutant mice. These results may help in disentangling the multifaced complexity of sensory abnormalities in autism and open avenues for the development of peripherally targeted treatments for tactile sensory deficits exhibited in ASD.

Electrophysiological and Behavioral Evidence for Hyper- and Hyposensitivity in Rare Genetic Syndromes Associated with Autism

Our study reviewed abnormalities in spontaneous, as well as event-related, brain activity in syndromes with a known genetic underpinning that are associated with autistic symptomatology. Based on behavioral and neurophysiological evidence, we tentatively subdivided the syndromes on primarily hyper-sensitive (Fragile X, Angelman) and hypo-sensitive (Phelan-McDermid, Rett, Tuberous Sclerosis, Neurofibromatosis 1), pointing to the way of segregation of heterogeneous idiopathic ASD, that includes both hyper-sensitive and hypo-sensitive individuals. This segmentation links abnormalities in different genes, such as FMR1, UBE3A, GABRB3, GABRA5, GABRG3, SHANK3, MECP2, TSC1, TSC2, and NF1, that are causative to the above-mentioned syndromes and associated with synaptic transmission and cell growth, as well as with translational and transcriptional regulation and with sensory sensitivity. Excitation/inhibition imbalance related to GABAergic signaling, and the interplay of tonic and phasic inhibition in different brain regions might underlie this relationship. However, more research is needed. As most genetic syndromes are very rare, future investigations in this field will benefit from multi-site collaboration with a common protocol for electrophysiological and event-related potential (EEG/ERP) research that should include an investigation into all modalities and stages of sensory processing, as well as potential biomarkers of GABAergic signaling (such as 40-Hz ASSR).

Evoked Potentials and EEG Analysis in Rett Syndrome and Related Developmental Encephalopathies: Towards a Biomarker for Translational Research

Rett syndrome is a debilitating neurodevelopmental disorder for which no disease-modifying treatment is available. Fortunately, advances in our understanding of the genetics and pathophysiology of Rett syndrome has led to the development of promising new therapeutics for the condition. Several of these therapeutics are currently being tested in clinical trials with others likely to progress to clinical trials in the coming years. The failure of recent clinical trials for Rett syndrome and other neurodevelopmental disorders has highlighted the need for electrophysiological or other objective biological markers of treatment response to support the success of clinical trials moving forward. The purpose of this review is to describe the existing studies of electroencephalography (EEG) and evoked potentials (EPs) in Rett syndrome and discuss the open questions that must be addressed before the field can adopt these measures as surrogate endpoints in clinical trials. In addition to summarizing the human work on Rett syndrome, we also describe relevant studies with animal models and the limited research that has been carried out on Rett-related disorders, particularly methyl-CpG binding protein 2 (MECP2) duplication syndrome, CDKL5 deficiency disorder, and FOXG1 disorder.

Sensory evoked potentials in patients with Rett syndrome through the lens of animal studies: Systematic review

OBJECTIVE

Systematically review the abnormalities in event related potential (ERP) recorded in Rett Syndrome (RTT) patients and animals in search of translational biomarkers of deficits related to the particular neurophysiological processes of known genetic origin (MECP2 mutations).

METHODS

Pubmed, ISI Web of Knowledge and BIORXIV were searched for the relevant articles according to PRISMA standards.

RESULTS

ERP components are generally delayed across all sensory modalities both in RTT patients and its animal model, while findings on ERPs amplitude strongly depend on stimulus properties and presentation rate. Studies on RTT animal models uncovered the abnormalities in the excitatory and inhibitory transmission as critical mechanisms underlying the ERPs changes, but showed that even similar ERP alterations in auditory and visual domains have a diverse neural basis. A range of novel approaches has been developed in animal studies bringing along the meaningful neurophysiological interpretation of ERP measures in RTT patients.

CONCLUSIONS

While there is a clear evidence for sensory ERPs abnormalities in RTT, to further advance the field there is a need in a large-scale ERP studies with the functionally-relevant experimental paradigms.

SIGNIFICANCE

The review provides insights into domain-specific neural basis of the ERP abnormalities and promotes clinical application of the ERP measures as the non-invasive functional biomarkers of RTT pathophysiology.

Altered trajectories of neurodevelopment and behavior in mouse models of Rett syndrome

Rett Syndrome (RTT) is a genetic disorder that is caused by mutations in the x-linked gene coding for methyl-CpG-biding-protein 2 (MECP2) and that mainly affects females. Male and female transgenic mouse models of RTT have been studied extensively, and we have learned a great deal regarding RTT neuropathology and how MeCP2 deficiency may be influencing brain function and maturation. In this manuscript we review what is known concerning structural and coinciding functional and behavioral deficits in RTT and in mouse models of MeCP2 deficiency. We also introduce our own corroborating data regarding behavioral phenotype and morphological alterations in volume of the cortex and striatum and the density of neurons, aberrations in experience-dependent plasticity within the barrel cortex and the impact of MeCP2 loss on glial structure. We conclude that regional structural changes in genetic models of RTT show great similarity to the alterations in brain structure of patients with RTT. These region-specific modifications often coincide with phenotype onset and contribute to larger issues of circuit connectivity, progression, and severity. Although the alterations seen in mouse models of RTT appear to be primarily due to cell-autonomous effects, there are also non-cell autonomous mechanisms including those caused by MeCP2-deficient glia that negatively impact healthy neuronal function. Collectively, this body of work has provided a solid foundation on which to continue to build our understanding of the role of MeCP2 on neuronal and glial structure and function, its greater impact on neural development, and potential new therapeutic avenues.

CRISPR-Cas9 human gene replacement and phenomic characterization in Caenorhabditis elegans to understand the functional conservation of human genes and decipher variants of uncertain significance

Our ability to sequence genomes has vastly surpassed our ability to interpret the genetic variation we discover. This presents a major challenge in the clinical setting, where the recent application of whole-exome and whole-genome sequencing has uncovered thousands of genetic variants of uncertain significance. Here, we present a strategy for targeted human gene replacement and phenomic characterization, based on CRISPR-Cas9 genome engineering in the genetic model organism Caenorhabditis elegans, that will facilitate assessment of the functional conservation of human genes and structure-function analysis of disease-associated variants with unprecedented precision. We validate our strategy by demonstrating that direct single-copy replacement of the C. elegans ortholog (daf-18) with the critical human disease-associated gene phosphatase and tensin homolog (PTEN) is sufficient to rescue multiple phenotypic abnormalities caused by complete deletion of daf-18, including complex chemosensory and mechanosensory impairments. In addition, we used our strategy to generate animals harboring a single copy of the known pathogenic lipid phosphatase inactive PTEN variant (PTEN-G129E), and showed that our automated in vivo phenotypic assays could accurately and efficiently classify this missense variant as loss of function. The integrated nature of the human transgenes allows for analysis of both homozygous and heterozygous variants and greatly facilitates high-throughput precision medicine drug screens. By combining genome engineering with rapid and automated phenotypic characterization, our strategy streamlines the identification of novel conserved gene functions in complex sensory and learning phenotypes that can be used as in vivo functional assays to decipher variants of uncertain significance.

Developing a sense of touch

The sensation of touch is mediated by mechanosensory neurons that are embedded in skin and relay signals from the periphery to the central nervous system. During embryogenesis, axons elongate from these neurons to make contact with the developing skin. Concurrently, the epithelium of skin transforms from a homogeneous tissue into a heterogeneous organ that is made up of distinct layers and microdomains. Throughout this process, each neuronal terminal must form connections with an appropriate skin region to serve its function. This Review presents current knowledge of the development of the sensory microdomains in mammalian skin and the mechanosensory neurons that innervate them.

Peripheral Mechanosensory Neuron Dysfunction Underlies Tactile and Behavioral Deficits in Mouse Models of ASDs

Patients with autism spectrum disorders (ASDs) commonly experience aberrant tactile sensitivity, yet the neural alterations underlying somatosensory dysfunction and the extent to which tactile deficits contribute to ASD characteristics are unknown. We report that mice harboring mutations in Mecp2, Gabrb3, Shank3, and Fmr1 genes associated with ASDs in humans exhibit altered tactile discrimination and hypersensitivity to gentle touch. Deletion of Mecp2 or Gabrb3 in peripheral somatosensory neurons causes mechanosensory dysfunction through loss of GABAA receptor-mediated presynaptic inhibition of inputs to the CNS. Remarkably, tactile defects resulting from Mecp2 or Gabrb3 deletion in somatosensory neurons during development, but not in adulthood, cause social interaction deficits and anxiety-like behavior. Restoring Mecp2 expression exclusively in the somatosensory neurons of Mecp2-null mice rescues tactile sensitivity, anxiety-like behavior, and social interaction deficits, but not lethality, memory, or motor deficits. Thus, mechanosensory processing defects contribute to anxiety-like behavior and social interaction deficits in ASD mouse models. PAPERCLIP.

Enhancement of postsynaptic GABAA and extrasynaptic NMDA receptor-mediated responses in the barrel cortex of Mecp2-null mice

Rett syndrome (RTT) is a neurodevelopmental disorder that results from mutations in the X-linked gene for methyl-CpG-binding protein 2 (MECP2). The underlying cellular mechanism for the sensory deficits in patients with RTT is largely unknown. This study used the Bird mouse model of RTT to investigate sensory thalamocortical synaptic transmission in the barrel cortex of Mecp2-null mice. Electrophysiological results showed an excitation/inhibition imbalance, biased toward inhibition, due to an increase in efficacy of postsynaptic GABAA receptors rather than alterations in inhibitory network and presynaptic release properties. Enhanced inhibition impaired the transmission of tonic sensory signals from the thalamus to the somatosensory cortex. Previous morphological studies showed an upregulation of NMDA receptors in the neocortex of both RTT patients and Mecp2-null mice at early ages [Blue ME, Naidu S, Johnston MV. Ann Neurol 45: 541-545, 1999; Blue ME, Kaufmann WE, Bressler J, Eyring C, O'Driscoll C, Naidu S, Johnston MV. Anat Rec (Hoboken) 294: 1624-1634, 2011]. Although AMPA and NMDA receptor-mediated excitatory synaptic transmission was not altered in the barrel cortex of Mecp2-null mice, extrasynaptic NMDA receptor-mediated responses increased markedly. These responses were blocked by memantine, suggesting that extrasynaptic NMDA receptors play an important role in the pathogenesis of RTT. The results suggest that enhancement of postsynaptic GABAA and extrasynaptic NMDA receptor-mediated responses may underlie impaired somatosensation and that pharmacological blockade of extrasynaptic NMDA receptors may have therapeutic value for RTT.

MeCP2+/- mouse model of RTT reproduces auditory phenotypes associated with Rett syndrome and replicate select EEG endophenotypes of autism spectrum disorder

Impairments in cortical sensory processing have been demonstrated in Rett syndrome (RTT) and Autism Spectrum Disorders (ASD) and are thought to contribute to high-order phenotypic deficits. However, underlying pathophysiological mechanisms for these abnormalities are unknown. This study investigated auditory sensory processing in a mouse model of RTT with a heterozygous loss of MeCP2 function. Cortical abnormalities in a number of neuropsychiatric disorders, including ASD are reflected in auditory evoked potentials and fields measured by EEG and MEG. One of these abnormalities, increased latency of cortically sourced components, is associated with language and developmental delay in autism. Additionally, gamma-band abnormalities have recently been identified as an endophenotype of idiopathic autism. Both of these cortical abnormalities are potential clinical endpoints for assessing treatment. While ascribing similar mechanisms of idiopathic ASD to Rett syndrome (RTT) has been controversial, we sought to determine if mouse models of RTT replicate these intermediate phenotypes. Mice heterozygous for the null mutations of the gene MeCP2, were implanted for EEG. In response to auditory stimulation, these mice recapitulated specific latency differences as well as select gamma and beta band abnormalities associated with ASD. MeCP2 disruption is the predominant cause of RTT, and reductions in MeCP2 expression predominate in ASD. This work further suggests a common cortical pathophysiology for RTT and ASD, and indicates that the MeCP2+/- model may be useful for preclinical development targeting specific cortical processing abnormalities in RTT with potential relevance to ASD.

Effects of sedation on auditory brainstem response in Rett syndrome

Prolongation of the I-V interpeak latency intervals have been reported in Rett syndrome and other neurodevelopmental disorders. It has been suggested that the use of sedation may account for differences in the interpeak latency intervals when comparisons are made across diagnostic groups if sedated control groups are not used for the basis of comparison. This study examined the effects of sedation on auditory brainstem response interpeak latency intervals (i.e., I-III, III-V, and I-V) in two groups: (1) a group with Rett syndrome who were positive for mutations in the MECP2 gene and (2) a group negative for mutations in the MECP2 gene but who were severely to profoundly delayed with other causes of mental retardation. To further assess the effects of sedation, a third group of sedated and nonsedated female participants, taken from an in-house normative auditory brainstem response database was also included. An analysis of variance indicated (1) longer I-V interpeak latency intervals in the sedated participants with Rett syndrome; (2) longer III-V interpeak latency intervals in the mutation-positive participants as compared to non-Rett syndrome, mutation-negative participants; and (3) no significant effects of sedation on the I-III, III-V, or I-V interpeak latency intervals among the normative group participants, according to t tests. The findings suggest a possible biological basis for the discrepancy in the literature on auditory brain stem responses in Rett syndrome, and warrant cautious interpretation of auditory brainstem responses findings in sedated subjects with Rett syndrome, as well as in those with mental retardation and seizures.

Neurophysiology of Rett syndrome

Rett syndrome is a neurodevelopmental disorder that in most cases is consequent to a mutation in the MECP2 gene. The central nervous system is the primary organ system involved in Rett syndrome. Neurophysiologic evaluations provide information concerning the developmental aspects of Rett syndrome and the character and extent of involvement of the central, peripheral, and autonomic nervous system pathways. Evoked potentials typically demonstrate intactness of peripheral auditory and visual pathways and suggest dysfunction of central or "higher" cortical pathways. Somatosensory evoked potentials can be characterized by "giant" responses, suggesting cortical hyperexcitability. Cortical hyperexcitability is further suggested by the findings of the electroencephalogram (EEG), which are primarily characterized by a loss of expected developmental features; the appearance of focal, multifocal, and generalized epileptiform abnormalities; and the occurrence of rhythmic slow (theta) activity, primarily in the frontal-central regions. Epileptic seizures are reported to occur frequently in Rett syndrome. However, many events presumed to be seizures have no EEG correlate during video-EEG monitoring. Impairment of the autonomic nervous system in Rett syndrome is suggested by an increased incidence of long Q-T intervals during electrocardiographic recordings and diminished heart rate variability. Autonomic nervous system dysfunction can contribute to the increased incidence of sudden unexpected death in Rett syndrome.

Neurophysiology of Rett syndrome

Neurophysiological evaluations have been widely applied in the study of Rett syndrome (RS) to provide information concerning the developmental aspects of RS; the character and extent of involvement of the central, peripheral, and autonomic nervous system pathways; and evaluation of the clinical symptomatology of RS. The electroencephalogram (EEG) is invariably abnormal and shows characteristic, though not diagnostic, changes: loss of expected developmental features; the appearance of focal, multifocal, and generalized epileptiform abnormalities; and the occurrence of rhythmic slow (theta) activity primarily in the frontal-central regions. Epileptic seizures are reported to occur frequently in RS, and partial and generalized seizures may be experienced by RS girls. However, many events presumed to be seizures have no EEG correlate during video-EEG monitoring, suggesting the possibility of a nonepileptic mechanism. Such monitoring may be necessary to determine appropriate use of antiepileptic drugs. Evoked potentials typically demonstrate intact peripheral auditory and visual pathways and suggest dysfunction of central or "higher" cortical pathways. Somatosensory-evoked potentials may be characterized by "giant" responses, suggesting cortical hyperexcitability. An increased incidence of long QT intervals during electrocardiographic recordings and diminished heart-rate variability, suggesting impairment of the autonomic nervous system, are described in RS. With the discovery of the genetic basis of RS, neurophysiological studies will provide parameters for phenotype-genotype correlations and characterization of animal models.

Neurophysiology of Rett syndrome

Neurophysiological studies on Rett syndrome (RTT) are reviewed, and pathophysiology of RTT is discussed. The electroencephalography (EEG), sensory evoked potentials (SEP), sleep-wake rhythm study and polysomnography (PSG) study showed age-dependent characteristics. PSG revealed the brainstem and midbrain monoaminergic systems are deranged from early developmental stage, that is serotonin and noradrenaline systems seem to be hypoactive and dopaminergic system is also hypoactive associated with receptor supersensitivity. These monoaminergic systems are known to influence the maturation of the higher neuronal systems at specific areas and at specific ages. Particularly the synaptogenesis of the cerebral cortex is modulated by region or layer specifically from an early stage of the development. The observations made in EEG and SEP studies also suggested specific subcortical and cortical involvements taking place during the development. The age-dependent appearance of characteristic clinical features of RTT, and the variation of the clinical severities, e.g. classical, variant, form fruste, etc., can also be explained by the specific features of these monoaminergic systems. Furthermore, analysis of the components of rapid eye movement sleep suggested the onset of RTT lies between 36 gestational weeks to 3-4 months postnatally. The discovery of the mutations of methyl-CpG-binding protein 2 (MECP2) gene as the causative gene of RTT is an epoch helping not only to understand the pathophysiology of RTT but also various neurodevelopmental disorders.

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).

Auditory brainstem response findings in Rett syndrome: stability over time

OBJECTIVE

To determine whether changes consistent with the presence of white matter neurodegenerative disease would be observed in the auditory brainstem response (ABR) in Rett syndrome in conjunction with age advancement.

STUDY DESIGN

Initial and follow-up ABR findings were analyzed in a sample of 27 female patients with Rett syndrome. The interval between ABR tests ranged from 1 to 9 years.

RESULTS

No significant group differences consistent with neurodegenerative disease were observed in the wave I-III, III-V, or I-V interpeak latency intervals between the initial and follow-up test sessions.

CONCLUSIONS

Rett syndrome is not a disorder characterized by progressive white matter degeneration affecting the integrity of the brainstem central auditory pathways.

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.

Brainstem frequency-following responses in Rett syndrome

The brainstem frequency-following response (FFR) is a short-latency evoked response that reflects waveform properties of periodic auditory stimuli. Unlike neural activity evoked by transient stimuli, the FFR originates in phase-locked neurons that provide unique information concerning the early processing of auditory inputs. FFRs elicited by a pure tone were recorded from 9 Rett syndrome patients (age 26-55 years, mean = 34.4 years) and compared with those of 18 normal infants (age 2-10 months, mean = 5.0 months), and 113 young adult (age 18-30 years, mean = 22.2 years) controls. The Rett syndrome pattern indicated considerable intersubject latency variability and poor intrasubject repeat reliability except for brief FFR components which were consistently synchronized. The pattern observed in Rett syndrome was similar in certain respects to that observed in infants, but both patterns differed from those of adults, who showed larger amplitudes and consistent waveform synchrony. Clinical and neuropathologic data indicate developmental arrest rather than a neurodegenerative process in Rett syndrome. The present results are consistent with this interpretation. Neurophysiologic studies may identify markers that are distinctive in Rett syndrome and make it possible to monitor changes with age and disease process.

Neurophysiological observations on corticospinal projections to the upper limb in subjects with Rett syndrome

The aim of the present study was to investigate the excitability of corticospinal neurons and the integrity of their projections to the alpha motor neurons through the corticospinal tract in subjects of different ages with Rett syndrome. Electromagnetic stimulation of the motor cortex and cervical motor roots was used to evoke motor action potentials in the biceps brachii and hypothenar muscles. The phasic stretch reflex in the biceps brachii was also recorded to study the excitability of spinal alpha motor neurons. Motor cortex stimulation evoked motor action potentials at low threshold and with abnormally short latencies and prolonged durations. In contrast cervical motor root stimulation resulted in responses of normal latency and duration. The phasic stretch reflex had a low threshold, short latency and prolonged duration. It is concluded that in Rett syndrome the corticospinal pathway is intact. The results suggest disordered synaptic control of the Betz cell of the motor cortex and/or the spinal alpha motor neuron, although the involvement of the latter might be a consequence of dysfunction in supraspinal descending motor pathways.

Peripheral nerve involvement in the Rett syndrome

Clinical and neurophysiological studies were performed on 4 girls aged 9 to 19 years with the Rett syndrome (RS) (stages II and III). In addition to constant foot deformity (pes planus) and brisk tendon reflexes in the lower limbs, they showed muscular atrophy of the calves and mild atrophy of the extensor digitorum brevis muscle. Motor NCVs were within normal values for age, but distal latencies in two girls and compound action potential in one were abnormal, suggesting mild distal, predominantly motor neuropathy. The upper limbs showed nothing abnormal except for mild hand deformities. Neuropathology in 2 autopsy cases (aged 11 and 17 years, stage IV) and sural nerve biopsies in three girls aged 2, 3 and 17 years, all clinical stage II-III without inanition, revealed mild distal axonopathy without demyelination. Selected axons in sural nerve of a girl aged 17 and in the dental pulp of a girl aged 9, both at clinical stage II, showed increased numbers of neurofilaments. Skeletal muscle in two girls aged 11 and 17 years (both stage IV) exhibited mild alterations in the sarcoplasmic reticulum with occasional disordered myofibrils and tubular structures in the Z-filaments, but no neurogenic atrophy or abnormal mitochondria. The reported data do not support spinal denervation (motor neuron degeneration) in RS, but suggest late onset denervation due to mild distal axonopathy of hitherto unknown origin.

Cranial computed tomographic and magnetic resonance imaging studies on the Rett syndrome

We reported here 5 cases with the Rett syndrome (RS) who fulfilled the criteria for RS. The changes in CT and MRI findings with the clinical course were studied. Cortical atrophy especially in the frontal area, progressed gradually with the clinical progression after 2 years old in CT. In axial sections on MRI, frontal atrophy was also demonstrated. The white matter, basal ganglia, thalamus and hypothalamus area were all normal. In midline sagittal sections, corpus callosum was hypoplastic, and widening of prepontine cistern and narrowing of the brainstem were demonstrated in some cases. There was no significant atrophy of the cerebellum, amygdala and hippocampus. Follow-up studies including CT and MRI are necessary to evaluate brain morphology in RS.

The Rett syndrome: gross motor disability and neural impairment in adults

Profile and variation of gross motor disability and neural impairments were studied in a series of 30 women, aged 22-44 years, fulfilling the diagnostic criteria for the Rett syndrome (RS). The sequential development of neurological signs and a movement disorder causing immobility in 80% were found. On the basis of acquired and sustained walking ability, the women could be divided into three groups: one comprising those 20% still walking (group III), one those 60% previously walking (group IVA) and the third those 20% who had never developed walking ability (group IVB). Spastic signs, seldom prominent, were found in all the three groups, while dystonic signs were most common in those previously walking, and weakness and wasting in the group that never acquired that skill. Early progressive scoliosis, peroneal weakness and excavated feet, interpreted as lower motor neuron signs mainly due to spinal tract impairment, were most extensive among those never able to walk. Loss of walking was considered a consequence of deranging combinations of weakness and dystonia. The patterns of neuromotor disturbances are discussed in relation to other manifestations of RS.

Rett syndrome: clinical peculiarities, diagnostic approach, and possible cause

The presentation and diagnosis of Rett syndrome at various ages and stages are reviewed. In addition to the "classic" form of this disorder, variability of phenotype exemplified by atypical patients is also examined. Characteristic peculiar manifestations of the condition are compiled from the author's personal experiences over a 28 year period. Speculations and ideas are presented on possible alternative causative mechanisms in an attempt to explain the origin of this disorder.

Neurophysiological findings in the Rett syndrome, II: Visual and auditory brainstem, middle and late evoked responses

Nine girls with the Rett syndrome (RS) were investigated neurophysiologically using evoked potentials techniques. Visual- (VER) and auditory-evoked responses, including the early (ABR), middle (MLR) and late components (ACR), were recorded. There was evidence of variable, multilevel impairment of the nervous system. While ABR and MLR indicated lesions at the brainstem and midbrain levels, the late responses and VER pointed to an intra-cerebral/cortical defect. It is suggested that the perceptual mechanisms still functioned and some discrimination properties remained.

Neurophysiological findings in the Rett syndrome, I: EMG, conduction velocity, EEG and somatosensory-evoked potential studies

Nine girls, aged 10 to 22 years, with confirmed Rett syndrome--eight as stage IV and one at stage III--were investigated neurophysiologically. EMG and neurography studies were performed, and somatosensory-evoked responses (SER) were recorded as well as EEG with topographic mapping. Even in advanced clinical stages, no major motor root involvement or demyelinating motor peripheral neuropathy was detected on EMG or neurography, but an axonopathy, possibly of secondary origin, was observed. EEG showed slowing and dominance of low frequency activity of subcortical origin, with or without epileptic discharges. SER findings suggested involvement of the spinal cord and the spinothalamic system. Neurophysiological investigations can be used by the clinician in the differential diagnosis of the Rett syndrome.

Rett syndrome: spinal cord neuropathology

The morphologic changes of the spinal cord in Rett syndrome are described in 2 young women who died at 20 and 30 years of age. Both patients had been in a severely disabled state for many years with tetraparesis and extreme muscle wasting. Degeneration and loss of spinal ganglion nerve cells, in addition to gliosis of both the white and gray matter of the spinal cord, were evident. The number of motor neurons appeared to be reduced and axonal changes suggestive of degeneration were observed in both the ascending and descending tracts.

The clinical recognition and differential diagnosis of Rett syndrome

Rett syndrome (RS) is characterized by progressive loss of intellectual functioning and fine and gross motor skills as well as development of stereotypic hand movement abnormalities, occurring after 6 to 18 months of normal development. Rett syndrome has been previously reported only in girls, but the possibility of the syndrome existing in male children cannot be currently excluded. Although the syndrome is thought to be relatively common, it was only described in the English literature 5 years ago. There is currently no marker for the syndrome; diagnosis is based on clinical criteria. The newly developed diagnostic criteria for RS are reviewed, with special attention given to the historical aspects of the diagnosis in the prenatal, perinatal, neonatal, and early childhood periods. Rett syndrome is characterized by a predictable, orderly progression of signs and symptoms. Four stages of RS have been described; each stage has special characteristics and offers different diagnostic challenges for the neurologist. Infantile autism is the most common incorrect diagnosis made for children with RS. The simultaneous regression of both motor and language skills, as well as the stereotypic hand movements, hyperventilation, bruxism, and seizures in early childhood are all typical in RS and help distinguish RS from infantile autism.

Peripheral nerve findings in Rett syndrome

Sural nerve and peroneus brevis muscle biopsies were studied in 12 patients with Rett syndrome, ten with the typical form of the disorder according to 1985 criteria, and two with atypical features. Ages ranged from 23 months to 25 years. All stages of the disease were represented. There was evidence of a mild axonal neuropathy in seven of 12 patients. Degenerative and occasional regenerative changes were seen in five sural nerve biopsies, including one from the youngest patient in the series, who was normally nourished and fully ambulatory. Occasional nonspecific ultrastructural abnormalities were present, including accumulation of pi granules in Schwann cells and Hirano bodies within axons. However, morphometric analysis of the four nerves in which these alterations were most prominent showed a normal density and size distribution of myelinated fibers. Enzyme histochemistry of the peroneus brevis biopsies demonstrated abnormal predominance of type II muscle fibers in three of the 12 biopsies and atrophy of type I fibers in one patient.

Rett syndrome: epidemiology and nosology--progress in knowledge 1986--a conference communication

Recent progress from pooled clinical experience is reviewed. The approximate number of documented cases of Rett syndrome (RS) as of October 1986 was 1,100. Three sister pairs, 3 monozygotic twin pairs (both twin girls affected), 3 dizygotic twin pairs (one twin girl affected) are known. Genetics are discussed based on these and other known occurrences. Early seizure-onset variants have been reported in a dozen cases representing various countries. Approximately 20 incomplete RS related cases, also termed "formes frustes," are documented as of October 1986. An actual Swedish series of 66 classical RS, 4 "formes frustes," 2 early seizure-onset variants and 9 other clinically near related atypical cases is summarized. A suggested late stage IV spinal cord impairment is discussed, as well as the whole adult multifacetted complex neurological pattern. Gastrointestinal pathology is discussed with reference to possible generalized neuropeptide disturbances.