Importance of Rett syndrome in child neurology

Using Precision Medicine to Disentangle Genotype-Phenotype Relationships in Twins with Rett Syndrome: A Case Report

Rett syndrome (RTT) is a paediatric neurodevelopmental disorder spanning four developmental stages. This multi-system disorder offers a unique window to explore genotype-phenotype relationships in a disease model. However, genetic prognosticators of RTT have limited clinical value due to the disorder's heterogeneity on multiple levels. This case report used a precision medicine approach to better understand the clinical phenotype of RTT twins with an identical pathogenic MECP2 mutation and discordant neurodevelopmental profiles. Targeted genotyping, objective physiological monitoring of heart rate variability (HRV) parameters, and clinical severity were assessed in a RTT twin pair (5 years 7 months old) with an identical pathogenic MECP2 mutation. Longitudinal assessment of autonomic HRV parameters was conducted using the Empatica E4 wristband device, and clinical severity was assessed using the RTT-anchored Clinical Global Impression Scale (RTT-CGI) and the Multi-System Profile of Symptoms Scale (MPSS). Genotype data revealed impaired BDNF function for twin A when compared to twin B. Twin A also had poorer autonomic health than twin B, as indicated by lower autonomic metrics (autonomic inflexibility). Hospitalisation, RTT-CGI-S, and MPSS subscale scores were used as measures of clinical severity, and these were worse in twin A. Treatment using buspirone shifted twin A from an inflexible to a flexible autonomic profile. This was mirrored in the MPSS scores, which showed a reduction in autonomic and cardiac symptoms following buspirone treatment. Our findings showed that a combination of a co-occurring rs6265 BDNF polymorphism, and worse autonomic and clinical profiles led to a poorer prognosis for twin A compared to twin B. Buspirone was able to shift a rigid autonomic profile to a more flexible one for twin A and thereby prevent cardiac and autonomic symptoms from worsening. The clinical profile for twin A represents a departure from the disorder trajectory typically observed in RTT and underscores the importance of wider genotype profiling and longitudinal objective physiological monitoring alongside measures of clinical symptoms and severity when assessing genotype-phenotype relationships in RTT patients with identical pathogenic mutations. A precision medicine approach that assesses genetic and physiological risk factors can be extended to other neurodevelopmental disorders to monitor risk when genotype-phenotype relationships are not so obvious.

A transcranial magnetic stimulation study for the characterization of corticospinal pathway plasticity in children with neurological disorders

We aimed to investigate cortical and radicular TMS-evoked motor evoked potentials (MEPs) in children with neurological disorders (n = 57, mean age: 5.45 years) and agematched healthy controls (n = 46). Four TMS parameters were analyzed: MEP amplitudes, the latencies of MEP, the latency jump (cortical MEP latency at rest - cortical active-MEP latency at with slightly contracted targeted muscle), and central motor conduction time. Children with neurological disorders were categorized according to the two major types of neuronal plasticity; excessive plasticity: 29 children with cerebral palsy and impaired plasticity: 28 children with neurodegenerative diseases, stroke, and central nervous system infections. The active-MEP abnormalities (absent and prolonged latencies) were correlated with the location of cortical involvement on MRI patterns. We obtained a significantly increased rate of abnormal cortical active-MEPs in children with impaired plasticity (21/28, 75%) compared with excessive plasticity (18/29, 62%). The rate of absent MEP response is three times more in children with impaired plasticity (43%) than in children with excessive plasticity (14%). A more reduced latency jump was measured in children with impaired plasticity compared to children with excessive plasticity. TMS-evoked active-MEPs and latency jumping are valuable parameters for characterizing neuronal plasticity in children with neurological disorders.

Substantial acetylcholine reduction in multiple brain regions of Mecp2-deficient female rats and associated behavioral abnormalities

Rett syndrome (RTT) is a neurodevelopmental disorder with X-linked dominant inheritance caused mainly by mutations in the methyl-CpG-binding protein 2 (MECP2) gene. The effects of various Mecp2 mutations have been extensively assessed in mouse models, but none adequately mimic the symptoms and pathological changes of RTT. In this study, we assessed the effects of Mecp2 gene deletion on female rats (Mecp2^+/−) and found severe impairments in social behavior [at 8 weeks (w), 12 w, and 23 w of age], motor function [at 16 w and 26 w], and spatial cognition [at 29 w] as well as lower plasma insulin-like growth factor (but not brain-derived neurotrophic factor) and markedly reduced acetylcholine (30%–50%) in multiple brain regions compared to female Mecp2^+/+ rats [at 29 w]. Alternatively, changes in brain monoamine levels were relatively small, in contrast to reports on mouse Mecp2 mutants. Female Mecp2-deficient rats express phenotypes resembling RTT and so may provide a robust model for future research on RTT pathobiology and treatment.

Are dopamine receptor and transporter changes in Rett syndrome reflected in Mecp2-deficient mice?

We tested the claim that the dopaminergic dysfunction of Rett Syndrome (RTT) also occurs in Mecp2-deficient mice that serve as a model of the syndrome. We used positron emission tomography (PET) to image dopamine D₂ receptors (D₂R) and transporters (DAT) in women with RTT and in Mecp2-deficient mice, and D₁R and D₂R density was measured in postmortem human tissue by autoradiography. Results showed 1) significantly reduced D₂R density in the striatum of women with RTT compared to control subjects. 2) PET imaging of mouse striatum similarly demonstrated significant reductions in D₂R density of 7-10 week-old hemizygous (Mecp2-null) and heterozygous (HET) mice compared to wild type (WT) mice. With age, the density of D₂R declined in WT mice but not HET mice. 3) In contrast, postmortem autoradiography revealed no group differences in the density of D₁R and D₂R in the caudate and putamen of RTT versus normal control subjects. 4) In humans and in the mouse model, PET revealed only marginal group differences in DAT. The results confirm that dopaminergic dysfunction in RTT is also present in Mecp2-deficient mice and that reductions in D₂R more likely explain the impaired ambulation and progressive rigidity observed rather than alterations in DAT.

Effect of Serotonin 1A Agonists and Selective Serotonin Reuptake Inhibitors on Behavioral and Nighttime Respiratory Symptoms in Rett Syndrome

BACKGROUND

Rett syndrome is characterized by psychomotor regression during early childhood, autistic-like behaviors, and aberrant breathing patterns. Dysfunction of the serotonergic system has been postulated to play a role in the pathophysiology of these symptoms.

PATIENT DESCRIPTION

We present an 11-year-old girl with Rett syndrome who exhibited marked respiratory symptoms, including frequent apneic events during sleep. She had been treated for these respiratory symptoms using noninvasive positive pressure ventilation since age six years. Treatment with serotonin 1A receptor agonist was initiated at age eight years, whereas treatment using a selective serotonin reuptake inhibitor began at age nine years. Noninvasive positive pressure ventilation therapy was effective in reducing symptoms of sleep apnea, and administration of serotonergic agents resulted in amelioration of sleep apneic events even in the absence of noninvasive positive pressure ventilation. In addition, improvements in hand stereotypy and social skills were observed after initiation of serotonin-based therapy.

DISCUSSION

The respiratory difficulties our patient experienced during non-rapid eye movement (REM) sleep are characteristic of post-sigh central apnea. Exaggerated activity of expiratory neurons during such apneic events has been observed in mouse models of Rett syndrome. We suggest that prescribed serotonergic agents might serve to inhibit such activity, attenuating the imbalance between inspiratory and expiratory neurons. These agents might also be useful in the treatment of autistic-like behaviors caused by impaired serotonergic transmission in the brain.

Impact of Rett Syndrome Mutations on MeCP2 MBD Stability

Rett syndrome causing missense mutations in the methyl-CpG-binding domain (MBD) of methyl CpG-binding protein 2 (MeCP2) were investigated both in silico and in vitro to reveal their effect on protein stability. It is demonstrated that the vast majority of frequently occurring mutations in the human population indeed alter the MBD folding free energy by a fraction of a kcal/mol up to more than 1 kcal/mol. While the absolute magnitude of the change of the free energy is small, the effect on the MBD functionality may be substantial since the folding free energy of MBD is about 2 kcal/mol only. Thus, it is emphasized that the effect of mutations on protein integrity should be evaluated with respect to the wild-type folding free energy but not with the absolute value of the folding free energy change. Furthermore, it was observed that the magnitude of the effect is correlated neither with the burial of the mutation sites nor with the basic amino acid physicochemical property change. Mutations that strongly perturb the immediate structural features were found to have little effect on folding free energy, while very conservative mutations resulted in large changes of the MBD stability. This observation was attributed to the protein's ability to structurally relax and reorganize to reduce the effect of mutation. Comparison between in silico and in vitro results indicated that some Web servers perform relatively well, while the free energy perturbation approach frequently overpredicts the magnitude of the free energy change especially when a charged amino acid is involved.

Structural, Dynamical, and Energetical Consequences of Rett Syndrome Mutation R133C in MeCP2

Rett Syndrome (RTT) is a progressive neurodevelopmental disease affecting females. RTT is caused by mutations in the MECP2 gene and various amino acid substitutions have been identified clinically in different domains of the multifunctional MeCP2 protein encoded by this gene. The R133C variant in the methylated-CpG-binding domain (MBD) of MeCP2 is the second most common disease-causing mutation in the MBD. Comparative molecular dynamics simulations of R133C mutant and wild-type MBD have been performed to understand the impact of the mutation on structure, dynamics, and interactions of the protein and subsequently understand the disease mechanism. Two salt bridges within the protein and two critical hydrogen bonds between the protein and DNA are lost upon the R133C mutation. The mutation was found to weaken the interaction with DNA and also cause loss of helicity within the 141-144 region. The structural, dynamical, and energetical consequences of R133C mutation were investigated in detail at the atomic resolution. Several important implications of this have been shown regarding protein stability and hydration dynamics as well as its interaction with DNA. The results are in agreement with previous experimental studies and further provide atomic level understanding of the molecular origin of RTT associated with R133C variant.

Quantification of functional abilities in Rett syndrome: a comparison between stages III and IV

We aimed to evaluate the functional abilities of persons with Rett syndrome (RTT) in stages III and IV. The group consisted of 60 females who had been diagnosed with RTT: 38 in stage III, mean age (years) of 9.14, with a standard deviation of 5.84 (minimum 2.2/maximum 26.4); and 22 in stage IV, mean age of 12.45, with a standard deviation of 6.17 (minimum 5.3/maximum 26.9). The evaluation was made using the Pediatric Evaluation of Disability Inventory, which has 197 items in the areas of self-care, mobility, and social function. The results showed that in the area of self-care, stage III and stage IV RTT persons had a level of 24.12 and 18.36 (P=0.002), respectively. In the area of mobility, stage III had 37.22 and stage IV had 14.64 (P<0.001), while in the area of social function, stage III had 17.72 and stage IV had 12.14 (P=0.016). In conclusion, although persons with stage III RTT have better functional abilities when compared with stage IV, the areas of mobility, self-care, and social function are quite affected, which shows a great functional dependency and need for help in basic activities of daily life.

[Rett syndrome: double epidural catheter for the control of postoperative pain after scoliosis surgery. A literature review]

Rett syndrome is a severe and incapacitating neurological disease caused by a structural defect in the short arm of the X chromosome (Xq28). It affects females and consists of multiple and progressive neurological impairments that start from a young age, leading to lifelong disability and dependency. Scoliosis appears in more than 50% of patients and requires surgical correction in cases where the curvature is severe. Pre-anaesthetic assessment is essential in order to identify the risk factors and thus reduce the morbidity and mortality associated with the surgical procedure. We present the case of a patient affected by this syndrome and scoliosis, who was scheduled to have an instrumented thoracolumbar spine arthrodesis with general anaesthesia, which passed without incident. We evaluate the specific details of this syndrome, its potential complications, and its management from an anaesthetic point of view, emphasising the control of postoperative pain using a double epidural catheter with an infusion of local anaesthetics and fentanyl.

Use of buspirone and fluoxetine for breathing problems in Rett syndrome

Rett syndrome is a severe neurodevelopmental disease with a prevalence of 0.88 per 10,000 girls aged 5-18 years, and is often caused by mutations in methyl-cytosine-phosphate-guanine (CpG)-binding protein 2. Disorder of respiratory control is a prominent feature of Rett syndrome. Brainstem serotoninergic neurons are known to play an important role in the arrangement of breathing rhythm and pattern. We present a patient whose severe hyperventilation and apneic attacks resolved with the concomitant treatment of fluoxetine and buspirone. To our knowledge, we describe the first patient with Rett syndrome to receive fluoxetine for respiratory problems.

Linking epigenetics to human disease and Rett syndrome: the emerging novel and challenging concepts in MeCP2 research

Epigenetics refer to inheritable changes beyond DNA sequence that control cell identity and morphology. Epigenetics play key roles in development and cell fate commitments and highly impact the etiology of many human diseases. A well-known link between epigenetics and human disease is the X-linked MECP2 gene, mutations in which lead to the neurological disorder, Rett Syndrome. Despite the fact that MeCP2 was discovered about 20 years ago, our current knowledge about its molecular function is not comprehensive. While MeCP2 was originally found to bind methylated DNA and interact with repressor complexes to inhibit and silence its genomic targets, recent studies have challenged this idea. Indeed, depending on its interacting protein partners and target genes, MeCP2 can act either as an activator or as a repressor. Furthermore, it is becoming evident that although Rett Syndrome is a progressive and postnatal neurological disorder, the consequences of MeCP2 deficiencies initiate much earlier and before birth. To comprehend the novel and challenging concepts in MeCP2 research and to design effective therapeutic strategies for Rett Syndrome, a targeted collaborative effort from scientists in multiple research areas to clinicians is required.

Transcranial magnetic stimulation in children

Developmental disabilities (e.g. attention deficit disorder; cerebral palsy) are frequently associated with deviations of the typical pattern of motor skill maturation. Neurophysiologic tools, such as transcranial magnetic stimulation (TMS), which probe motor cortex function, can potentially provide insights into both typical neuromotor maturation and the mechanisms underlying the motor skill deficits in children with developmental disabilities. These insights may set the stage for finding effective interventions for these disorders. We review the literature pertaining to the use of TMS in pediatrics. Most TMS-evoked parameters show age-related changes in typically developing children and some of these are abnormal in a number of childhood-onset neurological disorders. Although no TMS-evoked parameters are diagnostic for any disorder, changes in certain parameters appear to reflect disease burden or may provide a measure of treatment-related improvement. Furthermore, TMS may be especially useful when combined with other neurophysiologic modalities (e.g. fMRI). However, much work remains to be done to determine if TMS-evoked parameters can be used as valid and reliable biomarkers for disease burden, the natural history of neurological injury and repair, and the efficacy of pharmacological and rehabilitation interventions.

The transcriptional repressor Mecp2 regulates terminal neuronal differentiation

Rett syndrome (RTT) is a severe neurodevelopmental disorder with features of autism that results from mutation of the gene encoding the transcriptional repressor methyl-CpG binding protein (MECP2). The consequences of loss of a transcription factor may be complex, affecting the expression of many proteins, thus limiting understanding of this class of diseases and impeding therapeutic strategies. This is true for RTT. Neither the cell biological mechanism(s) nor the developmental stage affected by MECP2 deficiency is known. In vivo analysis of the olfactory system demonstrates that Mecp2 deficiency leads to a transient delay in the terminal differentiation of olfactory neurons. This delay in maturation disrupts axonal targeting in the olfactory bulb, resulting in abnormal axonal projections, subglomerular disorganization, and a persistent reduction in glomerular size. These results indicate a critical cell biological function for Mecp2 in mediating the final stages of neuronal development.

Expression of MeCP2 in olfactory receptor neurons is developmentally regulated and occurs before synaptogenesis

Rett syndrome, a neurodevelopmental disorder hypothesized to be due to defective neuronal maturation, is a result of mutations in the mecp2 gene encoding the transcriptional repressor methyl-CpG binding protein (MeCP2). We utilized the olfactory system, which displays postnatal neurogenesis, as a model to investigate MeCP2 expression during development and after injury. MeCP2 expression increased postnatally, localizing to mature olfactory receptor neurons (ORNs) and sustentacular supporting cells. The timing of MeCP2 expression was defined by using detergent ablation (to remove the ORNs) and unilateral olfactory bulbectomy (to remove the ORN target), both of which increase neurogenesis. MeCP2 expression in the ORNs reached prelesioning levels as cells matured after ablation, whereas expression was not completely restored after bulbectomy, in which functional synaptogenesis cannot occur. Thus, MeCP2 expression correlates with the maturational state of ORNs, and precedes synaptogenesis. Identifying the time window of MeCP2 expression should help further clarify the biological defects in Rett syndrome.

Rett syndrome: investigation of nine patients, including PET scan

BACKGROUND

We describe nine females with Rett Syndrome (RS), aged 14 to 26 years. All had had developmental delay before the end of their first year and had subsequently regressed to profound dementia with apraxia, ataxia, irregular respirations and often also seizures.

METHODS

The Revised Gesell developmental assessment and Alpern-Boll Developmental Profile were used in modified form. Volumetric measurements of basal ganglia using MRI were compared with the findings in nine age-matched volunteer females. Positron emission scans with [18F]-6-fluorodopa and [11C]-raclopride were performed under light anesthesia with intravenous Propofol, and the findings were compared with those in healthy control girls. Bidirectional sequencing of the coding regions of the MECP2 gene was investigated in blood samples for mutational analyses.

RESULTS

The RS females functioned at a mental age level ranging from about 4 to 15 months. The scores correlated with height, weight and head circumference. Magnetic resonance scans of basal ganglia showed a significant reduction in the size of the caudate heads and thalami in the Rett cases. Positron emission scans demonstrated that the mean uptake of fluorodopa in RS was reduced by 13.1% in caudate and by 12.5% in putamen as compared to the controls, while dopamine D2 receptor binding was increased significantly by 9.7% in caudate and 9.6% in putamen. Mutations in the coding regions of the MECP2 gene were present in all nine patients. No significant correlation between type and location of mutation and volumetric changes or isotope uptake was demonstrable.

CONCLUSIONS

Our findings suggest a mild presynaptic deficit of nigrostriatal activity in Rett syndrome.

Discussant--pathophysiologies of Rett syndrome

Studies on sleep parameters of Rett syndrome revealed hypoactivity of the noradrenaline (NA) and the serotonin (5HT) neuron in early infancy while preserving the function of the dopamine (DA) and the cholinergic neurons of the pons normally. The sleep-wake cycle remains in its development at the level of 4 months of age. Polysomnographies also showed a decrease of the function of the nigrostriatal (NS)-DA neuron in early childhood and suggested the development of receptor supersensitivity in late childhood. Neurohistochemical and neuroimaging (PET) studies revealed the hypofunction of the NS-DA neuron with receptor supersensitivity and of involvement of the cholinergic neurons to the cortical pathology, whereas no substantial pathological or histochemical abnormalities were observed in the NA and the 5HT neurons in the brainstem. The decrease of tyrosine hydroxylase without neurodegenerative changes observed in the substantia nigra of Rett syndrome had similarity to the pathology caused by excitotoxic lesion of the pedunculopontine nuclei (PPN) observed in an animal experiments. Clinically the grade of disability of locomotion was shown to correlate to the grade of the disabilities of language. These clinical manifestations were also correlated to the specific loci of the mutation in the methyl binding domain of the MECP2 gene. In rodents the axons of the brainstem 5HT neuron involved in the morphogenesis of the brain in the early developmental course disappear in neonates without apoptotic or degenerative changes in the neurons. This period corresponds to the first 1.5-2 years in humans. Thus, in Rett syndrome, the primary lesion appears in the brainstem NA and 5HT neurons which affects development of synaptogenesis of the cortex and also dysfunction of the PPN. The latter causes dysfunction of the DA neuron and the cholinergic neuron in the midbrain. The mutation of the MECP2 gene may cause early transcription of the genes which prune the axons of the aminergic neurons for the developmental morphogenesis of the central nervous system in early infancy.