Possible hyperendorphinergic pathophysiology of the Rett syndrome

Rett Syndrome: The Emerging Landscape of Treatment Strategies

Rett syndrome (RTT) has enjoyed remarkable progress in achieving specific therapies. RTT, a unique neurodevelopmental disorder first described in 1966, progressed slowly until the landmark paper of Hagberg and colleagues in 1983. Thereafter, rapid advances were achieved including the development of specific diagnostic criteria and the active search for a genetic etiology, resulting 16 years later in identification of variants in the methyl-CpG-binding protein (MECP2) gene located at Xq28. Shortly thereafter, the NIH Office of Rare Diseases funded the RTT Natural History Study (NHS) in 2003, initiating the acquisition of natural history data on clinical features from a large population of individuals with RTT. This information was essential for advancement of clinical trials to provide specific therapies for this disorder. In the process, the International Rett Syndrome Association (IRSA) was formed (now the International Rett Syndrome Foundation-IRSF), which participated directly in encouraging and expanding enrollment in the NHS and, subsequently, in developing the SCOUT program to facilitate testing of potential therapeutic agents in a mouse model of RTT. The overall objective was to review clinical characteristics developed from the NHS and to discuss the status of specific therapies for this progressive neurodevelopmental disorder. The NHS study provided critical information on RTT: growth, anthropometrics, longevity, key comorbidities including epilepsy, breath abnormalities, gastroesophageal dysfunction, scoliosis and other orthopedic issues, puberty, behavior and anxiety, and progressive motor deterioration including the appearance of parkinsonian features. Phenotype-genotype correlations were noted including the role of X chromosome inactivation. Development of clinical severity and quality of life measures also proved critical for subsequent clinical trials. Further, development of biochemical and neurophysiologic biomarkers offered further endpoints for clinical trials. Initial clinical trials prior to the NHS were ineffective, but advances resulting from the NHS and other studies worldwide promoted significant interest from pharmaceutical firms resulting in several clinical trials. While some of these have been unrewarding such as sarizotan, others have been quite promising including the approval of trofinetide by the FDA in 2023 as the first agent available for specific treatment of RTT. Blarcamesine has been trialed in phase 3 trials, 14 agents have been studied in phase 2 trials, and 7 agents are being evaluated in preclinical/translational studies. A landmark study in 2007 by Guy et al. demonstrated that activation of a normal MECP2 gene in a null mouse model resulted in significant improvement. Gene replacement therapy has advanced through translational studies to two current phase 1/2 clinical trials (Taysha102 and Neurogene-401). Additional genetic therapies are also under study including gene editing, RNA editing, and X-chromosome reactivation. Taken together, progress in understanding and treating RTT over the past 40 years has been remarkable. This suggests that further advances can be expected.

Clinical trials and treatment prospects

Prospects for definitive therapeutic intervention for Rett syndrome (RS) have been elevated by the discovery of mutations in the methyl-CpG-binding protein 2 gene (MECP2) in more than 80% of females meeting clinical criteria for this disorder. As such, a review of previous clinical trials, descriptions of the status of clinical management for the prominent medical problems of RS, and a preview of an ongoing clinical trial conducted jointly at the Baylor College of Medicine and the University of Alabama at Birmingham are presented. The conduct of controlled clinical trials requires adherence to diagnostic criteria for RS; stratification by age, stage, and presence of MECP2 mutations; and use of clearly defined outcome measures. Previous clinical trials in RS have been conducted with L-carnitine, the ketogenic diet, and the opiate antagonist, naltrexone. The L-carnitine and naltrexone trials were double blind, placebo-controlled and us ed the motor behavioral analysis described in this review. All failed to provide evidence of dramatic improvements in the clinical features of RS. Specific recommendations are presented for clinical management of growth failure, breathing irregularities, seizures, ambulation, scoliosis, gastrointestinal function, self abuse, and habilitation/education although systematic evaluations of each in the context of RS have not been conducted. The only ongoing trial involves dietary supplementation with folate and betaine and is based on the finding that gene expression of some alleles of the agouti gene could be altered by dietary methyl supplementation. The availability of animal models expressing mutations in MECP2 should enhance the evaluation of innovative therapies for RS.

Reduced expression of neuropeptides can be related to respiratory disturbances in Rett syndrome

We immunohistochemically examined neurotransmitter systems, which function in the brainstem and are involved in neuronal organization of respiration, in an autopsy brain from a patient with Rett syndrome (RS). Immunoreactivity (IR) for tyrosine hydroxylase, a functional marker for catecholaminergic neurons, was severely reduced in the locus ceruleus, while that for tryptophan hydroxylase involved in serotonin synthesis was spared in the raphe nuclei. In the brainstem, IR for substance P (SP) was reduced in the parabrachial complex and that for methionine-enkephalin (met-enk) was affected in the parabrachial, hypoglossal, dorsal vagal and solitary nuclei. In addition, expressions of these neuropeptides were also disturbed in the basal ganglia. A widespread altered expression of antagonistic neuropeptides, SP and met-enk, may be involved in the pathogenesis of RS, especially in its respiratory manifestation.

Rett syndrome: neurobiological changes underlying specific symptoms

Rett syndrome (RS) is a progressive disorder that is predominant in females. It is associated with cortical atrophy, stereotyped hand movements mimicking hand-washing, severe mental deficiency, and cortical and extrapyramidal dysfunction. The cause of RS is unknown; no consistent genetic abnormalities, at either the cellular or mitochondrial levels, have been identified. The diagnosis still depends solely upon clinical evidence. The clinical progression of RS is consistent with an arrested neuronal development that may be due to either impaired cellular differentiation or the lack of appropriate trophic factors. Neuropathological studies have confirmed (1) a generalized brain atrophy involving the cerebrum and cerebellum; (2) a decrease in neuronal cell size and increased cell packing density throughout the brain; (3) a reduction in the number of basal forebrain cholinergic neurons; (4) a reduction in the concentration of melanin-containing neurons in the substantia nigra. Biochemical studies have identified (1) a decrease in cholinergic markers in the neocortex, hippocampus, thalamus and basal ganglia; (2) inconsistent and variable changes in biogenic amine biomarkers in post-mortem tissues and cerebrospinal fluid (CSF); (3) an elevation of beta-endorphin levels in the thalamus and glutamate levels in the CSF; (4) no evidence for mitochondrial dysfunction. These data suggest that there is a primary deficit in cholinergic function that might underlie some of the higher cognitive impairments and extrapyramidal dysfunction. Overall, the clinical, biochemical and neuropathological data suggest that RS is a neurodevelopmental disorder that has its greatest effects upon a limited number of neural systems during the first few years of postnatal life.

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.

Rett syndrome: controlled study of an oral opiate antagonist, naltrexone

HYPOTHESIS

The opiate antagonist, naltrexone, will be beneficial in Rett syndrome.

SUBJECTS

Twenty-five individuals fulfilling the criteria for Rett syndrome.

METHOD

Randomized, double-blind, placebo-controlled crossover trial with two treatment periods, 4 months each, and an intervening 1-month washout period. Clinical stage, motor and cognitive development, motor-behavioral analysis, neurophysiological parameters (computerized electroencephalographic analysis, breathing characteristics, quantification of stereotyped hand movements, and sleep characteristics), and cerebrospinal fluid beta-endorphin measurements were evaluated at baseline and at the end of each treatment period.

RESULTS

Only data from the first period of this study were analyzed due to significant sequence effects in the crossover design. This analysis indicated positive effects on certain respiratory characteristics including decreased disorganized breathing during wakefulness. Four (40%) of the individuals receiving naltrexone progressed one or more clinical stages versus none of the individuals receiving placebo. The adjusted (for baseline value and Rett stage) end of treatment psychomotor test age (Bayley Scales) was significantly higher for the placebo group. There was no significant change for the other parameters.

CONCLUSION

Naltrexone may modify some of the respiratory disturbance in Rett syndrome. Declines in motor function and more rapid progression of the disorder suggest a deleterious effect.

Neurochemical alterations in Rett syndrome

The Rett syndrome (RS) is a neurological disorder associated with severe mental deficiency and neurological manifestations of cortical and extrapyramidal dysfunction. The present report is (1) a postmortem brain study that compares the levels of choline acetyltransferase (ChAT) activity and the binding density of selected neurotransmitter receptors in four cases of RS and five normal controls of similar age and (2) a study of cerebrospinal fluid (CSF) concentrations of the endogenous tridecapeptide neurotensin in 12 RS patients and 8 controls of similar age. The level of ChAT activity was lower in many cortical and subcortical regions in the RS brains as compared to control levels. The number of NMDA, AMPA, mu opioid and neurotensin binding sites, as well as CSF concentrations of neurotensin, did not differ significantly from control levels. The results suggest that changes in specific neurotransmitter systems, particularly cholinergic neurons, in the thalamus, cerebellum and basal ganglia may underlie the progressive deterioration in motor and cognitive function characteristic of this disorder.

Cerebrospinal fluid beta-endorphin in Rett syndrome

An hypothesis of increased endorphinergic activity has been proposed to account for the characteristic symptoms of Rett syndrome. Cerebrospinal fluid samples from eight girls with Rett syndrome were analysed for beta-endorphin (beta-EP) immunoactivity and compared with samples from a control group of 15 children with acute leukaemia in remission. Severity of symptoms was not found to be related to beta-EP level. A group of early-treated adolescents with phenylketonuria had beta-EP levels similar to the Rett syndrome patients, but no symptoms resembling theirs. Therefore it is unlikely that increased levels of beta-EP are of primary pathogenetic significance. The conflicting findings of many earlier reports may be a result of differences between control groups.

Adult and developing human cerebella exhibit different profiles of opioid binding sites

The binding of [3H][D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO), [3H][D-Pen2,5]enkephalin (DPDPE), [3H]ethylketocyclazocine (EKC), and [3H][Met5]enkephalin (MET) was used to examine mu-, delta-, kappa-, and zeta-receptors, respectively, in the developing (birth to postnatal day 19) and adult human cerebellum. Specific and saturable binding of all ligands was recorded in developing brains, and of [3H]DAGO, [3H]DPDPE, and [3H]EKC in adult cerebellum; all data fit a single homogeneous binding site for each ligand. However, the ontogenic profile of opioid receptor subtypes differed. Delta- and kappa-receptor capacities were 7.8- and 3.6-fold, respectively, greater in infant cerebellum than in adults. The mu-receptor decreased over 7-fold in both binding affinity and capacity after day 2; by adulthood, the binding affinity was the same as in newborns but only one-half the binding capacity was recorded. The concentration of zeta-receptors was 20-fold greater in subjects 2-19 days of age than in newborns. These data demonstrate the presence, and distinct developmental profiles, of opioid receptors in human cerebellum. Although the function of mu-, delta-, and kappa-receptors in human cerebellum are unclear, the growth-related zeta-receptor is present at a time of cell replication and differentiation but is not detected in mature cerebellum.

Naltrexone therapy of apnea in children with elevated cerebrospinal fluid beta-endorphin

Previous studies have indicated increased immunoreactivity of the endogenous opioid peptide beta-endorphin in the cerebrospinal fluid (CSF) of infants under 2 years of age with apnea. To assess the role of endogenous opioids in the pathogenesis of apnea in children, the effect of oral treatment with the opioid antagonist naltrexone was studied in apneic infants, as well as in older apneic children, with demonstrated increases in CSF immunoreactive beta-endorphin (i-BE). In the 8 apneic infants with elevated i-BE in lumbar CSF (range, 55-155 pg/ml; normal, 17-52 pg/ml), no further apnea occurred during naltrexone therapy (1 mg/kg/day, by mouth). Five children (2-8 years old) with apnea of unknown cause had elevated CSF i-BE (range, 74-276 pg/ml) compared to 6 age-matched nonapneic children (range, 15-48 pg/ml). No apneic events occurred during naltrexone therapy, except in 1 child during stressful events, but apnea recurred in some patients after attempts to discontinue naltrexone treatment. Adverse effects of naltrexone included complaints of headaches in 2 children and symptoms of a narcotic withdrawal syndrome during the first 3 days of treatment in 1 child. Three children with Leigh's syndrome had elevated CSF i-BE (range, 104-291 pg/ml) and their apnea also responded to naltrexone. We conclude that elevated endogenous opioids contribute to the pathogenesis of apnea in children and may even result in physical dependence.