Development of language in Rett syndrome

Toward the Language Oscillogenome

Language has been argued to arise, both ontogenetically and phylogenetically, from specific patterns of brain wiring. We argue that it can further be shown that core features of language processing emerge from particular phasal and cross-frequency coupling properties of neural oscillations; what has been referred to as the language ‘oscillome.’ It is expected that basic aspects of the language oscillome result from genetic guidance, what we will here call the language ‘oscillogenome,’ for which we will put forward a list of candidate genes. We have considered genes for altered brain rhythmicity in conditions involving language deficits: autism spectrum disorders, schizophrenia, specific language impairment and dyslexia. These selected genes map on to aspects of brain function, particularly on to neurotransmitter function. We stress that caution should be adopted in the construction of any oscillogenome, given the range of potential roles particular localized frequency bands have in cognition. Our aim is to propose a set of genome-to-language linking hypotheses that, given testing, would grant explanatory power to brain rhythms with respect to language processing and evolution.

Language Impairments in ASD Resulting from a Failed Domestication of the Human Brain

Autism spectrum disorders (ASD) are pervasive neurodevelopmental disorders entailing social and cognitive deficits, including marked problems with language. Numerous genes have been associated with ASD, but it is unclear how language deficits arise from gene mutation or dysregulation. It is also unclear why ASD shows such high prevalence within human populations. Interestingly, the emergence of a modern faculty of language has been hypothesized to be linked to changes in the human brain/skull, but also to the process of self-domestication of the human species. It is our intention to show that people with ASD exhibit less marked domesticated traits at the morphological, physiological, and behavioral levels. We also discuss many ASD candidates represented among the genes known to be involved in the “domestication syndrome” (the constellation of traits exhibited by domesticated mammals, which seemingly results from the hypofunction of the neural crest) and among the set of genes involved in language function closely connected to them. Moreover, many of these genes show altered expression profiles in the brain of autists. In addition, some candidates for domestication and language-readiness show the same expression profile in people with ASD and chimps in different brain areas involved in language processing. Similarities regarding the brain oscillatory behavior of these areas can be expected too. We conclude that ASD may represent an abnormal ontogenetic itinerary for the human faculty of language resulting in part from changes in genes important for the “domestication syndrome” and, ultimately, from the normal functioning of the neural crest.

Language deficits in schizophrenia and autism as related oscillatory connectomopathies: An evolutionary account

Schizophrenia (SZ) and autism spectrum disorders (ASD) are characterised by marked language deficits, but it is not clear how these arise from gene mutations associated with the disorders. Our goal is to narrow the gap between SZ and ASD and, ultimately, give support to the view that they represent abnormal (but related) ontogenetic itineraries for the human faculty of language. We will focus on the distinctive oscillatory profiles of the SZ and ASD brains, in turn using these insights to refine our understanding of how the brain implements linguistic computations by exploring a novel model of linguistic feature-set composition. We will argue that brain rhythms constitute the best route to interpreting language deficits in both conditions and mapping them to neural dysfunction and risk alleles of the genes. Importantly, candidate genes for SZ and ASD are overrepresented among the gene sets believed to be important for language evolution. This translational effort may help develop an understanding of the aetiology of SZ and ASD and their high prevalence among modern populations.

Aspects of speech-language abilities are influenced by MECP2 mutation type in girls with Rett syndrome

This study investigates relationships between methyl-CpG-binding protein 2 gene (MECP2) mutation type and speech-language abilities in girls with Rett syndrome. Cross-sectional data on 766 girls, aged 15 years and under, with genetically confirmed Rett syndrome was obtained from the Australian Rett Syndrome Database (ARSD) (n = 244) and the International Rett Syndrome Phenotype Database (InterRett) (n = 522). Relationships between MECP2 mutation type and age of regression in speech-language abilities, and the level of speech-language abilities before and after this regression were investigated. The females had a median age of 4.95 years in the ARSD and 5.25 years in InterRett. The majority (89%, 685/766) acquired speech-language abilities in the form of babble or words at some point in time. Of those who acquired babble or words, 85% (581/685) experienced a regression in these abilities. Those with a p.Arg133Cys mutation were the most likely to use one or more words, prior to (RRR = 3.45; 95% CI 1.15-10.41) and after (RRR = 5.99; 95% CI 2.00-17.92), speech-language regression. Girls with Rett syndrome vary in their use of speech and language, and in their experience of speech-language regression and these variations are partly explained by genotype.

The shape of the human language-ready brain

Our core hypothesis is that the emergence of our species-specific language-ready brain ought to be understood in light of the developmental changes expressed at the levels of brain morphology and neural connectivity that occurred in our species after the split from Neanderthals–Denisovans and that gave us a more globular braincase configuration. In addition to changes at the cortical level, we hypothesize that the anatomical shift that led to globularity also entailed significant changes at the subcortical level. We claim that the functional consequences of such changes must also be taken into account to gain a fuller understanding of our linguistic capacity. Here we focus on the thalamus, which we argue is central to language and human cognition, as it modulates fronto-parietal activity. With this new neurobiological perspective in place, we examine its possible molecular basis. We construct a candidate gene set whose members are involved in the development and connectivity of the thalamus, in the evolution of the human head, and are known to give rise to language-associated cognitive disorders. We submit that the new gene candidate set opens up new windows into our understanding of the genetic basis of our linguistic capacity. Thus, our hypothesis aims at generating new testing grounds concerning core aspects of language ontogeny and phylogeny.

Early socio-communicative forms and functions in typical Rett syndrome

Rett syndrome (RTT) is a severe neurological disorder characterized by a developmental regression in motor and speech-language domains. There is, however, limited research on socio-communicative development of affected children before the onset of regression. We analyzed audio-video recordings made by parents of six 9- to 12-month old girls later diagnosed with typical RTT, applying the Inventory of Potential Communicative Acts (IPCA) to identify early communicative forms and functions. Each girl used at least one communicative form (e.g., body movement, eye gaze, or vocalizations) to gain attention and answer, but none were observed to make choices or request information. Varying numbers of children were observed to perform other communicative functions according to the IPCA including social convention, rejecting or requesting an object. Non-verbal forms (e.g., reaching, moving closer, eye contact, smiling) were more common than non-linguistic verbal forms (e.g., unspecified vocalizations, pleasure vocalizations, crying). (Pre-)linguistic verbal forms (e.g., canonical or variegated babbling, proto-words) were not used for communicative purposes. These data suggest that atypical developmental patterns in the socio-communicative domain are evident prior to regression in young individuals later diagnosed with RTT.

Changing the perspective on early development of Rett syndrome

We delineated the achievement of early speech-language milestones in 15 young children with Rett syndrome (MECP2 positive) in the first two years of life using retrospective video analysis. By contrast to the commonly accepted concept that these children are normal in the pre-regression period, we found markedly atypical development of speech-language capacities, suggesting a paradigm shift in the pathogenesis of Rett syndrome and a possible approach to its early detection.

Topological properties of large-scale structural brain networks in children with familial risk for reading difficulties

Developmental dyslexia is a neurobiological deficit characterized by persistent difficulty in learning to read in children and adults who otherwise possess normal intelligence. Functional and structural connectivity data suggest that developmental dyslexia could be a disconnection syndrome. However, whether abnormalities in connectivity exist in beginning readers at-risk for reading difficulties is unknown. Using graph-theoretical analysis, we investigated differences in global and regional topological properties of structural brain networks in 42 beginning readers with (FH+) and without (FH-) familial risk for reading difficulties. We constructed separate structural correlation networks based on measures of surface area and cortical thickness. Results revealed changes in topological properties in brain regions known to be abnormal in dyslexia (left supramarginal gyrus, left inferior frontal gyrus) in the FH+ group mainly in the network constructed from measures of cortical surface area. We also found alterations in topological properties in regions that are not often advertised as dyslexia but nonetheless play important role in reading (left posterior cingulate, hippocampus, and left precentral gyrus). To our knowledge, this is the first report of altered topological properties of structural correlation networks in children at risk for reading difficulty, and motivates future studies that examine the mechanisms underlying how these brain networks may mediate the influences of family history on reading outcome.

Early speech-language development in females with Rett syndrome: focusing on the preserved speech variant

AIM

Our aim was to contribute new findings related to the pre-regressional verbal development of females with a variant of Rett syndrome (RTT) as the loss of spoken language is one of the key clinical features of RTT, and it would be of particular interest to study the early speech-language development of females who are considered to have preserved some speech-language abilities.

METHOD

We analysed 461 minutes of audio-video recordings containing play situations and the daily routines of six females (aged 7 to 24 months; mean birthweight 3057g, SD 195g) with the preserved speech variant (PSV) of RTT. All videos were recorded by parents and analysed retrospectively after the diagnosis PSV was made.

RESULTS

From the age of 7 months onwards, we observed two types of vocalizations, appearing intermittently: (1) apparently normal sequences; and (2) atypical (i.e. inhalatory, pressed, or high-pitched crying-like) vocalizations. Some participants failed to reach the milestone of canonical babbling. We observed a limited phonological and lexical complexity and a restricted compositional variability. Volubility was reduced during the whole period under observation. Hand stereotypies with simultaneous atypical vocalizations appeared only during the second year of life.

INTERPRETATION

The intermittent character of normal versus abnormal verbal behaviours might contribute to an early identification of children with a possible genetic mutation, and provides evidence that speech-language functions are abnormal from the very beginning.

Communication and eating proficiency in 125 females with Rett syndrome: The Swedish Rett center survey

PURPOSE

To describe communication and eating proficiency and related factors in Swedish females with Rett syndrome, from a parental/caregiver view.

METHOD

Data from a questionnaire were used and approached descriptively, qualitatively and comparatively. Parents/caregivers of 125 females with Rett syndrome completed the survey. The mean age of the females was 19.6 years.

RESULTS

Communication: expressive language was limited and the most common way of expressing will was by producing motor acts, 50% of which were higher level of language (eye- and finger-pointing and gestures). Results also showed communicative behaviour was demonstrated during social activities. Eating proficiency: For eight specific types of eating difficulties, the majority of the females suffered from none, one or two of them. There were significant associations of some specific eating difficulties with breathing dysfunctions and food consistency, but not with dental or oral problems.

CONCLUSIONS

Results from this study have confirmed that verbal communication in Rett syndrome is limited. Despite a lack of verbal communication the females in this study were reported to need and enjoy social interplay. Outcomes concerning eating were generally good. For optimal intervention, it is recommended that family and professionals work together to evaluate eating situations and communication ability for females with Rett syndrome.

Rett syndrome

Rett syndrome (RS) is an X-linked neurodevelopmental disorder and the second most common cause of genetic mental retardation in females. Different mutations in MECP2 are found in up to 95% of typical cases of RS. This mainly neuronal expressed gene functions as a major transcription repressor. Extensive studies on girls who have RS and mouse models are aimed at finding main gene targets for MeCP2 protein and defining neuropathologic changes caused by its defects. Studies comparing autistic features in RS with idiopathic autism and mentally retarded patients are presented. Decreased dendritic arborization is common to RS and autism, leading to further research on similarities in pathogenesis, including MeCP2 protein levels in autistic brains and MeCP2 effects on genes connected to autism, like DLX5 and genes on 15q11-13 region. This area also is involved in Angelman syndrome, which has many similarities to RS. Despite these connections, MECP2 mutations in nonspecific autistic and mentally retarded populations are rare.

Early motor disturbances in Rett syndrome and its pathophysiological importance

Assessment of the development of motor function of Rett syndrome (RTT) revealed hypotonia with failure of crawling and disturbance in skillful hand manipulation are shown as early motor signs. Clinical evaluation has revealed the former as postural hypotonia with failure in locomotion and neurophysiological examinations have showed this to be due to hypofunction of the aminergic neurons of the brainstem. The latter signs are considered to indicate dysfunction of the corticospinal tract at higher levels. As the signs appear along with deceleration of head growth, dysfunction of the noradrenergic neuron, which is involved in synaptogenesis in the cerebral cortex, is postulated as the cause. The characteristic stereotyped hand movements appear in early childhood after loss of purposeful hand use and are underlain by rigid hypertonus. Neurophysiological examinations have indicated that these are due to hypofunction of the nigrostriatal (NS) dopamine (DA) neuron. By comparison with animal experimental work the neurohistochemical changes in the substantia nigra of the autopsied brain of RTT suggest a lesion caused by the dysfunction of the pedunculopontine nucleus, induced by dysfunction of the brainstem aminergic neurons which modulate postural tone and locomotion. Hypofunction of the aminergic neurons also cause 'leakage' of atonia into non-REM stages which lead to disturbances in the autonomic nervous system through inhibition of the reflex system. The grade of disturbance of locomotion closely matches the grade in abnormalities of higher cortical function as indicated by the development of meaningful words. The loci of missense mutation of methyl CPG binding domain of MECP 2gene which affect locomotion severely also markedly impaired their effects on the formation of the heterochromatin. Thus, dysfunction of the aminergic neurons of the brainstem which regulate postural tone and locomotion is proposed as the primary lesion.

Another patient with MECP2 mutation without classic Rett syndrome phenotype

Rett syndrome and Angelman syndrome are two neurodevelopmental disorders characterized by partial overlapping features. Rett syndrome is frequently caused by a mutation in methyl-CpG-binding protein (MECP2) gene, localized on chromosome Xq28, whereas Angelman syndrome is frequently caused by different genetic anomalies at chromosome 15q11-q13 (deletions, uniparental disomy, imprinting center mutations, ubiquitin E3 ligase [UBE3A] gene mutations). Recently, some patients with a clinical diagnosis of Angelman syndrome were found to have a mutation in MECP2 gene. This report describes another patient with an Angelman-like phenotype and with an MECP2 mutation.

Heterogeneity in residual function of MeCP2 carrying missense mutations in the methyl CpG binding domain

Rett syndrome is a neurodevelopmental disorder with severe mental retardation caused by mutations in the MECP2 gene. Mutations in the MECP2 gene are also associated with other genetic disorders, including X linked mental retardation in males. Missense mutations identified so far are present primarily in the methyl CpG binding domain (MBD) of MECP2. Here, the functional significance of 28 MBD missense mutations identified in patients were analysed by transient expression of the mutant proteins in cultured cells. The effects of mutations were evaluated by analysis of the affinity of MeCP2 to pericentromeric heterochromatin in mouse L929 cells and on transcriptional repressive activity of MeCP2 in Drosophila SL2 cells. These analyses showed that approximately one-third (9/28) of MBD missense mutations showed strong impairment of MeCP2 function. The mutation of the R111 residue, which directly interacts with the methyl group of methyl cytosine, completely abolished MeCP2 function and mutations affecting beta-sheets and a hairpin loop have substantial functional consequences. In contrast, mutations that showed marginal or mild impairment of the function fell in unstructured regions with no DNA interaction. Since each of these mutations is known to be pathogenic, the mutations may indicate residues that are important for specific functions of MeCP2 in neurones.

Describing the phenotype in Rett syndrome using a population database

BACKGROUND

Mutations in the MECP2 gene have been recently identified as the cause of Rett syndrome, prompting research into genotype-phenotype relations. However, despite these genetic advances there has been little descriptive epidemiology of the full range of phenotypes.

AIMS

To describe the variation in phenotype in Rett syndrome using four different scales, by means of a population database.

METHODS

Using multiple sources of ascertainment including the Australian Paediatric Surveillance Unit, the development of an Australian cohort of Rett syndrome cases born since 1976 has provided the first genetically characterised population based study of Rett syndrome. Follow up questionnaires were administered in 2000 to families and used to provide responses for items in four different severity scales.

RESULTS

A total of 199 verified cases of Rett syndrome were reported between January 1993 and July 2000; 152 families provided information for the follow up study. The mean score using the Kerr scale was 22.9 (SD 4.8) and ranged from 20.5 in those under 7 years to 24.2 in those over 17 years. The mean Percy score was 24.9 (SD 6.6) and also increased with age group from 23.0 to 26.9. The mean Pineda score was 16.3 (SD 4.5) and did not differ by age group. The mean WeeFIM was 29.0 (SD 11.9), indicating extreme dependence, and ranged from 18 to 75.

CONCLUSION

We have expanded on the descriptive epidemiology of Rett syndrome and shown different patterns according to the severity scale selected. Although all affected children are severely functionally dependent, it is still possible to identify some variation in ability, even in children with identified MECP2 mutations.

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.