Specificity of cerebellar vermian abnormalities in autism: a quantitative magnetic resonance imaging study

Fragile X syndrome and autism at the intersection of genetic and neural networks

Autism, an entirely behavioral diagnosis with no largely understood etiologies and no population-wide biomarkers, contrasts with fragile X syndrome (FXS), a single-gene disorder with definite alterations of gene expression and neuronal morphology. Nevertheless, the behavioral overlap between autism and FXS suggests some overlapping mechanisms. Understanding how the single-gene alteration in FXS plays out within complex genetic and neural network processes may suggest targets for autism research and illustrate strategies for relating autism to more singular genetic syndromes.

Growth in stature and head circumference in high-functioning autism and Asperger disorder during the first 3 years of life

Little effort has been made to characterize the developmental anatomic phenotype of autism; although there is evidence of an increased head circumference and brain size, few other physical characteristics have been studied. The head circumference, body length/height, and weight measurements of infants, who were later diagnosed with high-functioning autism (HFA, n = 16) and Asperger disorder (AsD, n = 12), were extracted from health records over the first 3 years of life and compared to the measurements of a matched normal control group (n = 19). Using linear mixed-effects models, no differences were found in the average growth rate for head circumference, stature, or weight between the children with HFA and AsD. However, a significantly higher growth rate in body length/height and weight was found for the combined group of children with HFA and AsD compared to the normal control group. A trend toward higher growth rate in head circumference was also found among the former group. The results indicate that growth dysregulation in autism is not specific to the brain but also involves growth in stature.

Autism spectrum disorder in Fragile X syndrome: differential contribution of adaptive socialization and social withdrawal

The present study extends our previous work on characterizing the profile of social behavior abnormalities in boys with Fragile X (FraX) and autism spectrum disorder (ASD) using clinically oriented behavioral rating scales and standardized instruments. The goal was to further distinguish behavioral parameters contributing to the diagnostic classification of FraX + ASD. The study design included two cohorts of boys with FraX (3-8 years), a larger main cohort for cross-sectional analyses (n = 56, 24 with ASD), and a longitudinal subset (n = 30, 11 with ASD) of the main cohort with up to 3 yearly observations. The focus was on the relative contribution of delayed adaptive socialization and social withdrawal, including item components of their corresponding rating instruments, to the diagnosis of ASD in boys with FraX. Using a combination of regression analyses, we demonstrated that: (1) as delayed socialization, social withdrawal is also a correlate of FraX + ASD; (2) items of social withdrawal scales representing avoidance were the main predictors of ASD status, particularly in older boys; (3) adaptive socialization skills reflecting rules of social behavior and recognition and labeling of emotions, linked to verbal reasoning abilities, were selectively associated with FraX + ASD; (4) adaptive socialization is the primary determinant over time of ASD status in boys with FraX; and (5) integrated adaptive socialization-social withdrawal models allow the identification of distinctive FraX + ASD subgroups. Altogether, our findings suggest that two distinct but interrelated social behavior abnormalities, one linked to impaired cognitive processes (delayed socialization) and the second one to disturbance in limbic circuits (avoidance), play a role in the development of ASD in boys with FraX.

Immunogold study of effects of prenatal exposure to lipopolysaccharide and/or valproic acid on the rat blood-brain barrier vessels

The involvement of blood microvessels, representing the anatomic site of the blood-brain barrier (BBB), in brain damage induced by prenatal exposure to lipopolysaccharide (LPS) and/or valproic acid (VPA) was studied in four-week-old rats. The immunogold procedure was applied for localization at the ultrastructural level of endogenous albumin and glucose transporter (GLUT-1) in three brain regions: cerebral cortex, cerebellum and hippocampus. Four groups of rats were used: (1) untreated control, (2) prenatally VPA-treated, (3) prenatally LPS-treated, and (4) prenatally LPS- and VPA-treated. The functional state of the BBB was evaluated as follows: (a) by its tightness, i.e., permeability to blood-borne albumin, and (b) by the expression of GLUT-1 in the endothelial cells (ECs). Using morphometry, the labelling density for GLUT-1 was recorded over luminal and abluminal plasma membranes of the ECs, also providing information on their functional polarity. No extensive increase of vascular permeability and/or any considerable dysfunction of the BBB in experimental groups nos. 2 and 3 were observed, although in solitary vascular profiles, increased endocytosis or even transcytosis of albumin by ECs was noted. In experimental group no. 4, some vascular profiles showed scanty leakage (microleakage), manifested by the presence of immunosignals for albumin in the perivascular area. Although some fluctuations in the expression of GLUT-1 occurred in all experimental groups, especially in group no. 3, a most pronounced and significant diminution of the labelling density, in all three regions of the brain, was observed in group no. 4. This finding suggests the synergistic action of prenatally applied LPS and VPA that affects specific transport functions of glucose in the microvascular endothelium. The diminished or disturbed supply of glucose to selected brain regions can be one of the factors leading to previously observed behavioral disturbances in similarly treated rats.

Autism at the beginning: microstructural and growth abnormalities underlying the cognitive and behavioral phenotype of autism

Autistic symptoms begin in the first years of life, and recent magnetic resonance imaging studies have discovered brain growth abnormalities that precede and overlap with the onset of these symptoms. Recent postmortem studies of the autistic brain provide evidence of cellular abnormalities and processes that may underlie the recently discovered early brain overgrowth and arrest of growth that marks the first years of life in autism. Alternative origins and time tables for these cellular defects and processes are discussed. These cellular and growth abnormalities are most pronounced in frontal, cerebellar, and temporal structures that normally mediate the development of those same higher order social, emotional, speech, language, speech, attention, and cognitive functions that characterize autism. Cellular and growth pathologies are milder and perhaps nonexistent in other structures (e.g., occipital cortex), which are known to mediate functions that are often either mildly affected or entirely unaffected in autistic patients. It is argued that in autism, higher order functions largely fail to develop normally in the first place because frontal, cerebellar, and temporal cellular and growth pathologies occur prior to and during the critical period when these higher order neural systems first begin to form their circuitry. It is hypothesized that microstructural maldevelopment results in local and short distance overconnectivity in frontal cortex that is largely ineffective and in a failure of long-distance cortical-cortical coupling, and thus a reduction in frontal-posterior reciprocal connectivity. This altered circuitry impairs the essential role of frontal cortex in integrating information from diverse functional systems (emotional, sensory, autonomic, memory, etc.) and providing context-based and goal-directed feedback to lower level systems.

Lessons from fragile X regarding neurobiology, autism, and neurodegeneration

The fragile X mental retardation 1 gene (FMR1) mutation causes two disorders: fragile X syndrome (FXS) in those with the full mutation and the fragile X-associated tremor/ataxia syndrome (FXTAS) in some older individuals with the premutation. FXS is caused by a deficiency of the FMR1 protein (FMRP) leading to dysregulation of many genes that create a phenotype with ADHD, anxiety, and autism. FXTAS is caused by the elevation of FMR1-mRNA to levels 2 to 8 times normal in the premutation. This causes an RNA gain of function toxicity leading to brain atrophy, white matter disease, neuronal and astrocytic inclusion formation, and subsequent ataxia, intention tremor, peripheral neuropathy, and cognitive decline. The neurobiology and pathophysiology of FXS and FXTAS are described in detail.

Immunity, neuroglia and neuroinflammation in autism

Autism is a complex neurodevelopmental disorder of early onset that is highly variable in its clinical presentation. Although the causes of autism in most patients remain unknown, several lines of research support the view that both genetic and environmental factors influence the development of abnormal cortical circuitry that underlies autistic cognitive processes and behaviors. The role of the immune system in the development of autism is controversial. Several studies showing peripheral immune abnormalities support immune hypotheses, however until recently there have been no immune findings in the CNS. We recently demonstrated the presence of neuroglial and innate neuroimmune system activation in brain tissue and cerebrospinal fluid of patients with autism, findings that support the view that neuroimmune abnormalities occur in the brain of autistic patients and may contribute to the diversity of the autistic phenotypes. The role of neuroglial activation and neuroinflammation are still uncertain but could be critical in maintaining, if not also in initiating, some of the CNS abnormalities present in autism. A better understanding of the role of neuroinflammation in the pathogenesis of autism may have important clinical and therapeutic implications.

Oculomotor studies of cerebellar function in autism

Histopathological, neuroimaging and genetic findings indicate cerebellar abnormalities in autism, but the extent of neurophysiological dysfunction associated with those findings has not been systematically examined. Suppression of intrusive saccades (square wave jerks) and the ability to sustain eccentric gaze, two phenomena requiring intact cerebellar function, were examined in 52 high-functioning individuals with autism and 52 age- and IQ-matched healthy subjects during visual fixation of static central and peripheral targets. Rates of intrusive saccades were not increased in autism during visual fixation, and foveopetal ocular drift was also not increased when subjects held an eccentric gaze. The absence of gross disturbances of visual fixation associated with cerebellar disease in individuals with autism, such as increased square wave jerk rates and foveopetal drift when holding eccentric gaze, indicates that the functional integrity of cerebellar--brainstem networks devoted to oculomotor control is preserved in autism despite reported anatomic variations. However, increased amplitude of intrusive saccades and reduced latency of target refixation after intrusive saccades were observed in individuals with autism, especially when subjects maintained fixation of remembered target locations without sensory guidance. The atypical metrics of intrusive saccades that were observed may be attributable to faulty functional connectivity in cortico-cerebellar networks.

Retards mentaux liés à l’X

X-linked mental retardation (XLMR) affects 1.8 per thousand male births and is usually categorized as "syndromic" (MRXS) or "non-specific" (MRX) forms according to the presence or absence of specific signs in addition to the MR. Up to 60 genes have been implicated in XLMR and certain mutations can alternatively lead to MRXS or MRX. Indeed the extreme phenotypic and allelic heterogeneity of XLMR makes the classification of most genes difficult. Therefore, following identification of new genes, accurate retrospective clinical evaluation of patients and their families is necessary to aid the molecular diagnosis and the classification of this heterogeneous group of disorders. Analyses of the protein products corresponding to XLMR genes show a great diversity of cellular pathways involved in MR. Common mechanisms are beginning to emerge : a first group of proteins belongs to the Rho and Rab GTPase signaling pathways involved in neuronal differentiation and synaptic plasticity and a second group is related to the regulation of gene expression. In this review, we illustrate the complexity of XLMR conditions and present recent data about the FMR1, ARX and Oligophrenin 1 genes.

Recent advances in fragile X: a model for autism and neurodegeneration

PURPOSE OF REVIEW

This review will describe recent developments in the neurobiology of fragile X syndrome (FXS), the association between FXS and autism, and involvement in premutation carriers.

RECENT FINDINGS

Metabotropic glutamate receptor 5 (mGluR5)-coupled pathways are dysregulated in individuals with FXS and this is thought to relate to the FXS phenotype. The mGluR5 model suggests that mGluR5 antagonists, including downstream effectors such as lithium, could be useful for treating FXS. Two forms of clinical involvement associated with the fragile X mental retardation 1 (FMR1) gene, autism and fragile X-associated tremor/ataxia syndrome (FXTAS), have received additional attention during the past year. One study has found that approximately 30% of individuals with FXS have autism; those with autism have lowered cognitive abilities, language problems, and behavioral difficulties compared to those with FXS alone. Furthermore, evidence is mounting that autism also occurs in some young males who have premutation alleles. Finally, males and occasional females with premutation alleles may develop a neurological syndrome with aging that consists of tremor, ataxia, peripheral neuropathy, and cognitive deficits. Significant brain atrophy and white-matter disease is usually seen. This new disorder (FXTAS) is thought to be related to elevated levels of abnormal FMR1 mRNA.

SUMMARY

Full-mutation forms of the gene (> 200 repeats) can cause autism, learning disabilities, anxiety disorders, and mental retardation. Disorders associated with premutation forms of the gene (55-200 repeats) include, in addition to autism, FXTAS in older males and females, and premature ovarian failure.

Histone modifications in Rett syndrome lymphocytes: a preliminary evaluation

Most cases of Rett syndrome (RTT) are associated with mutations in the coding region of the transcriptional regulator MeCP2. This gene appears to repress gene expression through chromatin conformational changes secondary to histone modifications, mainly histone deacetylation of core histones H3 and H4. There is limited and contradictory information about histone modifications in RTT tissues. The present study intended to provide a preliminary characterization of histone acetylation (AcH3, AcH4) and methylation (MeH3) in RTT, with emphasis on non-selected peripheral cells and molecular-neurologic correlations. We compared 17 females with RTT, 11 of them with MeCP2 mutations, with 10 gender-matched controls in terms of lymphocyte lysate immunoblotting-based levels. We found that immunoreactivities for MeCP2 and AcH3/AcH4 are variable in both control and RTT subjects. Despite this variability, RTT subjects with nonsense mutations showed the expected reduction in C-terminal MeCP2 immunoreactivity. Regardless of MeCP2 levels, both subjects with (RTTPos) and without (RTTNeg) mutations had decreased levels of AcH3. The latter reductions were mainly driven by decreases in levels of H3 acetylated at lysine residue 14 (AcH3K14) and independent of parallel, but milder, decreases in immunoreactivity for MeH3 lysine residues (MeH3K4/MeH3K9). Within our study sample, reductions in AcH3 were correlated with severity of head growth deceleration in the RTTPos group. This contrasted with the lack of significant association between location of MeCP2 mutation and severity of the RTT neurologic phenotype. We concluded that there were distinctive profiles of histone acetylation/methylation in RTT peripheral cells, which reflect pathogenetic mechanisms common to subjects with clinical features of this disorder, regardless of mutation status, and that these patterns may be relevant to neurologic dysfunction in RTT.

Autism spectrum disorder in fragile X syndrome: communication, social interaction, and specific behaviors

The present study extends our previous work on social behavior impairment in young males with fragile X syndrome (FraX). Specifically, we evaluated whether the autistic phenomenon in FraX is expressed as a range of behavioral impairments as in idiopathic autism (Aut). We also examined whether there are behaviors, identified as items of the Autism Diagnostic Interview-Revised (ADI-R), that in FraX predispose to or differentiate subjects with autism spectrum disorder (ASD) diagnosis. Finally, regression models were utilized to test the relative contribution of reduced communication and socialization skills to ADI-R scores and diagnoses. A cohort of 56 boys (3-8 years) with FraX was examined in terms of scores on measures of cognition (IQ was a co-variate in most analyses.), autistic behavior, problem/aberrant behavior, adaptive behavior, and language development. We found that, indeed, in terms of problem behavior and adaptive skills, there is a range of severity from FraX + Aut to FraX + PDD (Pervasive Developmental Disorder) to FraX + none. ADI-R items representing "Play" types of interaction appear to be "susceptibility" factors since they were abnormal across the FraX cohort. Integrated regression models demonstrated that items reflecting complex social interaction differentiated the FraX + ASD (Aut + PDD) subgroup from the rest of the FraX cohort, while abnormalities in basic verbal and non-verbal communication distinguished the most severely affected boys with FraX + Aut from the milder FraX + PDD cohort. Models incorporating language, adaptive communication, and adaptive socialization skills revealed that socialization was not only the main influence on scores but also a predictor of ASD diagnosis. Altogether, our findings demonstrate that the diagnosis of ASD in FraX reflects, to a large extent, an impairment in social interaction that is expressed with variable severity in young males with FraX.

Brain development in autism: early overgrowth followed by premature arrest of growth

Due to the relatively late age of clinical diagnosis of autism, the early brain pathology of children with autism has remained largely unstudied. The increased use of retrospective measures such as head circumference, along with a surge of MRI studies of toddlers with autism, have opened a whole new area of research and discovery. Recent studies have now shown that abnormal brain overgrowth occurs during the first 2 years of life in children with autism. By 2-4 years of age, the most deviant overgrowth is in cerebral, cerebellar, and limbic structures that underlie higher-order cognitive, social, emotional, and language functions. Excessive growth is followed by abnormally slow or arrested growth. Deviant brain growth in autism occurs at the very time when the formation of cerebral circuitry is at its most exuberant and vulnerable stage, and it may signal disruption of this process of circuit formation. The resulting aberrant connectivity and dysfunction may lead to the development of autistic behaviors. To discover the causes, neural substrates, early-warning signs and effective treatments of autism, future research should focus on elucidating the neurobiological defects that underlie brain growth abnormalities in autism that appear during these critical first years of life.

Imaging data in autism: from structure to malfunction

During the last two decades, neuroimaging studies have improved our knowledge of brain development and contributed to our understanding of disorders involving the developing brain. Differences in cerebral anatomy have been determined in autism spectrum disorder (ASD). Morphological studies by magnetic resonance imaging have provided evidence of structural differences in ASD compared with the normal population. This has enhanced our view of autism as a neurobiological disorder corresponding with different stages and events in brain development. Alterations in volume of the total brain and specifically the cerebellum, frontal lobe, and limbic system have been identified. There appears to be a pattern of increased and then decreased rate of brain growth over time. We integrate these observations with neurobehavioral findings to provide a developmental hypothesis of the pathophysiology of autism.

Cerebellar function in autism: functional magnetic resonance image activation during a simple motor task

BACKGROUND

The cerebellum is one of the most consistent sites of neuroanatomic abnormality in autism, yet it is still unclear how such pathology impacts cerebellar function. In normal subjects, we previously demonstrated with functional magnetic resonance imaging (fMRI) a dissociation between cerebellar regions involved in attention and those involved in a simple motor task, with motor activation localized to the anterior cerebellum ipsilateral to the moving hand. The purpose of the present investigation was to examine activation in the cerebella of autistic patients and normal control subjects performing this motor task.

METHODS

We studied eight autistic patients and eight matched normal subjects, using fMRI. An anatomic region-of-interest approach was used, allowing a detailed examination of cerebellar function.

RESULTS

Autistic individuals showed significantly increased motor activation in the ipsilateral anterior cerebellar hemisphere relative to normal subjects, in addition to atypical activation in contralateral and posterior cerebellar regions. Moreover, increased activation was correlated with the degree of cerebellar structural abnormality.

CONCLUSIONS

These findings strongly suggest dysfunction of the autistic cerebellum that is a reflection of cerebellar anatomic abnormality. This neurofunctional deficit might be a key contributor to the development of certain diagnostic features of autism (e.g., impaired communication and social interaction, restricted interests, and stereotyped behaviors).

The autistic brain: birth through adulthood

PURPOSE OF REVIEW

We discuss evidence of brain maldevelopment in the first years of life in autism and new neuroanatomical and functional evidence from later ages of development.

RECENT FINDINGS

Head circumference, an accurate indicator of brain size in children, was reported to jump from normal or below normal size in the first postnatal months in autistic infants to the 84 th percentile by about 1 year of age; this abnormally accelerated growth was concluded by 2 years of age. Infants with extreme head (and therefore brain) growth fell into the severe end of the clinical spectrum and had more extreme neuroanatomical abnormalities. In the frontal and temporal lobes in autism, there have been reports of abnormal increases in gray and white matter at 2 to 4 years; reduced metabolic measures; deviant diffusion tensor imaging results in white matter; underdeveloped cortical minicolumns; and reduced functional activation during socio-emotional, cognitive and attention tasks. Cerebellar abnormalities included abnormal volumes, reduced number and size of Purkinje neurons in the vermis and hemispheres, molecular defects, and reduced functional activation in posterior regions.

SUMMARY

A new neurobiological phenomenon in autism has been described that precedes the onset of clinical behavioral symptoms, and is brief and age-delimited to the first two years of life. The neurobiological defects that precede, trigger, and underlie it may form part of the developmental precursors of some of the anatomical, functional, and behavioral manifestations of autism. Future studies of the first years of life may help elucidate the factors and processes that bring about the unfolding of autistic behavior.