Effect of the deficits of fragile X mental retardation protein on cognitive status of fragile x males and females assessed by robust pedigree analysis

Ocular motor indicators of executive dysfunction in fragile X and Turner syndromes

Fragile X and Turner syndromes are two X-chromosome-related disorders associated with executive function and visual spatial deficits. In the present study, we used ocular motor paradigms to examine evidence that disruption to different neurological pathways underlies these deficits. We tested 17 females with fragile X, 19 females with Turner syndrome, and 40 females with neither disorder who comprised the comparison group. Group differences emerged for both the fragile X and Turner syndrome groups, each relative to the comparison group: Females with fragile X had deficits in generating memory-guided saccades, predictive saccades, and saccades made in the overlap condition of a gap/overlap task. Females with Turner syndrome showed deficits in generating memory-guided saccades, but not during either the predictive saccade or gap/overlap task. Females with Turner syndrome, but not females with fragile X, showed deficits in visually guided saccades and anti-saccades. These findings indicate that different brain regions are affected in the two disorders, and suggest that different pathways lead to the similar cognitive phenotypes described for fragile X and Turner syndromes.

Language comprehension in boys with fragile X syndrome and boys with Down syndrome

BACKGROUND

Fragile X syndrome (FXS) is the most common known inherited cause of intellectual disability, yet very few studies have explored the language comprehension skills of children with FXS. We examined the receptive vocabulary, grammatical morphology and syntax skills of boys with FXS (who were additionally classified as having autism, autism spectrum, or no autism) and compared them to boys with Down syndrome (DS) and typically developing (TD) boys at similar non-verbal developmental levels.

METHODS

The Vocabulary, Grammatical Morphology, and Elaborated Phrases and Sentences subtests of the Test for Auditory Comprehension of Language - 3rd Edition (TACL-3) were administered annually up to three times to assess the language comprehension skills of 35 boys with FXS without autism, 24 boys with FXS with autism spectrum, 19 boys with FXS with autism, 45 boys with DS and 40 TD boys at similar non-verbal cognitive levels.

RESULTS

After controlling for non-verbal cognition and maternal education levels, we found that the three groups of boys with FXS did not differ from each other but scored lower than the TD boys in language comprehension. The boys with DS scored lower in language comprehension than boys with FXS without autism and TD boys. For all of the groups, scores for receptive vocabulary, grammatical morphology and syntax did not differ.

CONCLUSIONS

Boys with FXS and boys with DS differed in receptive language levels, demonstrating unique language profiles for each syndrome. Language comprehension appears to be an important area to target in assessment and intervention for both populations.

Language development and fragile X syndrome: profiles, syndrome-specificity, and within-syndrome differences

Fragile X syndrome (FXS) is the leading inherited cause of mental retardation. In this article, we review what is known about the language and related problems of individuals with FXS. In doing so, we focus on the syndrome-specific features of the language phenotype and on the organismic (i.e., genetic and individual neurocognitive and behavioral) and environmental factors associated with within-syndrome variation in the phenotype. We also briefly review those aspects of the behavioral phenotype of FXS that are relevant for understanding syndrome-specific features of, and within-syndrome variability in, language. The review includes summaries of research on the prelinguistic foundations for language development and on each of the major components of language (i.e., vocabulary, morphosyntax, and pragmatics). Throughout the review, we point out implications of existing research for intervention as well as directions for future research.

Molecular and cognitive predictors of the continuum of autistic behaviours in fragile X

The distributions of scores for autistic behaviours obtained from the Autism Diagnostic Observation Scale-Generic (ADOS-G) were investigated in 147 males and females affected with the full mutation in the fragile X mental retardation 1 (FMR1) gene, in 59 individuals with the premutation, and in 42 non-fragile X relatives, aged 4-70 years. The scores representing communication and social interaction were continuously distributed across the two fragile X groups, and they were significantly elevated compared with the non-fragile X controls. Strong relationships were found between both these scores and FMRP deficits, but they became insignificant for social interaction, and the sum of social interaction and communication scores, when FSIQ was included as another predictor of autism scores. Other significant predictors of these scores in both sexes were those executive skills which related to verbal fluency, and to the regulation and control of motor behaviour. Overall, our data have shown that cognitive impairment, especially of verbal skills, best explains the comorbidity of autism and fragile X. This implies some more fundamental perturbations of specific neural connections which are essential for both specific behaviours and cognition. We also emphasize that FXS offers a unique molecular model for autism since FMRP regulates the translation of many other genes involved in synaptic formation and plasticity which should be natural targets for further exploration.

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.

Examination of the effect of the polymorphic CGG repeat in the FMR1 gene on cognitive performance

A CGG repeat sequence located in the 5' untranslated region of the FMR1 gene is polymorphic with respect to size and stability of the repeat during parent-offspring transmission. When expanded to over 200 repeats, the gene is hypermethylated and silenced, leading to fragile X syndrome (FXS). Recently, alleles with large unmethylated repeat tracts (premutations) have been associated with ovarian failure and a late-onset tremor/ataxia syndrome, symptoms unrelated to FXS. To further investigate the phenotype consequence of high repeat alleles, we have analyzed Wechsler adult intelligence scales-III (WAIS-III) measures on 66 males and 217 females with a wide range of repeat sizes. Among females only, we found that FMR1 repeat size and transcript level significantly explained approximately 4% of the variance in the Verbal IQ summary measure, suggesting that this polymorphism is one of many factors that influence variation in cognitive performance. Because of the well established association of increasing repeat size with decreasing age at menopause, we also investigated the reproductive stage and use of hormone replacement therapy (HRT) as a covariate to model verbal intelligence quotient (VIQ). We found that it explained an additional 5% of the variance in VIQ, but did not interact with FMR1 repeat and transcript level.

A comparison of phonological skills of boys with fragile X syndrome and Down syndrome

In this study, the authors compared the phonological accuracy and patterns of sound change of boys with fragile X syndrome, boys with Down syndrome, and typically developing mental-age-matched boys. Participants were 50 boys with fragile X syndrome, ages 3 to 14 years; 32 boys with Down syndrome, ages 4 to 13 years; and 33 typically developing boys, ages 2 to 6 years, who were matched for nonverbal mental age to both the boys with fragile X syndrome and the boys with Down syndrome. All participants were administered a standardized articulation test, and their sound accuracy, phonological process, and proportion of whole-word proximity scores were analyzed. Although boys with fragile X syndrome were delayed in their speech development, they did not differ from the typically developing, mental-age-matched boys in the percentage of correct early-, middle-, and late-developing consonants; phonological processes; or whole-word proximity scores. Furthermore, boys with fragile X syndrome had fewer errors on early-, middle-, and late-developing consonants; fewer syllable structure processes; and higher whole-word proximity scores than did boys with Down syndrome. Boys with Down syndrome also were delayed in their speech development, yet their phonological inventories, occurrences of phonological processes, and proportion of whole-word proximity scores indicated greater delays in their phonological development than the younger, typically developing boys. These results suggest that males with fragile X syndrome display phonological characteristics in isolated words similar to younger, typically developing children, whereas males with Down syndrome show greater delays as well as some developmental differences compared with both the males with fragile X syndrome and typically developing males.

Phenotypic variation and FMRP levels in fragile X

Data on the relationships between cognitive and physical phenotypes, and a deficit of fragile X mental retardation 1 (FMR1) gene-specific protein product, FMRP, are presented and discussed in context with earlier findings. The previously unpublished results obtained, using standard procedures of regression and correlations, showed highly significant associations in males between FMRP levels and the Wechsler summary and subtest scores and in females between these levels and the full-scale intelligence quotient (FSIQ), verbal and performance IQ, and some Wechsler subtest scores. The published results based on data from 144 extended families with fragile X, recruited from Australia and the United States within a collaborative NIH-supported project, were obtained using robust modification of maximum likelihood in pedigrees. The results indicated that processing speed, short-term memory, and the ability to control attention, especially in the context of regulating goal-directed behavior, may be primarily affected by the FMRP depletion. The effect of this depletion on physical phenotype was also demonstrated, especially on body and head height and extensibility of finger joints. It is recommended that further studies should rely on more accurate measures of FMRP levels, and use of larger samples, to overcome extensive variability in the data.

Effect of fragile X status categories and FMRP deficits on cognitive profiles estimated by robust pedigree analysis

The effect of the fragile X pre-mutation and full mutation categories, and FMRP deficits in these categories, on neurocognitive status, have been assessed in fragile X individuals from 144 extended families, which included fragile X individuals, as well as their non-fragile X relatives. Neuropsychological status was assessed by the Wechsler summary and subtest test scores. A modification of the maximum likelihood estimators for pedigree data that is resistant to outliers was used to analyze the data. The results have demonstrated the effect of large expansions of CGG repeat in the FMR1 (fragile X mental retardation 1) gene (full mutation) in decreasing full scale IQ (FSIQ), as well as several FSIQ-adjusted subtest scores in the performance domain. Moreover, the results have demonstrated significant cognitive deficits in male individuals with pre-mutation. FMRP depletion correlates strongly with neurocognitive status in the full mutation subjects. Evidence for the effect of FMRP in smaller expansions (pre-mutation) in reducing FSIQ, Performance and Verbal scores, as well as subtest scores in males, has also been obtained. The results are also suggestive of factors other than FMRP deficit which may determine some specific cognitive deficits in fragile X pre-mutation carriers. Genetic variance estimated from the models accounts for less than half of the total variance in FSIQ, and it varies widely between individual Wechsler subtests.