CGG-repeat length threshold for FMR1 RNA pathogenesis in a cellular model for FXTAS

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder that affects carriers of premutation alleles (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene. The presence of elevated levels of expanded mRNA found in premutation carriers is believed to be the basis for the pathogenesis in FXTAS, but the exact mechanisms by which the mRNA causes toxicity are not known. In particular, it is not clear whether there is a threshold for a CGG-repeat number below which no cellular dysregulation occurs, or whether toxicity depends on mRNA concentration. We have developed a doxycycline-inducible episomal system that allows us to study separately the effects of CGG-repeat number and mRNA concentration (at fixed CGG-repeat length) in neuroblastoma-derived SK cells. Our findings show that there is a CGG-repeat size threshold for toxicity that lies between 62 and 95 CGG repeats. Interestingly, for repeat sizes of 95 CGG and above, there is a clear negative correlation between mRNA concentration and cell viability. Taken together, our results provide evidence for an RNA-toxicity model with primary dependence on CGG-repeat size and secondary dependence on mRNA concentration, thus formally ruling out any simple titration model that operates in the absence of either protein-binding cooperativity or some form of length-dependent RNA structural transition.     

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.     

Risk indicators and psychopathology in traumatised children and adolescents with a history of sexual abuse

PURPOSE: Childhood sexual abuse (CSA) is widespread amongst South African (SA) children, yet data on risk factors and psychiatric consequences are limited and mixed. METHODS: Traumatised children and adolescents referred to our Youth Stress Clinic were interviewed to obtain demographic, sexual abuse, lifetime trauma and psychiatric histories. RESULTS: Data for 94 participants (59 female, 35 male; mean age 14.25 [8.25-19] years) exposed to at least one lifetime trauma were analysed. Sexual abuse was reported in 53% of participants (42.56% females, 10.63% males) with 64% of violations committed by perpetrators known to them. Multinomial logistic regression analysis revealed female gender (P=0.002) and single-parent families (P=0.01) to be significant predictors of CSA (62.5%). CSA did not predict exposure to other traumas. Sexually abused children had significantly higher physical and emotional abuse subscale scores and total CTQ scores than non-abused children. Depression (33%, X(2)=10.89, P=0.001) and PTSD (63.8%, X(2)=4.79, P=0.034) were the most prevalent psychological consequences of trauma and both were significantly associated with CSA. CONCLUSIONS: High rates of CSA predicted high rates of PTSD in this traumatised sample. Associations we found appear consistent with international studies of CSA and, should be used to focus future social awareness, prevention and treatment strategies in developing countries.     

Altered hypothalamus-pituitary-adrenal gland axis regulation in the expanded CGG-repeat mouse model for fragile X-associated tremor/ataxia syndrome

The human FMR1 gene contains an unstable CGG-repeat in its 5' untranslated region. The repeat length in the normal population is polymorphic (5-54 CGG-repeats). Individuals carrying lengths beyond 200 CGGs (i.e. the full mutation) show hypermethylation and as a consequence gene silencing of the FMR1 gene. The absence of the gene product FMRP causes the fragile X syndrome, the most common inherited form of mental retardation. Elderly carriers of the premutation (PM), which is defined as a repeat length between 55 and 200 CGGs, can develop a progressive neurodegenerative syndrome: fragile X-associated tremor/ataxia syndrome (FXTAS). The high FMR1 mRNA levels observed in cells from PM carriers have led to the hypothesis that FXTAS is caused by a pathogenic RNA gain-of-function mechanism. Apart from tremor/ataxia, specific psychiatric symptoms have been described in PM carriers with or without FXTAS. Since these symptoms could arise from elevated stress hormone levels, we investigated hypothalamic-pituitary-adrenal (HPA) axis regulation using a knock-in mouse model with an expanded CGG-repeat in the PM range (>98 repeats) in the Fmr1 gene, which shows repeat instability, and displays biochemical, phenotypic and neuropathological characteristics of FXTAS. We show elevated levels of corticosterone in serum and ubiquitin-positive inclusions in both the pituitary and adrenal gland of 100-week-old animals. In addition, we demonstrate ubiquitin-positive inclusions in the amygdala from aged expanded CGG-repeat mice. We hypothesize that altered regulation of the HPA axis and the amygdala and higher stress hormone levels in the mouse model for FXTAS may explain associated psychological symptoms in humans.     

Two experimental models for generating abdominal adhesions

PURPOSE: To develop dependable rat models for generating abdominal adhesions that allow for objective evaluation and quantification. METHODS: Two adhesion models were devised and compared with conventional side-wall models involving cecal abrasion and peritoneal excision or abrasion. model T (tissue): removal of a 2.5 by 2.5 cm segment of full-thickness abdominal wall with overlying skin closure, exposing the viscera to subcutaneous tissue; model M (mesh): removal of an identical segment, replacing the defect with a 2.5 by 2.5 cm polypropylene mesh sewn to the cut edges. This exposed the viscera directly to the mesh surface. Seven days after operation, the character and extent of the adhesions were assessed at autopsy. Results were expressed as the percent area of subcutaneous tissue involved (T) or of mesh surface involved (M). For model T the percent involvement of the circumference of the defect edge was also recorded. The extent of omental and intestinal adhesions were evaluated individually. RESULTS: The classical side-wall models showed inconsistent patterns of adhesion formation and were difficult to evaluate. Every animal from both models M and T developed extensive adhesions. The mean coverage of mesh surface (M) was 93% and subcutaneous surface (T) 82%. In model T the mean involvement of the defect cut edge was 80% of the circumference. All model T animals had both intestinal and omental adhesions whereas there were no intestinal attachments in model M. Tenacity of adhesions did not differ significantly between animals or models. CONCLUSIONS: Adhesion models M and T are consistent, predictable, and dependable. They each yield extensive adhesion coverage to a defined site, which allow for standardized measurement.     

Neuropsychological dysfunction among affected heterozygous fragile X females.

Fragile X (or Martin-Bell) syndrome is an X-linked disorder that often produces mental retardation in males, but usually affects heterozygous females to a lesser degree. Here we report the results of a brief neuropsychological examination of 20 heterozygous fra(x) girls and women and two control groups of 20 individuals each. One control group was composed of fra(x)-negative mothers (obligate carriers) and sisters of male probands with fra(x) syndrome, whereas the other was composed of 14 head-injured and six learning disabled women and girls. In addition to general intellectual impairment, several specific cognitive deficits were consistently found in individuals with the Martin-Bell syndrome, suggesting focal neuropsychological dysfunction. Significant differences were noted between fra(x) individuals and controls on most cognitive and neuropsychological measures studied. Over one-third of the fra(x) individuals demonstrated neuropsychological symptoms characteristic of the full developmental Gerstmann syndrome, whereas another third had three or four of the five signs of possible parietal lobe dysfunction. In our sample, there was an association between improved performance and increasing age. Differences among heterozygous individuals in number of focal symptoms may reflect some variability in the penetrance of the fra(x) gene, as well as in the functional organization of the brain.