The fragile X syndrome: the patients and their chromosomes

Therapeutic efficacy of the BKCa channel opener chlorzoxazone in a mouse model of Fragile X syndrome

Fragile X syndrome (FXS) is an X-linked neurodevelopmental disorder characterized by several behavioral abnormalities, including hyperactivity, anxiety, sensory hyper-responsiveness, and autistic-like symptoms such as social deficits. Despite considerable efforts, effective pharmacological treatments are still lacking, prompting the need for exploring the therapeutic value of existing drugs beyond their original approved use. One such repurposed drug is chlorzoxazone which is classified as a large-conductance calcium-dependent potassium (BKCa) channel opener. Reduced BKCa channel functionality has been reported in FXS patients, suggesting that molecules activating these channels could serve as promising treatments for this syndrome. Here, we sought to characterize the therapeutic potential of chlorzoxazone using the Fmr1-KO mouse model of FXS which recapitulates the main phenotypes of FXS, including BKCa channel alterations. Chlorzoxazone, administered either acutely or chronically, rescued hyperactivity and acoustic hyper-responsiveness as well as impaired social interactions exhibited by Fmr1-KO mice. Chlorzoxazone was more efficacious in alleviating these phenotypes than gaboxadol and metformin, two repurposed treatments for FXS that do not target BKCa channels. Systemic administration of chlorzoxazone modulated the neuronal activity-dependent gene c-fos in selected brain areas of Fmr1-KO mice, corrected aberrant hippocampal dendritic spines, and was able to rescue impaired BKCa currents recorded from hippocampal and cortical neurons of these mutants. Collectively, these findings provide further preclinical support for BKCa channels as a valuable therapeutic target for treating FXS and encourage the repurposing of chlorzoxazone for clinical applications in FXS and other related neurodevelopmental diseases.

Cerebral palsy due to chromosomal anomalies and continuous gene syndromes

When cerebral palsy is defined as a disorder of movement and posture that is due to nonprogressive disturbances that occur in the developing fetal and infant brain, a significant proportion-up to 10%--is the consequence of chromosomal anomalies and continuous gene syndromes. Abnormalities of chromosomes are constitutional or acquired. Acquired chromosomal abnormalities develop postnatally, affect only one clone of cells, and are implicated in the evolution of neoplasia. Constitutional abnormalities develop during gametogenesis or early embryogenesis and affect a significant portion of the subject's cells.

Chromosome fragility in the Lennox-Gastaut epilepsy syndrome

Chromosome fragility was sought in a well-defined cohort of patients with Lennox-Gastaut epilepsy syndrome. Twenty-five patients plus 25 age- and sex-matched normal controls had cytogenetic studies performed. All patients were taking sodium valproate, and four were also on hydantoin. Blood cultures from both patients and controls were set up simultaneously in an appropriate medium designed to elicit fragile-sites. Harvesting took place after 96 h by standard techniques. In addition to routine banded analysis, at least 50 cells were scored for chromosome breaks and gaps from each patient and control. Results showed no difference between the patients and the control group in the overall occurrence of fragility or the type of aberration detected. No fragile-X syndrome was detected. This study found no effect due to sodium valproate on the occurrence of chromosome aberrations.

Fragile X positivity in Chinese children with autistic spectrum disorder

Chromosome analysis was performed in 105 Chinese children (96 boys, 9 girls) with autistic spectrum disorder to assess fragile X positivity. Seventy percent of these autistic children were mentally retarded. None of the children in the infantile autism group (N = 75) had fragile X positivity. Two boys in the autistic condition group (N = 30) had clinical features and chromosomal positivity for fragile X syndrome. The low (2%) prevalence rate of fragile X positivity in children with different degrees of expressivity of autistic features may be related to other factors rather than to pure autistic characteristics per se.

New perspectives in autism, Part II: The differential diagnosis and neurobiology of autism

The clinical spectrum of autism spans a broad range of functions, but the core symptoms remain the same regardless of the intelligence of the child: the autistic type of social deficit that ranges from a lack of inclination to relate to extreme difficulty with the mechanics of social interactions, a global communication deficit that involves both verbal and nonverbal modes, and a severe cognitive deficit involving concept formation (abstraction) that is combined with an exceptional memory for factual information. These symptoms may vary dramatically in severity, but the basic deficits are identifiable regardless of IQ. Under-recognition of autism is a major problem at all IQs, but especially in patients with IQs above 50. No drugs have been found to significantly improve the core deficits in autism. Antipsychotics should be avoided except for short-term use. Antidepressants, anxiolytics, and anticonvulsants are important in the treatment of depression, affective modulation, situation-related stress, and seizures. Intensive social skills training is assuming a prominent role in behavior modification programs, and success with higher-functioning autistic children suggests that outcome can be improved by intensive training. The neurobiology of autism has also undergone dramatic changes. The psychogenic theories of etiology have been completely invalidated. Autism is now considered to be a neurological disorder resulting from an error in brain development. The precise location and nature of this deficit are still being actively debated and investigated. One theory emphasizes a dysfunction of the limbic system that results in an impairment in the acquisition of information. A second theory proposes a primary role for dysfunction of the cortical association network responsible for the processing of information.

Autism: familial aggregation and genetic implications

A review of the current literature suggests that genetic factors play an important role in the etiology of autism. It is likely that the etiology of currently idiopathic cases of autism will be shown to be heterogeneous, just as the few known etiologies are both environmental and genetic. Moreover, we would speculate that within the group of cases shown to have genetic etiologies, more than one genetic locus will be found. Some evidence suggests that quite often it is not autism itself that is inherited but rather some genetic abnormality of language or sociability that interacts with other factors to produce autism.

Individual variation and specific cognitive deficits in the fra(X) syndrome

Mental retardation has been associated with fra(X) but comprehensive psychological evaluation has rarely been applied to 2 major behavioral questions 1) the extent of individual variation in IQ among fra(X) males and the possibility of some fra(X) males being of normal IQ; and 2) whether there is a depression in general IQ or whether specific abilities are impaired. The problems of developing an effective battery of tests for assessing fra(X) are discussed. These questions were examined in 54 individuals, comprising fra(X) males, their obligate carrier mothers and those sisters shown to have the fra(X). Among noninstitutionalised males nonverbal IQ as measured on a Block Design test ranged from 100 to 0, and vocabulary scores while generally higher, ranged from 79-33. The males scored low on a digit span memory task, while performance on a memory of objects task was adequate. Despite lower overall scores, a similar pattern and variability emerged in institutionalised males. Daughters were extremely variable in performance and the mothers performed much better, supporting the view that women who have children are a selected subset of fra(X) syndrome individuals. The performance of one male is discussed in detail. His vocabulary and nonverbal IQ scores were normal, despite his having other specific cognitive deficits. The pattern of abilities and behavior seen in fra(X) may result in an overestimation of intelligence and underestimation of penetrance when based on clinical impressions rather than formal psychological assessment. The implications of this for molecular and for population genetic approaches to fra(X) are discussed.

Linkage analysis using multiple Xq DNA polymorphisms in normal families, families with the fragile X syndrome, and other families with X linked conditions

Multipoint linkage analysis was undertaken with eight Xq cloned DNA sequences which identify one or more restriction fragment length polymorphisms in 26 families. These families comprise seven phase known normal families with three or more males in the third generation, seven families segregating for haemophilia B, one large family with dyskeratosis congenita, and 11 families with the fragile X syndrome. Phase known meioses informative for three or more loci supported the order centromere--DXYS1--DXS107--DXS102, DXS51--F9--FRAXA--DXS15, DXS52, F8--Xqter in each group of families studied. One of the normal families was segregating for protan colour blindness and showed a phase known recombination which would support the order centromere--F9--DXS52--CBP--Xqter. With the exception of DXYS1, all of these sequences have been localised to Xq27----qter by in situ hybridisation or hybridisation to Xq fragment panels, and on this basis should lie within 20 cM of one another. No recombination was observed between the sequences localised to Xq28, namely DXS52, F8, and DXS15 (between DXS15 and DXS52 Z = 12.25 at theta = 0 with confidence limits of 0 to 5 cM). However, an excess of recombination was apparent in the region of FRAXA with maximal lod scores as follows: F9 versus FRAXA (Z = 2.05, theta = 0.19), DXS52 versus FRAXA (Z = 1.85, theta = 0.26), and DXS15 versus FRAXA (Z = 1.33, theta = 0.27). No consistent differences were observed in the frequency of recombination when families with the fragile X syndrome were compared with normal families or families segregating for other X linked conditions. These results are compared with other published work and support the conclusion that although measurable linkage exists between these flanking markers and FRAXA, the intervals as measured by the frequency of meiotic recombination will seriously limit their clinical usefulness.

Evidence against close linkage of the loci for fraXq of Martin-Bell syndrome and for factor IX

Linkage between the loci for fraXq of Martin-Bell syndrome and factor IX was studied in nine families exhibiting this syndrome by means of a restriction fragment length polymorphism at the factor IX locus. Computer analysis of the data indicates there to be no evidence for close linkage between the syndrome and the factor IX locus.

Families with more than one handicapped child

Clinical patterns, aetiological factors and utilization of services were studied in 40 families who had more than one handicapped child. By retrospective review of the case notes and interviewing consenting parents (90%), data relating to diagnosis, investigations, genetic counselling, social circumstances and help received were obtained. Both children had the same disorder in 31 families, however in only nine of these families did the children have a specific diagnosis. Non-specific mental retardation was the commonest disorder (40 children). Of the 673 special investigations performed, only 14 were diagnostic. Genetic counselling had been given to only 19 families. Thirty-two of the families had remained as stable units and of the children, 51 received attendance allowances and 21 had periods of respite care. The implications of these findings are discussed.

Molecular genetics of the human X chromosome

The human X chromosome will soon be mapped at 10 cM intervals. This will permit the localisation of any X linked disorder provided that informative families are available for linkage analysis. The location of RFLPs currently in use for clinical diagnosis is summarised. The next decade should witness the elucidation of the molecular basis of some of the more common defects, such as the muscular dystrophies and X linked mental retardation.

The genetic distance between the coagulation factor IX gene and the locus for the fragile X syndrome: clinical implications

In 3 families with the fragile-X [fra(X)] syndrome, we have identified a minimum of 4 recombinations in 9 meioses between the syndrome locus and the coagulation Factor IX gene. Two Factor IX intragenic restriction fragment length polymorphisms (RFLPs), produced with TaqI and XmnI, were used as markers. In lod score calculations, incomplete penetrance of the fra(X) mutation in males and females was taken into account by the computer program LIPED. The cumulative maximum lod score calculated from these data and from data previously reported was 2.75 at a recombination frequency of 20% (theta = 0.20). This indicates that the genetic distance between the Factor IX gene and the fra(X) locus is too great for Factor IX probes to be used alone for carrier detection in the fra(X) syndrome. Additional polymorphic loci more tightly linked to the fra(X) syndrome locus are required.

Genetic mapping of DNA segments relative to the locus for the fragile-X syndrome at Xq27.3

We have tested linkage between the locus for the fragile-X [fra(X)] syndrome at Xq27.3 and five polymorphic restriction sites identified by four DNA probes mapping distal to Xq26.1. A maximum distance of approximately 15 centimorgans (cM) between Xq27.3 and the marker loci mapping to this region was predicted based on the physical chromosome length. Close linkage between the disease and marker loci was excluded for probes DXS19 and DXS37 (theta = .05, Z = -2.94 and Z = -4.17, respectively). These marker loci were estimated to be less than five cM apart but approximately 40 cM proximal to the fragile site, indicating that there is a significantly greater frequency of recombination in this region of the X chromosome than expected from the physical length. Linkage results for the other marker loci and the fra(X) syndrome were inconclusive. However, the pX45d probe locus appears very closely linked to the factor IX locus (Z = 1.94 at theta = 0) and is approximately 20 cM proximal to Xq27.3. A relative map of the polymorphic restriction sites, fra(X) syndrome locus, and factor IX locus was constructed by maximizing lod scores over the Xq26.1----q27.3 region.

Manifestation of the fragile site Xq27 in fibroblasts. IV. Clones from a heterozygous female do not manifest this site homogeneously on either the early or late replicating X chromosome

Fibroblasts from a heterozygous carrier for the Martin-Bell syndrome, who manifests the fragile site Xq27, were cloned to separate the population carrying the primary defect on the active X chromosome from the population with this defect on the inactive X. Clones with this defect on the active X manifest the fra(X)(q27) whereas clones from the other population are fra(X)-negative (Steinbach et al. 1983b). In this project, the replication status of the X chromosome manifesting the fra(X)(q27) was studied in seven clones with this defect on the active X. The results obtained on the clones were very similar to the results obtained from uncloned fibroblasts and lymphocytes. In the clones the fragile site was found manifested on the early replicating X in 73 cells and on the late replicating X in four cells. Since the defect is located on the active X chromosome of these cells the manifestation of the fragile site on the late replicating X suggests that the defect and the fragile site cannot be identical. It is concluded that there is no obligate synteny of this defect and the manifested fragile site.