X-linked mental retardation

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.     

X-linked mental retardation

Genetic factors have an important role in the aetiology of mental retardation. However, their contribution is often underestimated because in developed countries, severely affected patients are mainly sporadic cases and familial cases are rare. X-chromosomal mental retardation is the exception to this rule, and this is one of the reasons why research into the genetic and molecular causes of mental retardation has focused almost entirely on the X-chromosome. Here, we review the remarkable recent progress in this field, its promise for understanding neural function, learning and memory, and the implications of this research for health care.     

X-linked mental retardation: many genes for a complex disorder

X-linked mental retardation (XLMR) is a common cause of moderate to severe intellectual disability in males. XLMR is very heterogeneous, and about two-thirds of patients have clinically indistinguishable non-syndromic (NS-XLMR) forms, which has greatly hampered their molecular elucidation. A few years ago, international consortia overcame this impasse by collecting DNA and cell lines from large cohorts of XLMR families, thereby paving the way for the systematic study of the molecular causes of XLMR. Mutations in known genes might already account for 50% of the families with NS-XLMR, and various genes have been pinpointed that seem to be of particular diagnostic importance. Eventually, even therapy of XLMR might become possible, as suggested by the unexpected plasticity of the neuronal wiring in the brain, and the recent successful drug treatment of a fly model for fragile X syndrome.     

X-linked non-specific mental retardation

Non-specific mental retardation is a very common and genetically heterogeneous disorder but, to date, only six genes related to this condition have been identified. Five of these six have been found in the past two years, through positional-cloning efforts of mapped X-linked families. The characteristics of the newly identified genes are providing insights into the molecular mechanisms of mental impairment and the development of cognitive functions.     

Balanced translocations in mental retardation

Over the past few decades, the knowledge on genetic defects causing mental retardation has dramatically increased. In this review, we discuss the importance of balanced chromosomal translocations in the identification of genes responsible for mental retardation. We present a database-search guided overview of balanced translocations identified in patients with mental retardation. We divide those in four categories: (1) balanced translocations that helped to identify a causative gene within a contiguous gene syndrome, (2) balanced translocations that led to the identification of a mental retardation gene confirmed by independent methods, (3) balanced translocations disrupting candidate genes that have not been confirmed by independent methods and (4) balanced translocations not reported to disrupt protein coding sequences. It can safely be concluded that balanced translocations have been instrumental in the identification of multiple genes that are involved in mental retardation. In addition, many more candidate genes were identified with a suspected but (as yet?) unconfirmed role in mental retardation. Some balanced translocations do not disrupt a protein coding gene and it can be speculated that in the light of recent findings concerning ncRNA’s and ultra-conserved regions, such findings are worth further investigation as these potentially may lead us to the discovery of novel disease mechanisms.     

Sensory impairment in mental retardation: a potential role for NGF

Sensory impairment is defined as the inability to interpret outside stimuli such as visual, auditory, verbal, sense of touch, taste or smell or feelings of pain. This leads to absence of sensation and neuronal coordination. The impairment may be caused by ageing and other physiological changes, accident or injuries or can be found in some cases of mental retardation (MR) also referred to as intellectual disability. Known cases of MR involving inability to accurately interpret an outside source or stimuli are: Fragile-X syndrome; Tuberous sclerosis complex (TSC) with associated autism spectrum disorder (ASD); Rett syndrome; Autism and ASD with or without MR; Chromosome 22q13.3 deletion syndrome; familial dysautonomia, Prader-Willi's syndrome, Williams syndrome. In this review we will discuss in particular form of ASD and altered sensory sensitivity. The role of NGF in causing pronociceptive activity and its role in peripheral sensitisation is discussed under the light of its involvement in forms of MR where loss of pain perception is a main feature due to mutations to NGF receptors or NGF genes during development. Other forms of MR with altered sensory impairment will be considered as well as additional potential mechanisms involved.     

Palmar dermatoglyphics in mental retardation

Palmar dermatoglyphics has been studied in 86 mentally retarded males versus 50 normal males. The important findings in brief are as follows:

1. Frequency of patterns in descending order (all ten taken together) were ulnar loops followed by whorls, in both the groups.
2. Highly significant differences were found between the two groups in righ c-d, a-d, left a-b, a-d and vertical distance from a-d to the axial triradius, significant differences in left b-d, c-d and both distances from axial triradius to a vertical dropped proximally from triradius a.

The finding of this work has been compared with other authors. These findings give a base to classify mental retardation from the dermatoglyphic point of view, thus to help in diagnosis of the disease in newly born individuals.     

The effect of a traditional dance training program on dynamic balance of individuals with mental retardation

The purpose of this study was to evaluate the influence of a Greek traditional dance training program on the dynamic balance of individuals with mental retardation (MR). A total of 17 individuals participated in this study. Ten individuals with mild or moderate MR and 7 individuals with mild or moderate MR who studied in special schools were assigned to intervention (MR-I) and control (MR-C) groups, respectively. Pretraining and posttraining exercise tests were performed to determine the dynamic balance ability. Dynamic balance ability was measured by means of a balance deck (Lafayette, Lafayette, IN, USA) in 30-, 45-, and 60-second intervals. The MR-I group underwent a 16-week Greek traditional dance training program at a frequency of 3 times per week and for a duration of 45 minutes per season. Posttraining results showed that the individuals with MR in the MR-I group improved during treatment, from their baseline scores on dynamic balance measurements (30 seconds: p < 0.01, 45 seconds: p < 0.05, 60 seconds: p < 0.05). The MR-C group did not show any improvement between the 2 measurements. In conclusion, individuals with MR may be able to improve their dynamic balance when performing a systematic and well-designed Greek traditional dance training program.     

X-linked mental retardation and epigenetics

The search for the genetic defects in constitutional diseases has so far been restricted to direct methods for the identification of genetic mutations in the patients' genome. Traditional methods such as karyotyping, FISH, mutation screening, positional cloning and CGH, have been complemented with newer methods including array-CGH and PCR-based approaches (MLPA, qPCR). These methods have revealed a high number of genetic or genomic aberrations that result in an altered expression or reduced functional activity of key proteins. For a significant percentage of patients with congenital disease however, the underlying cause has not been resolved strongly suggesting that yet other mechanisms could play important roles in their etiology. Alterations of the 'native' epigenetic imprint might constitute such a novel mechanism. Epigenetics, heritable changes that do not rely on the nucleotide sequence, has already been shown to play a determining role in embryonic development, X-inactivation, and cell differentiation in mammals. Recent progress in the development of techniques to study these processes on full genome scale has stimulated researchers to investigate the role of epigenetic modifications in cancer as well as in constitutional diseases. We will focus on mental impairment because of the growing evidence for the contribution of epigenetics in memory formation and cognition. Disturbance of the epigenetic profile due to direct alterations at genomic regions, or failure of the epigenetic machinery due to genetic mutations in one of its components, has been demonstrated in cognitive derangements in a number of neurological disorders now. It is therefore tempting to speculate that the cognitive deficit in a significant percentage of patients with unexplained mental retardation results from epigenetic modifications.     

Anthropology and mental retardation: Research approaches and opportunities

Although mental retardation is largely a sociocultural phenomenon, anthropological interest in this field has been slow to develop. In recent years, anthropological concepts and methods have been used in study of the community adaptation of mentally retarded persons and societal reactions to them. As an illustration, research developments at the Mental Retardation Research Center, UCLA, are discussed. The need for expanded, collaborative research by social and biomedical scientists is examined. The research puzzles include the links between poverty, ethnicity, schools, families and mental retardation, as well as the nature of intelligence and adaptation.     

Fragile X mental retardation syndrome: structure of the KH1-KH2 domains of fragile X mental retardation protein

Fragile X syndrome is the most common form of inherited mental retardation in humans, with an estimated prevalence of about 1 in 4000 males. Although several observations indicate that the absence of functional Fragile X Mental Retardation Protein (FMRP) is the underlying basis of Fragile X syndrome, the structure and function of FMRP are currently unknown. Here, we present an X-ray crystal structure of the tandem KH domains of human FMRP, which reveals the relative orientation of the KH1 and KH2 domains and the location of residue Ile304, whose mutation to Asn is associated with a particularly severe incidence of Fragile X syndrome. We show that the Ile304Asn mutation both perturbs the structure and destabilizes the protein.     

Diagnostic genome profiling in mental retardation

Mental retardation (MR) occurs in 2%-3% of the general population. Conventional karyotyping has a resolution of 5-10 million bases and detects chromosomal alterations in approximately 5% of individuals with unexplained MR. The frequency of smaller submicroscopic chromosomal alterations in these patients is unknown. Novel molecular karyotyping methods, such as array-based comparative genomic hybridization (array CGH), can detect submicroscopic chromosome alterations at a resolution of 100 kb. In this study, 100 patients with unexplained MR were analyzed using array CGH for DNA copy-number changes by use of a novel tiling-resolution genomewide microarray containing 32,447 bacterial artificial clones. Alterations were validated by fluorescence in situ hybridization and/or multiplex ligation-dependent probe amplification, and parents were tested to determine de novo occurrence. Reproducible DNA copy-number changes were present in 97% of patients. The majority of these alterations were inherited from phenotypically normal parents, which reflects normal large-scale copy-number variation. In 10% of the patients, de novo alterations considered to be clinically relevant were found: seven deletions and three duplications. These alterations varied in size from 540 kb to 12 Mb and were scattered throughout the genome. Our results indicate that the diagnostic yield of this approach in the general population of patients with MR is at least twice as high as that of standard GTG-banded karyotyping.     

Fetal face syndrome with mental retardation

Summary Two cases of a variant of the fetal face syndrome are described. The affected babies were born from two consecutive pregnancies to the same gypsy parents. In addition to the classical syndrome, both cases showed a severe feeding difficulties, failure to thrive and severe psychomotoric retardation and one of them cleft lip and palate.     

Dendritic spine abnormalities in mental retardation

Abnormalities in dendritic spine morphologies are often associated with mental retardation. Since dendritic spines are thought to represent a morphological correlate of neuronal plasticity, altered spine morphologies may underlie or contribute to cognitive deficits seen in mental retardation. Signaling cascades that are important for cytoskeletal regulation may have an impact upon spine morphologies. The Rho GTPase signaling pathway has been shown to be involved in the regulation of the cytoskeleton and to play fundamental roles in the structural plasticity of dendritic spines. Moreover, alterations in the Rho GTPase signaling pathway have been shown to contribute to mental retardation. Recently, different mental retardation-associated genes have been identified that encode modulators of the Rho GTPases. Disturbances in these genes can lead to mental retardation and—on the morphological level—to alterations in dendritic spines. Thus, getting more insight into the Rho GTPase signaling pathways, and the molecules involved, would not only help in understanding the basic mechanisms by which the morphologies of dendritic spines are modulated but may also allow the development of therapeutic strategies to counteract some aspects of mental retardation.