X linked mental retardation

Novel eGZ-motif formed by regularly extruded guanine bases in a left-handed Z-DNA helix as a major motif behind CGG trinucleotide repeats

The expansion of d(CGG) trinucleotide repeats (TRs) lies behind several important neurodegenerative diseases. Atypical DNA secondary structures have been shown to trigger TR expansion: their characterization is important for a molecular understanding of TR disease. CD spectroscopy experiments in the last decade have unequivocally demonstrated that CGG runs adopt a left-handed Z-DNA conformation, whose features remain uncertain because it entails accommodating GG mismatches. In order to find this missing motif, we have carried out molecular dynamics (MD) simulations to explore all the possible Z-DNA helices that potentially form after the transition from B- to Z-DNA. Such helices combine either CpG or GpC Watson-Crick steps in Z-DNA form with GG-mismatch conformations set as either intrahelical or extrahelical; and participating in BZ or ZZ junctions or in alternately extruded conformations. Characterization of the stability and structural features (especially overall left-handedness, higher-temperature and steered MD simulations) identified two novel Z-DNA helices: the most stable one displays alternately extruded Gs, and is followed by a helix with symmetrically extruded ZZ junctions. The G-extrusion favors a seamless stacking of the Watson-Crick base pairs; extruded Gs favor syn conformations and display hydrogen-bonding and stacking interactions. Such conformations could have the potential to hijack the MMR complex, thus triggering further expansion.

Molecular conformations and dynamics of nucleotide repeats associated with neurodegenerative diseases: double helices and CAG hairpin loops

Pathogenic DNA secondary structures have been identified as a common and causative factor for expansion in trinucleotide, hexanucleotide, and other simple sequence repeats. These expansions underlie about fifty neurological and neuromuscular disorders known as “anticipation diseases”. Cell toxicity and death have been linked to the pathogenic conformations and functional changes of the RNA transcripts, of DNA itself and, when trinucleotides are present in exons, of the translated proteins. We review some of our results for the conformations and dynamics of pathogenic structures for both RNA and DNA, which include mismatched homoduplexes formed by trinucleotide repeats CAG and GAC; CCG and CGG; CTG(CUG) and GTC(GUC); the dynamics of DNA CAG hairpins; mismatched homoduplexes formed by hexanucleotide repeats (GGGGCC) and (GGCCCC); and G-quadruplexes formed by (GGGGCC) and (GGGCCT). We also discuss the dynamics of strand slippage in DNA hairpins formed by CAG repeats as observed with single-molecule Fluorescence Resonance Energy Transfer. This review focuses on the rich behavior exhibited by the mismatches associated with these simple sequence repeat noncanonical structures.

E-motif formed by extrahelical cytosine bases in DNA homoduplexes of trinucleotide and hexanucleotide repeats

Atypical DNA secondary structures play an important role in expandable trinucleotide repeat (TR) and hexanucleotide repeat (HR) diseases. The cytosine mismatches in C-rich homoduplexes and hairpin stems are weakly bonded; experiments show that for certain sequences these may flip out of the helix core, forming an unusual structure termed an ‘e-motif’. We have performed molecular dynamics simulations of C-rich TR and HR DNA homoduplexes in order to characterize the conformations, stability and dynamics of formation of the e-motif, where the mismatched cytosines symmetrically flip out in the minor groove, pointing their base moieties towards the 5′-direction in each strand. TRs have two non-equivalent reading frames, (GCC)_n and (CCG)_n; while HRs have three: (CCCGGC)_n, (CGGCCC)_n, (CCCCGG)_n. We define three types of pseudo basepair steps related to the mismatches and show that the e-motif is only stable in (GCC)_n and (CCCGGC)_n homoduplexes due to the favorable stacking of pseudo GpC steps (whose nature depends on whether TRs or HRs are involved) and the formation of hydrogen bonds between the mismatched cytosine at position i and the cytosine (TRs) or guanine (HRs) at position i − 2 along the same strand. We also characterize the extended e-motif, where all mismatched cytosines are extruded, their extra-helical stacking additionally stabilizing the homoduplexes.

Structure and Dynamics of DNA and RNA Double Helices Obtained from the CCG and GGC Trinucleotide Repeats

Expansions of both GGC and CCG sequences lead to a number of expandable, trinucleotide repeat (TR) neurodegenerative diseases. Understanding of these diseases involves, among other things, the structural characterization of the atypical DNA and RNA secondary structures. We have performed molecular dynamics simulations of (GCC) _n and (GGC) _n homoduplexes in order to characterize their conformations, stability, and dynamics. Each TR has two reading frames, which results in eight nonequivalent RNA/DNA homoduplexes, characterized by CpG or GpC steps between the Watson-Crick base pairs. Free energy maps for the eight homoduplexes indicate that the C-mismatches prefer anti-anti conformations, while G-mismatches prefer anti-syn conformations. Comparison between three modifications of the DNA AMBER force field shows good agreement for the mismatch free energy maps. The mismatches in DNA-GCC (but not CCG) are extrahelical, forming an extended e-motif. The mismatched duplexes exhibit characteristic sequence-dependent step twist, with strong variations in the G-rich sequences and the e-motif. The distribution of Na⁺ is highly localized around the mismatches, especially G-mismatches. In the e-motif, there is strong Na⁺ binding by two G(N7) atoms belonging to the pseudo GpC step created when cytosines are extruded and by extrahelical cytosines. Finally, we used a novel technique based on fast melting by means of an infrared laser pulse to classify the relative stability of the different DNA-CCG and -GGC homoduplexes.

Structural characterization of a dimer of RNA duplexes composed of 8-bromoguanosine modified CGG trinucleotide repeats: a novel architecture of RNA quadruplexes

Fragile X syndrome and fragile X-associated tremor/ataxia syndrome (FXTAS) are neurodegenerative disorders caused by the pathogenic expansion of CGG triplet repeats in the FMR1 gene. FXTAS is likely to be caused by a 'toxic' gain-of-function of the FMR1 mRNA. We provide evidence for the existence of a novel quadruplex architecture comprising CGG repeats. The 8-bromoguanosine ((Br)G)-modified molecule GC(Br)GGCGGC forms a duplex in solution and self-associates via the major groove to form a four-stranded, antiparallel (GC(Br)GGCGGC)4 RNA quadruplex with (Br)G3:G6:(Br)G3:G6 tetrads sandwiched between mixed G:C:G:C tetrads. Self-association of Watson-Crick duplexes to form a four-stranded structure has previously been predicted; however, no experimental evidence was provided. This novel four-stranded RNA structure was characterized using a variety of experimental methods, such as native gel electrophoresis, NMR spectroscopy, small-angle X-ray scattering and electrospray ionization mass spectrometry.

Model-based evaluation of the impact of formulation and food intake on the complex oral absorption of mavoglurant in healthy subjects

PURPOSE

To compare the pharmacokinetics of intravenous (IV), oral immediate-release (IR) and oral modified-release (MR) formulations of mavoglurant in healthy subjects, and to assess the food effect on the MR formulation's input characteristics.

METHODS

Plasma concentration-time data from two clinical studies in healthy volunteers were pooled and analysed using NONMEM®. Drug entry into the systemic circulation was modelled using a sum of inverse Gaussian (IG) functions as an input rate function, which was estimated specifically for each formulation and food state.

RESULTS

Mavoglurant pharmacokinetics was best described by a two-compartment model with a sum of either two or three IG functions as input function. The mean absolute bioavailability from the MR formulation (0.387) was less than from the IR formulation (0.436). The MR formulation pharmacokinetics were significantly impacted by food: bioavailability was higher (0.508) and the input process was shorter (complete in approximately 36 versus 12 h for the fasted and fed states, respectively).

CONCLUSIONS

Modelling and simulation of mavoglurant pharmacokinetics indicate that the MR formulation might provide a slightly lower steady-state concentration range with lower peaks (possibly better drug tolerance) than the IR formulation, and that the MR formulation's input properties strongly depend on the food conditions at drug administration.

Distinctive structural motifs of RNA G-quadruplexes composed of AGG, CGG and UGG trinucleotide repeats

Trinucleotide repeats are microsatellite sequences that are polymorphic in length. Their expansion in specific genes underlies a number of neurodegenerative disorders. Using ultraviolet-visible, circular dichroism, nuclear magnetic resonance (NMR) spectroscopies and electrospray ionization mass spectrometry, the structural preferences of RNA molecules composed of two and four repeats of AGG, CGG and UGG in the presence of K⁺, Na⁺ and NH₄⁺ were analysed. (AGG)₂A, (AGG)₄A, p(UGG)₂U and p(UGG)₄U strongly prefer folding into G-quadruplexes, whereas CGG-containing sequences can adopt different types of structure depending on the cation and on the number of repeats. In particular, the two-repeat CGG sequence folds into a G-quadruplex in potassium buffer. We also found that each G-quadruplex fold is different: A:(G:G:G:G)A hexads were found for (AGG)₂A, whereas mixed G:C:G:C tetrads and U-tetrads were observed in the NMR spectra of G(CGG)₂C and p(UGG)₂U, respectively. Finally, our NMR study highlights the influence of the strand sequence on the structure formed, and the influence of the intracellular environment on the folding. Importantly, we highlight that although potassium ions are prevalent in cells, the structures observed in the HeLa cell extract are not always the same as those prevailing in biophysical studies in the presence of K⁺ ions.

Structural studies of CNG repeats

CNG repeats (where N denotes one of the four natural nucleotides) are abundant in the human genome. Their tendency to undergo expansion can lead to hereditary diseases known as TREDs (trinucleotide repeat expansion disorders). The toxic factor can be protein, if the abnormal gene is expressed, or the gene transcript, or both. The gene transcripts have attracted much attention in the biomedical community, but their molecular structures have only recently been investigated. Model RNA molecules comprising CNG repeats fold into long hairpins whose stems generally conform to an A-type helix, in which the non-canonical N-N pairs are flanked by C-G and G-C pairs. Each homobasic pair is accommodated in the helical context in a unique manner, with consequences for the local helical parameters, solvent structure, electrostatic potential and potential to interact with ligands. The detailed three-dimensional profiles of RNA CNG repeats can be used in screening of compound libraries for potential therapeutics and in structure-based drug design. Here is a brief survey of the CNG structures published to date.

Development of mavoglurant and its potential for the treatment of fragile X syndrome

INTRODUCTION

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. With no curative treatment available, current therapeutic approaches are aimed at symptom management. FXS is caused by silencing the FMR1 gene, which encodes FMRP; as loss of FMRP leads to the development of symptoms associated with FXS.

AREAS COVERED

In this evaluation, the authors examine the role of the metabotropic glutamate receptor 5 (mGluR5) in the pathophysiology of FXS, and its suitability as a target for rescuing the disease state. Furthermore, the authors review the evidence from preclinical studies of pharmacological interventions targeting mGluR5 in FXS. Lastly, the authors assess the findings from clinical studies in FXS, in particular the use of the Aberrant Behavior Checklist-Community Edition (ABC-C) and the recently developed ABC-C for FXS scale, as clinical endpoints to assess disease modification in this patient population.

EXPERT OPINION

There is cautious optimism for the successful treatment of the core behavioral and cognitive symptoms of FXS based on preclinical data in animal models and early studies in humans. However, the association between mGluR5-heightened responsiveness and the clinical phenotype in humans remains to be demonstrated. Many questions regarding the optimal treatment and outcome measures of FXS remain unanswered.

Crystal structures of CGG RNA repeats with implications for fragile X-associated tremor ataxia syndrome

The CGG repeats are present in the 5'-untranslated region (5'-UTR) of the fragile X mental retardation gene FMR1 and are associated with two diseases: fragile X-associated tremor ataxia syndrome (FXTAS) and fragile X syndrome (FXS). FXTAS occurs when the number of repeats is 55-200 and FXS develops when the number exceeds 200. FXTAS is an RNA-mediated disease in which the expanded CGG tracts form stable structures and sequester important RNA binding proteins. We obtained and analysed three crystal structures of double-helical CGG repeats involving unmodified and 8-Br modified guanosine residues. Despite the presence of the non-canonical base pairs, the helices retain an A-form. In the G-G pairs one guanosine is always in the syn conformation, the other is anti. There are two hydrogen bonds between the Watson-Crick edge of G(anti) and the Hoogsteen edge of G(syn): O6·N1H and N7·N2H. The G(syn)-G(anti) pair shows affinity for binding ions in the major groove. G(syn) causes local unwinding of the helix, compensated elsewhere along the duplex. CGG helical structures appear relatively stable compared with CAG and CUG tracts. This could be an important factor in the RNA's ligand binding affinity and specificity.

The FMR1 gene and fragile X-associated tremor/ataxia syndrome

The CGG-repeat present in the 5'UTR of the FMR1 gene is unstable upon transmission to the next generation. The repeat is up to 55 CGGs long in the normal population. In fragile X patients, a repeat length exceeding 200 CGGs (full mutation: FM) generally leads to methylation of the repeat and the promoter region, which is accompanied by silencing of the FMR1 gene. The gene product FMRP is involved in regulation of transport and translation of certain mRNA in the dendrite, thereby affecting synaptic plasticity. This is central to learning and memory processes. The absence of FMRP seen in FM is the cause of the mental retardation seen in fragile X patients. The premutation (PM) is defined as 55-200 CGGs. Female PM carriers are at risk of developing primary ovarian insufficiency. Recently it was discovered that elderly PM carriers might develop a progressive neurodegenerative disorder called fragile X-associated tremor/ataxia syndrome. Although arising from the mutations in the same gene, distinct mechanisms lead to fragile X syndrome (absence of FMRP) and FXTAS (toxic RNA gain of function). The pathogenic mechanisms thought to underlie these disorders are discussed, with a specific emphasis on FXTAS. This review gives insight on the implications of all possible repeat length categories seen in fragile X families.

The phenotypic consequences of MECP2 mutations extend beyond Rett syndrome

Although MECP2 was initially identified as the causative gene in classic Rett syndrome (RTT), the gene has now been implicated in several phenotypes that extend well beyond the clinically defined disorder. MECP2 mutations have been found in people with various disorders, including neonatal onset encephalopathy, X-linked recessive mental retardation (MRX), classic and atypical RTT, autism, and Angelman syndrome, as well as mildly affected females and normal carrier females. To make matters more complex, in approximately 20% of classic sporadic RTT cases and more than 50% of affected sister pairs, no mutation in MECP2 has been found. X-chromosome inactivation patterns can clearly affect the phenotypic expression in females, while the effect of the type and position of the mutation is more apparent in the broader phenotype than in RTT. Both males and females are at risk, although an excess of paternally derived mutations are found in most cases of classic RTT. Thus, because of the range of disparate phenotypes, the gene may account for a relatively large portion of mental retardation in the population.

[A practical diagnostic approach to mental deficiency in 2002]

The identification of an etiology in children with mental deficiency is a major challenge in routine pediatrics. As the result of a workshop leaded by the Société française de neurologie pédiatrique (SFNP), we propose a three steps diagnostic procedure, taking into account several frequent clinical observations leading to further targeted investigations. The yield of systematic imaging and biological screening remains very low, when performed for a non specific isolated mental retardation, without any characteristic clinical features. Yet, it is mandatory for an accurate genetic counseling to know not only the clinical diagnosis of developmental delay, but also the pathophysiology and the underlying molecular mechanism. The SFNP's proposal points out the necessity of a comprehensive clinical process including cautious neurodevelopmental assessment, reliable cognitive and adaptive skills evaluation, and collaboration between different specialists.

Genetic basis of cognitive disability

The importance of genetic influences on cognitive disability has been recognized for a long time, but molecular analysis has only recently begun to yield insights into the pathogenesis of this common and disabling condition. The availability of genome sequences has enabled the characterization of the chromosomal deletions and trisomies that result in cognitive disability, and mutations in rare single-gene conditions are being discovered. The molecular pathology of cognitive disability is turning out to be as heterogeneous as the condition itself, with unexpected complexities even in apparently simple gene-deletion syndromes. One remarkable finding from studies on X-linked mental retardation is that mutations in different small guanosine triphosphate (GTP)-binding proteins result in cognitive disability without other somatic features. Advances are also being made in cognitive disability with polygenic origins, such as dyslexia and autism. However, the genetic basis of mild intellectual disability has yet to be satisfactorily explained.

Genetic basis of cognitive disability

The importance of genetic influences on cognitive disability has been recognized for a long time, but molecular analysis has only recently begun to yield insights into the pathogenesis of this common and disabling condition. The availability of genome sequences has enabled the characterization of the chromosomal deletions and trisomies that result in cognitive disability, and mutations in rare single-gene conditions are being discovered. The molecular pathology of cognitive disability is turning out to be as heterogeneous as the condition itself, with unexpected complexities even in apparently simple gene-deletion syndromes. One remarkable finding from studies on X-linked mental retardation is that mutations in different small guanosine triphosphate (GTP)-binding proteins result in cognitive disability without other somatic features. Advances are also being made in cognitive disability with polygenic origins, such as dyslexia and autism. However, the genetic basis of mild intellectual disability has yet to be satisfactorily explained.

Genes for cognitive function: developments on the X

Developments in human genome research enabled the first steps toward a molecular understanding of cognitive function. That there are numerous genes on the X chromosome affecting intelligence at the lower end of the cognitive range is no longer in doubt. Naturally occurring mutations have so far led to the identification of seven genes accounting for a small proportion of familial nonspecific X-linked mental retardation. These new data indicate that normal expression of many more X-linked and autosomal genes contribute to cognitive function. The emerging knowledge implicating genes in intracellular signaling pathways provides the insight to identify as candidates other X-linked and autosomal genes regulating the normal development of cognitive function. Recent advances in unravelling the underlying molecular complexity have been spectacular but represent only the beginning, and new technologies will need to be introduced to complete the picture.

A new gene involved in X-linked mental retardation identified by analysis of an X;2 balanced translocation

X-linked forms of mental retardation (MR) affect approximately 1 in 600 males and are likely to be highly heterogeneous. They can be categorized into syndromic (MRXS) and nonspecific (MRX) forms. In MRX forms, affected patients have no distinctive clinical or biochemical features. At least five MRX genes have been identified by positional cloning, but each accounts for only 0.5%-1.0% of MRX cases. Here we show that the gene TM4SF2 at Xp11.4 is inactivated by the X breakpoint of an X;2 balanced translocation in a patient with MR. Further investigation led to identification of TM4SF2 mutations in 2 of 33 other MRX families. RNA in situ hybridization showed that TM4SF2 is highly expressed in the central nervous system, including the cerebral cortex and hippocampus. TM4SF2 encodes a member of the tetraspanin family of proteins, which are known to contribute in molecular complexes including beta-1 integrins. We speculate that through this interaction, TM4SF2 might have a role in the control of neurite outgrowth.

X linked severe mental retardation, craniofacial dysmorphology, epilepsy, ophthalmoplegia, and cerebellar atrophy in a large South African kindred is localised to Xq24-q27

To date over 150 X linked mental retardation (XLMR) conditions have been documented. We describe a five generation South African family with XLMR, comprising 16 affected males and 10 carrier females. The clinical features common to the 16 males included profound mental retardation (100%), mutism despite apparently normal hearing (100%), grand mal epilepsy (87.5%), and limited life expectancy (68.8%). Of the four affected males examined, all had mild craniofacial dysmorphology and three were noted to have bilateral ophthalmoplegia and truncal ataxia. Three of 10 obligate female carriers had mild mental retardation. Cerebellar and brain stem atrophy was shown by cranial imaging and postmortem examination. Linkage analysis shows the gene to be located between markers DXS424 (Xq24) and DXS548 (Xq27.3), with a maximum two point lod score of 3.10.

FRAXAC1 and DXS548 polymorphisms in the Chinese population

The fragile X syndrome is the most common inherited form of mental retardation. Haplotype studies using FRAXAC1 and DXS548 polymorphic markers flanking the fragile site have demonstrated linkage disequilibrium at the FMR1 locus. We investigated the association of the FRAXAC1, DXS548 and CGG alleles between normal subjects and mentally retarded (MR) patients of unspecified cause who do have fragile X syndrome. We have evaluated the FRAXAC1 site in 390 normal subjects and 321 MR patients and the DXS548 site in 146 normal and 319 MR subjects. Both FRAXAC1 and DXS548 alleles were determined by application of the polymerase chain reaction. When compared with Caucasians, the normal Chinese population has a different FRAXAC1 allele distribution. There are more AC18 repeat alleles and fewer AC19 repeat alleles. The DXS548 allele distributions were similar between Chinese and Caucasians. The same distribution pattern of FRAXAC1 alleles was found in both normal subjects and MR patients, but there were significant differences in the distribution patterns of DXS548 alleles. The FMR1 CGG-DXS548 and FRAXAC1-DXS548 haplotype distribution between normal subjects and MR patients also differed significantly. Our results suggest a possible association between DXS548 alleles and non-FRAXA mental retardation.

Another dystonia

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Hypoplastic left heart in cerebrocostomandibular syndrome

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High frequency of the haemochromatosis C282Y mutation in Hungary could argue against a Celtic origin of the mutation

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Analysis of an interstitial deletion in a patient with Kallmann syndrome, X-linked ichthyosis and mental retardation

Contiguous gene syndromes are an interesting clinical phenomenon, resulting from interstitial or terminal deletions of several adjacent genes. The phenotype results in a combination of two or more monogenic disorders and relates clinical findings to corresponding genotypes. We present the case of a male patient with Kallmann syndrome (KS), X-linked ichthyosis (XLI) and X-linked mental retardation (MRX). He was referred at the age of 15.4 years for delayed puberty and obesity. He had a previous history of pyloric stenosis, bilateral orchidopexy and surgical correction of a pes equinovarus adductus. On physical examination, generalised ichthyosis and hypoplastic external genitalia were found. KS was evident with hypogonadotropic hypogonadism, hyposmia and a hypoplastic anlage of the olfactory tract in magnetic resonance imaging. Lipoprotein electrophoresis, and lack of steroid sulfatase and arylsulfatase-C activity in leucocytes confirmed XLI. DNA investigation established an interstitial deletion in Xp22.3 involving the Kallmann (KAL) gene, the steroid sulfatase (STS) gene and a putative mental retardation locus (MRX). The novel MRX locus maps to a 1-Mb region between DXS1060 and GS1.

Characterisation of an inverted X chromosome (p11.2q21.3) associated with mental retardation using FISH

We report on a patient with a pericentric inversion of the X chromosome, 46,Y,inv(X) (p11.2q21.3), who was referred for cytogenetic analysis because of mild mental retardation, short stature, prepubescent macro-orchidism, and submucous cleft palate. The same chromosomal abnormality was found in the proband's mother. The inverted X chromosome was late replicating in all the mother's lymphocytes studied, indicative of a likely unbalanced inversion. We show, by fluorescence in situ hybridisation (FISH) using a panel of ordered yeast artificial chromosome (YAC) clones, that the Xp breakpoint is localised in Xp11.23 between DXS146 and DXS255 and that the Xq breakpoint is assigned to the X-Y homologous region in Xq21.3. YACs crossing the Xp and Xq breakpoints have been identified. One of these two breakpoints could be linked to the mental retardation in this patient as many non-specific mental retardation (MRX) loci have previously been located in the pericentromeric region of the X chromosome. Morever, the elucidation at the molecular level of this rearrangement will also indicate if cleft palate or prepubescent macro-orchidism, or both, in this boy are related to one of the two X breakpoints.

Distinct facial appearance with nasal hypoplasia, constipation, severe mental retardation and hypotonia in two unrelated young males

In this report we describe two unrelated young males with severe mental retardation, persisting hypotonia, and constipation. A maternal uncle of one of these two boys died at the age of 18 months and presented the same clinical symptoms. The triad mental retardation, hypotonia, constipation is a characteristic finding in the FG syndrome, an X-linked mental retardation syndrome. At the present time, there is increasing evidence that the FG syndrome-phenotype may be present in different XLMR conditions, e.g. the fragile X syndrome. In addition to the triad severe mental retardation, hypotonia, constipation, the present two male index patients had a characteristic facial appearance with nasal hypoplasia, relative microcephaly and pre- and postnatal overgrowth. The question is raised whether the present two males are examples of a specific entity within the FG-syndrome-like phenotype.

Further linkage evidence for localization of mutational sites for nonsyndromic types of X-linked mental retardation at the pericentromeric region

We used several microsatellite markers scattered along the X chromosome to search for linkage relationships in a large Sardinian pedigree segregating for nonspecific X-linked mental retardation (MRX). Markers DXS573 and AR, located at chromosomal subregions Xp11.4-p11.22 and Xq11.2-q12, respectively, were found to segregate in full concordance with the disease, leading to a LOD score of 4.21 at zero recombination value. Recombination with the disease was found with markers MAOB and DXS454 located at Xp11.4-p11.3 and Xq21.1-q22, respectively; accordingly, markers distal to Xp11.4 and Xq22 also segregated independently of the disease. These findings provide strong linkage evidence in favor of the localization of one MRX mutational site in the pericentromeric region of the human X chromosome, justifying the assignment of a new symbol (MRX26) to our pedigree. Finally, on the basis of the recombinational events observed in the Xq21-q22 region, we have been able to refine the assignment of marker DXS456 to Xq21.33-q22.

Linkage analysis in three families with nonspecific X-linked mental retardation

Nonspecific X-linked mental retardation (XLMR) is a common disorder. The number of genes involved in this condition is not known, but it is estimated to be more than 10. We present a clinical and linkage study on 3 families with XLMR. All families were analyzed using highly polymorphic markers covering the X chromosome; screening for the fragile X mutation was negative. The first family (MRX 36) consisted of 1 female and 4 male patients in 3 generations and 7 healthy individuals. Considering the female as an expressing heterozygous carrier, a maximum LOD score of 3.41 was reached in region Xp21.2-Xp22.1. Considering her phenotype to be unknown, a LODmax of 1.97 was reached in the same region. The second family consisted of 5 affected and 6 healthy males with mild to borderline mental retardation. Linkage analysis using an X-linked recessive model with full penetrance and no phenocopies excluded linkage over almost the entire X chromosome. Using alternative models, including an affecteds-only analysis, a LODmax of 1.49 was found in region Xq24-28. The third family, consisting of 4 male patients with moderate mental retardation in 1 generation yielded a LODmax of 0.9 in region Xp22.13-11.3. However, even in this small pedigree, exclusion mapping was able to exclude very large parts of the X chromosome and in this way identify a likely candidate region.

X chromosome inactivation and X-linked mental retardation

The expression of X-linked genes in females heterozygous for X-linked defects can be modulated by epigenetic control mechanisms that constitute the X chromosome inactivation pathway. At least four different effects have been found to influence, in females, the phenotypic expression of genes responsible for X-linked mental retardation (XLMR). First, non-random X inactivation, due either to stochastic or genetic factors, can result in tissues in which one cell type (for example, that in which the X chromosome carrying a mutant XLMR gene is active) dominates, instead of the normal mosaic cell population expected as a result of random X inactivation. Second, skewed inactivation of the normal X in individuals carrying a deletion of part of the X chromosome has been documented in a number of mentally retarded females. Third, functional disomy of X-linked genes that are expressed inappropriately due to the absence of X inactivation has been found in mentally retarded females with structurally abnormal X chromosomes that do not contain the X inactivation center. And fourth, dose-dependent overexpression of X-linked genes that normally "escape" X inactivation may account for the mental and developmental delay associated with increasing numbers of otherwise inactive X chromosomes in individuals with X chromosome aneuploidy.

Mental retardation: genetic findings, clinical implications and research agenda

The most important genetic advances in the field of mental retardation include the discovery of the novel genetic mechanism responsible for the Fragile X syndrome, and the imprinting involved in the Prader-Willi and Angelman syndromes, but there have also been advances in our understanding of the pathogenesis of Down syndrome and phenylketonuria. Genetic defects (both single gene Mendelizing disorders and cytogenetic abnormalities) are involved in a substantial proportion of cases of mild as well as severe mental retardation, indicating that the previous equating of severe mental retardation with pathology, and of mild retardation with normal variation, is a misleading over-simplication. Within the group in which no pathological cause can be detected, behaviour genetic studies indicate that genetic influences are important, but that their interplay with environmental factors, which are also important, is at present poorly understood. Research into the joint action of genetic and environmental influences in this group will be an important research area in the future.

Localisation of a new gene for non-specific mental retardation to Xq22-q26 (MRX35)

Non-specific mental retardation (MR) is a condition in which MR appears to be the only consistent manifestation. The X linked form (MRX) is genetically heterogeneous. We report clinical, cytogenetic, and linkage data on a family with X linked non-specific MR. Two point and multi-point linkage analysis with 18 polymorphic markers, covering the entire chromosome, showed close linkage to DXS1001 and DXS425 with a maximal lod score of 2.41 at 0% recombination. DXS178 and the gene for hypoxanthine phosphoribosyl-transferase (HPRT), located in Xq22 and Xq26 respectively, flank the mutation. All other chromosomal regions could be excluded with odds of at least 100:1. To our knowledge there is currently no other non-specific MR gene mapped to this region. Therefore, the gene causing MR in this family can be considered to be a new, independent MRX locus (MRX35).

Nonspecific X-linked mental retardation with macrocephaly and obesity: a further family

The phenotypic nonspecificity of many forms of X-linked mental retardation has hampered attempts to classify them into clinically homogeneous groups. One such condition, described by Clark and Baraitser [1987: Am J Med Genet 26:13-15], has been the subject of a single pedigree report to date. We now describe a further pedigree whose affected members share many manifestations with those reported by Clark and Baraitser, and we consider the possible distinction between this condition and Atkin-Flaitz syndrome.

Genetic analysis of a kindred with X-linked mental handicap and retinitis pigmentosa

A kindred is described in which X-linked nonspecific mental handicap segregates together with retinitis pigmentosa. Carrier females are mentally normal but may show signs of the X-linked retinitis pigmentosa carrier state and become symptomatic in their later years. Analysis of polymorphic DNA markers at nine loci on the short arm of the X chromosome shows that no crossing-over occurs between the disease and Xp11 markers DXS255, TIMP, DXS426, MAOA, and DXS228. The 90% confidence limits show that the locus is in the Xp21-q21 region. Haplotype analysis is consistent with the causal gene being located proximal to the Xp21 loci DXS538 and 5'-dystrophin on the short arm of the X chromosome. The posterior probability of linkage to the RP2 region of the X chromosome short arm (Xp11.4-p11.23) is .727, suggesting the possibility of a contiguous-gene-deletion syndrome. No cytogenetic abnormality has been identified.

A clinical and genetic study of X-linked recessive ichthyosis and contiguous gene defects

X-linked recessive ichthyosis (XLI) is caused by a deletion, or mutation, of the steroid sulphatase gene on the distal short arm of the X chromosome (Xp22.3). This region of the X chromosome is particularly susceptible to deletions. Such deletions can occasionally extend to involve neighbouring genes, causing a contiguous gene defect. Therefore, XLI may be associated with Kallmann's syndrome (KS), mental retardation, X-linked recessive chondrodysplasia punctata and short stature. We have reviewed 33 patients with XLI. Nine showed evidence of contiguous gene defects. A further four had neurological deficit sustained at the time of birth. This study highlights the importance of screening patients with X-linked recessive ichthyosis for neighbouring genetic disorders and, in particular, the early identification of KS, as delay in diagnosis may lead to infertility and osteoporosis. Parents should be warned about possible obstetric complications due to prolonged labour in future pregnancies.

New X-linked mental retardation (XLMR) syndrome with distinct facial appearance and growth retardation

We report on 2 brothers and their nephew with an apparently new X-linked mental retardation (XLMR) syndrome characterized by a distinct facial appearance, growth retardation, and severe mental retardation. The facial traits included triangular shape; bifrontal narrowness; malar flatness; blepharophimosis; very deeply set eyes; epicanthus inversus; bulbous nose; low hairline; low-set, deeply cupped, and protruding ears; short ill-defined philtrum; and thin tented upper lip. These facial anomalies are particularly striking and recognizable even at birth. The boys were small for gestational age and remained below -2 SD in growth parameters. With age, large joint contractures developed. Pectus excavatum was apparent at birth but became more obvious with age. Global developmental delay was evident in infancy. The brothers were nonverbal while their nephew spoke simple words. Optic atrophy, esotropia, nystagmus, and spastic diplegia were evident. They were self-abusive, hyperactive, and poorly coordinated. CT scans demonstrated atrophic hydrocephalus. No EEG abnormalities were detected. Karyotypes were 46,XY and fragile X negative. Routine chemistries; amino, organic, and uronic acids; oligosaccharides; lysosomal enzymes; and very long chain fatty acids were normal. Remarkable phenotypic similarity between these brothers and their nephew and lack of manifestations in their mothers makes X-linked recessive inheritance likely. This syndrome, which does not appear to have been reported previously, adds to the delineation of XLMR.

Mental retardation, congenital heart defect, cleft palate, short stature, and facial anomalies: a new X-linked multiple congenital anomalies/mental retardation syndrome: clinical description and molecular studies

We report on two brothers and their two maternal uncles with severe mental retardation, congenital heart defect, cleft or highly arched palate, short stature and craniofacial anomalies consisting of microcephaly, abnormal ears, bulbous nose, broad nasal bridge, malar hypoplasia, and micrognathia. Three of the four patients died at an early age. The mother of the two brothers had an atrial septal defect. She is assumed to be a manifesting carrier of a mutant gene, which is expressed in her two sons and two brothers. By multipoint linkage analysis it is found that the most likely location of the responsible gene is the pericentromeric region Xp21.3-q21.3 with DMD and DXS3 as flanking markers. Maximum information is obtained with marker DXS453 (Z = 1.20 at theta = 0.0).

Non-specific X-linked mental retardation: linkage analysis in MRX2 and MRX4 families revisited

We have previously reported linkage analysis in 3 families with non-specific X-linked mental retardation (XLMR). This used RFLPs and was limited by the relatively low informativeness and density of markers available. We have performed a new linkage analysis using microsatellites (including new Genethon markers) in the two most informative families. In the MRX2 family, a lod score of 2.61 at theta = 0.05 had previously been obtained with DXS85 in Xp22.2. We now report a tighter linkage with AFM 135xe7 (DXS989, z = 4.62 at theta = 0.00) and established the order DXS85-DXS207-DXS999 (AFM234 y12)-MRX2, DXS365, DXS1052 (AFM 163yh2), DXS989-DXS1065 (AFM224zf2), DMD 3'. The localization of MRX2 in Xp22.2-p22.1 is thus clearly different from the more distal MRX gene defined by patients with contiguous gene syndromes. In the MRX4 family, a maximum lod score of 2.53 at theta = 0.00 had been obtained with DXS159 in Xq13. Our present study did not show recombination from ALAS2 in Xp11.21 to DXS441 in Xq13.3 (z = 3.38 at theta = 0.00 for the latter marker) and the closest flanking markers are DXS255 in Xp11.22 and DXYS1 in Xq21.3. Reduced recombination around the centromere prevents precise mapping. The localisation of MRX4 overlaps with that of several other MRX families.

Towards identification of X-linked mental retardation genes: a proposal

Identification of X linked mental retardation (XLMR) genes that can only be broadly localised by linkage analysis will ultimately depend on systematic screening of many probands for mutations in many candidate genes. This would be more efficiently performed by analysis of mRNA (or illegitimate transcripts) by reverse transcriptase-polymerase chain reaction (RT-PCR). A scheme is proposed that associates standardized reporting of XLMR families, including small families that would not by themselves yield statistically significant linkage information, and deposit of a lymphoblastoid cell line for one proband of each family to an accessible repository.

Developmental disabilities: genetic implications

Knowledge and new techniques in genetics can aid in a better understanding of developmental disabilities. Through new treatments and better assessment skills, individuals with disorders such as phenylketonuria (PKU), Down syndrome, and fragile X syndrome are having diagnoses sooner and are living into adulthood. With the passage of public law 99.457, early intervention services are mandated for those who are 3 years old or younger and are developmentally delayed or at risk for delay. Organizations for paraprofessionals and professionals in the arena of developmental disabilities and genetics exist to create a forum for future action, awareness, and direction.

The genetics of mental retardation

Individuals affected by mental retardation are a clinically and aetiologically heterogeneous group. This heterogeneity is particularly highlighted when we consider the genetics of mental retardation. Recent advances in molecular genetic techniques have enabled us to understand more about the molecular basis of several genetic syndromes associated with mental retardation. In contrast, where there is no discrete cause, the interplay of genetic and environmental influences remains poorly understood.

X-linked mental retardation exhibiting linkage to DXS255 and PGKP1: a new MRX family (MRX14) with localization in the pericentromeric region

Gene localization was determined by linkage analysis in a large French family with X-linked mental retardation (MRX). Seven living affected males were clinically studied and the clinical picture was characterized by moderate to severe mental handicap with poor secondary speech acquisition. Seizures, slight microcephaly, simian crease, anteverted pinnae, and macroorchidism were observed in some patients only. Linkage analysis revealed no recombination between the MRX gene and two loci: DXS255 at Xp11.22 (Zmax = 3.31 at theta = 0.00) and PGKP1 at Xq11.2-q12 (Zmax = 3.08 at theta = 0.00). One recombination was observed between the gene and the two loci DXS164 at Xp21.2 and DXS441 at Xq13.3, respectively. These results suggested gene localization in the pericentromeric region of the X chromosome, and the LOD scores justified assignment of the symbol MRX14 to this family.

Smith-Fineman-Myers syndrome: report on a large family

Smith-Fineman-Myers syndrome is considered an X-linked mental retardation (XLMR) syndrome. Three families have been reported to date. However, none presented a typical pattern of X-linked inheritance. Here we reported on 5 males with Smith-Fineman-Myers syndrome with similar phenotypic expression as in those cases reported previously; they were distributed in 4 sibships of one large maternal kindred. This finding adds strong support to the hypothesis of the Smith-Fineman-Myers syndrome being a rare XLMR syndrome.

Origins of the fragile X syndrome mutation

The fragile X syndrome is a common cause of mental impairment. In view of the low reproductive fitness of affected males, the high incidence of the syndrome has been suggested to be the result of a high rate of new mutations occurring exclusively in the male germline. Extensive family studies, however, have failed to identify any cases of a new mutation. Alternatively, it has been suggested that a selective advantage of unaffected heterozygotes may, in part, explain the high incidence of the syndrome. Molecular investigations have shown that the syndrome is caused by the amplification of a CGG trinucleotide repeat in the FMR-1 gene which leads to the loss of gene expression. Further to this, genetic studies have suggested that there is evidence of linkage disequilibrium between the fragile X disease locus and flanking polymorphic markers. More recently, this analysis has been extended and has led to the observation that a large number of fragile X chromosomes appear to be lineage descendants of founder mutation events. Here, we present a study of the FRAXAC1 polymorphic marker in our patient cohort. We find that its allele distribution is strikingly different on fragile X chromosomes, confirming the earlier observations and giving further support to the suggestions of a fragile X founder effect.

The Juberg-Marsidi syndrome maps to the proximal long arm of the X chromosome (Xq12-q21)

Juberg-Marsidi syndrome (McKusick 309590) is a rare X-linked recessive condition characterized by severe mental retardation, growth failure, sensorineural deafness, and microgenitalism. Here we report on the genetic mapping of the Juberg-Marsidi gene to the proximal long arm of the X chromosome (Xq12-q21) by linkage to probe pRX214H1 at the DXS441 locus (Z = 3.24 at theta = .00). Multipoint linkage analysis placed the Juberg-Marsidi gene within the interval defined by the DXS159 and the DXYS1X loci in the Xq12-q21 region. These data provide evidence for the genetic distinction between Juberg-Marsidi syndrome and several other X-linked mental retardation syndromes that have hypogonadism and hypogenitalism and that previously. Finally, the mapping of the Juberg-Marsidi gene is of potential interest for reliable genetic counseling of at-risk women.