Screening of the ARX gene in 682 retarded males

Type 1 early infantile epileptic encephalopathy: A case report and literature review

BACKGROUND

Variants in the Aristaless-related homeobox (ARX) gene lead to a variety of phenotypes, with intellectual disability being a steady feature. Other features can include severe epilepsy, spasticity, movement disorders, hydranencephaly, and ambiguous genitalia in males. X-linked Ohtahara syndrome or Type 1 early infantile epileptic encephalopathy (EIEE1) is a severe early-onset epileptic encephalopathy with arrested psychomotor development caused by hemizygous mutations in the ARX gene, which encodes a transcription factor in fundamental brain developmental processes.

METHODS

We presented a case report of a 2-year-old boy who exhibited symptoms such as microcephaly, seizures, and severe multifocal epileptic abnormalities, and genetic techniques such as autozygosity mapping, Sanger sequencing, and whole-exome sequencing.

RESULTS

We confirmed that the patient had the NM_139058.3:c.84C>A; p.(Cys28Ter) mutation in the ARX gene.

CONCLUSION

The patient with EIEE1 had physical symptoms and hypsarrhythmia on electroencephalogram. Genetic testing identified a causative mutation in the ARX gene, emphasizing the role of genetic testing in EIEE diagnosis.

Further characterisation of ARX-related disorders in females due to inherited or de novo variants

The Aristaless-related homeobox (ARX) gene is located on the X chromosome and encodes a transcription factor that is essential for brain development. While the clinical spectrum of ARX-related disorders is well described in males, from X linked lissencephaly with abnormal genitalia syndrome to syndromic and non-syndromic intellectual disability (ID), its phenotypic delineation in females is incomplete. Carrier females in ARX families are usually asymptomatic, but ID has been reported in some of them, as well as in others with de novo variants. In this study, we collected the clinical and molecular data of 10 unpublished female patients with de novo ARX pathogenic variants and reviewed the data of 63 females from the literature with either de novo variants (n=10), inherited variants (n=33) or variants of unknown inheritance (n=20). Altogether, the clinical spectrum of females with heterozygous pathogenic ARX variants is broad: 42.5% are asymptomatic, 16.4% have isolated agenesis of the corpus callosum (ACC) or mild symptoms (learning disabilities, autism spectrum disorder, drug-responsive epilepsy) without ID, whereas 41% present with a severe phenotype (ie, ID or developmental and epileptic encephalopathy (DEE)). The ID/DEE phenotype was significantly more prevalent in females carrying de novo variants (75%, n=15/20) versus in those carrying inherited variants (27.3%, n=9/33). ACC was observed in 66.7% (n=24/36) of females who underwent a brain MRI. By refining the clinical spectrum of females carrying ARX pathogenic variants, we show that ID is a frequent sign in females with this X linked condition.

Screening of the duplication 24 pb of ARX gene in Moroccan patients with X-linked Intellectual Disability

OBJECTIVE

Intellectual Disability (ID) represents a neuropsychiatric disorder, which its etiopathogenesis remains insufficiently understood. Mutations in the Aristaless Related Homeobox gene (ARX) have been identified to cause syndromic and nonsyndromic (NS-ID). The most recurrent mutation of this gene is a duplication of 24pb, c.428-451dup. Epidemiological and genetic studies about ID in the Moroccan population remain very scarce, and none study is carried out on the ARX gene. This work aimed to study c.428-451dup (24 bp) mutation in the exon 2 of the ARX gene in 118 males' Moroccan patients with milder NS-ID to evaluate if the gene screening is a good tool for identifying NS-ID.

RESULTS

Our mutational analysis did not show any dup(24pb) in our patients. This is because based on findings from previous studies that found ARX mutations in 70% of families with NS-ID, and in most cases, 1.5-6.1% of individuals with NS-ID have this duplication. Since 1/118 = 0.0084 (0.84%) is not much different from 1.5%, then it is reasonable that this could a sample size artifact. A complete screening of the entire ARX gene, including the five exons, should be fulfilled. Further investigations are required to confirm these results.

Immunodeficiency in a Patient with 22q11.2 Distal Deletion Syndrome and a p.Ala7dup Variant in the MAPK1 Gene

The genetic basis for sporadic immunodeficiency in patients with 22q11.2 distal deletion syndrome is unknown. We report an adult with a type 1 (D-F) 22q11.2 distal deletion syndrome and recurrent severe infections due to herpes zoster virus, presenting mild T cell lymphopenia and diminished frequency of naive CD4⁺ T cells, but increased frequencies of central, effector, and terminally differentiated memory T cells. Antigen-specific CD4⁺ and CD8⁺ T cells to influenza, rotavirus, and SEB were conserved in the patient, but responses to tetanus toxoid were temporarily undetectable. Exomic sequencing identified the c.20_22dupCGG (NM_002745.4) variant in the remaining MAPK1 gene of the patient, which adds 1 alanine to the polyalanine amino-terminal tract of the protein (p.Ala7dup). The mother, unlike the father, was heterozygote for the variant. Western blot analysis with the patient's activated PBMCs showed a 91% reduction in the MAPK1 protein. Further studies will be necessary to determine whether or not the variant present in the remaining MAPK1 gene of the patient is pathogenic.

Wwox deletion leads to reduced GABA-ergic inhibitory interneuron numbers and activation of microglia and astrocytes in mouse hippocampus

The association of WW domain-containing oxidoreductase WWOX gene loss of function with central nervous system (CNS) related pathologies is well documented. These include spinocerebellar ataxia, epilepsy and mental retardation (SCAR12, OMIM: 614322) and early infantile epileptic encephalopathy (EIEE28, OMIM: 616211) syndromes. However, there is complete lack of understanding of the pathophysiological mechanisms at play. In this study, using a Wwox knockout (Wwox KO) mouse model (2 weeks old, both sexes) and stereological studies we observe that Wwox deletion leads to a significant reduction in the number of hippocampal GABA-ergic (γ-aminobutyric acid) interneurons. Wwox KO mice displayed significantly reduced numbers of calcium-binding protein parvalbumin (PV) and neuropeptide Y (NPY) expressing interneurons in different subfields of the hippocampus in comparison to Wwox wild-type (WT) mice. We also detected decreased levels of Glutamic Acid Decarboxylase protein isoforms GAD65/67 expression in Wwox null hippocampi suggesting lower levels of GABA synthesis. In addition, Wwox deficiency was associated with signs of neuroinflammation such as evidence of activated microglia, astrogliosis, and overexpression of inflammatory cytokines Tnf-a and Il6. We also performed comparative transcriptome-wide expression analyses of neural stem cells grown as neurospheres from hippocampi of Wwox KO and WT mice thus identifying 283 genes significantly dysregulated in their expression. Functional annotation of transcriptome profiling differences identified 'neurological disease' and 'CNS development related functions' to be significantly enriched. Several epilepsy-related genes were found differentially expressed in Wwox KO neurospheres. This study provides the first genotype-phenotype observations as well as potential mechanistic clues associated with Wwox loss of function in the brain.

Regulation of histone H3K4 methylation in brain development and disease

The growing list of mutations implicated in monogenic disorders of the developing brain includes at least seven genes (ARX, CUL4B, KDM5A, KDM5C, KMT2A, KMT2C, KMT2D) with loss-of-function mutations affecting proper regulation of histone H3 lysine 4 methylation, a chromatin mark which on a genome-wide scale is broadly associated with active gene expression, with its mono-, di- and trimethylated forms differentially enriched at promoter and enhancer and other regulatory sequences. In addition to these rare genetic syndromes, dysregulated H3K4 methylation could also play a role in the pathophysiology of some cases diagnosed with autism or schizophrenia, two conditions which on a genome-wide scale are associated with H3K4 methylation changes at hundreds of loci in a subject-specific manner. Importantly, the reported alterations for some of the diseased brain specimens included a widespread broadening of H3K4 methylation profiles at gene promoters, a process that could be regulated by the UpSET(KMT2E/MLL5)-histone deacetylase complex. Furthermore, preclinical studies identified maternal immune activation, parental care and monoaminergic drugs as environmental determinants for brain-specific H3K4 methylation. These novel insights into the epigenetic risk architectures of neurodevelopmental disease will be highly relevant for efforts aimed at improved prevention and treatment of autism and psychosis spectrum disorders.

Unraveling the pathogenesis of ARX polyalanine tract variants using a clinical and molecular interfacing approach

The Aristaless-related homeobox (ARX) gene is implicated in intellectual disability with the most frequent pathogenic mutations leading to expansions of the first two polyalanine tracts. Here, we describe analysis of the ARX gene outlining the approaches in the Australian and Portuguese setting, using an integrated clinical and molecular strategy. We report variants in the ARX gene detected in 19 patients belonging to 17 families. Seven pathogenic variants, being expansion mutations in both polyalanine tract 1 and tract 2, were identifyed, including a novel mutation in polyalanine tract 1 that expands the first tract to 20 alanines. This precise number of alanines is sufficient to cause pathogenicity when expanded in polyalanine tract 2. Five cases presented a probably non-pathogenic variant, including the novel HGVS: c.441_455del, classified as unlikely disease causing, consistent with reports that suggest that in frame deletions in polyalanine stretches of ARX rarely cause intellectual disability. In addition, we identified five cases with a variant of unclear pathogenic significance. Owing to the inconsistent ARX variants description, publications were reviewed and ARX variant classifications were standardized and detailed unambiguously according to recommendations of the Human Genome Variation Society. In the absence of a pathognomonic clinical feature, we propose that molecular analysis of the ARX gene should be included in routine diagnostic practice in individuals with either nonsyndromic or syndromic intellectual disability. A definitive diagnosis of ARX-related disorders is crucial for an adequate clinical follow-up and accurate genetic counseling of at-risk family members.

The c.429_452 duplication of the ARX gene: a unique developmental-model of limb kinetic apraxia

Background

The c.429_452dup24 of the ARX gene is a rare genetic anomaly, leading to X-Linked Intellectual Disability without brain malformation. While in certain cases c.429_452dup24 has been associated with specific clinical patterns such as Partington syndrome, the consequence of this mutation has been also often classified as “non-specific Intellectual Disability”. The present work aims at a more precise description of the clinical features linked to the c.429_452dup24 mutation.

Methods

We clinically reviewed all affected patients identified in France over a five-year period, i.e. 27 patients from 12 different families. Detailed cognitive, behavioural, and motor evaluation, as well as standardized videotaped assessments of oro-lingual and gestural praxis, were performed. In a sub-group of 13 ARX patients, kinematic and MRI studies were further accomplished to better characterize the motor impairment prevalent in the ARX patients group. To ensure that data were specific to the ARX gene mutation and did not result from low-cognitive functioning per se, a group of 27 age- and IQ-matched Down syndrome patients served as control.

Results

Neuropsychological and motor assessment indicated that the c.429_452dup24 mutation constitutes a recognizable clinical syndrome: ARX patients exhibiting Intellectual Disability, without primary motor impairment, but with a very specific upper limb distal motor apraxia associated with a pathognomonic hand-grip. Patients affected with the so-called Partington syndrome, which involves major hand dystonia and orolingual apraxia, exhibit the most severe symptoms of the disorder. The particular “reach and grip” impairment which was observed in all ARX patients, but not in Down syndrome patients, was further characterized by the kinematic data: (i) loss of preference for the index finger when gripping an object, (ii) major impairment of fourth finger deftness, and (iii) a lack of pronation movements. This lack of distal movement coordination exhibited by ARX patients is associated with the loss of independent digital dexterity and is similar to the distortion of individual finger movements and posture observed in Limb Kinetic Apraxia.

Conclusion

These findings suggest that the ARX c.429_452dup24 mutation may be a developmental model for Limb Kinetic Apraxia.

Novel mutation in ARX associated with early hand preference and a mild phenotype

The authors report detailed clinical and developmental assessment of 3 brothers who were found to carry a novel mutation in the ARX gene associated with a relatively mild phenotype of static global developmental delay and early hand preference. The decision of when to initiate specialized genetic testing for patients with apparently isolated developmental delay remains controversial, and this report of 3 brothers who presented with early hand preference and transient contralateral weakness may assist clinicians in prioritizing investigations in patients with a similar presentation.

Screening and cell-based assessment of mutations in the Aristaless-related homeobox (ARX) gene

ARX mutations cause a diverse spectrum of human disorders, ranging from severe brain and genital malformations to non-syndromic intellectual disability (ID). ARX is a transcription factor with multiple domains that include four polyalanine (pA) tracts, the first two of which are frequently expanded by mutations. We progressively screened DNA samples from 613 individuals with ID initially for the most frequent ARX mutations (c.304ins(GCG)(7)'expansion' of pA1 and c.429_452dup 'dup24bp' of pA2). Five hundred samples without pA1 or pA2 mutations had the entire ARX ORF screened by single stranded polymorphism conformation (SSCP) and/or denaturing high pressure liquid chromatography (dHPLC) analysis. Overall, eight families with six mutations in ARX were identified (1.31%): five duplication mutations in pA2 (0.82%) with three new clinical reports of families with the dup24bp and two duplications larger than the dup24bp mutation discovered (dup27bp, dup33bp); and three point mutations (0.6%), including one novel mutation in the homeodomain (c.1074G>T). Four ultraconserved regions distal to ARX (uc466-469) were also screened in a subset of 94 patients, with three unique nucleotide changes identified in two (uc466, uc467). The subcellular localization of full length ARX proteins was assessed for 11 variants. Protein mislocalization increased as a function of pA2 tract length and phenotypic severity, as has been previously suggested for pA1. Similarly, protein mislocalization of the homeodomain mutations also correlated with clinical severity, suggesting an emerging genotype vs cellular phenotype correlation.

Contractions in the second polyA tract of ARX are rare, non-pathogenic polymorphisms

Aristaless related homeobox (ARX) is a transcription factor containing highly conserved octapeptide, homeobox, acidic, and aristaless domains, as well as four polyA tracts. The most frequent ARX mutation found to date in patients with X-linked infantile spasms, Partington syndrome or X-linked mental retardation, is a duplication of 24 bp in exon 2, resulting in the expansion of the second polyA tract. Although the pathogenic role of this expansion has been well characterized, the effect of contractions in the same polyA tract is still debated since different reports have associated contractions to either mental retardation or a normal phenotype. Here, we report two unrelated girls with epilepsy and mental retardation who inherited from their unaffected parents, of either sex, a deletion of 24 bp (c.441_464del), resulting in a contraction of eight alanines in the second polyA tract of ARX. Segregation studies revealed the c.441_464del also in two healthy relatives of one of the patients. This finding supports the hypothesis that this contraction represents a rare, benign polymorphism.

ARX spectrum disorders: making inroads into the molecular pathology

The Aristaless-related homeobox gene (ARX) is one of the most frequently mutated genes in a spectrum of X-chromosome phenotypes with intellectual disability (ID) as their cardinal feature. To date, close to 100 families and isolated cases have been reported to carry 44 different mutations, the majority of these (59%) being a result of polyalanine tract expansions. At least 10 well-defined clinical entities, including Ohtahara, Partington, and Proud syndromes, X-linked infantile spasms, X-linked lissencephaly with ambiguous genitalia, X-linked myoclonic epilepsy and nonsyndromic intellectual disability have been ascertained from among the patients with ARX mutations. The striking intra- and interfamilial pleiotropy together with genetic heterogeneity (same clinical entities associated with different ARX mutations) are becoming a hallmark of ARX mutations. Although males are predominantly affected, some mutations associated with malformation phenotypes in males also show a phenotype in carrier females. Recent progress in the study of the effect of ARX mutations through sophisticated animal (mice) and cellular models begins to provide crucial insights into the molecular function of ARX and associated molecular pathology, thus guiding future inquiries into therapeutic interventions.

A novel de novo 27 bp duplication of the ARX gene, resulting from postzygotic mosaicism and leading to three severely affected males in two generations

The Aristaless Related Homeobox (ARX) gene is a Q(50) paired homeobox gene. These genes are important regulators of essential events during vertebrate embryogenesis, including the development of the central and peripheral nervous system. Mutations in ARX have been identified in at least 82 different families and sporadic cases, and are responsible for at least 8 clinically distinct disorders. The recurrent 24 bp duplication (dup) mutation, c.429_452dup(24 bp), is the most frequent ARX mutation, which accounts for 45% of all cases reported to date. Here we report a novel de novo, familial dup mutation of 27 bp, c.430_456dup(27 bp), which involves the same region of the ARX gene in exon 2, as the dup24 bp mutation. The female progenitor of this dup27 bp allele exhibits mosaicism, likely resulting from a postmitotic de novo mutation event early in embryonic development. Three males with the dup27 bp mutation presented with infantile spasms, two of whom died early in life. Their phenotype appeared more severe, when compared to the spectrum of clinical presentations associated with the dup24 bp mutation. We propose that this might be at least partly due to the single, extra alanine residue (A) (21A in dup27 vs. 20A in dup24), which takes polyalanine tract 2 of ARX beyond the maximum, naturally occurring limit of 20A found in the human genome.

Clinical study of two brothers with a novel 33 bp duplication in the ARX gene

Pathogenic variations of the ARX (aristaless-related homeobox) gene are associated with marked phenotypic pleiotropy. These phenotypes are X-linked neurological disorders that include brain and genital malformation and non-malformation syndromes. Typically, malformation phenotypes result from pathogenic variations that are predicted to truncate the ARX protein, or alter residues in the highly conserved homeodomain. While non-malformation phenotypes tend to be caused by pathogenic variations that are predicted to expand the first two polyalanine tracts of ARX, or alter residues outside of the homeodomain. The most common pathogenic variation of the ARX gene is a duplication of 24 bp, c.429_452 dup, which leads to an expansion of the second polyalanine tract of the ARX protein from 12 to 20 alanine residues. This pathogenic variation is associated with both sporadic and familial nonsyndromic mental retardation. Syndromic manifestations include mental retardation with hand dystonia (Partington syndrome), infantile spasms (West syndrome) and/or other epileptic seizures. Here, we report on a novel pathogenic variant of a tandem 33 bp duplication that is predicted to result in an expansion of polyalanine tract 2 in two brothers with mental retardation, epilepsy, dystonia, and the novel feature of intermittent hyperventilation. This pathogenic variation is predicted to result in a "non-homogeneous" polyalanine tract expansion that is longer than predicted expansion caused by the common 24 bp duplication. The location of the novel 33 bp duplication in the same region as the common 24 bp duplication supports this region as the ARX variation "hot spot."

Identification of Arx transcriptional targets in the developing basal forebrain

Mutations in the aristaless-related homeobox (ARX) gene are associated with multiple neurologic disorders in humans. Studies in mice indicate Arx plays a role in neuronal progenitor proliferation and development of the cerebral cortex, thalamus, hippocampus, striatum, and olfactory bulbs. Specific defects associated with Arx loss of function include abnormal interneuron migration and subtype differentiation. How disruptions in ARX result in human disease and how loss of Arx in mice results in these phenotypes remains poorly understood. To gain insight into the biological functions of Arx, we performed a genome-wide expression screen to identify transcriptional changes within the subpallium in the absence of Arx. We have identified 84 genes whose expression was dysregulated in the absence of Arx. This population was enriched in genes involved in cell migration, axonal guidance, neurogenesis, and regulation of transcription and includes genes implicated in autism, epilepsy, and mental retardation; all features recognized in patients with ARX mutations. Additionally, we found Arx directly repressed three of the identified transcription factors: Lmo1, Ebf3 and Shox2. To further understand how the identified genes are involved in neural development, we used gene set enrichment algorithms to compare the Arx gene regulatory network (GRN) to the Dlx1/2 GRN and interneuron transcriptome. These analyses identified a subset of genes in the Arx GRN that are shared with that of the Dlx1/2 GRN and that are enriched in the interneuron transcriptome. These data indicate Arx plays multiple roles in forebrain development, both dependent and independent of Dlx1/2, and thus provides further insights into the understanding of the mechanisms underlying the pathology of mental retardation and epilepsy phenotypes resulting from ARX mutations.

A longer polyalanine expansion mutation in the ARX gene causes early infantile epileptic encephalopathy with suppression-burst pattern (Ohtahara syndrome)

Early infantile epileptic encephalopathy with suppression-burst pattern (EIEE) is one of the most severe and earliest forms of epilepsy, often evolving into West syndrome; however, the pathogenesis of EIEE remains unclear. ARX is a crucial gene for the development of interneurons in the fetal brain, and a polyalanine expansion mutation of ARX causes mental retardation and seizures, including those of West syndrome, in males. We screened the ARX mutation and found a hemizygous, de novo, 33-bp duplication in exon 2, 298_330dupGCGGCA(GCG)9, in two of three unrelated male patients with EIEE. This mutation is thought to expand the original 16 alanine residues to 27 alanine residues (A110_A111insAAAAAAAAAAA) in the first polyalanine tract of the ARX protein. Although EIEE is mainly associated with brain malformations, ARX is the first gene found to be responsible for idiopathic EIEE. Our observation that EIEE had a longer expansion of the polyalanine tract than is seen in West syndrome is consistent with the findings of earlier onset and more-severe phenotypes in EIEE than in West syndrome.

Childhood autism and associated comorbidities

Autism is a heterogeneous neurodevelopmental disorder with a variety of different etiologies, but with a heritability estimate of more than 90%. Although the strong correlation between autism and genetic factors has been long established, the exact genetic background of autism is still unclear. This review refers to all the genetic syndromes that have been described in children with pervasive developmental disorders (tuberous sclerosis, fragile X, Down, neurofibromatosis, Angelman, Prader-Willi, Gilles de la Tourette, Williams, etc.). Issues covered include prevalence and main characteristics of each syndrome, as well as the possible base of its association with autism in terms of contribution to the current knowledge on the etiology and genetic base of pervasive developmental disorders.

A novel mutation of the ARX gene in a male with nonsyndromic mental retardation

ARX (Aristaless-related homeobox gene) is located at Xp22. It contains 5 exons and encodes a 562-amino acid protein. The protein contains 4 polyalanine tracts, 3 of which are encoded in exon 2 and 1 in exon 4. Mutations in the ARX gene have been found in X-linked infantile spasms syndrome, Partington syndrome (mental retardation with dystonic movements of the hands), X-linked lissencephaly with abnormal genitalia, X-linked myoclonus epilepsy with spasticity and intellectual disability, and in nonsyndromic X-linked mental retardation. The most common mutation in ARX (seen in X-linked infantile spasms syndrome, Partington syndrome, and X-linked mental retardation) is a 24-bp duplication in exon 2 resulting in expansion of a polyalanine tract. Truncating mutations (deletions, frameshift, non-sense) have been found in X-linked lissencephaly with abnormal genitalia, as well as homeodomain missense mutations in X-linked myoclonus epilepsy with spasticity and intellectual disability. The authors report a novel 24-bp in-frame deletion within exon 2 of the ARX gene in a male child with X-linked mental retardation and review the spectrum of ARX mutations. This mutation results in a contraction of the second polyalanine repeat.

Mutation screening of the Aristaless-related homeobox (ARX) gene in Thai pediatric patients with delayed development: first report from Thailand

Mutations in the Aristaless-related homeobox gene, ARX, have been a cause of X-linked mental retardation (XLMR) and are responsible for a vast phenotypic spectrum including syndromic and non-syndromic forms of mental retardation. Since the gene was initially identified, it has been generally screened in several patients with XLMR. This study is the first report of ARX mutational screening in Thai pediatric patients with delayed development. Two hundred and fifty-one patients participated in this study. Two hundred and three of the 251 patients were initially referred for molecular diagnosis of the Fragile XA syndrome and had negative test results. The remaining 48 patients were specifically recruited for the ARX mutational analysis and had previously reported phenotypes of the ARX mutations. Screening for the c.428_451 dup mutation was performed in all samples. Screening for other point mutations in all coding exons was performed in all 48 patients recruited for the ARX mutational analysis and in 29 patients initially referred for diagnosis of the Fragile XA syndrome who had two or more affected males in the family suggesting an X-linked inheritance pattern. Two patients were found to have the c.428_451 dup mutation. Considering genotype-phenotype correlation, we suggest screening of the most common mutation, the c.428_451 dup mutation by PCR, in patients with infantile spasm syndrome, Partington syndrome and non-syndromic X-linked mental retardation. Screening in patients who have negative Fragile XA test results should be considered when no other known causes of mental retardation are identified especially in families with suggestive X-linked inheritance pattern.

MRX87 family with Aristaless X dup24bp mutation and implication for polyAlanine expansions

Background

Cognitive impairments are heterogeneous conditions, and it is estimated that 10% may be caused by a defect of mental function genes on the X chromosome. One of those genes is Aristaless related homeobox (ARX) encoding a polyA-rich homeobox transcription factor essential for cerebral patterning and its mutations cause different neurologic disorders. We reported on the clinical and genetic analysis of an Italian family with X-linked mental retardation (XLMR) and intra-familial heterogeneity, and provided insight into its molecular defect.

Methods

We carried out on linkage-candidate gene studies in a new MRX family (MRX87). All coding regions and exon-intron boundaries of ARX gene were analysed by direct sequencing.

Results

MRX87 patients had moderate to profound cognition impairment and a combination of minor congenital anomalies. The disease locus, MRX87, was mapped between DXS7104 and DXS1214, placing it in Xp22-p21 interval, a hot spot region for mental handicap. An in frame duplication of 24 bp (ARXdup24) in the second polyAlanine tract (polyA_II) in ARX was identified.

Conclusion

Our study underlines the role of ARXdup24 as a critical mutational site causing mental retardation linked to Xp22. Phenotypic heterogeneity of MRX87 patients represents a new observation relevant to the functional consequences of polyAlanine expansions enriching the puzzling complexity of ARXdup24-linked diseases.

Mutation screening of the ARX gene in patients with autism

Mutations in the Aristaless related homeobox (ARX) gene are associated with a broad spectrum of disorders, including nonsyndromic X-linked mental retardation, sometimes associated with epilepsy, as well as syndromic forms with brain abnormalities and abnormal genitalia. Furthermore, ARX mutations have been described in a few patients with autism or autistic features. In this study, we screened the ARX gene in 226 male patients with autism spectrum disorders and mental retardation; 42 of the patients had epilepsy. The mutation analysis was performed by direct sequencing of all exons and flanking regions. No ARX mutations were identified in any of the patients tested. These findings indicate that mutations in the ARX gene are very rare in autism.

Aristaless-related homeobox gene, the gene responsible for West syndrome and related disorders, is a Groucho/transducin-like enhancer of split dependent transcriptional repressor

Aristaless-related homeobox gene (ARX) is an important paired-type homeobox gene involved in the development of human brain. The ARX gene mutations are a significant contributor to various forms of X-chromosome-linked mental retardation with and without additional features including epilepsy, lissencephaly with abnormal genitalia, hand dystonia or autism. Here we demonstrate that the human ARX protein is a potent transcriptional repressor, which binds to Groucho/transducin-like enhancer of split (TLE) co-factor proteins and the TLE1 in particular through its octapeptide (Engrailed homology repressor domain (eh-1) homology) domain. We show that the transcription repression activity of ARX is modulated by two strong repression domains, one located within the octapeptide domain and the second in the region of the polyalanine tract 4, and one activator domain, the aristaless domain. Importantly, we show that the transcription repression activity of ARX is affected by various naturally occurring mutations. The introduction of the c.98T>C (p.L33P) mutation results in the lack of binding to TLE1 protein and relaxed transcription repression. The introduction of the two most frequent ARX polyalanine tract expansion mutations increases the repression activity in a manner dependent on the number of extra alanines. Interestingly, deletions of alanine residues within polyalanine tracts 1 and 2 show low or no effect. In summary we demonstrate that the ARX protein is a strong transcription repressor, we identify novel ARX interacting proteins (TLE) and offer an explanation of a molecular pathogenesis of some ARX mutations, including the most frequent ARX mutations, the polyalanine tract expansion mutations, c.304ins(GCG)7 and c.428_451dup.

Mutation screening of brain-expressed X-chromosomal miRNA genes in 464 patients with nonsyndromic X-linked mental retardation

MiRNAs are small noncoding RNAs that control the expression of target genes at the post-transcriptional level and have been reported to modulate various biological processes. Their function as regulatory factors in gene expression renders them attractive candidates for harbouring genetic variants with subtle effects on IQ. In an attempt to investigate the potential role of miRNAs in the aetiology of X-linked mental retardation, we have examined all 13 known, brain-expressed X-chromosomal miRNAs in a cohort of 464 patients with non-syndromic X-linked MR and found four nucleotide changes in three different pre-miRNA hairpins. All the observed changes appear to be functionally neutral which, taken together with the rarity of detected nucleotide changes in miRNA genes, may reflect strong selection and thus underline the functional importance of miRNAs.

Mutational screening of ARX gene in Brazilian males with mental retardation of unknown etiology

ARX gene mutations have been known as important causes of developmental and neurological disorders and are responsible for a large spectrum of abnormal phenotypes, includeing syndromic as well as nonsyndromic forms of mental retardation. We have screened the entire coding and flanking intronic sequences of ARX gene in 143 mentally impaired males in order to investigate the contribution of ARX mutations to mental retardation in the population of Rio de Janeiro, Brazil. Three sequence variants were identified: one patient had the most recurrent mutation already observed in ARX gene, the c.428_451dup(24 bp), two patients presented the c.1347C>T (p.G449G) in exon 4, and one patient had the intronic variant c.1074-3T>C. Although two of these alterations were considered polymorphisms, the known pathogenic variant c.428_451dup(24 bp) was found at a high rate (4.8%) among X-linked mental retardation (XLMR) families. Our results, the first in Latin America, reinforce the idea that ARX mutations are relevant to mental retardation and are indicative that molecular screening of exon 2 should be considered in males with mental retardation of unknown etiology, associated or not with neurological manifestations, especially in familial cases.

A new paradigm for West syndrome based on molecular and cell biology

Symptomatic West syndrome has heterogeneous backgrounds. Recently, two novel genes, ARX and CDKL5, have been found to be responsible for cryptogenic West syndrome or infantile spasms. Both are located in the human chromosome Xp22 region and are mainly expressed and play roles in fetal brain. Moreover, several genes responsible for brain malformations including lissencephaly, which is frequently associated with West syndrome or infantile spasms, have been found, and the mechanisms responsible for the neural network disorders in these brain malformations are rapidly being determined. Findings of animal and in vitro studies and mutation analyses in humans are delineating the molecular and cellular basis of West syndrome. Mutations of the ARX gene controlling the development of GABAergic interneurons exhibit pleiotropic effects including lissencephaly with a strong genotype-phenotype correlation. An expansion mutation of the first polyalanine tract of ARX is more strongly related to infantile spasms than is that of the second polyalanine tract. Although the phenotype of CDKL5 mutation is similar to Rett syndrome caused by MECP2 mutation, the former is characterized by early-onset seizures and association with West syndrome. Lissencephaly caused by LIS1 or DCX mutation frequently results in West syndrome, while lissencephaly due to ARX mutation is associated with the most severe form of epilepsy but never results in West syndrome nor infantile spasms. Both LIS1 and DCX participate in the development of GABAergic interneurons as well as pyramidal neurons, while ARX participates only in that of interneurons. Individuals with lissencephaly due to ARX mutation lack non-pyramidal or GABAergic interneurons. ARX is crucial for the development of GABAergic interneuron, so abnormal interneurons in patients with ARX mutation are thought to be implicated in the pathological mechanism, even though brain MRI is normal. Abnormal interneurons appear to play an essential role in the pathogenesis of West syndrome or infantile spasms, which can be considered an interneuronopathy.

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.

ARX: a gene for all seasons

The Aristaless-related homeobox gene, ARX, is an important transcription factor with a crucial role in forebrain, pancreas and testes development. At least fifty-nine mutations have been described in the ARX gene in seven X-chromosome linked disorders involving mental retardation. Recent studies with ARX screening suggest that the gene is mutated in 9.5% of X-linked families with these disorders. Two different polyalanine expansion mutations represent 46% of all currently known mutations and show considerable pleiotropy. The ARX gene is emerging as one of the more important disease-causing genes on the X chromosome and ought to be considered for routine screening. Although the normal Arx protein is known to be a bifunctional transcriptional activator and repressor, the complete biochemical characterization of the normal and mutated ARX awaits further investigation. Pax4 was identified as one of the ARX target genes, and both proteins have crucial functions in endocrine mouse pancreas alpha-cell and beta-cell lineage specification.

TheARX mutations: A frequent cause of X-linked mental retardation

The ARX gene mutations have been demonstrated to cause different forms of mental retardation (MR). Beside FMR1, in families with X-linked mental retardation (XLMR), the ARX dysfunction was demonstrated to be among the most frequent causes of this heterogeneous group of disorders. Nevertheless, in sporadic cases of MR, ARX mutations are extremely rare. In order to evaluate the frequency of ARX mutation in XLMR, we performed mutational analysis of ARX in 165 mentally retarded probands negative for FRAXA and belonging to families in which the condition segregates as an X-linked condition. The same recurrent mutation, an in frame 24 bp insertion (c.428-451 dup (24 bp)), was identified in five patients. In one family, the mother of two affected boys was found not to carry the mutation detected in her sons. These data suggest the presence of germline mosaicism for the mutation in the mother. Our results confirm the significant contribution of ARX mutations in the etiology of MR, especially in this group of patients selected for XLMR (3%). These data, together with those reported in the literature, imply that screening for c.428-451 dup (24 bp) mutation should be recommended in all patients with suspected XLMR.

Screening of ARX in mental retardation families: Consequences for the strategy of molecular diagnosis

Mutations in the human ARX gene have been shown to cause nonsyndromic X-linked mental retardation (MRX) as well as syndromic forms such as X-linked lissencephaly with abnormal genitalia (XLAG), Partington syndrome and X-linked infantile spasm. The most common causative mutation, a duplication of 24 bp, was found in families with a variety of phenotypes, but not in the more severe XLAG phenotypes. The aim of the study was to access the frequency of ARX mutations in families with established or putative X-linked mental retardation (XLMR) collected by the European XLMR Consortium. We screened the entire coding region of ARX for mutations in 197 novel XLMR families by denaturing high-performance liquid chromatography, and we identified eight mutations (six c.428_451dup24, one insertion and one novel missense mutation p.P38S). To better define the prevalence of ARX mutations, we included previously reported results of 157 XLMR families. Together, these data showed the relatively high rate (9.5%) of ARX mutations in X-linked MR families and an expectedly low rate in families with affected brother pairs (2.2%). This study confirms that the frequency of ARX mutations is high in XLMR, and the analysis of ARX in MRX should not be limited to duplication.

ARX mutation c.428-451dup (24bp) in a Brazilian family with X-linked mental retardation

The recently identified gene ARX (Aristalles-Related Homeobox) codifies the ARX protein, an important transcript factor that belongs to one of the three largest classes of homeoproteins, the paired (Prd) class. Several mutations have been identified in ARX gene, which is responsible for a wide spectrum of phenotypes, including syndromic as well as non syndromic forms of mental retardation. One of the mutations, the c.428-451 dup (24 bp) is the most frequent identified in the ARX gene. This duplication leads to an expansion of the second polyalanine tract of ARX protein. We have reported the identification of a Brazilian family segregating the c.428-451 dup (24 bp) in ARX gene.

X linked mental retardation: a clinical guide

Mental retardation is more common in males than females in the population, assumed to be due to mutations on the X chromosome. The prevalence of the 24 genes identified to date is low and less common than expansions in FMR1, which cause Fragile X syndrome. Systematic screening of all other X linked genes in X linked families with mental retardation is currently not feasible in a clinical setting. The phenotypes of genes causing syndromic and non-syndromic mental retardation (NLGN3, NLGN4, RPS6KA3(RSK2), OPHN1, ATRX, SLC6A8, ARX, SYN1, AGTR2, MECP2, PQBP1, SMCX, and SLC16A2) are first discussed, as these may be the focus of more targeted mutation analysis. Secondly, the relative prevalence of genes causing only non-syndromic mental retardation (IL1RAPL1, TM4SF2, ZNF41, FTSJ1, DLG3, FACL4, PAK3, ARHGEF6, FMR2, and GDI) is summarised. Thirdly, the problem of recurrence risk where a molecular genetics diagnosis has not been made and what proportion of the male excess of mental retardation is due to monogenic disorders of the X chromosome are discussed.

Maternal mosaicism for mutations in the ARX gene in a family with X linked mental retardation

We describe two brothers with mental retardation (MR) due to a c.428_451dup24 in the ARX gene. The mother did not apparently carry the mutation, as determined by dHPLC and by fragment size analysis. Using semiquantitative fluorescent PCR, we show however that 4% of her lymphocytes and 24% of her fibroblasts harbored the duplication. We thus show that the mother displays somatic mosaicism for the duplication thereby highlighting the need to reconsider the molecular screening in sporadic cases of MR.

XLMR in MRX families 29, 32, 33 and 38 results from the dup24 mutation in the ARX (Aristaless related homeobox) gene

Background

X-linked mental retardation (XLMR) is the leading cause of mental retardation in males. Mutations in the ARX gene in Xp22.1 have been found in numerous families with both nonsyndromic and syndromic XLMR. The most frequent mutation in this gene is a 24 bp duplication in exon 2. Based on this fact, a panel of XLMR families linked to Xp22 was tested for this particular ARX mutation.

Methods

Genomic DNA from XLMR families linked to Xp22.1 was amplified for exon 2 in ARX using a Cy5 labeled primer pair. The resulting amplicons were sized using the ALFexpress automated sequencer.

Results

A panel of 11 families with X-linked mental retardation was screened for the ARX 24dup mutation. Four nonsyndromic XLMR families – MRX29, MRX32, MRX33 and MRX38 – were found to have this particular gene mutation.

Conclusion

We have identified 4 additional XLMR families with the ARX dup24 mutation from a panel of 11 XLMR families linked to Xp22.1. This finding makes the ARX dup24 mutation the most common mutation in nonsyndromic XLMR families linked to Xp22.1. As this mutation can be readily tested for using an automated sequencer, screening should be considered for any male with nonsyndromic MR of unknown etiology.

X-linked mental retardation: further lumping, splitting and emerging phenotypes

X-linked mental retardation (XLMR) is a very heterogeneous condition, subdivided in two categories mainly based on clinical features: syndromic XLMR (MRXS) and non-syndromic XLMR (MRX). Although it was thought that 20-25% of mental retardation (MR) in males was caused by monogenetic X-linked factors, recent estimations are lower: in the range of 10-12%. The number of identified genes involved in XLMR has been rapidly growing in the past years. Subsequently, an increasing number of patients and families have been reported in which mutations in XLMR genes have been identified. It was observed previously, that mutations in several of XLMR genes can result in syndromic and in non-syndromic phenotypes. This observation has been confirmed for the more recently identified genes. Therefore, in this review, focus has been given on the clinical data and on phenotype-genotype correlations for those genes implicated in both non-syndromic and syndromic XLMR.

Monogenic X-linked mental retardation: is it as frequent as currently estimated? The paradox of the ARX (Aristaless X) mutations

Mental retardation affects 30 to 50% more males than females, and X-linked mental retardation (XLMR) is thought to account for the major part of this sex bias. Nonsyndromic XLMR is very heterogeneous, with more than 15 genes identified to date, each of them accounting for a very small proportion of nonsyndromic families. The Aristaless X (ARX) gene is an exception since it was found mutated in 11 of 136 such families, with a highly recurrent mutation (dup24) leading to an expansion of a polyalanine tract in the protein. The rather high frequency of dup24 reported in families with clear X-linked MR (6.6%) contrasts with the very low prevalence of this mutation observed in sporadic male MR (0.13%). We conclude that monogenic XLMR has much lower prevalence in male MR (< 10%) than the 23% that would be required to account for a 30% male excess of mental retardation.

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.