ARX mutations in X-linked lissencephaly with abnormal genitalia

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.

Dual effects of ARX poly-alanine mutations in human cortical and interneuron development

Mutations in ARX , an X-linked gene, are implicated in a wide spectrum of neurological disorders including patients who have intellectual disability and epilepsy. Mouse models have shown that Arx is critical for cortical development and interneuron migration, however they do not recapitulate the full phenotype observed in patients. Moreover, the epilepsy in many patients with poly-alanine tract expansion (PAE) mutations in ARX show pharmacoresistance, emphasizing the need to develop new treatments. Here, we used human neural organoid models to study the consequences of PAE mutations, one of the most prevalent mutations in ARX . We found that PAE mutations result in an early increase in radial glia cells and intermediate progenitor cells, and premature differentiation leading to a loss of cortical neurons at later timepoints. Moreover, ARX expression is upregulated in CO derived from patient at 30 DIV which alters the expression of CDKN1C , SFRP1 , DLK1 and FABP7 , among others. We also found a cell autonomously enhanced interneuron migration, which can be rescued by CXCR4 inhibition. Furthermore, ARX PAE assembloids had hyper-activity and synchrony evident from the early stages. These data provide novel insights to the pathogenesis of these and likely related human neurological disorders and identifies a critical window for therapeutic interventions.

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.

Diagnostic pitfalls in patients with malformations of cortical development

Malformations of cortical development (MCDs) are a major source of morbidity and mortality in the pediatric patient cohort. Correct diagnosis of the cause is essential for symptom management, disease prognosis and family counselling but is frequently hampered due to numerous potential pitfalls in the diagnostic process. This review highlights potential problems that either prevent the establishment of a diagnosis or are the sources of diagnostic errors. The focus is placed on hereditary causes of MCDs and strategies will be proposed to circumvent potential diagnostic pitfalls. Errors may occur during variant detection, filtering, or interpretation in relation to patient's phenotype. Based on detailed clinical assessment suitable targeted and untargeted methods to identify pathogenic variants with context-dependent filtering and evaluation approaches will be discussed.

Lissencephaly: Update on diagnostics and clinical management

Lissencephaly represents a spectrum of rare malformations of cortical development including agyria, pachygyria and subcortical band heterotopia. The progress in molecular genetics has led to identification of 31 lissencephaly-associated genes with the overall diagnostic yield over 80%. In this review, we focus on clinical and molecular diagnosis of lissencephaly and summarize the current knowledge on histopathological changes and their correlation with the MRI imaging. Additionally we provide the overview of clinical follow-up recommendations and available data on epilepsy management in patients with lissencephaly.

Aristaless-Related Homeobox (ARX): Epilepsy Phenotypes beyond Lissencephaly and Brain Malformations

The Aristaless-related homeobox (ARX) transcription factor is involved in the development of GABAergic and cholinergic neurons in the forebrain. ARX mutations have been associated with a wide spectrum of neurodevelopmental disorders in humans and are responsible for both malformation (in particular lissencephaly) and nonmalformation complex phenotypes. The epilepsy phenotypes related to ARX mutations are West syndrome and X-linked infantile spasms, X-linked myoclonic epilepsy with spasticity and intellectual development and Ohtahara and early infantile epileptic encephalopathy syndrome, which are related in most of the cases to intellectual disability and are often drug resistant. In this article, we shortly reviewed current knowledge of the function of ARX with a particular attention on its consequences in the development of epilepsy during early childhood.

Constraint and conservation of paired-type homeodomains predicts the clinical outcome of missense variants of uncertain significance

The need to interpret the pathogenicity of novel missense variants of unknown significance identified in the homeodomain of X-chromosome aristaless-related homeobox (ARX) gene prompted us to assess the utility of conservation and constraint across these domains in multiple genes compared to conventional in vitro functional analysis. Pathogenic missense variants clustered in the homeodomain of ARX contribute to intellectual disability (ID) and epilepsy, with and without brain malformation in affected males. Here we report novel c.1112G>A, p.Arg371Gln and c.1150C>T, p.Arg384Cys variants in male patients with ID and severe seizures. The third case of a male patient with a c.1109C>T, p.Ala370Val variant is perhaps the first example of ID and autism spectrum disorder (ASD), without seizures or brain malformation. We compiled data sets of pathogenic variants from ClinVar and presumed benign variation from gnomAD and demonstrated that the high levels of sequence conservation and constraint of benign variation within the homeodomain impacts upon the ability of publicly available in silico prediction tools to accurately discern likely benign from likely pathogenic variants in these data sets. Despite this, considering the inheritance patterns of the genes and disease variants with the conservation and constraint of disease variants affecting the homeodomain in conjunction with current clinical assessments may assist in predicting the pathogenicity of missense variants, particularly for genes with autosomal recessive and X-linked patterns of disease inheritance, such as ARX. In vitro functional analysis demonstrates that the transcriptional activity of all three variants was diminished compared to ARX-Wt. We review the associated phenotypes of the published cases of patients with ARX homeodomain variants and propose expansion of the ARX-related phenotype to include severe ID and ASD without brain malformations or seizures. We propose that the use of the constraint and conservation data in conjunction with consideration of the patient phenotype and inheritance pattern may negate the need for the experimental functional validation currently required to achieve a diagnosis.

Arx Expression Suppresses Ventralization of the Developing Dorsal Forebrain

Early brain development requires a tight orchestration between neural tube patterning and growth. How pattern formation and brain growth are coordinated is incompletely understood. Previously we showed that aristaless-related homeobox (ARX), a paired-like transcription factor, regulates cortical progenitor pool expansion by repressing an inhibitor of cell cycle progression. Here we show that ARX participates in establishing dorsoventral identity in the mouse forebrain. In Arx mutant mice, ventral genes, including Olig2, are ectopically expressed dorsally. Furthermore, Gli1 is upregulated, suggesting an ectopic activation of SHH signaling. We show that the ectopic Olig2 expression can be repressed by blocking SHH signaling, implicating a role for SHH signaling in Olig2 induction. We further demonstrate that the ectopic Olig2 accounts for the reduced Pax6 and Tbr2 expression, both dorsal specific genes essential for cortical progenitor cell proliferation. These data suggest a link between the control of dorsoventral identity of progenitor cells and the control of their proliferation. In summary, our data demonstrate that ARX functions in a gene regulatory network integrating normal forebrain patterning and growth, providing important insight into how mutations in ARX can disrupt multiple aspects of brain development and thus generate a wide spectrum of neurodevelopmental phenotypes observed in human patients.

Detection of Aristaless-related homeobox protein in ovarian sex cord-stromal tumors

OBJECTIVE

To examine the potential of ARX as a novel biomarker of ovarian endometriosis and other ovarian pathologies.

METHODS

The mRNA level of ARX in ovarian endometriosis and normal endometrium samples was determined by real-time PCR, while the protein level was determined by Western blotting and immunohistochemical staining. Immunohistochemical analysis was performed on nearly 200 tissue samples of different ovarian pathologies. GraphPad Prism was used for statistical analysis.

RESULTS

The expression of ARX was significantly increased in ovarian endometriosis samples as compared to normal endometrium. Also Western blotting data showed higher ARX levels in the ovarian endometriosis samples versus normal endometrium. Immunohistochemical analysis revealed that the protein is localized in the ovarian stroma and does not originate from endometriosis. Further immunohistochemical analysis performed on several different non-neoplastic and neoplastic ovarian tissue samples revealed that in the non-neoplastic ovary ARX protein is present only in the stromal cells and their derivates (luteinized stromal cells, theca and Leydig cells) and not in granulosa cells, oocites, surface epithelium or rete ovarii, while all stromal and sex cord tumors showed strong nuclear staining for ARX. All other primary or metastatic epithelial tumors of the ovary were ARX negative.

CONCLUSIONS

ARX is not associated with endometriosis and cannot be used as a biomarker for ovarian endometriosis. ARX is present in ovarian stroma and cells derived from ovarian stroma as well as in all types of sex cord-stromal tumors of the ovary and could thus be used as a marker for sex cord-stromal differentiation in ovarian tumors.

X-Linked Lissencephaly With Absent Corpus Callosum and Abnormal Genitalia: An Evolving Multisystem Syndrome With Severe Congenital Intestinal Diarrhea Disease

X-linked lissencephaly with abnormal genitalia is a rare and devastating syndrome. The authors present an infant with a multisystem phenotype where the intestinal manifestations were as life limiting as the central nervous system features. Severe chronic diarrhea resulted in failure to thrive, dehydration, electrolyte derangements, long-term hospitalization, and prompted transition to palliative care. Other multisystem manifestations included megacolon, colitis, pancreatic insufficiency hypothalamic dysfunction, hypothyroidism, and hypophosphatasia. A novel aristaless-related homeobox gene mutation, c.1136G>T/p.R379L, was identified. This case contributes to the clinical, histological, and molecular understanding of the multisystem nature of this disorder, especially the role of ARX in the development of the enteroendocrine system.

INTERNEURONOPATHIES AND THEIR ROLE IN EARLY LIFE EPILEPSIES AND NEURODEVELOPMENTAL DISORDERS

GABAergic interneurons control the neural circuitry and network activity in the brain. The advances in genetics have identified genes that control the development, maturation and integration of GABAergic interneurons and implicated them in the pathogenesis of epileptic encephalopathies or neurodevelopmental disorders. For example, mutations of the Aristaless-Related homeobox X-linked gene (ARX) may result in defective GABAergic interneuronal migration in infants with epileptic encephalopathies like West syndrome (WS), Ohtahara syndrome or X-linked lissencephaly with abnormal genitalia (XLAG). The concept of "interneuronopathy", i.e. impaired development, migration or function of interneurons, has emerged as a possible etiopathogenic mechanism for epileptic encephalopathies. Treatments that enhance GABA levels, may help seizure control but do not necessarily show disease modifying effect. On the other hand, interneuronopathies can be seen in other conditions in which epilepsy may not be the primary manifestation, such as autism. In this review, we plan to outline briefly the current state of knowledge on the origin, development, and migration and integration of GABAergic interneurons, present neurodevelopmental conditions, with or without epilepsy, that have been associated with interneuronopathies and discuss the evidence linking certain types of interneuronal dysfunction with epilepsy and/or cognitive or behavioral deficits.

ARX polyalanine expansion mutations lead to migration impediment in the rostral cortex coupled with a developmental deficit of calbindin-positive cortical GABAergic interneurons

The Aristaless-related homeobox gene (ARX) is indispensable for interneuron development. Patients with ARX polyalanine expansion mutations of the first two tracts (namely PA1 and PA2) suffer from intellectual disability of varying severity, with seizures a frequent comorbidity. The impact of PA1 and PA2 mutations on the brain development is unknown, hindering the search for therapeutic interventions. Here, we characterized the disturbances to cortical interneuron development in mice modeling the two most common ARX polyalanine expansion mutations in human. We found a consistent ∼40-50% reduction of calbindin-positive interneurons, but not Stt+ or Cr+ interneurons, within the cortex of newborn hemizygous mice (p=0.024) for both mutant strains compared to wildtype (p=0.011). We demonstrate that this was a consequence of calbindin precursor cells being arrested or delayed at the ventral subpallium en route of tangential migration. Ex-vivo assay validated this migration deficit in PA1 cells (p=0.0002) suggesting that the defect is contributed by intrinsic loss of Arx function within migrating cells. Both humans and mice with PA1 mutations present with severe clinical features, including intellectual disability and infantile spasms. Our data further demonstrated the pathogenic mechanism was robustly shared between PA1 and PA2 mutations, as previously reported including Arx protein reduction and overlapping transcriptome profiles within the developing mouse brains. Data from our study demonstrated that cortical calbindin interneuron development and migration is negatively affected by ARX polyalanine expansion mutations. Understanding the cellular pathogenesis contributing to disease manifestation is necessary to screen efficacy of potential therapeutic interventions.

Molecular regulation of steroidogenesis in endocrine Leydig cells

Steroid hormones regulate essential physiological processes and inadequate levels are associated with various pathological conditions. Consequently, the process of steroid hormone biosynthesis is finely regulated. In the testis, the main steroidogenic cells are the Leydig cells. There are two distinct populations of Leydig cells that arise during development: fetal and adult Leydig cells. Fetal Leydig cells are responsible for masculinizing the male urogenital tract and inducing testis descent. These cells atrophy shortly after birth and do not contribute to the adult Leydig cell population. Adult Leydig cells derive from undifferentiated precursors present after birth and become fully steroidogenic at puberty. The differentiation of both Leydig cell populations is controlled by locally produced paracrine factors and by endocrine hormones. In fully differentially and steroidogenically active Leydig cells, androgen production and hormone-responsiveness involve various signaling pathways and downstream transcription factors. This review article focuses on recent developments regarding the origin and function of Leydig cells, the regulation of their differentiation by signaling molecules, hormones, and structural changes, the signaling pathways, kinases, and transcription factors involved in their differentiation and in mediating LH-responsiveness, as well as the fine-tuning mechanisms that ensure adequate production steroid hormones.

Reinitiation of mRNA translation in a patient with X-linked infantile spasms with a protein-truncating variant in ARX

Mutations in the Aristaless-related homeobox gene (ARX) lead to a range of X-linked intellectual disability phenotypes, with truncating variants generally resulting in severe X-linked lissencephaly with ambiguous genitalia (XLAG), and polyalanine expansions and missense variants resulting in infantile spasms. We report two male patients with early-onset infantile spasms in whom a novel c.34G>T (p.(E12*)) variant was identified in the ARX gene. A similar variant c.81C>G (p.(Y27*)), has previously been described in two affected cousins with early-onset infantile spasms, leading to reinitiation of ARX mRNA translation resulting in an N-terminal truncated protein. We show that the novel c.34G>T (p.(E12*)) variant also reinitiated mRNA translation at the next AUG codon (c.121-123 (p.M41)), producing the same N-terminally truncated protein. The production of both of these truncated proteins was demonstrated to be at markedly reduced levels using in vitro cell assays. Using luciferase reporter assays, we demonstrate that transcriptional repression capacity of ARX was diminished by both the loss of the N-terminal corepressor octapeptide domain, as a consequence of truncation, and the marked reduction in mutant protein expression. Our study indicates that premature termination mutations very early in ARX lead to reinitiation of translation to produce N-terminally truncated protein at markedly reduced levels of expression. We conclude that even low levels of N-terminally truncated ARX is sufficient to improve the patient's phenotype compared with the severe phenotype of XLAG that includes malformations of the brain and genitalia normally seen in complete loss-of-function mutations in ARX.

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.

The diverse genetic landscape of neurodevelopmental disorders

Advances in genetic tools and sequencing technology in the past few years have vastly expanded our understanding of the genetics of neurodevelopmental disorders. Recent high-throughput sequencing analyses of structural brain malformations, cognitive and neuropsychiatric disorders, and localized cortical dysplasias have uncovered a diverse genetic landscape beyond classic Mendelian patterns of inheritance. The underlying genetic causes of neurodevelopmental disorders implicate numerous cell biological pathways critical for normal brain development.

Arx polyalanine expansion in mice leads to reduced pancreatic α-cell specification and increased α-cell death

ARX/Arx is a homeodomain-containing transcription factor necessary for the specification and early maintenance of pancreatic endocrine α-cells. Many transcription factors important to pancreas development, including ARX/Arx, are also crucial for proper brain development. Although null mutations of ARX in human patients result in the severe neurologic syndrome XLAG (X-linked lissencephaly associated with abnormal genitalia), the most common mutation is the expansion of the first polyalanine tract of ARX, which results primarily in the clinical syndrome ISSX (infantile spasms). Mouse models of XLAG, ISSX and other human ARX mutations demonstrate a direct genotype-phenotype correlation in ARX-related neurologic disorders. Furthermore, mouse models utilizing a polyalanine tract expansion mutation have illustrated critical developmental differences between null mutations and expansion mutations in the brain, revealing context-specific defects. Although Arx is known to be required for the specification and early maintenance of pancreatic glucagon-producing α-cells, the consequences of the Arx polyalanine expansion on pancreas development remain unknown. Here we report that mice with an expansion mutation in the first polyalanine tract of Arx exhibit impaired α-cell specification and maintenance, with gradual α-cell loss due to apoptosis. This is in contrast to the re-specification of α-cells into β- and δ-cells that occurs in mice null for Arx. Overall, our analysis of an Arx polyalanine expansion mutation on pancreatic development suggests that impaired α-cell function might also occur in ISSX patients.

Novel NDE1 homozygous mutation resulting in microhydranencephaly and not microlyssencephaly

Lissencephaly is characterized by deficient cortical lamination. Recently homozygous NDE1 mutations were reported in three kindred afflicted with extreme microcephaly with lissencephaly or microlissencephaly. Another severe developmental defect that involves the brain is microhydranencephaly which manifests with microcephaly, motor and mental retardation and brain malformations that include gross dilation of the ventricles with complete absence of the cerebral hemispheres or severe delay in their development. In the three related patients with microhydranencephaly that we had reported previously, we identified a homozygous deletion that encompasses NDE1 exon 2 containing the initiation codon. The mutation is predicted to result in a null allele. Herein we compare the clinical phenotypes of our research patients to those reported as microlissencephaly. The clinical findings in our patients having the fourth NDE1 mutation reported so far widen the spectrum of brain malformations resulting from mutations in NDE1.

Fragile X and X-linked intellectual disability: four decades of discovery

X-Linked intellectual disability (XLID) accounts for 5%-10% of intellectual disability in males. Over 150 syndromes, the most common of which is the fragile X syndrome, have been described. A large number of families with nonsyndromal XLID, 95 of which have been regionally mapped, have been described as well. Mutations in 102 X-linked genes have been associated with 81 of these XLID syndromes and with 35 of the regionally mapped families with nonsyndromal XLID. Identification of these genes has enabled considerable reclassification and better understanding of the biological basis of XLID. At the same time, it has improved the clinical diagnosis of XLID and allowed for carrier detection and prevention strategies through gamete donation, prenatal diagnosis, and genetic counseling. Progress in delineating XLID has far outpaced the efforts to understand the genetic basis for autosomal intellectual disability. In large measure, this has been because of the relative ease of identifying families with XLID and finding the responsible mutations, as well as the determined and interactive efforts of a small group of researchers worldwide.

Developmental mechanisms for the generation of telencephalic interneurons

Interneurons, which release the neurotransmitter γ-aminobutyric acid (GABA), are the major inhibitory cells of the central nervous system (CNS). Despite comprising only 20-30% of the cerebral cortical neuronal population, these cells play an essential and powerful role in modulating the electrical activity of the excitatory pyramidal cells onto which they synapse. Although interneurons are present in all regions of the mature telencephalon, during embryogenesis these cells are generated in specific compartments of the ventral (subpallial) telencephalon known as ganglionic eminences. To reach their final destinations in the mature brain, immature interneurons migrate from the ganglionic eminences to developing telencephalic structures that are both near and far from their site of origin. The specification and migration of these cells is a complex but precisely orchestrated process that is regulated by a combination of intrinsic and extrinsic signals. The final outcome of which is the wiring together of excitatory and inhibitory neurons that were born in separate regions of the developing telencephalon. Disruption of any aspect of this sequence of events during development, either from an environmental insult or due to genetic mutations, can have devastating consequences on normal brain function.

Corpus callosum agenesis, severe mental retardation, epilepsy, and dyskinetic quadriparesis due to a novel mutation in the homeodomain of ARX

We report on a patient with agenesis of the corpus callosum (ACC), severe mental retardation, infantile spasms and subsequent intractable epilepsy, spastic/dyskinetic quadriparesis, severe limb contractures, and scoliosis. This complex, newly described phenotype, is due to a novel non-conservative missense mutation in the ARX homeodomain (c.1072A>T; p.R358W), inherited from the unaffected mother. Differently from previously reported non-conservative mutations falling within the same domain, p.R358W did not cause XLAG. It is therefore possible that differences in clinical manifestations between our patient and those with XLAG, are related to the different position of the amino acid substitution in the homeodomain, or to the different chemical properties introduced by the substitution itself. To test the hypothesis that the patient's mother was asymptomatic because of non-random X chromosome inactivation (XCI), we performed DNA methylation studies of the human androgen receptor gene, demonstrating skewing of the XCI ratio (85:15). The complex phenotype described here combines different traits that had previously been linked to various ARX mutations, including conservative missense mutations in the homeodomain and expansion in the first ARX polyalanine tract and contributes to the expanding pleiotropy associated with ARX mutations.

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.

Mutations in ARX Result in Several Defects Involving GABAergic Neurons

Genetic investigations of X-linked mental retardation have demonstrated the implication of ARX in a wide spectrum of disorders extending from phenotypes with severe neuronal migration defects, such as lissencephaly, to mild or moderate forms of mental retardation without apparent brain abnormalities, but with associated features of dystonia and epilepsy. These investigations have in recent years directed attention to the role of this gene in brain development. Analysis of its spatio-temporal localization profile revealed expression in telencephalic structures at all stages of development, mainly restricted to populations of GABA-containing neurons. Furthermore, studies of the effects of ARX loss of function either in humans or in lines of mutant mice revealed varying defects, suggesting multiple roles of this gene during development. In particular, Arx has been shown to contribute to almost all fundamental processes of brain development: patterning, neuronal proliferation and migration, cell maturation and differentiation, as well as axonal outgrowth and connectivity. In this review, we will present and discuss recent findings concerning the role of ARX in brain development and how this information will be useful to better understand the pathophysiological mechanisms of mental retardation and epilepsy associated with ARX mutations.

Mutations in the nuclear localization sequence of the Aristaless related homeobox; sequestration of mutant ARX with IPO13 disrupts normal subcellular distribution of the transcription factor and retards cell division

Background

Aristaless related homeobox (ARX) is a paired-type homeobox gene. ARX function is frequently affected by naturally occurring mutations. Nonsense mutations, polyalanine tract expansions and missense mutations in ARX cause a range of intellectual disability and epilepsy phenotypes with or without additional features including hand dystonia, lissencephaly, autism or dysarthria. Severe malformation phenotypes, such as X-linked lissencephaly with ambiguous genitalia (XLAG), are frequently observed in individuals with protein truncating or missense mutations clustered in the highly conserved paired-type homeodomain.

Results

We have identified two novel point mutations in the R379 residue of the ARX homeodomain; c.1135C>A, p.R379S in a patient with infantile spasms and intellectual disability and c.1136G>T, p.R379L in a patient with XLAG. We investigated these and other missense mutations (R332P, R332H, R332C, T333N: associated with XLAG and Proud syndrome) predicted to affect the nuclear localisation sequences (NLS) flanking either end of the ARX homeodomain. The NLS regions are required for correct nuclear import facilitated by Importin 13 (IPO13). We demonstrate that missense mutations in either the N- or C-terminal NLS regions of the homeodomain cause significant disruption to nuclear localisation of the ARX protein in vitro. Surprisingly, none of these mutations abolished the binding of ARX to IPO13. This was confirmed by co-immunoprecipitation and immmuno fluorescence studies. Instead, tagged and endogenous IPO13 remained bound to the mutant ARX proteins, even in the RanGTP rich nuclear environment. We also identify the microtubule protein TUBA1A as a novel interacting protein for ARX and show cells expressing mutant ARX protein accumulate in mitosis, indicating normal cell division may be disrupted.

Conclusions

We show that the most likely, common pathogenic mechanism of the missense mutations in NLS regions of the ARX homeodomain is inadequate accumulation and distribution of the ARX transcription factor within the nucleus due to sequestration of ARX with IPO13.

New trends in neuronal migration disorders

Neuronal migration disorders are an heterogeneous group of disorders of nervous system development and they are considered to be one of the most significant causes of neurological and developmental disabilities and epileptic seizures in childhood. In the last ten years, molecular biologic and genetic investigations have widely increased our knowledge about the regulation of neuronal migration during development. One of the most frequent disorders is lissencephaly. It is characterized by a paucity of normal gyri and sulci resulting in a "smooth brain". There are two pathologic subtypes: classical and cobblestone. Classical lissencephaly is caused by an arrest of neuronal migration whereas cobblestone lissencephaly caused by overmigration. Heterotopia is another important neuronal migration disorder. It is characterized by a cluster of disorganized neurons in abnormal locations and it is divided into three main groups: periventricular nodular heterotopia, subcortical heterotopia and marginal glioneural heterotopia. Polymicrogyria develops at the final stages of neuronal migration, in the earliest phases of cortical organization; bilateral frontoparietal form is characterized by bilateral, symmetric polymicrogyria in the frontoparietal regions. Bilateral perisylvian polymicrogyria causes a clinical syndrome which manifests itself in the form of mild mental retardation, epilepsy and pseudobulbar palsy. Schizencephaly is another important neuronal migration disorder whose clinical characteristics are extremely variable. This review reports the main clinical and pathophysiological aspects of these disorders paying particular attention to the recent advances in molecular genetics.

Aristaless-related homeobox gene disruption leads to abnormal distribution of GABAergic interneurons in human neocortex: evidence based on a case of X-linked lissencephaly with abnormal genitalia (XLAG)

X-linked lissencephaly with abnormal genitalia (XLAG) is a rare disorder caused by mutations in the aristaless-related homeobox (ARX) gene, located on Xp22.13. Arx-null mice show loss of tangential migration of GABAergic interneurons, presumably being related to caudal ganglionic eminence tangential migration. In the present study, we investigated a subpopulation of GABAergic interneurons in the brain of an infant with XLAG, who had a novel nonsense mutation of the ARX gene, compared with those of age-matched normal controls and Miller-Dieker syndrome. We performed immunocytochemistry for interneuron and migration markers. We found that glutamic acid decarboxylase (GAD)- and calretinin (CR)-containing cells were significantly reduced in the neocortex and located in the white matter and neocortical subventricular zone, while neuropeptide Y- or cholecystokinin-containing cells were normally distributed. Moreover, in the neocortical subventricular region, the GAD- and CR-containing cells expressed the radial migration marker Mash-1 as well as nestin. Our findings suggest that ARX protein controls not only the tangential migration of GABAergic interneurons from the ganglionic eminence, but also may serve to induce radial migration from the neocortical subventricular zone.

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.

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.

Molecular pathology of expanded polyalanine tract mutations in the Aristaless-related homeobox gene

The Aristaless-related homeobox gene (ARX) is one of the major genes causing X-linked mental retardation. We have been interested in the pathogenic mechanism of expanded polyalanine tract mutations in ARX. We showed that the c.304ins(GCG)7 mutation causing an increase from 16 to 23 alanines increased the propensity of ARX protein aggregation and a shift from nuclear to cytoplasmic localization. We proposed that mislocalization of ARX via cytoplasmic aggregation and subsequent degradation leads to a partial loss of function, contributing to the pathogenesis. We identified importin 13 (IPO13), a mediator of nuclear import for a variety of proteins, as a novel ARX interacting protein. We predicted that the transport of ARX by IPO13 from the cytoplasm to the nucleus might be disrupted by expanded polyalanine tract mutations, but our data showed that in both yeast and mammalian cells these mutant ARX proteins were still able to interact with IPO13. We established the nuclear localization regions of the ARX homeodomain that were required for the interaction with IPO13 and correct localization of the full-length ARX transcription factor to the nucleus.

Inactivation of Arx, the murine ortholog of the X-linked lissencephaly with ambiguous genitalia gene, leads to severe disorganization of the ventral telencephalon with impaired neuronal migration and differentiation

ARX loss-of-function mutations cause X-linked lissencephaly with ambiguous genitalia (XLAG), a severe neurological condition that results in profound brain malformations, including microcephaly, absence of corpus callosum, and impairment of the basal ganglia. Despite such dramatic defects, their nature and origin remain largely unknown. Here, we used Arx mutant mice as a model to characterize the cellular and molecular mechanisms underlying the basal ganglia alterations. In these animals, the early differentiation of this tissue appeared normal, whereas subsequent differentiation was impaired, leading to the periventricular accumulation of immature neurons in both the lateral ganglionic eminence and medial ganglionic eminence (MGE). Both tangential migration toward the cortex and striatum and radial migration to the globus pallidus and striatum were greatly reduced in the mutants, causing a periventricular accumulation of NPY+ or calretinin+ neurons in the MGE. Arx mutant neurons retained their differentiation potential in vitro but exhibited deficits in morphology and migration ability. These findings imply that cell-autonomous defects in migration underlie the neuronal localization defects. Furthermore, Arx mutants lacked a large fraction of cholinergic neurons and displayed a strong impairment of thalamocortical projections, in which major axon fiber tracts failed to traverse the basal ganglia. Altogether, these results highlight the critical functions of Arx in promoting neural migration and regulating basal ganglia differentiation in mice, consistent with the phenotype of XLAG patients.

Lissencephaly, abnormal genitalia and refractory epilepsy: case report of XLAG syndrome

INTRODUCTION: X-linked lissencephaly with ambiguous genitalia (XLAG) is a recently described genetic disorder caused by mutation in the aristaless-related homeobox (ARX) gene (Xp22.13). Patients present with lissencephaly, agenesis of the corpus callosum, refractory epilepsy of neonatal onset, acquired microcephaly and male genotype with ambiguous genitalia. CASE REPORT: Second child born to healthy nonconsanguineous parents, presented with seizures within the first hour of life that remained refractory to phenobarbital, phenytoin and midazolam. Examination identified microcephaly, axial hypotonia, pyramidal signs and ambiguous genitalia. EEG showed disorganized background activity and seizures starting at the right midtemporal, central and occipital regions. MRI showed diffuse pachygyria, moderate thickening of the cortex, enlarged ventricles, agenesis of the corpus callosum and septum pellucidum. Karyotype showed a 46,XY genotype. Additional findings were hypercalciuria, vesicoureteral reflux, patent ductus arteriosus and chronic diarrhea.

Central and gonadal hypogonadism in X-linked lissencephaly

OBJECTIVE

To directly test gonadal function in a patient with X-linked lissencephaly with ambiguous genitalia (XLAG) in light of lack of previous functional data.

STUDY DESIGN AND RESULTS

We studied an infant who failed to increase testosterone levels in response to hCG stimulation.

CONCLUSION

In XLAG, the gonads are not only structurally dysgenetic but also functionally abnormal.

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.

The role of ARX in cortical development

The ARX protein (encoded by the aristaless-related homeobox gene) is a member of the paired class of homeoproteins. More precisely, it is a member of the Aristaless subclass of proteins with a glutamine residue (Q) at the critical position 50 of the homeodomain (Q50). Through identification of diverse inherited or de novo mutations, genetic investigations of X-linked mental retardation conditions have demonstrated the implication of ARX in a wide spectrum of disorders extending from phenotypes with severe neuronal migration defects, such as lissencephaly, to mild forms of X-linked mental retardation without apparent brain abnormalities. These investigations have recently directed attention to the role of this gene in brain development. Analysis of its spatiotemporal localization profile have revealed expression mainly in telencephalic structures at all stages of development. Interestingly, in adult, ARX expression becomes restricted to a population of GABAergic neurons. Although the identification of the target genes regulated by ARX remains a crucial step to better understanding its role during brain development, studies of the role of ARX orthologs in different models have indicated that it is essential for important developmental processes such as proliferation, cell differentiation and neuronal migration.

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.

ZFH4 protein is expressed in many neurons of developing rat brain

The zinc finger-homeodomain (ZFH) transcription factors contain a zinc finger motif and a homeodomain that might regulate neural and mesenchymal cell differentiation. We have cloned the ZFH4 gene that encodes a protein with structures closely related to ATBF1. In order to study the expression pattern of ZFH4 in the developing rat brain, we raised an antibody against a glutathione-S-transferase (GST) fusion protein of ZFH4. Western blotting with this antibody identified a gene product of 390 kDa in the normal rat brain. Levels of the protein were high in the brainstem at embryonic and neonatal periods and in the midbrain and diencephalon in neonatal rat brain. In addition, the corresponding mRNA of 12.5 kb was detected by Northern blotting. An immunolocalization study showed that postmitotic neurons in the brainstem were the major site of ZFH4 expression, and the levels of expression varied depending on age and anatomical sites. Expression was transient and weak in precursor cells at early neurogenesis. Although ZFH4 levels decreased after birth, ZFH4 continued to be expressed in the mature neurons including DOPA decarboxylase-positive neurons. High levels of expression were also detected in non-neuronal cells of the subcommissural organ, but the expression was almost undetectable throughout precursor cells to mature neurons in the cerebral cortex and hippocampus. The spatial and temporal expression patterns closely resembled those of ATBF1, and we detected neurons that expressed ZFH4, ATBF1, or both. We postulate that ZFH4 participates in the regulation of neural cell maturation or of region-specific differentiation of the brain.

A polyalanine tract expansion in Arx forms intranuclear inclusions and results in increased cell death

A growing number of human disorders have been associated with expansions of a tract of a single amino acid. Recently, polyalanine (polyA) tract expansions in the Aristaless-related homeobox (ARX) protein have been identified in a subset of patients with infantile spasms and mental retardation. How alanine expansions in ARX, or any other transcription factor, cause disease have not been determined. We generated a series of polyA expansions in Arx and expressed these in cell culture and brain slices. Transfection of these constructs results in nuclear protein aggregation, filamentous nuclear inclusions, and an increase in cell death. These inclusions are ubiquitinated and recruit Hsp70. Coexpressing Hsp70 decreases the percentage of cells with nuclear inclusions. Finally, we show that expressing mutant Arx in mouse brains results in neuronal nuclear inclusion formation. Our data suggest expansions in one of the ARX polyA tracts results in nuclear protein aggregation and an increase in cell death; likely underlying the pathogenesis of the associated infantile spasms and mental retardation.

Genetics of disorders of cortical development

Since the advent of MR imaging, cortical malformations have become an increasingly recognized cause of epilepsy and neurologic impairment. Improved radiographic characterization of cortical malformations has been requisite to defining their genetics, and a large portion of these disorders are now known to have a genetic basis. Uncovering genetic etiologies has provided insight into phenotypic diversity, revealed the importance of de novo mutations, and resulted in improved radiographic-genetic correlation. This article provides an overview of major cerebral cortical malformations and focuses on the genetic mechanisms of their causation.

Genetics of the epilepsies

PURPOSE OF REVIEW

This article reviews the most significant advances in the field of genetics of the epilepsies during the past year, with emphasis on newly identified genes and functional studies leading to new insights into the pathophysiology of epilepsy.

RECENT FINDINGS

Mutations in the chloride channel gene CLCN2 have been associated with the most common forms of idiopathic generalized epilepsies. A mutation in the ATP1A2 sodium potassium ATPase pump gene has been described in a family in which familial hemiplegic migraine and benign familial infantile convulsions partly co-segregate. The leucine-rich, glioma-inactivated 1 gene (LGI1) (also known as epitempin) was found to be responsible for autosomal-dominant lateral temporal lobe epilepsy in additional families. The serine-threonine kinase 9 gene (STK9) was identified as the second gene associated with X-linked infantile spasms. Mutations in the Aristaless-related homeobox gene (ARX) have been recognized as a cause of X-linked infantile spasms and sporadic cryptogenic infantile spasms. A second gene underlying progressive myoclonus epilepsy of Lafora, NHLRC1, was shown to code for a putative E3 ubiquitin ligase.

SUMMARY

Genes associated with idiopathic generalized epilepsies remain within the ion channel family. Mutations in non-ion channel genes are responsible for autosomal-dominant lateral temporal lobe epilepsy, a form of idiopathic focal epilepsy, malformations of cortical development, and syndromes that combine X-linked mental retardation and epilepsy. Most genetic epilepsies have a complex mode of inheritance, and genes identified so far account only for a minority of families and sporadic cases. Functional studies are leading to a better understanding of the mechanisms underlying hyperexcitability and seizures.