Targeted loss of Arx results in a developmental epilepsy mouse model and recapitulates the human phenotype in heterozygous females

Abstract

Mutations in the X-linked aristaless-related homeobox gene (ARX) have been linked to structural brain anomalies as well as multiple neurocognitive deficits. The generation of Arx-deficient mice revealed several morphological anomalies, resembling those observed in patients and an interneuron migration defect but perinatal lethality precluded analyses of later phenotypes. Interestingly, many of the neurological phenotypes observed in patients with various ARX mutations can be attributed, in part, to interneuron dysfunction. To directly test this possibility, mice carrying a floxed Arx allele were generated and crossed to Dlx5/6(CRE-IRES-GFP)(Dlx5/6(CIG)) mice, conditionally deleting Arx from ganglionic eminence derived neurons including cortical interneurons. We now report that Arx(-/y);Dlx5/6(CIG) (male) mice exhibit a variety of seizure types beginning in early-life, including seizures that behaviourally and electroencephalographically resembles infantile spasms, and show evolution through development. Thus, this represents a new genetic model of a malignant form of paediatric epilepsy, with some characteristics resembling infantile spasms, caused by mutations in a known infantile spasms gene. Unexpectedly, approximately half of the female mice carrying a single mutant Arx allele (Arx(-/+);Dlx5/6(CIG)) also developed seizures. We also found that a subset of human female carriers have seizures and neurocognitive deficits. In summary, we have identified a previously unrecognized patient population with neurological deficits attributed to ARX mutations that are recapitulated in our mouse model. Furthermore, we show that perturbation of interneuron subpopulations is an important mechanism underling the pathogenesis of developmental epilepsy in both hemizygous males and carrier females. Given the frequency of ARX mutations in patients with infantile spasms and related disorders, our data unveil a new model for further understanding the pathogenesis of these disorders.

The Rhox genes

“…Most studies on Arx have focused on its role in the brain, as it is commonly mutated in mental retardation patients, and studies in Arx-mutant mice have shown that ARX is crucial for development of the forebrain (Kitamura et al 2002, Gecz et al 2006. Indeed, because of the dramatic brain defects that result from its loss, Arx-null mice typically undergo epileptic seizures and perinatal death, precluding analysis of ARX's role in fertility (Kitamura et al 2002, Marsh et al 2009). However, this has not prevented an analysis of the role of ARX in the developing male gonad in embryos.…”

Getting rid of the catastrophe: frontier research in infantile spasms

“…The ARX knockin mouse model with a triple repeat expansion previously described in human patients manifests spasms in early life, and subsequent expression of other seizures and cognitive and behavioral deficits [23]. Immunohistochemistry revealed reduction of calbindin-and calretinin-labeled cells in the neocortex of ARX knockout mice [22] and reduction of calbindin and neuropeptide Y (NPY) interneurons as well as cholinergic neurons in the cortex and hippocampus of ARX knockin mice [23]. These findings may suggest that the impairment of GABAergic interneurons (interneuronopathy) [24] is an important mechanism underlying the pathogenesis of these developmental epilepsies.…”

“…Loss of ARX gene function has been associated with a variety of neurologic syndromes in humans involving mental retardation and epilepsy, including IS [21]. The conditional ARX knockout mouse model, in which loss of ARX function is observed in interneurons, manifests limbic seizures in early life and spasm-like seizures in adulthood [22]. The ARX knockin mouse model with a triple repeat expansion previously described in human patients manifests spasms in early life, and subsequent expression of other seizures and cognitive and behavioral deficits [23].…”

Genetic Epilepsy Syndromes Without Structural Brain Abnormalities: Clinical Features and Experimental Models

“…To date, ARX mutations have been identified in about 10 different clinical conditions, with or without brain malformations [89][90][91]. Malformation phenotypes, including X-linked lissencephaly with abnormal genitalia (XLAG), X-linked lissencephaly with abnormal genitalia with severe hydrocephalus, and Proud syndrome (agenesis of the corpus callosum with abnormal genitalia) are associated with protein truncation mutations and missense mutations in the homeobox [89,92].…”