Arx is a direct Dlx target gene in the developing forebrain and modulates its GABAergic migration activity

The Arx transcription factor is expressed in the developing ventral telencephalon and subsets of its derivatives. Mutation of human ARX
ortholog causes neurological disorders including epilepsy, lissencephaly, and mental retardation. We have isolated the mouse Arx
endogenous enhancer modules that control its tightly compartmentalized forebrain expression. Interestingly, they are scattered downstream
of its coding region and partially included within the introns of the downstream PolA1 gene. These enhancers are ultraconserved
noncoding sequences that are highly conserved throughout the vertebrate phylum. Functional characterization of the Arx GABAergic
enhancer element revealed its strict dependence on the activity of Dlx transcription factors. Dlx overexpression induces ectopic expression
of endogenous Arx and its isolated enhancer, whereas loss of Dlx expression results in reduced Arx expression, suggesting that Arx
is a key mediator of Dlx function. To further elucidate the mechanisms involved, a combination of gain-of-function studies in mutant Arx
or Dlx tissues was pursued. This analysis provided evidence that, although Arx is necessary for the Dlx-dependent promotion of interneuron
migration, it is not required for the GABAergic cell fate commitment mediated by Dlx factors. Although Arx has additional
functions independent of the Dlx pathway, we have established a direct genetic relationship that controls critical steps in the development
of telencephalic GABAergic neurons. These findings contribute elucidating the genetic hierarchy that likely underlies the etiology of a
variety of human neurodevelopmental disorders.