Novel ADAR2 variants in children with seizures, intellectual disability, and motor delay have reduced RNA editing

The <i>ADARB1</i> gene encodes the adenosine deaminase acting on RNA 2 (ADAR2) RNA editing enzyme, which edits the <i>GRIA2</i> transcript Q/R editing site with almost 100% efficiency in the nervous system. The edited <i>GRIA2</i> ^<i>R</i> transcript encodes the GLUA2 R subunit isoform of tetrameric α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, which is essential to prevent seizures associated with aberrantly elevated AMPA receptor cation permeability. Rare biallelic variants in <i>ADARB1</i> cause severe infant and childhood seizures and developmental delays in seven cases we previously described. Here, we report two new homozygous <i>ADARB1</i> variants and study ADAR2 variant editing activities at the <i>GRIA2</i> Q/R site and other editing sites in cell cultures. One new variant in the second double-stranded RNA binding domain (dsRBD II) retains up to 60% editing activity, whereas another, in the deaminase domain, eliminates RNA editing activity. Reduced <i>GRIA2</i> Q/R site editing increases AMPA receptor permeability by upregulating the expression of the GLUA2 Q isoform and reducing overall GLUA2 subunit levels, resulting in AMPA receptors that lack GLUA2 and are calcium-permeable. Because failure to edit the <i>GRIA2</i> Q/R site leads to failure of intron 11 splicing, we also examined the effects of ADAR2 variants on the splicing of a mouse <i>Gria2</i>-based reporter and concluded that ADAR2 variants affect splicing only through their effects on RNA editing activity. To expand the number of variants in <i>ADARB1</i>, some variants reported in ClinVar have also been analyzed by in silico methods to predict which are likely to be most deleterious and associated with seizures in patients.