The phenotypic and genotypic spectrum of individuals with mono- or biallelic ANK3 variants

Abstract

ANK3 encodes ankyrin-G, a protein involved in neuronal development and signaling. Alternative splicing gives rise to three ankyrin-G isoforms comprising different domains with distinct expression patterns. Mono- or biallelic ANK3 variants are associated with non-specific syndromic intellectual disability in 14 individuals (seven with monoallelic and seven with biallelic variants). In this study, we describe the clinical features of 13 additional individuals and review the data on a total of 27 individuals (16 individuals with monoallelic and 11 with biallelic ANK3 variants) and demonstrate that the phenotype for biallelic variants is more severe. The phenotypic features include language delay (92%), autism spectrum disorder (76%), intellectual disability (78%), hypotonia (65%), motor delay (68%), attention deficit disorder (ADD) or attention deficit hyperactivity disorder (ADHD) (57%), sleep disturbances (50%), aggressivity/self-injury (37.5%), and epilepsy (35%). A notable phenotypic difference was presence of ataxia in three individuals with biallelic variants, but in none of the individuals with monoallelic variants. While the majority of the monoallelic variants are predicted to result in a truncated protein, biallelic variants are almost exclusively missense. Moreover, mono- and biallelic variants appear to be localized differently across the three different ankyrin-G isoforms, suggesting isoform-specific pathological mechanisms.

Keywords: aggressivity; ankyrin‐G; autism spectrum disorder; epilepsy; hypotonia; intellectual disability; language delay; neurodevelopmental disorder; sleep disturbances.

Figure 1 –

Schematic overview of the ankyrin-G isoforms and variants. The color of the protein background bar and the lines indicating variant position illustrate whether a region and variant is present in all three protein isoforms (green), the 270 kDa and 480 kDa isoforms (yellow), or solely the 480 kDa isoform (red). The ankyrin repeats comprise the membrane-binding region, the ZU5 and UPA domains comprise the spectrin-binding domain, and the regulatory region comprise the death domain and the unstructured C-terminal region, with or without the serine-rich domain and the giant exon. A) The 480 kDa ankyrin-G isoform (NM_020987.5) as well as the position of the biallelic (left) and monoallelic (right) ANK3 variants. The microdeletion is not included. Compound heterozygous variants are in trans with the variant with the same proband number in superscript. Homozygous variants are in blue. B) The 270 kDa (left) and 190 kDa (right) isoforms.

Figure 2 –

Structural models of the ANK3 wild-type (WT) and variant domains. The WT ankyrin-repeat domain (ANKRD) (A) and spectrin-binding domain (SBD)-death motifs (B) with variant positions represented as pink spheres. (C) The p.(Arg31Gln) variant induces a conformational change affecting the position of the N-terminal (N). (D) p.(Arg541Gly) and (E) p.(Ala573Glu) alter the surface electrostatic charges. (F) p.(Pro1224Leu) destabilizes the local structure of the second ZU5 domain due to abnormal flexibility and the loss of an α-helix.