Heterozygous loss of function of IQSEC2/Iqsec2 leads to increased activated Arf6 and severe neurocognitive seizure phenotype in females.

Matilda R Jackson,Jozef Gecz,Laura Määttänen,Claire C Homan,Irma Jarvela,Alison Gardner,Karagh E Loring,Cheryl Shoubridge,Monica Hn Thai,Maria Arvio,Marie Shaw

doi:10.26508/lsa.201900386

Abstract

Clinical presentations of mutations in the IQSEC2 gene on the X-chromosome initially implicated to cause non-syndromic intellectual disability (ID) in males have expanded to include early onset seizures in males as well as in females. The molecular pathogenesis is not well understood, nor the mechanisms driving disease expression in heterozygous females. Using a CRISPR/Cas9-edited Iqsec2 KO mouse model, we confirm the loss of Iqsec2 mRNA expression and lack of Iqsec2 protein within the brain of both founder and progeny mice. Both male (52%) and female (46%) Iqsec2 KO mice present with frequent and recurrent seizures. Focusing on Iqsec2 KO heterozygous female mice, we demonstrate increased hyperactivity, altered anxiety and fear responses, decreased social interactions, delayed learning capacity and decreased memory retention/novel recognition, recapitulating psychiatric issues, autistic-like features, and cognitive deficits present in female patients with loss-of-function IQSEC2 variants. Despite Iqsec2 normally acting to activate Arf6 substrate, we demonstrate that mice modelling the loss of Iqsec2 function present with increased levels of activated Arf6. We contend that loss of Iqsec2 function leads to altered regulation of activated Arf6-mediated responses to synaptic signalling and immature synaptic networks. We highlight the importance of IQSEC2 function for females by reporting a novel nonsense variant c.566C > A, p.(S189*) in an elderly female patient with profound intellectual disability, generalised seizures, and behavioural disturbances. Our human and mouse data reaffirm IQSEC2 as another disease gene with an unexpected X-chromosome heterozygous female phenotype. Our Iqsec2 mouse model recapitulates the phenotypes observed in human patients despite the differences in the IQSEC2/Iqsec2 gene X-chromosome inactivation between the species.

Highlights

X-linked intellectual disability is a common, clinically complex disease arising from mutations in more than 140 genes on the X-chromosome [1], affecting between 1/600 and 1/1,000 males and a substantial number of females [2]
There is a growing list of X-chromosome genes which are subject to X-inactivation or escape X-inactivation, including, for example, PHF6, CLCN4, ALG13, ARX, or USP9X, DDX3X, which display distinct phenotypes in males and females depending on the functional severity of the variant, as well as manifesting in a more severe female phenotype than the heterozygous state would predict [4, 5, 6, 7, 8, 9, 10]
Injection of the CRISPR/Cas9 guides was performed as a fee for service (South Australian Genome Editing facility, University of Adelaide, Adelaide) [21]

Summary

Introduction

X-linked intellectual disability is a common, clinically complex disease arising from mutations in more than 140 genes on the X-chromosome [1], affecting between 1/600 and 1/1,000 males and a substantial number of females [2]. The sex determination system used is XX/XY, with dosage compensation in females as a result of random inactivation of one of the two X chromosomes in every cell. We contend that the IQ motif and Sec domain 2 protein (IQSEC2) (NM_001111125) (MIM 300522) is another X-chromosome disease gene in which we see a severe female phenotype because of heterozygous loss-of-function mutation

Methods

Results

Conclusion