Paving Therapeutic Avenues for FOXG1 Syndrome: Untangling Genotypes and Phenotypes from a Molecular Perspective

Abstract

Development of the central nervous system (CNS) depends on accurate spatiotemporal control of signaling pathways and transcriptional programs. Forkhead Box G1 (FOXG1) is one of the master regulators that play fundamental roles in forebrain development; from the timing of neurogenesis, to the patterning of the cerebral cortex. Mutations in the FOXG1 gene cause a rare neurodevelopmental disorder called FOXG1 syndrome, also known as congenital form of Rett syndrome. Patients presenting with FOXG1 syndrome manifest a spectrum of phenotypes, ranging from severe cognitive dysfunction and microcephaly to social withdrawal and communication deficits, with varying severities. To develop and improve therapeutic interventions, there has been considerable progress towards unravelling the multi-faceted functions of FOXG1 in the neurodevelopment and pathogenesis of FOXG1 syndrome. Moreover, recent advances in genome editing and stem cell technologies, as well as the increased yield of information from high throughput omics, have opened promising and important new avenues in FOXG1 research. In this review, we provide a summary of the clinical features and emerging molecular mechanisms underlying FOXG1 syndrome, and explore disease-modelling approaches in animals and human-based systems, to highlight the prospects of research and possible clinical interventions.

Keywords: FOXG1; FOXG1 syndrome; Rett syndrome; brain development; disease modelling; hiPSCs; neurodevelopmental disorders; organoids.

Figure 1

Functions of FOXG1. FOXG1 is located in 14q12 in humans and contains only one exon (i). FOXG1 protein domains: FOXG1 consists of a Forkhead domain (FKHD), a 20-residue Groucho (Gro)-binding domain (GBD), and a 10-residue histone demethylase (KDM5B)-binding domain (JBD) (ii). FOXG1 plays important roles in many neurodevelopmental processes through its interaction with DNA and protein interaction partners (iii). C-term: C-terminus; N-term: N-terminus; TF: transcription factor; UTR: untranslated region. Illustration was created with Biorender.com.

Figure 2

Mutation hotspots of FOXG1. FOXG1 gene (i) and protein (ii) domains, and the distribution of variants in a schematic illustration depicting the N-terminal domain, Forkhead binding domain (FKHD), Groucho-binding domain (GBD), JARID1B-binding domain (JBD), and C-terminal domain. The mutations are distributed in all parts of the gene, affecting all protein domains. The most severe phenotypes are observed upon mutations in the N-terminal domain and FBD (shown in red) There are two mutation hotspots in the N-terminal region (arrows) that lead to frame shifts starting upstream of the FBD, GBD, and JBD. Potential variants arising from the mutations in the two hotspots, c.256dupC and c.460dupG, are shown in (iii). The variants in the C-terminal domain, including GBD and JBD, cause milder phenotypes of FOXG1 syndrome (shown in yellow). Number of variants observed in each protein domain are noted on the illustration [4,5,11,12]. UTR: untranslated region. Illustration was created with Biorender.com.

Figure 3

Approaches and potentials of human-based disease modelling for FOXG1 syndrome. (A) Upon genetic screening for FOXG1 syndrome, specific mutations are identified. Patient-derived somatic cells (fibroblast and other cell types) are reprogrammed to a pluripotent state (iPSC). The mutations in these cells are ‘corrected’ using a CRISPR/Cas9 approach. In parallel, iPSCs from healthy donors are used to introduce the patient-specific mutations using CRISPR/Cas9 genome editing approaches. iPSCs carrying the disease-related mutation can be differentiated into neural cell types that are affected in the disease. (B) In another approach, iPSCs carrying patient-specific mutations are used to generate cerebral organoids to model neurodevelopment. (C) High throughput sequencing approaches and functional assays are employed on these differentiated neurons and cerebral organoids to recapitulate the syndrome in the human model system. (D) Potential targets and biomarkers obtained from these studies would contribute to drug discoveries and treatment of symptoms in patients. Illustration was created with Biorender.com.