AAV-mediated FOXG1 gene replacement for FOXG1 c.632T>C (p.Ile211Thr): forkhead domain missense variant with potential dominant-negative considerations

For FOXG1 c.632T>C (p.Ile211Thr), a pathogenic missense variant within the forkhead domain (FHD), AAV-mediated gene replacement represents the primary therapeutic concept under preclinical investigation. Unlike FOXG1 truncating variants where haploinsufficiency is the dominant mechanism, this missense variant in the DNA-binding FHD (residues 181-275) may produce a stable but functionally impaired protein that could exert dominant-negative effects by competing with wild-type FOXG1 for DNA binding sites. This mechanistic distinction has important implications for gene therapy design: simple gene addition may be insufficient if the mutant protein interferes with transgene-derived wild-type FOXG1. Any therapeutic strategy for this variant must therefore consider whether allele-specific silencing of the mutant allele is needed alongside or instead of gene addition.

FOXG1 p.Ile211Thr occurs within the forkhead domain, the critical DNA-binding region of the FOXG1 transcription factor. Structural studies of forkhead domains indicate that Ile211 participates in the hydrophobic core maintaining the winged-helix fold; substitution with the polar threonine residue is predicted to destabilize the domain structure and impair DNA binding. ClinVar classifies this variant as Pathogenic. In preclinical gene therapy development, Bhavsar et al. have shown AAV-FOXG1 delivery to the developing mouse brain can partially rescue microcephaly and behavioral phenotypes in Foxg1+/- heterozygous mice, establishing proof-of-concept for gene supplementation in haploinsufficiency models (preclinical data presented at gene therapy conferences). FOXG1 syndrome has gained attention from the gene therapy community, with advocacy organizations (FOXG1 Research Foundation) actively funding therapeutic development. The disorder affects GABAergic interneuron development and cortical patterning, making early intervention critical before irreversible neurodevelopmental damage is established.

No clinical-stage gene therapy program for FOXG1 syndrome exists as of early 2026. The missense nature of p.Ile211Thr raises a specific concern that truncating-variant studies may not fully predict: if the mutant protein retains partial DNA-binding capacity, it could act as a dominant-negative, occupying FOXG1 target promoters without activating transcription. In this scenario, gene addition alone may be insufficient — allele-specific knockdown (e.g., via RNAi or ASO co-treatment) might be required. FOXG1 dosage sensitivity is a theoretical concern, as FOXG1 functions as a transcription factor where both under- and overexpression during neurodevelopment could be detrimental, though FOXG1 overexpression phenotypes are less well-characterized than for MECP2. AAV tropism for the diverse CNS cell types affected in FOXG1 syndrome (cortical neurons, GABAergic interneurons) poses delivery challenges. The therapeutic window is narrow — most neurological damage occurs during prenatal and early postnatal brain development, and it is unclear whether postnatal gene therapy can meaningfully reverse established structural brain abnormalities such as simplified gyral patterns and corpus callosum hypoplasia.