Ectopic HCN4 Provides a Target Biomarker for the Genetic Spectrum of mTORopathies.

Background And Objectives: Pathogenic variants in PI3K-AKT-mTOR pathway and GATOR1 complex genes resulting in hyperactivation of mechanistic target of rapamycin (mTOR) complex 1 are a major cause of drug-resistant epilepsy and focal cortical malformations (FCM). Resective neurosurgery is often required to achieve seizure control in patients with mTORopathies due to lack of effectiveness of nonsurgical therapies, including antiseizure medication and mTOR inhibitors. Elevated hyperpolarization-activated cyclic nucleotide-gated potassium channel isoform 4 (HCN4) has been proposed as a key marker in some mTOR-related brain malformations. This study aimed to investigate HCN4 as a biomarker in the brain across the genetic spectrum of mTORopathies in humans.

Methods: Our study investigated the relative steady-state levels and cellular localization of HCN4 in resected human brain tissue from 18 individuals with mTORopathies (3 individuals with tuberous sclerosis complex (TSC) due to variants, 5 individuals with focal cortical dysplasia type IIA (FCD IIA) due to genetic variants in , , and , and 10 individuals with FCD IIB due to variants in , , , or ).

Results: Elevated HCN4 was observed to be highly restricted to abnormal cell types (dysmorphic neurons and balloon cells) in brain tissue from all mTORopathy tissues ( < 0.0001) compared with those in controls, regardless of genetic cause or variant allele frequency. Elevated HCN4 was not observed in controls or individuals with non-mTOR-related focal epilepsy due to pathogenic variants in , , or .

Discussion: HCN4 provides a biomarker for the genetic spectrum of mTORopathies and may present a potential therapeutic target for seizure control in mTOR-related epilepsy.