Fibroblast growth factor homologous factors (FHFs) are intracellular proteins which regulate voltage-gated sodium (Na v ) channels in the brain and other tissues. FHF dysfunction has been linked to neurological disorders including epilepsy. Here, we describe two sibling pairs and three unrelated males who presented in infancy with intractable focal seizures and severe developmental delay. Whole-exome sequencing identified hemi-and heterozygous variants in the N-terminal domain of the A isoform of FHF2 (FHF2A). The X-linked FHF2 gene (also known as FGF13) has alternative first exons which produce multiple protein isoforms that differ in their N-terminal sequence. The variants were located at highly conserved residues in the FHF2A inactivation particle that competes with the intrinsic fast inactivation mechanism of Na v channels. Functional characterization of mutant FHF2A co-expressed with wild-type Na v 1.6 (SCN8A) revealed that mutant FHF2A proteins lost the ability to induce rapid-onset, long-term blockade of the channel while retaining pro-excitatory properties. These gain-of-function effects are likely to increase neuronal excitability consistent with the epileptic potential of FHF2 variants. Our findings demonstrate that FHF2 variants are a cause of infantile-onset developmental and epileptic encephalopathy and underline the critical role of the FHF2A isoform in regulating Na v channel function.Voltage-activated sodium channels (Na v ) play an essential role in the generation and spread of action potentials in excitable tissues. 1,2 Variants in Na v channels and their regulatory partners are a major cause of infantile-onset developmental and epileptic encephalopathies (DEEs). 3,4 DEEs are typically associated with developmental delay or regression, treatment-resistant seizures, and electroencephalographic abnormalities. 5,6 The developmental consequences of DEEs are due to frequent epileptiform activity in combination with the direct effects of the genetic variant. 7,8 Fibroblast growth factor homologous factors (FHFs) are intracellular proteins that bind to the C-terminal domain of Na v channels to modulate their function and location. [9][10][11][12] FHFs were initially identified due to their homology with fibroblast growth factors (FGFs). 13 However, FHFs are not secreted by cells and have only limited ability to activate FGF receptors. 9,14,15 There are four FHF genes in mammals (often referred to by their FGF names): FHF1 (FGF12 [MIM: 601513]), FHF2 (FGF13 [MIM: 300070]), FHF3 (FGF11 [MIM: 601514]), and FHF4 (FGF14 [MIM: 601515]). 9 The FHF genes have multiple transcription initiation sites. The alternative first exons produces multiple isoforms with variable N-terminal domains. 16 The isoforms differ in their localization and ability to regulate Na v channels. 9 FHF2, located at Xq26.3-q27.1, is highly expressed in the developing and adult brain. 9,13 FHF2 is also expressed in endocrine tissues, ovaries, skeletal muscle, and the myocardium of the developing heart. [17][18][19] FHF2 has been implicated...

Mutations in the TRIM8 gene have been described in patients with severe developmental delay, intellectual disability and epilepsy. Only six patients have been described to date. All the previous mutations were truncating variants clustered in the C-terminus of the protein. A previous patient with TRIM8-related epileptic encephalopathy was reported to have nephrotic syndrome. Here we describe the clinical, radiological and histological features of an 8-yearold male patient with a TRIM8 mutation who, in contrast to previous patients, had only mild intellectual disability and well-controlled epilepsy. The patient was found to have proteinuria at 2 years of age. Renal biopsy findings were suggestive of focal segmental glomerulosclerosis. His kidney function declined and peritoneal dialysis was started at 5 years of age. He underwent renal transplant at 7 years of age. Trio-based whole genome sequencing identified a novel de novo heterozygous frameshift mutation in TRIM8 (NM_030912.2) c.1198_1220del, p.(Tyr400ArgfsTer2). This patient is further evidence that TRIM8 mutations cause a syndrome with both neurological and renal features. Our findings suggest the spectrum of TRIM8-related disease may be wider than previously thought with the possibility of milder neurodevelopmental problems and/or a more severe, progressive renal phenotype. We highlight the need for proteinuria screening in patients with TRIM8 mutations.

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