Abstract
Disruption of Fgf13 Causes Synaptic Excitatory–Inhibitory Imbalance and Genetic Epilepsy and Febrile Seizures Plus
Puranam RS, He XP, Yao L, Le T, Jang W, Rehder CW, Lewis DV, McNamara JO. J Neurosci 2015;35:8866–8881.
We identified a family in which a translocation between chromosomes X and 14 was associated with cognitive impairment and a complex genetic disorder termed “Genetic Epilepsy and Febrile Seizures Plus” (GEFS+). We demonstrate that the breakpoint on the X chromosome disrupted a gene that encodes an auxiliary protein of voltage-gated Na+ channels, fibroblast growth factor 13 (Fgf13). Female mice in which one Fgf13 allele was deleted exhibited hyperthermia-induced seizures and epilepsy. Anatomic studies revealed expression of Fgf13 mRNA in both excitatory and inhibitory neurons of hippocampus. Electrophysiological recordings revealed decreased inhibitory and increased excitatory synaptic inputs in hippocampal neurons of Fgf13 mutants. We speculate that reduced expression of Fgf13 impairs excitability of inhibitory interneurons, resulting in enhanced excitability within local circuits of hippocampus and the clinical phenotype of epilepsy. These findings reveal a novel cause of this syndrome and underscore the powerful role of FGF13 in control of neuronal excitability.
Commentary
Genetic epilepsy with febrile seizures plus (GEFS+) is a dominantly inherited disorder that is characterized by febrile seizures that sometimes persist beyond 6 years of age and the development of epilepsy. Mutations in at least 6 genes have been shown to cause GEFS+, and additional loci have been mapped, indicating that more genes are yet to be identified. Among the known GEFS+ genes, mutations in the voltage-gated sodium channel SCN1A are the most frequently observed, accounting for about 10% of cases (1).
In a recent study by Puranam and colleagues, which is the focus of this commentary, fibroblast growth factor 13 (FGF13) was identified as a new GEFS+ gene. The proband of the small pedigree reported in this study was a 21-year-old male who experienced a simple febrile seizure at the age of 6 and subsequently developed temporal lobe seizures and cognitive deficits. His 19-year-old sibling experienced 4 febrile seizures between 1.5 to 2 years of age and developed afebrile seizures at age 4 yet appears to be seizure-free now. Interestingly, the underlying genetic defect was determined to be a disruption of FGF13 (on the X chromosome) due to a balanced translocation between the X chromosome and chromosome 14. This translocation was inherited from the mother who had a history of febrile seizures as an infant. Consistent with expression on the X chromosome, male Fgf13 knock-out mice die at embryonic day 12.5, while heterozygous female mice exhibit a normal lifespan and 50% reduction in Fgf13 expression. Consistent with clinical presentation, mutant mice have spontaneous seizures and are susceptible to hyperthermia-induced seizures (a model of febrile seizure susceptibility). Fgf13 expression was observed in both excitatory and inhibitory neurons of the adult mouse hippocampus, and hippocampal slices from Fgf13 knockout mice exhibited decreased inhibition and increased excitation when compared to wild-type mice.
Fibroblast growth factors (FGFs) comprise a family of polypeptides that are highly conserved in both amino acid sequence and gene structure and are involved in a broad range of cellular activities. FGF13, along with FGF11, FGF12, and FGF14, belong to a subset of FGFs termed “fibroblast growth factor homologous factors” (FHFs) that are primarily expressed in the nervous system. Several lines of evidence support a mechanistic overlap between Fgf13 and SCN1A-associated epilepsy. Firstly, FGF11–FGF14 are involved in the regulation of sodium channel function; and FGF13 has been shown to bind directly to the C-terminus of SCN1A, thereby affecting channel trafficking and modulation of channel activity (2). Secondly, as demonstrated in this manuscript, hippocampal slices from Fgf13 knockout mice exhibit decreased inhibitory and increased excitatory synaptic inputs. Similar alterations in neuronal excitability have been observed in Scn1a mutants, suggesting that reduced excitability of inhibitory interneurons is likely a consequence of mutations in both genes. In addition, neuronal cultures from rat embryonic cortex exhibit increased GAD-immunopositive neurons when treated with Fgf13 (3), suggesting that reduced Fgf13 expression would lead to decreased GABA levels.
While the authors convincingly demonstrate that FGF13 is the causal gene in this GEFS+ pedigree, several questions remain: First, the frequency of FGF13 mutations in GEFS+ is unknown, and additional studies will be required to establish the contribution of FGF13 to this disorder. Furthermore, different mutations in the same epilepsy gene are often associated with different types of epilepsy. Therefore, it remains to be seen whether FGF13 dysfunction might underlie additional epilepsy subtypes. Second, the identification of FGF13 as an epilepsy gene raises the possibility that mutations in other members of this gene family might also lead to altered neuronal excitability and epilepsy. Accordingly, mice deficient in Fgf12 and Fgf14 exhibit neurological abnormalities such as ataxia and changes in hippocampal morphology and activity (reviewed by Zhang et al. [4]). In addition, increased expression of the related fibroblast growth factor FGF2 has been observed following seizure induction, implying a possible role in epileptogenesis (5). Interestingly, FGF2 is evolutionarily closely related to the FHFs (reviewed by Ornitz and Itoh [6]). Inclusion of the members of this gene family in future genetic testing is therefore warranted. Third, while the three affected family members described in this manuscript exhibited the general features of GEFS+, it is possible that distinct clinical features might emerge as more families are identified with FGF13 mutations. For example, there appears to be a difference in the age-dependence of susceptibility to hyperthermia-induced seizures between Fgf13 and Scn1a mouse lines. Specifically, Fgf13 knockout mice were reported to be susceptible to hyperthermia-induced seizures at postnatal day 15 (P15) but not at later time points (P30 and P60). In contrast, heterozygous Scn1a knockout mice are not susceptible to hyperthermia-induced seizures at P17–P18, but exhibit increased susceptibility at P22–P23 and P30–P46 (7). We also observe greater susceptibility to hyperthermia-induced seizures in older mice that express the SCN1A R1648H GEFS+ mutation (personal observation).
In summary, this study has established an important general role for FGF13 in the regulation of neuronal excitability, and more specifically, in the development of epilepsy. Furthermore, in addition to regulating ion channel activity, FGF13 is also involved in axonal development and neuronal migration (8). Therefore, the Fgf13 knock-out mice will provide the opportunity to more closely examine the range of clinically relevant phenotypes that are likely to be associated with FGF13 dysfunction.