Chris Pappas

A Role of SCN9A in Human Epilepsies, As a Cause of Febrile Seizures and As a Potential Modifier of Dravet Syndrome

A follow-up study of a large Utah family with significant linkage to chromosome 2q24 led us to identify a new febrile seizure (FS) gene, SCN9A encoding Nav1.7. In 21 affected members, we uncovered a potential mutation in a highly conserved amino acid, p.N641Y, in the large cytoplasmic loop between transmembrane domains I and II that was absent from 586 ethnically matched population control chromosomes. To establish a functional role for this mutation in seizure susceptibility, we introduced the orthologous mutation into the murine Scn9a ortholog using targeted homologous recombination. Compared to wild-type mice, homozygous Scn9a N641Y/N641Y knockin mice exhibit significantly reduced thresholds to electrically induced clonic and tonic-clonic seizures, and increased corneal kindling acquisition rates. Together, these data strongly support the SCN9A p.N641Y mutation as disease-causing in this family. To confirm the role of SCN9A in FS, we analyzed a collection of 92 unrelated FS patients and identified additional highly conserved Nav1.7 missense variants in 5% of the patients. After one of these children with FS later developed Dravet syndrome (severe myoclonic epilepsy of infancy), we sequenced the SCN1A gene, a gene known to be associated with Dravet syndrome, and identified a heterozygous frameshift mutation. Subsequent analysis of 109 Dravet syndrome patients yielded nine Nav1.7 missense variants (8% of the patients), all in highly conserved amino acids. Six of these Dravet syndrome patients with SCN9A missense variants also harbored either missense or splice site SCN1A mutations and three had no SCN1A mutations. This study provides evidence for a role of SCN9A in human epilepsies, both as a cause of FS and as a partner with SCN1A mutations.

A locus for febrile seizures (FEB3) maps to chromosome 2q23-24.

Febrile seizures are the most common form of childhood seizures, occurring in 2% to 5% of North American children. We report a large Utah family with 21 members affected by febrile seizures inherited as an autosomal dominant trait. All had generalized tonic–clonic seizures with onset associated with fever, consistent with the consensus febrile seizure phenotype, and none had febrile seizures beyond 6 years of age. Eighteen affected individuals had recurrent febrile seizures. Eight individuals developed afebrile seizures between ages 5 and 13 years. Afebrile seizures consisted of generalized tonic–clonic, generalized tonic, generalized atonic, simple partial, and partial complex seizure types and were associated with abnormal electroencephalographic findings in 5 individuals, all of whom were intellectually normal. We undertook linkage analysis in this family, defining the disease phenotype as febrile seizures alone. Linkage analysis in epilepsy candidate gene/loci regions failed to show evidence for linkage to febrile seizures. However, a genomewide scan and subsequent fine mapping revealed significant evidence for a new febrile seizure locus (FEB3) on chromosome 2q23‐24 with linkage to the marker D2S2330 (LOD score 8.08 at θ = 0.001). Haplotype analysis defined a critical 10‐cM region between markers D2S141 and D2S2345 that contains the FEB3 locus.

Mouse models of human KCNQ2 and KCNQ3 mutations for benign familial neonatal convulsions show seizures and neuronal plasticity without synaptic reorganization

The childhood epilepsy syndrome of benign familial neonatal convulsions (BFNC) exhibits the remarkable feature of clinical remission within a few weeks of onset and a favourable prognosis, sparing cognitive abilities despite persistent expression of the mutant KCNQ2 or KCNQ3 potassium channels throughout adulthood. To better understand such dynamic neuroprotective plasticity within the developing brain, we introduced missense mutations that underlie human BFNC into the orthologous murine Kcnq2 (Kv7.2) and Kcnq3 (Kv7.3) genes. Mutant mice were examined for altered thresholds to induced seizures, spontaneous seizure characteristics, hippocampal histology, and M‐current properties of CA1 hippocampal pyramidal neurons. Adult Kcnq2A306T/+ and Kcnq3G311V/+ heterozygous knock‐in mice exhibited reduced thresholds to electrically induced seizures compared to wild‐type littermate mice. Both Kcnq2A306T/A306T and Kcnq3G311V/G311V homozygous mutant mice exhibited early onset spontaneous generalized tonic‐clonic seizures concurrent with a significant reduction in amplitude and increased deactivation kinetics of the neuronal M‐current. Mice had recurrent seizures into adulthood that triggered molecular plasticity including ectopic neuropeptide Y (NPY) expression in granule cells, but without hippocampal mossy fibre sprouting or neuronal loss. These novel knockin mice recapitulate proconvulsant features of the human disorder yet show that inherited M‐current defects spare granule cells from reactive changes in adult hippocampal networks. The absence of seizure‐induced pathology found in these epileptic mouse models parallels the benign neurodevelopmental cognitive profile exhibited by the majority of BFNC patients.

Electroconvulsive seizure thresholds and kindling acquisition rates are altered in mouse models of human KCNQ2 and KCNQ3 mutations for benign familial neonatal convulsions

Purpose:  Benign familial neonatal convulsions (BFNC) is caused by mutations in the KCNQ2 and KCNQ3 genes, which encode subunits of the M‐type potassium channel. The purpose of this study was to examine the effects of orthologous BFNC‐causing mutations on seizure thresholds and the acquisition of corneal kindling in mice with heterozygous expression of the mutations.

Evaluation of heterodimeric guanylyl cyclase genes as candidates for human hypertension.

Objective Both physiologic and pharmacological data have implicated the nitric oxide (NO) signaling cascade in the regulation of blood pressure in humans and its impairment in the pathogenesis of hypertension. In biological systems, the principal receptor for NO is NO-stimulated guanylyl cyclase. NO-stimulated guanylyl cyclases are obligate heterodimers (α/β). The genes for guanylyl cyclase subunits α1, β1, and β2 are likely candidates for causing hypertension in the Dahl rat as their expression is altered and their gene loci are closely linked to known quantitative trait loci for blood pressure in Dahl rat crosses. The objective of the current study was to test whether markers near guanylyl cyclase subunit genes were linked to hypertension in Caucasians. Design To test for linkage of genetic markers in or near the guanylyl cyclase genes to hypertension in Caucasians, a sample of 124 Utah hypertensive sib pairs was genotyped. Results Four highly polymorphic markers in or near the human guanylyl cyclase subunits homologous to the rat α1 (human chromosome 8), rat β1 (human chromosome 4), and rat β2 (human chromosme 13) genes showed no evidence of excess allele sharing in the set of hypertensive sibships. Conclusion We conclude that the heterodimeric guanylyl cyclase subunit loci do not appear to be linked to hypertension in Caucasians.