Nancy D. Merner

Arrhythmogenic right ventricular cardiomyopathy type 5 is a fully penetrant, lethal arrhythmic disorder caused by a missense mutation in the TMEM43 gene.

Autosomal-dominant arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) causes sudden cardiac death and is characterized by clinical and genetic heterogeneity. Fifteen unrelated ARVC families with a disease-associated haplotype on chromosome 3p (ARVD5) were ascertained from a genetically isolated population. Identification of key recombination events reduced the disease region to a 2.36 Mb interval containing 20 annotated genes. Bidirectional resequencing showed one rare variant in transmembrane protein 43 (TMEM43 1073C-->T, S358L), was carried on all recombinant ARVD5 ancestral haplotypes from affected subjects and not found in population controls. The mutation occurs in a highly conserved transmembrane domain of TMEM43 and is predicted to be deleterious. Clinical outcomes in 257 affected and 151 unaffected subjects were compared, and penetrance was determined. We concluded that ARVC at locus ARVD5 is a lethal, fully penetrant, sex-influenced morbid disorder. Median life expectancy was 41 years in affected males compared to 71 years in affected females (relative risk 6.8, 95% CI 1.3-10.9). Heart failure was a late manifestation in survivors. Although little is known about the function of the TMEM43 gene, it contains a response element for PPAR gamma (an adipogenic transcription factor), which may explain the fibrofatty replacement of the myocardium, a characteristic pathological finding in ARVC.

Exome Sequencing Identifies FUS Mutations as a Cause of Essential Tremor

Essential tremor (ET) is a common neurodegenerative disorder that is characterized by a postural or motion tremor. Despite a strong genetic basis, a gene with rare pathogenic mutations that cause ET has not yet been reported. We used exome sequencing to implement a simple approach to control for misdiagnosis of ET, as well as phenocopies involving sporadic and senile ET cases. We studied a large ET-affected family and identified a FUS p.Gln290(∗) mutation as the cause of ET in this family. Further screening of 270 ET cases identified two additional rare missense FUS variants. Functional considerations suggest that the pathogenic effects of ET-specific FUS mutations are different from the effects observed when FUS is mutated in amyotrophic lateral sclerosis cases; we have shown that the ET FUS nonsense mutation is degraded by the nonsense-mediated-decay pathway, whereas amyotrophic lateral sclerosis FUS mutant transcripts are not.

KIF1A, an axonal transporter of synaptic vesicles, is mutated in hereditary sensory and autonomic neuropathy type 2

Hereditary sensory and autonomic neuropathy type II (HSANII) is a rare autosomal-recessive disorder characterized by peripheral nerve degeneration resulting in a severe distal sensory loss. Although mutations in FAM134B and the HSN2 exon of WNK1 were associated with HSANII, the etiology of a substantial number of cases remains unexplained. In addition, the functions of WNK1/HSN2 and FAM134B and their role in the peripheral nervous system remain poorly understood. Using a yeast two-hybrid screen, we found that KIF1A, an axonal transporter of synaptic vesicles, interacts with the domain encoded by the HSN2 exon. In parallel to this screen, we performed genome-wide homozygosity mapping in a consanguineous Afghan family affected by HSANII and identified a unique region of homozygosity located on chromosome 2q37.3 and spanning the KIF1A gene locus. Sequencing of KIF1A in this family revealed a truncating mutation segregating with the disease phenotype. Subsequent sequencing of KIF1A in a series of 112 unrelated patients with features belonging to the clinical spectrum of ulcero-mutilating sensory neuropathies revealed truncating mutations in three additional families, thus indicating that mutations in KIF1A are a rare cause of HSANII. Similarly to WNK1 mutations, pathogenic mutations in KIF1A were almost exclusively restricted to an alternatively spliced exon. This study provides additional insights into the molecular pathogenesis of HSANII and highlights the potential biological relevance of alternative splicing in the peripheral sensory nervous system.

Genetically encoded impairment of neuronal KCC2 cotransporter function in human idiopathic generalized epilepsy

The KCC2 cotransporter establishes the low neuronal Cl− levels required for GABAA and glycine (Gly) receptor‐mediated inhibition, and KCC2 deficiency in model organisms results in network hyperexcitability. However, no mutations in KCC2 have been documented in human disease. Here, we report two non‐synonymous functional variants in human KCC2, R952H and R1049C, exhibiting clear statistical association with idiopathic generalized epilepsy (IGE). These variants reside in conserved residues in the KCC2 cytoplasmic C‐terminus, exhibit significantly impaired Cl−‐extrusion capacities resulting in less hyperpolarized Gly equilibrium potentials (EGly), and impair KCC2 stimulatory phosphorylation at serine 940, a key regulatory site. These data describe a novel KCC2 variant significantly associated with a human disease and suggest genetically encoded impairment of KCC2 functional regulation may be a risk factor for the development of human IGE.

The natural history of a genetic subtype of arrhythmogenic right ventricular cardiomyopathy caused by a p.S358L mutation in TMEM43

To determine the phenotype and natural history of a founder genetic subtype of autosomal dominant arrhythmogenic right ventricular cardiomyopathy (ARVC) caused by a p.S358L mutation in TMEM43. The age of onset of cardiac symptoms, clinical events and test abnormalities were studied in 412 subjects (258 affected and 154 unaffected), all of which occurred in affected males significantly earlier and more often than unaffected males. Affected males were hospitalized four times more often than affected females (pu2009≤u20090.0001) and died younger (pu2009≤u20090.001). The temporal sequence from symptoms onset to death was prolonged in affected females by 1–2 decades. The most prevalent electrocardiogram (ECG) manifestation was poor R wave progression (PRWP), with affected males twice as likely to develop PRWP as affected females (pu2009≤u20090.05). Left ventricular enlargement (LVE) occurred in 43% of affected subjects, with 11% fulfilling criteria for dilated cardiomyopathy. Ventricular ectopy on Holter monitor was common and occurred early: the most diagnostically useful clinical test. No symptom or test could rule out diagnosis. This ARVC subtype is a sex‐influenced lethal arrhythmogenic cardiomyopathy, with a unique ECG finding, LV dilatation, heart failure and early death, where molecular pre‐symptomatic diagnosis has the greatest clinical utility.

Genetic analysis of the FUS/TLS gene in essential tremor.

Although essential tremor (ET) has a genetic basis, specific genes have not been identified. Recently, in a large ET family (FET1) from Quebec, a non‐sense mutation (p.Q290X) in the amyotrophic lateral sclerosis (ALS) gene fused in sarcoma/translated in liposarcoma (FUS/TLS) was identified by exome sequencing. No confirmatory studies have been published.

VAMP1 Mutation Causes Dominant Hereditary Spastic Ataxia in Newfoundland Families

Our group previously described and mapped to chromosomal region 12p13 a form of dominantly inherited hereditary spastic ataxia (HSA) in three large Newfoundland (Canada) families. This report identifies vesicle-associated membrane protein 1 (VAMP1), which encodes a critical protein for synaptic exocytosis, as the responsible gene. In total, 50 affected individuals from these families and three independent probands from Ontario (Canada) share the disease phenotype together with a disruptive VAMP1 mutation that affects axa0critical donor site for the splicing of VAMP1 isoforms. This mutation leads to the loss of the only VAMP1 isoform (VAMP1A) expressed in the nervous system, thus highlighting an association between the well-studied VAMP1 and a neurological disorder. Given the variable phenotype seen in the affected individuals examined here, we believe that VAMP1 should be tested for mutations in patients with either ataxia or spastic paraplegia.

Recurrent missense mutations in TMEM43 (ARVD5) due to founder effects cause arrhythmogenic cardiomyopathies in the UK and Canada.

AIMSnAutosomal dominant arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) (in the group of arrhythmogenic cardiomyopathies) is a common cause of sudden cardiac death in young adults. It is both clinically and genetically heterogeneous, with 12 loci (ARVC/D1-12) and eight genes identified, the majority of which encode structural proteins of cardiac desmosomes. The most recent gene identified, TMEM43, causes disease due to a missense mutation in a non-desmosomal gene (p.S358L) in 15 extended families from Newfoundland, Canada. To determine whether mutations in TMEM43 cause ARVC/D and arrhythmogenic cardiomyopathy in other populations, we fully re-sequenced TMEM43 on 143 ARVC/D probands (families) from the UK and 55 probands (from 55 families) from Newfoundland.nnnMETHODS AND RESULTSnBidirectional sequencing of TMEM43 including intron-exon boundaries revealed 33 variants, the majority located in non-coding regions of TMEM43. For the purpose of validation, families of probands with rare, potentially deleterious coding variants were subjected to clinical and molecular follow-up. Three missense variants of uncertain significance (p.R28W, p.E142K, p.R312W) were located in highly conserved regions of the TMEM43 protein. One variant (p.R312W) also co-segregated with relatives showing clinical signs of disease. Genotyping and expansion of the disease-associated haplotype in subjects with the p.R312W variant from Newfoundland, Canada, and the UK suggest common ancestry.nnnCONCLUSIONnAlthough the p.R312W variant was found in controls (3/378), identification of an ancestral disease p R312W haplotype suggests that the p.R312W variant is a pathogenic founder mutation.

Profound, prelingual nonsyndromic deafness maps to chromosome 10q21 and is caused by a novel missense mutation in the Usher syndrome type IF gene PCDH15.

We studied a consanguineous family (Family A) from the island of Newfoundland with an autosomal recessive form of prelingual, profound, nonsyndromic sensorineural hearing loss. A genome-wide scan mapped the deafness trait to 10q21-22 (max LOD score of 4.0; D10S196) and fine mapping revealed a 16u2009Mb ancestral haplotype in deaf relatives. The PCDH15 gene was mapped within the critical region and was an interesting candidate because truncating mutations cause Usher syndrome type IF (USH1F) and two missense mutations have been previously associated with isolated deafness (DFNB23). Sequencing of the PCDH15 gene revealed 33 sequencing variants. Three of these variants were homozygous exclusively in deaf siblings but only one of them was not seen in ethnically matched controls. This novel c.1583 T>A transversion predicts an amino-acid substitution of a valine with an aspartic acid at codon 528 (V528D). Like the two DFNB23 mutations, the V528D mutation in Family A occurs in a highly conserved extracellular cadherin (EC) domain of PCDH15 and is predicted to be more deleterious than the previously identified DFNB23 missense mutations (R134G and G262D). Physical assessment, vestibular and visual function testing in deaf adults ruled out syndromic deafness because of Usher syndrome. This study validates the DFNB23 designation and supports the hypothesis that missense mutations in conserved motifs of PCDH15 cause nonsyndromic hearing loss. This emerging genotype–phenotype correlation in USH1F is similar to that in several other USH1 genes and cautions against a prognosis of a dual sensory loss in deaf children found to be homozygous for hypomorphic mutations at the USH1F locus.

Variable neurologic phenotype in a GEFS+ family with a novel mutation in SCN1A

PURPOSEnTo describe the spectrum of clinical disease in a mutliplex family with an autosomal dominant form of generalized epilepsy with febrile seizures plus (GEFS+) and determine its genetic etiology.nnnMETHODSnMedical and family history was obtained on 11 clinically affected individuals and their relatives across three generations through medical chart review and home visits. A candidate gene approach including haplotype analysis and direct sequencing was used.nnnRESULTSnAn epilepsy-associated haplotype was identified on 2q24. Direct sequencing of the entire SCN1A gene identified seven sequence variants. However, only one of these, c.1162 T>C, was not found in population controls. This transition in exon 8 of SCN1A predicts a substitution (Y388H) of a highly conserved tyrosine residue in the loop between transmembrane segments S5 and S6 of the sodium channel protein (Na(v)1.1). Clinical features in mutation carriers of this novel missense mutation were highly variable, ranging from febrile seizures to severe refractory epilepsy.nnnCONCLUSIONnA novel missense mutation in the pore-forming region of the sodium channel gene SCN1A causes GEFS+ with a variable phenotype that includes mood and anxiety disorders, as well as ataxia, expanding the GEFS+ spectrum to include neuropsychiatric disease.