Background: The SCN8A gene on chromosome 12q13 encodes the voltage gated sodium channel Nav1.6, which is widely expressed in neurons of the CNS and PNS. Mutations in the mouse ortholog of SCN8A result in ataxia and other movement disorders.
Methods: We screened the 26 coding exons of SCN8A in 151 patients with inherited or sporadic ataxia.
Results: A 2 bp deletion in exon 24 was identified in a 9 year old boy with mental retardation, pancerebellar atrophy, and ataxia. This mutation, Pro1719ArgfsX6, introduces a translation termination codon into the pore loop of domain 4, resulting in removal of the C-terminal cytoplasmic domain and predicted loss of channel function. Three additional heterozygotes in the family exhibit milder cognitive and behavioural deficits including attention deficit hyperactivity disorder (ADHD). No additional occurrences of this mutation were observed in 625 unrelated DNA samples (1250 chromosomes).
Conclusions: The phenotypes of the heterozygous individuals suggest that mutations in SCN8A may result in motor and cognitive deficits of variable expressivity, but the study was limited by lack of segregation in the small pedigree and incomplete information about family members. Identification of additional families will be required to confirm the contribution of the SCN8A mutation to the clinical features in ataxia, cognition and behaviour disorders.
- attention deficit hyperactivity disorder
- sodium channel
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Voltage gated sodium channels play an essential role in the initial phase of the action potential during neuronal firing. The human genes SCN1A through SCN11A encode a small family of sodium channel α subunits, which are large pore-forming transmembrane proteins.1,2 Association with small β subunits modifies the channel activity and trafficking of the α subunits. SCN8A on chromosome 12q13 encodes the neuronal channel Nav1.6 that is widely distributed in neurons of the central and peripheral nervous systems. In the mouse, mutations in SCN8A result in ataxia, tremor, muscle weakness, and dystonia.3–6 Homozygosity for a null allele of mouse SCN8A results in progressive paralysis during postnatal week 3,5 the period when Nav1.6 becomes localised at the nodes of Ranvier.7,8 Electrophysiological recordings from SCN8A homozygous null mice have detected altered neuronal firing patterns, including reduced repetitive firing in cerebellar Purkinje cells, and reduced resurgent current in cerebellar Purkinje cells, prefrontal cortex pyramidal neurons, and spiny sensory neurons.9–12
Mutations in the sodium channels SCN1A and SCN2A are responsible for inherited and sporadic epilepsies.4,13,14 Severe myoclonic epilepsy of infancy (OMIM 607208) results from haploinsufficiency of SCN1A in patients who are heterozygous for null mutations.15–17 Many of the observed null mutations in SCN1A cause protein truncation.16 Deletion of only the C-terminal cytoplasmic domain of SCN1A results in disease of comparable severity to that resulting from truncations near the N terminus of the protein, indicating that the C terminus is essential for channel function.4 Haploinsufficiency has also been observed for the cardiac sodium channel SCN5A in a patient with a conduction disorder.18
Recent studies have investigated the contribution of mutations in neuronal sodium channels to cognition and psychiatric disorders. Several potentially causal heterozygous missense mutations were found in the channels SCN1A, SCN2A, and SCN3A in families with autism.19 In a study of suicide, transmission disequilibrium was observed for a single nucleotide polymorphism in an intron of SCN8A.20 Long term potentiation, a cellular component of learning, is accompanied by changes in sodium channel kinetics.21 Most recently, inhibitors of sodium channels were found to have a therapeutic effect in a rat model of anxiety.22
In order to identify disease associated mutations of human SCN8A, we screened 151 unrelated patients with inherited or sporadic ataxia.23 We identified four members of a family who are heterozygous for a protein truncation allele of human SCN8A. The data suggest that loss of function mutations in SCN8A may result in both cognitive and motor deficits.
METHODS AND RESULTS
Detection of a frameshift mutation in SCN8A causing protein truncation
Mutation screening was carried out for 151 patients with inherited or sporadic ataxia. These patients did not carry mutations in the known ataxia genes SCA1, SCA2, SCA3, SCA6, SCA7, SCA8, SCA10, SCA12, and SCA14.23–25 Twenty six coding exons of SCN8A, exons 1–24 plus 10a and 10b, were amplified by PCR from genomic DNA using the primers described in table 1. Heteroduplex analysis was carried out by conformation sensitive gel electrophoresis (CSGE).26 The PCR product containing the first 519 bp of exon 24 from one individual contained two slowly migrating conformers in addition to the wild type product, indicative of heterozygosity (fig 1A). This PCR product was purified with the Qiaquick gel extraction kit (Qiagen, Valencia, CA) and sequenced by the University of Michigan DNA Sequencing Core. The product was then cloned into the TOPO TA vector (Invitrogen, Carlsbad, CA) and 10 individual clones were sequenced from both strands. Two different alleles were recovered, one with wild type sequence and one with a 2 bp deletion of nucleotides 5156 and 5157 of the coding sequence (fig 1B). This deletion shifts the reading frame and causes a premature stop codon just downstream of the deletion. This mutation, Pro1719ArgfsX6, is predicted to truncate the protein in the pore region of transmembrane domain 4, resulting in deletion of the entire cytoplasmic C-terminal domain (fig 1C). Similar mutations in other voltage gated sodium channels result in complete loss of channel activity.4 Exon 24 was also screened by CSGE in 262 additional ataxia patients of northern European origin and 137 patients with other neurological disorders, also of northern European origin. All experiments were carried out with appropriate institutional IRB approval.
Genotyping assay for Pro1719ArgfsX6 in exon 24
To detect the 2 bp deletion in exon 24, a 318 bp genomic fragment containing the mutated sequence was amplified from genomic DNA using a forward primer in intron 23 (TTG CCT TAA TGA TGT CCT TGC CTG) and a reverse primer in the coding sequence of exon 24 (TAC AAA GAA GAA GAT GCC CAC TGA). Product length was determined on an ABI sequencer using GeneMapper software. DNA from 75 Centre d’Etudes du Polymorphisme Humain (CEPH) parents was genotyped.
The proband was a 9 year old boy with marked delay of cognitive and motor development and stimulant responsive attention deficit disorder. His MRI showed moderate pancerebellar atrophy that was accentuated in the vermal and parasagittal regions, as well as optic nerve hypoplasia, but no cerebral abnormalities. Neurological examination revealed bilateral esophoria, strabismatic amblyopia, and unsustained gaze evoked nystagmus on horizontal gaze. The proband had normal strength, tone, and reflexes, but notable ataxia of speech, dysmetria in the upper extremities resulting in nearly illegible handwriting, and mildly ataxic gait with wide base and en bloc turning. The motor abnormalities are consistent with the observed cerebellar malformation. Karyotype data were not available.
Inheritance of the sodium channel mutation
DNA from four additional family members was analysed (fig 2). The father (I-1) is homozygous for the wild type allele (fig 2) and exhibits no clinical abnormalities; he is the custodial parent. The proband’s mother (I-2) is heterozygous for Pro1719ArgfsX6, as are the maternal aunt (I-3) and first cousin (II-5) (fig 2). The mother has a history of emotional instability with mild cognitive impairment, as does the maternal aunt. The first cousin has been diagnosed with attention deficit hyperactivity disorder (ADHD). None of these family members was available for formal clinical evaluation (including MRI) to assess possible subclinical cerebellar atrophy. Due to the incomplete information on the other heterozygotes, it is unclear whether they exhibit a milder version of the abnormalities seen in the proband, resulting from haploinsufficiency of SCN8A, or if some of the proband’s symptoms are caused by an unrelated developmental disorder.
DNA was unavailable from other family members, but anecdotal evidence of behavioural deficits was reported for the maternal grandmother and her two brothers, now in their 60s, and her son, aged 35 years. These individuals attended special education classes as children and require assisted living situations as adults. The family is of Norwegian and Swedish background.
Family members were genotyped for three microsatellite markers and one SNP from chromosome 12q13.3. The Pro1719ArgfsX6 mutation is carried on a chromosome segment with haplotype D12S1663-1, D12S368-3, D12S83-5, and the A allele of rs303802 (fig 2). The allele frequencies for these markers are 0.30, 0.07, 0.30, and 0.8, respectively.
Exon 24 of SCN8A does not contain a hotspot for mutation
The CpT dinucleotide deletion in Pro1719ArgfsX6 removed the final C in a C6 repeat. To determine whether this is a site of genome instability, we screened a total of 625 DNA samples (1250 chromosomes). The samples included 151 ataxia patients from the current study, 262 additional ataxia patients, 137 patients with other neurological disorders, and 75 CEPH parents. No additional occurrence of Pro1719ArgfsX6 was identified, indicating that this is a rare allele in the northern European population represented by our samples.
P1719fsX1724 is the first reported mutation in human SCN8A. Several lines of evidence indicate that the loss of the C-terminal domain in the SCN8A mutant allele results in loss of function. This domain of the voltage gated sodium channels has been highly conserved during evolution and is thought to play a role in channel inactivation. An interaction site for the sodium channel β1 subunit has been localised within the C-terminal domain.27 Biophysical studies of similarly located mutations in the channels SCN1A and SCN5A have been carried out in transfected mammalian cells; the results demonstrate that truncation of the C-terminal domain greatly reduces or eliminates channel activity.28,29 Many additional mutations in SCN1A that result in severe myoclonic epilepsy of infancy are deletions of the C-terminal domain.4 The reduced level of Nav1.6 in heterozygous individuals is expected to reduce neuronal excitability, resulting in altered firing patterns. Electrophysiological studies of neurons from heterozygous null mice would be of great value for understanding the consequences of reduced Nav1.6 in different types of neurons. Preliminary studies have detected impaired learning and increased anxiety in mice heterozygous for an SCN8A null mutation (B McKinney, M Meisler, and G Murphy, unpublished observations).
The tissue specific expression of SCN8A in the nervous system, the predicted loss of channel activity by the mutant allele, and the low population frequency of the mutant allele, are all consistent with a causal role for this mutation in the neurological deficits of the proband. It is not unexpected that a reduction in the amount of the Nav1.6 sodium channel, due to heterozygosity for a loss of function allele, would have clinical consequences. Haploinsufficiency for the closely related channel SCN1A (Nav1.1) results in a severe seizure disorder,4 demonstrating that levels of expression in a null heterozygote can be insufficient for normal function. The threshold for uncompromised motor function in the mouse is between 10% and 50% of normal levels of Nav1.6.6
The severity of cognitive defects in family members who are heterozygous for the SCN8A mutation varies from mental retardation in the proband to ADHD in his first cousin. If 50% is close to the threshold for normal SCN8A function, then variation in genetic background or environment may be expected to influence cognitive outcome. SCN8A expression is particularly high in the cerebellum, and targeted inactivation of SCN8A in Purkinje cells is sufficient to produce ataxia.30 On the other hand, cerebellar malformation has not been observed in Scn8a null or heterozygous mice. The ataxic features of the proband are consistent with the moderate pancerebellar atrophy revealed by MRI, but the lack of MRIs for the other heterozygotes in the family preclude firm conclusions regarding the role of the SCN8A mutation in the cerebellar atrophy.
The CpT dinucleotide deletion in the SCN8A mutation P1719fsX1724 occurs in the context of a C6 nucleotide sequence. Deletion of one or two nucleotides from C4 to C6 repeats are responsible for the common deafness allele of connexin 26 and RAI1 mutations in patients with Smith-Magenis syndrome. However, no other mutations in this C6 run were observed among 625 unrelated individuals, indicating that this is not a highly unstable site.
Follow up studies will be required to assess the prevalence of SCN8A mutations and their significance in disorders of cognition and behaviour. In future screening, it will be worthwhile to include the recently described 5′ non-coding exons and promoter region of the gene,31 which were not included in the present study. Families with both motor and cognitive features should be investigated in future studies.
We are grateful to the family for their cooperation. We thank Drs Jeffrey Innis and Donna Martin for helpful comments on the manuscript.
Published Online First 19 October 2005
This work was supported by NIH grants NS34509 (MHM) and NS040389 (LPWR), The Wilson Family Medical Foundation (MHM), and the National Ataxia Foundation (LPWR)
Competing interests: none declared
Ethics approval: blood samples were obtained with Internal Review Board approved consent from subjects at the University of Minnesota
The patient details described in this report are published with consent
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