The phenotype of SCN8A developmental and epileptic encephalopathy

Objective

To delineate the electroclinical features of SCN8A infantile developmental and epileptic encephalopathy (EIEE13, OMIM #614558).

Methods

Twenty-two patients, aged 19 months to 22 years, underwent electroclinical assessment.

Results

Sixteen of 22 patients had mildly delayed development since birth. Drug-resistant epilepsy started at a median age of 4 months, followed by developmental slowing, pyramidal/extrapyramidal signs (22/22), movement disorders (12/22), cortical blindness (17/22), sialorrhea, and severe gastrointestinal symptoms (15/22), worsening during early childhood and plateauing at age 5 to 9 years. Death occurred in 4 children, following extreme neurologic deterioration, at 22 months to 5.5 years. Nonconvulsive status epilepticus recurred in 14 of 22 patients. The most effective antiepileptic drugs were oxcarbazepine, carbamazepine, phenytoin, and benzodiazepines. EEG showed background deterioration, epileptiform abnormalities with a temporo-occipital predominance, and posterior delta/beta activity correlating with visual impairment. Video-EEG documented focal seizures (FS) (22/22), spasm-like episodes (8/22), cortical myoclonus (8/22), and myoclonic absences (1/22). FS typically clustered and were prolonged (<20 minutes) with (1) cyanosis, hypomotor, and vegetative semiology, sometimes unnoticed, followed by (2) tonic-vibratory and (3) (hemi)-clonic manifestations ± evolution to a bilateral tonic-clonic seizure. FS had posterior-temporal/occipital onset, slowly spreading and sometimes migrating between hemispheres. Brain MRI showed progressive parenchymal atrophy and restriction of the optic radiations.

Conclusions:

SCN8A developmental and epileptic encephalopathy has strikingly consistent electroclinical features, suggesting a global progressive brain dysfunction primarily affecting the temporo-occipital regions. Both uncontrolled epilepsy and developmental compromise contribute to the profound impairment (increasing risk of death) during early childhood, but stabilization occurs in late childhood.

Gene Panel Testing in Epileptic Encephalopathies and Familial Epilepsies

In recent years, several genes have been causally associated with epilepsy. However, making a genetic diagnosis in a patient can still be difficult, since extensive phenotypic and genetic heterogeneity has been observed in many monogenic epilepsies. This study aimed to analyze the genetic basis of a wide spectrum of epilepsies with age of onset spanning from the neonatal period to adulthood. A gene panel targeting 46 epilepsy genes was used on a cohort of 216 patients consecutively referred for panel testing. The patients had a range of different epilepsies from benign neonatal seizures to epileptic encephalopathies (EEs). Potentially causative variants were evaluated by literature and database searches, submitted to bioinformatic prediction algorithms, and validated by Sanger sequencing. If possible, parents were included for segregation analysis. We identified a presumed disease-causing variant in 49 (23%) of the 216 patients. The variants were found in 19 different genes including SCN1A, STXBP1, CDKL5, SCN2A, SCN8A, GABRA1, KCNA2, and STX1B. Patients with neonatal-onset epilepsies had the highest rate of positive findings (57%). The overall yield for patients with EEs was 32%, compared to 17% among patients with generalized epilepsies and 16% in patients with focal or multifocal epilepsies. By the use of a gene panel consisting of 46 epilepsy genes, we were able to find a disease-causing genetic variation in 23% of the analyzed patients. The highest yield was found among patients with neonatal-onset epilepsies and EEs.

The role of SLC2A1 mutations in myoclonic astatic epilepsy and absence epilepsy, and the estimated frequency of GLUT1 deficiency syndrome

The first mutations identified in SLC2A1, encoding the glucose transporter type 1 (GLUT1) protein of the blood-brain barrier, were associated with severe epileptic encephalopathy. Recently, dominant SLC2A1 mutations were found in rare autosomal dominant families with various forms of epilepsy including early onset absence epilepsy (EOAE), myoclonic astatic epilepsy (MAE), and genetic generalized epilepsy (GGE). Our study aimed to investigate the possible role of SLC2A1 in various forms of epilepsy including MAE and absence epilepsy with early onset. We also aimed to estimate the frequency of GLUT1 deficiency syndrome in the Danish population. One hundred twenty patients with MAE, 50 patients with absence epilepsy, and 37 patients with unselected epilepsies, intellectual disability (ID), and/or various movement disorders were screened for mutations in SLC2A1. Mutations in SLC2A1 were detected in 5 (10%) of 50 patients with absence epilepsy, and in one (2.7%) of 37 patient with unselected epilepsies, ID, and/or various movement disorders. None of the 120 MAE patients harbored SLC2A1 mutations. We estimated the frequency of SLC2A1 mutations in the Danish population to be approximately 1:83,000. Our study confirmed the role of SLC2A1 mutations in absence epilepsy with early onset. However, our study failed to support the notion that SLC2A1 aberrations are a cause of MAE without associated features such as movement disorders.

The phenotypic spectrum of SCN8A encephalopathy

OBJECTIVE

SCN8A encodes the sodium channel voltage-gated α8-subunit (Nav1.6). SCN8A mutations have recently been associated with epilepsy and neurodevelopmental disorders. We aimed to delineate the phenotype associated with SCN8A mutations.

METHODS

We used high-throughput sequence analysis of the SCN8A gene in 683 patients with a range of epileptic encephalopathies. In addition, we ascertained cases with SCN8A mutations from other centers. A detailed clinical history was obtained together with a review of EEG and imaging data.

RESULTS

Seventeen patients with de novo heterozygous mutations of SCN8A were studied. Seizure onset occurred at a mean age of 5 months (range: 1 day to 18 months); in general, seizures were not triggered by fever. Fifteen of 17 patients had multiple seizure types including focal, tonic, clonic, myoclonic and absence seizures, and epileptic spasms; seizures were refractory to antiepileptic therapy. Development was normal in 12 patients and slowed after seizure onset, often with regression; 5 patients had delayed development from birth. All patients developed intellectual disability, ranging from mild to severe. Motor manifestations were prominent including hypotonia, dystonia, hyperreflexia, and ataxia. EEG findings comprised moderate to severe background slowing with focal or multifocal epileptiform discharges.

CONCLUSION

SCN8A encephalopathy presents in infancy with multiple seizure types including focal seizures and spasms in some cases. Outcome is often poor and includes hypotonia and movement disorders. The majority of mutations arise de novo, although we observed a single case of somatic mosaicism in an unaffected parent.

Clinical and molecular characterization of two patients with overlapping de novo microdeletions in 2p14-p15 and mild mental retardation

Here, we present two patients with overlapping de novo microdeletions in chromosome 2p14-p15, mild mental retardation concerning especially language development, as well as mild dysmorphic features. Patient 1 also presented with generalized seizures, sensorineural hearing loss, and relative microcephaly. In patient 1, molecular karyotyping detected a 2.23-Mb deletion in chromosome 2p14-p15 including 11 known genes. The second patient, with a 2.84-Mb microdeletion containing 15 genes, was identified in the DECIPHER database. The two deleted regions overlap by a stretch of 1.6 Mb that contains 10 genes, several of which have functions in neuronal development. This report illustrates the power of databases such as DECIPHER and MRNET in assessing the pathogenicity of copy-number variations (CNVs).

Transmitted cytogenetic abnormalities in patients with mental retardation: Pathogenic or normal variants?

Knowing the origin of cytogenetic abnormalities detected in individuals with mental retardation and dysmorphic features is essential to genetic counselling of affected families. To illustrate this, we report on six families with transmitted cytogenetic abnormalities and discuss the genotype-phenotype correlations, including the possibility of the abnormalities being normal genomic variants. The abnormalities were detected using metaphase HR-CGH; their size was estimated to range from 1.6 to 7.5 Mb using tiling path array-CGH and real-time PCR. The abnormalities were transmitted through two to four generations and included interstitial deletions of 1p31.3-p32.1, 2q13, 10q11.21-q11.23, and 13q31.1; a duplication of 1p34.1-p34.2; and in one family both a deletion of 18q21.1 and a duplication of 4q35.1-q35.2. The probands were mentally retarded and had nonspecific dysmorphic features except for one patient with the Bohring-Opitz syndrome. We considered the abnormalities in two families to be clinically significant: In one family, the proband's brain abnormality was comparable to previously reported abnormalities in individuals with a similar duplication of 1p31-p32. Congenital heart disease was previously mapped to the chromosomal region of 18q that was affected in the proband of another family. The carrier parents in both families had mild clinical features. In two families the abnormalities were considered as coincidental findings, and in two further families the abnormalities were insufficient to explain the phenotypes of the probands but possibly were related to a milder phenotype in other family members. These cases illustrate the need for careful assessment of the extended family in order to interpret the phenotypic consequences of abnormalities identified using array-CGH.

Additional chromosomal abnormalities in patients with a previously detected abnormal karyotype, mental retardation, and dysmorphic features

The detection of chromosomal abnormalities in patients with mental retardation (MR) and dysmorphic features increases with improvements of molecular cytogenetic methods. We report on six patients referred for detailed characterization of chromosomal abnormalities (four translocations, one inversion, one deletion) detected by conventional cytogenetics, in whom metaphase CGH revealed imbalances not involved in the initially detected rearrangements. The detected abnormalities were validated by real-time PCR. Parents were investigated by CGH in four cases. The genomic screening revealed interstitial deletions of 2q33.2-q34, 3p21, 4q12-q13.1, 6q25, 13q22.2-q31.1, and 14q12. The estimated minimum sizes of the deletions ranged from 2.65 to 9.27 Mb. The CGH assay did not reveal imbalances that colocalized with the breakpoints of the inversion or the translocations. The deletion of 6q included ESR1, in which polymorphisms are associated with variation of adult height. FOXG1B, known to be involved in cortical development, was located in the 14q deletion. The results illustrate that whole-genome molecular cytogenetic analysis of phenotypically affected patients with abnormal conventional karyotypes may detect inapparent molecular cytogenetic abnormalities in patients with microscopic chromosomal abnormalities and that these data provide additional information of clinical importance.

Evaluation of the present breeding programme against copper toxicosis in Danish Bedlington terriers

A breeding programme to eradicate copper toxicosis in Danish Bedlington terriers has been established based on a DNA marker test. Genotyping of both parents is compulsory and after 1 January 2000, only homozygous non-carriers are used for breeding. In this study, two groups of Bedlington terriers were genotyped at 18 microsatellite loci. One group represented the original population of Bedlington terriers before introducing the breeding programme (n = 23); the other represented a group of homozygous non-carriers (n = 24) available for breeding after year 2000. Allele numbers, allele frequencies, observed heterozygosities (Ho), expected heterozygosities (He), locus-specific coefficients of inbreeding (Fl) and Nei's genetic distance (D) was calculated. Individual coefficients of inbreeding (Fi) were calculated from the pedigrees and an assignment test was performed. Four rare alleles were lost in the group of homozygous non-carriers. No significant differences were observed between the mean values of allele numbers, Ho, He, Fl and Fi of the two populations of dogs. Nei's genetic distance between the two populations was 0.06 and 88% of the homozygous non-carriers were assigned correctly in the assignment test. The overall diversity of the breed was low (Ho = 0.41) and the breeders were advised to include the heterozygous carriers again.

Microsatellite marker C04107 as a diagnostic marker for copper toxicosis in the Danish population of Bedlington terriers

The linkage phase of marker C04107 was evaluated before implementation of the marker in a diagnostic test. Blood samples from 68 dogs were collected and genotyped by PCR. Two alleles were detected with sizes of 160 bp and 164 bp and allele frequencies of 0.45 and 0.55 respectively. Genotyping revealed that 35 dogs were heterozygous (51.5%), 22 dogs were homozygous for the normal allele (32.3%) and 11 dogs were homozygous for the disease allele (16.2%). Liver biopsies were taken from 14 selected dogs and the copper content was evaluated histologically. Biopsies from 8 dogs homozygous for the disease allele showed many copper granules along with single cell necrosis, haemosiderosis and cellular infiltration. In liver biopsies from 6 dogs genotyped to be heterozygous or homozygous for the normal allele, copper granules were absent or moderate in number and no lesions were present. The survey demonstrates that the linkage phase of marker C04107 in the Danish population of Bedlington terriers is similar to the linkage phase detected in other countries. Thus, the marker can be used in a diagnostic test for copper toxicosis in Denmark.

Microsatellite marker C04107 as a diagnostic marker for copper toxicosis in the Danish population of Bedlington terriers

The linkage phase of marker C04107 was evaluated before implementation of the marker in a diagnostic test. Blood samples from 68 dogs were collected and genotyped by PCR. Two alleles were detected with sizes of 160 bp and 164 bp and allele frequencies of 0.45 and 0.55 respectively. Genotyping revealed that 35 dogs were heterozygous (51.5%), 22 dogs were homozygous for the normal allele (32.3%) and 11 dogs were homozygous for the disease allele (16.2%). Liver biopsies were taken from 14 selected dogs and the copper content was evaluated histologically. Biopsies from 8 dogs homozygous for the disease allele showed many copper granules along with single cell necrosis, haemosiderosis and cellular infiltration. In liver biopsies from 6 dogs genotyped to be heterozygous or homozygous for the normal allele, copper granules were absent or moderate in number and no lesions were present. The survey demonstrates that the linkage phase of marker C04107 in the Danish population of Bedlington terriers is similar to the linkage phase detected in other countries. Thus, the marker can be used in a diagnostic test for copper toxicosis in Denmark.

Screening for familial hypercholesterolaemia by measurement of apolipoproteins in capillary blood

A total of 3025 families with schoolchildren aged 6-8 years were offered pilot screening for familial hypercholesterolaemia by measurement of the concentrations of apolipoproteins A-1 and B in the children's capillary blood and by analysis of their family histories of early ischaemic heart disease. The concentrations of the apolipoproteins were determined by double rocket immunoelectrophoresis of an eluate of blood spotted on filter paper. Results were available from 2085 children. Because their B:A-1 ratio was above the 97.5 centile and their concentration of B was above the 99th centile, 54 children (2.6%) were selected to have their apolipoprotein concentrations reassessed. The 17 children (0.8%) whose values were persistently above the chosen cut off points, and all of their available first and second degree relatives, had fasting determinations of serum lipid concentrations carried out. Raised serum concentrations of low density lipoprotein cholesterol and an autosomal dominant pattern of hypercholesterolaemia were found in 12 children and 10 families, respectively, suggesting a higher incidence of familial hypercholesterolaemia than the reported 1:500. Further investigations among family members disclosed hypercholesterolaemia in 29 relatives. A family history of early ischaemic heart disease was elicited by questionnaire, and was positive in only five of the 12 schoolchildren with hypercholesterolaemia. We conclude that analysis of apolipoproteins from capillary blood spotted on filter paper is suitable for screening for familial hypercholesterolaemia, and that this method is more efficient than screening based on family history.