Anna-Kaisa Anttonen

The gene disrupted in Marinesco-Sjögren syndrome encodes SIL1, an HSPA5 cochaperone.

We identified the gene underlying Marinesco-Sjögren syndrome, which is characterized by cerebellar ataxia, progressive myopathy and cataracts. We identified four disease-associated, predicted loss-of-function mutations in SIL1, which encodes a nucleotide exchange factor for the heat-shock protein 70 (HSP70) chaperone HSPA5. These data, together with the similar spatial and temporal patterns of tissue expression of Sil1 and Hspa5, suggest that disturbed SIL1-HSPA5 interaction and protein folding is the primary pathology in Marinesco-Sjögren syndrome.

SCN2A mutation associated with neonatal epilepsy, late-onset episodic ataxia, myoclonus, and pain

Background: Inherited and de novo mutations in sodium channel genes underlie a variety of channelopathies. Mutations in SCN2A, encoding the brain sodium channel NaV1.2, have previously been reported to be associated with benign familial neonatal infantile seizures, febrile seizures plus, and intractable epilepsy of infancy. Methods: We evaluated the clinical characteristics in a patient with a neonatal-onset complex episodic neurologic phenotype. We screened SCN2A for mutations and carried out in vitro electrophysiologic analyses to study the consequences of the identified mutation. We studied the developmental expression of NaV1.2 in cerebellum by immunohistochemical analysis. Results: The patient presented with neonatal-onset seizures and variable episodes of ataxia, myoclonia, headache, and back pain after 18 months of age. The patient carries a de novo missense mutation (p.Ala263Val) in SCN2A, which leads to a pronounced gain-of-function, in particular an increased persistent Na+ current. Immunohistochemical studies suggest a developmentally increasing expression of NaV1.2 in granule cell axons projecting to Purkinje neurons. Conclusions: These results can explain a neuronal hyperexcitability resulting in seizures and other episodic symptoms extending the spectrum of SCN2A-associated phenotypes. The developmentally increasing expression of NaV1.2 in cerebellum may be responsible for the later onset of episodic ataxia.

Novel SIL1 mutations and exclusion of functional candidate genes in Marinesco-Sjögren syndrome

Marinesco–Sjögren syndrome (MSS) is a rare autosomal recessively inherited neurodegenerative disorder characterized by cerebellar ataxia, cataracts, mental retardation, and progressive myopathy. Recently, mutations in the SIL1 gene, which encodes an endoplasmic reticulum (ER) resident cochaperone, were identified as a major cause of MSS. We here report four novel mutations in SIL1, including the first missense substitution p.Leu457Pro described in MSS. In addition, we excluded three functional candidate genes, HSPA5, HYOU1, and AARS, as causative genes in SIL1 mutation-negative patients. To understand the mechanisms of disturbed SIL1 function, we studied the subcellular localization of the missense mutant Leu457Pro protein in COS-1 cells. Moreover, we studied a mutant protein lacking the putative C-terminal ER retrieval signal. In contrast to the wild-type proteins localization to ER and Golgi apparatus, both mutant proteins formed aggregates within the ER depending on the expression level. These data imply that aggregation of mutant proteins may contribute to MSS pathogenesis. The genetic background of a subgroup of patients with MSS remains uncovered.

Two novel CLN6 mutations in variant late‐infantile neuronal ceroid lipofuscinosis patients of Turkish origin

Neuronal ceroid lipofuscinoses (NCLs) are the most common neurodegenerative childhood‐onset disorders characterized by autosomal recessive inheritance, epileptic seizures, progressive psychomotor deterioration, visual failure, and premature death. At least seven subtypes of childhood‐onset NCLs have been identified of which the late‐infantile–onset forms (LINCLs) are genetically the most heterogeneous with four underlying genes identified. A variant form of LINCL (vLINCL) present in Turkish patients has been considered a distinct clinical and genetic entity (CLN7). However, we recently showed that mutations in the CLN8 gene account for a subset of Turkish vLINCL. Toward identifying the CLN7 gene we here screened the known NCL loci for homozygosity in nine Turkish vLINCL families. These loci were excluded in seven families that are likely to represent the ‘true’ Turkish vLINCL. In two families, we identified two novel homozygous mutations in the CLN6 gene: an intronic base substitution (c.542 + 5G > T) affecting the splicing of the transcript and a nonsense mutation (c.663C > G) creating a stop codon at tyrosine 221. These data indicate that CLN6 mutations, in addition to those of CLN8, should be considered a diagnostic alternative in Turkish vLINCL patients. The genetic background of the ‘true’ Turkish vLINCL, CLN7, remains to be defined.

Dravet syndrome: New potential genetic modifiers, imaging abnormalities, and ictal findings

Dravet syndrome is an autosomal dominant epileptic encephalopathy of childhood, which is caused mainly by SCN1A and PCHD19 mutations. Although Dravet syndrome is well recognized, the causes of acute encephalopathy are still elusive, and reported data on ictal electroencephalography (EEG) and structural brain abnormalities are scarce.

Digenic mutations in severe myoclonic epilepsy of infancy

The clinical features of severe myoclonic epilepsy of infancy (SMEI) resemble those of mitochondrial diseases, although most patients have the sodium channel (SCN1A) mutation. We describe a patient with SMEI and enlarged muscle mitochondria associated with mutations in mitochondrial polymerase gamma 1 (POLG1) and SCN1A. Due to increased risk of valproate-induced liver failure in patients with POLG1 mutations, we recommend POLG1 gene analysis for SMEI patients before valproate administration.

Pitfalls in genetic testing : the story of missed SCN1A mutations

Sanger sequencing, still the standard technique for genetic testing in most diagnostic laboratories and until recently widely used in research, is gradually being complemented by next‐generation sequencing (NGS). No single mutation detection technique is however perfect in identifying all mutations. Therefore, we wondered to what extent inconsistencies between Sanger sequencing and NGS affect the molecular diagnosis of patients. Since mutations in SCN1A, the major gene implicated in epilepsy, are found in the majority of Dravet syndrome (DS) patients, we focused on missed SCN1A mutations.

The Finnish disease heritage database (FinDis) update-a database for the genes mutated in the Finnish disease heritage brought to the next-generation sequencing era.

The Finnish Disease Heritage Database (FinDis) (http://findis.org) was originally published in 2004 as a centralized information resource for rare monogenic diseases enriched in the Finnish population. The FinDis database originally contained 405 causative variants for 30 diseases. At the time, the FinDis database was a comprehensive collection of data, but since 1994, a large amount of new information has emerged, making the necessity to update the database evident. We collected information and updated the database to contain genes and causative variants for 35 diseases, including six more genes and more than 1,400 additional disease‐causing variants. Information for causative variants for each gene is collected under the LOVD 3.0 platform, enabling easy updating. The FinDis portal provides a centralized resource and user interface to link information on each disease and gene with variant data in the LOVD 3.0 platform. The software written to achieve this has been open‐sourced and made available on GitHub (http://github.com/findis‐db), allowing biomedical institutions in other countries to present their national data in a similar way, and to both contribute to, and benefit from, standardized variation data. The updated FinDis portal provides a unique resource to assist patient diagnosis, research, and the development of new cures.

Marinesco-Sjögren syndrome due to SIL1 mutations with a comment on the clinical phenotype

BACKGROUND Marinesco-Sjögren syndrome is an autosomal recessive cerebellar ataxia, characterised by cerebellar ataxia, myopathy, cataracts and intellectual disability, due to mutations in the SIL1 gene. METHODS The clinical features and two novel SIL1 mutations of four Dutch patients with Marinesco-Sjögren syndrome are described and compared to the literature on genetically proven Marinesco-Sjögren patients. RESULTS The core phenotype of this syndrome appears homogeneous, but: [1] cataract can develop later than the motor and cognitive signs; [2] myopathy is an early feature that seems progressive during the course of the disease; [3] serum creatine kinase is normal or only mildly elevated; [4] peripheral neuropathy is absent; and [5] a variable degree of intellectual disability is present in most Marinesco-Sjögren patients. CONCLUSIONS Because the late appearance of some hallmarks and the uncertainty as to whether incomplete phenotypes occur, SIL1 mutation analysis is helpful early in the diagnostic work-up of children with suspected inherited ataxias.

Dysfunctional ADAM22 implicated in progressive encephalopathy with cortical atrophy and epilepsy

Objective: To identify the molecular genetic basis of a syndrome characterized by rapidly progressing cerebral atrophy, intractable seizures, and intellectual disability. Methods: We performed exome sequencing in the proband and whole-genome single nucleotide polymorphism genotyping (copy number variant analysis) in the proband-parent trio. We used heterologous expression systems to study the functional consequences of identified mutations. Results: The search for potentially deleterious recessive or de novo variants yielded compound heterozygous missense (c.1202G>A, p.Cys401Tyr) and frameshift deletion (c.2396delG, p.Ser799IlefsTer96) mutations in ADAM22, which encodes a postsynaptic receptor for LGI1. The deleterious effect of the mutations was observed in cell surface binding and immunoprecipitation assays, which revealed that both mutant proteins failed to bind to LGI1. Furthermore, immunoprecipitation assays showed that the frameshift mutant ADAM22 also did not bind to the postsynaptic scaffolding protein PSD-95. Conclusions: The mutations identified abolish the LGI1-ADAM22 ligand-receptor complex and are thus a likely primary cause of the probands epilepsy syndrome, which is characterized by unusually rapidly progressing cortical atrophy starting at 3–4 months of age. These findings are in line with the implicated role of the LGI1-ADAM22 complex as a key player in nervous system development, specifically in functional maturation of postnatal synapses. Because the frameshift mutation affects an alternatively spliced exon with highest expression in postnatal brain, the combined effect of the mutations is likely to be hypomorphic rather than complete loss of function. This is compatible with the longer survival of the patient compared to Lgi1−/− and Adam22−/− mice, which develop lethal seizures during the first postnatal weeks.