Distinct neurodevelopmental and epileptic phenotypes associated with gain- and loss-of-function GABRB2 variants.

BACKGROUND: Variants in GABRB2, encoding the ß2 subunit of the γ-aminobutyric acid type A (GABAA) receptor, can result in a diverse range of conditions, ranging from febrile seizures to severe developmental and epileptic encephalopathies. However, the mechanisms underlying the risk of developing milder vs more severe forms of disorder remain unclear. In this study, we conducted a comprehensive genotype-phenotype correlation analysis in a cohort of individuals with GABRB2 variants. METHODS: Genetic and electroclinical data of 42 individuals harbouring 26 different GABRB2 variants were collected and accompanied by electrophysiological analysis of the effects of the variants on receptor function. FINDINGS: Electrophysiological assessments of α1ß2γ2 receptors revealed that 25/26 variants caused dysfunction to core receptor properties such as GABA sensitivity. Of these, 17 resulted in gain-of-function (GOF) while eight yielded loss-of-function traits (LOF). Genotype-phenotype correlation analysis revealed that individuals harbouring GOF variants suffered from severe developmental delay/intellectual disability (DD/ID, 74%), movement disorders such as dystonia or dyskinesia (59%), microcephaly (50%) and high risk of early mortality (26%). Conversely, LOF variants were associated with milder disease manifestations. Individuals with these variants typically exhibited fever-triggered seizures (92%), milder degrees of DD/ID (85%), and maintained ambulatory function (85%). Notably, severe movement disorders or microcephaly were not reported in individuals with loss-of-function variants. INTERPRETATION: The data reveals that genetic variants in GABRB2 can lead to both gain and loss-of-function, and this divergence is correlated with distinct disease manifestations. Utilising this information, we constructed a diagnostic flowchart that aids in predicting the pathogenicity of recently identified variants by considering clinical phenotypes. FUNDING: This work was funded by the Australian National Health & Medical Research Council, the Novo Nordisk Foundation and The Lundbeck Foundation.

Paternal germline mosaicism of a SCN2A mutation results in Ohtahara syndrome in half siblings.

Ohtahara syndrome is a devastating early infantile epileptic encephalopathy caused by mutations in different genes. We describe a patient with Ohtahara syndrome who presented on the first day of life with refractory tonic seizures and a suppression-burst pattern on EEG. The patient developed severe microcephaly, and never achieved any developmental milestones. He died at the age of 5 years. A de novo missense mutation (c. 4007C>A, p.S1336Y) in SCN2A was found. Interestingly, the father has another son with Ohtahara syndrome from a different mother. The half brother carries the same SCN2A mutation, strongly suggesting paternal gonadal mosaicism of the mutation. The broad clinical spectrum of SCN2A mutations now includes Ohtahara syndrome. This is the first report of familial Ohtahara syndrome due to a germline mosaic SCN2A mutation. Somatic mosaicism, including germline, has been described in several epileptic encephalopathies such as Dravet syndrome, KCNQ2 neonatal epileptic encephalopathy, SCN8A epileptic encephalopathy and STXBP1 related Ohtahara syndrome. Mosaicism should be considered as one of the important inheritance patterns when counseling parents with a child with these devastating diseases.

Mercury intoxication in a 2-year-old girl: a diagnostic challenge for the physician.

A 2-year-old girl presented with hypertension, anorexia and vomiting, restlessness, insomnia and acrodynia. Her blood pressure upon arrival was 145/98 mmHg. Ultrasound of the abdomen, CT scan of chest, abdomen and pelvis, and echocardiogram, were normal. Urinary levels of catecholamines were elevated, urine level of mercury was found to be high (33.2 microg/g creatinine), although blood level was normal (>0.5 microg/dl, reference value 0-4 microg/dl). Following a 1-month course of oral treatment with dimercaptosuccinic acid (DMSA) the child's symptoms and signs resolved, and urinary mercury and catecholamines levels normalized. Mercury intoxication should be suspected in a patient with severe hypertension, personality changes and acrodynia. Normal blood levels of mercury do not exclude this diagnosis, and catecholamine levels may serve as a surrogate marker for confirmation of the diagnosis and to evaluate response to treatment.