Authors :
Presenting Author: Annie Ting-Gee Chiu, MBBS, FHKCPaed – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Austin Health, Melbourne, Victoria, Australia
Mark Bennett, PhD – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Victoria, Australia
Harshini Thiyagarajah, BSc (Hons) – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Austin Health, Melbourne, Victoria, Australia
Amy Schneider, MGenCouns – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Austin Health, Melbourne, Victoria, Australia
Sian Macdonald, MSc – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Austin Health, Melbourne, Victoria, Australia
Tom Witkowski, BSc (Hons) – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Austin Health, Melbourne, Victoria, Australia
Edith Fuerte, BSc – Department of Cell and Molecular Biology, Center for Pediatric Neurological Disease Research, St. Jude Children's Research Hospital, Memphis, Tennessee, USA
Talia Allan, MSc – Austin Health & The University of Melbourne
Nico Lieffering, BBmedSc (Hons) – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Austin Health, Melbourne, Victoria, Australia
Blake Robinson, Pre-med – Department of Cell and Molecular Biology, Center for Pediatric Neurological Disease Research, St. Jude Children's Research Hospital, Memphis, Tennessee, USA
Christy LaFlamme, MSc – Department of Cell and Molecular Biology, Center for Pediatric Neurological Disease Research, St. Jude Children's Research Hospital, Memphis, Tennessee, USA
Soham Sengupta, PhD – Department of Cell and Molecular Biology, Center for Pediatric Neurological Disease Research, St. Jude Children's Research Hospital, Memphis, Tennessee, USA
Clara Chung, MBBS, MMed – Department of Clinical Genetics, Liverpool Hospital, Liverpool, New South Wales, Australia
Michael Cardamone, MBBS, PhD – Department of Paediatric Neurology, Sydney Children’s Hospital, Sydney, New South Wales, Australia
Cassandra Gray, BSc (Hons) – Centre for Population Genomics, Garvan Institute of Medical Research, University of New South Wales and Murdoch Children's Research Institute, Melbourne, Australia
Piero Perucca, MD, PhD – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Austin Health, Heidelberg, Victoria, Australia
Samuel Berkovic, MD – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Austin Health, Melbourne, Victoria, Australia
Heather Mefford, MD, PhD – Department of Cell and Molecular Biology, Center for Pediatric Neurological Disease Research, St. Jude Children's Research Hospital, Memphis, Tennessee, USA
Michael Hildebrand, PhD – Epilepsy Research Centre, Department of Medicine, University of Melbourne, Austin Health, Melbourne, Victoria, Australia; Neuroscience Group, Murdoch Children’s Research Institute, Parkville, Victoria, Australia
Ingrid Scheffer, AO MBBS PhD FRACP FAES FAA FRS – Epilepsy Research Centre and Royal Children’s Hospital, The University of Melbourne, Austin Health, Florey and Murdoch Children’s Research Institutes
Rationale:
RNU genes encode small nuclear RNAs (snRNAs), components of the spliceosome responsible for removing introns from pre-messenger RNA. Genome sequencing recently identified pathogenic variants in RNU4-2 and RNU2-2 in 0.5% and 0.1% of patients with neurodevelopmental disorders respectively1,2. We hypothesized that RNU2-2 variants may cause developmental and epileptic encephalopathies (DEEs).
Methods:
We analysed 672 patients with DEEs of unknown cause for recurrent RNU2-2 pathogenic variants n.4G >A, n.35A >G and n.35A >C. We performed Sanger sequencing in 498 individuals, and reanalysed genome sequencing data in 304. An additional patient was recruited. We interviewed parents using a validated seizure questionnaire, obtained EEG and neuroimaging results and medical records. We analysed phenotypic features and natural history of affected patients. The study was approved by Austin Health Human Research Ethics Committee (Project No. H2007/02961).Results:
We identified recurrent pathogenic RNU2-2 variants in 4/672 (0.6%) patients with DEEs. Including the additional patient, two had n.4G >A and three had n.35A >G. Median seizure onset was 24 months. First seizure was febrile in three. All had explosive seizure onset and multiple seizure types: tonic-clonic (5), focal impaired consciousness (n=5), hemiclonic (n=3), hypermotor (n=2), tonic (n=2), myoclonic (n=1), atonic (n=1) and absence (n=1). Convulsive status epilepticus occurred in three patients and nonconvulsive status epilepticus in two. EEG showed sleep-activated multifocal discharges (n=4). MRI brain showed hippocampal sclerosis (n=3). One patient had childhood-onset DEE with spike-wave activation in sleep, evolving into Lennox-Gastaut syndrome at 18 years of age. Developmental regression (n=4) or plateauing (n=1) occurred. They had severe to profound impairment. Comorbidities included hyperventilation (n=3), obstructive sleep apnoea (n=3), autism spectrum disorder (n=3), hypotonia (n=3), choreoathetosis (n=1), sleep (n=5) and gastrointestinal (n=3) problems. Dysmorphic features included microcephaly, prominent eyebrows, broad nasal root, full cheeks and wide mouth.
Conclusions:
RNU2-2 causes a distinctive severe DEE, with high rates of status epilepticus, hemiclonic seizures, multiple seizure types, hippocampal sclerosis and hyperventilation. It shares phenotypic overlap with Dravet syndrome and Rett syndrome. Our findings highlight the importance of analysing non-coding regions in patients with unsolved DEEs, and the emerging role of snRNAs in genetic epilepsies.
Funding: The National Health and Medical Research Council (NHMRC) postgraduate scholarship, Australia