Somatic mosaicism in focal epilepsy. Recent findings highlighted the role of somatic parental mosaicism in epileptic encephalopathies. However, somatic mosaicism has also emerged over the last few years as a prominent mechanism in the pathogenesis of lesional focal epilepsies, including focal cortical dysplasia (FCD) type 2 and hemimegalencephaly. Previous studies have identified the role of mosaicism of genes such as MTOR, TSC1/TSC2, and genes encoding components of the PI3K/AKT pathway in patients with epilepsy secondary to brain malformations. A recent study in Annals of Neurology has identified a new unrelated genetic cause of refractory non-lesional focal epilepsy, which leads us to wonder what role mosaicism may be playing in focal epilepsies without obvious findings on MRI.
Ion channels and brain malformations. When the “channelopathy” concept first emerged – the idea that dysfunction of neuronal ion channels leads to neurological disease including epilepsy – it seemed implausible that such dysfunction could lead to malformations of cortical development. However, recent research has suggested that ion channel dysfunction may indeed be linked with brain malformations. In 2017, we saw convincing evidence that germline de novo variants in GRIN2B can cause malformations of cortical development. Some suggestive, but less conclusive, evidence has also linked SCN1A and SCN2A to brain malformations. Now Fry and collaborators demonstrate that de novo pathogenic variants in GRIN1 can also cause significant polymicrogyria, expanding the phenotypic spectrum of GRIN1-related disorders. As a disclaimer, I am also a co-author on the publication by Fry and collaborators. Continue reading
Silence. You might wonder why you hear very little about ARX in exome studies these days. The X-chromosomal aristaless related homeobox gene was one of the first genes for epilepsies and brain malformations to be discovered. Mutations in ARX can be identified in male patients with a variety of neurodevelopmental disorders including idiopathic West Syndrome – accordingly, it’s on the differential list for patients with Infantile Spasms without a known cause. Let me tell you about the problems that the ARX gene poses for exome sequencing. Continue reading
Success rate. What is in an exome? There are lots of rare and unknown variants that are hard to make sense of unless we can ask a specific question or have further data to limit the number of genes that we look at. Genetic studies in recessive diseases with limited candidate genes to consider might represent one of the most straightforward cases. In a recent paper in BMC Neurology, the genetic cause of a recessive epilepsy/intellectual disability syndrome in a consanguineous family presenting with primary microcephaly was solved using a single exome of an affected individual. Was this just lucky or can this strategy be applied to any recessive family with a reasonable chance? Continue reading
Complexity. Structural genomic variants or copy number variations (CNV) are known genetic risk factors for various epilepsy syndromes. In fact, CNVs might represent the single most studied type of genetic alterations across a very broad range of epilepsy syndromes. There is, however, a group of patients that is usually not investigated in genetic studies: patients with presumable lesional epilepsies or questionable findings on Magnetic Resonance Imaging (MRI). Many of these epilepsies are usually thought to be secondary to the identified lesion, and genetic risk factors are not considered. In a recent study in the European Journal of Human Genetics last week, we investigated the role of CNVs in a cohort of patients with complex epilepsy phenotypes that were not easily classified into existing categories. Many of patients included had definite or questionable findings on MRI. The results of our study made us wonder whether the boundary between lesional and genetic epilepsies needs to redrawn. Continue reading
Variability. It has been rumored for quite some time, but only now is solid evidence present to show this phenomenon: the high degree of genomic diversity of human neurons. In a recent paper in Science, the genomic diversity among frontal brain neurons is explored on a cell-by-cell basis. The results are breathtaking: up to 40% of frontal cortex neurons have altered genomic material affected by large deletions or duplications. This study provides the linchpin for a plethora of new investigations aiming to understand the impact of this phenomenon in health and disease. Continue reading
Communication breakdown. The Greek orator Demosthenes was said to treat his speech impediment by talking with pebbles in his mouth and shouting above the roar of the ocean waves. US Vice President Joe Biden, brutally nicknamed Joe Impedimenta in school, worked on his stuttering reading Emerson and Yeats aloud. Hollywood actor Samuel L. Jackson overcame blocks and pauses while talking by interjecting his trade mark profanity. Given the list of famous people who stutter including Isaac Newton, Charles Darwin, and Theodore Roosevelt, I feel in pretty good company. I am a person who stutters myself, even though my speech impediment is currently mild. Stuttering is a neurodevelopmental disorder whose genetic architecture is entirely unexplored on the molecular level but clinically shares resemblance with many other neurodevelopmental disorders that we have written about on this blog. Today is International Stuttering Awareness Day. I have thought back and forth about whether I want to write this post given my personal involvement as a person who stutters and the resulting lack of objectivity. However, I finally decided to do so in order to put stuttering where it belongs – on a research blog about neurogenetics. Continue reading
Radial migration. The fact that neurons find their place in the cortex during development is nothing short of a miracle. Many neurons originate in the subventricular zone, i.e. the area lining the ventricles. During brain development, these neurons subsequently climb outwards to their final positions using radial glia cells as scaffolds. If this delicate process is disturbed, neurons may be misplaced. Periventricular nodular heterotopia (PVNH) is a condition in which defects in neuronal migration result in ectopic neuronal nodules lining the ventricles. These nodules may result in a broad range of epilepsies, ranging from mild seizure disorders to intractable epilepsy with intellectual disability. Many cases of PVNH are assumed to be genetic, and FLNA and ARFGEF2 as known causative genes. However, the cause remains unknown in a significant number of patients. In a recent paper in Brain, C6orf70 is identified as a new candidate for PVNH using a clever combination of array CGH and exome sequencing. Continue reading
Tuberous Sclerosis. Tuberous Sclerosis Complex (TSC) is a neurodevelopmental disorder caused by lack of function of the TSC1 or TSC2 tumor suppressor gene. With respect to the Central Nervous System, this disease is characterized by so-called tubers, benign tumors consisting of dysplastic neurons that are highly epileptogenic. Accordingly, TSC is one of the most common causes of West Syndrome. However, there is also evidence for neurological dysfunction beyond tubers. Increasing evidence suggests that the mutations alone can result in abnormalities of neuronal networks, resulting in epilepsy, intellectual disability or autism. The thalamus appears to be a key structure that is affected by this dysfunction. Now, a recent study in Cell explores the effects of TSC1 deletions at different developmental stages with respect to neuronal development in the thalamus. Continue reading
Share or be shared. During the last two weeks, the RES consortium has approved a new data sharing policy that will allow us to work with increased transparency and accountability within our upcoming projects. This new data sharing policy is a consequent extension of the previous protocols we had in earlier consortia – with one major difference. This time, it’s in writing. While we are getting ready to tackle the large dataset on epileptic encephalopathies released by the Sanger Institute, we took a moment to talk about how things should be running.