The final frontier. The last five years have seen enormous progress in understanding the genetic basis of sporadic severe, treatment-resistant epilepsies due to de novo mutations. However, there has been much less progress in understanding the basis of familial epilepsy, which has historically been the major focus of epilepsy genetics. Particularly small families with mild epilepsies are challenging to solve, with the exception of rare families with pathogenic variants in known epilepsy genes. Exome-first approaches in familial epilepsy are particularly challenging given the sheer amount of variants segregating in small families by chance. In a recent publication by the Epi4K Consortium, a novel approach is presented to identify the genetic basis of familial epilepsies, overcoming the limited power of small families by analyzing rare variants in probands in a case/control study design. Here are some fascinating insights from this study. Continue reading
Issue 8/2015. This week’s review of the relevant publications in the field is about a novel risk factor for focal epilepsies, a gene involved in mRNA transport from the cell nucleus, and a small, confirmatory study on exome sequencing in Infantile Spasms.
GABAergic. Let’s start out with a provocative statement. There is a single gene that may explain more cases of Lennox-Gastaut Syndrome (LGS) and Infantile Spasms (IS) than you would expect, rivalling SCN1A for the most common gene found in this group of patients. It’s a gene that you are probably aware of but that you may think to be a very rare finding. In a recent publication in Annals of Neurology, the Epi4K consortium published their recent analysis of copy number variations that were derived from exome data. Combining de novo mutations and copy number variations points to GABRB3 as a major player in LGS and IS, explaining probably more than 2% of patients. Let’s find out about the twilight zone, strategies to obtain structural variants from exomes, and the re-emergence of the 15q duplication syndrome. Continue reading
CNV. Structural genomic variations or Copy Number Variations (CNVs) significantly contribute to the genetic architecture of many neurodevelopmental disorders. However, given the enormous variation in the human genome in healthy individuals, the precise contribution of CNVs remains poorly understood. In a recent publication in PLOS Genetics, we were able to assess the microdeletion architecture in more than 1,000 patients with Genetic Generalized Epilepsy (GGE) compared to more than 5,000 controls. We found that microdeletions occur almost twice as often in GGE patients compared to controls, an analysis that revealed both known suspects and interesting candidates. Continue reading
Heritable. Many epilepsy syndromes have signature EEG traits, and these traits are thought to have a strong genetic component. The endophenotype concept suggests that using these epilepsy-related traits in genetic studies will facilitate gene discovery, a concept that has failed us so far in epilepsy research, unfortunately. Now, in a recent publication in Brain, we were able to demonstrate that variants in CHD2 predispose to photosensitivity, an abnormal cortical response to flickering light. Finally, after several decades of persisting difficulties, there is some progress in the field of EEG genetics. Continue reading
Pyridoxal 5’-phosphate (PLP). PNPO deficiency is a rare neurometabolic disease that presents with severe neonatal epilepsy responsive to pyridoxal phosphate. While the classical clinical presentation is well described, there might be milder versions of this potentially treatable neurometabolic disease that have not been recognized so far. In a recent publication in Brain, the phenotypic spectrum of PNPO deficiency is revisited. In addition to the classical neonatal phenotype, the authors identify patients with later onset and atypical response to pyridoxal phosphate. In addition, they identify a rare, potentially causative PNPO variant that probably gets stuck in most exome filtering pipelines. Continue reading
Living in Cologne is a little tough at the moment. Currently, we are in the middle of the Cologne Carnival, the world’s oldest carnival, which started in 1829. Until the upcoming Wednesday the entire city is one big festival. In addition to the 1 million Cologne citizens probably another million tourists will join. Due to this (positive) distraction I will write less than usual. However, I still consider this week’s publications noteworthy. Continue reading
Missing heritability. The concept of missing heritability is often invoked to demonstrate that existing genetic techniques only identify a fraction of the overall genetic risk for human diseases including the epilepsies. This statement implicitly assumes that we have a good and solid understanding of what the magnitude of genetic risk actually is. However, when looking at the epidemiological studies that have investigated familial risk of epilepsy, some of these studies have inherent problems, including small sample sizes, different phenotype definitions, recruitment bias, and lack of controls. A recent study in Brain now reassesses the familial risk of epilepsy in a population-based cohort of the Rochester Epidemiology Project. There are few instant classics in the field of epilepsy genetics – this study is one of them. 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
Variations on Copy Numbers. In the third issue of our series on the papers of the week I will focus on the detection and annotation of the most common form of structural variation encountered in genomes. Deletions, duplications and inversions are frequent events, which are surprisingly hard to deal with using sequencing-based tools. Hence, this is an area of active development.