CNV. There are different forms of genetic variation and historically, our ability to query the entire exome or genome is a relatively recent development. However, the first type of genetic variation that could be assessed in the epilepsies in large cohorts were copy number variations (CNV), small gains or losses of chromosomal materials. In a recent study, the entire Epi25 cohort was analyzed for CNVs, giving a long-needed update on the role of the structural genomic variations in various forms of epilepsies and highlighting that the overall landscape of CNVs in the epilepsies is well understood and delineated. With up to 3% of individuals with epilepsies carrying some of the recurrent CNVs, this type of genomic variation remains a rare, but important source of genetic morbidity in the epilepsies. 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
A productive week in epilepsy genetics. Scientists and editors were certainly busy this week reporting novel variants and deletions as well the experimental and statistical advances for their interpretation.
A de novo GRIN2A missense mutation in early-onset epileptic encephalopathy. We and others have associated variants affecting the GRIN2A gene with a range of childhood focal epilepsy syndromes. Continue reading
New Year – new papers. The United Nations has declared 2014 the International Year of Family Farming and Crystallography.
But for epilepsy genetics it will be the year of genotyping and sequencing. Hopefully, these studies will translate into with major insights in epilepsy genetics.
Back from Israel. This week I will send you on holidays with a lot of interesting papers for the the time off. Hopefully you will find time to read some of these papers and won’t get overwhelmed by the amount of studies this week. Here we go:
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.
GEFS+, meet CNV. Microduplications at 17q12 have been identified in various neurodevelopmental disorders and in some unaffected individuals, a pattern familiar from other structural genomic variants such as microdeletions at 16p13.11 and 15q11.2. In contrast to the corresponding microdeletion, most 17q12 microduplications are inherited. This suggests that the microduplication is a risk factor, but does not fully explain the phenotype. In a recent paper in Neurology, Hardies and collaborators look at the families of 17q12 microduplication carriers with epilepsy. And this is when they noticed something strange. Continue reading
The enigmatic deletion. Amongst the various microdeletions implicated in human epilepsy, the 16q13.11 microdeletion is one of the structural variations that poses significant difficulties in understanding its associated risk and phenotypes. Now a recent paper in PLOS One investigates a large cohort of patients with various neurodevelopmental disorders for microdeletions in the 16p13.11 region. And particularly the finding regarding the sex distribution of symptomatic deletion carriers is remarkable. Continue reading
Why does this child with speech delay get an EEG? My first encounter with Landau-Kleffner-Syndrome and continuous spikes and waves during slow sleep (CSWS) was in medical school when my pediatric neurology attending faced me with this very question. I looked at him and basically had no idea. This is when I learned about the spectrum of rolandic epilepsies and how epilepsy interacts with speech. This concept is best explained by going back to the most common epilepsy in children, Benign Rolandic Epilepsy (BRE). And the genetics of BRE and the rolandic spectrum has been anything else but straightforward. Continue reading
Microdeletions in seizure disorders. In a recent paper in Nature, Golzio and colleagues identified KCTD13 as the main driver for the neurodevelopmental phenotype of the 16p11.2 microdeletion. Small losses of chromosomal material as found in microdeletions usually affect several neighbouring genes. Many deletions are due to the particular duplication architecture of the human genome and are canonical, i.e. they always have the same size and include the same genes. The same duplication architecture also makes these variants relatively common, and the full impact of microdeletion-associated genetic morbidity has startled the neurogenetics. The recent five years have led to the identification of several epilepsy-related microdeletions including variants at 15q13.3, 16p13.11 and 15q11.2. There are further microdeletions that are usually found in patients with autism or intellectual disability and to a lesser extent in patients with epilepsy. The 16p11.2 microdeletion, the first microdeletion to be identified through a large-scale association study, is one of these variants.
From deletion to causative genes. For many microdeletions, the statistical evidence for the association with a particular phenotype is often beyond reasonable doubt given that several thousands samples can be included nowadays. The identification of the underlying causative gene, however, is extremely difficult. It is technically challenging and time-consuming to investigate all included genes functionally through conventional model systems. The function of many genes included in microdeletions are not related to ion channels, the best known pathological substrate in epilepsies, and hampers testing effects through established electrophysiological techniques. Finally, microdeletions only lead to hemizygosity, i.e. the second copy of a gene should still be expressed at lower level, requiring model system looking for a quantitative rather than qualitative change. The bottom line is that epilepsy researchers are stuck without suitable model systems, which would allow for a medium-size throughput screening for genes in these deletions. This is where Danio rerio comes into play.
The zebrafish as a model for neurodevelopmental disorders. The zebrafish (Danio rerio) is a good model system for genetic and developmental research. The technologies for genetic manipulation are highly advanced. In addition, embryos are transparent and develop externally. Furthermore, a zebrafish develops quickly and produces a large number of offspring. For her studies on developmental genetics using the zebrafish as a model system, Christiane Nüsslein-Volhard received the Nobel Prize for Medicine in 1995.
Screening of the candidate genes of the 16p11.2 microdeletion. Golzio and coworkers focussed on a peculiar aspect of the 16p11.2 microdeletion as an outcome parameter for their genetic screening – macrocephaly, i.e. an enlarged head circumference. In contrast, patients with the corresponding 16p11.2 microduplication often show microcephaly, i.e. a reduced head circumference. Golzio and colleagues deviced a system to measure head circumference in zebrafish embryos and then overexpressed the 29 genes contained in the 16p11.2 microdeletion in the developing embryo. Strikingly, only KCTD13 resulted in microcephaly. Macrocephaly was seen when KCTD13 was knocked-out with a morpholino. This demonstrated that up- or downregulation of KCTD13 affects head size. The authors went on to show that these differences in head size are driven by differences in neuronal proliferation. KCTD13 is highly expressed in the human forebrain and recent studies have suggested a role for excessive neurons in the frontal lobe in autism.
Application to epilepsy research. The authors combine a clever screening strategy with a convincing follow-up study, highlighting the potential of zebrafish studies in neurogenetics. However, head circumference is not identical with autism and only represents a surrogate parameter. Therefore, even though the authors emphasize the role of head circumference as an essential part of the 16p11.2 phenotypes, it only represents a minor aspect of it. Nevertheless, the authors demonstrate that Danio rerio is a good model system for medium-throughput screening strategies, and epilepsy models in zebrafish do exist, suggesting that this study design might help decipher the plethora of candidate genes arising from the genetic studies in EuroEPINOMICS.