Converging thoughts. During late 2013, I had several unrelated discussions about the possible role of genetic modifiers of SCN1A in Dravet Syndrome. To some extent, SCN1A is a paradox. One the one hand, the connection between Dravet Syndrome and SCN1A is one of the clearest connections between gene and disease that we see in genetic epilepsies. On the other hand, we see a remarkable phenotypic heterogeneity in families, and some presumably pathogenic SCN1A variants can also be identified in unaffected control individuals. This leaves us with the question whether there are genetic modifiers in Dravet Syndrome that might help provide some insight into additional mechanisms of disease. This post is a collection of 10 individual thoughts that emerged during the discussions last year. 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.
The treatment options for epilepsy must undoubtedly be improved. More than 20 antiepileptic drugs are licensed but in 30% of patients seizures are not controlled, despite treatment with a number of anti epileptic drugs and the response to medication is difficult to predict. Antiepileptic medications can cause severe adverse reactions and increase the risk of fetal malformations in women taking them during pregnancy. The differences in drug response and the occurrence of rare adverse reactions are believed to be caused by variants in the genetic makeup of individuals. Knowledge of these variants would help us to predict drug response and adverse drug reactions. This personalized treatment would help us to select medications for each individual.
Climate change. In the era of exome and genome sequencing, it might be worthwhile revisiting the merit of family studies in epilepsy research. Seizure disorders are known to have a highly diverse genetic architecture. When singleton studies identify a single, unique gene finding, this discovery usually does not provide much information about the potential causal role of the variant given the high degree of genomic noise. In contrast, family studies are usually considered more robust, as segregation of variants can be traced. Here is the inconvenient truth: unless the family is very large, segregation of possibly monogenic variants adds little information given the vast amount of variants present in our genomes. Continue reading
The river of genetic variants. The era of high-throughput sequencing has given us several unexpected insights into the human genome. One of these insights is the observation that mutations or variations can occur in parts of our genome without any major consequences. Every individual is a “knockout” for at least two genes in the human genome. This means that in every individual, both copies of a single gene are disrupted through mutations or small deletions or duplications. In addition, there are dozens, if not hundreds, of genes with disruptive mutations that affect only a single copy of the gene. Similar mutations in specific disease-associated genes, however, will invariably result in an early onset genetic disorder. This comparison already shows that the genes in the human genome differ with respect to the amount of disruptive genetic variation they can tolerate. A recent study in PLOS Genetics now tries to catalogue the genes in the human genome by assessing their mutation intolerance based on the genetic variation seen in large-scale exome datasets. Many genes for neurodevelopmental disorders are highly intolerant to mutations. Furthermore, some genes for monogenic epilepsies show surprising results in this assessment. Continue reading
Susceptibility. Two weeks ago, we published a post on rare variants in SCN9A as potential susceptibility genes for Dravet Syndrome with mutations in SCN1A. Ever since reading the article by Mulley and collaborators, I had tried to come up with an idea of what the genetic architecture might look like if both de novo variants and inherited variants contribute. I wanted to follow up on my earlier post with this brief back-of-the-envelope calculation. Continue reading
Exome no more. Over the last 15 months, we have repeatedly discussed how exome sequencing or genome sequencing is applied to neurodevelopmental disorders in order to discover new candidate genes and to assess the role of known candidate genes. We have also wondered sometimes whether exome sequencing is the most straightforward approach. Now – outpacing the two large international consortia using exome sequencing in epileptic encephalopathies – a recent study in Nature Genetics uses a different approach to uncover the genetic basis in 10% of patients with epileptic encephalopathies. Targeted resequencing or gene panel analysis is a hybrid technology between candidate gene sequencing and next generation sequencing and focuses only on a subset of candidate genes. While their study provides a comprehensive overview over the genetics of rare epilepsy syndromes, it raises the question whether the era of large-scale exome sequencing is coming to a natural end. Continue reading
Trio-sequencing your clinic. From the perspective of a child neurology clinic, I often wonder how much information we would gain if we performed trio exome sequencing for de novo mutations systematically in all our patients with difficult-to-treat epilepsies. Many of these patients have epilepsies that are difficult to classify and they have not been included in our existing EuroEPINOMICS working groups on defined syndromes. Now, a recent publication in Epilepsia gives us an idea what we will find if we perform family-based exome sequencing in patients with unclassified epileptic encephalopathies. Basically, you will find SCN1A and CDKL5 plus mutations in several genes that are likely pathogenic. But there is much more to this issue, which motivated me to come up with a classification scheme for epilepsy-related de novo events. Continue reading
Clinical genome sequencing. While exome and genome sequencing is widely used as a research tool, these technologies are also routinely applied in a clinical setting. As with many other data-rich diagnostic tests in medicine, there is an ongoing question on how to deal with potentially relevant findings that turn up indicentally. Now the American College of Medical Genetics and Genomics (ACMG) has released their long-expected recommendations on data return of incidental findings in clinical exome and genome sequencing. Their recommendations provide an interesting basis for discussion on what to do with genetic findings that are found by chance. Continue reading
Red flags. Despite the availability of a large panel of metabolic and genetic tests as well as high-resolution neuroimaging, the cause of disease in the vast majority of patients remains unknown. This situation also applies for intellectual disability, where there is little to offer in terms of diagnostic procedures once patients are negative for array comparative genomic hybridization (array CGH). In clinical practice, we often hope that some minor clinical or biochemical features may lead us to the correct diagnosis, but in the majority of cases, these investigations lead nowhere. Now, in two back-to-back publications in the American Journal of Human Genetics, two papers describe PGAP2 mutations in patients with non-syndromal intellectual disability with elevated alkaline phosphatase. Continue reading