An inconvenient truth – segregation of monogenic variants in small families

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

Why I am still struggling with SCN9A in Dravet Syndrome

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

Validation of rare variants – the power of finding anything at all

How much? Last week, we discussed the probability of finding de novo variants in patients with epileptic encephalopathies, but our calculations were only half the story. Genes that are identified through genome-wide sequencing technologies are often validated in additional cohorts. In many cases, we will only be able to establish a given gene as causative if we find another patient with a mutation in this gene. I was therefore asked to write an additional post on power calculations for rare variants in validation cohorts. Let me tell you the story how I stumbled across a little bit of almost forgotten high school math. Continue reading

Exome study in IGE questions channelopathy concept

IGE and the hunt for rare variants. Idiopathic Generalized Epilepsy (IGE) or Genetic Generalized Epilepsy (GGE) is one of the most common epilepsy subtypes. Family studies and twin studies suggest that genetic factors play an important role. Some families with mutations in GABRG2, GABRA1 and EFHC1 are known, and recurrent microdeletions are found in 3% of sporadic patients. For the majority of patients, the genetic basis remains unknown, but a heterogeneous pattern of rare variants is expected. Much effort is currently spent on genetic studies in IGE including the EuroEPINOMICS CoGIE study. A recent paper now reports the first exome sequencing in IGE to identify rare variants…

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