G proteins. Intracellular signaling in neurons can occur through various mechanisms including so-called second messengers. G proteins constitute an important part of the signaling cascade that translates the signal from membrane-bound receptors. On neurons, GABA-B receptors or alpha-2 adrenergic receptors use signal transduction through the so-called G alpha-o proteins, which are particularly abundant in the CNS and encoded by the GNAO1 gene. Now a recent paper in the American Journal of Human Genetics describes de novo mutations in Ohtahara Syndrome and movement disorders.
Ohtahara Syndrome and genes. Ohtahara Syndrome is a severe early onset epilepsy that usually starts in the first few weeks of life and usually evolves into therapy-refractory epilepsy and severe cognitive impairment. Tonic spasms are often the predominant seizure type. The EEG usually shows a suppression-burst pattern, which is indicative of a severe impairment in cerebral function. Some genes for Ohtahara Syndrome have been identified in recent years including STXBP1 and CASK. Mutations in other genes including ARX and KCNQ2 may also result in Ohtahara-like phenotypes. Also, various neurometabolic disorders and brain malformations can results in Ohtahara Syndrome. However, in a large subset of cases, metabolic tests and neuroimaging is normal. Hence, a genetic cause is assumed.
The G protein. The human genome encodes more than 1000 G protein coupled receptors that are involved in virtually any organ system. G proteins are trimeric, i.e. they consist of three subunits. The “G” in G protein stands for guanosine nucleotide, which is guanosine triphosphate (GTP) and guanosine diphosphate (GDP). In the inactive state, G proteins are tightly attached to a membrane-bound receptor and bind GDP. Once the receptor is activated, the G protein exchanges GDP by GTP and breaks into two parts, the alpha subunit and the beta/gamma subunit. Both subunits are then capable of activating downstream signaling cascades. In the case of the G alpha-o proteins, the alpha subunit can more or less directly inhibit calcium channels and activate potassium channels. This links G alpha-o activation to neuronal exitability.
GNAO1 mutations. Nakamura and colleagues identified four patients with Ohtahara Syndrome and de novo mutations in GNAO1. Two of their patients had additional movement disorders including dystonia and chorea and athetosis. In contrast to epileptic seizures, which usually arise in the cortex, these movement disorders arise in the extrapyramidal system, which includes the basal ganglia and other nuclei. This observation indicates that G alpha-o is also involved in other systems and that the phenotype may well extend beyond epileptic encephalopathy. Mice lacking G alpha-o also show a broad range of neurological findings including tremor, motor control impairment and seizures. A common feature of the four patients described by Nakamura and colleagues is the early onset in the first weeks of life and the severe EEG finding of suppression-burst activity, the electrographic hallmark of Ohtahara syndrome. All four patients developed tonic seizures during the course of the epilepsy that were refractory to treatment.
Discovered as we speak. When I gave a presentation in Berlin last week, I pointed out that we are currently living in the most dynamic phase of gene discovery in epileptic encephalopathies that we have ever witnessed. High-throughput sequencing in large patient cohorts generates hundreds of candidate genes. Only a year ago, there was widespread skepticism that these gene findings were little more than random events, as there is also a similar mutation rate in control individuals. At times, I felt that it would be an insurmountable task to tell signal from noise. However, new genes for epilepsies are materializing out of this noise pretty much every week: they are confirmed virtually “as we speak”. GNAO1 is such a gene. A GNAO1 de novo mutation was first observed in a patient with schizophrenia and then in a patient with epileptic encephalopathy in the Epi4K study (trio cj). Neither finding really proves that this gene was causative given the broad phenotype. However, with the study by Nakamura and colleagues, the pathogenic role of this gene is confirmed. There is some remaining question, as deletions of the genes are also seen in controls, but the findings in the Toronto Database Genomic Variants refer to a 2007 study that might have generated false positives. The DECIPHER database includes two patients with one partial and one complete deletion of GNAO1 and adjacent genes. However, the phenotypes are not available. The Exome Variant Server reveals some missense mutations found in controls, but no frameshift or splice site mutations.
Summary for EuroEPINOMICS. GNAO1 is a novel gene for early-onset epileptic encephalopathy that may be associated with an extrapyramidal movement disorder. As with other genes for the epileptic encephalopathies, GNAO1 mutations seem to be rare. Nakamura and colleagues identified two of their mutations only by screening a cohort of 450 individuals, suggesting that the frequency of GNAO1 mutations within the epileptic encephalopathies is not much higher than 1%. GNAO1 encephalopathy might therefore be another “1% disease” in the genetic architecture of the epileptic encephalopathies as already demonstrated for other causative genes. It will be helpful to see whether a particular phenotype can be delineated, given that this gene has also been found in a patient with schizophrenia. This will finally help us understand how GNAO1 results in epilepsy and might lead to novel strategies for intervention. G proteins in the Central Nervous System are highly redundant, which might be a strategy to compensate for the loss of G alpha-o.