Rett. We have written very little about MECP2 on Beyond the Ion Channel. MECP2 is the gene for Rett Syndrome, a neurodegenerative disorder almost exclusively affecting females. Classical Rett Syndrome is characterized by developmental regression in the first two years of life and the development of distinctive hand movements, which historically led to Rett Syndrome being considered a recognizable entity. This blog post is the introduction to our MECP2 Epilepsiome page. However, in 2016, a time when many genes are re-defined by exome studies, I was wondering whether Rett Syndrome is still the classical syndrome that I initially learned about.
The Wild West. The diagnostic genetic testing landscape in 2016 is a paradox. In theory genetic testing has never been more widely available clinically, with over 20 diagnostic laboratories in the US alone offering a variety of genetic testing options for patients with epilepsy, ranging from single gene testing to NGS panels to whole exome sequencing. However, access to and reimbursement of genetic services varies widely, with no consensus on an approach to testing or professional guidelines to aide clinicians. Here is our brief guide to epilepsy genetic test selection for busy clinicians. Continue reading
Inhibition. GABA is the main inhibitory neurotransmitter in the the Central Nervous System. Given that epilepsy is typically associated with increased excitability, all mechanisms related to GABA signaling are of natural interest to the epilepsy community. Almost 15 years ago, mutations in GABRA1, coding for alpha-1 subunit of the GABA-A receptor, have been identified in familial Juvenile Myoclonic Epilepsy, but there has been relative silence around this gene since. Now, two publications highlight the other side of GABRA1 as a gene for epileptic encephalopathies, putting the GABA receptor into the spotlight again.
The story of SCN1A. Variants in SCN1A were first reported in association with epilepsy in 2000, when familial heterozygous SCN1A missense variants were identified in two large families with GEFS+. The phenotype was characterized by incomplete penetrance and significant variable expressivity between family members, making it clear from the beginning that the SCN1A story would not be simple. Within the next few years, we learned that SCN1A variants could cause a wide spectrum of epilepsy phenotypes, including GEFS+, Dravet syndrome, intractable childhood epilepsy with generalized tonic-clonic seizures, and, less frequently, infantile spasms and simple febrile seizures. As it became clear that SCN1A variants played an important role in genetic epilepsies, focus turned towards understanding the mechanism underlying seizure genesis, as well as identifying management and therapy options. Even after 15 years of study, our understanding of SCN1A-related epilepsy is still evolving. Keep reading to learn more about the most recent discoveries related to SCN1A. Continue reading
VUS – The dreaded variant of uncertain significance. With the advent of next generation sequencing panels and exome sequencing, what used to be an occasional laboratory finding in epilepsy has now become a daily occurrence. Lab reports detailing multiple VUS findings for an individual patient have become a routine part of clinical practice. How do you, as a healthcare provider, explain the meaning and implications of VUS findings to patients and families in a way that is understandable to them? Continue reading
The story of TBC1D24. As with many epilepsy genes, the TBC1D24 story increases in complexity over time. Initially described to be associated with autosomal recessive familial infantile myoclonic epilepsy by Falace and colleagues and with autosomal recessive focal epilepsy by Corbett and colleagues in 2010, pathogenic variants in TBC1D24 have since been identified as a major cause of DOORS syndrome and have also been identified in individuals with familial malignant migrating partial seizures of infancy, progressive myoclonus epilepsy, early-onset epileptic encephalopathy, and autosomal dominant and autosomal recessive non-syndromic hearing loss. However, little is known about a potential genotype-phenotype correlation of TBC1D24-related disorders, as well as the underlying mechanism. Keep reading to learn more about recent discoveries related to TBC1D24. Continue reading
The story of PCDH19. The clinical features and unique inheritance pattern of PCDH19-related epilepsy were first described in 1971, and the clinical entity was coined Epilepsy in Females with Mental Retardation (EFMR), due to the presence of epilepsy and cognitive disability that seemed to be limited to females. Pathogenic PCDH19 variants were identified in females in 2008, and it soon became clear that PCDH19 is a major player in the genetic basis of epilepsy, with more than 100 patients with PCDH19 variants described to date. The inheritance pattern is one of the most striking features of this condition. Heterozygous females are affected, while hemizygous transmitting males are spared. At the cellular level, the disease mechanism seems to be loss of function. However, at the tissue level, the current hypothesis for the underlying mechanism is gain of function, resulting from the co-existence of two different PCDH19-expressing neuronal populations in females and mosaic males. Keep reading to learn more about recent discoveries related to PCDH19. Continue reading
Unknown significance. Quite possibly the two most dreaded words in clinical genetics. To some these two words should seldom be used let alone act as qualifiers for testing results. What are the rules of assessment? How do laboratories determine what constitutes enough evidence to say that a variant, previously known as mutation, is of known significance? Continue reading
The story of SCN2A. Pathogenic SCN2A variants were first described in patients with autosomal dominant benign familial neonatal/infantile seizures (BFNIS) in the early 2000s. More recently, it has became clear that variants in SCN2A can cause a wider spectrum of epilepsy phenotypes, ranging from milder phenotypes, such as BFNIS, to severe phenotypes, such as infantile spasms and severe early onset epileptic encephalopathies, including Ohtahara syndrome and West syndrome. In addition, SCN2A variants have been identified in a small number of patients with autism, schizophrenia, and intellectual disability without seizures. Despite now having earned its title as a well-established epilepsy gene, we still have a lot to learn about SCN2A. Keep reading to learn more about the expansion of the SCN2A-associated epilepsy phenotype and other recent discoveries about SCN2A. Continue reading
Continued expansion of the KCNT1 phenotype. In 2012, de novo heterozygous KCNT1 variants were first described in six individuals with migrating partial seizures of infancy (MPSI) (Barcia et al, 2012). In the same edition of Nature Genetics, Heron and colleagues (2012) described 3 families with frontal lobe epilepsy with prominent psychiatric features were also identified to have heterozygous disease-causing variants in KCNT1. Within the last 5 years, de novo and inherited heterozygous KCNT1 variants have been found in a number of patients with MPSI and ADNFLE. Yet, there have been no clear genotype-phenotype correlations established. Recently, several studies have identified KCNT1 variants in patients with other types of epilepsy. Keep reading to learn more about the expansion of the KCNT1-associated epilepsy phenotype. Continue reading