Mutations in V-ATPase proton pump implicated in epilepsy syndrome

Proton pumps are important enzymes, not only for the stomach, where they maintain the acidity needed to digest food, but elsewhere in the body. Genetic mutations perturbing one type of proton pump have been implicated in several diseases, including myopathy, osteopetrosis and hearing loss.

Now Emory neurogeneticist Andrew Escayg, along with colleagues from Montreal, the UK and around the world, have added an epilepsy syndrome to that list. It doesn’t really have a name yet, besides the gene involved: ATP6V0C. Their findings were recently published in Brain.

Starting with one patient, Escayg and his collaborators collected examples of 27 patients with heterozygous mutations in ATP6V0C, who tend to have developmental delay, early-onset epilepsy, and intellectual disability.

V-ATPase structure. *ATP6V0C* encodes a protein forming the c ring (red)

“What’s distinctive about this group of patients is that they often have cardiac abnormalities or structural alterations in the brain visible on MRI,” Escayg says. “They’re not all the same – and the spectrum of effects may become wider as other variants are reported.”

ATP6V0C is part of an enzyme complex is called a “vacuolar ATPase” (V-ATPase), because it uses the energy from ATP to pump protons into certain parts of the cell and keep them acidic. Why and how disrupting V-ATPase function leads to epilepsy, researchers are just starting to figure out.

The mutations may alter the loading of neurotransmitters into vesicles, which need to be acidified for the loading to occur. Or they may affect other aspects of brain development. Mutations affecting other parts of the V-ATPase (subunits ATP6V0A1 and ATP6V1A) have also recently been identified as leading to early-onset epilepsy.

Posted on October 7, 2022 by Quinn Eastman in Neuro Leave a comment

Fragile X files — expanded

A genetic disorder caused by silencing of a gene on the X chromosome, fragile X syndrome affects about one child in 5,000, and is more common and more severe in boys. It often causes mild to moderate intellectual disabilities as well as behavioral and learning challenges.

Amy Talboy, MD

The gene responsible for fragile X syndrome, the most common inherited form of intellectual disability, was identified more than 25 years ago. Emory genetics chair Stephen Warren played a major role in achieving that milestone. His work led to insights into the molecular details of learning and memory, and nationwide clinical trials — which have a more complicated story.

Treating the molecular basis of a neurodevelopmental disorder, instead of simply addressing symptoms, is a lofty goal – one that remains unfulfilled. Now a new study, supported by the National Institute of Neurological Disorders and Stroke, is reviving a pharmacological strategy that Warren had a hand in developing.

“This is a very well thought out approach to studying changes in language and learning in children who are difficult to test,” says Amy Talboy, medical director of Emory’s Down Syndrome and Fragile X clinics, who is an investigator in the NINDS study. “It could change how we conduct these types of studies in the future.” Read more

Posted on October 23, 2018 by Quinn Eastman in Neuro Leave a comment

Insight into brain + learning via ‘friend of fragile X’ gene

We can learn a lot about somebody from the friends they hang out with. This applies to people and also to genes and proteins. Emory scientists have been investigating a gene that we will call — spoiler alert — “Friend of fragile X.”

Fragile X syndrome is the most common inherited form of intellectual disability, studied by research teams around the world with drug discovery and clinical trials in mind. It is caused by a disruption of the gene FMR1.

In an independent form of inherited intellectual disability found in a small number of Iranian families, a gene called ZC3H14 is mutated. Two papers from Ken Moberg, PhD, associate professor of cell biology, Anita Corbett, PhD, professor of biology and colleagues show that FMR1 and ZC3H14 are, in effect, friends.

The findings provide new insight into the function of FMR1 as well as ZC3H14; the evidence comes from experiments performed in fruit flies and mice. The most recent paper is in the journal Cell Reports (open access), published this week.

The scientists found that the proteins encoded by FMR1 and ZC3H14 stick together in cells and they hang out in the same places. The two proteins have related functions: they both regulate messenger RNA in neurons, which explains their importance for learning and memory.

The fragile X protein (FMRP) was known to control protein production in response to signals arriving in neurons, but the Cell Reports paper shows that FMRP is also regulating the length of “tails” attached to messenger RNAs – something scientists did not realize, even after years of studying FMRP and fragile X, Moberg says.

To be sure, FMRP interacts with many proteins and appears to be a critical gatekeeper. Emory geneticist Peng Jin, who has conducted his share of research on this topic, says that “FMRP must be very social and has a lot of friends.” More here.

Posted on August 11, 2017 by Quinn Eastman in Neuro Leave a comment

More on NMDA receptor variants + epilepsy/ID

NMDA receptors are complex electrochemical machines, important for signaling between brain cells. Rare mutations in the corresponding genes cause epilepsy and intellectual disability.

Pre-M1 helices in multi-subunit NMDA receptor. Adapted from Ogden et al PLOS Genetics (2017).

In Emory’s Department of Pharmacology, the Traynelis and Yuan labs have been harvesting the vast amounts of information now available from public genome databases, to better understand how changes in the NMDA receptor genes relate to function. (Take a “deeper dive” into their November 2016 publication on this topic here.)

Their recent paper in PLOS Genetics focuses on a particular region in the NMDA receptor, called the pre-M1 helix (see figure). It also includes experiments on whether drugs now used for Alzheimer’s disease, such as memantine, could be repurposed to have beneficial effects for patients with certain mutations. The in vitro data reported here could inform clinical use. Read more

Posted on January 31, 2017 by Quinn Eastman in Neuro Leave a comment

Deep dive into NMDA receptor variation

The study of human genetics has often focused on mutations that cause disease. When it comes to genetic variations in healthy people, scientists knew they were out there, but didn’t have a full picture of their extent. That is changing with the emergence of resources such as the Exome Aggregation Consortium or ExAC, which combines sequences for the protein-coding parts of the genome from more than 60,000 people into a database that continues to expand.

Rare mutations in the NMDA receptor genes cause epilepsy (GRIN2A) or intellectual disability (GRIN2B). Shown in blue are agonist binding domains of the receptors, where several disease-causing mutations can be found.

At Emory, the labs of Stephen Traynelis and Hongjie Yuan have published an analysis of ExAC data, focusing on the genes encoding two NMDA receptor subunits, GRIN2A and GRIN2B. These receptors are central to signaling between brain cells, and rare mutations in the corresponding genes cause epilepsy (GRIN2A) or intellectual disability (GRIN2B). GRIN2B mutations have also been linked with autism spectrum disorder.

Steve Traynelis and Hongjie Yuan

The new paper in the American Journal of Human Genetics makes a deep dive into ExAC data to explore the link between normal variation in the healthy population and regions of the proteins that harbor disease-causing mutations.

In addition, the paper provides a detailed look at how 25 mutations that were identified in individuals with neurologic disease actually affect the receptors. For some patients, this insight could potentially guide anticonvulsant treatment with a repurposed Alzheimer’s medication. Also included are three new mutations from patients identified by whole exome sequencing, one in GRIN2A and two in GRIN2B.

“This is one of the first analyses like this, where we’re mapping the spectrum of variation in a gene onto the structure of the corresponding protein,” says Traynelis, PhD, professor of pharmacology at Emory University School of Medicine. “We’re able to see that the disease mutations cluster where variation among the healthy population disappears.”

Heat map of agonist binding domain for GRIN2A. From Swanger et al AJHG (2016).

Postdoctoral fellow Sharon Swanger, PhD is first author of the paper, and Yuan, MD, PhD, assistant professor of pharmacology, is co-senior author.

It’s not always obvious, looking at the sequence of a given mutation, how it’s going to affect NMDA receptor function. Only introducing the altered gene into cells and studying protein function in the lab provides that information, Traynelis says.

NMDA receptors are complicated machines: mutations can affect how well they bind their ligands (glutamate and glycine), how they open and shut, or how they are processed onto the cell surface. On top of that complexity, mutations that make the receptors either stronger or weaker can both lead the brain into difficulty; within each gene, both types of mutation are associated with similar disorders. With some GRIN2A mutations, the functional changes identified in the lab were quite strong, but the effect on the brain was less dramatic (mild intellectual disability or speech disorder), suggesting that other genetic factors contribute to outcomes.

Clinical relevance

Traynelis and Yuan previously collaborated with the NIH’s Undiagnosed Disease Program to show that the Alzheimer’s medication memantine can be repurposed as an anticonvulsant, for a child with intractable epilepsy coming from a mutation in the GRIN2A gene. (Nature Communications, Annals of Clinical and Translational Neurology)

Memantine is an NMDA receptor antagonist, aimed at counteracting the overactivation of the receptor caused by the mutation. Memantine has also been used to treat children with epilepsy associated with mutations in the related GRIN2D gene. However, memantine doesn’t work on all activating mutations, and could have effects on the unmutated NMDA receptors in the brain as well. Traynelis reports that his clinical colleagues are developing guidelines for physicians on the use of memantine for children with GRIN gene mutations.

This study and related investigations were supported by funding from the Eunice Kennedy Shriver National Institute of Child Health and Human Development (R01HD082373), the National Institute of Neurological Disorders and Stroke (R24NS092989), the Atlanta Clinical & Translational Science Institute (UL1TR000454), and CURE Epilepsy: Citizens United for Research in Epilepsy.

Posted on November 11, 2016 by Quinn Eastman in Neuro 2 Comments

Let’s not elope

Elopement may sound cute, because the word evokes a starry-eyed couple running away to get married. Elopement also refers to when a child runs or wanders from a safe, supervised environment. It can be a worrisome concern among the parents and caregivers of children with autism spectrum disorder and/or intellectual disability.

Here is a straightforward post from Seattle Children’s on elopement. Cathy Rice, now director of Emory Autism Center and previously at the CDC, has published two papers on elopement.

This May, Nathan Call, director of Severe Behavior Programs at Marcus Autism Center, and colleagues published a retrospective review of their behavioral treatments for elopement, extending back to 2003. This is a companion to their 2015 analysis of treatment for pica, the ingestion of inedible substances. Call is also assistant professor of pediatrics at Emory University School of Medicine.

He summarized their approach by saying: “Individualizing treatment based upon the reason each child elopes seems to work very well.” The paper makes it clear that the reasons for a child eloping were a mixed bag: for some it was “access to preferred tangible items,” for others it was access to attention or other reasons.

Elopement in children with certain conditions can be challenging to research in a real-world context due to the high stakes involved; the consequences of not intervening can be catastrophic. For instance, Call recounted an incident involving a child whose fascination with balloons was so intense it affected his awareness of property values and safety. The child’s impulse to elope was triggered twice, each time leading to near-tragic accidents with vehicles, once because a colorful balloon display at a local real estate had irresistibly caught his attention. This highlights the critical need for vigilant supervision and safety strategies in such cases.

The 11 children in the review were ages 5 to 12, and 7 had a diagnosis of autism spectrum disorder – others had Down syndrome or intellectual disability. Read more

Posted on October 14, 2016 by Quinn Eastman in Neuro Leave a comment

Fragile X regulation is a finely tuned machine

A PNAS paper published MondayÂ demonstrates the kinds of insights that can be gleaned from a large scale sequencing project examining the fragile X gene.

Most children (boys, usually) who have fragile X syndrome have a particular mutation. An expanded â€œtriplet repeatâ€ stretch of DNA, which is outside the protein-coding region of the gene, puts the entire gene to sleep.

At Emory,Â geneticist Steve Warren, cell biologist Gary Bassell and colleagues have been identifying less common changesÂ in the fragile X gene by looking in boys who are developmentally delayed, but donâ€™t have the triplet repeat expansion. The first author of the paper is former postdoc Joshua Suhl, now at Booz Allen Hamilton in Massachusetts.

The authors describe two half-brothers who have the same genetic variant, which changes how production of the FMRP protein is regulated.Â These examples show that the fragile X gene is so central to how neurons function that several kinds of geneticÂ glitches in it can make this finely tuned machine break down.

â€œThis is a hot area and not much is known about it,â€ Warren says. Read more

Posted on November 10, 2015 by Quinn Eastman in Neuro Leave a comment

Fragile X syndrome: building a case for a treatment strategy

New research in mice strengthens a potential strategy for treating fragile X syndrome, the most common inherited form of intellectual disability and a major single-gene cause of autism spectrum disorder.

The results, published April 23 inÂ Cell Reports, suggest that a drug strategy targeting a form of the enzyme PI3 (phosphoinositide-3) kinase could improve learning and behavioral flexibility in people with fragile X syndrome. The PI3 kinase strategy represents an alternative to one based on drugs targeting mGluR5 glutamate receptors, which have had difficulty showing benefits in clinical trials.

Research led by Emory scientistsÂ Gary Bassell, PhD and Christina Gross, PhD had previously found that the p110Î² form of PI3 kinase is overactivated in the brain in a mouse fragile X model, and in blood cells from human patients with fragile X syndrome.

Now they have shown that dialing back PI3 kinase overactivation by using genetic tools can alleviate some of the cognitive deficits and behavioral alterations observed in the mouse model. Drugs that target the p110Î² form of PI3 kinase are already in clinical trials for cancer.

“Further progress in this direction could lead to a clinical trial in fragile X,” says Bassell, who is chair of Cell Biology at Emory University School of Medicine. “The next step is to test whether this type of drug can be effective in the mouse model and in human patient cells.” Read more

Posted on April 24, 2015 by Quinn Eastman in Neuro Leave a comment

Point mutation in fragile X gene reveals separable functions in brain

A new paper in PNAS from geneticist Steve Warren and colleagues illustratesÂ the complexity of the protein disrupted in fragile X syndrome. It touches on how proposed drug therapies that address one aspect of fragile X syndrome may not be able to compensate for all of them. [For a human side of this story, read/listen to this recent NPR piece from Jon Hamilton.]

Fragile X syndrome is the most common single-gene disorder responsible for intellectual disability. Most patients with fragile X syndrome inherit it because a repetitive stretch of DNA, which is outside the protein-coding portion of the fragile X gene, is larger than usual. The expanded number of CGG repeats silences the entire gene.

However, simple point mutations affecting the fragile X protein are possible in humans as well. In the PNAS paper, Warren’s team describes what happens with a particularly revealing mutation, which allowed researchers to dissect fragile X protein’s multifaceted functions. Read more

Posted on January 5, 2015 by Quinn Eastman in Neuro Leave a comment

Socialization relative strength in fragile X longitudinal study

A studyÂ published in Pediatrics this week tracks “adaptive behavior” asÂ children and adolescents with fragile X syndrome are growing up. This isÂ the largest longitudinal study to date in fragile X, which isÂ the leading inherited cause of intellectual disability and the leading single-gene risk factor for autism spectrum disorder.

Adaptive behavior covers a range of everyday social and practical skills, including communication, socialization, and completing tasks of daily living such as getting dressed. In this study, socialization emerged as a relative strength in boys with fragile X, in that it did not decline as much as the other two domains of adaptive behavior measured: communication and daily living skills.

The lead author of the paper is Cheryl Klaiman, formerly of the Stanford University Center for Interdisciplinary Brain Sciences, now senior psychologist at Marcus Autism Center.

The “socialization as relative strength in fragile X” findings meshes with a growing awareness in the autism field, summarized nicely here by Jessica Wright at the Simons Foundation Autism Research Initiative, thatÂ fragile X syndrome symptoms are often distinct from those in autism spectrum disorder.

One key distinction between the disorders, for example, is in social interactions. Children with autism and those with fragile X syndrome both shy away from social contact, have trouble making friends and avert their gaze when people look at them.

But children with fragile X syndrome often sneak a peek when the other person turns his back, researchers say. Children with autism, in contrast, seem mostly uninterested in social interactions.

â€œChildren with fragile X syndrome all have very severe social anxiety that plays a big role in the perception that they have autism,â€ saysÂ Stephen Warren, professor of human genetics at Emory University School of Medicine in Atlanta. â€œThey are actually interested in their environment; they are just very shy and anxious about it.â€

Posted on July 29, 2014 by Quinn Eastman in Neuro Leave a comment