Common Genetic Variation Contributes to Migraines in Families

What's the science?

Migraine is an extremely common disorder affecting up to 20% of adults in developed countries. Migraines tend to run in families, however, the genetic risk underlying this is not well understood. A typical way to study genetic risk is to look at rare genetic mutations, which often put a person at high risk for a disease disease. However, ‘common’ genetic variations (i.e. single nucleotide polymorphisms) in the population could have a significant contribution to risk of migraine that runs in families. This week in Neuron, Gormley and colleagues test whether a genetic risk score (of common genetic variants) is associated with migraines that aggregate in families.

How did they do it?

They used a dataset including 1589 families (a total of 8319 family members) with genotype information. They generated a polygenic risk score for each individual using a set of SNPs (single nucleotide polymorphisms; commonly occurring changes in the genetic code), that were previously shown to be associated with migraine risk. They tested for the association between this polygenic risk score and risk of different migraine subtypes including migraine with aura, migraine without aura and a rare form of migraine (hemiplegic migraine) in families (using ‘logistic mixed modelling’ which controlled for sex, age and genetic relatedness) compared to a large group of controls. They also compared the risk of migraine explained by the genetic risk score in families to a large sample population of unrelated individuals with migraine.  

What did they find?

They found that the genotypes associated with risk for migraine (from the polygenic risk score) were enriched (i.e. more common) in families with all forms of migraine compared to controls. The strongest association was for the migraine with aura, followed by hemiplegic migraine and migraine without aura. In particular, a subtype of migraine hemiplegic migraine had the strongest enrichment of polygenic risk. Polygenic risk score explained 3.5% of the variance in families (for presence of migraine) compared to 1.6% variance explained (for presence of migraine) in the general population. When compared to people with migraine in the general population, family members with migraine showed a greater burden of polygenic risk (more risk variants in their genome) and this was strongest for hemiplegic migraine followed by migraine with typical aura and migraine without aura. Furthermore, in a genetic transmission test they found that in families, offspring with migraine received a higher percentage of common migraine risk variants from their parents then would be expected by chance, suggesting that these common genetic risk variants are over-transmitted to family members with migraine.

What's the impact?

This is the first study to show that common genetic variation known to associate with migraine risk, is enriched (more common) in families compared to those with migraines in the general population. Before this study, the contributions of common genetic variants to migraine risk in families was unclear. We now know that common genetic risk variants that contribute to all forms of migraine risk are transmitted through families.

Gormley et al., Common Variant Burden Contributes to the Familial Aggregation of Migraine in 1589 Families. Neuron (2018). Access the original scientific publication here.

A New Technique Using Holography and Optics to Precisely Control Neuronal Activity

What's the science?

Different information in the brain is encoded by the firing of specific neurons, but also by the timing, rate of that firing, and synchrony of firing with other neurons. Therefore, in order to understand and control neural activity, we must be able to control each of these parameters. The ideal experimental technique would have both high spatial and high temporal resolution over a large volume of brain tissue, and be able to create as well as remove existing activity. Two-photon optogenetics is a technique used to stimulate specific neurons: genes are inserted into neurons, which allows these neurons to produce light-sensitive proteins (opsins), and then light is shone on these neurons, generating action potentials. However, there are limitations to how precise the timing of stimulation, number of neurons stimulated and location of stimulation can be with optogenetics. This week in Nature Neuroscience, Mardinly and colleagues engineered a new technique that allows for precise temporal and spatial control of neural activity using special opsins and holography (a technique that can illuminate the entire cell body/soma of a neuron) to ‘write’ neural activity, along with simultaneous volumetric calcium imaging to ‘read’ neural activity.

How did they do it?

They used an experimental setup that incorporated holography, which involves lasers and bending light to illuminate multiple specific neurons in 3D space, allowing for fast excitation of neurons at precise spatial locations.

Experiment Set 1 → First, they studied the characteristics of several opsins with the goal of developing two opsins: one that could generate large excitatory currents quickly to activate cells and one that generated large inhibitory currents to suppress cells. Using patch clamping (in anesthetized mice) and brain slice recording techniques in mice, they tested several opsins for the amplitude and kinetics of the photocurrent elicited. They mutated the chosen opsins to optimally generate or suppress activity. They then tested the precision and speed with which neurons with this opsin elicited action potentials.

Experiment Set 2 → Next, they used holography (to stimulate or suppress neurons) and calcium imaging (to ‘read’ (i.e. record) the resulting neural activity) at the same time. Mice who expressed neurons with both chosen opsins ran on a treadmill with their heads fixed in place, and neurons in the primary somatosensory cortex were activated or suppressed (depending on the opsin) using a holographic technique. Neurons were activated or suppressed one at a time, and later in random groupings, to test whether the activity of multiple different types of neurons could successfully be altered at the same time.

What did they find?

Experiment Set 1 → After testing several excitatory opsins, they mutated the fastest opsin tested to create a highly potent but still equally fast version, that they call ST-ChroME. After genetic mutation of this opsin they observed that it required low laser power (i.e. light shone) to produce action potentials with short latency and low temporal variability (which is desirable) during brain slice recordings. Using patch-clamp recordings, they found that neurons with ST-ChroME exhibited action potentials reliably (89% of the time). ST-eGtACR1 was the opsin found to be the most effective at suppressing neural activity. Using patch-clamp recording, the firing rate of neurons with ST-eGtACR1 was reduced to 8% of normal firing rate during holographic suppression.  

Experiment Set 2 → Next, mice ran on a treadmill and neuronal activity was simultaneously stimulated and recorded. Neurons (with ST-ChroME) activated one at a time produced reliable calcium signals, indicating they could be reliably activated in vivo. In vivo optical suppression resulted in a reliably reduced calcium response. Finally, when entire groups of neurons were stimulated, calcium signalling was as expected, indicating neurons of different sizes or functions can be activated simultaneously. Up to 50 neurons could be activated at the same time, allowing for thousands of light-evoked action potentials per second.

What's the impact?

In this study, optogenetic techniques (newly engineered opsins—proteins activated by light) were combined with holography (bending of light to be distributed over a 3D volume) to stimulate neurons precisely. By combining holography with calcium imaging, the authors facilitated spatially and temporally precise simultaneous stimulation and recording (‘write’ and ‘read’) of neuronal activity respectively. Holography allowed for specific neurons at various different locations in a 3D volume of the brain to be activated at the same time. This study expands on previous optogenetic techniques which lacked a high level of spatial and temporal precision and will help us to perform a wide variety of experiments where the activity of multiple neurons can be altered with high spatial and temporal precision.

A. R. Mardinly et al., Precise multimodal optical control of neural ensemble activity. Nature Neuroscience (2018). Access the original scientific publication here.

A Polymorphism in GAD65 Modulates Anxiety Vulnerability in Women

What's the science?

Anxiety disorders are common and debilitating and occur more frequently in women. Understanding the brain chemistry and genetic predisposition to anxiety will be critical for developing treatments. A brain region called the pregenual anterior cingulate cortex is known to be involved in anxiety and to regulate amygdala activity (another brain region involved in anxiety and emotion). The balance of neurotransmitters, GABA (inhibitory) and glutamate (excitatory), in the pregenual anterior cingulate is thought to be important for modulating brain activity and anxiety. Different genetics affecting the synthesis of GABA in the brain could affect anxiety, however this is not yet clear. This week in The Journal of Neuroscience, Colic and colleagues test whether a genetic polymorphism (change in the code of a gene) can alter GABA levels in the brain and also influence brain activity and anxiety.

How did they do it?

They sorted 105 healthy participants (mean age = 27) into two groups based on their genotype for the GAD65 gene (carriers vs. non-carriers of a G allele) which is responsible for GABA synthesis. Carriers of the G allele of this genotype are associated with drastic increases in GABA transcription (production) in peripheral cells. They scanned the participants (mean age of 27) using an imaging technique called MR Spectroscopy which can measure GABA/glutamate neurotransmitter ratios in the brain. They used resting state fMRI to measure brain activity at rest in the pregenual anterior cingulate cortex of these participants. They then tested a) the effect of genotype on brain activity, b) GABA/glutamate levels and c) whether there was any genotype by sex interaction (whether genotype affects these brain measures depending on sex) d) whether measured vulnerability to anxiety using a harm avoidance scale (correlates with anxiety) from the Temperament and Character Inventory was correlated with these brain measures.

What did they find?

Resting brain activity in the pregenual anterior cingulate was significantly lower in carriers of the G allele, but there was no interaction effect of GAD65 or sex on brain activity. However, there was a significant interaction between sex and genotype in the pregenual anterior cingulate on GABA/ glutamate levels, with female G allele carriers showing higher GABA levels. The level of reported harm avoidance was negatively correlated with the level of GABA in the pregenual anterior cingulate cortex in females (and not in males), suggesting that the lower GABA could be underlying anxiety vulnerability in females. To confirm the relationships between genotype, brain activity, GABA and harm avoidance they performed a ‘moderated mediation analysis’ including genotype as a predictor and harm avoidance as an outcome, brain activity and GABA as mediators, and sex as a moderator. Genotype significantly predicted harm avoidance, mediated by GABA/ glutamate levels in females only.

Brain, Servier Medical Art, image by BrainPost, CC BY-SA 3.0

What's the impact?

This is the first study to show that a polymorphism in a gene producing GABA is associated with differences in GABA levels in the brain. Further, these polymorphisms affect anxiety in females (not in males), and this relationship is mediated by GABA levels in the brain. This study highlights the importance of studying genetic differences that could influence brain chemistry or activity and that these differences may differ depending on sex. We now know that female vulnerability to anxiety could be related to genetic predisposition and inhibitory neurotransmitter levels in the pregenual anterior cingulate cortex.

Colic et al., GAD65 promoter polymorphism rs2236418 modulates harm avoidance in women via inhibition/excitation balance in the rostral ACC. The Journal of Neuroscience (2018). Access the original scientific publication here.