OUTLOOK GENE THERAPY Excitatory neurons release neurotransmitters that electrically stimulate neighbouring cells, whereas inhibitory neurons release neurotrans- mitters that suppress electrical activity. A seizure is a period of runaway electrical activity during which the normal balance between excitation and inhibition is lost. Current anti-seizure drugs either dampen excitatory mechanisms or boost inhibitory ones. But they do so indiscriminately, producing wide-ranging side effects by affecting CREATIVES BISHOP/UCL HEALTH DAVID neural circuits throughout the nervous system. Current gene-therapy strategies, by contrast, use harmless viruses to introduce one or two therapeutic genes into the defined volume of tissue from which focal epilepsy emanates. “It is more personal, more targeted, and prob- ably has fewer side effects because we treat the tissue that needs to be treated, instead of treating the whole body,” says Merab Kokaia, a neuroscientist working on this approach at Lund University in Sweden. The strategies in development target focal epilepsy, but treating generalized epilepsy is a longer-term possibility. Elizabeth Nicholson and Dimitri Kullmann at University College London. RESTORING BALANCE The brains of people with epilepsy contain NEUROLOGY increased amounts of neuropeptide Y (NPY), a chemical that certain neurons release when they are especially active. NPY acts on five receptors, Y1 to Y5, some of which are excitatory and some Repairs for a inhibitory. The levels of some of these receptors are also altered in epilepsy: notably, levels of Y2, which strongly inhibits neurotransmitter release, are higher. Overall, the accumulation of runaway brain NPY and the altered levels of its receptors seem to represent an adaptive response — an intrinsic bid to hold back runaway brain activity. Gene therapy could damp down epilepsy seizures in people In 2004, investigators used a viral vector to for whom current drugs are ineffective. deliver the NPY gene into the brains of rats that had been manipulated to display a form of epileptic activity1. The resulting overexpres- BY LIAM DREW people with epilepsy, each one selected to quell sion of NPY caused a reduction in seizure the rampage of electrical activity that causes frequency. Other animal experiments also he seizures of around one-third of epileptic seizures. The most advanced projects showed that overexpressing the neuropeptide people with epilepsy are resistant to avail- are now being readied for clinical trials. galanin likewise suppressed seizures. able medicines — a statistic that haunts Kokaia, who was already working on NPY Tneurology. It has been this way for decades. The EXCITATION AND INHIBITION and epilepsy at the time, became interested medicines have got better by becoming safer Epilepsy comes in many forms. It is defined in the therapeutic potential of this approach and causing fewer side effects. But still there by the repeated occurrence of seizures — but and started experimenting with introduc- are people for whom the drugs simply don’t these seizures can vary in their nature, inten- ing genes for neuropeptides, their receptors work — and for them, epilepsy can be ruinous. sity and frequency. And the disorder can arise or both. He found that overexpressing NPY “There’s stigma; they can’t drive; they have from numerous causes, progress in different alone decreased seizure frequency, but simul- difficulty holding down jobs; they have diffi- ways and affect distinct parts of the brain. taneously overexpressing it with the inhibi- culty maintaining relationships,” says Dimitri Crucially, epilepsy can either be focal, with tory Y2 receptor dramatically heightened the Kullmann, a neurologist and neuroscientist at seizures beginning in a specific brain region, anti-seizure effect2. “What we are trying to do University College London (UCL). or generalized, with seizures developing across is boost the natural response of the brain by Currently, the main hope for people with wide spans of the brain. Focal epilepsy is more gene therapy,” says Kokaia. severe drug-resistant epilepsy is surgery. Some- common, and it can be further subcategorized In 2015, Kokaia co-founded CombiGene in one whose seizures arise from a well-defined according to whether the seizures remain focal Lund, Sweden, to commercially develop this region of the brain might be offered an opera- or spread to become generalized. There is also technique. In the past two years, CombiGene tion to remove that region. This is a drastic pro- variation in the size of the seizure-generating has confirmed the anti-seizure effects of the cedure, but not especially rare; it is carried out focus and whether it is discrete, and therefore NPY–Y2 combination therapy, now called about 500 times every year in the United States. potentially removable, or enmeshed with vital CG01, in further rodent models of epilepsy. Kullmann is hoping that gene therapy can brain tissue, and thus inoperable. And the company has successfully introduced make such surgery unnecessary. His group and Brains essentially work by relaying electri- the NPY and Y2 genes into brain tissue that was others are investigating the potential benefits cal signals from neuron to neuron through surgically removed from people with epilepsy. of introducing different genes into the brains of the release of chemical neurotransmitters. Experiments using such tissue also ended S10 | NATURE | VOL 564 | 13 DECEMBER 2018©2018 Spri nger Nature Li mited. All ri ghts reserved. ©2018 Spri nger Nature Li mited. All ri ghts reserved. GENE THERAPY OUTLOOK interest in the second neuropeptide, galanin. tool — DREADDs (designer receptors exclu- Whereas NPY suppressed neurotransmis- sively activated by designer drugs) — into a sion in human tissue, galanin did nothing — therapy. DREADDs are genetically engineered human neurons lack functional receptors for it. receptors that, like opsins, sit silently in neurons Kullmann’s move into the epilepsy field was unless they are activated by a stimulus, but in serendipitous. His group was investigating the this case, the stimulus is a drug rather than light. ESTHER KROOK-MAGNUSON KROOK-MAGNUSON ESTHER voltage-gated potassium channel Kv1.1 — a Both Kullmann and Kokaia have found that type of ion channel that electrically quiets neu- inhibitory DREADDs can suppress seizures rons — as part of work on an entirely different when the genes encoding them are inserted neurological condition, episodic ataxia. The into the seizure foci of epileptic animals using group made a virus that transferred the Kv1.1 viral vectors. If the therapy were translated to gene into neurons. Because a neighbouring humans, people might take the activating drug laboratory was routinely using rodent models regularly in a similar way to current epilepsy of epilepsy, Kullmann and colleagues thought it medicines — but with the advantage that the might be worth testing Kv1.1 in these animals. DREADDs would not inhibit brain tissue out- The effect, published in 2012, was a dramatic side the region where the DREADD is situated. Brain cells could be manipulated using light. reduction in seizure frequency3. After see- Alternatively, people might receive the drug ing this effect in three separate animal mod- automatically through an implanted, seizure- awkwardly located foci make them ineligible for els, Kullmann and UCL colleague Stephanie activated drug-delivery system, or simply take surgery, says CombiGene chief executive Jan Schorge developed a viral vector that intro- the drug when they feel the first indications of Nilsson. And, more speculatively, if it is success- duces a modified Kv1.1 gene specifically into a seizure. ful, gene therapy could potentially be adopted excitatory neurons, and does not integrate the Kullmann is also exploring an ion channel by some people instead of conventional drugs. gene into the cell’s genome. that was originally identified in nematode But for the time being, CombiGene and In principle, CG01 or Kv1.1 could provide worms. In nematodes, the glutamate-gated Kullmann’s team are long-term suppression of epileptic seizures chloride (GluCl) channel is inhibitory and is “What we are planning safety and following a single injection, with the genes activated by the neurotransmitter glutamate. trying to do tolerability trials that continually generating products that calm the But in mammals, glutamate is the main excita- is boost the will involve only peo- neurons in which they are expressed. tory neurotransmitter that is responsible for natural response ple with drug-resist- driving excess activity during seizures, and ant epilepsy who are TRIGGERED ACTIVATION of the brain by none of its receptors is inhibitory. gene therapy” awaiting surgery. This Several alternative approaches are mainly Kullmann and his colleague Andreas Lieb is not because people based on converting widely used basic- were interested in using an engineered version in this group are the research technologies into clinical tools. These of the GluCl channel that is activated by a drug, sole intended recipients of gene therapy — approaches are more complicated, but hold but then they learnt that mutations in GluCl rather, they present a unique opportunity. potential advantages over CG01 or Kv1.1. can change its glutamate sensitivity. If they The virus is likely to be given during presur- Opsins, for example, are membrane picked a mutated channel that was insensitive gical investigations of the seizure locus, then proteins that are activated by light, and the