The Big Hope for Huntington's

HUNTINGTON’S DISEASE OUTLOOK

TREATMENTS The big hope for Huntington’s A quarter of a century after its discovery, researchers are finally unlocking ways to neutralize the gene behind Huntington’s disease.

BY LIAM DREW

n September 2014, at a meeting of the European Huntington’s Disease Network, Sarah Tabrizi announced the launch of a Idrug trial. Tabrizi, a neurologist and director of the University College London Huntington’s Disease Centre, would be working with Ionis Pharmaceuticals of Carlsbad, California, to test the safety and tolerability of a drug candi- date called IONIS-HTTRx in people for the first time. The drug had been designed to reduce the amount of protein being made by the gene that causes Huntington’s disease. That gene is huntingtin (HTT). Inheriting just one mutated copy brings about a progres- sive neurodegeneration that typically begins in a person’s forties. The condition’s most distinctive symptom is involuntary, jerky limb movements. This is preceded by subtler psychiatric symptoms and followed by a disabling dementia. IONIS-HTTRx is an antisense oligonucleotide (ASO): an artificial chain of 12–25 nucleotides that is designed to prevent the production of protein from a specific gene. In the trial, people in the initial stages of Huntington’s disease Sarah Tabrizi is helping to move innovative potential treatments for Huntington’s disease into the clinic. would receive four monthly injections of ASO directly into their cerebrospinal fluid (CSF) production of the protein had, on average, In 2013, it partnered with Ionis to develop through a lumbar puncture. IONIS-HTTRx decreased by about 40%. IONIS-HTTRx and, after the initial trial, Roche was expected to diffuse into the brain, where it “People started crying,” says Jeff Carroll, a acquired the drug for US$45 million. The com- would suppress the production of the protein neuroscientist who investigates Huntington’s panies will continue to collaborate. If all goes huntingtin in neurons. As well as ensuring that disease at Western Washington University in to plan, a large phase III trial of IONIS-HTTRx the drug had no adverse effects, Tabrizi and Bellingham. “Everybody who works in Hun- will commence later in 2018. Ionis would use a new assay to measure levels tington’s disease long enough meets families and Assessing where the project stands at present, of mutant huntingtin. gets to know them, so it Tabrizi says she has become fond of a quote The Huntington’s disease community was “It’ll never becomes very personal.” from a speech by Winston Churchill: “Now, this excited about the trial — a way of silencing reach the Carroll’s connection is not the end. It is not even the beginning of the HTT has been sought since the gene was dis- brain. It’s to his work runs particu- end. But it is, perhaps, the end of the beginning.” covered in 1993. But the path to using ASOs never going to larly deep. He began his as treatments had been rocky and the brain work.” career in neuroscience THE BEGINNING OF THE BEGINNING is notoriously difficult to target with drugs. after his mother was The molecular basis of ASO technology is the Tabrizi recalls that after her presentation, col- diagnosed with Huntington’s disease. Then, in stuff of secondary-school textbooks. In double- leagues told her, “It’ll never reach the brain. It’s 2003, he discovered that he, too, had the muta- stranded nucleic acids, the base guanine binds never going to work.” tion for the condition. Looking at Tabrizi’s slide, to cytosine, and thymine (in DNA) or uracil (in In December 2017, however, a press Carroll thought, “This is a graph that is chang- RNA) binds to adenine. This pairing enables release revealed that the doubters had been ing my life.” DNA both to replicate and to supply cells with wrong — the trial had been a success. And in Tabrizi emphasizes that the trial did not instructions for making protein. March 2018, Tabrizi unveiled the resulting data show that IONIS-HTTRx is able to treat Hun- ASOs are designed to be complementary to at the final session of the 13th Annual Hun- tington’s disease. It demonstrated only that the the messenger RNAs of specific genes, which tington’s Disease Therapeutics Conference. drug was safe and well-tolerated, and — cru- act as templates for protein production. When Her most important slide showed decreases cially — that it engaged its target in the brain. a cell is flooded with a particular ASO, the ASO in the level of mutant huntingtin in trial par- “What we now have to do,” she says, “is to move will bind to its target mRNA, preventing it ticipants’ CSF — indicative of reduced levels quickly forward to larger, longer trials to test from guiding protein synthesis. of the toxic protein in their brains — that were whether the drug slows disease progression.” Forty years ago, researchers at Harvard proportional to the amounts of drug the vol- These trials will be run by pharmaceuti- University in Cambridge, Massachusetts, 1 DAVID BISHOP FOR UCL CREATIVES BISHOP FOR DAVID unteers had received. At the two highest doses, cal company Roche of Basel, Switzerland. demonstrated that an ASO made of DNA

OUTLOOK HUNTINGTON’S DISEASE could stop the replication of a virus by blocking But after advances in treating HIV infection ASOs that target HTT mRNA could reverse viral protein production. And a year later, in that maintained the immune response, the Huntington’s-disease-like symptoms in mouse 1979, it was shown2 that the binding of ASOs to drug fell out of use. Other early ASOs were models of the condition, alongside a demonstra- mRNA not only prevented protein synthesis, clinical failures. In the mid-1990s, several tion that ASOs downregulate huntingtin but also triggered the degradation of mRNA. companies who had invested heavily in ASO production in the brains of rhesus macaques. Given that ASOs could, in principle, sup- technology withdrew from the field. After this proof-of-concept work, Ionis press the expression of any gene, these find- The frustration extended to basic research. designed and validated an ASO that would ings raised hopes of a fresh approach to the The first published report of an attempt to work in people, established the best way to treatment of many diseases. And conditions suppress the production of huntingtin in mice deliver it to the brain — optimizing the lum- such as Huntington’s, caused by faulty genes using ASOs described a failed experiment3. bar-puncture procedure, for example — and that produce proteins with toxic effects, were That study’s lead researcher, Ole Isacson, who then determined the parts of the brain that seen as particularly promising targets. works on neurodegeneration at the Harvard the drug was most likely to reach. Finally, There was, however, a considerable prob- Stem Cell Institute in Cambridge, recalls the with Tabrizi, Roche and the CHDI Founda- lem: DNA makes a lousy drug. A good drug conflicted time that followed the description tion (a US non-profit organization that funds tends to distribute of HTT in 1993. “People were so enthralled by research on Huntington’s disease), it developed evenly through the “People were genetics,” he says. “They thought if you had the the assay5 to track levels of mutant huntingtin. body, so that suffi- so enthralled gene, you had the cure. Those of us with direct cient amounts reach by genetics. experience of working on disease models didn’t THE END OF THE BEGINNING the desired target. To They thought get that feeling.” The optimism that surrounds IONIS-HTTRx achieve this, the drug if you had the Bennett concurs. The discovery of HTT stems from the well-established link between must persist for a sub- gene, you had piqued his interest in Huntington’s disease, reduced levels of mutant huntingtin and stantial amount of time. the cure.” but “at the time, we weren’t ready”, he says. improvements in symptoms in animal models On this score, ASOs Instead, Ionis focused on improving the sta- of Huntington’s disease. Yet some researchers face a major problem when given to mam- bility of ASOs. Second-generation ASOs were are concerned that IONIS-HTTRx suppresses mals, which produce high levels of enzymes developed in the late 1990s and early 2000s by not only the production of mutant huntingtin, called nucleases that digest nucleic acids. incorporating further chemical modifications but also synthesis of the normal protein. Also, to be effective, ASOs must bind to tar- that increased resistance to digestion by nucle- This is because selectively targeting get mRNAs both tightly and with specificity. ases. Isacson says that the enhanced stability mRNA from the mutated copy of HTT Complementary base pairing means that ASOs of present ASOs, which can act for months, — leaving mRNA from the normal copy have preferred-partner mRNAs. However, compared with the short-lived molecules that untouched — poses a huge technical challenge. because they are highly charged molecules, he used in 1997, is “one of the most remarkable The mutation that causes Huntington’s disease ASOs can also bind to mRNAs to which they improvements in technology that I’ve seen”. is an overlong run of the nucleotide triplet are not perfectly complementary — thereby Only after Ionis had developed such ASOs did CAG (see page S36): normal HTT contains affecting the expression of other genes — as Bennett begin to tackle genetic disorders affect- 17–35 consecutive such triplets, whereas in well as to proteins. Both events could give rise ing the brain. people with the condition, at least one copy of to off-target effects and toxicity issues. To do so, Ionis forged a collaboration, in 2003, the gene has 36 or more in a row. Consequently, Making ASO-based treatments a reality with Don Cleveland, a neuroscientist at the an ASO of around 20 nucleotides that targets has required the creation of molecules that University of California, San Diego, that aimed CAG repeats would bind to both normal and retained nucleic acid’s property of complemen- to treat a genetic form of amyotrophic lateral disease-causing versions of HTT mRNA. And, tary base pairing, while otherwise overhauling sclerosis, or motor-neuron disease. Then, in problematically, more than 50 other human ASO chemistry. 2006, buoyed by progress they had made using genes also contain 10 or more CAG repeats, Ionis has been a central player in this mouse models of that condition, Ionis and which means that targeting the sequence could pursuit since its foundation in 1989. Frank Cleveland began to work on Huntington’s dis- produce unwanted side effects. 4 Bennett, vice-president of research, set up ease. In 2012, they published studies showing An alternative approach to targeting HTT AND MARKETING COMMUNICATIONS RHYS LOGAN/WWU the company’s laboratories and later led the development of IONIS-HTTRx. He stresses that Ionis was “founded on an idea for a technology”. Unlike most start-ups, it did not license technology from elsewhere. And the technology that it, like other companies developing ASOs, has produced has undergone a substantial evolution. Initially, the company focused on treatments for viral infections and cancer. It used first-generation ASOs, the backbones of which had already been chemically modified to differ from those of DNA, which fraction- ally increased their resistance to nucleases. In 1998, Ionis’s drug fomivirsen (Vitravene) became the first ASO to be approved by the US Food Jeffrey Carroll’s and Drug Administration. It research is was injected into the eyes of motivated by people with AIDS to treat a a personal viral infection that can cause connection to blindness in those with com- Huntington’s promised immune systems. disease.

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HUNTINGTON’S DISEASE OUTLOOK

mRNA is being developed by Wave Life Sciences in Cambridge, Massachusetts. Its strategy takes RNA INTERFERENCE advantage of functionally insignificant differ- ences that can often be found between a per- Another way to halt huntingtin son’s two copies of HTT. If their disease-causing gene differs from their normal copy by a single Antisense oligonucleotides (ASOs) are not modifying tissues to produce RNA might nucleotide — an A, for example, instead of a the only way to suppress protein synthesis reduce the need for repeated dosing, but the C — an ASO can be designed to target only the by targeting messenger RNAs. Cells already procedure cannot be reversed if side effects HTT mRNA containing the substituted nucleo- have an intrinsic mechanism for stifling develop. And, unlike ASOs, which need tide, a mutation known as a single nucleotide mRNA that involves producing short only to be introduced to the cerebrospinal polymorphism (SNP). “We could make those molecules of RNA that are complementary fluid, these vectors must be administered SNPs into therapeutic targets for drugs,” says to mRNAs. When a short RNA binds to in the vicinity of the cells to be treated. In Paul Bolno, Wave’s chief executive. an mRNA, protein synthesis is prevented Huntington’s disease, researchers have Wave was founded on an innovative means through a phenomenon known as RNA focused on the striatum — an area in the of manufacturing ASOs, in which the intrin- interference (RNAi). middle of the brain that is most-visibly sic symmetry — or ‘handedness’ — of each Most approaches to harnessing RNAi affected by the condition. nucleotide is specified during ASO synthesis. involve introducing a gene into cells that Controversy exists as to whether But the company’s SNP-targeting approach to then constantly produces a desired short suppressing huntingtin in the striatum Huntington’s disease also requires technical RNA to permanently suppress synthesis of alone will halt Huntington’s disease. This innovation in genotyping. If a physician were a target protein. Three companies — Spark brain structure has an important role in to prescribe a drug that suppresses a gene on Therapeutics in Philadelphia, Pennsylvania; the condition’s progression, and Pedro the basis of it containing a particular SNP, he or UniQure in Lexington, Massachusetts; Gonzalez-Alegre, a neurologist at the she would need to be certain that the SNP is in and Voyager Therapeutics in Cambridge, University of Pennsylvania’s Perelman the patient’s disease-causing version: inadvert- Massachusetts — are pursuing RNAi School of Medicine in Philadelphia, thinks ently suppressing the normal copy while leav- approaches to treating Huntington’s that “improving one key brain region will ing mutated HTT unaffected could accelerate disease. have benefits beyond that area itself”. But the disease. In conventional gene sequencing, Each is developing a virus-based ultimately, he concedes, “we will answer this DNA from a person’s two copies of a gene is vector — which is incapable of replicating question only when we do it in humans.” combined — it’s possible to discover which and therefore poses no threat to health — Perhaps, he suggests, the strong RNA-based mutations the person has, but not on which that delivers a gene encoding a short RNA suppression of huntingtin in the striatum chromosome (of the pair) they are found. To that inhibits huntingtin production. However, might be supplemented with ASOs to more determine that in Huntington’s disease, the this one-off genetic treatment raises modestly suppress the protein throughout sequencing reaction must follow the same potential safety concerns. Permanently the brain and body. L.D. strand of DNA from the region of CAG repeats to the SNP that is being used to differentiate between versions of the gene. Wave says that despairingly cites two almost identical studies6,7 for Huntington’s disease and, potentially, its sequencing platform does exactly this. But in mice that gave radically different results. brain diseases in general. She can now smile to meet regulatory approval, the error rate will “Right now, we don’t know enough,” he says. about the doubters that she encountered on have to be essentially zero. “And you can only get so much from mice.” announcing the trial. “This is science,” she says. The company has now begun separate Another question about the long-term “When you’re trying to develop new therapies, phase I trials of two ASOs that each target one future of ASOs concerns the plausibility of giv- you’re always going to have sceptics, and you of the two most common SNPs in mutated ing lumbar punctures to patients on a regular have to just carry on with what you believe in. HTT. The approach represents a personalized basis, potentially, for decades. ASOs that can And I believed in this.” route to treating Huntington’s disease — only cross the blood–brain barrier, which do not Carroll, like many other researchers and people with the targeted SNPs can benefit. need to be introduced directly into the CSF, people who have been touched by Hunting- Unfortunately, about 30% of those with the are still in early development. And there are ton’s disease, has been riding a fresh wave of condition have neither SNP. Bolno says that methods other than a lumbar puncture for hope since Tabrizi revealed results of the ASO Wave is looking for further SNP targets. getting drugs into the CSF: some people with trial in March. “Ever since I got my diagnosis,” The need for specificity is contentious. multiple sclerosis, for example, use implanted he says, “I have operated under the assump- Bolno points to studies in mice, in which pumps for the task. Tabrizi says that finding an tion that I’ll die at the same time my mum switching off the production of normal hun- alternative delivery system is a “post-approval did — that I’ll get sick when she did. Seeing tingtin causes deleterious effects, as evidence problem” — given that no current treatment that graph was the first time I’ve believed that that suppressing both forms of huntingtin in stops the condition, she notes, people with it could be better.” ■ people might have unwanted consequences. Huntington’s disease accept that they’ll need to Ionis and Tabrizi, however, disagree. visit a hospital regularly to receive treatment. Liam Drew is a science writer based in Although studies in mice show that normal Other potential genetic treatments hold the London. huntingtin is crucial for early development, advantage of having to be administered only 1. Zamecnik, P. C. & Stephenson, M. L. Proc. Natl Acad. they emphasize that, in adult animals, its func- once. At the moment, the most viable such Sci. USA 75, 280–284 (1978). 2. Donis-Keller, H. Nucleic Acids Res. 7, 179–192 tion is much less important. Besides, they point alternative is RNA interference (see ‘Another (1979). out, ASOs do not reduce levels of huntingtin to way to halt huntingtin’). Looking further 3. Haque, N. & Isacson, O. Exp Neurol. 144, 139–46 zero. They cite other studies in mice in which ahead, the gene-editing tool CRISPR could (1997). 4. Kordasiewicz, H. B. et al. Neuron 74, 1031–1044 lowering but not totally removing huntingtin correct HTT directly (S42). But although the (2012). had no adverse effects. What’s more, they say, enthusiasm that surrounds this technique is 5. Wild, E. J. et al. J. Clin. Invest. 125, 1979–1986 rhesus macaques given IONIS-HTTRx for up valid, the history of ASOs shows that much (2015). 6. Dietrich, P., Johnson, I. M., Alli, S. & Dragatsis, I. PLoS to nine months showed no detrimental effects. work is needed to move exciting ideas into Genet. 13, e1006846 (2017). Both Ionis and Wave are working with the clinic. 7. Wang, G., Liu, X., Gaertig, M. A., Li, S. & Li, X. J. Proc. Carroll to resolve this pivotal issue. Carroll “ASOs are reaching prime time,” says Tabrizi, Natl Acad. Sci. USA 113 3359–3364 (2016).

©2018 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. ©2018 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts re31ser vMAYed. 2018 | VOL 557 | NATURE | S41