Tominersen

近日，一项来自于麻省理工学院和哈佛大学博德研究所的研究表明，业内一直以来关于亨廷顿舞蹈症的开发方式可能搞错了。 相关研究发表于Cell上。 业内开发方向 一直以来，亨廷顿舞蹈症的病因被认为是亨廷顿基因（HTT）突变。正常情况下，该基因内包含一段CAG（胞嘧啶、腺嘌呤和鸟嘌呤）三核苷酸重复序列，能够编码一种功能不明，但似乎非常重要的蛋白质，当这段CAG重复序列的数量在患者中年时期异常增加时，就会导致亨廷顿病的发生。此时被称为纹状体投射神经元（SPN）的神经细胞开始死亡。这会引发一系列运动和认知症状，最终导致患者失去生命。 然而为什么CAG重复序列增加就会导致亨廷顿病发，一直是研究领域的空白。 虽然如此，业内的开发方向基本上围绕着HTT的突变蛋白展开。 例如诺华在去年12月看上了PTC Therapeutics的PTC518，这是一种HTT剪接调节剂，作为小分子药物具有穿透血脑屏障的能力，可以显著减少突变HTT蛋白。 另外还有uniQure公司的AMT-130，能够通过AAV5载体携带专门沉默HTT基因表达的基因来治疗该疾病。 虽然说方法多种多样，但是核心主旨仍然是围绕沉默，抑制突变的HTT蛋白展开。然而现在的这项研究却表明，抑制HTT蛋白或许为时已晚。 为时已晚 为了探究CAG重复片段是如何导致亨廷顿病的，以Steven McCarroll为首的研究人员检查了处于疾病不同阶段去世的患者大脑中的纹状体投射神经元。令他们惊讶的是，他们发现患者一生中亨廷顿基因的CAG重复片段长度会增加，起初增长缓慢，但在达到约80个重复片段后迅速增加。 而后期进展会越来越快，一开始是以10年为单位增长，接下来是以年为单位增长，到最后就会以月为单位迅速增长。 不过，这种重复增长在基因突变至超过150个重复片段之前对患者影响不大。而150个重复片段以后，原本在纹状体投射神经元中沉默的其他基因开始激活，包括参与程序性细胞死亡的基因。 因此，只有在重复的CAG序列足够长之后，这种蛋白才会呈现出毒性。 但是研究人员认为，一旦出现了毒性，想要拯救患者可能就为时已晚了。降低突变蛋白拯救不了已经进入迈向死亡倒计时的细胞。 所以降低亨廷顿突变蛋白表达可能就不是治疗的理想方法，而且实际上早先有许多围绕降低亨廷顿蛋白突变的疗法临床失败，很可能就是这个原因。 作者认为，治疗可以侧重于减缓重复片段的增长；考虑到重复片段达到有毒长度需要很长时间，即使只是适度减缓体细胞扩张，也可能大大推迟亨廷顿病症状的出现。 总结 总的来说，这项研究或许可以改变业内对于亨廷顿病治疗方法的开发方向，从原本的抑制突变蛋白，转向减少重复片段增长。 这也可能为我们解答了，为什么此前罗氏和Ionis开发的tominersen明明降低了脑脊液中突变亨廷顿蛋白的水平，但是仍然没有获益的情况。Wave Life Sciences公司的WVE-003被武田放弃的理由也可能与此有关。 不过，这也意味着许多在研管线，可能因此出现大洗牌或大裁撤，例如上文提到的诺华和PTC预付10亿美元的合作。 参考来源： https://www.cell.com/action/showPdf?pii=S0092-8674%2824%2901379-5

Most clinical trials for Huntington’s disease have run aground in the last few years, but academic researchers haven’t been dissuaded from trying to shift how the rare genetic disorder is understood. A handful of papers released in the last 12 months suggest those shifts are coming sooner rather than later. The latest scientific study, published in Cell on Thursday , comes from researchers at the Broad Institute of MIT and Harvard. They claim previous research may have been looking at the disease the wrong way and offer a potential new framework for treatments. Led by Steve McCarroll and Sabina Berretta, the group looked at postmortem brain tissue of Huntington’s patients and compared it to tissue from individuals without Huntington’s, aiming to see how a specific DNA mutation influences the disease’s pathology. They assert, contrary to early assumptions, that Huntington’s patients don’t slowly deteriorate. Rather, the mutation builds up and repeats in patients over time and then suddenly becomes toxic, leading to rapid decline. “Once toxicity starts, it’s fast,” McCarroll said in a call with reporters last week. “It’s months, not decades. It’s about 100 times faster than we thought.” McCarroll and Berretta’s group says it might be more effective, then, to treat Huntington’s patients before any toxicity appears in order to delay the disease’s onset — maybe indefinitely down the road. That could amount to a functional cure, where individuals with the genetic mutation never suffer from symptoms because they end up living normal lives and die from natural causes before their DNA becomes toxic. But McCarroll cautions it’s still early days, and the paper offers few solutions to put new screening and drug development procedures into practice beyond the general idea that people who carry the mutant gene should be treated earlier. It’s also unlikely there will be any near-term clinical results for drugs targeting these underlying genetic errors at earlier stages in patients’ diseases. The most prominent company attempting to do so — Triplet Therapeutics — shut down in 2022. Still, McCarroll says he has been encouraged by the interest he’s seen in some corners of the biopharma industry. “It takes years of work to develop new therapies, but we’ve been sharing these results in the scientific community at conferences for a year and a half,” McCarroll said. “I think it’s fair to say that companies have been really interested in it. And I can’t speak for them, but I can say, broadly, many companies are starting or expanding programs to try to do this.” Huntington’s researchers have known since the 1990s that the disease is caused by an inherited mutation in the huntingtin gene (HTT). The mutation causes a “triplet repeat,” where three letters in the HTT gene are erroneously duplicated over and over again and result in DNA mismatches that produce toxic proteins. In Huntington’s, the letters that repeat are a sequence of CAG. Healthy individuals typically have HTT genes consisting of 10 to 35 CAG repeats, while anyone with 36 or more CAG repeats has the mutation and, as a result, Huntington’s disease. But what McCarroll and Berretta found is that the mutation doesn’t become toxic until the CAG sequence repeats 150 times, which can take decades. It’s an assertion that fits within what’s been known about Huntington’s, given that most patients only start presenting symptoms between the ages of 30 and 50. After the discovery of the HTT gene in the 1990s, the conventional wisdom held that the mutation progressively caused brain cells to die off slowly, accumulating enough for Huntington’s symptoms to usually start appearing by middle age. Drug developers, in turn, largely tried to make treatments designed to reduce the buildup of the toxic mutant Huntingtin proteins. Some successfully did reduce the buildup, but patients still weren’t improving. Other recent attempts, such as Roche and Ionis’ antisense therapy tominersen, tried to reduce the buildup by interfering with the RNA that encoded such toxic proteins, preventing the mutated DNA from killing more neurons. In 2021, tominersen flopped a Phase 3 study in perhaps the field’s most high-profile setback . (The companies have since launched a new Phase 2 trial aiming to enroll younger patients.) The McCarroll-Berretta paper implies that those approaches were essentially doomed to fail, because they attempted to treat patients after the rapid toxicity began. By getting therapies to patients earlier, namely before their CAG sequences reach 150 repeats, developers could ultimately be more successful in treating Huntington’s. Irina Antonijevic, the former chief medical officer of Triplet Therapeutics, which was attempting to develop antisense and siRNA drugs for Huntington’s, told Endpoints News in an interview last summer that the theory behind their paper — how toxic repeats with runaway expansion drive Huntington’s pathology — had been floated for some time. Not everyone was on board immediately, she noted. “When this was first discussed more than 10 years ago, it was pooh-poohed,” Antonijevic said. Over time, more scientists eventually started noting the theory’s merits and tested whether it held up. But to claim a specific toxicity threshold and prove it in an experiment using human brain tissue, which McCarroll and Berretta did in their study, is “mind-blowing,” she said. “This paper takes it even further to say, “What is the threshold?” Antonijevic said. “‘How long do the repeats have to be to really become detrimental for the cells?’” Triplet Therapeutics’ approach came the closest to what the new paper describes, not just for Huntington’s but other genetic disorders with triplet repeats (hence where the startup got its name). The company aimed to target a gene called MSH3, which is involved in DNA mismatch repair, with its lead antisense program for Huntington’s. Ideally, the drug would have knocked down the MSH3 gene to prevent the CAGs from expanding and delay the onset of symptoms. But the drug never started clinical trials. Triplet ultimately shut down in late 2022 after the Roche failure and the broader biotech downturn cast a chill over Huntington’s development. The company was unable to raise a Series B round after bringing in a $59 million Series A in 2019. Former CEO Nessan Bermingham told Endpoints the new paper validated the company’s work, but said there’s still more that needs to be done, such as designing a clinical trial for people who carry the mutation but don’t have symptoms. “When you intervene, do you need to really intervene before they show any phenotypic representation of the disease?” Bermingham said. “Or when you start to see the actual disease present or manifest?” McCarroll and Berretta’s paper comes about a year after researchers in the UK, led by Gillian Bates at University College London, published their own Huntington’s paper in Brain using mouse models and came to a similar conclusion: Earlier treatments for Huntington’s are likely to be the most promising, and MSH3 is currently the most viable target. The Bates paper also argues that the rate of disease progression is capped after the CAG sequences surpass 185 repeats, and that treatments trying to prevent more expansion will be completely ineffective if given after this time. But the papers differ in one key aspect. Bates contends that CAG repeats in the brain are not a requirement for Huntington’s to develop, given what’s known about how the disease develops in young children. She and her colleagues note how, in extremely rare pediatric cases of Huntington’s, infants who present with symptoms before they turn 2 years old are born with mutant sequences of at least 200 CAG repeats — which typically takes decades to happen in older patients. Asked what he thinks about the differences between their two papers during the call with reporters, McCarroll essentially demurred: “Our results suggest that [the CAG repeats] are necessary and sufficient,” he said. Bates declined to comment for this story, citing scheduling difficulties.