Clinical trial of gene therapy for the treatment of achromatopsia

色视是一种严重的遗传性视网膜疾病，其发病率约为1/30000，其特征是出生时就缺乏日光视力，畏光和缓慢进行性的视锥感光细胞丧失。大约一半的ACHM病例是由CNGB3基因突变引起的，CNGB3基因编码锥体特异性环核苷酸门控通道的两个亚基之一，这是光转导级联的重要组成部分。到目前为止，这种遗传性视网膜营养不良还没有成功的治疗方法。然而，三项AAV2介导的基因治疗的独立临床试验，包括我们团队进行的一项试验，显示了一种快速进展的遗传性视网膜营养不良的视网膜敏感性和视力的改善：由RPE65缺乏引起的Leber先天性黑色素症。这些试验表明，基因补充疗法可能是治疗隐性遗传性视网膜疾病的合适策略。CNGB3基因突变引起的色盲有几个特征，使其成为证明原则基因治疗试验的良好候选疾病。这种疾病可以是静止的或进展缓慢的，这创造了一个很大的机会之窗，治疗有望带来临床益处。更重要的是，可能恢复以前缺失的视锥功能，并预期随后日间视力的改善，应该能够对治疗效果进行快速和有力的评估。在之前使用CNGB3缺乏的小鼠模型的研究中，我们已经能够显示成功的基因补充，从而显著挽救了视锥细胞光感受器的功能和存活(Carvalho等，Hum Mol Genet(2000)20：3161-75)。这项研究是迄今为止报道的由于光感受器缺陷而导致的视网膜营养不良动物模型最有效的挽救之一，表明这种疾病可能特别适合基因补充治疗。对于第一个里程碑(持续2年)，我们将在UCL基因治疗联盟的生产设施中生产符合GMP标准的基因治疗载体(AAV2/8.hCAR.hCNGB3)。媒介毒性研究和治疗效果研究将根据既定方案在内部进行。成功完成这一里程碑将被定义为获得监管机构的许可开始临床试验。第二个里程碑(持续3年)将是临床试验的完成，包括对试验对象的1年随访。成功完成这一里程碑将被定义为视锥派生视觉功能的任何持续改善(由一系列心理物理、电生理和功能磁共振技术确定)，即大于每次测试的重测差异，以及没有毒性，如III级或IV级眼部不良事件或非眼部不良事件的定义。目前，已发现150多个基因，当突变时，可导致遗传性视网膜变性。由于这些疾病大多是由光感受器细胞缺陷引起的，因此有效和安全地将基因转移到光感受器是发展视网膜基因治疗的关键。动物实验表明，与目前临床使用的AAV2载体相比，AAV2/8伪型载体为转导细胞提供了更高的效率，并导致了更高水平的转基因表达。因此，这项试验的结果不仅与基因疗法治疗色盲有关，而且成功的结果将为未来许多其他遗传性视网膜营养不良的基因疗法的发展铺平道路。

Achromatopsia (ACHM) is a severe inherited retinal disorder with a population frequency around 1/30000, characterised by the absence of daylight vision from birth, photophobia and a slowly progressing loss of cone photoreceptors. Approximately half of ACHM cases are caused by mutations in the CNGB3 gene, which encodes one of two subunits of the cone-specific cyclic nucleotide-gated channel, an essential component of the phototransduction cascade. Thus far no successful treatments exist for this inherited retinal dystrophy. However, three independent clinical trials of AAV2-mediated gene therapy, including one performed by our group, have shown improvements in retinal sensitivity and vision in a rapidly progressing form of inherited retinal dystrophy: Leber congenital amaurosis caused by RPE65-deficiency. These trials have suggested that gene supplementation therapy can be a suitable strategy for the treatment of recessively inherited retinal disease. Achromatopsia caused by mutations in the CNGB3 gene has several characteristics that make it a good candidate disease for a proof of principle gene therapy trial. The disorder can be stationary or slow-progressing, which creates a large window of opportunity where treatment would be expected to lead to clinical benefit. More importantly, a potential restoration of previously absent cone function and the expected ensuing improvement in daylight vision should allow a rapid and robust assessment of treatment efficacy. In a previous study using a murine model of CNGB3 deficiency, we have been able to show successful gene supplementation and consequently substantial rescue of cone photoreceptor function and survival (Carvalho et al, Hum Mol Genet (2000) 20: 3161-75). This study constitutes one of the most effective rescues of an animal model of retinal dystrophy due to a photoreceptor defect reported to date, suggesting that this disorder may be particularly amenable to gene supplementation therapy.For the first milestone (duration 2 years) we will produce the gene therapy vector (AAV2/8.hCAR.hCNGB3) to GMP standards at the production facility of the UCL gene therapy consortium. Vector toxicity studies and treatment efficacy studies will be performed in-house, according to established protocols. Successful completion of this milestone will be defined as permission from the regulatory authorities to commence a clinical trial.The second milestone (duration 3 years) will be the completion of the clinical trial, including a 1 year follow-up of the trial subjects. Successful completion of this milestone will be defined as any sustained improvement in cone-derived visual function (as determined by an array of psychophysical, electrophysiological, and fMRI techniques), that is greater that the test-retest variation for each test, and the absence of toxicity, as defined by a Grade III or IV ocular adverse event, or a non-ocular SUSAR.Currently, more than 150 genes have been identified that, when mutated, can give rise to inherited retinal degeneration. As most of these forms of the disease are caused by photoreceptor cell defects, an efficient and safe method of gene transfer to the photoreceptors is essential in the development of retinal gene therapy. Animal experiments have shown that the AAV2/8 pseudotyped vector gives transduces the photoreceptor cells with higher efficiency and leads to higher levels of transgene expression than the AAV2 vector that has been used in the clinic thus far. The results of this trial will therefore not only be of relevance for gene therapy to treat achromatopsia, but a successful outcome will pave the way for the future development of gene therapy for many other inherited retinal dystrophies.