Clinical trial of gene therapy for the treatment of achromatopsia

Achromatopsia （ACHM）是一种严重的遗传性视网膜疾病，发病率约为1/30000，其特征是出生时就没有日光视力，恐光和缓慢进展的锥体光感受器丧失。大约一半的ACHM病例是由CNGB3基因突变引起的，该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，三项由aav2介导的基因治疗的独立临床试验，包括我们小组进行的一项试验，显示了在一种快速进展的遗传性视网膜营养不良形式中视网膜敏感性和视力的改善：由rpe65缺乏症引起的Leber先天性黑朦。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。由CNGB3基因突变引起的色盲有几个特点，使其成为一个很好的候选疾病，用于原理证明基因治疗试验。这种疾病可以是静止的，也可以是缓慢进展的，这就创造了一个很大的机会窗口，治疗有望带来临床效益。更重要的是，先前缺失的视锥细胞功能的潜在恢复和预期的日光视力的改善应该允许对治疗效果进行快速和有力的评估。在先前使用CNGB3缺乏小鼠模型的研究中，我们已经能够成功地展示基因补充，从而大量挽救视锥细胞光感受器功能和存活（Carvalho等人，Hum Mol Genet(2000) 20: 3161-75）。这项研究是迄今为止报道的最有效的视网膜营养不良动物模型之一，这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）。病媒毒性研究和治疗效果研究将根据既定方案在内部进行。这一里程碑的成功完成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（为期3年）将是临床试验的完成，包括对试验对象的1年随访。这一里程碑的成功完成将被定义为锥体衍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成像技术确定），大于每次测试的测试-重测试变异，并且没有毒性（由III级或IV级眼部不良事件或非眼部SUSAR定义）。目前，超过150个基因已经被确定，当发生突变时，可以引起遗传性视网膜变性。由于这些疾病的大多数形式是由光感受器细胞缺陷引起的，因此在视网膜基因治疗的发展中，一种有效且安全的基因转移到光感受器的方法是必不可少的。动物实验表明，与目前临床上使用的AAV2载体相比，AAV2/8伪型载体对光感受器细胞的转导效率更高，转基因表达水平也更高。因此，这项试验的结果不仅与基因治疗色盲相关，而且成功的结果将为许多其他遗传性视网膜营养不良症的基因治疗的未来发展铺平道路。

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.