A novel antisense oligonucleotide treatment, potentially capable of addressing up to 25% of Leber’s congenital amaurosis (LCA) cases, has shown basic proof-of-concept in an induced pluripotent stem cell model of the severe childhood blindness. Led by researchers at UCL’s Institute of Ophthalmology and Morefields Eye Hospital, London, the study investigated the pathogenesis and treatment of a form of LCA caused by mutations in the CEP290 gene. CEP290, (centrosomal protein of 290kDa) is a protein present in the centrosome and, in photoreceptors, is present in the connecting cilium. Loss of the protein has been shown to reduce ciliogenesis while mutations in CEP290 have been associated with several ciliopathies, including Joubert syndrome, nephronophthisis, Meckel-Gruber syndrome and Senior-Loken syndrome. In LCA, correction of a CEP290 splicing error using an antisense morpholino may provide a potential therapeutic option. In their paper, published in Cell Stem Cell, the UK based research team also uncovered potential mechanisms to explain the retina-specific phenotype of CEP290 LCA patients.
While LCA is estimated to affect somewhere between 1 in 30,000 and 1 in 81,000 individuals, the most common “causative” LCA gene is CEP290 which is thought to be responsible for between 15% and 25% of all LCA cases. The most common of the characterized CEP290 mutations is the intronic c.2991+1665A>G change resulting in an aberrant splicing followed by inclusion of a 128 base pair exon sequence containing a premature stop codon (p.C998X) between exons 26 and 27. This particular mutation has been specifically associated with LCA retinal dystrophy and not with other syndromic pathologies. According to the research team, targeting the aberrant splicing with an antisense oligonucleotide may bring about therapeutic benefit by either adjusting levels of the naturally occurring splice isoforms or by inducing exon skipping of the premature stop codon and associated sequence. The research was conducted using fibroblasts from an LCA patient with the CEP290 mutation. The patient’s cells were used to generate iPSC-derived RPE and three-dimensional (3D) optic cups which allowed the research team to investigate disease mechanisms as well as test the antisense strategies.
Results of the study indicated that tissue-specific differences in splicing may partly explain the retinal specific phenotype of the common CEP290 mutation. The researchers proposed that higher levels of aberrant splicing in the retina may lead to a greater deficit of CEP290 protein, compared to other cell types, which might potentially explain the poorer phenotype seen in the retina, as opposed to the retina being more vulnerable to reduced levels of CEP290. In addressing the therapeutic opportunity, the researchers found that treatment of the optic cups with the antisense morpholino was capable of blocking aberrant splicing and successfully restored expression of the wild-type CEP290, enabling normal cilia-based protein trafficking. Commenting on the research results Prof. Mike Cheetham, principal investigator on the study at UCL Institute of Ophthalmology, stated, “Our findings using stem cells grown into organoids in the lab provide direct evidence of tissue-specific differences how genes are used by cells that are likely to contribute to the relationship between genetic inheritance and the manifestation of symptoms that affect patients. It has been suggested that photoreceptors are more vulnerable to the CEP290-LCA fault than other cell types, but our results suggest instead that photoreceptors produce significantly less CEP290 than other cells. We think this may also apply to other genes but, importantly, we now have the means to correct this, and this understanding was only possible with stem cells derived from patients.”