CEP290 mutations cause a spectrum of ciliopathies from Leber congenital amaurosis type 10 (LCA10) to embryo-lethal Meckel syndrome (MKS). Using panel-based molecular diagnosis testing for inherited retinal diseases, we identified two individuals with some preserved vision despite biallelism for presumably truncating CEP290 mutations. The first one carried a homozygous 1 base-pair deletion in exon 17, introducing a premature termination codon (PTC) in exon 18 (c.1666del; p.Ile556Phefs*17). mRNA analysis revealed a basal exon skipping (BES) of exon 18, providing mutant cells with the ability to escape protein truncation, while disrupting the reading frame in controls. The second individual harbored compound heterozygous nonsense mutations in exon 8 (c.508A>T, p.Lys170*) and exon 32 (c.4090G>T, p.Glu1364*), respectively. Some CEP290 lacking exon 8 were detected in mutant fibroblasts but not in controls whereas some skipping of exon 32 occurred in both lines, but with higher amplitude in the mutant. Considering that the deletion of either exon maintains the reading frame in either line, skipping in mutant cells likely involves nonsense-associated altered splicing (NAS) alone (exon 8), or with BES (exon 32). Skipping of PTC-containing exons in mutant cells allowed production of CEP290 isoforms with preserved ability to assemble into a high molecular weight complex and to interact efficiently with proteins important for cilia formation and intraflagellar trafficking. In contrast, studying LCA10 and MKS fibroblasts we show moderate to severe cilia alterations, providing support for a correlation between disease severity and the ability of cells to express shortened, yet functional, CEP290 isoforms.

Mutations in CEP290 encoding a centrosomal protein important to cilia formation cause a spectrum of diseases, from isolated retinal dystrophies to multivisceral and sometimes embryo–lethal ciliopathies. In recent years, endogenous and/or selective non-canonical exon skipping of mutant exons have been documented in attenuated retinal disease cases. This observation led us to consider targeted exon skipping to bypass protein truncation resulting from a recurrent mutation in exon 36 (c.4723A > T, p.Lys1575*) causing isolated retinal ciliopathy. Here, we report two unrelated individuals (P1 and P2), carrying the mutation in homozygosity but affected with early-onset severe retinal dystrophy and congenital blindness, respectively. Studying skin-derived fibroblasts, we observed basal skipping and nonsense associated–altered splicing of exon 36, producing low (P1) and very low (P2) levels of CEP290 products. Consistent with a more severe disease, fibroblasts from P2 exhibited reduced ciliation compared to P1 cells displaying normally abundant cilia; both lines presented however significantly elongated cilia, suggesting altered axonemal trafficking. Antisense oligonucleotides (AONs)-mediated skipping of exon 36 increased the abundance of the premature termination codon (PTC)-free mRNA and protein, reduced axonemal length and improved cilia formation in P2 but not in P1 expressing higher levels of skipped mRNA, questioning AON-mediated exon skipping to treat patients carrying the recurrent c.4723A > T mutation.

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