The role of alternative splicing in CEP290-related disease pathogenesis

Primary ciliopathies are a group of inherited developmental disorders resulting from defects in the primary cilium. Mutations in CEP290 (Centrosomal protein of 290kDa) are the most frequent cause of recessive ciliopathies (incidence up to 1:15,000). Pathogenic variants span the full length of this large (93.2kb) 54 exon gene, causing phenotypes ranging from isolated inherited retinal dystrophies (IRDs; Leber Congenital Amaurosis, LCA) to a pleiotropic range of severe syndromic multi-organ ciliopathies affecting retina, kidney and brain. Most pathogenic CEP290 variants are predicted null (37% nonsense, 42% frameshift), but there is no clear genotype-phenotype association. Almost half (26/53) of the coding exons in CEP290 are in-phase "skiptic" (or skippable) exons. Variants located in skiptic exons could be removed from CEP290 transcripts by skipping the exon, and nonsense-associated altered splicing (NAS) has been proposed as a mechanism that attenuates the pathogenicity of nonsense or frameshift CEP290 variants. Here, we have used in silico bioinformatic techniques to study the propensity of CEP290 skiptic exons for NAS. We then used CRISPR-Cas9 technology to model CEP290 frameshift mutations in induced pluripotent stem cells (iPSCs) and analysed their effects on splicing and ciliogenesis. We identified exon 36, a hotspot for LCA mutations, as a strong candidate for NAS that we confirmed in mutant iPSCs that exhibited sequence-specific exon skipping. Exon 36 skipping did not affect ciliogenesis, in contrast to a larger frameshift mutant that significantly decreased cilia size and incidence in iPSCs. We suggest that sequence-specific NAS provides the molecular basis of genetic pleiotropy for CEP290-related disorders.