Résumé
Usher syndrome (USH) is the most prevalent form of the syndromic inherited retinal dystrophy (IRD) retinitis pigmentosa (RP) associated with hearing defects. USH is classified into three clinical forms of which USH type 2 (USH2) is the most common. The highest prevalent gene for USH2 is USH2A. Mutations in USH2A are as well the most frequent cause of non-syndromic RP. Interestingly, there exist two recurrent mutations in USH2A (c.2276G>T and c.2299delG) which account for 50% of patients with a clear genotype-phenotype correlation associated to these mutations. Thus, some mutations known as “retina specific alleles” (c.2276G>T) will give rise to isolated RP without ear defects. The reason behind this phenomenon is currently not well understood. Similarly, the pathophysiology associated with USH2A mutations and the function of the USH2A gene product, Usherin, is unknown, hindering the development of treatments. The rapid advances in somatic cell reprogramming and differentiation techniques in recent years, have led to the development of elaborated protocols for obtaining retinal organoids, which will allow the elucidation of the still unknown pathophysiology associated with USH2A.We focused our work on the characterization of two patient iPSC-derived retinal organoids at early stages. The first patient presented with USH2 (USH2A-USH) and was homozygous for the c.2299delG mutation. The second patient presented with isolated RP (USH2A-RP) and was compound heterozygous for the c.2299delG and c.2276G>T mutations. We also differentiated the isogenic iPSC lines, previously corrected using CRISPR/Cas9, together with a wild-type (WT) iPSC line. The characterization at early stages of differentiation shows a dysfunction in the development of photoreceptors in USH2A-RP retinal organoids, which is reverted to WT levels upon CRISPR/Cas9 correction. Interestingly, we do not observe the same phenotype in the USH2A-USH retinal organoids, suggesting a differential disease mechanisms associated with these two common mutations. This characterization allows us to obtain a first hint into the USH2A-associated pathophysiology at early stages of the development, helping to achieve a suitable retinal model for the study of these diseases.