Background: Leber Congenital Amaurosis (LCA) and Early Onset Retinal Dystrophy (EORD), are primary causes of inherited childhood blindness. Both are autosomal recessive diseases, with more than 25 genes explaining approximately ~70% of cases. However, the genetic cause for many cases remains unclear. Sequencing studies from genetically isolated populations with increased prevalence of a disorder has proven useful for rare variant studies, making Costa Rica an ideal place to study LCA/EORD genetics. Materials and Methods: Twenty-eight affected children (25 LCA, three EORD) and their immediate family members, totaling 52 individuals (30 affected) from 22 families, were sequenced. Whole exome sequencing was performed on all affected individuals. Available parents were analyzed either by WES or Sanger sequencing to determine transmission. Results: All affected individuals demonstrated compound heterozygous or homozygous mutations in known Inherited Retinal Disease (IRD) genes. Twelve variants were identified in at least one individual in three genes, RDH12, RPE65, and USH2A. Four recurrent RPE65 mutations were observed in 97% of individuals and 95% of families. All patients with LCA and two of the three individuals with EORD had biallelic mutations in RPE65; one child with EORD had a homozygous RDH12 mutation. Conclusions: These data suggest that the majority of LCA/EORD in Costa Rica is due to four founder effects in RPE65 which have been maintained in this genetically isolated population. This finding is of great clinical significance due to the availability of gene therapy recently approved in US and European Union for patients with biallelic RPE65 deficits.

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