Genetic background of congenital aniridia

Abstract

Congenital aniridia is a complex disease, characterized mainly by iris and foveal hypoplasia, but patients show great clinical variability with overlapping of different anterior and posterior segment anomalies. More than 90% of patients carry heterozygous variants in the PAX6 gene, a highly conserved transcriptional regulator that plays a key role in normal ocular development. Loss‐of‐function variants causing premature stop codons are the most frequent within the broad mutational spectrum of PAX6, with more than 750 different mutations identified to date. 11p13 microdeletions altering PAX6 or its downstream regulatory region have been found in ~25% of patients. Iris abnormalities are also present in various aniridia‐mimicking conditions, such as FOXC1 and PITX2‐related anterior segment dysgenesis, among other minor genes. Therefore, this is now recognized that genetic and clinical diagnosis of patients with suspected aniridia is challenging. Genetic testing has proven to be crucial for differential diagnosis. Here, we update best practices to improve genetic testing and clinical management of aniridia using more cost‐effective next‐generation sequencing analysis.Recent advances in genomic sequencing contributed to increasing diagnostic yield in congenital aniridia and allied conditions, from 70 to 95% as we demonstrated in a cohort of Spanish patients, and also in a “naïve” cohort of Hungarian patients without previous genetic testing. NGS allowed us to identify pathogenic PAX6 variants in patients with an atypical or incomplete presentation of aniridia, such as isolated foveal or optic nerve hypoplasia. In addition, genomic approaches contributed to elucidating new molecular mechanisms underlying the etiopathogenesis of congenital aniridia involving non‐coding PAX6 variants and revealed an unexpected role of mosaic variants in the intrafamilial variable expressivity of aniridia.Emerging long‐read sequencing techniques may provide insight into hidden sources of variation at PAX6 locus in some patients with unexplained genetic causes of aniridia. Nanopore‐based long‐read whole genome sequencing unveiled cryptic balanced chromosomal structural variants that directly disrupt PAX6 or its regulatory elements.In conclusion, NGS‐based genetic testing is crucial to ensure a correct and timely clinical diagnosis, and provide better prognosis and management of patients with aniridia or aniridia‐like, besides the further benefits to families by allowing prenatal and preimplantation genetic testing. Genetic findings would also be critical in defining eligible patients for future clinical trials.