Genetics and epidemiology of aniridia: Updated guidelines for genetic study

New horizons in aniridia management: Clinical insights and therapeutic advances

Congenital aniridia is a rare genetic eye disorder characterized by the complete or partial absence of the iris from birth. Various theories and animal models have been proposed to understand and explain the pathogenesis of aniridia. In the majority of cases, aniridia is caused by a mutation in the PAX6 gene, which affects multiple structures within the eye. Treating these ocular complications is challenging and carries a high risk of side effects. However, emerging approaches for the treatment of aniridia-associated keratopathy, iris abnormalities, cataract abnormalities, and foveal hypoplasia show promise for improved outcomes. Genetic counseling plays a very important role to make informed choices. We also provide an overview of the newer diagnostic and therapeutic approaches such as next generation sequencing, gene therapy, in vivo silencing, and miRNA modulation.

A novel PAX6 variant as the cause of aniridia in a Chinese patient with SRRRD

BACKGROUND

The genotype characteristics and their associated clinical phenotypes in patients with aniridia were analyzed to explore pathogenic variants using whole-exome sequencing.

METHODS

One patient with aniridia was enrolled at the Beijing Tongren Hospital. Comprehensive ophthalmic and general examinations were performed on the patient. DNA was extracted from the patient, and whole-exome sequencing was performed to identify the causative variant. The pathogenicity of the variant was predicted using in silico analysis and evaluated according to American College of Medical Genetics and Genomics guidelines. Relationships between genetic variants and clinical features were analyzed.

RESULTS

In addition to the classical aniridia phenotype showing complete iris aplasia, foveal hypoplasia, and ectopic lentis, the patient also exhibited spontaneous reattachment rhegmatogenous retinal detachment (SRRRD). Whole-exome sequencing identified a novel heterozygous variant, exon8:c.640_646del:p.R214Pfs*28.

CONCLUSIONS

The present study broadens the range of genetic variants described in aniridia and presents an aniridia patient with SRRRD.

ABE8e Corrects Pax6-Aniridic Variant in Humanized Mouse ESCs and via LNPs in Ex Vivo Cortical Neurons

INTRODUCTION

Aniridia is a rare congenital vision-loss disease caused by heterozygous variants in the PAX6 gene. There is no vision-saving therapy, but one exciting approach is to use CRISPR/Cas9 to permanently correct the causal genomic variants. Preclinical studies to develop such a therapy in animal models face the challenge of showing efficacy when binding human DNA. Thus, we hypothesized that a CRISPR gene therapy can be developed and optimized in humanized mouse embryonic stem cells (ESCs) that will be able to distinguish between an aniridia patient variant and nonvariant chromosome and lay the foundation for human therapy.

METHODS

To answer the challenge of binding human DNA, we proposed the "CRISPR Humanized Minimally Mouse Models" (CHuMMMs) strategy. Thus, we minimally humanized Pax6 exon 9, the location of the most common aniridia variant c.718C > T. We generated and characterized a nonvariant CHuMMMs mouse, and a CHuMMMs cell-based disease model, in which we tested five CRISPR enzymes for therapeutic efficacy. We then delivered the therapy via lipid nanoparticles (LNPs) to alter a second variant in ex vivo cortical primary neurons.

RESULTS

We successfully established a nonvariant CHuMMMs mouse and three novel CHuMMMs aniridia cell lines. We showed that humanization did not disrupt Pax6 function in vivo, as the mouse showed no ocular phenotype. We developed and optimized a CRISPR therapeutic strategy for aniridia in the in vitro system, and found that the base editor, ABE8e, had the highest correction of the patient variant at 76.8%. In the ex vivo system, the LNP-encapsulated ABE8e ribonucleoprotein (RNP) complex altered the second patient variant and rescued 24.8% Pax6 protein expression.

CONCLUSION

We demonstrated the usefulness of the CHuMMMs approach, and showed the first genomic editing by ABE8e encapsulated as an LNP-RNP. Furthermore, we laid the foundation for translation of the proposed CRISPR therapy to preclinical mouse studies and eventually patients with aniridia.

PAX6 disease models for aniridia

Aniridia is a pan-ocular genetic developmental eye disorder characterized by complete or partial iris and foveal hypoplasia, for which there is no treatment currently. Progressive sight loss can arise from cataracts, glaucoma, and aniridia-related keratopathy, which can be managed conservatively or through surgical intervention. The vast majority of patients harbor heterozygous mutations involving the PAX6 gene, which is considered the master transcription factor of early eye development. Over the past decades, several disease models have been investigated to gain a better understanding of the molecular pathophysiology, including several mouse and zebrafish strains and, more recently, human-induced pluripotent stem cells (hiPSCs) derived from aniridia patients. The latter provides a more faithful cellular system to study early human eye development. This review outlines the main aniridia-related animal and cellular models used to study aniridia and highlights the key discoveries that are bringing us closer to a therapy for patients.