Aniridia is a rare congenital eye disorder occurring once in 40,000-100,000 births, with no influence of race or gender (Hingorani et al., 2012). Although named for the lack of iris, the disorder is actually panocular; affecting the whole eye. Consequently, vision loss is attributable to three main causes in disparate regions of the eye: 1) hypomorphic development of the retinal fovea, 2) progressive corneal clouding, and 3) progressive glaucoma. Of these vision-threatening phenotypes, glaucoma is the best managed, leaving hypomorphic fovea and progressive corneal clouding in need of a new vision-saving therapy. Two thirds of all aniridia cases are inherited through autosomal dominant transmission, with almost complete penetrance and variable expressivity, and one third of cases occur sporadically (Brauner et al., 2008; Ton et al., 1991). In 90% of aniridia cases, whether inherited or sporadic, mutations have been found in the transcription factor PAX6 (paired box gene 6) (Hingorani et al., 2012). Studies in human and mouse have shown that aniridia is a haploinsufficiency disorder; thus, loss of one functional-PAX6 allele reduces PAX6 (human protein) levels sufficiently to cause developmental damage to the eye (Glaser et al., 1992; Hill et al., 1991; Ton et al., 1991).
Our goal is to develop an rAAV (recombinant adeno-associated virus)-based gene therapy for aniridia. Our experiments are the first towards rAAV-based gene therapy for aniridia in mouse.
For augmentation gene therapy, we are delivering additional PAX6 encoded in rAAV directly to the mouse eye. Because the transcription factor PAX6 may need to be carefully regulated, we have designed MiniPromoters from the human PAX6 gene and tested them in the mouse eye (Hickmott et al., 2016 in revision). Successfully using PAX6 to treat aniridia will not only provide a treatment for this disorder, but further clear the path for other rare disorders, and for the first time open up many transcription factor diseases for gene therapy.
For genome-editing therapy, we are using the bacterial CRISPR (clustered regularly interspaced short palindromic repeats) /cas9 (CRISPR-associated nuclease 9) system. It will be delivered in rAAV to edit genomic DNA in vivo in a mouse model of aniridia. Thus, we will merge the safe and efficient access to variety of cell types achieved by AAV, with the precision of CRISPR/cas9 editing. This base-pair substitution strategy would be applicable to 62–86% of classical aniridia patients. The final therapy would be “precision medicine”, in that a different CRISPR/cas9 gene-editing assay would be needed for each patient.
Corso-Díaz, X., Borrie, A.E., Bonaguro, R.J., Schuetz, J.M., Rosenberg, T., Jensen, H., Brooks, B.P., MacDonald, I., Pasutto, F., Walter, M., Gronskov, K., Brooks-Wilson, A.R., and Simpson, E.M. Absence of NR2E1 mutations in Patients with Aniridia. Molecular vision 18:2770-2782. (2012) PMID 23213277