Pre-clinical and clinical studies of NTBC and other compounds as potential treatments for albinism

National Eye Institute

Investigators

Linked publications, trials & patents

Abstract

1. Effect of NTBC in mouse models of OCA 2, OCA3 and OCA4 Our previously-published work had established that nitisinone (NTBC) can increase melanin pigmentation in a mouse model of OCA1B (Onojafe et al., Invest Ophthalmol Vis Sci. 2018). However, treatment of a mouse model of OCA3 with oral NTBC does not have a favorable effect on melanin production and minimally affects the number of pigmented melanosomes in the iris stroma (Onojafe et al., Invest Ophthalmol Vis Sci. 2019). We further hypothesized that NTBC might improve melanization in mouse models of OCA2 (melanocyte-specific transporter protein) and of OCA4 (the so-called "underwhite" allele of SLC45A2). Plasma tyrosine concentration was increased in NTBC-treated OCA2 and OCA4 mice after a month of nitisinone treatment with no overt toxic side-effects. Fur pigmentation was augmented in both mouse lines. Iris pigmentation was augmented predominantly in OCA4 mice. Electron microscopy also confirmed a small increase in choroidal pigmentation, whereas, no effect was observed in the RPE in both lines (manuscript in preparation). 2. High-throughput drug screening to identify compounds that regulate Tyr activity We used purified, truncated Tyr protein (previously tested and validated to have equivalent enzymatic activity to full length protein) in a high-throughput drug screening. In collaboration with NCATS, we screened 34,000 compounds from the Genesis Drug Collection, the Natural Products Library, and the NCATS Pharmaceutical Collection. We identified >100 new inhibitors and a few activators of tyrosinase. After validation in a secondary enzymatic screen in vitro and in zebrafish in vivo, we have tested three doses of the top candidate compound on the OCA1B mouse model. The drug was well tolerated when administered i.p. for 30 days. Preliminary analysis indicated a modest increase in hair melanin in treated mice, but no convincing changes in eye melanin. 3. In vitro disease-in-a-dish modeling of OCA We developed a disease-in-a-dish model of oculocutaneous albinism type 1A (OCA1A) and type 2 (OCA2) using induced pluripotent stem cell (iPSC) technology. OCA patient fibroblasts were reprogrammed to iPSCs and differentiated to retinal pigment epithelium (OCA-RPE) using a developmentally-guided differentiation protocol. Tissue morphology and physiology as well as expression and functionality of the visual cycle apparatus were comparable to control pigmented iPSC-derived RPE. Furthermore, pigmentation defects comprising immature melanosomes in the OCA-RPE cells in vitro faithfully replicated tissue characteristics in vivo. We are currently testing gene replacement and CRISPR-mediated gene editing approaches with the goal to revert the phenotype and restore pigmentation in OCA-RPE. 4. Suprachoroidal gene replacement therapy We are currently conducting pilot studies of suprachoroidal delivery of an AAV-tyrosinase gene therapy construct in a rat model of oculocutaneous albinism, type 1.