Oculocutaneous albinism type 1A (OCA1A) is a rare recessive genetic condition caused by mutations in TYROSINASE (TYR) that results in pigmentation defects of the skin, hair and eyes. This study was performed to understand melanosome biogenesis and maturation defects in an OCA1A in vitro model using retinal pigment epithelium (RPE) derived from TYR knockout human induced pluripotent stem cells (iPSC). CRISPR-Cas9 was used to knockout the TYR gene in iPSC to generate an isogenic pair. A developmentally guided protocol was used to differentiate the isogenic iPSC pair towards RPE monolayer tissue. Monolayer organization, melanosome formation and maturation were studied using electron microscopy. Loss of TYR protein was studied using Western blot and immuno-fluorescence staining. RPE cellular morphology and junction integrity was studied using immunofluorescence staining and transepithelial resistance measurements. An isogenic pair comprising of untargeted control and TYR knockout iPSC were successfully differentiated towards RPE monolayer tissue with polygonal cell morphology. TYR knockout RPE exhibited significantly reduced TYR protein, increased presence of immature pre-melanosomes and a complete lack of mature melanosomes. We observed abnormal junctional localization of β-catenin staining pattern, as has been reported previously for albino mouse RPE- and OCA1A patient-derived RPE. Differentiation of TYR-deficient iPSC toward RPE displayed pigmentation defects and absence of mature melanosomes, whereas melanosome biogenesis was not affected, because pre-melanosomes were still observed. These observations were also similar to what was observed in OCA1A patient-derived RPE monolayer tissue, independently confirming the validity of these previous findings.

Oculocutaneous albinism type IA (OCA1A) is a rare autosomal recessive disorder caused by variants in the TYR gene, resulting in complete loss of tyrosinase activity and absence of melanin production. In this study, we report a novel missense variant, TYR (NM_000372.5):c.143A>C (p.Gln48Pro), found within exon 1 and mapped to chr11: g.89178096A>C (GRCh38/hg38). This variant was identified through exome sequencing in a 4-year-old Iranian girl from a consanguineous family presenting with features of OCA1A. This discovery expands the mutational spectrum of OCA1 and underscores the importance of genetic screening in diagnosing rare inherited disorders. Future studies involving functional assays are necessary to elucidate the molecular mechanisms of this variant and its potential impact on melanogenesis.

Purpose: Oculocutaneous albinism type 1A (OCA1A), with lifelong absent melanin in skin, hair, and eyes, is the most severe type of albinism with greatest ametropia and poorest vision. We evaluated the relationship between age when spectacles were begun and visual outcome, in addition to status of refraction, in OCA1A. Methods: After IRB approval, a retrospective review of 70 consecutive charts of patients with OCA1A identified 24 fitting inclusion criterion of BCVA recorded at age 10-12 years. Exclusion criteria were those with other vision-threatening diagnoses and patients seen for a single visit. We recorded sex, age at beginning glasses, and refraction and BCVA at age 10-12 and most recent visit. Data were arbitrarily grouped by those initiating glasses at ≤ age 12 months and > age 12 months. Results: Regression analysis showed a larger degree of astigmatism was weakly associated with worse vision at age 10-12 years. A weakly positive relationship was found between poorer BCVA at last visit and older age at which glasses were initiated. All receiving glasses by age 1 and only half receiving glasses when older had improved visual acuity from age 10-12 years to last follow up. Conclusion: Additional study of a larger sample of this rare disorder is needed to determine if early glasses wear improves later BCVA.

Precise single-base editing in Xenopus tropicalis would greatly expand the utility of this true diploid frog for modeling human genetic diseases caused by point mutations. Here, we report the efficient conversion of C-to-T or G-to-A in X. tropicalis using the rat apolipoprotein B mRNA editing enzyme catalytic subunit 1-XTEN-clustered regularly interspaced short palindromic repeat-associated protein 9 (Cas9) nickase-uracil DNA glycosylase inhibitor-nuclear localization sequence base editor [base editor 3 (BE3)]. Coinjection of guide RNA and the Cas9 mutant complex mRNA into 1-cell stage X. tropicalis embryos caused precise C-to-T or G-to-A substitution in 14 out of 19 tested sites with efficiencies of 5-75%, which allowed for easy establishment of stable lines. Targeting the conserved T-box 5 R237 and Tyr C28 residues in X. tropicalis with the BE3 system mimicked human Holt-Oram syndrome and oculocutaneous albinism type 1A, respectively. Our data indicate that BE3 is an easy and efficient tool for precise base editing in X. tropicalis.-Shi, Z., Xin, H., Tian, D., Lian, J., Wang, J., Liu, G., Ran, R., Shi, S., Zhang, Z., Shi, Y., Deng, Y., Hou, C., Chen, Y. Modeling human point mutation diseases in Xenopus tropicalis with a modified CRISPR/Cas9 system.