Molecular characterization of SLC24A5 variants and evaluation of Nitisinone treatment efficacy in a zebrafish model of OCA6

Abstract

Skin pigmentation is a highly heterogeneous trait with diverse consequences worldwide. SLC24A5, encoding a potent K+‐dependent Na+/Ca2+ exchanger, is among the known color‐coding genes that participate in melanogenesis by maintaining pH in melanosomes. Deficient SLC24A5 activity results in oculocutaneous albinism (OCA) type 6 in humans. In this study, by utilizing a exome sequencing (ES) approach, we identified two new variants [p. (Gly110Arg) and p. (IIe189Ilefs*1)] of SLC24A5 cosegregating with the OCA phenotype, including nystagmus, strabismus, foveal hypoplasia, albinotic fundus, and vision impairment, in three large consanguineous Pakistani families. Both of these variants failed to rescue the pigmentation in zebrafish slc24a5 morphants, confirming the pathogenic effects of the variants. We also phenotypically characterized a commercially available zebrafish mutant line (slc24a5ko) that harbors a nonsense (p.Tyr208*) allele of slc24a5. Similar to morphants, homozygous slc24a5ko mutants had significantly reduced melanin content and pigmentation. Next, we used these slc24a5ko zebrafish mutants to test the efficacy of nitisinone, a compound known to increase ocular and fur pigmentation in OCA1 (TYR) mutant mice. Treatment of slc24a5ko mutant zebrafish embryos with varying doses of nitisinone did not improve melanin production and pigmentation, suggesting that treatment with nitisinone is unlikely to be therapeutic in OCA6 patients.

UBA2 variants underlie a recognizable syndrome with variable aplasia cutis congenita and ectrodactyly

“…As wide phenotypic variability is observed in both fish and human UBA2 / uba2- related phenotypes, additional studies are warranted to define potential modifiers. Morpholinos (MOs) have been used in reverse genetic studies in a range of animal models 31 , 32 . However, MOs may be hard to interpret as they typically result in more severe phenotypes 33 .…”

In vitro disease modeling of oculocutaneous albinism type 1 and 2 using human induced pluripotent stem cell-derived retinal pigment epithelium

“…Animal models of various types of OCA are well characterized and recapitulate pigmentation defects robustly, shedding light on the cellular nature of these defects ( OCA1A : Onojafe et al, 2011 ; Zhou et al, 2015 ; Wu et al, 2020 ; OCA2 : Rinchik et al., 1993 ; Ishikawa et al., 2015 ; Wu et al, 2020 ; OCA3 : Onojafe et al, 2018 ; OCA4 : Winkler et al., 2014 ; OCA6 : Yousaf et al., 2020 ; OA1 : Incerti et al, 2000 ). Most of these studies are focused on melanosome biogenesis defects in melanocytes and very few on RPE defects.…”

Clinical and Mutation Spectrum of Autosomal Recessive Non-Syndromic Oculocutaneous Albinism (nsOCA) in Pakistan: A Review

“…OCA4 is frequent in the Japanese population with a rate of 24% of overall OCA followed by different countries in Asia and Europe. OCA type 5 is only reported in a consanguineous Pakistani family [ 22 ], while cases of OCA6 and OCA7 are reported in China, eastern India, and Atlantic Island [ 23 , 24 , 25 ], but the exact prevalence has not been documented. Syndromic OCA is also variable in prevalence.…”