Interaction between G Protein-Coupled Receptor 143 and Tyrosinase: Implications for Understanding Ocular Albinism Type 1.

Developmental eye defects in X-linked ocular albinism type 1 are caused by G-protein coupled receptor 143 (GPR143) mutations. Mutations result in dysfunctional melanosome biogenesis and macromelanosome formation in pigment cells, including melanocytes and retinal pigment epithelium. GPR143, primarily expressed in pigment cells, localizes exclusively to endolysosomal and melanosomal membranes unlike most G protein-coupled receptors, which localize to the plasma membrane. There is some debate regarding GPR143 function and elucidating the role of this receptor may be instrumental for understanding neurogenesis during eye development and for devising therapies for ocular albinism type I. Many G protein-coupled receptors require association with other proteins to function. These G protein-coupled receptor-interacting proteins also facilitate fine-tuning of receptor activity and tissue specificity. We therefore investigated potential GPR143 interaction partners, with a focus on the melanogenic enzyme tyrosinase. GPR143 coimmunoprecipitated with tyrosinase, while confocal microscopy demonstrated colocalization of the proteins. Furthermore, tyrosinase localized to the plasma membrane when coexpressed with a GPR143 trafficking mutant. The physical interaction between the proteins was confirmed using fluorescence resonance energy transfer. This interaction may be required in order for GPR143 to function as a monitor of melanosome maturation. Identifying tyrosinase as a potential GPR143 binding protein opens new avenues for investigating the mechanisms that regulate pigmentation and neurogenesis.

Chediak-Higashi syndrome with progressive visual loss.

BACKGROUND: The change of visual function in Chediak-Higashi syndrome has not been well described. CASES: The visual function of a 12-year-old Japanese girl with ocular albinism due to Chediak-Higashi syndrome was followed by periodic ophthalmological examinations. OBSERVATIONS: A lack of pigmentation in the iris and ocular fundus, and pigmentary degeneration of the peripheral retina were observed. The visual loss and the constriction of visual field progressed with increasing age. The electroretinogram was extinguished at 12 years of age. CONCLUSIONS: The constriction of the visual field may be due to the pigmentary degeneration of the ocular fundus. Ophthalmologists should be aware that progressive visual loss and the constriction of visual field can occur in patients with Chediak-Higashi syndrome as they grow older.

Clinical findings in Japanese patients with Waardenburg syndrome type 2.

PURPOSE: To determine the visual characteristics of Japanese subjects with the Waardenburg syndrome type 2. METHODS: The visual functions of 11 albino patients who were identified from the screening of 240 children attending a school for children with a hearing deficit were studied. The ophthalmological examinations included eye position, visual acuity, biomicroscopy, ophthalmoscopy, visual field by confrontation or Goldmann's perimetry, stereoacuity by the Titmus test, and color vision by the Ishihara pseudoisochromatic plates. RESULTS: A combination of congenital sensory deafness and partial ocular albinism without lateral displacement of the lacrimal puncta was observed in 11 (4.6%) of the students with hearing deficit. All these children had sectorial heterochromia irides with local retinal hypopigmentation. Lid deformities were not present. The retinal vasculature was normal, and macular hypoplasty was not found. Other than 1 eye with hyperopic amblyopia, no serious visual disturbance was found in these patients. CONCLUSIONS: The 11 students were classified as having Waardenburg syndrome type 2. None had a critical visual deficit, and all had partial heterochromia irides and retinal hypopigmentation.

Apparent digenic inheritance of Waardenburg syndrome type 2 (WS2) and autosomal recessive ocular albinism (AROA).

Waardenburg syndrome (WS) is a clinically and genetically heterogeneous disease accounting for >2% of the congenitally deaf population. It is characterized by deafness in association with pigmentary anomalies and various defects of neural crest-derived tissues. At least four types are recognized (WS1, WS2, WS3 and WS4) on the basis of clinical and genetic criteria. Two previously described families seemed to delineate a new subtype characterized by WS2 in conjunction with ocular albinism (OA). Since mutations in the MITF gene are responsible for some instances of WS2, we screened for mutations in one of the WS2-OA families and discovered a 1 bp deletion in exon 8 of MITF. OA previously has been associated with compound heterozygosity for a mutant TYR allele and the TYR(R402Q) allele, a functionally significant polymorphism that is associated with moderately reduced tyrosinase catalytic activity. In this family, all of the individuals with the OA phenotype are either homozygous or heterozygous for TYR(R402Q), and heterozyous for the 1 bp deletion in MITF This suggests that the WS2-OA phenotype may result from digenic interaction between a gene for a transcription factor (MITF) and a gene that it regulates (TYR).