Genetic insights into Tietz albinism-deafness syndrome: A new dominant-negative mutation in MITF.

Tietz albinism-deafness syndrome (TADS) is a rare and severe manifestation of Waardenburg syndrome that is primarily linked to mutations in MITF. In this report, we present a case of TADS resulting from a novel c.637G>C mutation in MITF (p.Glu213Gln; GenBank Accession number: NM_000248). A 3-year-old girl presented with congenital generalized hypopigmentation of the hair, skin, and irides along with complete sensorineural hearing loss. Histopathological and electron microscopy investigations indicated that this variant did not alter the number of melanocytes in the skin but significantly impaired melanosome maturation within melanocytes. Comprehensive melanin analysis revealed marked reductions in both eumelanin (EM) and pheomelanin (PM) rather than changes in the EM-to-PM ratio observed in oculocutaneous albinism. We conducted an electrophoretic mobility shift assay to investigate the binding capability of the identified variant to DNA sequences containing the E-box motif along with other known variants (p.Arg217del and p.Glu213Asp). Remarkably, all three variants exhibited dominant-negative effects, thus providing novel insights into the pathogenesis of TADS. This study sheds light on the genetic mechanisms underlying TADS and offers a deeper understanding of this rare condition and its associated mutations in MITF.

Impact of a SLC24A5 variant on the retinal pigment epithelium of a Japanese patient with oculocutaneous albinism type 6.

Oculocutaneous albinism (OCA) 6 is a non-syndromic type of OCA that has distinct ocular symptoms and variable cutaneous hypopigmentation. The causative gene of OCA6 is SLC24A5, which encodes NCKX5, a K+ -dependent Na+ /Ca2+ exchanger 5. NCKX5 is involved in the maturation of melanosomes, but its function is still unclear. In this study, we characterized a Japanese patient with OCA6. Genetic analysis revealed compound heterozygous variants in SLC24A5, c.590 + 1dupG, and c.598G>A (p.G200R). To clarify the functional significance of the missense variant, we generated a knock-in (KI) mouse model carrying the mouse homolog of the G200R variant using the CRISPR/Cas9 system. Chemical analysis showed decreased amounts of eumelanin in the hair and skin of KI mice, while levels of benzothiazine units in pheomelanin were significantly increased in their hair. Retinal pigment was also decreased in KI mice. Notably, a histopathologic study revealed a significant pigment loss in the retinal pigment epithelium (RPE) but not in the choroid. Immunohistochemically, the expression of NCKX5 in the RPE was decreased but was maintained in the choroid of KI mice. These findings could explain the difference in phenotypic severity between eye symptoms and hypopigmentation in the skin/hair.

Expression of discoidin domain receptor 1 and E-cadherin in epidermis affects melanocyte behavior in rhododendrol-induced leukoderma mouse model.

Vitiligo is a depigmentation disease characterized by gradual loss of melanin and melanocytes from the epidermis. The mechanism of melanocyte loss is not yet known. In this report, we showed that the expression of discoidin domain receptor 1 and E-cadherin, known adhesion molecules, was variable or absent in the epidermis of rhododendrol-induced leukoderma (RDIL) mice during the depigmentation process. Our findings suggest that melanocyte damage by rhododendrol promotes reduction of adhesion molecules not only in melanocytes but also in keratinocytes, and this is associated with the detachment of melanocytes from the basal layer.

Genome-wide association study identifies CDH13 as a susceptibility gene for rhododendrol-induced leukoderma.

Racemic RS-4-(4-hydroxyphenyl)-2-butanol (rhododendrol; trade name: Rhododenol [RD]), which is used in topical skin-lightening cosmetics, was unexpectedly reported in Japan to induce leukoderma or vitiligo called RD-induced leukoderma (RIL) after repeated application. To our knowledge, no studies have investigated chemical-induced vitiligo pathogenesis on a genome-wide scale. Here, we conducted a genome-wide association study (GWAS) for 147 cases and 112 controls. CDH13, encoding a glycosylphosphatidylinositol-anchored protein called T-cadherin (T-cad), was identified as the strongest RIL susceptibility gene. RD sensitivity was remarkably increased by T-cad knockdown in cultured normal human melanocytes. Furthermore, we confirmed tyrosinase upregulation and downregulation of the anti-apoptotic molecules (BCL-2 and BCL-XL), suggesting that T-cad is associated with RD via tyrosinase or apoptotic pathway regulation. Finally, monobenzyl ether of hydroquinone sensitivity also tended to increase with T-cad knockdown, suggesting that the T-cad could be a candidate susceptibility gene for RIL and other chemical-induced vitiligo forms. This is the first GWAS for chemical-induced vitiligo, and it could be a useful model for studying the disease's genetic aspects.

NGS-based targeted resequencing identified rare subtypes of albinism: Providing accurate molecular diagnosis for Japanese patients with albinism.

Albinism, which is commonly inherited as an autosomal recessive trait, is characterized by a reduction or absence of melanin in the eyes, skin, and hair. To date, more than 20 causal genes for albinism have been identified; thus, the accurate diagnosis of albinism requires next-generation sequencing (NGS). In this study, we analyzed 46 patients who tested negative for oculocutaneous albinism (OCA)1-4 and Hermansky-Pudlak syndrome (HPS)1 based on conventional analysis, in addition to 28 new Japanese patients, using NGS-based targeted resequencing. We identified a genetic background for albinism in 18 of the 46 patients (39%), who were previously tested negative according to the conventional analysis. In addition, we unveiled a genetic predisposition toward albinism in 23 of the 28 new patients (82%). We identified six patients with rare subtypes of albinism, including HPS3, HPS4, and HPS6, and found 12 novel pathological mutations in albinism-related genes. Furthermore, most patients who were not diagnosed with albinism by the NGS analysis showed mild manifestations of albinism without apparent eye symptoms and harbored only one heterozygous mutation, occasionally in combination with skin-color associated gene variants.

A 4-bp deletion promoter variant (rs984225803) is associated with mild OCA4 among Japanese patients.

Oculocutaneous albinism (OCA) type 4 is one of the most common types of albinism among Japanese population. In some patients who were clinically diagnosed with OCA, we have found a heterozygous pathological mutation in the coding region of SLC45A2, the gene responsible for OCA4, not leading to a DNA-based diagnosis. In this study, we evaluated pathological variants in the promoter region of SLC45A2 in these patients. The results indicated that the majority of the patients had a 4-bp deletion in the said region (c.-492_489delAATG; GenBank accession number: NM_016180; rs984225803) in the contralateral allele. These patients displayed a mild phenotype, especially regarding eye manifestations. The results of the luciferase reporter assay and electrophoretic mobility shift assay supported the pathological role of the variant. In addition, four of 220 alleles in Japanese normal control subjects also showed the deletion variant, indicating that this variant could possibly be a skin color-associated variant.

Leukoderma induced by rhododendrol is different from leukoderma of vitiligo in pathogenesis: A novel comparative morphological study.

BACKGROUND: Rhododendrol (rhododenol), an inhibitor of tyrosinase activity, is used as a skin-whitening component. Many cases of leukoderma after the application have been reported, termed rhododenol-induced leukoderma (RIL). The aim of this study was to clarify the pathogenesis of RIL morphologically through comparison with vitiligo. METHODS: We examined 14 cases of RIL and 15 cases of vitiligo using routine histopathology and immunohistochemistry. Thirteen cases of RIL, six cases of vitiligo and specimens of the RIL mouse model were evaluated by electron microscopy. RESULTS: There were common findings in RIL and vitiligo at the light-microscopic level: (a) vacuolar changes in the dermo-epidermal junction, (b) melanophages in the papillary dermis, (c) perifollicular lymphocyte infiltration, (d) loss or decrease of basal melanin pigment and (e) decrease of melanocytes in the lesions. The ultrastructural observations showed specific findings of RIL: (a) remaining melanocytes in depigmented lesions, (b) inhomogeneous melanization in melanocytes and (c) degenerated melanosomes in melanocytes. Some of the findings were observed in a RIL mouse model. Furthermore, it is notable that cell organelles of melanocytes were intact in our RIL cases. CONCLUSION: Morphological changes of RIL targeting melanosomes in melanocytes without degeneration of organelles reflect the reversible clinical course of most cases.

Characterization of melanosomes and melanin in Japanese patients with Hermansky-Pudlak syndrome types 1, 4, 6, and 9.

Hermansky-Pudlak syndrome (HPS) is an autosomal recessive disorder characterized by oculocutaneous albinism (OCA), a bleeding tendency, and ceroid deposition. Most of the causative genes for HPS encode subunits of the biogenesis of lysosome-related organelles complex (BLOC). In this study, we identified one patient each with HPS4, HPS6, and HPS9 by whole-exome sequencing. Next, we analyzed hair samples from the three patients and representative patients with HPS1 and controls using electron microscopy and chemical methods. All HPS patients had fewer, smaller, and more immature melanosomes than healthy controls. Further, all patients showed reduced total melanin content and increased levels of benzothiazine-type pheomelanin. The results of this study demonstrate the impact of the dysfunctions of BLOCs on the maturation of melanosomes and melanin levels and composition through analysis of their hair samples.

Microsatellite polymorphism located immediately upstream of the phosphatidylinositol glycan, class K gene (PIGK) affects its expression, which correlates with tyrosinase activity in human melanocytes.

BACKGROUND: Glycosylphosphatidylinositol (GPI) acts as a membrane anchor and a post-translational modifier for more than 150 proteins (called GPI-anchored proteins: GPI-APs). However, little study has been done to explore the role of GPI-APs in melanocytes. METHODS: The relationship between the mRNA expression of the genes which play essential roles in GPI anchoring system [phosphatidylinositol glycan, class A, and class K gene (PIGA, PIGK)] and melanogenesis-related genes (MITF, TYRP1, TYRP2, and TYR) as well as DOPA oxidase activities were evaluated in 13 different normal human epidermal melanocytes (NHEMs). A short tandem repeat (STR) polymorphism located in the predicted promoter region of PIGK was genotyped in the NHEMs. RNA interference experiment of PIGK was also conducted using one of the NHEMs. RESULTS: PIGK mRNA expression in NHEMs were strongly in inverse correlation with TYR mRNA and DOPA oxidase activities. NHEMs with the STR polymorphism revealed a low level of PIGK expression. However, a transient knockdown of PIGK in NHEM failed to reveal significant changes in the expression of TYR mRNA and DOPA oxidase activity. CONCLUSIONS: This report firstly demonstrated that inadequate protein-GPI anchoring caused by suppression of PIGK might affect the expression or function of some GPI-APs associated with tyrosinase activity.