Heterologous expression of tyrosinase recapitulates the misprocessing and mistrafficking in oculocutaneous albinism type 2: effects of altering intracellular pH and pink-eyed dilution gene expression

Novel compound heterozygous mutations in OCA2 gene were identified in a Chinese family with oculocutaneous albinism

BACKGROUND

Oculocutaneous albinism (OCA) is a group of rare autosomal recessive disorders characterized by clinical genetic heterogeneity. OCA type II (OMIM: 203200) is the most common subtype among African and African Americans, primarily caused by pathogenic variants in the OCA2 (HGNC ID: 8101) gene. In this study, we presented a Chinese family with OCA and reported two novel variants in the OCA2 gene.

METHODS

Whole-exome sequencing (WES) was performed to identify pathogenic variants in the proband. The candidate variants were subsequently validated using Sanger sequencing and QPCR assay. Additionally, bioinformatics analyses were employed to predict the deleteriousness and conservation of the identified mutations.

RESULTS

In the 16-year-old male proband, two novel compound heterozygous OCA2 variants, NM_000275.3: c.1640T>G (NP_000266.2: p.L547R) and an exons 10-19 deletion variant, were identified. Meanwhile, a reported heterozygous variant c.1441G>A/p.A481T (NM_000275.3, NP_000266.2) in the OCA2 gene was also found in the proband. Sanger sequencing confirmed that the two variants c.1441G>A/p.A481T and c.1640T>G/p.L547R were inherited from his father. Moreover, qPCR assay revealed that the exons 10-19 deletion was inherited from the mother, his sister also carried this variant. Fortunately, the variant was not detected in the amniotic fluid of the proband's sister. Multiple online bioinformatics tools predicted the variant c.1640T>G to be damaging, leading to the replacement of a highly conserved leucine with an arginine. The gross exon 10-19 deletion in the OCA2 gene resulted in a truncated, non-functional protein losing the 3-9 transmembrane α-helices domains. According to the American College of Medical Genetics and Genomics classification, these three variants in the OCA2 gene were evaluated as likely pathogenic.

CONCLUSION

This study has identified two novel compound variants in the OCA2 gene and a previously reported variant in a Chinese family with OCA. By expanding the mutation spectrum of the OCA2 gene, our findings contribute to a better understanding of the genetic basis of OCA.

Oculocutaneous albinism: epidemiology, genetics, skin manifestation, and psychosocial issues

Oculocutaneous albinism (OCA) is a group of rare, inherited disorders associated with reduced melanin biosynthesis. Clinical manifestations of the eight known subtypes of OCA include hypopigmented skin, eyes, and hair and ocular manifestations, such as decreased visual acuity and nystagmus. OCA affects people globally but is most prevalent in African countries. Individuals with oculocutaneous albinism lack UV protection and are prone to skin damage and skin cancers. For many African albino individuals, there are significant challenges in seeking treatment for skin cancer and preventing sun damage due to psychosocial factors and poor education. This review summarizes the current understanding of the epidemiology, genetics, and clinical manifestations of OCA. We also discuss the medical and psychosocial challenges that affect individuals with OCA and the current landscape of albinism treatment modalities. The extent of the psychosocial challenges needs to be better understood and additional educational interventions may improve quality of life for people with albinism.

A community-science approach identifies genetic variants associated with three color morphs in ball pythons (Python regius)

Color morphs in ball pythons (Python regius) provide a unique and largely untapped resource for understanding the genetics of coloration in reptiles. Here we use a community-science approach to investigate the genetics of three color morphs affecting production of the pigment melanin. These morphs-Albino, Lavender Albino, and Ultramel-show a loss of melanin in the skin and eyes, ranging from severe (Albino) to moderate (Lavender Albino) to mild (Ultramel). To identify genetic variants causing each morph, we recruited shed skins of pet ball pythons via social media, extracted DNA from the skins, and searched for putative loss-of-function variants in homologs of genes controlling melanin production in other vertebrates. We report that the Albino morph is associated with missense and non-coding variants in the gene TYR. The Lavender Albino morph is associated with a deletion in the gene OCA2. The Ultramel morph is associated with a missense variant and a putative deletion in the gene TYRP1. Our study is one of the first to identify genetic variants associated with color morphs in ball pythons and shows that pet samples recruited from the community can provide a resource for genetic studies in this species.

Mahogunin Ring Finger 1 regulates pigmentation by controlling the pH of melanosomes in melanocytes and melanoma cells

Mahogunin Ring Finger 1 (MGRN1) is an E3-ubiquitin ligase absent in dark-furred mahoganoid mice. We investigated the mechanisms of hyperpigmentation in Mgrn1-null melan-md1 melanocytes, Mgrn1-KO cells obtained by CRISPR-Cas9-mediated knockdown of Mgrn1 in melan-a6 melanocytes, and melan-a6 cells depleted of MGRN1 by siRNA treatment. Mgrn1-deficient melanocytes showed higher melanin content associated with increased melanosome abundance and higher fraction of melanosomes in highly melanized maturation stages III-IV. Expression, post-translational processing and enzymatic activity of the rate-limiting melanogenic enzyme tyrosinase measured in cell-free extracts were comparable in control and MGRN1-depleted cells. However, tyrosinase activity measured in situ in live cells and expression of genes associated with regulation of pH increased upon MGRN1 repression. Using pH-sensitive fluorescent probes, we found that downregulation of MGRN1 expression in melanocytes and melanoma cells increased the pH of acidic organelles, including melanosomes, strongly suggesting a previously unknown role of MGRN1 in the regulation of melanosomal pH. Among the pH regulatory genes upregulated by Mgrn1 knockdown, we identified those encoding several subunits of the vacuolar adenosine triphosphatase V-ATPase (mostly Atp6v0d2) and a calcium channel of the transient receptor potential channel family, Mucolipin 3 (Mcoln3). Manipulation of expression of the Mcoln3 gene showed that overexpression of Mcoln3 played a significant role in neutralization of the pH of acidic organelles and activation of tyrosinase in MGRN1-depleted cells. Therefore, lack of MGRN1 led to cell-autonomous stimulation of pigment production in melanocytes mostly by increasing tyrosinase specific activity through neutralization of the melanosomal pH in a MCOLN3-dependent manner.

Current landscape of Oculocutaneous Albinism in Japan

Oculocutaneous albinism (OCA), which is roughly divided into non-syndromic and syndromic OCA, is a group of autosomal recessive disorders caused by mutations in genes associated with pigmentation. Patients with OCA have hypopigmentation and ocular manifestations such as photophobia, amblyopia, and nystagmus. Hermansky-Pudlak syndrome (HPS), the most common syndromic OCA, is characterized by the additional features of a bleeding tendency and other critical systemic comorbidities such as pulmonary fibrosis and immunodeficiency. NGS-based gene analyses have identified several new causative genes for OCA and have detected rare subtypes of OCA with high accuracy including Japanese patients. In our survey of 190 Japanese OCA patients/families, OCA4 is the most common subtype (25.3%) followed by OCA1 (20.0%), HPS1 (14.7%), and OCA2 (8.4%). Similar to the A481T variant in OCA2, which is associated with a mild form of OCA2 and skin color variation, the c.-492_489delAATG variant located in the promoter region of SLC45A2 has been uniquely identified in Japanese patients with a mild form of OCA4. Further, rare OCA subtypes, including OCA3, HPS2, HPS3, HPS4, HPS5, HPS6, and HPS9, have also been identified in Japanese patients. The clinical characteristics and underlying molecular mechanisms of each subtype of OCA are concisely summarized in this review.

Membrane transport proteins in melanosomes: Regulation of ions for pigmentation

Melanosomes are unique organelles in melanocytes that produce melanin, the pigment for skin, hair, and eye color. Tyrosinase is the essential and rate-limiting enzyme for melanin production, that strictly requires neutral pH for activity. pH maintenance is a result of the combinational function of multiple ion transport proteins. Thus, ion homeostasis in melanosomes is crucial for melanin synthesis. Defect of the ion transport system causes various pigmentation phenotypes, from mild effect to severe disorders such as albinism. In this review, we summarize the up-to-date knowledge of the ion transport system, such as transport function, structure, and the physiological roles and mechanisms of the ion transport proteins in melanosomes. In addition, we propose a model of melanosomal ion transport system-how the functional coupling of multiple transport proteins modulates and maintains ion homeostasis. We discuss melanin synthesis in terms of the ion transport system.

Uveal melanoma

Uveal melanoma (UM) is the most common primary intraocular malignancy in adults. UMs are usually initiated by a mutation in GNAQ or GNA11, unlike cutaneous melanomas, which usually harbour a BRAF or NRAS mutation. The annual incidence in Europe and the USA is ~6 per million population per year. Risk factors include fair skin, light-coloured eyes, congenital ocular melanocytosis, ocular melanocytoma and the BAP1-tumour predisposition syndrome. Ocular treatment aims at preserving the eye and useful vision and, if possible, preventing metastases. Enucleation has largely been superseded by various forms of radiotherapy, phototherapy and local tumour resection, often administered in combination. Ocular outcomes are best with small tumours not extending close to the optic disc and/or fovea. Almost 50% of patients develop metastatic disease, which usually involves the liver, and is usually fatal within 1 year. Although UM metastases are less responsive than cutaneous melanoma to chemotherapy or immune checkpoint inhibitors, encouraging results have been reported with partial hepatectomy for solitary metastases, with percutaneous hepatic perfusion with melphalan or with tebentafusp. Better insight into tumour immunology and metabolism may lead to new treatments.

Novel compound heterozygous mutations in OCA2 gene associated with non-syndromic oculocutaneous albinism in a Chinese Han patient: a case report

Background

Oculocutaneous albinism (OCA) is a group of rare genetically heterogeneous disorders. The present study aimed to identify the genetic cause of a Chinese Han family with non-syndromic oculocutaneous albinism (OCA).

Case presentation

Here, we report an 11-month-old male proband from a Chinese Han non-consanguineous family, who presented with milky skin, yellow white hair, nystagmus, astigmatism, and hypermetropia. We performed the targeted next-generation sequencing (NGS) on the proband and identified two novel compound heterozygous variants (c.1865 T > C (p.Leu622Pro) and exons 17–21 deletion) in OCA2 gene associated with OCA type 2 (OCA2, OMIM 203200). Meanwhile, a previously reported heterozygous mutation (c.4805G > A) in MYO7 gene related with Usher syndrome type 1B was found. The online tools SIFT, PolyPhen-2, and Mutation Taster predicted variant c.1865 T > C was probably damaging. The residue p.Leu622 was in a highly conserved region among species by CLUSTALW. Three-dimensional homology model with I-TASSER indicated that p.Leu622Pro variant disturbed the formation of the α-helix, resulting in a random coil structure. The gross deletion (exons 17–21) in OCA2 gene has was not been reported previously. These two novel variants in OCA2 gene were inherited from each parent respectively, after verification by Sanger sequencing and quantitative PCR (qPCR) in the family.

Conclusions

This study indicates the two novel compound heterozygous mutations in OCA2 gene may be responsible for clinical manifestations of OCA2. It expands the mutation spectrum of OCA2 gene and is helpful to screen for large deletions with targeted NGS protocol in monogenic disease. It also assists the genetic counselling, carrier screening and personalized healthcare of the disease.

Electronic supplementary material

The online version of this article (10.1186/s12881-019-0850-7) contains supplementary material, which is available to authorized users.

[Oculocutaneous and ocular albinism]

Albinism can be divided into oculocutaneous albinism (OCA) and ocular albinism (OA). In the differential diagnostics these can be distinguished from rarer syndromes with partial albinism, which are frequently associated with susceptibility to infections and neurological symptoms. The OCA is an autosomal recessive inherited disease of melanin biosynthesis, which leads to complete or partial loss of melanin in the skin, hair follicles and eyes. Of the seven currently known subtypes (OCA 1-7), four are well-characterized (OCA 1-4). These are based on gene mutations, which code for tyrosinase, a key enzyme in melanin synthesis and for further proteins. These play an important role in the catalytic activity of tyrosinase and the structure and function of melanosomes. In the presence of these subtypes, the clinical symptoms and the course of the disease show a pronounced variability, especially in the type and extent of pigmentation of the skin and hair as well as the severity of eye involvement, which makes the phenotypic classification difficult. Treatment priorities are a consistent protection from UV light for prophylaxis against skin cancer and regular preventive investigations. The ocular alterations typical for albinism necessitate timely diagnostics and care by institutions specialized in ophthalmology. Novel strategies for systemic treatment of subtypes of albinism are in preclinical testing. The OA without skin involvement shows X‑linked inheritance, is much rarer and is characterized by reduced pigmentation of the retina and iris, nystagmus and macular hypoplasia, sometimes with substantial loss of visual acuity. The typical ocular symptoms of OA can be manifested to a varying extent in all forms of OCA.

OCA2 rs4778137 polymorphism predicts survival of breast cancer patients receiving neoadjuvant chemotherapy

BACKGROUND

Genome-wide association study (GWAS) studies have showed that single nucleotide polymorphisms (SNPs) in OCA2 gene were associated with the survival of breast cancer patients treated with adjuvant chemotherapy. To further explain the association between OCA2 SNPs and breast cancer survival, we investigated the predictive value of rs4778137 located in OCA2 in local advanced breast cancer patients receiving neoadjuvant chemotherapy.

PATIENTS AND METHODS

A case-cohort with 150 breast cancer patients was performed to evaluate the effects of the OCA2 rs4778137 on breast cancer survival. The association between rs4778137 genotypes and pathological complete response (pCR, defined that the postoperative pathology indicating no residual invasive breast cancer in the breast or the axillary lymph node) were analyzed. Logistic regression analysis was performed to identify the independent predictors of pCR. Survival was assessed by Kaplan-Meier method and Cox regression analysis according to the rs4778137 genotypes.

RESULTS

The differences between pCR and the rs4778137 genotypes were statistically significant (p < 0.05). The patients with genotype GG harbored a better disease-free survival (HR: 2.358, p = 0.000) and overall survival (HR: 1.578, p = 0.008) than the patients with genotype CC in rs4778137. The further Univariate and Multivariate survival analysis revealed that SNP rs4778137 was an independent predictive factor of disease-free survival (p = 0.000/p = 0.001) and overall survival (p = 0.006/p = 0.045).

CONCLUSION

The OCA2 rs4778137 may be a predictor for the clinical response and survival in local advanced breast cancer patients who received neoadjuvant chemotherapy.

An intracellular anion channel critical for pigmentation

Intracellular ion channels are essential regulators of organellar and cellular function, yet the molecular identity and physiological role of many of these channels remains elusive. In particular, no ion channel has been characterized in melanosomes, organelles that produce and store the major mammalian pigment melanin. Defects in melanosome function cause albinism, characterized by vision and pigmentation deficits, impaired retinal development, and increased susceptibility to skin and eye cancers. The most common form of albinism is caused by mutations in oculocutaneous albinism II (OCA2), a melanosome-specific transmembrane protein with unknown function. Here we used direct patch-clamp of skin and eye melanosomes to identify a novel chloride-selective anion conductance mediated by OCA2 and required for melanin production. Expression of OCA2 increases organelle pH, suggesting that the chloride channel might regulate melanin synthesis by modulating melanosome pH. Thus, a melanosomal anion channel that requires OCA2 is essential for skin and eye pigmentation.

Ion transport in pigmentation

Skin melanocytes and ocular pigment cells contain specialized organelles called melanosomes, which are responsible for the synthesis of melanin, the major pigment in mammals. Defects in the complex mechanisms involved in melanin synthesis and regulation result in vision and pigmentation deficits, impaired development of the visual system, and increased susceptibility to skin and eye cancers. Ion transport across cellular membranes is critical for many biological processes, including pigmentation, but the molecular mechanisms by which it regulates melanin synthesis, storage, and transfer are not understood. In this review we first discuss ion channels and transporters that function at the plasma membrane of melanocytes; in the second part we consider ion transport across the membrane of intracellular organelles, with emphasis on melanosomes. We discuss recently characterized lysosomal and endosomal ion channels and transporters associated with pigmentation phenotypes. We then review the evidence for melanosomal channels and transporters critical for pigmentation, discussing potential molecular mechanisms mediating their function. The studies investigating ion transport in pigmentation physiology open new avenues for future research and could reveal novel molecular mechanisms underlying melanogenesis.

Pigmented-MDCK (P-MDCK) cell line with tunable melanin expression: an in vitro model for the outer blood-retinal barrier

Retinal pigment epithelium, which forms the outer blood-retinal barrier, is a critical barrier for transport of drugs to the retina. The purpose of this study was to develop a pigmented MDCK (P-MDCK) cell line as a rapidly established in vitro model for the outer blood-retinal barrier to assess the influence of melanin pigment on solute permeability. A melanin synthesizing P-MDCK cell line was developed by lentiviral transduction of human tyrosinase and p-protein genes in MDCK (NBL-2) cells. Melanin content, tyrosinase activity (conversion of L-dopa to dopachrome), and transepithelial electrical resistance (TEER) were measured. Expression of tyrosinase protein and p-protein in P-MDCK cells was confirmed by confocal microscopy. Effect of l-tyrosine (0 to 2 mM) in culture medium on melanin synthesis in P-MDCK cells was evaluated. Cell uptake and transepithelial transport of pigment-binding chloroquine (Log D = 1.59) and a negative control salicylic acid (Log D = -1.14) were investigated. P-MDCK cells expressed tyrosinase and p-protein. Tyrosinase activity was 4.5-fold higher in P-MDCK cells compared to wild type MDCK cells. The transepithelial electrical resistance stabilized by day 4 in both cell types, with the TEER being 958 ± 33 and 964 ± 58 Ω·cm(2) for P-MDCK and wild type cells, respectively. Melanin content in P-MDCK cells depended on the concentration of l-tyrosine in culture medium, and increased from 3 to 54 μg/mg protein with an increase in l-tyrosine content from 0 to 2 mM. When the cells were grown in 2 mM l-tyrosine, uptake of chloroquine was 2.3-fold higher and the transepithelial transport was 2.2-fold lower in P-MDCK cells when compared to wild type MDCK cells. No significant difference was observed for both cell uptake and transport of salicylic acid. We developed a P-MDCK cell line with tunable melanin synthesis as a rapidly developing surrogate for retinal pigment epithelium.

Human hair melanins: what we have learned and have not learned from mouse coat color pigmentation

Hair pigmentation is one of the most conspicuous phenotypes in humans. Melanocytes produce two distinct types of melanin pigment: brown to black, indolic eumelanin and yellow to reddish brown, sulfur-containing pheomelanin. Biochemically, the precursor tyrosine and the key enzyme tyrosinase and the tyrosinase-related proteins are involved in eumelanogenesis, while only the additional presence of cysteine is necessary for pheomelanogenesis. Other important proteins involved in melanogenesis include P protein, MATP protein, α-MSH, agouti signaling protein (ASIP), MC1R (the receptor for MSH and ASIP), and SLC7A11, a cystine transporter. Many studies have examined the effects of loss-of-function mutations of those proteins on mouse coat color pigmentation. In contrast, much less is known regarding the effects of mutations of the corresponding proteins on human hair pigmentation except for MC1R polymorphisms that lead to pheomelanogenesis. This perspective will discuss what we have/have not learned from mouse coat color pigmentation, with special emphasis on the significant roles of pH and the level of cysteine in melanosomes in controlling melanogenesis. Based on these data, a hypothesis is proposed to explain the diversity of human hair pigmentation.

Molecular cloning, sequence characteristics, and polymorphism analyses of the tyrosinase-related protein 2 / DOPAchrome tautomerase gene of black-boned sheep (Ovis aries)

Tyrosinase-related protein 2 (TYRP2) plays a pivotal role in the biosynthesis of eumelanin. Black-boned sheep have excessive melanin and eumelanin, resulting in dark (black) muscles and organs. This study was designed to investigate the effects of variants of the TYRP2 gene on black traits and coat colour of black-boned sheep. Melanin traits were measured in three populations of sheep (Nanping black-boned, Nanping normal, and Romney Marsh) and compared in this study. From the TYRP2 cDNA, all 8 exons and their flanking regions were amplified and characterized. Fifteen single nucleotide polymorphisms (SNPs) were identified in the exons and their flanking regions. Five exonic polymorphic sites, including two synonymous (c.93T>G and c.1140C>T) and three non-synonymous mutations (c.163C>T (p.R55W), c.605G>A (p.R202H), and c.1141A>G (p.T381A)), were retrieved. PCR-RFLP analysis of c.605G>A showed that the frequencies of allele G in the Nanping black-boned, Nanping normal, and Romney Marsh sheep were 0.632, 0.603, and 0.886, respectively. Sheep with the GG genotype had significantly (P < 0.05) lower tyrosinase activity, alkali-soluble melanin content, and ratio of eumelanin : total melanin than sheep with GA and AA genotypes when measured across all investigated samples but not when samples within each population of sheep were compared. However, there was no association of TYRP2 genotype at a single SNP position with coat colour across populations. Nonetheless, the two breeds with higher overall tyrosinase activity did produce darker and more varied coat colours than the breed with lower tyrosinase activity.

Differential gene expression of TRPM1, the potential cause of congenital stationary night blindness and coat spotting patterns (LP) in the Appaloosa horse (Equus caballus)

The appaloosa coat spotting pattern in horses is caused by a single incomplete dominant gene (LP). Homozygosity for LP (LP/LP) is directly associated with congenital stationary night blindness (CSNB) in Appaloosa horses. LP maps to a 6-cM region on ECA1. We investigated the relative expression of two functional candidate genes located in this LP candidate region (TRPM1 and OCA2), as well as three other linked loci (TJP1, MTMR10, and OTUD7A) by quantitative real-time RT-PCR. No large differences were found for expression levels of TJP1, MTMR10, OTUD7A, and OCA2. However, TRPM1 (Transient Receptor Potential Cation Channel, Subfamily M, Member 1) expression in the retina of homozygous appaloosa horses was 0.05% the level found in non-appaloosa horses (R = 0.0005). This constitutes a >1800-fold change (FC) decrease in TRPM1 gene expression in the retina (FC = -1870.637, P = 0.001) of CSNB-affected (LP/LP) horses. TRPM1 was also downregulated in LP/LP pigmented skin (R = 0.005, FC = -193.963, P = 0.001) and in LP/LP unpigmented skin (R = 0.003, FC = -288.686, P = 0.001) and was downregulated to a lesser extent in LP/lp unpigmented skin (R = 0.027, FC = -36.583, P = 0.001). TRP proteins are thought to have a role in controlling intracellular Ca(2+) concentration. Decreased expression of TRPM1 in the eye and the skin may alter bipolar cell signaling as well as melanocyte function, thus causing both CSNB and LP in horses.

Glycolipid-dependent sorting of melanosomal from lysosomal membrane proteins by lumenal determinants

Melanosomes are lysosome-related organelles that coexist with lysosomes in mammalian pigment cells. Melanosomal and lysosomal membrane proteins share similar sorting signals in their cytoplasmic tail, raising the question how they are segregated. We show that in control melanocytes, the melanosomal enzymes tyrosinase-related protein 1 (Tyrp1) and tyrosinase follow an intracellular Golgi to melanosome pathway, whereas in the absence of glycosphingolipids, they are observed to pass over the cell surface. Unexpectedly, the lysosome-associated membrane protein 1 (LAMP-1) and 2 behaved exactly opposite: they were found to travel through the cell surface in control melanocytes but followed an intracellular pathway in the absence of glycosphingolipids. Chimeric proteins having the cytoplasmic tail of Tyrp1 or tyrosinase were transported like lysosomal proteins, whereas a LAMP-1 construct containing the lumenal domain of Tyrp1 localized to melanosomes. In conclusion, the lumenal domain contains sorting information that guides Tyrp1 and probably tyrosinase to melanosomes by an intracellular route that excludes lysosomal proteins and requires glucosylceramide.

Chemical genetic screening identifies tricyclic compounds that decrease cellular melanin content

A screen of a library of 2,000 drugs and natural products in murine melanocytes identified 10 tricyclic antidepressants (TCAs) as compounds that potently decreased intracellular melanin content. The rank order of potency of these compounds for decreasing melanin content was different than their relative potencies as antidepressants. These compounds had no effect on either the level or the enzymatic activity of cellular tyrosinase (Tyr). Increased presence of both Tyr and melanin in the culture media was observed in treated melanocytes. Immunofluorescence localization revealed that these compounds decreased intracellular melanin content by disrupting the intracellular trafficking of Tyr gene family proteins. In treated melanocytes, Tyr, Tyr-related protein 1, and dopachrome tautomerase accumulated in enlarged granules distributed throughout the cytoplasm. Colocalization of Tyr with lysosome-associated membrane protein 1 was observed within many of these granules. Partial colocalization of Tyr with the Hermansky-Pudlak syndrome 1 gene product observed in control melanocytes was abolished by TCA treatment. Our results show that these compounds decreased intracellular melanin content by altering the trafficking of Tyr gene family proteins and inducing abnormal secretion of Tyr. Results from our screening have implications for the design of products for skin lightening and treatment of hyperpigmentation.

Pigmentation in Black-boned sheep (Ovis aries): association with polymorphism of the MC1R gene

Variations in vertebrate skin and hair color are due to varied amounts of eumelanin (brown/black) and phaeomelanin (red/yellow) produced by the melanocytes. The melanocortin 1 receptor (MC1R) is a regulator of eumelanin and phaeomelanin production in the melanocytes, and MC1R mutations causing coat color changes are known in many vertebrates. We have sequenced the entire coding region of the MC1R gene in Black-boned, Nanping indigenous and Romney Marsh sheep populations and found two silent mutation sites of A12G and G144C, respectively. PCR-RFLP of G144C showed that frequency of allele G in Black-boned, Nanping indigenous and Romney Marsh sheep was 0.818, 0.894 and 0, respectively. Sheep with GG genotype had significantly higher (P < 0.05) tyrosinase activity than sheep with CC genotype in the all investigated samples. Moreover, there was significant effect of MC1R genotype on coat color, suggesting that MC1R gene could affect coat color but not black traits. There would be merit in further studies using molecular techniques to elucidate the cause of black traits in these Black-boned sheep.

[Phenotype of the visual system in oculocutaneous and ocular albinism]

In spite of albinism being one of the visual impairments which has been known for over a century, it has only been known for a few decades that albinism is correlated to severe cerebral morphological developmental alterations. The increasing knowledge about the role of melanin in the development and orientation of cerebral neurons not only renders more insight into albinism, but also a greater insight in the physiological neuronal and cerebral development in man. Concerning the morphological and visual phenotype there are new clinical findings which enlarge the known spectrum of albinism. In a representative group of 506 persons with oculocutaneous and ocular albinism who are in care at the Department of Ophthalmology at the University of Saarland (UKS), we present a staging of morphological findings of the iris, retinal pigment epithelium and macula, and of the optic nerve head which has been in use for 10 years. Albinism may present with a remarkably mild ocular phenotype and a near to normal functional phenotype. We present correlations between molecular genetic types of albinism, ocular phenotype and visual function. Of great importance concerning later visual acuity is the dysplasia of the optic nerve head (ONH), which is a frequent finding in albinism. The appearance of the ONH should always be included in any clinical description of an albinism patient. It is highly possible that due to a moderate phenotype there are still many patients who have not been diagnosed yet. Visual acuity of 30/20 to 20/20 and no nystagmus do not rule out albinism. In addition, when performing albino VEPs in phenotypically normal children with infantile strabismus, small ONHs, but normal visual acuity and no nystagmus, the classical atypical chiasmal crossing is sometimes found. Therefore, the number of persons having undiagnosed albinism is probably quite high, perhaps there even is a very broad transition zone from normal to albinotic.

Oculocutaneous albinism

Oculocutaneous albinism (OCA) is a group of inherited disorders of melanin biosynthesis characterized by a generalized reduction in pigmentation of hair, skin and eyes. The prevalence of all forms of albinism varies considerably worldwide and has been estimated at approximately 1/17,000, suggesting that about 1 in 70 people carry a gene for OCA. The clinical spectrum of OCA ranges, with OCA1A being the most severe type with a complete lack of melanin production throughout life, while the milder forms OCA1B, OCA2, OCA3 and OCA4 show some pigment accumulation over time. Clinical manifestations include various degrees of congenital nystagmus, iris hypopigmentation and translucency, reduced pigmentation of the retinal pigment epithelium, foveal hypoplasia, reduced visual acuity usually (20/60 to 20/400) and refractive errors, color vision impairment and prominent photophobia. Misrouting of the optic nerves is a characteristic finding, resulting in strabismus and reduced stereoscopic vision. The degree of skin and hair hypopigmentation varies with the type of OCA. The incidence of skin cancer may be increased. All four types of OCA are inherited as autosomal recessive disorders. At least four genes are responsible for the different types of the disease (TYR, OCA2, TYRP1 and MATP). Diagnosis is based on clinical findings of hypopigmentation of the skin and hair, in addition to the characteristic ocular symptoms. Due to the clinical overlap between the OCA forms, molecular diagnosis is necessary to establish the gene defect and OCA subtype. Molecular genetic testing of TYR and OCA2 is available on a clinical basis, while, at present, analysis of TYRP1 and MATP is on research basis only. Differential diagnosis includes ocular albinism, Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, Griscelli syndrome, and Waardenburg syndrome type II. Carrier detection and prenatal diagnosis are possible when the disease causing mutations have been identified in the family. Glasses (possibly bifocals) and dark glasses or photocromic lenses may offer sufficient help for reduced visual activity and photophobia. Correction of strabismus and nystagmus is necessary and sunscreens are recommended. Regular skin checks for early detection of skin cancer should be offered. Persons with OCA have normal lifespan, development, intelligence and fertility.

Identification of Novel Pigmentation Modulators by Chemical Genetic Screening

There is a continual need for compounds that effectively modulate melanin synthesis. To identify novel pigmentation modulators and their cellular targets, chemical genetic screenings were performed with triazine-based combinatorial libraries that include various linkers as intrinsic components of the small molecules in the library. The linker provides a ready means of attachment to beads, eliminating several common time-consuming downstream steps in the isolation of cellular targets for the small molecules of interest. Twelve compounds were identified as novel pigmentation modulators from various screenings performed in normal and albino murine melanocytes and zebrafish. Target identification by affinity chromatography revealed unexpected roles for prohibitin and mitochondrial F1F0-adenotriphosphatase in the regulation of mammalian pigmentation. The identification of prohibitin, a "scaffold protein", as a propigmentation effector represents a novel mechanism by which propigmentary signals are transduced. Results from our screenings provide potential active agents and targets for the medical and aesthetic treatment of disorders of pigmentation.