Human tyrosinase gene, mapped to chromosome 11 (q14----q21), defines second region of homology with mouse chromosome 7

The biochemistry of melanogenesis: an insight into the function and mechanism of melanogenesis-related proteins

Melanin is an amino acid derivative produced by melanocyte through a series of enzymatic reactions using tyrosinase as substrate. Human skin and hair color is also closely related to melanin, so understanding the mechanisms and proteins that produce melanin is very important. There are many proteins involved in the process of melanin expression, For example, proteins involved in melanin formation such as p53, HNF-1α (Hepatocyte nuclear factor 1α), SOX10 (Sry-related HMg-Box gene 10) and pax3 (paired box gene 3), MC1R(Melanocortin 1 Receptor), MITF (Microphthalmia-associated transcription factor), TYR (tyrosinase), TYRP1 (tyrosinase-related protein-1), TYRP2 (tyrosinase-related protein-2), and can be regulated by changing their content to control the production rate of melanin. Others, such as OA1 (ocular albinism type 1), Par-2 (protease-activated receptor 2) and Mlph (Melanophilin), have been found to control the transfer rate of melanosomes from melanocytes to keratinocytes, and regulate the amount of human epidermal melanin to control the depth of human skin color. In addition to the above proteins, there are other protein families also involved in the process of melanin expression, such as BLOC, Rab and Rho. This article reviews the origin of melanocytes, the related proteins affecting melanin and the basic causes of related gene mutations. In addition, we also summarized the active ingredients of 5 popular whitening cosmetics and their mechanisms of action.

Natural tyrosinase enzyme inhibitors: A path from melanin to melanoma and its reported pharmacological activities

The skin containing melanin pigment acts as a protective barrier and counteracts the UVR and other environmental stressors to maintain or restore disrupted cutaneous homeostasis. The production of melanin pigment is dependent on tyrosine levels. L-tyrosine and L-dihydroxyphenylalanine (L-DOPA) can serve both as a substrates and intermediates of melanin synthetic pathway and as inducers and positive regulators of melanogenesis. The biosynthesis of melanin is stimulated upon exposure to UVR, which can also stimulate local production of hormonal factors, which can stimulate melanoma development by altering the chemical properties of eu- and pheomelanin. The process of melanogenesis can be altered by several pathways. One involves activation of POMC, with the production of POMC peptides including MSH and ACTH, which increase intracellular cAMP levels, which activates the MITF, and helps to stimulate tyrosinase (TYR) expression and activity. Defects in OCA1 to 4 affects melanogenic activity via posttranslational modifications resulting in proteasomal degradation and reducing pigmentation. Further, altering, the MITF factor, helps to regulate the expression of MRGE in melanoma, and helps to increase the TYR glycosylation in ER. CRH stimulates POMC peptides that regulate melanogenesis and also by itself can stimulate melanogenesis. The POMC, P53, ACTH, MSH, MC1R, MITF, and 6-BH4 are found to be important regulators for pigmentation. Melanogenesis can affect melanoma behaviour and inhibit immune responses. Therefore, we reviewed natural products that would alter melanin production. Our special focus was on targeting melanin synthesis and TYR enzyme activity to inhibit melanogenesis as an adjuvant therapy of melanotic melanoma. Furthermore, this review also outlines the current updated pharmacological studies targeting the TYR enzyme from natural sources and its consequential effects on melanin production.

The retinal pigmentation pathway in human albinism: Not so black and white

Albinism is a pigment disorder affecting eye, skin and/or hair. Patients usually have decreased melanin in affected tissues and suffer from severe visual abnormalities, including foveal hypoplasia and chiasmal misrouting. Combining our data with those of the literature, we propose a single functional genetic retinal signalling pathway that includes all 22 currently known human albinism disease genes. We hypothesise that defects affecting the genesis or function of different intra-cellular organelles, including melanosomes, cause syndromic forms of albinism (Hermansky-Pudlak (HPS) and Chediak-Higashi syndrome (CHS)). We put forward that specific melanosome impairments cause different forms of oculocutaneous albinism (OCA1-8). Further, we incorporate GPR143 that has been implicated in ocular albinism (OA1), characterised by a phenotype limited to the eye. Finally, we include the SLC38A8-associated disorder FHONDA that causes an even more restricted "albinism-related" ocular phenotype with foveal hypoplasia and chiasmal misrouting but without pigmentation defects. We propose the following retinal pigmentation pathway, with increasingly specific genetic and cellular defects causing an increasingly specific ocular phenotype: (HPS1-11/CHS: syndromic forms of albinism)-(OCA1-8: OCA)-(GPR143: OA1)-(SLC38A8: FHONDA). Beyond disease genes involvement, we also evaluate a range of (candidate) regulatory and signalling mechanisms affecting the activity of the pathway in retinal development, retinal pigmentation and albinism. We further suggest that the proposed pigmentation pathway is also involved in other retinal disorders, such as age-related macular degeneration. The hypotheses put forward in this report provide a framework for further systematic studies in albinism and melanin pigmentation disorders.

Genotype-phenotype associations in Danish patients with ocular and oculocutaneous albinism

BACKGROUND

The study aimed to describe genotype-phenotype associations in patients with oculocutaneous and ocular-only albinism and to evaluate a set of diagnostic criteria proposed recently by Kruijt et al.

MATERIALS AND METHODS

Genotype-phenotype associations in patients with a clinical diagnosis of albinism were studied based on imaging of hair and ocular features (nystagmus, iris color and translucency, fundus pigmentation and foveal development) and self-evaluated skin type. Patients were sub-grouped based on genetic findings.

RESULTS

Patients with biallelic variants in TYR (n = 29), OCA2 (n = 22), other albinism genes (n = 13) or monoallelic variants in GPR143 (n = 13) were included as were 15 patients with a pure clinical diagnosis but no genetic findings. In descending order the most common findings were: foveal hypoplasia (any hypoplasia 95.2%, severe 88.0%), nystagmus (93.5%), iris translucency (any translucency 80.2%, moderate to severe 31.5%), misrouting on VEP (80.0%): fundus hypopigmentation (any hypopigmentation: 75.8%, severe 30.1%), fair skin type (73.8%), blue irides (62.0%), blonde hair (57.5%), and unpigmented eye lashes (39.1%). There were no phenotypic differences between the different genetic subgroups of albinism but patients with a pathogenic haplotype in TYR in combination with a classic variant had less iris translucency than patients with two classic variants in TYR.

CONCLUSIONS

Ocular developmental features were the most common findings whereas phenotypic features related to pigmentation were less common findings but there were no genotype-phenotype correlations. All patients with a genetically confirmed diagnosis of albinism fulfilled the diagnostic criteria by Kruijt irrespective of genetic subtype.

Comprehensive Review of the Genetics of Albinism

Introduction

It is important to understand albinism, since it is a disorder associated with visual impairment, predisposition to malignant melanomas, and social stigma. The main objective of this article is to review the genetics and biologic mechanisms of the non-syndromic albinism subtypes and to describe associated clinical manifestations. We also discuss research on its treatments.

Methods

A review of the published literature on albinism subtypes was performed, spanning basic laboratory research, published case reports, and experiences of people with albinism.

Results

Clear progress has been made in comprehending the causes of albinism; research has shed light on the complexity of the disorder and has led to the molecular classification of subtypes.

Discussion

Despite the increase in knowledge with regards to albinism, gaps still exist. It is important to continue the pursuit of unraveling the mechanism of the disorder and to monitor the frequency of the subtypes worldwide in order to aid in the development of treatments. Furthermore, disseminating knowledge of albinism is crucial for future progress.

Implications for practitioners

Albinism is a disorder characterized by hypopigmentation of the hair, skin, and eyes, with accompanying ocular abnormalities that remain relatively stable throughout life. The disorder is defined by a spectrum of pigmentation where albinism is more evident among individuals of dark complexion than their lighter-pigmented peers. Patients with albinism require protection against sun exposure and special resources to address visual impairments. When albinism patients are diagnosed and properly accommodated, they generally report a positive quality of life.

LEF-1 Regulates Tyrosinase Gene Transcription In Vitro

TYR, DCT and MITF are three important genes involved in maintaining the mature phenotype and producing melanin; they therefore participate in neural crest cell development into melanocytes. Previous studies have revealed that the Wnt signaling factor lymphoid enhancer-binding factor (LEF-1) can enhance DCT and MITF gene expression. However, whether LEF-1 also affects TYR gene expression remains unclear. In the present study, we found that LEF-1 regulated TYR transcription in vitro. LEF-1 overexpression increased TYR gene promoter activity, whereas LEF-1 knockdown by RNA interference significantly decreased TYR expression. Moreover, the core GTTTGAT sequence (-56 to -50) within the TYR promoter is essential for the effect of LEF-1 on TYR expression, and chromatin immunoprecipitation (ChIP) assay indicated that endogenous LEF-1 interacts with the TYR promoter. In addition, we observed a synergistic transactivation of the TYR promoter by LEF-1 and MITF. These data suggest that Wnt signaling plays an important role in regulating melanocyte development and differentiation.

Tyrosinase gene mutations in the Chinese Han population with OCA1

Oculocutaneous albinism (OCA) is a heterogeneous autosomal recessive genetic disorder that affects melanin synthesis. OCA results in reduced or absent pigmentation in the hair, skin and eyes. Type 1 OCA (OCA1) is the result of tyrosinase (TYR) gene mutations and is a severe disease type. This study investigated TYR mutations in a Chinese cohort with OCA1. This study included two parts: patient genetic study and prenatal genetic diagnosis. A total of 30 OCA1 patients were subjected to TYR gene mutation analysis. Ten pedigrees were included for prenatal genetic diagnosis. A total of 100 unrelated healthy Chinese individuals were genotyped for controls. The coding sequence and the intron/exon junctions of TYR were analysed by bidirectional DNA sequencing. In this study, 20 mutations were identified, four of which were novel. Of these 30 OCA1 patients, 25 patients were TYR compound heterozygous; two patients carried homozygous TYR mutations; and three were heterozygous. Among the ten prenatally genotyped fetuses, three fetuses carried compound heterozygous mutations and seven carried no mutation or only one mutant allele of TYR and appeared normal at birth. In conclusion, we identified four novel TYR mutations and showed that molecular-based prenatal screening to detect TYR mutations in a fetus at risk for OCA1 provided essential information for genetic counselling of couples at risk.

Inhibition of peroxisome proliferator-activated receptor gamma prevents the melanogenesis in murine B16/F10 melanoma cells

The purpose of this study was to investigate if PPARγ plays a role in the melanogenesis. B16/F10 cells were divided into five groups: control, melanin stimulating hormone (α-MSH), α-MSH+retinol, α-MSH+GW9662 (PPARγ antagonist), and GW9662. Cells in the control group were cultured in the Dulbecco's modified Eagle's medium (DMEM) for 48 hrs. To initiate the melanogenesis, cells in all α-MSH groups were cultured in medium containing α-MSH (10 nM) for 48 hrs. Cells were treated simultaneously with retinol (5 μM) in the α-MSH+retinol group. Instead of retinol, GW9662 (10 μM) was cocultured in the α-MSH+GW9662 group. Cells in the final group were cultured in the DMEM with GW9662. All the analyses were carried out 48 hours after treatments. The α-MSH was able to increase cell number, melanin production, and the activity of tyrosinase, the limiting enzyme in melanogenesis. These α-MSH-induced changes were prevented either by retinol or by GW9662. Further analyses of the activities of antioxidant enzymes including glutathione, catalase, and the superoxide dismutase (SOD) showed that α-MSH treatment raised the activity of SOD which was dependent on PPARγ level. According to our results, the α-MSH-induced melanogenesis was PPARγ dependent, which also modulated the expression of SOD.

Report of a novel OCA2 gene mutation and an investigation of OCA2 variants on melanoma risk in a familial melanoma pedigree

BACKGROUND

Oculocutaneous albinism type 2 (OCA2) is caused by mutations of the OCA2 gene. Individuals affected by OCA2 as well as other types of albinism are at a significantly increased risk for sun-induced skin-cancers, including malignant melanoma (MM).

OBJECTIVE

To identify the molecular etiology of oculocutaneous albinism in a previously uncharacterized melanoma pedigree and to investigate the relationship between two OCA2 variants and melanoma predisposition in this pedigree.

METHODS

DNA and RNA were isolated from the peripheral blood of seven patients in a familial melanoma pedigree. Electron microscopy was performed on the individual with clinical oculocutaneous albinism. OCA2, TYRP1, MC1R, CDKN2A/p16, CDKN2A/p19ARF, and CDK4 genes were sequenced in affected individuals. The relationship between OCA2 variants and melanoma was assessed using a pedigree likelihood-based method.

RESULTS

The proband was determined to be an OCA2 compound heterozygous mutation carrier with a previously reported conservative missense mutation (V443I) and a novel non-conservative missense mutation (L734R). The pedigree contained individuals diagnosed with both cutaneous and iris melanoma. Based on co-segregation analysis, the odds of these OCA2 variants being high penetrance loci for melanoma was: 1.3-to-1 if we include the iris melanoma as affected and 6.5-to-1 if we only consider cutaneous melanoma as affected.

CONCLUSION

The discovery of this novel OCA2 variant adds to the body of evidence on the detrimental effects of OCA2 gene mutations on pigmentation, supports existing GWAS data on the relevance of the OCA2 gene in melanoma predisposition, and may ultimately assist in the development of targeted molecular therapies in the treatment of OCA and melanoma.

Synthesis of new 1,3,4-thiadiazole and 1,2,3,4-oxathiadiazole derivatives from carbohydrate precursors and study of their effect on tyrosinase enzyme

5-(1,2,3,4-Tetrahydroxybutyl)-2-methylfuran-3-carbohydrazide (2) was condensed with a variety of ketones to afford carbohydrazide derivatives 3-6. Acetylation of 3-5 afforded the acetyl derivatives 7-9, while periodate oxidation of 3-6 afforded the formyl derivatives 10-13. Acid catalyzed condensation of thiosemicarbazide or o-tolylthiosemicarbazide with the prepared aldehydes 10-12 gave thiosemicarbazone derivatives 14-19. Cyclization of the latter with acetic anhydride afforded 4,5-dihydro-1,3,4-thiadiazolyl derivatives 20-25. On the other hand, condensation of p-tosylhydrazine with the prepared aldehydes 10-12 afforded p-tosylhydrazone derivatives 26-28. Cyclization of 26-28 with acetic anhydride afforded 1,2,3,4-oxathiadiazole derivatives 29-31 respectively. Moreover, the obtained results regarding to the effect of some of the prepared compounds on tyrosinase enzyme showed that the majority of these compounds having an inhibitory effect; especially compounds 12, 16, 17, and 28.

Inhibition of Melanogenesis in Murine B16/F10 Melanoma Cells byLigusticum sinensis Oliv

Ligusticum sinensis Oliv. (LSO) is an herbal drug commonly used as a topical treatment of epidermal hyperdepigmentation in Chinese medicine. However, the mechanism underlying the depigmentation by LSO is still unclear. The purpose of this study was to investigate the effects of LSO on the process of melanogenesis and its possible underlying mechanism. Suppressed DOPA oxidase activity of mushroom tyrosinase was first noted when incubated with aqueous extracts of LSO, demonstrating the direct inhibitory effect of LSO on mushroom tyrosinase. Further experiments were carried out in murine B16/F10 melanoma cells and the effects of LSO extract on melanin formation, tyrosinase activity and tyrosinase gene expression were tested. Under conditions without affecting the viability of murine B16/F10 melanoma cells, LSO extract significantly reduced the cellular melanin content in a dose-dependent manner. The DOPA oxidase activity of tyrosinase in B16/F10 cells was dose-dependently inhibited by LSO treatment, possibly mediated by the suppressed tyrosinase mRNA expression in LSO-treated B16/F10 cells. In conclusion, the inhibitory effect of LSO on melanogenesis is likely associated with decreased DOPA oxidase activity of tyrosinase that is most likely the result of the down-regulation of tyrosinase mRNA expression.

New insights into the active site structure and catalytic mechanism of tyrosinase and its related proteins

Tyrosinases are widely distributed in nature. They are copper-containing oxidases belonging to the type 3 copper protein family, together with catechol oxidases and haemocyanins. Tyrosinases are essential enzymes in melanin biosynthesis and therefore responsible for pigmentation of skin and hair in mammals, where two more enzymes, the tyrosinase-related proteins (Tyrps), participate in the pathway. The structure and catalytic mechanism of mammalian tyrosinases have been extensively studied but they are not completely understood because of the lack of information on the tertiary structure. The availability of crystallographic data of one plant catechol oxidase and one bacterial tyrosinase has improved the model of the three-dimensional structure of the active site of the enzyme. Furthermore, sequence comparison of tyrosinase and the Tyrps reveals that the three orthologue proteins share many key structural features, because of their common origin from an ancestral gene, although the specific residues responsible for their different catalytic capabilities have not been identified yet. This review summarizes our current knowledge of tyrosinase and Tyrps structure and function and describes the catalytic mechanism of tyrosinase and Dct/Tyrp2, which are better characterized.

Molecular basis of type IA (tyrosinase negative) oculocutaneous albinism

Type IA (Tyrosinase negative) oculocutaneous albinism (OCA) is produced by mutations of the tyrosinase gene. We have found a total of 13 different mutations associated with type IA OCA. Analysis of the distribution of the 9 missense mutations shows that most of these mutations cluster in three areas of the gene. All but one of these mutations involve amino acids that are conserved between the mouse and human. Two clusters involve the copper A and copper B binding sites, and could disrupt the metal ion-protein interaction necessary for enzyme function. The third cluster is in exon I and could represent an important functional domain of the enzyme such as the tyrosine binding site. The deletion or insertion frameshift mutations are distributed throughout the coding region and do not appear to cluster. We conclude that a diverse number of mutations are responsible for type IA OCA and many individuals are compound heterozygotes for mutations responsible for this genetic disease (Table 3).

Tyrosinase is not detected in human catecholaminergic neurons by immunohistochemistry and Western blot analysis

Catecholaminergic neurons of the primate substantia nigra (SN) pars compacta (SNc) and the locus coeruleus contain neuromelanin (NM) granules as characteristic structures underlying the pigmentation of these brain areas. Due to a phylogenetic appearance NM granules are absent in the rodent brain, but gradually become present in primates until they reach a maximal expression in humans. Although a possible mechanism of pigment formation may be autoxidation of the NM precursors dopamine or noradrenalin, several groups have suggested an enzymatic formation of NM mediated by tyrosinase or a related enzyme. Since tyrosinase mRNA is suggested to be expressed in the SN of mice and humans, we reinvestigated the expression of tyrosinase in the human SNc and the locus coeruleus at the protein level by immunohistochemistry and Western blot analysis, but could not detect tyrosinase in these brain regions.

Tyrosinase and ocular diseases: Some novel thoughts on the molecular basis of oculocutaneous albinism type 1

Tyrosinase (TYR) is a multifunctional copper-containing glycoenzyme (approximately 80 kDa), which plays a key role in the rate-limiting steps of the melanin biosynthetic pathway. This membrane-bound protein, possibly evolved by the fusion of two different copper-binding proteins, is mainly expressed in epidermal, ocular and follicular melanocytes. In the melanocytes, TYR functions as an integrated unit with other TYR-related proteins (TYRP1, TYRP2), lysosome-associated membrane protein 1 (LAMP1) and melanocyte-stimulating hormone receptors; thus forming a melanogenic complex. Mutations in the TYR gene (TYR, 11q14-21, MIM 606933) cause oculocutaneous albinism type 1 (OCA1, MIM 203100), a developmental disorder having an autosomal recessive mode of inheritance. In addition, TYR can act as a modifier locus for primary congenital glaucoma (PCG) and it also contributes significantly in the eye developmental process. Expression of TYR during neuroblast division helps in later pathfinding by retinal ganglion cells from retina to the dorsal lateral geniculate nucleus. However, mutation screening of TYR is complicated by the presence of a pseudogene-TYR like segment (TYRL, 11p11.2, MIM 191270), sharing approximately 98% sequence identity with the 3' region of TYR. Thus, in absence of a full-proof strategy, any nucleotide variants identified in the 3' region of TYR could actually be present in TYRL. Interestingly, despite extensive search, the second TYR mutation in 15% of the OCA1 cases remains unidentified. Several possible locations of these "uncharacterized mutations" (UCMs) have been speculated so far. Based on the structure of TYR gene, its sequence context and some experimental evidences, we propose two additional possibilities, which on further investigations might shed light on the molecular basis of UCMs in TYR of OCA1 patients; (i) partial deletion of the exons 4 and 5 region of TYR that is homologous with TYRL and (ii) variations in the polymorphic GA complex repeat located between distal and proximal elements of the human TYR promoter that can modulate the expression of the gene leading to disease pathogenesis.

Distinct distal regulatory elements control tyrosinase expression in melanocytes and the retinal pigment epithelium

Pigment cells of mammals are characterized by two different developmental origins: cells of the retinal pigment epithelium (RPE) originate from the optic cup of the developing forebrain, whereas melanocytes arise from the neural crest. The pigmentation gene tyrosinase is expressed in all pigment cells but differentially regulated in melanocytes and RPE. The tyrosinase promoter does not confer strong expression in pigment cells in vivo, while inclusion of a distal regulatory element at position -15 kb is necessary and sufficient to provide strong expression in melanocytes. Nevertheless, the regulatory elements responsible for correct spatial and temporal tyrosinase expression in the RPE remained unidentified so far. In this report, we show that a 186 kb BAC containing the tyrosinase gene provides transgene expression in both RPE and melanocytes indicating the presence of regulatory sequences required for expression in the RPE. A deletion analysis of the BAC was performed demonstrating that a RPE-regulatory element resides between -17 and -75 kb. Using multi-species comparative genomic analysis we identified three conserved sequences within this region. When tested in transgenic mice one of these sequences located at -47 kb targeted expression to the RPE. In addition, deletion of this regulatory element within a tyrosinase::lacZ BAC provided evidence that this sequence is not only sufficient but also required for correct spatial and temporal expression in the RPE. The identification of this novel element demonstrates that tyrosinase gene expression is controlled by separate distal regulatory sequences in melanocytes and RPE.

Establishment of tyrosinase sequence database in normally pigmented Indians and Japanese for rapid determination of novel mutations

BACKGROUND

Many mutations of the tyrosinase gene have been reported in oculocutaneous albinism type I (OCA1) patient. In the future, a greater number of novel mutations will be found as the search for pathological mutations in the tyrosinase genes of OCA patients from various ethnic origins. For rapid determination in future whether an observed mutation is a polymorphism or a novel pathological one, sequence databases of the gene of various ethnic people are needed.

OBJECTIVE

We established a sequence database of the tyrosinase gene of Japanese as well as Indian people.

METHOD

We collected DNA from 109 Japanese and 103 Indians with normal pigmentation and analyzed their tyrosinase gene using a direct sequencing method.

RESULT

The database shows an apparent difference between the two ethnic groups in polymorphisms of the tyrosinase gene namely, Q402 allele, Y192 allele and IV2+24 insT were found in the Indian population, but not in the Japanese. On the other hand, some Japanese had IV2-21 insT but none of the Indians did. The database supports the notion that the tyrosinase gene evolved and extended separately in the two ethnic groups. And the developing database confirmed that the reported mutations causing Indian and Japanese OCA were not among the polymorphisms in the database, which conversely gives genetical proof of the "genuine" pathological mutations.

CONCLUSION

Eventually, the sequence database we established will contribute to demonstrating novel mutations of albinism in Indians and Japanese.

Genetic disorders of pigmentation

More than 127 loci are actually known to affect pigmentation in mouse when they are mutated. From embryogenesis to transfer of melanin to the keratinocytes or melanocytes survival, any defect is able to alter the pigmentation process. Many gene mutations are now described, but the function of their product protein and their implication in melanogenesis are only partially understood. Each genetic pigmentation disorder brings new clues in the understanding of the pigmentation process. According to the main genodermatoses known to induce hypo- or hyperpigmentation, we emphasize in this review the last advances in the understanding of the physiopathology of these diseases and try to connect, when possible, the mutation to the clinical phenotype.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Ocular genetics: current understanding

Over the past decade, there has been an exponential increase in our knowledge of heritable eye conditions. Coincidentally, our ability to provide accurate genetic diagnoses has allowed appropriate counseling to patients and families. A summary of our current understanding of ocular genetics will prove useful to clinicians, researchers, and students as an introduction to the subject.

Sequences Associated With Human Iris Pigmentation

To determine whether and how common polymorphisms are associated with natural distributions of iris colors, we surveyed 851 individuals of mainly European descent at 335 SNP loci in 13 pigmentation genes and 419 other SNPs distributed throughout the genome and known or thought to be informative for certain elements of population structure. We identified numerous SNPs, haplotypes, and diplotypes (diploid pairs of haplotypes) within the OCA2, MYO5A, TYRP1, AIM, DCT, and TYR genes and the CYP1A2-15q22-ter, CYP1B1-2p21, CYP2C8-10q23, CYP2C9-10q24, and MAOA-Xp11.4 regions as significantly associated with iris colors. Half of the associated SNPs were located on chromosome 15, which corresponds with results that others have previously obtained from linkage analysis. We identified 5 additional genes (ASIP, MC1R, POMC, and SILV) and one additional region (GSTT2-22q11.23) with haplotype and/or diplotypes, but not individual SNP alleles associated with iris colors. For most of the genes, multilocus gene-wise genotype sequences were more strongly associated with iris colors than were haplotypes or SNP alleles. Diplotypes for these genes explain 15% of iris color variation. Apart from representing the first comprehensive candidate gene study for variable iris pigmentation and constituting a first step toward developing a classification model for the inference of iris color from DNA, our results suggest that cryptic population structure might serve as a leverage tool for complex trait gene mapping if genomes are screened with the appropriate ancestry informative markers.

Identification and Characterization of a DNase Hypersensitive Region of the Human Tyrosinase Gene

Mutations of the tyrosinase gene produce oculocutaneous albinism type 1 (OCA1). Most affected individuals are compound heterozygotes with different maternal and paternal mutations, but a substantial number of presumed tyrosinase alleles in these individuals have no identifiable mutation in the coding or proximal promoter region of the gene. This suggests that mutations in other regions of the gene, such as regulatory regions that are removed from the direct proximity of the coding sequence, may account for these currently unidentifiable mutations. The mouse tyrosinase gene has a distal enhancer or locus control region (LCR) that provides position-independent stimulation of gene expression, and a homologous regulatory region (HR) of the human gene could be the site of some of these mutations. We report a region 9 kb upstream of the human tyrosinase transcriptional start site that may be involved in regulation of this gene. Analysis of this region shows DNase I hypersensitivity in a cell lineage-specific pattern, a pattern indicative of regulatory regions of a gene. This region also has significant enhancer function when reporter vectors containing it are transfected into either human or mouse melanocyte cell lines, and elimination of specific sequences with homology to the mouse core enhancer in this region extinguishes the enhancer function. We believe that this region of homology contains sequences critical in the regulation of the human tyrosinase gene and is a candidate for the location of OCA1 mutations.

A novel mutation of the tyrosinase gene causing oculocutaneous albinism type 1 (OCA1)

Tyrosinase is a rate-limiting enzyme in the melanin biosynthetic pathway and a complete defect of the enzyme activity caused by homozygous mutations of the tyrosinase gene is well known to result in tyrosinase-negative oculocutaneous albinism (OCA1A) patients who never develop any melanin pigment in the skin, hair and eyes throughout life. In this paper, we report a novel missense substitution, R239W(CGG --> TGG) of the tyrosinase gene in a patient with tyrosinase-negative OCA.

The cream dilution gene, responsible for the palomino and buckskin coat colours, maps to horse chromosome 21

The colour locus historically referred to as C in the horse is linked to microsatellites markers on horse chromosome 21. Preliminary results demonstrated linkage of Ccr, thought to be the cream dilution variant of the C locus, to HTG10. An analysis of horse chromosome 21 using additional families confirmed and established a group of markers linked to Ccr. This work also improved the resolution of previously reported linkage maps for this chromosome. Linkage analysis unambiguously produced the map order: SGCV16-(19.1 cM)-HTG10-(3.8 cM)-LEX60/COR73-(1.3 cM)-COR68-(4.5 cM)- Ccr-(11.9 cM)-LEX31. Comparative and synteny data suggested that the horse C locus is not tyrosinase (TYR).

Identification of rab7 as a melanosome-associated protein involved in the intracellular transport of tyrosinase-related protein 1

The melanosome is a unique secretory granule of the melanocyte in which melanin pigments are synthesized by tyrosinase gene family glycoproteins. Melanogenesis is a highly regulated process because of its inherent toxicity. An understanding of the various regulatory mechanisms is important in delineating the pathophysiology involved in pigmentary disorders and melanoma. We have purified and analyzed the total melanosomal proteins from B16 mouse melanoma tumors in order to identify new proteins that may be involved in the control of the melanogenesis process. Melanosomal proteins were resolved by two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis, a predominant spot (27 kDa with isoelectric point 5.8-6.4) was excised and digested with cyanogen bromide, and the fragments were sequenced. Synthetic oligonucleotide primers were synthesized corresponding to the peptide sequences, and reverse transcriptase polymerase chain reaction amplification of total RNA from B16 cells was carried out. Sequencing of one of the polymerase-chain-reaction-mediated clones demonstrated 80%-97% sequence homology of 200 bp nucleotide with GTP-binding proteins at the 3'-untranslated region. GTP-binding assay on two-dimensional gels of melanosomal proteins showed the presence of several (five to six) small GTP-binding proteins, suggesting that small GTP-binding proteins are associated with the melanosome. Among the known GTP-binding proteins with similar molecular weight and isoelectric point ranges, rab3, rab7, and rab8 were found to be present in the melanosomal fraction by immunoblotting. Confocal immunofluorescence microscopy showed that rab7 is colocalized with the tyrosinase-related protein 1 around the perinuclear area as well as, in part, in the perikaryon, thereby suggesting that rab7 might be involved in the intracellular transport of tyrosinase-related protein 1. Tyrosinase-related protein 1 transport was blocked by the treatment of B16 cells with antisense oligonucleotide to rab7. We suggest (i) that rab7 is a melanosome-associated molecule, (ii) that tyrosinase-related protein 1 is present in late-endosome delineated granules, and (iii) that rab7 is involved in the transport of tyrosinase-related protein 1 from the late-endosome delineated granule to the melanosome.

One-allele system in the Korean for MboI-RFLP in exon 1 of the human tyrosinase (TYR) gene

Oculocutaneous albinism is an autosomal recessive genetic disorder. Several types of oculocutaneous albinism are caused by mutation in related genes. Oculocutaneous albinism 1 is associated with the tyrosinase gene. The human tyrosinase gene (TYR) encodes tyrosinase, a key enzyme in melanin biosynthesis. As exon 1 of the gene shows an MboI-RFLP within codon 192 in Caucasians, we studied allele frequencies of MboI 192 polymorphism in 200 chromosomes from 100 unrelated normal Korean individuals. As a result, only one allele system, the presence of the MboI 192 site, was detected in the Korean.

Molecular basis of albinism: mutations and polymorphisms of pigmentation genes associated with albinism

Albinism, caused by a deficiency of melanin pigment in the skin, hair, and eye (oculocutaneous albinism [OCA]), or primarily in the eye (ocular albinism [OA]), results from mutations in genes involved in the biosynthesis of melanin pigment. The lack of melanin pigment in the developing eye leads to fovea hypoplasia and abnormal routing of the optic nerves. These changes are responsible for the nystagmus, strabismus, and reduced visual acuity common to all types of albinism. Mutations in six genes have been reported to be responsible for different types of oculocutaneous and ocular albinism, including the tyrosinase gene (TYR) and OCA1 (MIM# 203100), the OCA2 gene and OCA2 (MIM# 203200), the tyrosinase-related protein-1 gene (TYRP1) and OCA3 (MIM# 203290), the HPS gene and Hermansky-Pudlak syndrome (MIM# 203300), the CHS gene (CHS1), and Chediak-Higashi syndrome (MIM# 214500), and the X-linked ocular albinism gene and OA1 (MIM#300500). The function of only two of the gene products is known tyrosinase and tyrosinase-related protein-1 both of which are enzymes in the melanin biosynthetic pathway. Continued mutational analysis coupled with function/structure studies should aid our understanding of the function of the remaining genes and their role in albinism. Mutation and polymorphism data on these genes are available from the International Albinism Center Albinism Database web site (http://www.cbc.umn.edu/tad).

A primary male autosomal linkage map of the horse genome

A primary male autosomal linkage map of the domestic horse (Equus caballus) has been developed by segregation analysis of 140 genetic markers within eight half-sib families. The family material comprised four Standardbred trotters and four Icelandic horses, with a total of 263 offspring. The marker set included 121 microsatellite markers, eight protein polymorphisms, five RFLPs, three blood group polymorphisms, two PCR-RFLPs, and one single strand conformation polymorphism (SSCP). One hundred markers were arranged into 25 linkage groups, 22 of which could be assigned physically to 18 different chromosomes (ECA1, ECA2, ECA3, ECA4, ECA5, ECA6, ECA7, ECA9, ECA10, ECA11, ECA13, ECA15, ECA16, ECA18, ECA19, ECA21, ECA22, and ECA30). The average distance between linked markers was 12.6 cM and the longest linkage group measured 103 cM. The total map distance contained within linkage groups was 679 cM. If the distances covered outside the ends of linkage groups and by unlinked markers were included, it was estimated that the marker set covered at least 1500 cM, that is, at least 50% of the genome. A comparison of the relationship between genetic and physical distances in anchored linkage groups gave ratios of 0.5-0.8 cM per Mb of DNA. This would suggest that the total male recombinational distance in the horse is 2000 cM; this value is lower than that suggested by chiasma counts. The present map should provide an important framework for future genome mapping in the horse.

Rufous oculocutaneous albinism in southern African Blacks is caused by mutations in the TYRP1 gene

Oculocutaneous albinism (OCA) is the most common autosomal recessive disorder among southern African Blacks. There are three forms that account for almost all OCA types in this region. Tyrosinase-positive OCA (OCA2), which is the most common, affects approximately 1/3,900 newborns and has a carrier frequency of approximately 1/33. It is caused by mutations in the P gene on chromosome 15. Brown OCA (BOCA) and rufous OCA (ROCA) account for the majority of the remaining phenotypes. The prevalence of BOCA is unknown, but for ROCA it is approximately 1/8,500. Linkage analysis performed on nine ROCA families showed that ROCA was linked to an intragenic marker at the TYRP1 locus (maximum LOD score = 3.80 at straight theta=.00). Mutation analysis of 19 unrelated ROCA individuals revealed a nonsense mutation at codon 166 (S166X) in 17 (45%) of 38 ROCA chromosomes, and a second mutation (368delA) was found in an additional 19 (50%) of 38 chromosomes; mutations were not identified in the remaining 2 ROCA chromosomes. In one family, two siblings with a phenotypically unclassified form of albinism were found to be compound heterozygotes for mutations (S166X/368delA) at the TYRP1 locus and were heterozygous for a common 2.7-kb deletion in the P gene. These findings have highlighted the influence of genetic background on phenotype, in which the genotype at one locus can be influenced by the genotype at a second locus, leading to a modified phenotype. ROCA, which in southern African Blacks is caused by mutations in the TYRP1 gene, therefore should be referred to as "OCA3," since this is the third locus that has been shown to cause an OCA phenotype in humans.

Molecular basis of congenital hypopigmentary disorders in humans: a review

Many specific gene products are sequentially made and utilized by the melanocyte as it emigrates from its embryonic origin, migrates into specific target sites, synthesizes melanin(s) within a specialized organelle, transfers pigment granules to neighboring cells, and responds to various exogenous cues. A mutation in many of the respective encoding genes can disrupt this process of melanogenesis and can result in hypopigmentary disorders. Following are examples highlighting this scenario. A subset of neural crest derived cells emigrate from the dorsal surface of the neural tube, become committed to the melanoblast lineage, and are targeted along the dorsal lateral pathway. The specific transcription factors PAX3 and MITF (microphthalmia transcription factor) appear to play a regulatory role in early embryonic development of the pigment system and in associated diseases (the Waardenburg syndromes). During the subsequent development and commitment of the melanoblast, concomitant expression of the receptors for fibroblasts growth factor (FGFR2), endothelin-B (EDNRB), and steel factor (cKIT) also appears essential for the continued survival of migrating melanoblasts. Lack or dysfunction of these receptors result in Apert syndrome, Hirschsprung syndrome and piebaldism, respectively. Once the melanocyte resides in its target tissue, a plethora of melanocyte specific enzymes and structural proteins are coordinately expressed to form the melanosome and to convert tyrosine to melanin within it. Mutations in the genes encoding these proteins results in a family of congenital hypopigmentary diseases called oculocutaneous albinism (OCA). The tyrosinase gene family of proteins (tyrosinase, TRP1, and TRP2) regulate the type of eumelanin synthesized and mutations affecting them result in OCA1, OCA3, and slaty (in the murine system), respectively. The P protein, with 12 transmembrane domains localized to the melanosome, has no assigned function as of yet but is responsible for OCA2 when dysfunctional. There are other genetically based syndromes, phenotypically resembling albinism, in which the synthesis of pigmented melanosomes, as well as specialized organelles of other cell types, is compromised. The Hermansky-Pudlak syndrome (HPS) and the Chediak-Higashi syndrome (CHS) are two such disorders. Eventually, the functional melanocyte must be maintained in the tissue throughout life. In some cases it is lost either normally or prematurely. White hair results in the absence of melanocytes repopulating the germinative hair follicle during subsequent anagen stages. Vitiligo, in contrast, results from the destruction and removal of the melanocyte in the epidermis and mucous membranes.

Detection of early micrometastases in subcutaneous fat of primary malignant melanoma patients by identification of tyrosinase-mRNA

To improve survival following melanoma excision, wide margin resection including subcutaneous fat, is indicated. Subcutaneous fat, resected with primary melanomas, was investigated using the tyrosinase PCR method originally described for detection of melanoma cells in peripheral blood. Identification of tyrosinase transcripts was possible in 4 melanoma patients (3 Clark level IV, 1 level III), while negative results were obtained from 6 other patients (2 level IV, 3 level III and 1 level I). In control experiments, after infiltration of the reference cell line SkMel into normal fat tissue, the PCR was positive in 33% (3/9) with 10(2) infiltrated cells, 69% (9/13) with 10(4) cells and in 100% (7/7) with 10(6) cells. We conclude that detection of melanoma cells in subcutaneous fat tissue is possible by tyrosinase PCR. Follow-up studies should show whether such positive tyrosinase PCR can define patients at very high risk of tumour relapse.

Type I oculocutaneous albinism associated with a full-length deletion of the tyrosinase gene

Type I oculocutaneous albinism is an autosomal recessive disorder in which the biosynthesis of melanin is reduced or absent in skin, hair, and eyes because of deficient activity of tyrosinase (EC 1.14.18.1). Type I oculocutaneous albinism is caused by mutations in the tyrosinase structural gene, TYR; however, no large TYR gene deletions have been identified previously in humans. Here we report a patient with type IB oculocutaneous albinism who is a compound heterozygote for TYR allele containing a mutation that is likely to affect pre-RNA splicing and a paternally inherited allele in which the TYR gene is completely deleted, the first such allele described to date. Aside from the albinism in the proband, his phenotype and that of his normally pigmented father is otherwise normal, suggesting that this TYR deletion does not involve other functionally important contiguous genes.

Identification of lymph node metastases by use of polymerase chain reaction (PCR) in melanoma patients

Diagnosis of clinically suspected lymph node metastases in melanoma patients can be confirmed with high sensitivity and specificity by fine needle aspiration (FNA) cytology. However, small lymph nodes or haemorrhagic metastases may yield negative or unevaluable cytology. We tested whether the sensitivity and specificity of presurgical diagnosis could be improved by a polymerase chain reaction (PCR) method, identifying tyrosine-mRNA in samples obtained by fine needle aspiration (FNA-PCR). PCR was positive in 17 of 18 histopathologically proven melanoma metastases, while conventional cytopathology detected 16 of 18. 4 of 9 disease-free melanoma patients with negative FNA cytology had positive PCR results, but controls gave negative results. FNA-PCR analysis cannot be recommended as superior to conventional FNA cytological examination. Whether the positive FNA-PCR in four of the nine clinically unsuspicious regional lymph nodes correlates with earlier disease progression or indicates lower specificity of the method will need further investigation.

Inhibitory effect of magnesium L-ascorbyl-2-phosphate (VC-PMG) on melanogenesis in vitro and in vivo

BACKGROUND

An inhibitory effect of ascorbic acid (AsA) on melanogenesis has been described. However, AsA is quickly oxidized and decomposed in aqueous solution and thus is not generally useful as a depigmenting agent.

OBJECTIVE

Our purpose was to examine the effect on pigmentation of magnesium-L-ascorbyl-2-phosphate (VC-PMG), a stable derivative of AsA.

METHODS

Percutaneous absorption of VC-PMG was examined in dermatomed human skin, and its effect on melanin production by mammalian tyrosinase and human melanoma cells in culture was also measured. A 10% VC-PMG cream was applied to the patients.

RESULTS

VC-PMG suppressed melanin formation by tyrosinase and melanoma cells. In situ experiments demonstrated that VC-PMG cream was absorbed into the epidermis and that 1.6% remained 48 hours after application. The lightening effect was significant in 19 of 34 patients with chloasma or senile freckles and in 3 of 25 patients with normal skin.

CONCLUSION

VC-PMG is effective in reducing skin hyperpigmentation in some patients.

Genetic linkage analysis of schizophrenia using chromosome 11q13-24 markers in Israeli pedigrees

It is generally agreed that there is a genetic component in the etiology of schizophrenia which may be tested by the application of linkage analysis to multiply-affected families. One genetic region of interest is the long arm of chromosome 11 because of previously reported associations of genetic variation in this region with schizophrenia, and because of the fact that it contains the locus for the dopamine D2 receptor gene. In this study we have examined the segregation of schizophrenia with microsatellite dinucleotide repeat DNA markers along chromosome 11q in 5 Israeli families multiply-affected for schizophrenia. The hypothesis of linkage under genetic homogeneity of causation was tested under a number of genetic models. Linkage analysis provided no evidence for significant causal mutations within the region bounded by INT and D11S420 on chromosome 11q. It is still possible, however, that a gene of major effect exists in this region, either with low penetrance or with heterogeneity.

The genetic susceptibility to Gilles de la Tourette syndrome in a large multiple affected British kindred: linkage analysis excludes a role for the genes coding for dopamine D1, D2, D3, D4, D5 receptors, dopamine beta hydroxylase, tyrosinase, and tyrosine hydroxylase

Segregation analyses have shown that Gilles de la Tourette Syndrome (GTS) is transmitted as an autosomal dominant gene disorder indicating that classical linkage analysis should be able to identify susceptibility loci. Previous studies of GTS have included investigations of neuroreceptor function, neurotransmitters, and their metabolites as well as neurotransmitter-related enzymes in an attempt to determine the pathophysiology of GTS. The neurotransmitter systems most often thought to be involved in GTS include those involving adrenaline, noradrenaline, and dopamine. We have carried out research to test the hypothesis that genes encoding proteins in the catecholamine pathways may contribute to the genetic etiology of GTS. Polymorphic markers at or near the D1, D2, D3, D4, D5 neuroreceptor gene loci as well as at the genes encoding dopamine beta hydroxylase (DBH), tyrosinase (TY) and tyrosine hydroxylase (TH) were studied in one large multiple affected pedigree. The linkage results of this investigation exclude a major role of these candidate genes in the etiology of GTS in the pedigree.

The expression of tyrosinase, tyrosinase-related proteins 1 and 2 (TRP1 and TRP2), the silver protein, and a melanogenic inhibitor in human melanoma cells of differing melanogenic activities

The expression of various melanogenic proteins, including tyrosinase, the tyrosinase-related proteins 1 (TRP1) and 2 (TRP2/DOPAchrome tautomerase), and the silver protein in human melanocytes was studied in six different human melanoma cell lines and compared to a mouse derived melanoma cell line. Analysis of the expression of tyrosinase, TRP1, TRP2, and the silver protein using flow cytometry revealed that in general there was a positive correlation between melanin formation and the expression of those melanogenic enzymes. Although several of the melanoma cell lines possessed significant activities of TRP2, the levels of DOPAchrome tautomerase in extracts of human cells were relatively low compared to those in murine melanocytes. Melanins derived from melanotic murine JB/MS cells, from melanotic human Ihara cells and HM-IY cells, from sepia melanin, and from C57BL/6 mouse hair were chemically analyzed. JB/MS cells, as well as Ihara cells and HM-TY cells, possessed significant amounts of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) derived melanins, this being dependent on the activity of TRP2. Kinetic HPLC assays showed that 5,6-dihydroxyindole (DHI) produced during melanogenesis was metabolized quickly to melanin in pigmented KHm-1/4 cells, whereas DHI was stable in amelanotic human SK-MEL-24 cells. A melanogenic inhibitor that has been purified from SK-MEL-24 cells that suppressed oxidation of DHI in the presence or absence of tyrosinase, but had no effect on DHICA oxidation. The sum of these results suggests that the expression of melanogenic enzymes as well as the activity of a melanogenic inhibitor are critical to the production of melanin synthesis in humans.

Microphthalmia-associated transcription factor as a regulator for melanocyte-specific transcription of the human tyrosinase gene

Tyrosinase is a rate-limiting enzyme in melanin biosynthesis and is specifically expressed in differentiated melanocytes. We have identified the enhancer element in the 5'-flanking region of the human tyrosinase gene that is responsible for its pigment cell-specific transcription and have termed it tyrosinase distal element (TDE) (positions -1861 to -1842). Transient expression assays showed that TDE confers efficient expression of a firefly luciferase reporter gene linked to the tyrosinase gene promoter in MeWo pigmented melanoma cells but not in HeLa cells, which do not express tyrosinase. TDE was specifically bound by nuclear proteins of MeWo and HeLa cells, the binding properties of which were indistinguishable in gel mobility shift assays. TDE contains the CATGTG motif in its center, and mutation analysis indicates that the CA dinucleotides of this motif are crucial for protein binding and pigment cell-specific enhancer function. The CATGTG motif is consistent with the consensus sequence recognized by a large family of transcription factors with a basic helix-loop-helix structure, which prompted us to examine the possible involvement of a ubiquitous transcription factor, USF, and a novel factor, microphthalmia-associated transcription factor (MITF), recently cloned as the human homolog of the mouse microphthalmia (mi) gene product. The mi phenotype is associated with a mutant mi locus and characterized by small eyes and loss of melanin pigments. Both USF and MITF are predicted to contain a basic helix-loop-helix structure and a leucine zipper structure. We provide evidence that USF binds to TDE, whereas we were unable to detect the DNA-binding activity of MITF. Transient coexpression assays showed that MITF specifically transactivates the promoter activity of the tyrosinase gene through the CATGTG motif of TDE but not the promoter of the ubiquitously expressed heme oxygenase gene, while USF is able to activate both promoters. These results indicate that MITF is a cell-type-specific factor that is capable of activating transcription of the tyrosinase gene.

Report from the Maryland Epidemiology Schizophrenia Linkage Study: no evidence for linkage between schizophrenia and a number of candidate and other genomic regions using a complex dominant model

Our collaborative group has undertaken a linkage study of schizophrenia, using a systematic sample of patients admitted to Maryland hospitals. An initial sample of 39 families, each having two or more affecteds, was available for genotyping candidate genes, candidate regions, and highly polymorphic markers randomly distributed throughout the genome. We used a single complex dominant model (with a disease gene frequency of 0.005 and age-dependent penetrance for affected phenotype: for under 35, penetrance = .45; for 35 and older, penetrance = .85). We report here 130 markers, which met the exclusion criteria of LOD score < -2.00 at theta > 0.01 in at least 10 informative families, and no evidence for heterogeneity. We also report here markers that were tested as candidates for linkage to the schizophrenic phenotype. They were selected based on the following criteria: a) proximity to reported chromosomal rearrangements (both 5q and 11q), b) suggestions of linkage from other families (5q), or c) presence of a candidate gene (5q, 11q, 3q: Dopamine receptors 1, 2, and 3, respectively). We also tested for mutations of codon 717 in exon 17 of the amyloid precursor protein (APP) gene and were unable to detect the C to T substitution in our schizophrenic group.

Microphthalmia-associated transcription factor as a regulator for melanocyte-specific transcription of the human tyrosinase gene

Tyrosinase is a rate-limiting enzyme in melanin biosynthesis and is specifically expressed in differentiated melanocytes. We have identified the enhancer element in the 5'-flanking region of the human tyrosinase gene that is responsible for its pigment cell-specific transcription and have termed it tyrosinase distal element (TDE) (positions -1861 to -1842). Transient expression assays showed that TDE confers efficient expression of a firefly luciferase reporter gene linked to the tyrosinase gene promoter in MeWo pigmented melanoma cells but not in HeLa cells, which do not express tyrosinase. TDE was specifically bound by nuclear proteins of MeWo and HeLa cells, the binding properties of which were indistinguishable in gel mobility shift assays. TDE contains the CATGTG motif in its center, and mutation analysis indicates that the CA dinucleotides of this motif are crucial for protein binding and pigment cell-specific enhancer function. The CATGTG motif is consistent with the consensus sequence recognized by a large family of transcription factors with a basic helix-loop-helix structure, which prompted us to examine the possible involvement of a ubiquitous transcription factor, USF, and a novel factor, microphthalmia-associated transcription factor (MITF), recently cloned as the human homolog of the mouse microphthalmia (mi) gene product. The mi phenotype is associated with a mutant mi locus and characterized by small eyes and loss of melanin pigments. Both USF and MITF are predicted to contain a basic helix-loop-helix structure and a leucine zipper structure. We provide evidence that USF binds to TDE, whereas we were unable to detect the DNA-binding activity of MITF. Transient coexpression assays showed that MITF specifically transactivates the promoter activity of the tyrosinase gene through the CATGTG motif of TDE but not the promoter of the ubiquitously expressed heme oxygenase gene, while USF is able to activate both promoters. These results indicate that MITF is a cell-type-specific factor that is capable of activating transcription of the tyrosinase gene.

Molecular basis of oculocutaneous albinism

Oculocutaneous albinism (OCA) is a complex group of genetic disorders that have historically been defined by clinical and biochemical methods. Recent advances in the molecular biology of pigmentation have greatly increased our understanding of the complexity of this group of disorders. To date, two different types of OCA (OCA1 and OCA2) have been mapped to specific chromosomal regions. Mutations have been found in the tyrosinase locus associated with OCA1 and the human homologue to the murine pink-eyed dilution locus associated with OCA2. Analysis of these genes and their mutations will allow us to better define and categorize the different types of albinism. Further, the analysis of these genes and their mutations will provide information on the role of these gene products in melanin biosynthesis and the effect specific mutations have on the pathogenesis of albinism.

Structural organization of the human tyrosinase gene and sequence analysis and characterization of its promoter region

Tyrosinase is the principal enzyme in the biosynthesis of melanin. The expression of tyrosinase is tissue-specific and appears to be regulated by various hormonal and environmental factors. Elucidation of the genomic structure and molecular basis of control of tyrosinase gene expression will greatly enhance our understanding of the regulation of human pigmentation. To this end, we have isolated and performed restriction mapping of recombinant cosmid and lambda phage clones containing the human tyrosinase gene, sequenced a 2.2-kilobase (kb) region of its promoter, and determined the potential regions regulating the tyrosinase gene expression in transient-expression system. The human tyrosinase gene is comprised of five exons and four introns. Based on our restriction mapping studies, the gene spans a distance of over 65-kb on chromosome 11 (q14-->q21). We constructed a series of plasmids (pHTY-CAT) that contain 5' sequential deletions of the human tyrosinase 5' flanking sequence fused to the reporter gene, chloramphenicol acetyltransferase (CAT). The plasmids were used to locate promoter regions that are potential regulators of tyrosinase gene expression in a transient expression system using melanoma cell lines. In human melanoma cells, the plasmid construct with a -2020 base pair (bp) promoter yielded the highest CAT activity. When the deletions reached -1739 bp, the CAT activity was dramatically reduced, indicating that important enhancer elements for transcription control are present between -1739 and -2020 bp. Further deletions up to -550 bp also resulted in dramatic decreases of CAT activity. However, when the deletion included -550 bp of the 5' flanking sequence, there was 26 percent of the CAT activity compared to that of the -2020 bp promoter. Deletions beyond -550 bp also showed markedly decreased CAT activity. Based on our data, we suggest that human tyrosinase gene expression is governed by both tissue-specific and multiple regulatory elements.