Molecular basis of tyrosinase-negative oculocutaneous albinism. A single base mutation in the tyrosinase gene causing arginine to glutamine substitution at position 59

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.

Human equivalent doses of L-DOPA rescues retinal morphology and visual function in a murine model of albinism

L-DOPA is deficient in the developing albino eye, resulting in abnormalities of retinal development and visual impairment. Ongoing retinal development after birth has also been demonstrated in the developing albino eye offering a potential therapeutic window in humans. To study whether human equivalent doses of L-DOPA/Carbidopa administered during the crucial postnatal period of neuroplasticity can rescue visual function, OCA C57BL/6 J-c2J OCA1 mice were treated with a 28-day course of oral L-DOPA/Carbidopa at 3 different doses from 15 to 43 days postnatal age (PNA) and for 3 different lengths of treatment, to identify optimum dosage and treatment length. Visual electrophysiology, acuity, and retinal morphology were measured at 4, 5, 6, 12 and 16 weeks PNA and compared to untreated C57BL/6 J (WT) and OCA1 mice. Quantification of PEDF, βIII-tubulin and syntaxin-3 expression was also performed. Our data showed impaired retinal morphology, decreased retinal function and lower visual acuity in untreated OCA1 mice compared to WT mice. These changes were diminished or eliminated when treated with higher doses of L-DOPA/Carbidopa. Our results demonstrate that oral L-DOPA/Carbidopa supplementation at human equivalent doses during the postnatal critical period of retinal neuroplasticity can rescue visual retinal morphology and retinal function, via PEDF upregulation and modulation of retinal synaptogenesis, providing a further step towards developing an effective treatment for albinism patients.

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-expressing neuronal cell line as in vitro model of Parkinson's disease

Oxidized metabolites of dopamine known as dopamine quinone derivatives are thought to play a pivotal role in the degeneration of nigrostriatal dopaminergic neurons in Parkinson’s disease. Although such quinone derivatives are usually produced via the autoxidation of catecholamines, tyrosinase, which is a key enzyme in melanin biosynthesis via the production of DOPA and subsequent molecules, can potentially accelerate the induction of catecholamine quinone derivatives by its oxidase activity. We have developed neuronal cell lines in which the expression of human tyrosinase was inducible. Overexpression of tyrosinase resulted in increased intracellular dopamine content in association with the formation of melanin pigments in neuronal somata, which eventually causes apoptotic cell death. This cellular model will provide a useful tool for detailed analyses of the neurotoxicity of oxidized catechol metabolites.

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).

Molecular bases of tyrosinase-negative oculocutaneous albinism: a single base insertion or a missense point mutation in the tyrosinase gene

We have identified two different mutations in the tyrosinase genes of Japanese patients with tyrosinase-negative oculocutaneous albinism (OCA). One is a single base insertion in the exon 2 of the tyrosinase gene that shifts the reading frame and introduces a premature termination codon (TGA) after the amino acid residue 298 (codon 316). The other is a G to A transition at residue 312, leading to a single amino acid substitution, arginine at position 59 (codon 77) to glutamine. The promoter activity of the patients' tyrosinase genes was evaluated in the cell-free transcription system prepared from pigmented melanoma cells, indicating that the patients' genes were accurately transcribed in vitro. It is therefore conceivable that the tyrosinase gene is expressed in their melanocytes. Furthermore, transient expression of the mutated genes indicates that the truncated tyrosinase or the tyrosinase containing glutamine 59 is unable to form melanin in melanocytes. We therefore propose that these mutations in the tyrosinase genes lead to a phenotype of tyrosinase-negative OCA.

Inhibition of the formation of the neurotoxin 5-S-cysteinyl-dopamine by polyphenols

The death of nigral neurons in Parkinson's disease is thought to involve the formation of the endogenous neurotoxin, 5-S-cysteinyl-dopamine. In the present study, we show that the polyphenols, (+)-catechin and caffeic acid, which contain a catechol moiety, inhibit tyrosinase-induced formation of 5-S-cysteinyl-dopamine via their capacity to undergo tyrosinase-induced oxidation to yield cysteinyl-polyphenol adducts. In contrast, the inhibition afforded by the flavanone, hesperetin, was not accompanied by the formation of cysteinyl-hesperetin adducts, indicating that it may inhibit via direct interaction with tyrosinase. Whilst the stilbene resveratrol also inhibited 5-S-cysteinyl-dopamine formation, this was accompanied by the formation of dihydrobenzothiazine, a strong neurotoxin. Our data indicate that the inhibitory effects of polyphenols against 5-S-cysteinyl-dopamine formation are structure-dependent and shed further light on the mechanisms by which polyphenols exert protection against neuronal injury relevant to neurodegenerative diseases.

Neuroendocrine functions of melanocytes: beyond the skin-deep melanin maker

The skin is armored with "dead cells", the stratum corneum, and is continuously exposed to external stressful environments, such as atmospheric oxygen, solar radiations, and thermal and chemical insults. Melanocytes of neural crest origin are located in the skin, eye, inner ear, and leptomeninges. Melanin pigment in the skin is produced by melanocytes under the influence of various endogenous factors, derived from neighboring keratinocytes and underlying fibroblasts. The differentiation and functions of melanocytes are regulated at multiple processes, including transcription, RNA editing, melanin synthesis, and the transport of melanosomes to keratinocytes. Impairment at each step causes the pigmentary disorders in humans, with the historical example of oculocutaneous albinism. Moreover, heterozygous mutations in the gene coding for microphthalmia-associated transcription factor, a key regulator for melanocyte development, are associated with Waardenburg syndrome type 2, an auditory-pigmentary disorder. Sun tanning, melasma, aging spots (lentigo senilis), hair graying, and melanoma are well-known melanocyte-related pathologies. Melanocytes therefore have attracted much attention of many ladies, makeup artists and molecular biologists. More recently, we have shown that lipocalin-type prostaglandin D synthase (L-PGDS) is expressed in melanocytes but not in other skin cell types. L-PGDS generates prostaglandin D2 and also functions as an inter-cellular carrier protein for lipophilic ligands, such as bilirubin and thyroid hormones. Thus, melanocytes may exert hitherto unknown functions through L-PGDS and prostaglandin D2. Here we update the neuroendocrine functions of melanocytes and discuss the possible involvement of melanocytes in the control of the central chemosensor that generates respiratory rhythm.

Increased dopamine and its metabolites in SH-SY5Y neuroblastoma cells that express tyrosinase

Oxidized metabolites of dopamine, known as dopamine quinone derivatives, are thought to play a pivotal role in the degeneration of dopaminergic neurons. Although such quinone derivatives are usually produced via the autoxidation of catecholamines, tyrosinase, which is a key enzyme in melanin biosynthesis via the production of DOPA and subsequent molecules, may potentially accelerate the induction of catecholamine quinone derivatives by its oxidase activity. In the present study, we developed neuronal cell lines in which the expression of human tyrosinase was inducible. Overexpression of tyrosinase in cultured cell lines resulted in (i) increased intracellular dopamine content; (ii) induction of oxidase activity not only for DOPA but also for dopamine; (iii) formation of melanin pigments in cell soma; and (iv) increased intracellular reactive oxygen species. Interestingly, the expressed tyrosinase protein was initially distributed in the entire cytoplasm and then accumulated to form catecholamine-positive granular structures by 3 days after the induction. The granular structures consisted of numerous rounded, dark bodies of melanin pigments and were largely coincident with the distribution of lysosomes. This cellular model that exhibits increased dopamine production will provide a useful tool for detailed analyses of the potentially noxious effects of oxidized catecholamine metabolites.

Identification of the immunodominant regions of the melanoma antigen tyrosinase by anti-tyrosinase monoclonal antibodies

Tyrosinase, the critical enzyme in melanin synthesis, is also found to be expressed in most malignant melanomas and can serve as a target for the immune response by both CD4+ and CD8+ T-cells. Therefore it could be used as a potential target for therapeutic intervention in tyrosinase-positive melanomas. In order to develop serological reagents for the immunodetection of human tyrosinase and to find the most immunogenic region of the protein, we have raised a panel of monoclonal antibodies (MAbs) against recombinant tyrosinase expressed and purified from bacteria. Epitope mapping revealed the 79 amino acid long stretch between 163 and 241 residues to be the most immunodominant region of the tyrosinase. This region could be further divided into three parts by binding different MAbs. These MAbs were very useful tools for the detection of tyrosinase expression from different constructs in tissue culture cells by immunocytochemistry and in melanocytes by immunohistochemistry. Some of the MAbs that recognized epitopes between 163 and 204 amino acids also recognized an additional distinct protein of about 70 kDa seen on Western blot analysis of transfected and non-transfected COS-7 cells. One of these, the MAb 4B1, was used in immunohistochemistry, and cross reaction with the basement membrane of the human tissue was observed. The analysis of the 4B1 MAb epitope showed that the C-terminal part of that region almost entirely overlaps with the sequence of the recently reported basement membrane protein beta-netrin.

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).

R278TER and P431L mutations of the tyrosinase gene exist in Japanese patients with tyrosinase-negative oculocutaneous albinism

We examined the tyrosinase gene of two Japanese patients with tyrosinase-negative oculocutaneous albinism by allele-specific amplification analysis on two known point mutations in Japanese, and the results indicated that they were compound heterozygouts, namely, one allele of the tyrosinase gene harbored one of two known mutations and another allele probably had a mutation unknown in Japanese patients. Therefore, we have cloned and sequenced the tyrosinase gene of the two patients and identified two different point mutations. One is a nonsense mutation, codon 278CGA (Arg) to TGA (TER), and the other is a substitution mutation, codon 431CCA (Pro) to CTA (Leu). However, these same mutations have already been observed in a Guyanan and a Moroccan Jewish patient, and in an Indo-Pakistani patient, respectively.

Mutations of the tyrosinase gene in three Korean patients with type I oculocutaneous albinism

Oculocutaneous albinism (OCA) is an inherited disorder of the melanin pigmentary system, characterized by a decrease or an absence of melanin in the skin, hair, and eyes. Type I (tyrosinase-deficient) OCA results from mutations of the tyrosinase (TYR) gene encoding tyrosinase, the enzyme that catalyzes at least the first two steps of melanin biosynthesis. We have analyzed the TYR gene in three Korean patients with severe type I OCA. Two patients were compound heterozygotes for the Arg (CGG) to Gln (CAG) mutation at position 77 and a C insertion mutation at position 310. The other was a compound heterozygote for a C insertion mutation at position 310 and the Asp (GAT) to Asn (AAT) mutation at position 383. These mutations were easily detected by restriction enzyme digestion or by SSCP analysis. Such methods of mutation analysis thus provide a basis for a screening system for the TYR gene mutations in Korean patients with type I OCA.

Insertion of a novel transposable element in the tyrosinase gene is responsible for an albino mutation in the medaka fish, Oryzias latipes

In the medaka fish (Oryzias latipes) many mutants for body color have been isolated. A typical example is the recessive oculocutaneous albino mutant i, which has amelanotic skin and red-colored eyes with no tyrosinase activity. To cast light on the molecular basis of the albino mechanism, we performed Southern blot analysis of genomic DNA from the mutant with an authentic tyrosinase gene probe; the results demonstrate that an extra 1.9 kb fragment is present inside the first exon. The insertion is responsible for the oculocutaneous albinism. About 80 copies of this fragment are present in the genomes of albino-i and wild-type fish; these repeated sequences are here designated Tol1 elements and the particular element found in the tyrosinase gene of albino-i is denoted Tol1-tyr. The nucleotide sequence of Tol1-tyr shows that the fragment (i) carries terminal inverted repeats of 14 bp, and (ii) is flanked by duplicated 8 bp segments of the host chromosome. These are properties of DNA-mediated transposable elements. Comparison of the nucleotide sequence of Tol1-tyr with other sequences in DNA databases, with special attention to sequences of transposable elements known to date, did not reveal any similarity. Thus, Tol1 constitutes a hitherto unknown family of DNA transposable elements.

Detection of point mutations in human tyrosinase gene by improved allele-specific amplification

Allele-specific amplification (ASA) is a simple and non-radioactive technique for detecting known point mutations that produce genetic diseases. Although this technique is based on the specific amplification of the target allele by a polymerase chain reaction (PCR) with allele-specific primers, the specificity of the amplification may depend on various PCR conditions. To avoid non-specific amplification which leads to false-positive results in ASA, we modified both the normal and mutant allele-specific primers so that they would have one constant base mismatch, located at the penultimate 3' position. We confirmed that our modification could inhibit such unfavorable amplification by using as templates genomic DNAs of patients affected with tyrosinase-negative oculocutaneous albinism (OCA). We then analyzed new patients affected with tyrosinase-negative OCA, and based the diagnosis on both the results of a clinical examination and those of a hair bulb test using ASA with the modified allele-specific primers. The results indicated that more than 3 alleles of the tyrosinase gene with a pathological mutation existed in Japanese patients.

Optimal physical performance in athletes: key roles of dopamine in a specific neurotransmitter/hormonal mechanism

It is proposed that exercise training leads to resetting of the central autonomic nervous system (ANS) status, modifies neuroendocrine function and consistently results in upgraded efficiency of physiological/metabolic regulations. The initiating neurotransmitter mechanism is widely held to be due, essentially, to activation of certain brain cholinergic neurons (amygdala n.), stimulation of the hypothalamic-pituitary-adrenocortical pathway, and to cortisol as the dominant peripheral effector of overall improved efficiency. This thesis raises certain questions. The present analysis, based on studies of sedentary and exercise trained humans, proposes the following: that (1) the ANS profile in exercise consists in enhanced dopaminergic (DA) relative to noradrenergic (NA) activity and increased vagal tone; (2) DA is the principal catecholamine neuromodulator/neurotransmitter of the brain, directly involved in motor control in the striatum and is key to the mechanism underlying increased and maintained efficiency of exercise trained humans; (3) DA is a major participant in many aspects of motor function which include the regulation of cardiovascular and renal function (heart rate, blood pressure, and other), muscle tone, visual processing, calcium homeostasis, protein synthesis and conceivably the optimal utilization of food intake; (4) the peripheral actions of DA reflect and are functionally interrelated to the observed global activation of brain DA systems in exercising animals, and probably man; (5) that a different enzyme profile evolves in exercise training which may potentiate DA synthesis and preserve the structural and functional integrity of central DA neurons; (6) that a shift to enhanced DA vs. NA activity occurs in exercise trained Whites which resembles the norm for sedentary Africans and confers distinct physiological advantages; (7) there is unequivocal evidence for the physiological efficiency of a DA dominated ANS profile which can be correlated to the low incidence of DA related diseases in aging Africans; (8) data suggests that the superiority of top-class African athletes in distance running and their endurance capacity are related to an inherent neurophysiological advantage, to efficient DA and protein synthesis, a decreased rate of DA decline during aging and to improved calcium homeostasis, inter alia. Throughout this study, the term sedentary refers to subjects not undergoing specific exercise training of defined intensity and duration.

Analysis of tyrosinase mutations associated with tyrosinase-related oculocutaneous albinism (OCA1)

Mutations of the tyrosinase gene associated with a partial or complete loss of enzymatic activity are responsible for tyrosinase related oculocutaneous albinism (OCA1). A large number of mutations have been identified and their analysis has provided insight into the biology of tyrosinase and the pathogenesis of these different mutations. Missense mutations produce their effect on the activity of an enzyme by altering an amino acid at a specific site. The location of these mutations in the peptide can be used to indicate potential domains important for enzymatic activity. Missense mutations of the tyrosinase polypeptide cluster in four regions, suggesting that these are important functional domains. Two of the potential domains involve the copper binding sites while the others are likely involved in substrate binding. More critical analysis of the copper binding domain of tyrosinase can be gained by analyzing the structure of hemocyanin, a copper-binding protein with a high degree of homology to tyrosinase in the copper binding region. This analysis indicates a single catalytic site in tyrosinase for all enzymatic activities.

Prenatal diagnosis of oculocutaneous albinism by analysis of the fetal tyrosinase gene

Tyrosinase-negative oculocutaneous albinism, the most severe subtype of a heterogeneous group of albinism, is an autosomal recessive trait caused by mutations in the tyrosinase gene. Prenatal diagnosis had been made previously only by evaluating fetal skin obtained by biopsy, an invasive procedure that cannot be performed earlier than 19 weeks of gestation. A pregnant mother of a 9-year-old Japanese boy with tyrosinase-negative oculocutaneous albinism wanted a prenatal diagnosis. Polymerase chain reaction amplification and allele-specific oligonucleotide hybridization revealed that the child is homozygous and the parents heterozygous for the pathologic mutation of the tyrosinase gene in exon 2 (single base insertion) but not for the one in exon 1. Prenatal diagnosis was made by analyzing the tyrosinase gene in fetal cells obtained by amniocentesis at 14 weeks of gestation, which demonstrated that the fetus was heterozygous for mutant tyrosinase gene. Pregnancy was therefore continued and a normal male infant was born. This procedure, the analysis of the fetal genomic tyrosinase DNA, is a rapid and reliable approach to the prenatal diagnosis of oculocutaneous albinism at a relatively early stage of pregnancy and is safer and less invasive than previous methods using fetal skin biopsy.

Prenatal diagnosis of tyrosinase-negative oculocutaneous albinism by an electron microscopic dopa reaction test of fetal skin

An electron microscopic DOPA reaction test of fetal skin was used for the prenatal diagnosis of tyrosinase-negative oculocutaneous albinism (OCA). The subject was a 34-year-old Japanese woman in her second pregnancy. Her first child, born in 1982, had been previously examined and confirmed to have tyrosinase-negative OCA. The parents requested a prenatal diagnosis and we sampled skin from the upper trunk of the fetus. On conventional electron microscopy, the development of melanosomes in interfollicular melanocytes had progressed no further than stage II. Fetal skin samples incubated with L-DOPA solution indicated a lack of tyrosinase activity and showed that the melanosomes had not progressed beyond stage II. In skin samples from the trunks of three Japanese fetuses aborted for other reasons at 19-20 weeks of gestation, most premature melanosomes were further melanized to stage IV after incubation with L-DOPA solution. A prenatal diagnosis of tyrosinase-negative OCA was made. The parents requested a termination and skin biopsies of the abortus confirmed the diagnosis. This study shows that tyrosinase is normally present in melanocytes of the fetal epidermis at 20 weeks' gestation, and that the electron microscopic DOPA reaction test of a fetal skin biopsy specimen is safe and practical, and provides reliable information for making a prenatal diagnosis of tyrosinase-negative OCA in the second trimester.

Analysis of tyrosinase gene mutations using direct automated infrared fluorescence DNA sequencing of amplified exons

The ability to correctly diagnose the molecular cause of genetic diseases is becoming increasingly important in medicine. This requires an efficient method for the analysis of the DNA sequence of specific genes and the detection of mutations in affected individuals. We report a method to determine the mutations responsible for tyrosinase related albinism (OCA1) using a combination of polymerase chain reaction-single stranded conformational polymorphism (PCR-SSCP) analysis and direct DNA cycle sequencing using fluorescently labeled oligonucleotides and an automated DNA sequencer based on infrared fluorescence technology. This method allows DNA from several individuals to be sequenced quickly and simultaneously so that the specific location of each mutation and the carrier status of family members can be determined.

Do pigmented naevi in albinism provide evidence of tyrosinase positivity?

We investigated a 4-year-old Japanese boy with oculocutaneous albinism who had a solitary pigmented mole measuring 5 mm in diameter on his back. An electron microscopic tyrosine incubation test and a DOPA reaction test clearly demonstrated the presence of tyrosinase activity in the patient's hypopigmented skin. The presence of tyrosinase activity was confirmed by tests on hair bulb samples. Histopathological evidence showed that the mole was a typical compound cellular naevus with melanin pigmentation. Although no reports to date have focused on the relationship between pigmented naevi in albinism and tyrosinase activity, our findings suggest that the occurrence of pigmented naevi in an albino may indicate the presence of tyrosinase activity.

Analysis of mutations in the copper B binding region associated with type I (tyrosinase-related) oculocutaneous albinism

Mutations of the tyrosinase gene are responsible for type I (tyrosinase-related) oculocutaneous albinism (OCA), an autosomal recessive genetic syndrome with a broad phenotypic spectrum. Mutant tyrosinase alleles can be associated with no melanin synthesis (I-A, tyrosinase-negative OCA), small to moderate amounts of melanin (I-B, yellow OCA) or unusual pigment patterns (I-TS, temperature-sensitive OCA). A total of 26 mutations of this gene have been described in type I OCA. Analysis of all known mis-sense mutations (n = 17) shows that most cluster in three areas of the coding region. Two clusters involve the copper A or copper B binding sites and may disrupt the metal ion-protein interaction necessary for enzyme function and the third cluster is located in exon I. Computer modeling of the secondary structure of the copper binding regions based on homology with the known crystal structure of hemocyanin show that they both consist of two alpha helices containing three histidine ligands that complex to a single copper atom. Mutations in the copper B binding region lie in the region between the two alpha helices that consists of a loop structure. These mutations may affect tyrosinase activity by either altering the position of the alpha helical domains and thus preventing proper copper binding to the histidine ligands, or affecting a catalytic or substrate binding site located between the two alpha helical domains.

Mutations of the tyrosinase gene in oculocutaneous albinism

Since our first report showing that the phenotype of tyrosinase-negative or type IA oculocutaneous albinism (OCA) is a consequence of a mutation in the tyrosinase gene (Tomita et al., Biochem. Biophys. Res. Commun., 164:990-996, 1989), a number of mutations were found in the tyrosinase gene of OCA patients. However, to establish the molecular basis of OCA in each patient, we must carry out several important experiments as summarized here. First, we should confirm that the cloned or amplified genomic DNA segments are not derived from the pseudogene or related gene. It should be noted that the putative tyrosinase pseudogene contains the sequence almost identical to exons 4 and 5, including their exon/intron boundaries of the authentic tyrosinase gene. Thus, the mutations, detected in exon 4 or 5 amplified from genomic DNA, must be carefully analyzed to exclude a possibility that the mutation is located in the pseudogene. Second, it is of significance to confirm the promoter activity of the patients' tyrosinase gene. Accordingly, we established the cell-free transcription system derived from melanoma cells where the cloned tyrosinase gene is faithfully transcribed. Finally, transient expression assay of mutant tyrosinase is invaluable to conclude that OCA phenotypes are associated with the mutant tyrosinase alleles. I also discuss the implications of a cluster of mutation sites in exon 1 coding for the amino-terminus of tyrosinase.

Proteolysis with trypsin of mammalian tyrosinase isoforms from B16 mouse melanoma

In spite of the central role of tyrosinase in mammalian pigmentation, few data are available on its structure and structure-function relationships based on direct analysis of the protein. A number of reasons have been invoked to account for this situation, including the problems for its purification and its resistance to proteases. However, no study on the effects of proteases on purified tyrosinase has been reported. We have purified the melanosomal and cytosolic tyrosinases from B16 mouse melanoma and analyzed their susceptibility to trypsin digestion. Both isoforms are sensitive to trypsin, and display similar peptide maps and kinetics of proteolysis, suggesting that they are products of the same gene. The peptide maps and the kinetics of appearance of the fragments were consistent with the sequential removal of N-terminal peptides, leading to a core of 55.3 kDa for the melanosomal form and 48.6 kDa for the cytosolic enzyme. This core was apparently resistant to further proteolysis and catalytically inactive. The difference in molecular weight for the core of the cytosolic and melanosomal forms is the same as that calculated for the native isoforms. The kinetics of enzyme inactivation indicate that the tyrosine hydroxylase and Dopa oxidase activities of tyrosinase are lost at the same rate, and should therefore display similar if not identical structural requirements. The results are discussed in terms of the relationship of both isoforms and of the putative protein sequences deduced from the cDNA clones proposed for tyrosinase.

Tyrosinase gene mutations in type I (tyrosinase-deficient) oculocutaneous albinism define two clusters of missense substitutions

Type I (tyrosinase-deficient) oculocutaneous albinism (OCA) results from mutations of the gene encoding tyrosinase, the enzyme that catalyzes the first 2 steps of melanin pigment biosynthesis. In type IA (tyrosinase-negative) OCA tyrosinase enzymatic activity is completely absent, and in type IB ("yellow") OCA tyrosinase activity is greatly reduced. Here, we describe 11 novel mutations of the tyrosinase gene in Caucasian patients with these 2 forms of type I OCA. Type I OCA in Caucasians appears to result from a great variety of different uncommon alleles. More than 80% of the known missense substitutions associated with type I OCA cluster within 2 relatively small regions of the tyrosinase polypeptide, suggesting that these may correspond to functionally important sites within the enzyme.

Cloning and sequence analysis of the tyrosinase gene from a patient with tyrosinase-positive oculocutaneous albinism

Tyrosinase is synthesized on membrane-bound ribosomes and transported into melanosomes through smooth endoplasmic reticulum and Golgi apparatus. Melanin polymers are produced only in melanosomes but never in smooth endoplasmic reticulum or Golgi apparatus, indicating that posttranslational modifications of tyrosinase are completed with melanosomes where tyrosinase becomes an active form. Based on a working hypothesis that tyrosinase-positive oculocutaneous albinism is a consequence of the structurally altered tyrosinase due to a point mutation in the gene of its gene coding for a glycosylation site or a membrane-binding site, which leads to the impairment in the posttranslational modification of tyrosinase and its catalytic activity, we have cloned the tyrosinase gene of one patient affected with tyrosinase-positive oculocutaneous albinism and determined its nucleotide sequence. Thus demonstrated all exons' nucleotide sequence of the patient's tyrosinase gene was found to be identical to that of the wild-type gene. The results indicate that the patient's tyrosinase itself is not altered. We therefore propose that the molecular basis for the development of tyrosinase-positive oculocutaneous albinism exists as a defect in other proteins required for the activation of tyrosinase or in other regions of the tyrosinase gene.

Downstream region of the human tyrosinase-related protein gene enhances its promoter activity

We have cloned and sequenced the human genomic DNA segments encoding the 5'-flanking region and the first two exons of the tyrosinase-related protein (TRP) gene, a pigment cell-specific gene. Functional analysis of its promoter suggests that the downstream region of the TRP gene, including the first intron, enhances the transient expression of the luciferase gene under control of the TRP gene promoter about 16- to 20-fold. This enhancer-like activity is detected not only in melanoma cells but also in HeLa cells whose TRP gene expression is assumed to be repressed. We suggest a possibility that the downstream region is not sufficient to confer pigment cell-specific expression, but is required for efficient transcription of the TRP gene in pigment cells.

Brain tumors predominantly express the neurofibromatosis type 1 gene transcripts containing the 63 base insert in the region coding for GTPase activating protein-related domain

Neurofibromatosis type 1 (NF1) is an autosomal dominant neurocutaneous disorder. A part of the gene for NF1 was cloned and its deduced protein has a domain functionally related to mammalian ras GTP-ase-activating protein (GAP). Human tissues examined express two types of NF1 mRNAs: an originally identified species of NF1 mRNA (type I) and another one containing the 63 base insert in the region coding for GAP-related domain (type II). However relative levels of both mRNAs seem to change under certain conditions. Human brain expresses type I mRNA predominantly, while type II is preferentially expressed in most primary brain tumors (13/16 tumors analyzed). We suggest that higher levels of type II mRNA may be related to the genesis of brain tumors.

Molecular analysis of an extended family with type IA (tyrosinase-negative) oculocutaneous albinism

We have analyzed the tyrosinase coding region of three individuals having Type IA OCA within an extended family using genomic DNA amplification and dideoxy sequencing. Two of the affected individuals are dizygotic twins. All three have a common missense mutation at codon 81 (Pro----Leu) within exon I. The twins have a second missense mutation at codon 371 (Asn----Thr) within exon III and the third individual has a second missense mutation at codon 47 (Gly----Asp) within exon I. For each of these three individuals, the loss of enzyme function is the result of two different mutations, showing that they are compound heterozygotes of two mutant tyrosinase alleles.