Human Albinism and Animal Models of Albinism

Ten phenotypic forms of oculocutaneous albinism (OCA) and four forms of ocular albinism (OA) have been identified in man. All have optic neuronal decussation defects at the optic chiasm. Thus any proposed animal model for these disorders must share optic neuronal decussation defects in addition to hypopigmentation. Three, tyrosinase-negative (ty-neg), yellow mutant (ym), and platinum (pt), OCA appear to be allelic in humans. Two, ty-neg and pt, OCA appear to be analogous to c-locus mutants c/c and c^p/c^p in mice, but no homologue is known in mice for ym OCA. Tyrosinase-positive (ty-pos) OCA, which is nonallelic with ty-neg OCA, shares many morphological and biochemical features with pink-eyed mice. Chediak-Higashi syndrome (CHS) and Hermansky-Pudlak syndrome (HPS) appear to be due to genes acting extrinsic to the melanin pathway. CHS is homologous with beige in mice. HPS was investigated in northwestern Puerto Rico, where it affects approximately 1 in 2,000 persons. Approximately 68% of 37 deceased HPS patients died from sequelae of ceroid storage disease, restrictive lung disease between ages 35 and 46 years (43%), and granulomatous colitis (8%) or hemorrhage (16%). The most accurate and consistent diagnostic feature of HPS is lack of platelet dense bodies. HPS patients with ceroid storage disease had high urinary levels of long-chain isoprenoid alcohols, dolichols, similar to that seen in the neuronal-ceroid lipofuscinoses (Batten disease). Dolichols are constituents of lysosomes, and their elevation in HPS suggests that this syndrome carries a lysosomal defect. There is no degradative pathway for ceroid and dolichols, which are eliminated by exocytosis. The exocytic process is thought to involve a thioendoproteinase. Pale-ear mice have been proposed as a model for HPS; their platelets lack dense bodies, and they are depigmented. Leupeptin, a thioendoproteinase inhibitor, administered to 100-day-old pale-eared and black wild-type C57 mice for 10 days resulted in the accumulation of ceroid in tissues in the same pattern as that in HPS, but granulomas of gut or fibrosis of lungs were not seen. Determinations of homology between mice and men at the molecular level is now possible with the isolation of mouse tyrosinase by Yamamoto et al. and isolation by Kwon et al. of human tyrosinase mapping at the c-locus in mice.     

Rescue of the albino phenotype by introduction of a functional tyrosinase gene into mice.

The c-locus of the mouse is thought to encode tyrosinase, the key enzyme for melanin synthesis in melanocytes of the skin and the eye. Recently, a mouse cDNA was isolated and shown to confer tyrosine activity on a cell line which expressed no specialized functions for melanin synthesis. To verify that the isolated tyrosinase gene is encoded at the genetically well characterized c-locus, a minigene was assembled from tyrosinase cDNA and tyrosinase genomic DNA and used for generation of transgenic mice. Following microinjection of this construct into fertilized eggs of an albino mouse strain, transgenic mice were obtained which showed pigmentation in skin and eyes. By in situ hybridization, we show expression of the transgene in melanocytes of the hairbulb and in the pigmented cell layers of the eye. We conclude that we have rescued the albino mutation (c/c) by introduction and expression of a functional tyrosinase gene.     

Oculocutaneous albinism type 1: link between mutations,tyrosinase conformational stability,and enzymatic activity

Oculocutaneous albinism type 1 (OCA1) is an autosomal recessive disorder caused by mutations in the tyrosinase gene. Two subtypes of OCA1 have been described: severe OCA1A with complete absence of tyrosinase activity and less severe OCA1B with residual tyrosinase activity. Here, we characterize the recombinant human tyrosinase intramelanosomal domain and mutant variants, which mimic genetic changes in both subtypes of OCA1 patients. Proteins were prepared using site‐directed mutagenesis, expressed in insect larvae, purified by chromatography, and characterized by enzymatic activities, tryptophan fluorescence, and Gibbs free energy changes. The OCA1A mutants showed very low protein expression and protein yield and are enzymatically inactive. Mutants mimicking OCA1B were biochemically similar to the wild type, but exhibited lower specific activities and protein stabilities. The results are consistent with clinical data, which indicates that OCA1A mutations inactivate tyrosinase and result in severe phenotype, while OCA1B mutations partially inactivate tyrosinase and result in OCA1B albinism.     

The tyrosinase gene in gorillas and the albinism of 'Snowflake'

The sequence of the tyrosinase (Tyr) gene coding tracts has been obtained for the gorilla (Gorilla gorilla gorilla). The five exons of the gene were sequenced in three gorillas and in a normally pigmented human. The tyrosinase gene has been found to be a very conserved locus with a very low substitution rate. Some nucleotide and amino acid differences were found between the gorilla and human tyrosinase coding sequences. One of the gorillas included in the study is the only known case of albinism in a gorilla ('Snowflake'). Mutations of the TYR gene lead to Oculocutaneous Albinism type 1 (OCA1), the most common type of albinism in humans (OMIM accession number 203100). The TYR gene encodes the tyrosinase enzyme (E.C. 1.14.18.1), whose activity was found to be completely lacking in 'Snowflake', indicating that a mutation in the Tyr gene is the likely cause of his albinism. Nonetheless, no nucleotide changes were detected that could account for the lack of Tyr product or tyrosinase activity in Snowflake, and explanations of these findings are discussed.     

Tyrosinase gene mutations associated with type IB ("yellow") oculocutaneous albinism

We have identified three different tyrosinase gene mutant alleles in four unrelated patients with type IB ("yellow") oculocutaneous albinism (OCA) and thus have demonstrated that type IB OCA is allelic to type IA (tyrosinase negative) OCA. In an inbred Amish kindred, type IB OCA results from homozygosity for a Pro----Leu substitution at codon 406. In the second family, type IB OCA results from compound heterozygosity for a type IA OCA allele (codon 81 Pro----Leu) and a novel type IB allele (codon 275 Val----Phe). In the third patient, type IB OCA results from compound heterozygosity for the same type IB allele (codon 275 Val----Phe) and a novel type IB OCA allele. In a fourth patient, type IB OCA results from compound heterozygosity for the codon 81 type IA OCA allele and a type IB allele that contains no identifiable abnormalities; dysfunction of this type IB allele apparently results from a mutation either well within one of the large introns or at some distance from the tyrosinase gene. In vitro expression of the Amish type IB allele in nonpigmented HeLa cells demonstrates that the Pro----Leu substitution at codon 406 greatly reduces but does not abolish tyrosinase enzymatic activity, a finding consistent with the clinical phenotype.     

Oculocutaneous albinism with TYRP1 gene mutations in a Caucasian patient

Non-syndromic oculocutaneous albinism (OCA) is a clinically and genetically heterogeneous autosomal recessive disorder with mutations identified in several genes: OCA1 (tyrosinase, TYR), OCA2 (OCA2), OCA3 (tyrosinase-related protein 1, TYRP1), and OCA4 (membrane-associated transporter protein, MATP). OCA3 was thought to be restricted to black populations, where it was clinically described as rufous or brown albinism, until the recent report of a homozygous TYRP1 mutation in Caucasian patients from a consanguineous Pakistani family. Here, we describe a German patient of Caucasian origin, with a light-yellow skin, yellow-gold hair with orange highlights, fair eyelashes, several pigmented naevi, and no tendency to tan, only to burn. Eye-colour is blue-green with substance defects of the iris. Molecular analysis did not reveal any mutation in the TYR and OCA2 genes. Two mutations were found in the TYRP1 gene: a missense mutation (c.1066G>A/p.Arg356Glu) that was inherited from the mother, and a de novo single-base deletion (c.106delT/p.Leu36X). This finding suggests that mutation screening should be extended to the TYRP1 gene in patients from all ethnic origins, at least in cases where no mutations have been identified in the other OCA genes.     

Tyrosinase gene mutations in oculocutaneous albinism 1 (OCA1): definition of the phenotype

Oculocutaneous albinism (OCA) is a common human genetic condition resulting from mutations in at least twelve different genes. OCA1 results from mutations of the tyrosinase gene and presents with the life-long absence of melanin pigment after birth (OCA1A) or with the development of minimal-to-moderate amounts of cutaneous and ocular pigment (OCA1B). Other types of OCA have variable amounts of cutaneous and ocular pigment. We hypothesized that white hair at birth indicates OCA1 and tested this in a sample of 120 probands with OCA and white hair at birth. We found that 102 (85%) of the probands had OCA1 with one or two identifiable tyrosinase gene mutations, with 169 (83%) of the 204 OCA1 tyrosinase gene alleles having identifiable mutations and 35 (17%) having no identifiable change in the coding, splice junction, or proximal promoter regions of the gene. The inability to identify the mutation was more common with OCA1B (24/35, 69%) than with OCA1A (11/35, 31%) alleles. Seven probands with no tyrosinase gene mutations were found to have OCA2 with one or two P gene mutations, and in eleven, no mutations were detected in either gene. We conclude that (1) the presence of white hair at birth is a useful clinical tool suggesting OCA1 in a child or adult with OCA, although OCA2 may also have this presentation; (2) the molecular analysis of the tyrosinase and P genes are necessary for precise diagnosis; and (3) the presence of alleles without identifiable mutations of the tyrosinase gene, particularly in OCA1B, suggests that more complex mutation mechanisms of this gene are common in OCA.Electronic database Information: accession numbers and URLs for data presented in this article are as follows:Albinism Database, , for a list of published mutations of the tyrosinase geneOnline Mendelian Inheritance in Man (OMIM), , for OCA1 (MIM 203100), OCA2 (MIM 203200)     

Effects of tyrosinase activity on the cytotoxicity of 3,4-dihydroxybenzylamine and buthionine sulfoximine in human melanoma cells

The rationale for melanoma specific dihydroxybenzene containing antitumor agents is based in part upon the ability of the enzyme tyrosinase to oxidize these pro drugs to toxic intermediates. In situ tyrosinase activity was demonstrated to be affected by both cell density and time from plating in pigmented melanoma cells. Phenylthiourea, which completely inhibited tyrosinase activity with minimal cytotoxicity was found to block the growth inhibitory activity of the antitumor dopamine analog 3,4-dihydroxybenzylamine (3,4-DHBA) (NSC 263475). The antioxidant dithioerythritol was also found to inhibit tyrosinase activity and to block the growth inhibitory effects of 3,4-DHBA in pigmented melanoma cell lines. Buthionine sulfoximine (BSO) was shown to be cytotoxic to melanoma cells and its growth inhibitory effects appears to correlate with tyrosinase levels. Furthermore, BSO was shown to potentiate the growth inhibitory effects of 3,4-DHBA on marginally pigmented human melanoma cell lines.     

Variation in the tyrosinase gene associated with a white humpback whale (Megaptera novaeangliae)

Tyrosinase-negative oculocutaneous albinism (OCA1A) is characterized by lifelong white hair and skin, a phenotype that has been described in most mammalian species worldwide. Tyrosinase is the key enzyme in melanin biosynthesis, and mutations in the tyrosinase gene result in OCA1A. We examined sequence variation at exon 1 of the tyrosinase gene in 66 humpback whale samples collected from the east coast of Australia, including an anomalously white humpback whale known as "Migaloo." We identified 3 novel variants, including a cytosine deletion that results in a premature stop codon in exon 1. The deletion truncates the tyrosinase protein including the putative catalytic domains that are essential for tyrosinase enzymatic activity. Migaloo was homozygous for this deletion, suggesting that the albino phenotype is a consequence of inactive tyrosinase caused by the frameshift in the tyrosinase gene.     

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 missense 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 a helicies 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 a helices that consists of a loop structure. These mutations may affect tyrosinase activity by either altering the position of the a helical domains and thus preventing proper copper binding to the histidine ligands, or affecting a catalytic or substrate binding site located between the two a helical domains.     

The etiology of oculocutaneous albinism (OCA) type II: the pink protein modulates the processing and transport of tyrosinase

Oculocutaneous albinism (OCA) is caused by reduced or deficient melanin pigmentation in the skin, hair, and eyes. OCA has different phenotypes resulting from mutations in distinct pigmentation genes involved in melanogenesis. OCA type 2 (OCA2), the most common form of OCA, is an autosomal recessive disorder caused by mutations in the P gene, the function(s) of which is controversial. In order to elucidate the mechanism(s) involved in OCA2, our group used several antibodies specific for various melanosomal proteins (tyrosinase, Tyrp1, Dct, Pmel17 and HMB45), including a specific set of polyclonal antibodies against the p protein. We used confocal immunohistochemistry to compare the processing and distribution of those melanosomal proteins in wild type (melan-a) and in p mutant (melan-p1) melanocytes. Our results indicate that the melanin content of melan-p1 melanocytes was less than 50% that of wild type melan-a melanocytes. In contrast, the tyrosinase activities were similar in extracts of wild type and p mutant melanocytes. Confocal microscopy studies and pulse-chase analyses showed altered processing and sorting of tyrosinase, which is released from melan-p1 cells to the medium. Processing and sorting of Tyrp1 was also altered to some extent. However, Dct and Pmel17 expression and subcellular localization were similar in melan-a and in melan-p1 melanocytes. In melan-a cells, the p protein showed mainly a perinuclear pattern with some staining in the cytoplasm where some co-localization with HMB45 antibody was observed. These findings suggest that the p protein plays a major role in modulating the intracellular transport of tyrosinase and a minor role for Tyrp1, but is not critically involved in the transport of Dct and Pmel17. This study provides a basis to understand the relationship of the p protein with tyrosinase function and melanin synthesis, and also provides a rational approach to unveil the consequences of P gene mutations in the pathogenesis of OCA2.     

Tyrosinase activity (TH, DO, PAGE-defined isozymes) and melanin production in regenerating hairbulb melanocytes of lethal yellow (Ay/a), black (a/a), agouti (AwJ/AwJ/) and albino (a/a/c2J/c2J) mice (C57BL/6J)

We compared tyrosinase activity (TH, DO, and native PAGE-defined isozymes) and melanin production in particulate and soluble fractions of hairbulb melanocytes of lethal yellow (Ay/a C/C), nonagouti black (a/a C/C), and albino (a/a c2J/c2J) of 3-, 6-, 9-, and 12-day regenerating hairbulbs. With respect to tyrosine hydroxylase (TH) and dopa oxidase (DO) activities, Ay/a melanocytes possessed only 25-35% of the activity of a/a; there were no genotype differences in either the subcellular distribution of activity in soluble and particulate fractions or in the relative increases of activity over the 12-day developmental period. TH data on wild-type agouti (AwJ/AwJ) mice over the 3-11 day regeneration interval showed an activity intermediate between that of a/a and Ay/a; the rate of TH increase reflected black and yellow phases of the agouti hair cycle. Analyses of the number and densities of dopa-sensitive bands following native PAGE of 3-, 6-, 9-, and 12-day hairbulb fractions of a/a and Ay/a mice suggested stage-dependent patterns. A comparison of rates and amounts of melanin production in 3-, 6-, 9-, and 12-day fractions showed consistent melanin production in Ay/a to be 10-20% that of a/a; however, fold increases in melanin production over the four stages were similar between genotypes. Overall, tyrosinase activity data support the notion that agouti locus modification of tyrosinase activity is a graded or quantitative rather than a qualitative phenomenon.     

Mutations of the tyrosinase gene in Indo-Pakistani patients with type I (tyrosinase-deficient) oculocutaneous albinism (OCA).

Oculocutaneous albinism (OCA) is a group of autosomal recessive disorders characterized by deficient synthesis of melanin pigment. Type I (tyrosinase-deficient) OCA results from mutations of the tyrosinase gene (TYR gene) encoding tyrosinase, the enzyme that catalyzes the first two steps of melanin biosynthesis. Mutations of the TYR gene have been identified in a large number of patients, most of Caucasian ethnic origin, with various forms of type I OCA. Here, we present an analysis of the TYR gene in eight Indo-Pakistani patients with type I OCA. We describe four novel TYR gene mutations and a fifth mutation previously observed in a Caucasian patient.     

Novel and recurrent mutations in the tyrosinase gene and the P gene in the German albino population

Albinism is a heterogeneous group of genetic disorders resulting from deficiencies in pigmentation. Clinically, it is divided into ocular (OA) and oculocutaneous albinism (OCA). OCA involves lack of pigment in the skin, hair, and eyes and results from mutations in the tyrosinase gene or in the P gene. OA mainly affects pigmentation in the visual system and may be a mild form of OCA or may be caused by other genetic defects. Clinical diagnosis of albinism type is difficult, because of the observed range of phenotypic variation. Thus, genetic analysis may be helpful with respect to a more accurate diagnosis. Here, we report the mutational profile, determined by genetic analysis of the tyrosinase and P genes, of a large German albino population. We have revealed a total of 42 distinct mutations, 19 of which are novel. Of the 74 unrelated patients screened, 32 (43%) had mutations in the tyrosinase gene, 16 (22%) had P gene mutations, and 26 (35%) patients had no detectable genetic abnormalities. This defines a population of albino patients who are tyrosinase-gene- and P-gene-negative and who thus may represent a good study group for searching for additional genes associated with albinism.     

The Tyrosinase Gene in Gorillas and the Albinism of ‘Snowflake’

The sequence of the tyrosinase (Tyr) gene coding tracts has been obtained for the gorilla (Gorilla gorilla gorilla). The five exons of the gene were sequenced in three gorillas and in a normally pigmented human. The tyrosinase gene has been found to be a very conserved locus with a very low substitution rate. Some nucleotide and amino acid differences were found between the gorilla and human tyrosinase coding sequences. One of the gorillas included in the study is the only known case of albinism in a gorilla (‘Snowflake’). Mutations of the TYR gene lead to Oculocutaneous Albinism type 1 (OCA1), the most common type of albinism in humans (OMIM accession number 203100). The TYR gene encodes the tyrosinase enzyme (E.C. 1.14.18.1), whose activity was found to be completely lacking in ‘Snowflake’, indicating that a mutation in the Tyr gene is the likely cause of his albinism. Nonetheless, no nucleotide changes were detected that could account for the lack of Tyr product or tyrosinase activity in Snowflake, and explanations of these findings are discussed.