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.     

Evolution of the Tyrosinase Related Gene (TYRL) in Primates

Tyrosinase is the major enzyme responsible for the formation of melanin pigment and is found throughout the animal kingdom. In humans, the tyrosinase gene (TYR) maps to the long arm of chromosome 11 at band q14→q21, while a tyrosinase related gene (TYRL) maps to the short arm of chromosome 11 at pll.2°Cen. We and others have found that the TYRL locus contains sequences that are similar to exons IV and V of the authentic tyrosinase gene but lacks sequences of exons I, II, and III. In an attempt to understand the evolution of the human tyrosinase gene, we have analyzed TYR and TYRL in primates and have found that exons IV and V of the chimpanzee and gorilla TYR are very similar to the human, with the gorilla sequence being more similar than the chimpanzee. We have also found that the gorilla but not the chimpanzee contains a TYRL locus similar to the human TYRL locus.     

A Tyrosinase missense mutation causes albinism in the Wistar rat

Tyrosinase serves as a key enzyme in the synthesis of melanin. In humans mutations in the TYR gene are associated with type 1 oculocutaneous albinism (OCA1) that leads to reduced or absent pigmentation of skin, hair and eye. Various mutations causing OCA in man, mouse, rabbit and cattle have been identified throughout the Tyrosinase gene including nonsense, missense, frameshift and splice site alterations. Here we report a missense substitution at codon R299H in exon 2 of the Tyr gene in the albino Wistar rat. As this very exchange has already been described in OCA patients, our findings reinforce the significance of this region for normal catalytic activity of tyrosinase protein.     

Identification of a tyrosinase (TYR) exon 4 deletion in albino ferrets (Mustela putorius furo)

Albinism is due to a lack of pigmentation in hair, skin and eye, and has been shown to occur in several animal species. Mutations of the tyrosinase (TYR) gene account for albinism in domestic cats, rabbits, cattle, mice and rats. In this study, we demonstrate that a TYR mutation accounts for albinism in the ferret (Mustela putorius furo). The coding sequence of the five exons of TYR was determined in genomic DNA from wild-type pigmented 'sable' coloured and albino ferrets. It was not possible to amplify TYR exon 4 in albino ferrets originating from different breeds. The deletion of exon 4 in albino ferrets was confirmed by Southern blot hybridization of genomic DNA from albino and pigmented ferrets. This is the first report of a deletion of a TYR exon in a non-human mammal.     

人与大猩猩,黑猩猩和猩猩亲缘关系的探讨

有关人锆超科的系统发育仍然存在刍议。争论焦点在与大猩猩和黑猩猩哪 个关系更近一点。酪氨酸酶是黑色素合成中的关键酶,酪氨酶基因的突变将导致白化病。测定了人猿科中大猩猩,黑猩猩、猩猩和长臂锆产基因全部５个外显子的ＤＮＡ序列。     

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.     

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

The enzyme tyrosinase (monophenol,L-dopa:oxygen oxidoreductase; EC 1.14.18.1) catalyzes the first two steps in the conversion of tyrosine to melanin, the major pigment found in melanocytes. Some forms of oculocutaneous albinism, characterized by the absence of melanin in skin and eyes and by a deficiency of tyrosinase activity, may result from mutations in the tyrosinase structural gene. A recently isolated human tyrosinase cDNA was used to map the human tyrosinase locus (TYR) to chromosome 11, region q14----q21, by Southern blot analysis of somatic cell hybrid DNA and by in situ chromosomal hybridization. A second site of tyrosinase-related sequences was detected on the short arm of chromosome 11 near the centromere (p11.2----cen). Furthermore, we have confirmed the localization of the tyrosinase gene in the mouse at or near the c locus on chromosome 7. Comparison of the genetic maps of human chromosome 11 and mouse chromosome 7 leads to hypotheses regarding the evolution of human chromosome 11.     

The tyrosinase gene and oculocutaneous albinism type 1 (OCA1): A model for understanding the molecular biology of melanin formation

Through the last century there has been a steady progression in our understanding of the biology of melanin biosynthesis. Much of this work includes the analysis of coat color mutations of the mouse and albinism in man. Our understanding has been greatly enhanced in the last 10 years, as the molecular pathogenesis of albinism has been better understood. Different mutations of the tyrosinase gene (TYR) , and their association with oculocutaneous albinism type 1 (OCA1) has provided insight into the biology of tyrosinase, including protein trafficking and structure/function analysis. Several questions still remain, including cryptic mutations that affect tyrosinase activity and the minimum amount of pigment required for normal optic development. The next 10 years should prove just as exciting as the last.     

Mutational analysis of TYR gene and its structural consequences in OCA1A

Oculocutaneous albinism type 1A (OCA1A) is the most severe form of albinism characterized by a complete lack of melanin production throughout life and is caused by mutations in the TYR gene. TYR gene codes tyrosinase protein to its relation with melanin formation by knowing the function of these SNPs. Based on the computational approaches, we have analyzed the genetic variations that could change the functional behaviour by altering the structural arrangement in TYR protein which is responsible for OCA1A. Consequences of mutation on TYR structure were observed by analyzing the flexibility behaviour of native and mutant tyrosinase protein. Mutations T373K, N371Y, M370T and P313R were suggested as high deleterious effect on TYR protein and it is responsible for OCA1A which were also endorsed with previous in vivo experimental studies. Based on the quantitative assessment and flexibility analysis of OCA1A variants, T373K showed the most deleterious effect. Our analysis determines that certain mutations can affect the dynamic properties of protein and can lead to disease conditions. This study provides a significant insight into the underlying molecular mechanism involved in albinism associated with OCA1A.     

Evolution of RH genes in hominoids: characterization of a gorilla RHCE-like gene

The human RH locus is responsible for the expression of the Rh blood group antigens. It consists of two closely linked genes, RHD and RHCE, that exhibit 92% similarity between coding regions. These observations suggest that they are derived from a relatively recent duplication event. Previously a study of nonhuman primate RH-like genes demonstrated that ancestral RH gene duplication occurred in the common ancestor of man, chimpanzees and gorillas. By amplification of intron 3 and intron 4 of gorilla RH-like genes, we have now shown that, like man, gorillas possess two types of RH intron 3 (RHCE intron 3 being 289 bp longer than the RHD intron 3) and two types of intron 4 (RHCE intron 4 being 654 bp longer than the RHD intron 4). Here we report the characterization of a cDNA encoded by a gorilla RH-like gene which possesses introns 3 and 4 of the RHCE type. A comparison of this gorilla RHCE-like coding sequence with previously characterized human and ape cDNA sequences suggests that RH genes experienced complex recombination events after duplication in the common ancestor of humans, chimpanzees and gorillas.     

Mutations of the tyrosinase gene in patients with oculocutaneous albinism from various ethnic groups in Israel.

We have analyzed the tyrosinase (TYR) gene in 38 unrelated patients with oculocutaneous albinism (OCA), derived from several different ethnic groups of the diverse population of Israel. We detected TYR gene mutations in 23 of the 34 patients with apparent type I (i.e., tyrosinase-deficient) OCA and in none of the patients with other clinical forms of albinism. Among Moroccan Jews with type IA (i.e., tyrosinase-negative) OCA, we detected a highly predominant mutant allele containing a missense substitution, Gly47Asp (G47D). This mutation occurs on the same haplotype as in patients from the Canary Islands and Puerto Rico, suggesting that the G47D mutation in these ethnically distinct populations may stem from a common origin.     

Albinism in the domestic cat (Felis catus) is associated with a tyrosinase (TYR) mutation

Albino phenotypes are documented in a variety of species including the domestic cat. As albino phenotypes in other species are associated with tyrosinase (TYR) mutations, TYR was proposed as a candidate gene for albinism in cats. An Oriental and Colourpoint Shorthair cat pedigree segregating for albinism was analysed for association with TYR by linkage and sequence analyses. Microsatellite FCA931, which is closely linked to TYR and TYR sequence variants were tested for segregation with the albinism phenotype. Sequence analysis of genomic DNA from wild-type and albino cats identified a cytosine deletion in TYR at position 975 in exon 2, which causes a frame shift resulting in a premature stop codon nine residues downstream from the mutation. The deletion mutation in TYR and an allele of FCA931 segregated concordantly with the albino phenotype. Taken together, our results suggest that the TYR gene corresponds to the colour locus in cats and its alleles, from dominant to recessive, are as follows: C (full colour) > c(b) (burmese) > or = c(s) (siamese) > c (albino).     

不同品种实验兔虹膜酪氨酸酶基因序列和表达量的变化

目的从酪氨酸酶基因序列和表达量两个方面探讨酪氨酸酶与家兔虹膜颜色表型的关系。方法通过PCR扩增和测序检测4个具有不同颜色性状的家兔品种的酪氨酸酶基因外显子序列多态性;通过荧光定量PCR检测酪氨酸酶基因表达水平。结果白化品种日本大耳白兔和獭兔的TYR基因序列在第1118个碱基处都由C突变为A,并导致编码蛋白在373位,即最后一个N-糖基化位点发生由Thr到Lys的突变。白毛黑眼兔和青紫兰兔在第870个碱基处全部发生由A到T的无义突变。在白毛黑眼兔种群的所有个体和獭兔种群的部分个体中都发现TYR基因序列在第91个碱基处发生G到A的突变,导致氨基酸序列第31位处Val到Met的变异。经内参基因GAPDH的校正,TYR基因在白毛黑眼兔和青紫兰兔中表达水平显著高于在日本大耳白兔和獭兔中的表达水平（P〈0.01）。而在白毛黑眼兔和青紫兰兔之间、日本大耳白兔和獭兔之间,TYR基因的表达差异没有显著性。结论家兔TYR基因突变可能大幅度降低TYR基因表达,导致酪氨酸酶功能低下,从而影响虹膜颜色表型。     

眼皮肤白化病分型及珊基因新突变的鉴定

目的了解我国眼皮肤白化病（oculocutaneous albinism,OCA）的分型和相关基因突变类型,探讨新突变可能的分子致病机制。方法应用PCR方法扩增TYR基因,经DNA序列测定检出突变,采用错配引物PCR进行新突变的群体筛查,结合生物信息学方法探讨一种新突变的致病性和可能的分子致病机制。结果10名患者中有5人存在2个突变TYR等位基因,共计8种突变类型,其中c．71G〉A（C24Y）和c．841G〉T（E281X）是OCA1A致病性新突变;C24极可能参与二硫键形成,C24Y将导致酪氨酸酶肽链内此二硫键消失,进而引起蛋白空间构象变化和功能异常而致病。结论从基因水平初步了解了我国OCA1所占的比例,探讨了TYR基因C24Y的致病性并初步阐明了其致病的分子机制。本结果丰富了人类TYR基因突变类型,为我国OCA分型诊断、产前基因诊断和遗传咨询等积累了有价值的数据资料。     

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.     

Tyrosinase and tyrosinase related protein 1 alleles specify domestic cat coat color phenotypes of the albino and brown loci

The genes encoding enzymes of the tyrosinase family are strong candidates for coat color variation in mammals. To investigate their influence in domestic cat coat color, we determined the complete nucleotide coding sequence of the domestic cat genes tyrosinase (TYR)--a plausible candidate gene for the albino (C) locus, and tyrosinase related protein 1 (TYRP1)--a candidate gene for the brown (B) locus. Sequence variants between individuals exhibiting variation in pigmentation were submitted to association studies. In TYR, two nonsynonymous substitutions encoding TYR-G301R and TYR-G227W were associated with the siamese and burmese phenotypes of the albino locus, respectively. TYRP1 was mapped on chromosome D4 within 5 cM of a highly polymorphic microsatellite, previously found to be fixed in a cat breed selected for the chocolate (b) allele of the B locus, which reinforced TYRP1 as a candidate gene for the B locus in the domestic cat. Two DNA polymorphisms, one leading to a TYRP1-A3G substitution in the signal peptide and another to an in-frame insertion TYRP1-421ins17/18 caused by a donor splice site mutation in intron 6, were associated with the chocolate (b) allele. A premature UAG stop codon at position 100 of TYRP1 was associated with a second allele of the B locus, cinnamon (b(l)). The results provide very strong evidence that the specific nucleotide variants of feline TYR (chromosome D1) are causative of the siamese (c(s)) and burmese (c(b)) alleles of the albino locus, as well as nucleotide variants of TYRP1 (chromosome D4) as specifying the chocolate (b) and cinnamon (b(l)) alleles of the B locus.     

Albinism in the American mink (Neovison vison) is associated with a tyrosinase nonsense mutation

Albino phenotypes are documented in various species including the American mink. In other species the albino phenotypes are associated with tyrosinase (TYR) gene mutations; therefore TYR was considered the candidate gene for albinism in mink. Four microsatellite markers were chosen in the predicted region of the TYR gene. Genotypes at the markers Mvi6025 and Mvi6034 were found to be associated with the albino phenotype within an extended half-sib family. A BAC clone containing Mvi6034 was mapped to chromosome 7q1.1-q1.3 by fluorescent in situ hybridization. Subsequent analysis of genomic TYR sequences from wild-type and albino mink samples identified a nonsense mutation in exon 1, which converts a TGT codon encoding cysteine to a TGA stop codon (c.138T>A, p.C46X; EU627590). The mutation truncates more than 90% of the normal gene product including the putative catalytic domains. The results indicate that the nonsense mutation is responsible for the albino phenotype in the American mink.     

Investigation of the RH locus in gorillas and chimpanzees

The human Rh blood-group system is encoded by two homologous genes,RhD andRhCE. TheRH genes in gorillas and chimpanzees were investigated to delineate the phylogeny of the humanRH genes. Southern blot analysis with an exon 7-specific probe suggested that gorillas have more than twoRH genes, as has recently been reported for chimpanzees. Exon 7 was well conserved between humans, gorillas, and chimpanzees, although the exon 7 nucleotide sequences from gorillas were more similar to the humanD gene, whereas the nucleotide sequences of this exon in chimpanzees were more similar to the humanCE gene. The intron between exon 4 and exon 5 is polymorphic and can be used to distinguish the humanD gene from theCE gene. Nucleotide sequencing revealed that the basis for the intron polymorphism is anAlu element inCE which is not present in theD gene. Examination of gorilla and chimpanzee genomic DNA for this intron polymorphism demonstrated that theD intron was present in all the chimpanzees and in all but one gorilla. TheCE intron was found in three of six gorillas, but in none of the seven chimpanzees. Sequence data suggested that theAlu element might have previously been present in the chimpanzeeRH genes but was eliminated by excision or recombination. Conservation of theRhD gene was also apparent from the complete identity between the 3′-noncoding region of the human D cDNA and a gorilla genomic clone, including anAlu element which is present in both species. The data suggest that at least twoRH genes were present in a common ancestor of humans, chimpanzees, and gorillas, and that additionalRH gene duplication has taken place in gorillas and chimpanzees. TheRhCE gene appears to have diverged more thanRhD among primates. In addition, theRhD gene deletion associated with the Rh-negative phenotype in humans seems to have occurred after speciation. Correspondence to: C.M. Westhoff     

Rh gene evolution in primates: study of intron sequences

By amplification and sequencing of RH gene intron 4 of various primates we demonstrate that an Alu-Sx-like element has been inserted in the RH gene of the common ancestor of humans, apes, Old World monkeys, and New World monkeys. The study of mouse and lemur intron 4 sequences allowed us to precisely define the insertion point of the Alu-Sx element in intron 4 of the RH gene ancestor common to Anthropoidea. Like humans, chimpanzees and gorillas possess two types of RH intron 4, characterized by the presence (human RHCE and ape RHCE-like genes) or absence (human RHD and ape RHD-like genes) of the Alu-Sx element. This led us to conclude that in the RH common ancestor of humans, chimpanzees, and gorillas, a duplication of the common ancestor gene gave rise to two genes, one differing from the other by a 654-bp deletion encompassing an Alu-Sx element. Moreover, most of chimpanzees and some gorillas posses two types of RHD-like intron 4. The introns 4 of type 1 have a length similar to that of human RHD intron 4, whereas introns 4 of type 2 display an insertion of 12 bp. The latest insertion was not found in the human genome (72 individuals tested). The study of RH intron 3 length polymorphism confirmed that, like humans, chimpanzees and gorillas possess two types of intron 3, with the RHD-type intron 3 being 289 bases shorter than the RHCE intron 3. By amplification and sequencing of regions encompassing introns 3 and 4, we demonstrated that chimpanzee and gorilla RH-like genes displayed associations of introns 3 and 4 distinct to those found in man. Altogether, the results demonstrate that, as in humans, chimpanzee and gorilla RH genes experienced intergenic exchanges.     

Mutations in the human orthologue of the mouse underwhite gene (uw) underlie a new form of oculocutaneous albinism, OCA4

Oculocutaneous albinism (OCA) affects approximately 1/20,000 people worldwide. All forms of OCA exhibit generalized hypopigmentation. Reduced pigmentation during eye development results in misrouting of the optic nerves, nystagmus, alternating strabismus, and reduced visual acuity. Loss of pigmentation in the skin leads to an increased risk for skin cancer. Two common forms and one infrequent form of OCA have been described. OCA1 (MIM 203100) is associated with mutations of the TYR gene encoding tyrosinase (the rate-limiting enzyme in the production of melanin pigment) and accounts for approximately 40% of OCA worldwide. OCA2 (MIM 203200), the most common form of OCA, is associated with mutations of the P gene and accounts for approximately 50% of OCA worldwide. OCA3 (MIM 203290), a rare form of OCA and also known as "rufous/red albinism," is associated with mutations in TYRP1 (encoding tyrosinase-related protein 1). Analysis of the TYR and P genes in patients with OCA suggests that other genes may be associated with OCA. We have identified the mouse underwhite gene (uw) and its human orthologue, which underlies a new form of human OCA, termed "OCA4." The encoded protein, MATP (for "membrane-associated transporter protein") is predicted to span the membrane 12 times and likely functions as a transporter.