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.     

Type 2 oculocutaneous albinism (OCA2) in Zimbabwe and Cameroon: distribution of the 2.7-kb deletion allele of the P gene

In previous studies, we characterized a 2.7-kb interstitial deletion allele of the P gene associated with tyrosinase-positive oculocutaneous albinism (OCA2) in African Americans and Africans. In this study, we investigated the frequency of this allele among OCA2 subjects in two African countries, Zimbabwe and Cameroon. The deletion allele was most common in Zimbabwe, comprising nearly all (92%) mutant alleles, which is the highest incidence reported so far. In addition, the deletion allele was widespread but less common among OCA2 Cameroonians and accounted for 65% of the mutant alleles. Received: 20 March 1997 / Accepted: 8 May 1997     

An intragenic deletion of the P gene is the common mutation causing tyrosinase-positive oculocutaneous albinism in southern African Negroids.

Tyrosinase-positive oculocutaneous albinism (OCA2), an autosomal recessive disorder of the melanin biosynthetic pathway, is the most common recessive disorder occurring in southern African Bantu-speaking Negroids, with an overall prevalence of 1/3,900. The OCA2 gene, P, has been mapped to chromosome 15q11-q13, and recently alterations in the P gene have been identified in OCA2 individuals. An intragenic deletion has been described and proposed to be of African origin because of its occurrence in four unrelated African American OCA2 individuals and in two individuals, one from Zaire and the other from Cameroon. This study shows that the intragenic deletion is a common cause of OCA2 in southern African Negroids (114/146 [.78]; OCA2 chromosomes) and is associated with one common haplotype (43/55 [.78]; OCA2 chromosomes), confirming the African origin of this allele. On the basis of haplotype data, it would appear that at least seven additional, less frequent OCA2 mutations occur in this population.     

MC1R mutations modify the classic phenotype of oculocutaneous albinism type 2 (OCA2)

The heterogeneous group of disorders known as oculocutaneous albinism (OCA) shares cutaneous and ocular hypopigmentation associated with common developmental abnormalities of the eye. Mutations of at least 11 loci produce this phenotype. The majority of affected individuals develop some cutaneous melanin; this is predominantly seen as yellow/blond hair, whereas fewer have brown hair. The OCA phenotype is dependent on the constitutional pigmentation background of the family, with more OCA pigmentation found in families with darker constitutional pigmentation, which indicates that other genes may modify the OCA phenotype. Sequence variation in the melanocortin-1 receptor (MC1R) gene is associated with red hair in the normal population, but red hair is unusual in OCA. We identified eight probands with OCA who had red hair at birth. Mutations in the P gene were responsible for classic phenotype of oculocutaneous albinism type 2 (OCA2) in all eight, and mutations in the MC1R gene were responsible for the red (rather than yellow/blond) hair in the six of eight who continued to have red hair after birth. This is the first demonstration of a gene modifying the OCA phenotype in humans.     

A 122.5-kilobase deletion of the P gene underlies the high prevalence of oculocutaneous albinism type 2 in the Navajo population

Oculocutaneous albinism (OCA) is a genetically heterogeneous disorder. There are four known types of OCA: OCA1-OCA4. The clinical manifestations of all types of OCA include skin and hair hypopigmentation and visual impairment. Although there are a few documented observations of high frequency of albinism among Native Americans, including the Hopi, Zuni, Kuna, Jemez, Laguna, San Juan, and Navajo, no causative molecular defect has been previously reported. In the present study, we show that albinism in one Native American population, the Navajo, is caused by a LINE-mediated 122.5-kilobase deletion of the P gene, thus demonstrating that albinism in this population is OCA2. This deletion appears to be Navajo specific, because this allele was not detected in 34 other individuals with albinism who listed other Native American origins, nor has it been reported in any other ethnic group. The molecular characterization of this deletion allele allowed us to design a three-primer polymerase chain reaction system to estimate the carrier frequency in the Navajo population by screening 134 unrelated normally pigmented Navajos. The carrier frequency was found to be approximately 4.5%. The estimated prevalence of OCA2 in Navajos is between approximately 1 per 1,500 and 1 per 2,000. We further estimate that this mutation originated 400-1,000 years ago from a single founder.     

The mouse p (pink-eyed dilution) and human P genes, oculocutaneous albinism type 2 (OCA2), and melanosomal pH

Recessive mutations of the mouse p (pink-eyed dilution) gene lead to hypopigmentation of the eyes, skin, and fur. Mice lacking a functional p protein have pink eyes and light gray fur (if non-agouti) or cream-colored fur (if agouti). The human orthologue is the P protein. Humans lacking a functional P protein have oculocutaneous albinism type 2 (OCA2). Melanocytes from p-deficient mice or OCA2 individuals contain small, minimally pigmented melanosomes. The mouse and human proteins are predicted to have 12 membrane spanning domains and possess significant sequence homology to a number of membrane transport proteins, some of which are involved in the transport of anions. The p protein has been localized to the melanosome membrane. Recently, it has been shown that melanosomes from p protein-deficient melanocytes have an abnormal pH. Melanosomes in cultured melanocytes derived from wild-type mice are typically acidic, whereas melanosomes from p protein-deficient mice are non-acidic. Melanosomes and related endosome-derived organelles (i.e., lysosomes) are thought to have an adenosine triphosphate (ATP)-driven proton pump that helps to generate an acidic lumen. To compensate for the charge of these protons, anions must also be transported to the lumen of the melanosome. In light of these observations, a model of p protein function is presented in which the p protein, together with the ATP-driven proton pump, regulates the pH of the melanosome.     

Oculocutaneous albinism (OCA2) in sub-Saharan Africa: distribution of the common 2.7-kb P gene deletion mutation

Oculocutaneous albinism (OCA2) is the most common autosomal recessive disorder in the South African Negroid population, occurring with a prevalence of 1/3900 individuals. The OCA2 locus, P, has been mapped to chromosome 15q11–q13 and a 2.7-kb interstitial deletion has been found to be the common mutation in Africa. This study reports the detection of the deletion allele in OCA2-affected individuals from the southern African, Zambian and Central African Republic (CAR) Negroid populations (0.77, 131/170 OCA2 chromosomes; 0.79, 11/14; 0.33, 4/12, respectively). Normally pigmented individuals from different African countries were also tested. The deletion mutation was found at a frequency of 0.013 (10/780) in the normally pigmented southern African Negroid population and at a lower frequency in individuals from central Africa (0.002; 2/834), including individuals from Zambia, Cameroon, Zaire and the CAR. The study confirms the African origin of this deletion allele. Haplotype analysis suggests that the deletion mutation probably occurred only once and that it arose before the divergence of these African populations, which is estimated to be about 2000– 3000 years ago. The unusually high frequency of OCA2 mutations, in particular the 2.7-kb deletion, suggests some selective agent or genetic drift. Received: 24 September 1996 / Revised: 8 November 1996     

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.     

眼皮肤白化病患者酪氨酸酶基因突变的研究

目的：对临床诊断为眼皮肤白化病（OCA）患者的酪氨酸酶（TYR）基因进行突变筛查,了解我国大陆OCA患者TYR基因突变类型,探讨基因突变对人TYR蛋白结构和功能的影响。方法：应用PCR技术,扩增患者及其父母的TYR基因外显子、外显子-内含子交界区及启动子区;以DNA序列测定技术,进行突变筛查与鉴定;利用生物信息学方法,对突变引起蛋白结构和功能的改变进行预测与分析。结果：在15名患者的30个TYR等位基因内,查明11种突变;其中错义突变5种（W400L、R299H、E294K、R77Q和K142M）,无义突变3种（R116X、R278X和G295X）,插入突变2种（929insC和232insGGG）,剪切位点突变1种（IVS1-3 C〉G）;对4个突变W400L、R299H、929insC、232insGGG的生物信息学分析显示,突变的致病性与蛋白结构和功能的改变相关。结论：W400L占本研究所检出全部OCA1突变等位基因的30．0％（9／30）,可能为中国大陆人群中较常见的TYR基因突变类型;应用生物信息学分析方法对TYR基因突变的致病性做出一些合理可能的解释是可行的。     

A single base deletion in the SLC45A2 gene in a Bullmastiff with oculocutaneous albinism

Oculocutaneous albinism type 4 (OCA4) in humans and similar phenotypes in many animal species are caused by variants in the SLC45A2 gene, encoding a putative sugar transporter. In dog, two independent SLC45A2 variants are known that cause oculocutaneous albinism in Doberman Pinschers and several small dog breeds respectively. For the present study, we investigated a Bullmastiff with oculocutaneous albinism. The affected dog was highly inbred and resulted from the mating of a sire to its own grandmother. We obtained whole genome sequence data from the affected dog and searched specifically for variants in candidate genes known to cause albinism. We detected a single base deletion in exon 6 of the SLC45A2 gene (NM_001037947.1:c.1287delC) that has not been reported thus far. This deletion is predicted to result in an early premature stop codon. It was confirmed by Sanger sequencing and perfectly co‐segregated with the phenotype in the available family members. We genotyped 174 unrelated dogs from diverse breeds, all of which were homozygous wildtype. We therefore suggest that SLC45A2:c.1287delC causes the observed oculocutaneous albinism in the affected Bullmastiff.     

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.     

Functional characterization of two novel splicing mutations in the OCA2 gene associated with oculocutaneous albinism type II

Oculocutaneous albinism (OCA) is characterized by hypopigmentation of the skin, hair and eye, and by ophthalmologic abnormalities caused by a deficiency in melanin biosynthesis. OCA type II (OCA2) is one of the four commonly-recognized forms of albinism, and is determined by mutation in the OCA2 gene.     

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.     

High‐resolution array‐CGH in patients with oculocutaneous albinism identifies new deletions of the TYR,OCA2, and SLC45A2 genes and a complex rearrangement of the OCA2 gene

Oculocutaneous albinism (OCA) is caused by mutations in six different genes, and their molecular diagnosis encompasses the search for point mutations and intragenic rearrangements. Here, we used high‐resolution array‐comparative genome hybridization (CGH) to search for rearrangements across exons, introns and regulatory sequences of four OCA genes: TYR, OCA2, TYRP1, and SLC45A2. We identified a total of ten new deletions in TYR, OCA2, and SLC45A2. A complex rearrangement of OCA2 was found in two unrelated patients. Whole‐genome sequencing showed deletion of a 184‐kb fragment (identical to a deletion previously found in Polish patients), whereby a large portion of the deleted sequence was re‐inserted after severe reshuffling into intron 1 of OCA2. The high‐resolution array‐CGH presented here is a powerful tool to detect gene rearrangements. Finally, we review all known deletions of the OCA1–4 genes reported so far in the literature and show that deletions or duplications account for 5.6% of all mutations identified in the OCA1–4 genes.     

Homozygous tyrosinase gene mutation in an American black with tyrosinase-negative (type IA) oculocutaneous albinism

We have identified a tyrosinase gene mutation in an American black with classic, tyrosinase-negative oculocutaneous albinism. This mutation results in an amino acid substitution (Cys----Arg) at codon 89 of the tyrosinase polypeptide. The proband is homozygous for the substitution, suggesting that this mutation may be frequently associated with tyrosinase-negative oculocutaneous albinism in blacks.     

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.     

Three different frameshift mutations of the tyrosinase gene in type IA oculocutaneous albinism.

Mutations in the gene for the pigment-producing enzyme tyrosinase are responsible for type IA (tyrosinase-negative) oculocutaneous albinism (OCA). Most reported mutations have been single base substitutions. We now report three different frameshift mutations in three unrelated individuals with type IA OCA. The first individual has a single base deletion within a series of five guanidines, resulting in a premature stop codon in exon I on one allele and a missense mutation at codon 382 in exon III on the homologous allele. The second individual is a genetic compound of two separate frameshift mutations, including both the same exon I single base deletion found in the first individual and a deletion of a thymidine-guanidine pair, within the sequence GTGTG, forming a termination codon (TAG) in exon I on the homologous allele. The third individual has a single base insertion in exon I on one allele and a missense mutation at codon 373 in exon III on the homologous allele. The two missense mutations occur within the copper Bbinding region and may interfere with either copper binding to the enzyme or oxygen binding to the copper. These five different mutations disrupt tyrosinase function and are associated with a total lack of melanin biosynthesis.