C pigment locus mutants of the fowl produce enzymatically inactive tyrosinase-like molecules

Mutations of TYR and MITF Genes are Associated with Plumage Colour Phenotypes in Geese

The polymorphism of microphthalmia-associated transcription factor (MITF) and tyrosinase (TYR) genes have been proposed to play a vital role in coat colour genesis in mammals, but their role remains ambiguous in geese at best. Here, we cloned and sequenced 1,397 bp coding region of MITF gene and a 588 bp fragment of TYR exon 1 for polymorphism analysis among 157 domestic geese showing three types of plumage colour. We detected a total of three SNPs (c.280T>C, c.345G>A, and c.369G>A) in TYR and six haplotypes (H1–H6). Among them, haplotypes H1, H2, H3, and H5 were significantly associated with white plumage trait of Zhedong White Geese. However, only diplotype H1H1 and H3H5 were significantly associated with white plumage trait of Zhedong White Geese (p<0.01). We only detected one SNP (c.1109C>T) for MITF gene and found that genotype CT and TT were significantly associated with white plumage trait of Zhedong White Geese. Briefly, our study suggested an association between polymorphisms of TYR and MITF genes and the plumage colour trait in domestic geese.

Mapping of the Recessive White Locus and Analysis of the Tyrosinase Gene in Chickens

An F(2) chicken population of 265 individuals, obtained from an intercross between the Japanese Game (colored plumage) and the White Plymouth Rock (the recessive white) and genotyped for microsatellite markers, was used for determining the locus of the gene responsible for the recessive white plumage phenotype in chickens. Two hundred twenty-five markers were mapped in 28 linkage groups. Linkage analysis revealed that the recessive white gene was mapped to chromosome 1. Detailed analysis using additional markers uncovered a significant linkage between 2 new markers, mapped to the flanking region of the tyrosinase gene, which is associated with skin and plumage color. The sequence of the tyrosinase gene was investigated in recessive white chickens and colored chickens. There were no obvious differences in the tyrosinase gene exons between the recessive white chicken and the colored chicken. However, sequence analysis of tyrosinase intron 4 in the recessive white chicken revealed a presence of an insertion of an avian retroviral sequence. The White Plymouth Rock and the F(2) generation with white plumage were identified as homozygous carriers of the retroviral sequence. Expression of the normal transcript containing exon 5 was substantially decreased in the recessive white chicken compared with the colored chicken. Some abnormal tyrosinase gene transcripts were expressed in the skin of the White Plymouth Rock: reverse transcription PCR products amplified from exon 3 to intron 4 and from retroviral sequence 3' long terminal repeat to exon 5. Based on these results, it was confirmed that an avian retroviral sequence insertion in the tyrosinase gene was the cause of recessive white phenotype in chickens.

Quantitative effects of an intronic retroviral insertion on the transcription of the tyrosinase gene in recessive white chickens

Recently, we reported the complete association of a retroviral insertion in intron 4 of the tyrosinase gene and the recessive white mutation (c) in chickens. The mutant allele carrying the retroviral insertion produced, in skin samples of 10-week-old chickens, aberrant tyrosinase transcripts that did not contain exon 5. In the present study, we performed serial molecular and statistical analyses on embryos and 10-week-old chickens to characterize the quantitative effect of the retroviral insertion on the expression pattern of tyrosinase in different tissues (skin and retina). By using quantitative real-time RT-PCR, we observed that the expression level of tyrosinase was significantly lower in recessive white chickens than in wild-type coloured chickens, but that this pattern was age- and tissue-dependent. The differential expression in skin was not significant in embryos, whereas it was highly significant in 10-week-old chickens. Furthermore, there was no difference in the expression of tyrosinase in the retinal pigment epithelium of animals with different genotypes; this corresponds to phenotypic data, which show pigmented eyes in both genotypes. These findings show that the retroviral insertion disturbs tyrosinase expression in the recessive white mutant chickens, and suggests that the regulation of tyrosinase expression in chickens differs between embryos and growing animals, as well as between skin and retina.

Complete association between a retroviral insertion in the tyrosinase gene and the recessive white mutation in chickens

BACKGROUND

In chickens, three mutant alleles have been reported at the C locus, including the albino mutation, and the recessive white mutation, which is characterized by white plumage and pigmented eyes. The albino mutation was found to be a 6 bp deletion in the tyrosinase (TYR) gene. The present work describes an approach to identify the structural rearrangement in the TYR gene associated with the recessive white mutation.

RESULTS

Molecular analysis of the chicken TYR gene has revealed a major structural difference (Restriction Fragment Length Polymorphism, RFLP) in the genomic DNA of the recessive white chicken. A major size difference of 7.7 kb was found in intron 4 of the TYR gene by long-range PCR. Molecular cloning and sequencing results showed the insertion of a complete avian retroviral sequence of the Avian Leukosis Virus (ALV) family. Several aberrant transcripts of the tyrosinase gene were found in 10 week old recessive white chickens but not in the homozygous wild type colored chicken. We established a rapid genotyping diagnostic test based on the discovery of this retroviral insertion. It shows that all homozygous carriers of this insertion had a white plumage in various chicken strains. Furthermore, it was possible to distinguish heterozygous carriers from homozygous normal chickens in a segregating line.

CONCLUSION

In this study, we conclude that the insertion of a complete avian retroviral sequence in intron 4 of the tyrosinase gene is diagnostic of the recessive white mutation in chickens. This insertion causes aberrant transcripts lacking exon 5, and we propose that this insertion is the causal mutation for the recessive white allele in the chicken.

Autosomal albino chicken mutation (ca/ca) deletes hexanucleotide (-deltaGACTGG817) at a copper-binding site of the tyrosinase gene

We compared tyrosinase cDNA sequences from a line of autosomal albino and Black Silky chickens isolated from cultured melanocytes by reverse transcription-polymerase chain reaction (RT-PCR). Both sources produce a single DNA fragment of predicted normal tyrosinase size. Direct sequencing of the PCR product showed three mutated sites in the tyrosinase gene of the albino chicken. Two silent point mutations and a deletion of six nucleotides (-deltaGACTGG) at 817 bp in the tyrosinase cDNA sequence were observed when compared with the White Leghorn and Black Silky cDNA sequences. The deduced albino chicken tyrosinase protein lacks two amino acids, aspartic acid and tryptophan. The position of these amino acids is consistent with one of the potential copper-binding sites that should be indispensable for function of the enzyme. We speculate that the six-base deletion is responsible for the inactive tyrosinase in this line of albino chickens.

Role of Mitf in differentiation and transdifferentiation of chicken pigmented epithelial cell

Mitf encodes a basic helix-loop-helix-leucine-zipper (bHLHzip) protein that is known to function in the development of melanocytes, pigmented epithelial cells (PECs), osteoclasts, and mast cells. In this paper, we report on the isolation, expression, and overexpression of the chicken Mitf and discuss the role of its protein product in the differentiation and transdifferentiation of PECs. Northern blotting showed that chicken Mitf is predominantly expressed in embryonic retinal pigmented epithelium (PE), but is expressed at low levels in other tissues. A 5' RACE analysis revealed differences in the 5' region Mitf nRNA in PE and other tissues. Immunological analysis revealed that Mitf, the protein encoded by Mitf, is first detected in the nuclei of the optic vesicle cells at embryonic stage 13 in a restricted region covered with mesenchymal cells. From stage 14 to 24, the specific staining is observable in the PE and precursor of the PE, the outer layer of the optic cup. In embryos at stages later than stage 29 the signals for Mitf in the future iris, ciliary body, and posterior retinal regions become faint. These results show that expression of Mitf starts at the optic vesicle stage at which no other marker genes for PECs such as mmp115 and tyrosinase are expressed. Dedifferentiation of cultured retinal PECs (rPECs) was induced by phenylthiourea and testicular hyaluronidase, bFGF, or TGF-beta. Mitf expression was inhibited by these factors and reactivated during redifferentiation of the dedifferentiated cells into rPECs, showing the correlation between Mitf expression and rPEC differentiation. Retrovirus-mediated overexpression of Mtif inhibited bFGF-induced dedifferentiation and transdifferentiation of rPECs to both lens and neural cells. These findings showed that downregulation of Mitf expression is essential for the transdifferentiation of rPEC. Mitf overexpression caused hyperpigmentation in cultured rPECs and suppressed the changes in gene expression induced by bFGF. Mitf overexpression promoted expression of mmp115 and tyrosinase in bFGF-treated rPECs suggesting a critical role for Mitf in rPEC differentiation. Mitf overexpression, however, did not promote expression of another rPEC-specific gene, pP344, in bFGF-treated rPECs. This result suggests the presence of other regulatory genes promoting rPEC differentiation. The expression patterns of pax6 and Mitf are complementary both in vivo in vitro. Overexpression of Mitf inhibited expression of pax6 in cultured rPECs. These observations suggest that Mitf regulates pax6 expression negatively.

Albinism in White Leghorn chickens

The appearance in 1988 of an oculocutaneous albino chick in a Single Comb White Leghorn line suggested a new mutational event. This line was closed in 1949, and has been reproduced each spring since then. Subsequent matings indicated that the mutation occurred at the C pigment locus. A mating of the Wisconsin albino (WIA) to cre/cre (red-eyed white) birds showed the mutation to be incompletely recessive to cre. No segregation was apparent when mated to ca/ca (recessive albinism) birds. These data indicate that the WIA mutation is identical to, or very similar to, the previously described tyrosinase-negative ca mutation at the C locus.

Phenotypic, embryonic, and neonatal effects of a gene for sex-linked imperfect albinism (Sal-s) in chickens

Gross phenotypic observations, histology, and tissue culture showed that the gene for sex-linked imperfect albinism that occurred at the University of Saskatchewan (Sal-s), allows a small amount of melanin pigment to be deposited in eyes and feathers. Melanin pigment accumulates in retinal pigment epithelial and cultured neural crest cells, but neural crest cells pigmenting the feathers transfer their pigment as it is produced, and this is seen as a constant amount of color in successive generations of feathers. Despite differences from early reports, it would appear that the phenotype produced by Sal-s is essentially the same as that produced by other Sal mutations. Albinos have a high incidence of lesions in the regions of the navel, the hocks, and the nares, similar to those associated with other hypomelanic mutations in the chicken. Yolk contents appear to be used more slowly by albinos late in incubation. The increased size of the yolk sacs probably contributes directly to producing the navel lesions and indirectly to variation in hatch weight. Albinos have small bursae of Fabricius, reduced hatchability, and early growth.

Separation of pigmented and albino melanocytes and the concomitant evaluation of endogenous peroxide content using flow cytometry

Flow cytometry (FCM) has been used extensively to analyze various biological properties of the cell. In this report, we describe a method by which FCM was used to determine the light scattering profile of a mixed population of pigmented and non-pigmented melanocytes, plus its subsequent use for the sorting and separation of the two cell types. In addition, the relative peroxide content in pigmented and non-pigmented melanocytes was compared by flow cytometry. Cultured avian melanocytes from a pigmented control and from three genetically distinct albino sources were studied. FCM analysis of forward versus side light scatter within a mixed suspension of pigmented and amelanotic melanocytes distinguished two overlapping populations of cells. Sorting of these two populations demonstrated that the population exhibiting much side and minimal forward light scatter was primarily pigmented melanocytes, while conversely the population exhibiting less side and more forward scatter was principally non-pigmented cells. These two melanocyte types also demonstrated differences in levels of endogenous peroxides. The intracellular content of peroxide in the two subpopulations of cells was measured utilizing the nonfluorescent compound, 2',7'-dichlorofluorescein diacetate (DCFH-DA), which within the cell is oxidized by intracellular peroxides to a fluorescent dichlorofluorescein (DCF). Non-pigmented albino melanocytes had the highest quantity of endogenous peroxides, while heavily pigmented cells had considerably less peroxide-related fluorescence. The amount of this DCF fluorescence could be enhanced by increasing concentrations of DCF used in the assay. These flow cytometric methods are useful for isolating and culturing subpopulations of melanocytes expressing various pigment levels and to investigate the relationship between melanin and its precursors with hydrogen and lipid peroxides in melanocytes.

Expression of virally transduced mouse tyrosinase in tyrosinase-negative chick embryo melanocytes in culture

A cDNA encoding mouse tyrosinase was inserted into a plasmid containing the provirus of a replication competent avian leukosis virus (ALV). A viral stock produced from the plasmid was used to infect cultured tyrosinase-negative (ca/ca) unpigmented chick embryo melanocytes. Five days after infection many cells were producing very dark discrete melanosomes.

Activation of tyrosinase in mouse melanoma cell cultures

Tyrosinase activity increased in Cloudman S-91 mouse melanoma cell homogenates incubated at 37 degrees C for a minimum of 8 h. Enzyme activity continued to increase for 48 h at which time the maximal level of activation was observed. Activation did not occur at 4 degrees C and did not occur in the cytosol fraction of the cell, suggesting that the response was localized to melanosomes. The activated enzyme was resistant to solubilization with the nonionic detergent, Triton X-100, and preparation of homogenates in this detergent did not inhibit the temperature-dependent activation of the melanosomal fraction of the cell. The activation process increased the Vmax of tyrosinase 10-fold and lowered the Km by a factor of 2 as determined by the tyrosine hydroxylase assay. The increase in tyrosinase activity was detectable by three assay methods: tyrosine hydroxylation, melanin synthesis, and by tyrosine decarboxylation. The formation of melanin, however, was found to be 1/20 that of either tyrosine hydroxylation or decarboxylation, a finding which suggests that the melanin pathway may be blocked at 5,6-dihydroxyindole. The "self-activation" response could not be mimicked by incubating cell homogenates with cyclic AMP-dependent protein kinase. Activated tyrosinase could be inhibited by the addition of fresh cell extracts, a finding which suggests that tyrosinase inhibitors may be present in these cells.

Tyrosinase and acid phosphatase activities in melanocytes from avian albinos

Two forms of cutaneous albinism in the chicken were investigated for the presence and distribution of tyrosinase and acid phosphatase in melanocytes in situ and in culture. In sex-linked recessive tyrosinase-positive albinism, sal, melanocytes in regenerating feathers and neural tube-derived cultures contained morphologically normal and abnormal premelanosomes. Tyrosinase was localized primarily to the abnormal premelanosomes and probably not to the normal ones. The cells possessed, in addition, vacuoles with membranous inclusions, located in the dendrites, and capped by dopa-positive vesicles (capping vesicles). Acid phosphatase colocalized with tyrosinase in the abnormal premelanosomes and capping vesicles. Tyrosinase activity in extracts of cultured sal melanocytes equalled that of e+ control melanocytes. A tyrosinase antiserum, raised against hamster tyrosinase (Pomerantz), precipitated 2 proteins, 68 kD and 82 kD, which had a precursor-product relationship. The amount of immunoprecipitate was the same in sal and control extracts, but in sal extracts the lower-molecular-weight protein was twice as abundant as the higher-molecular-weight protein. Melanocytes in regenerating feathers from an autosomal recessive, tyrosinase-negative albino, ca, also contained morphologically normal and abnormal premelanosomes. In culture, ca melanocytes had no formal premelanosomes but only dopa-negative multivesicular bodies with wispy filamentous material. Tyrosinase activity and immunoprecipitable tyrosinase were absent. These results suggest that: the tyrosinase-positive albino, sal, has an aberration in both its tyrosinase and acid phosphatase profiles and the tyrosinase-negative albino, ca, lacks functionally and antigenically normal tyrosinase.

Gene controlling a differentiation step in the quail melanocyte

Albino mutation in animals blocks pigmentation owing to a deficiency in tyrosinase, although it does not affect the differentiation of colorless melanocytes from the neural crest. In the albino Japanese quail (al, sex-linked), it was demonstrated that morphologically normal melanocytes differentiated from neural crest cells in culture and that these cells contained unmelanized melanosomes as expected for the mutant cells. The mutant melanocytes, however, were shown to exhibit tyrosinase activity in the Golgi-endoplasmic reticulum-lysosome region and in the Golgi vesicles. Our results seem to indicate that the mutation at the al locus affects the transport of tyrosinase from the Golgi area to melanosomes.

Detection of melanogenic proteins in cultured chick-embryo melanocytes

The phorbol ester, 12-0-tetradecanoylphorbol-13-acetate (TPA), was used as a reversible inhibitor of melanogenesis. Chick-melanocyte cultures of the black genotype, E/E, were grown in conditioned medium plus TPA. After growth in TPA and after its removal, the cells were pulse labeled with 3H-leucine. The membrane fraction, which included all tyrosinase activity as well as both mature and immature melanosomes, was solubilized with Triton X-100. The proteins were separated using two-dimensional electrophoresis and visualized by fluorography. One defined melanogenic protein, tyrosinase, was isolated, and its location was determined in the two-dimensional protein pattern. The protein patterns for both the TPA-inhibited cells and the cells in which the TPA effects were reversed after removal were compared. In addition to tyrosinase, at least nine TPA-sensitive proteins were found. These were designated as being putative melanogenic proteins which, along with tyrosinase, may be responsible for melanin-granule synthesis.