Molecular basis of dark-eyed albinism in the mouse

Identification of genes related to white and black plumage formation by RNA-Seq from white and black feather bulbs in ducks

To elucidate the genes involved in the formation of white and black plumage in ducks, RNA from white and black feather bulbs of an F(2) population were analyzed using RNA-Seq. A total of 2,642 expressed sequence tags showed significant differential expression between white and black feather bulbs. Among these tags, 186 matched 133 annotated genes that grouped into 94 pathways. A number of genes controlling melanogenesis showed differential expression between the two types of feather bulbs. This differential expression was confirmed by qPCR analysis and demonstrated that Tyr (Tyrosinase) and Tyrp1 (Tyrosinase-related protein-1) were expressed not in W-W (white feather bulb from white dorsal plumage) and W-WB (white feather bulb from white-black dorsal plumage) but in B-B (black feather bulb from black dorsal plumage) and B-WB (black feather bulb from white-black dorsal plumage) feather bulbs. Tyrp2 (Tyrosinase-related protein-2) gene did not show expression in the four types of feather bulbs but expressed in retina. C-kit (The tyrosine kinase receptor) expressed in all of the samples but the relative mRNA expression in B-B or B-WB was approximately 10 fold higher than that in W-W or W-WB. Additionally, only one of the two Mitf isoforms was associated with plumage color determination. Downregulation of c-Kit and Mitf in feather bulbs may be the cause of white plumage in the duck.

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.

The Tyr (albino) locus of the laboratory mouse

The albino mouse was already known in ancient times and was apparently selectively bred in Egypt, China, and Japan. Thus, it is not surprising that the c or albino locus (now the Tyr locus) was among the first used to demonstrate Mendelian inheritance in mammals at the dawn of the past century. This locus is now known to encode tyrosinase, the rate-limiting enzyme in the production of melanin pigment, and the molecular basis of the albino ( Tyr(c)) mutation is known. Here we describe the congenic series of Tyr-locus alleles, from wild type to null ( albino). We compare eye and skin pigmentation phenotypes and the genetic lesions that cause each. We suggest that this panel of congenic mutants contains rich, untapped resources for the study of many questions of basic cell biological interest.

Molecular basis of the extreme dilution mottled mouse mutation: a combination of coding and noncoding genomic alterations

Tyrosinase is the rate-limiting enzyme in melanin biosynthesis. It is an N-glycosylated, copper-containing transmembrane protein, whose post-translational processing involves intracytoplasmic movement from the endoplasmic reticulum to the Golgi and, eventually, to the melanosome. The expression of the tyrosinase (Tyr) gene is controlled by several regulatory regions including a locus control region (LCR) located 15 kb upstream from the promoter region. The extreme dilution mottled mutant mice (Tyrc-em) arose spontaneously at the MRC Institute in Harwell (United Kingdom) from a chinchilla-mottled mutant (Tyrc-m) stock, whose molecular basis corresponds to a rearrangement of 5'-upstream regulatory sequences including the LCR of the Tyr gene. Tyrc-em mice display a variegated pigmentation pattern in coat and eyes, in agreement with the LCR translocation, but also show a generalized hypopigmented phenotype, not seen in Tyrc-m mice. Genomic analyses of Tyrc-em mice showed a C1220T nucleotide substitution within the Tyr encoding region, resulting in a T373I amino acid change, which abolishes an N-glycosylation sequon located in the second metal ion binding site of the enzyme. Tyrosinase from Tyrc-em displayed a reduced enzymatic activity in vivo and in vitro, compared with wild-type enzyme. Deglycosylation studies showed that the mutant protein has an abnormal glycosylation pattern and is partially retained in the endoplasmic reticulum. We conclude that the phenotype of the extreme dilution mottled mouse mutant is caused by a combination of coding and noncoding genomic alterations resulting in several abnormalities that include suboptimal gene expression, abnormal protein processing, and reduced enzymatic activity.

Sequences Associated With Human Iris Pigmentation

To determine whether and how common polymorphisms are associated with natural distributions of iris colors, we surveyed 851 individuals of mainly European descent at 335 SNP loci in 13 pigmentation genes and 419 other SNPs distributed throughout the genome and known or thought to be informative for certain elements of population structure. We identified numerous SNPs, haplotypes, and diplotypes (diploid pairs of haplotypes) within the OCA2, MYO5A, TYRP1, AIM, DCT, and TYR genes and the CYP1A2-15q22-ter, CYP1B1-2p21, CYP2C8-10q23, CYP2C9-10q24, and MAOA-Xp11.4 regions as significantly associated with iris colors. Half of the associated SNPs were located on chromosome 15, which corresponds with results that others have previously obtained from linkage analysis. We identified 5 additional genes (ASIP, MC1R, POMC, and SILV) and one additional region (GSTT2-22q11.23) with haplotype and/or diplotypes, but not individual SNP alleles associated with iris colors. For most of the genes, multilocus gene-wise genotype sequences were more strongly associated with iris colors than were haplotypes or SNP alleles. Diplotypes for these genes explain 15% of iris color variation. Apart from representing the first comprehensive candidate gene study for variable iris pigmentation and constituting a first step toward developing a classification model for the inference of iris color from DNA, our results suggest that cryptic population structure might serve as a leverage tool for complex trait gene mapping if genomes are screened with the appropriate ancestry informative markers.

Influence of tyrosinase levels on pigment accumulation in the retinal pigment epithelium and on the uncrossed retinal projection

To study the relationship among tyrosinase activity, melanin production, and the routing of retinal ganglion cell (RGC) axons at the optic chiasm, we analysed mice with varying doses of the tyrosinase gene. These include the dark-eyed albino (Tyrc44H), a radiation-induced hypomorphic allele of tyrosinase; and transgenic mice carrying 1 or 2 alleles of a tyrosinase minigene on both wild-type (Tyr+) and albino (Tyrc) backgrounds. Melanization of the retinal pigment epithelium (RPE) occurred gradually even at <2% wild-type tyrosinase activity and was sensitive to tyrosinase activity up to <35% of wild-type levels, beyond which melanin synthesis appeared to be saturated. Overexpression of tyrosinase led to tyrosinase activity above wild type level, but did not increase melanin production. Although a loss of melanin because of a mutation in tyrosinase is associated with a decrease in the number of uncrossed fibers, elevating tyrosinase levels does not appear to cause an increase in the size of the uncrossed retinal projection. Our results suggest that replacing less than 35% of wild-type tyrosinase activity is sufficient to restore normal pigmentation of the RPE, and potentially, to allay visual defects.

Mechanisms of mutation induction in germ cells of the mouse as assessed by the specific locus test

Mouse germ cell specific locus mutagenesis data and a molecular characterization of mutant alleles have been reviewed to arrive at an understanding of the mechanism of mutation induction in mammals. (a) The spermatogenic stage specificity for the sensitivity to mutation induction by 20 chemical mutagens is considered. (b) The effects of a saturable repair process and its recovery over time are examined for the mutagenic efficiency of ethylnitrosourea. (c) The mutagenic events following methylnitrosourea and chlorambucil are shown to be mainly deletions. In contrast the mutations recovered after ethylnitrosourea treatment are almost exclusively base pair substitutions. (d) It is emphasized that to date very few specific locus experiments have been designed to test for mutagenic events outside the interval stem cell spermatogonia-mature spermatozoa. A specific locus mutation has recently been shown to be due to loss of heterozygosity via mitotic recombination in an early zygote stage and suggests a broader range of possible mechanisms of mutation when these stages are considered. (e) With the cloning of all 7 marker loci mutation analysis at the molecular level will allow a more direct assessment of the mutation process in future studies.

The tyrosinase tail mediates sorting to the lysosomal compartment in MDCK cells via a di-leucine and a tyrosine-based signal

Tyrosinase is a type I membrane protein found in melanosomes, which are lysosomal-like organelles and specific for pigment cells. A mutation of mouse tyrosinase, platinum (cp), leads to truncation of tyrosinase's cytosolic tail, and results in misrouting to the cell periphery. In this study, we expressed chimeras of wild-type and mutant cytosolic tails of mouse tyrosinase fused to rat lysosome-associated membrane protein-1 luminal and transmembrane domain to study sorting of tyrosinase in Madin-Darby canine kidney cells. The study shows that the mouse tyrosinase cytosolic tail is necessary and sufficient to mediate sorting of a heterologous type I membrane protein to compartments of the lysosomal lineage. Whereas deletions of 7 or 10 C-terminal amino acids of the tail still result in sorting to lysosomes, a deletion mutant corresponding to platinum (cp) tail fails to sort correctly and corroborates the in situ findings in cp homozygous mutant mice. Correct sorting of tyrosinase-lysosome-associated membrane protein-1 chimeras is mediated by the interplay of a di-leucine signal and a tyrosine motif of the Y-X-X-O type.

Regulation of the tyrosinase promoter in transgenic mice: expression of a tyrosinase-lacZ fusion gene in embryonic and adult brain

The enzyme tyrosinase is indispensable for pigmentation and the gene is expressed mainly in pigment cells. Regulatory elements, at -12 to -15 kb (enhancer) and within the 270 bp directly upstream of the transcription start site, have been described recently and their importance demonstrated in transgenic experiments. We were interested in tyrosinase promoter activity during development and used beta-galactosidase as reporter gene. Transgenic mice were generated carrying a tyrosinase-lacZ fusion gene, containing 6.1 kb of tyrosinase 5' sequences. In transgenic embryos, beta-galactosidase activity was detected along the entire neural tube, with the most prominent expression in the developing telencephalon, and also in the adult brain. Equivalent expression was observed in the developing retina. Tyrosinase protein was identified in embryonic and adult brain, but no DOPAoxidase or tyrosine hydroxylase activity was detected. From our results we conclude that 1) tyrosinase protein is present in embryonic and adult mouse brain and 2) the tyrosinase promoter can direct expression of a reporter gene to pigment cells and neural tissues.

Transforming growth factor-beta1 inhibits basal melanogenesis in B16/F10 mouse melanoma cells by increasing the rate of degradation of tyrosinase and tyrosinase-related protein-1

Current evidence suggests that melanogenesis is controlled by epidermal paracrine modulators. We have analyzed the effects of transforming growth factor-beta1 (TGF-beta1) on the basal melanogenic activities of B16/F10 mouse melanoma cells. TGF-beta1 treatment (48 h) elicited a concentration-dependent decrease in basal tyrosine hydroxylase and 3,4-dihydroxyphenylalanine (Dopa) oxidase activities, to less than 30% of the control values but had no effect on dopachrome tautomerase activity (TRP-2). The inhibition affected to similar extents the Dopa oxidase activity associated to tyrosinase-related protein-1 (TRP-1) and tyrosinase. This inhibition was noticeable between 1 and 3 h after the addition of the cytokine, and maximal after 6 h of treatment. The decrease in the enzymatic activity was paralleled by a decrease in the abundance of the TRP-1 and tyrosinase proteins. TGF-beta1 mediated this effect by increasing the rate of degradation of tyrosinase and TRP-1. Conversely, after 48 h of treatment, the expression of the tyrosinase gene decreased only slightly, while TRP-1 and TRP-2 gene expression was not affected. An increased rate of proteolytic degradation of TRP-1 and tyrosinase seems the main mechanism accounting for the inhibitory effect of TGF-beta1 on the melanogenic activity of B16/F10 cells.

Tyrosinase, the key enzyme in melanin synthesis, is expressed in murine brain

Tyrosinase is one of the key enzymes in mammalian melanin synthesis. The pigment is produced in two different cell types: the pigmented epithelial cell of the retina, and the melanocyte, a cell of neural-crest origin. We recently showed that a fusion gene between regulatory sequences of tyrosinase gene (tyr) and the beta-galactosidase gene (lacZ), when introduced into transgenic mice, resulted in embryonic expression in presumptive pigment cells but also in cells populations along the entire neural tube. This expression in the developing brain was striking, and we therefore asked whether this would still be detectable after birth. Transgenic mice carrying the tyr-lacZ fusion gene showed beta-galactosidase expression in adult brain. On Western blots, we detected tyrosinase-specific bands of 65-68 kDa in brain and eye. Using an affinity-purified antibody, we showed that detection of tyrosinase is specific and competed off by the presence of the cognate tyrosinase-derived peptide. However, neither tyrosine hydroxylase nor Dopa oxidase activity were detected in protein extracts of brain. We therefore suggest that tyrosinase is present in brain but either not functional or catalyzing different reactions compared to pigment cells.

Tyrosinase is a new marker for cell populations in the mouse neural tube

Tyrosinase, the key enzyme in melanin synthesis, is expressed in pigment cells derived from both neural crest and neuroectoderm. The present study was performed to detect tyrosinase promoter activity and tyrosinase gene expression during murine brain development. Mouse tyrosinase 5' region (6.1 Kb) was used to direct lacZ expression in transgenic mice. During embryogenesis, the transgene reproduced tyrosinase expression in pigment cells but was also observed in embryonic neuroectoderm and migrating neural crest cells. Both tyrosinase and lacZ were detected in cell populations often organized in columnar arrangements and found throughout the entire neural tube, in the cranial region as well as in the spinal chord. In the developing brain, the highest density of positive cells was localized to ventricular and subventricular zones and to evaginations of the neural tube such as optic vesicle, pineal gland, and olfactory bulbs. These results demonstrate that tyrosinase promoter activity and tyrosinase expression are not restricted to differentiated pigment cells. We suggest that tyrosinase is a new marker for cell populations in the neural tube, and that expression is correlated to regions undergoing rapid cell proliferation.

Early death due to defective neonatal lung liquid clearance in alpha-ENaC-deficient mice

The amiloride-sensitive epithelial sodium channel, ENaC, is a heteromultimeric protein made up of three homologous subunits (alpha, beta and gamma) (1,2). In vitro, assembly and expression of functional active sodium channels in the Xenopus oocyte is strictly dependent on alpha-ENaC--the beta and gamma subunits by themselves are unable to induce an amiloride-sensitive sodium current in this heterologous expression system (2). In vivo, ENaC constitutes the limiting step for sodium absorption in epithelial cells that line the distal renal tubule, distal colon and the duct of several exocrine glands. The adult lung expresses alpha, beta and gamma ENaC (3,4), and an amiloride-sensitive electrogenic sodium reabsorption has been documented in upper and lower airways (3-7), but it is not established whether this sodium transport is mediated by ENaC in vivo. We inactivated the mouse alpha-ENaC gene by gene targeting. Amiloride-sensitive electrogenic Na+ transport was abolished in airway epithelia from alpha-ENaC(-/-) mice. Alpha-ENaC(-/-) neonates developed respiratory distress and died within 40 h of birth from failure to clear their lungs of liquid. This study shows that ENaC plays a critical role in the adaptation of the newborn lung to air breathing.