A distal tyrosinase upstream element stimulates gene expression in neural-crest-derived melanocytes of transgenic mice: position-independent and mosaic expression

Dermal Delivery of Constructs Encoding Cre Recombinase to Induce Skin Tumors in PtenLoxP/LoxP;BrafCA/+ Mice

Current genetically-engineered mouse melanoma models are often based on Tyr::CreER^T2-controlled MAPK pathway activation by the BRAF^V600E mutation and PI3K pathway activation by loss of PTEN. The major drawback of these models is the occurrence of spontaneous tumors caused by leakiness of the Tyr::CreER^T2 system, hampering long-term experiments. To address this problem, we investigated several approaches to optimally provide local delivery of Cre recombinase, including injection of lentiviral particles, DNA tattoo administration and particle-mediated gene transfer, to induce melanomas in Pten^LoxP/LoxP;Braf^CA/+ mice lacking the Tyr::CreER^T2 allele. We found that dermal delivery of the Cre recombinase gene under the control of a non-specific CAG promoter induced the formation of melanomas, but also keratoacanthoma and squamous cell carcinomas. Delivery of Cre recombinase DNA under the control of melanocyte-specific promoters in Pten^LoxP/LoxP;Braf^CA/+ mice resulted in sole melanoma induction. The growth rate and histological features of the induced tumors were similar to 4-hydroxytamoxifen-induced tumors in Tyr::CreER^T2;Pten^LoxP/LoxP;Braf^CA/+ mice, while the onset of spontaneous tumors was prevented completely. These novel induction methods will allow long-term experiments in mouse models of skin malignancies.

Genetic variation in regulatory DNA elements: the case of OCA2 transcriptional regulation

Mutations within the OCA2 gene or the complete absence of the OCA2 protein leads to oculocutaneous albinism type 2. The OCA2 protein plays a central role in melanosome biogenesis, and it is a strong determinant of the eumelanin content in melanocytes. Transcript levels of the OCA2 gene are strongly correlated with pigmentation intensities. Recent studies demonstrated that the transcriptional level of OCA2 is to a large extent determined by the noncoding SNP rs12913832 located 21.5 kb upstream of the OCA2 gene promoter. In this review, we discuss current hypotheses and the available data on the mechanism of OCA2 transcriptional regulation and how this is influenced by genetic variation. Finally, we will explore how future epigenetic studies can be used to advance our insight into the functional biology that connects genetic variation to human pigmentation.

A subpopulation of smooth muscle cells, derived from melanocyte-competent precursors, prevents patent ductus arteriosus

BACKGROUND

Patent ductus arteriosus is a life-threatening condition frequent in premature newborns but also present in some term infants. Current mouse models of this malformation generally lead to perinatal death, not reproducing the full phenotypic spectrum in humans, in whom genetic inheritance appears complex. The ductus arteriosus (DA), a temporary fetal vessel that bypasses the lungs by shunting the aortic arch to the pulmonary artery, is constituted by smooth muscle cells of distinct origins (SMC1 and SMC2) and many fewer melanocytes. To understand novel mechanisms preventing DA closure at birth, we evaluated the importance of cell fate specification in SMC that form the DA during embryonic development. Upon specific Tyr::Cre-driven activation of Wnt/β-catenin signaling at the time of cell fate specification, melanocytes replaced the SMC2 population of the DA, suggesting that SMC2 and melanocytes have a common precursor. The number of SMC1 in the DA remained similar to that in controls, but insufficient to allow full DA closure at birth. Thus, there was no cellular compensation by SMC1 for the loss of SMC2. Mice in which only melanocytes were genetically ablated after specification from their potential common precursor with SMC2, demonstrated that differentiated melanocytes themselves do not affect DA closure. Loss of the SMC2 population, independent of the presence of melanocytes, is therefore a cause of patent ductus arteriosus and premature death in the first months of life. Our results indicate that patent ductus arteriosus can result from the insufficient differentiation, proliferation, or contractility of a specific smooth muscle subpopulation that shares a common neural crest precursor with cardiovascular melanocytes.

Microphthalmia-associated transcription factor modulates expression of NADPH oxidase type 4: a negative regulator of melanogenesis

How signaling via reactive oxygen species (ROS) influences skin pigmentation is unclear. We have investigated how NADPH oxidase-derived ROS modulates the expression of the key pigment "melanin" synthesizing enzymes in B16 mouse melanoma cells. A melanin inducer α-melanocyte-stimulating hormone (α-MSH) caused ROS generation that was inhibited by the NADPH oxidase inhibitor Diphenyleneiodonium (DPI) and was insensitive to antagonists of other ROS-producing enzyme systems including mitochondrial enzymes, cycloxygenase, and xanthine oxidase. NADPH oxidase 4 (Nox4) was found to be the most abundant isoform expressed in B16 cells, and its gene levels, as well as ROS generation, were enhanced by α-MSH. Interestingly, silencing Nox4 gene expression with Nox4 siRNA augmented melanin formation under basal conditions and after α-MSH stimulation, demonstrating that constitutive or stimulated Nox4-dependent ROS inhibits melanin formation. This process may be mediated by targeting the promoter region of a melanin synthesizing enzyme tyrosinase, because Nox4 siRNA enhanced tyrosinase promoter activity. Moreover, inhibition of tyrosinase mRNA expression in Nox4 siRNA-treated cells by blocking de novo mRNA and protein synthesis with actinomycin D and cycloheximide respectively indicates that Nox4 repression induces melanogenesis by increasing tyrosinase gene expression. We also found that α-MSH activated its downstream signal transducer microphthalmia-associated transcription factor (MITF) to stimulate Nox4 gene expression. We thus identified a novel mechanism by MITF signaling that in turn stimulates Nox4 to drive ROS generation, thereby repressing melanin synthesis. Such sequence of actions appears to act as an internal feedback mechanism to fine-tune melanin synthesis in response to exogenous challenges such as UV radiation.

A 3,387 bp 5'-flanking sequence of the goat alpha-S1-casein gene provides correct tissue-specific expression of human granulocyte colony-stimulating factor (hG-CSF) in the mammary gland of transgenic mice

A new expression vector containing the 1,944 bp 5'-flanking regulatory region together with exon 1 and intron 1 of the goat alpha-S1-casein gene (CSN1S1), the full-sized human granulocyte colony-stimulating factor gene (hGCSF) and the 3'-flanking sequence of the bovine CSN1S1, was created. The vector DNA was used for generation of four mouse transgenic lines. The transgene was integrated into chromosomes 8 and 12 of two founders as 2 and 5 copies, respectively. Tissue-specific secretion of hG-CSF into the milk of transgenic mice was in the range of 19-40 μg/ml. RT-PCR analysis of various tissues of the transgenic mice demonstrated that expression of hGCSF was detected in only the mammary gland in the progeny of all founders. Moreover, cells were shown to be positive for hG-CSF by immunofluorescent analysis in the mammary glands but not in any other tissues. There were no signs of mosaic expression in the mammary gland. Trace amounts of hG-CSF were detected in the serum of females of two transgenic lines during lactation only. However, no transgenic mice showed any changes in hematopoiesis based on the number of granulocytes in blood. Immunoblotting of hG-CSF in the milk of transgenic mice revealed two forms, presumably the glycosylated and non-glycosylated forms. The hematopoietic activity of hG-CSF in the milk of transgenic females is comparable to that of recombinant G-CSF. In general, the data obtained in this study show that the new expression vector is able to provide correct tissue-specific expression of hG-CSF with high biological activity in transgenic mice.

Differential gene expression of TRPM1, the potential cause of congenital stationary night blindness and coat spotting patterns (LP) in the Appaloosa horse (Equus caballus)

The appaloosa coat spotting pattern in horses is caused by a single incomplete dominant gene (LP). Homozygosity for LP (LP/LP) is directly associated with congenital stationary night blindness (CSNB) in Appaloosa horses. LP maps to a 6-cM region on ECA1. We investigated the relative expression of two functional candidate genes located in this LP candidate region (TRPM1 and OCA2), as well as three other linked loci (TJP1, MTMR10, and OTUD7A) by quantitative real-time RT-PCR. No large differences were found for expression levels of TJP1, MTMR10, OTUD7A, and OCA2. However, TRPM1 (Transient Receptor Potential Cation Channel, Subfamily M, Member 1) expression in the retina of homozygous appaloosa horses was 0.05% the level found in non-appaloosa horses (R = 0.0005). This constitutes a >1800-fold change (FC) decrease in TRPM1 gene expression in the retina (FC = -1870.637, P = 0.001) of CSNB-affected (LP/LP) horses. TRPM1 was also downregulated in LP/LP pigmented skin (R = 0.005, FC = -193.963, P = 0.001) and in LP/LP unpigmented skin (R = 0.003, FC = -288.686, P = 0.001) and was downregulated to a lesser extent in LP/lp unpigmented skin (R = 0.027, FC = -36.583, P = 0.001). TRP proteins are thought to have a role in controlling intracellular Ca(2+) concentration. Decreased expression of TRPM1 in the eye and the skin may alter bipolar cell signaling as well as melanocyte function, thus causing both CSNB and LP in horses.

The tyrosinase enhancer is activated by Sox10 and Mitf in mouse melanocytes

The terminal differentiation of melanocytes is associated with the transcriptional activation of genes responsible for pigment production such as tyrosinase. Pigment cell-specific transcription factors, such as Mitf, as well as specific proximal and distal regulatory elements (DRE) are implicated in the tight control of tyrosinase expression during development and adulthood. Proper tyrosinase expression in melanocytes depends upon the presence of a DRE that is located at -15 kb and provides enhancer activity via a central element termed core-enhancer. In this report, we show that the transcription factors Sox10, Mitf and USF-1 are able to activate the core-enhancer in luciferase reporter assays. Comparative sequence analysis identified evolutionarily motifs resembling Sox10 binding sites that were required for full enhancer activity in melanoma cells and in tyrosinase::lacZ transgenic mice. Sox10 was able to bind the DRE in vitro and mutation of the conserved motifs abolished the enhancer transactivation mediated by Sox10. In addition, two highly conserved CAGCTG E-box motifs were identified that were also required for enhancer activity and for transactivation by Mitf. The results suggest that Sox10 directly, and Mitf, most likely indirectly, activate the tyrosinase enhancer, underlining the contribution of Sox10 to tyrosinase gene regulation in melanocytes.

Distinct distal regulatory elements control tyrosinase expression in melanocytes and the retinal pigment epithelium

Pigment cells of mammals are characterized by two different developmental origins: cells of the retinal pigment epithelium (RPE) originate from the optic cup of the developing forebrain, whereas melanocytes arise from the neural crest. The pigmentation gene tyrosinase is expressed in all pigment cells but differentially regulated in melanocytes and RPE. The tyrosinase promoter does not confer strong expression in pigment cells in vivo, while inclusion of a distal regulatory element at position -15 kb is necessary and sufficient to provide strong expression in melanocytes. Nevertheless, the regulatory elements responsible for correct spatial and temporal tyrosinase expression in the RPE remained unidentified so far. In this report, we show that a 186 kb BAC containing the tyrosinase gene provides transgene expression in both RPE and melanocytes indicating the presence of regulatory sequences required for expression in the RPE. A deletion analysis of the BAC was performed demonstrating that a RPE-regulatory element resides between -17 and -75 kb. Using multi-species comparative genomic analysis we identified three conserved sequences within this region. When tested in transgenic mice one of these sequences located at -47 kb targeted expression to the RPE. In addition, deletion of this regulatory element within a tyrosinase::lacZ BAC provided evidence that this sequence is not only sufficient but also required for correct spatial and temporal expression in the RPE. The identification of this novel element demonstrates that tyrosinase gene expression is controlled by separate distal regulatory sequences in melanocytes and RPE.

A conserved transcriptional enhancer that specifies Tyrp1 expression to melanocytes

Pigment cells of mammals originate from two different lineages: melanocytes arise from the neural crest, whereas cells of the retinal pigment epithelium (RPE) originate from the optic cup of the developing forebrain. Previous studies have suggested that pigmentation genes are controlled by different regulatory networks in melanocytes and RPE. The promoter of the tyrosinase-related family gene Tyrp1 has been shown to drive detectable transgene expression only to the RPE, even though the gene is also expressed in melanocytes as evident from Tyrp1-mutant mice. This indicates that the regulatory elements responsible for Tyrp1 gene expression in the RPE are not sufficient for expression in melanocytes. We thus searched for a putative melanocyte-specific regulatory sequence and demonstrate that a bacterial artificial chromosome (BAC) containing the Tyrp1 gene and surrounding sequences is able to target transgenic expression to melanocytes and to rescue the Tyrp1b (brown) phenotype. This BAC contains several highly conserved non-coding sequences that might represent novel regulatory elements. We further focused on a sequence located at -15 kb, which we identified as a melanocyte-specific enhancer as shown by cell culture and transgenic mice experiments. In addition, we show that the transcription factor Sox10 can activate this conserved enhancer. The presence of a distal Tyrp1 regulatory element, which specifies melanocyte-specific expression, supports the idea that separate regulatory sequences can mediate differential gene expression in melanocytes and RPE.

Spatiotemporal gene control by the Cre-ERT2 system in melanocytes

The organ-specific and temporal control of gene activation/inactivation is a key issue in the understanding of protein function during normal and pathological development and during oncogenesis. We generated transgenic mice bearing a tamoxifen-dependent Cre recombinase (Tyr::Cre-ERT2) gene expressed under the control of a 6.1 kb murine tyrosinase promoter in order to facilitate targeted spatiotemporally controlled somatic recombination in melanoblasts/melanocytes. Cre-ERT2 production was detected in tissues containing melanocytes. After tamoxifen induction at various times during embryogenesis and adulthood in a Cre-responsive reporter mouse strain, genetic recombination was detected in the melanoblasts and melanocytes of the skin. Thus, the Tyr::Cre-ERT2 transgenic mice provides a valuable tool for following this cell lineage and for investigating gene function in melanocyte development and transformation.

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.

Regional abnormalities in retinal development are associated with local ocular hypopigmentation

The retinal pigment epithelium (RPE) plays a key role in regulating retinal development. The critical enzyme in pigment production is tyrosinase. Transgenic mice with a tyrosinase construct where the locus control region was deleted (YRT4) display a variegated phenotype of tyrosinase expression. Their central retina is largely pigment free, whereas more peripheral regions are heavily pigmented. We have used this model to ask whether the influence of pigmented RPE over the retina during development is fundamentally governed by local interactions or is global. Our data show that YRT4 eyes have intermediate melanin content and relatively low tyrosinase activity compared with wild-type and albino animals. Rod counts are comparable to those in pigmented mice in peripheral regions but similar to those in albinos centrally. Anterograde labelling of retinal pathways demonstrates the presence of relatively normal ipsilateral chiasmatic projection in YRT4 mice, comparable with that in pigmented animals and consistent with the peripheral pigmented origin of this pathway. Examination of cellular proliferation levels during retinal development reveals that YRT4 mice display an extended period of mitosis, similar to that found in albinos. Hence, our results show that the regulatory influence of the RPE over the developing retina depends on localized interactions between these tissues.

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.

Genes and chromosomes: control of development

The past decade has witnessed immense progress in research into the molecular basis behind the developmental regulation of genes. Sets of genes functioning under hierarchical control have been identified, evolutionary conserved systems of genes effecting the cell-to-cell transmission of transmembrane signals and assigned a central role in morphogenesis have been intensively studied; the concept of genomic regulatory networks coordinating expression of many genes has been introduced, to mention some of the major breakthroughs. It should be noted that the temporal and tissue-specific parameters of gene expression are correctly regulated in development only in the context of the chromosome and that they are to a great extent dependent on the position of the gene on the chromosome or the interphase nucleus. Moreover epigenetic inheritance of the gene states through successive cell generations has been conducted exclusively at the chromosome level by virtue of cell or chromosome memory. The ontogenetic memory is an inherent property of the chromosome and cis-regulation has a crucial role in its maintenance.

Tyrosinase-Cre mice for tissue-specific gene ablation in neural crest and neuroepithelial-derived tissues

This study describes the derivation of two new lines of transgenic mice that express Cre recombinase under the control of tyrosinase transcriptional elements. To determine the suitability of the Tyrosinase-Cre transgene for tissue-specific gene ablation studies, a fate map of Cre expression domains was determined using the Z/AP reporter strain. It was shown that Cre-expressing cells contribute to a wide array of neural crest and neuroepithelial-derived lineages. The melanocytes of the harderian gland and eye choroid, sympathetic cephalic ganglia, leptomeninges of the telencephalon, as well as cranial nerves (V), (VII), and (IX) are derived either fully or partly from Cre-expressing cephalic crest. The cells contributing to the cranial nerves were the first to exhibit Cre expression at E10.5 as they were migrating into the branchial arches. The melanocytes, chromaffin cells of the adrenal medulla, and dorsal root ganglia are derived from trunk neural crest that either express Cre or were derived from Cre-expressing precursors. An array of brain tissue including the basal forebrain, hippocampus, olfactory bulb, and the granule cell layer of the lateral cerebellum, as well as the retinal pigmented epithelium and glia of the optic nerve originate from Cre-expressing neuroepithelial cells.

Identification and functional validation of a 5' upstream regulatory sequence in the human tyrosinase gene homologous to the locus control region of the mouse tyrosinase gene

Comparison analysis of the sequences of the mouse and human genomes has proven a powerful approach in identifying functional regulatory elements within the non-coding regions that are conserved through evolution between homologous mammalian loci. Here, we applied computational analysis to identify regions of homology in the 5' upstream sequences of the human tyrosinase gene, similar to the locus control region (LCR) of the mouse tyrosinase gene, located at -15 kb. We detected several stretches of homology within the first 30 kb 5' tyrosinase gene upstream sequences of both species that include the proximal promoter sequences, the genomic region surrounding the mouse LCR, and further upstream segments. We cloned and sequenced a 5' upstream regulatory sequence found between -8 and -10 kb of the human tyrosinase locus (termed h5'URS) homologous to the mouse LCR sequences, and confirmed the presence of putative binding sites at -9 kb, homologous to those described in the mouse tyrosinase LCR core. Finally, we functionally validated the presence of a tissue-specific enhancer in the h5'URS by transient transfection analysis in human and mouse cells, as compared with homologous DNA sequences from the mouse tyrosinase locus. Future experiments in cells and transgenic animals will help us to understand the in vivo relevance of this newly described h5'URS sequence as a potentially important regulatory element for the correct expression of the human tyrosinase gene.

Melanocytes and pigmentation are affected in dopachrome tautomerase knockout mice

The tyrosinase family comprises three members, tyrosinase (Tyr), tyrosinase-related protein 1 (Tyrp1), and dopachrome tautomerase (Dct). Null mutations and deletions at the Tyr and Tyrp1 loci are known and phenotypically affect coat color due to the absence of enzyme or intracellular mislocalization. At the Dct locus, three mutations are known that lead to pigmentation phenotype. However, these mutations are not null mutations, and we therefore set out to generate a null allele at the Dct gene locus by removing exon 1 of the mouse Dct gene. Mice deficient in Dct [Dct(tm1(Cre)Bee)] lack Dct mRNA and dopachrome tautomerase protein. They are viable and do not show any abnormalities in Dct-expressing sites such as skin, retinal pigment epithelium, or brain. However, the mice show a diluted coat color phenotype, which is due to reduced melanin content in hair. Primary melanocytes from Dct knockout mice are viable in culture and show a normal distribution of tyrosinase and tyrosinase-related protein 1. In comparison to the knockout, the slaty mutation (Dct(slt)/Dct(slt)) has less melanin and affects growth of primary melanocytes severely. In summary, we have generated a knockout of the Dct gene in mice with effects restricted to pigment production and coat color.

Genomic domains and regulatory elements operating at the domain level

The sequencing of the complete genomes of several organisms, including humans, has so far not contributed much to our understanding of the mechanisms regulating gene expression in the course of realization of developmental programs. In this so-called "postgenomic" era, we still do not understand how (if at all) the long-range organization of the genome is related to its function. The domain hypothesis of the eukaryotic genome organization postulates that the genome is subdivided into a number of semiindependent functional units (domains) that may include one or several functionally related genes, with these domains having well-defined borders, and operate under the control of special (domain-level) regulatory systems. This hypothesis was extensively discussed in the literature over the past 15 years. Yet it is still unclear whether the hypothesis is valid or not. There is evidence both supporting and questioning this hypothesis. The most conclusive data supporting the domain hypothesis come from studies of avian and mammalian beta-globin domains. In this review we will critically discuss the present state of the studies on these and other genomic domains, paying special attention to the domain-level regulatory systems known as locus control regions (LCRs). Based on this discussion, we will try to reevaluate the domain hypothesis of the organization of the eukaryotic genome.

Cre-mediated recombination in the skin melanocyte lineage

Organ-specific expression of a Cre recombinase allows the analysis of gene function in a particular tissue or cell type. Using a 6.1 kb promoter from the mouse tyrosinase gene, we generated and characterized two lines of transgenic mice that express Cre recombinase in melanoblasts. Utilizing a Cre-responsive reporter mouse strain, genetic recombination was detected in the melanoblasts of the skin from embryonic day 11.5. In addition, Cre-expression was detected in the skin and eyes of mice. Cre transgene activity was occasionally detected in the brain and peripheral nerves but not in other tissues. When Tyr::Cre mice were crossed with mice carrying a homozygous loxP conditional mutation for the insulin-like growth factor receptor gene (Igf1r), Cre-melanoblast-specific recombination pattern was confirmed and no abnormal phenotype was observed. In conclusion, Tyr::Cre transgenic mice provide a valuable tool to follow the cell lineage and to examine gene function in melanocyte development and transformation.

Gene transfer strategies in animal transgenesis

Position effects in animal transgenesis have prevented the reproducible success and limited the initial expectations of this technique in many biotechnological projects. Historically, several strategies have been devised to overcome such position effects, including the progressive addition of regulatory elements belonging to the same or to a heterologous expression domain. An expression domain is thought to contain all regulatory elements that are needed to specifically control the expression of a given gene in time and space. The lack of profound knowledge on the chromatin structure of expression domains of biotechnological interest, such as mammary gland-specific genes, explains why most standard expression vectors have failed to drive high-level, position-independent, and copy-number-dependent expression of transgenes in a reproducible manner. In contrast, the application of artificial chromosome-type constructs to animal transgenesis usually ensures optimal expression levels. YACs, BACs, and PACs have become crucial tools in animal transgenesis, allowing the inclusion of distant key regulatory sequences, previously unknown, that are characteristic for each expression domain. These elements contribute to insulating the artificial chromosome-type constructs from chromosomal position effects and are fundamental in order to guarantee the correct expression of transgenes.

Artificial chromosome transgenesis in pigmentary research

Pigmentary genes were among the first mammalian genes to be studied, mostly because of the obvious phenotypes associated with their mutations. In 1990, tyrosinase, encoding the limiting enzyme in the melanin synthesis pathway, was eventually assigned to the c (albino) locus by classical rescue experiments driven by functional constructs in transgenic mice. These pioneer reports triggered the study of the regulation of endogenous tyrosinase gene expression by combining different amounts of upstream regulatory and promoter regions and testing their function in vivo in transgenic animals. However, faithful and reproducible transgenic expression was not achieved until the entire tyrosinase expression domain was transferred to the germ-line of mice using artificial-chromosome-type transgenes. The use of these large tyrosinase transgenic constructs and the ease with which they could be manipulated in vitro enabled the discovery of previously unknown but fundamental regulatory regions, such as the tyrosinase locus control region (LCR), whose presence was required in order to guarantee position-independent and copy-number-dependent expression of tyrosinase transgenes, with an expression level, per copy, comparable to that of an endogenous wild-type allele. Subsequently, functional dissection of elements present within this LCR through the generation of new artificial-chromosome type tyrosinase transgenes has revealed the existence of different regulatory activities. The existence of some of these units had been suggested previously by standard-type transgenic analyses. In this review, we will discuss both independent approaches and conclude that optimal tyrosinase transgene expression requires the use of its complete expression domain.

Functional conservation of the promoter regions of vertebrate tyrosinase genes

Tyrosinase is the key enzyme for synthesizing melanin pigments, which primarily determine mammalian skin coloration. Considering the important roles of pigments in the evolution and the adaptation of vertebrates, phylogenetic changes in the coding and flanking regulatory sequences of the tyrosinase gene are particularly intriguing. We have now cloned cDNA encoding tyrosinase from Japanese quail and snapping turtle. These nonmammalian cDNA are highly homologous to those of the mouse and human tyrosinases, whereas the 5' flanking sequences are far less conserved except for a few short sequence motifs. Nevertheless, we demonstrate that the 5' flanking sequences from the quail or turtle tyrosinase genes are capable of directing the expression of a fused mouse tyrosinase cDNA when introduced into cultured mouse albino melanocytes. This experimental method, which reveals the functional conservation of regulatory sequences in one cell type (the melanocyte), may be utilized to evaluate phylogenetic differences in mechanisms controlling specific gene expression in many other types of cells. We also provide evidence that the 5' flanking sequences from these nonmammalian genes are functional in vivo by producing transgenic mice. Phylogenetic changes of vertebrate tyrosinase promoters and the possible involvement of conserved sequence motifs in melanocyte-specific expression of tyrosinase are discussed.

Variegated expression and delayed retinal pigmentation during development in transgenic mice with a deletion in the locus control region of the tyrosinase gene

Deletion of the tyrosinase locus control region (LCR) in transgenic mice results in variegated expression in the skin. Here we investigate the pigmentation pattern of other tissues that express tyrosinase: iris, choroid, and retina in the same animals. A mosaic distribution of pigmentation appears in the iris and choroid. Interestingly, a markedly different mosaic pattern is found in the retina, where central areas contain little or no melanin while pigmentation rises to normal levels towards periphery. Further, there is a temporal delay in the initiation and accumulation of pigment in retinal pigmented epithelium (RPE) cells during development, and patterns of adult retinal melanisation in these mice appear arrested at a stage found in early embryogenesis in wild-type mice. These results demonstrate that the tyrosinase LCR is needed for the correct establishment and maintenance of this expression domain throughout development, but particularly during the later stages of retinal melanisation.

Sequence and chromosomal context effects on variegated expression of keratin 5/lacZ constructs in stratified epithelia of transgenic mice

The expression of transgene loci in mammals often occurs in a heterocellular fashion resulting in variegated patterns of expression. We have examined the effect of chromosomal integration site, copy number, and transcriptionally activating sequences on the variegation of a keratin 5-lacZ (K5Z) construct in the stratified epithelia of transgenic mice. lacZ expression in these mice is always mosaic, and the beta-gal activity per cell is usually higher in the lines with a higher proportion of expressing cells. Similar constructs, in which cDNAs were exchanged by lacZ sequences, showed no variegation. Also, when a strongly active, nonvariegating construct was coinjected with K5Z, most transgenic lines showed an almost homogeneous lacZ expression. The comparison of transgene arrays of different copies inserted at the same locus (obtained by using a lox/Cre system) showed that the reduction of copy number does not lead to an increase in the proportion of cells that express the transgene. Finally, in most of the variegating or nonexpressing lines the transgenes were located both at intermediate positions and at peritelomeric regions in the long chromosome arms. These findings suggest that the probability and efficiency of expression of K5Z genes depend on both long range chromosomal influences and on sequences in the transgene array.

Increased transgene expression by the mouse tyrosinase enhancer is restricted to neural crest-derived pigment cells

In this study, we have addressed the impact of the mouse tyrosinase enhancer on regulated expression from the mouse tyrosinase promoter during embryonic development. Stable and transient transgenic experiments using the reporter gene lacZ reveal that (1) expression is detected in neural crest-derived melanoblasts from E11.5 onward, (2) the enhancer does not increase transgenic expression in optic cup-derived pigment cells of the retinal pigment epithelium (RPE), and (3) expression in the telencephalon is not any longer detected. The importance of the enhancer for expression in pigment cells of the eye was further investigated in adult mice using an attenuated diphtheria toxin A gene. This demonstrated that in presence of the enhancer the transgene expression is specifically targeted to neural crest-derived melanocytes of the choroid and not, or slightly, to the RPE. This suggests that tyrosinase is differentially regulated in the two pigment cell lineages, and that this promoter can be used to target expression preferentially to the neural crest-derived melanocyte lineage.

The Fugu rubripes tyrosinase gene promoter targets transgene expression to pigment cells in the mouse

The regulation of the mouse tyrosinase gene expression is controlled by a highly conserved element at -100 bp, the M-box, and an enhancer at -12 kb. In most vertebrates, the length of intergenic sequences makes it difficult to analyze the whole gene and the complete regulatory region. We took advantage of the compact Fugu genome to identify regulatory regions involved in pigment cell-specific expression. We isolated the Fugu tyrosinase gene, and identified putative cis-acting regulatory elements within the promoter. We then asked whether the Fugu promoter sequence functions in mouse pigment cells. We showed that E11.5 transgenic embryos bearing 6 kb or 3 kb of Fugu tyrosinase 5' sequence fused to the reporter gene lacZ revealed melanoblast and RPE-specific expression. This is the first evidence that the tyrosinase promoter is active at midgestation in melanoblasts, long before the onset of pigmentation.

Distal upstream tyrosinase S/MAR-containing sequence has regulatory properties specific to subsets of melanocytes

A distal upstream regulatory element of the mouse tyrosinase gene has locus control region (LCR)-like activity and is required for position-independent expression of linked genes. It consists of a DNAse I hypersensitive site, which has enhancer activity in neural crest-derived melanocytes, embedded within a scaffold/matrix attachment region (S/MAR), both of which are necessary for LCR activity. To address the role of the S/MAR in position-independent expression, we assessed the ability of a fragment containing most of the S/MAR to insulate a transgene from position effects. The S/MAR sequence showed a striking cell type specificity in its function in all six multicopy transgenic lines, dampening position effects considerably in cutaneous melanocytes while allowing no expression in other neural crest-derived melanocytes, and causing elevated expression in ocular melanocytes derived from the neural tube. The specificity of transgene expression in the eye suggested the presence of both positive and negative regulatory elements in this enhancer/S/MAR region, which was confirmed by transient transfection analyses. This is the first known regulatory element to exhibit different activities in melanocytes of different developmental origins.

Stable and full rescue of the pigmentation in a medaka albino mutant by transfer of a 17 kb genomic clone containing the medaka tyrosinase gene

In the medaka Oryzias latipes, several albino strains have mutations in the tyrosinase gene that have been fully characterized at the molecular level. A genomic clone from wild-type medaka containing the 5 kb tyrosinase gene with its five exons, 10 kb of upstream sequences and 2 kb downstream sequences was introduced into fertilized eggs from a tyrosinase-negative albino strain. We show that the injection of this genomic clone predominantly conferred mosaic expression ending before the hatching stage. A minority of juveniles retained a variable number of pigmented cells, including four individuals keeping one pigmented eye through adulthood. Two of these could be mated, and one of these transmitted the transgene resulting in complete rescue of pigmentation to 16% of its offspring. The resulting transgenic line harbors a single copy of the wild-type tyrosinase gene and all fish are wild-type with respect to pigmentation. These experiments suggest that the tyrosinase genomic clone, or a future shorter version of it, can be used in fish to routinely detect transgenic lines. The apparent faithful and systematic expression of the tyrosinase transgene is most probably due to the presence of a locus control region (LCR) in the injected clone.

Inhibition of melanin synthesis by cystamine in human melanoma cells

In studies to determine whether pigmentation can be regulated physiologically by thiols, human melanoma cells (MM418c5) and melanocytes were found to become depigmented when cultured continuously in 50 microM cystamine. Cystamine was depleted from the culture medium and the treatment was nontoxic and reversible. Cysteamine, dithiothreitol, and phenylthiourea were less effective, and glutathione, cysteine, and cystine were inactive. Tyrosinase (dopa oxidase) activity was not greatly affected except for induction of a lag period. In contrast, tyrosinase activity in an amelanotic melanoma cell line (MM96L) was rapidly inhibited without consumption of cystamine/cysteamine, in association with the generation of free thiol in the culture medium, and could be enhanced by the cystine transport inhibitor, glutamate. Tyrosinase expressed by a recombinant vaccinia virus was inhibited by cystamine treatment of MM96L and HeLa cells. Cystamine treatment lowered the degree of cross-linking of the pigmentation antigen gp75/TRP-1 in MM418c5 cells. Tyrosinase protein and mRNA levels in MM418c5 cells were not affected by cystamine. The results show that cystamine at a concentration close to physiologic levels has multiple effects on the melanogenic pathway. In amelanotic cells, tyrosinase has a short half-life and is readily inhibited by cystamine/cysteamine whereas tyrosinase in the more mature melanosomes of the pigmented cell appears to be less accessible to proteolytic and thiol attack. Inhibition of melanin synthesis in the latter cell type may arise to a significant degree from reduction of cystamine to cysteamine, which sequesters quinones.

Brachyury-related transcription factor Tbx2 and repression of the melanocyte-specific TRP-1 promoter

Previous work has demonstrated that two key melanocyte-specific elements termed the MSEu and MSEi play critical roles in the expression of the melanocyte-specific tyrosinase-related protein 1 (TRP-1) promoter. Both the MSEu and MSEi, located at position -237 and at the initiator, respectively, bind a melanocyte-specific factor termed MSF but are also recognized by a previously uncharacterized repressor, since mutations affecting either of these elements result in strong up-regulation of TRP-1 promoter activity in melanoma cells. Here we demonstrate that repression mediated by the MSEu and MSEi also operates in melanocytes. We also report that both the MSEu and MSEi are recognized by the brachyury-related transcription factor Tbx2, a member of the recently described T-box family, and that Tbx2 is expressed in melanocyte and melanoblast cell lines but not in melanoblast precursor cells. Although Tbx2 and MSF each recognize the TRP-1 MSEu and MSEi motifs, it is binding by Tbx-2, not binding by MSF, that correlates with repression. Several lines of evidence tend to point to the brachyury-related transcription factor Tbx2 as being the repressor of TRP-1 expression: both the MSEu and MSEi bind Tbx2, and mutations in either element that result in derepression of the TRP-1 promoter diminish binding by Tbx2; the TRP-1 promoter, but not the tyrosinase, microphthalmia, or glyceraldehyde-3-phosphate dehydrogenase (G3PDH) promoter, is repressed by Tbx2 in cotransfection assays; a high-affinity consensus brachyury/Tbx2-binding site is able to constitutively repress expression of the heterologous IE110 promoter; and a low-affinity brachyury/Tbx2 binding site is able to mediate Tbx2-dependent repression of the G3PDH promoter. Although we cannot rule out the presence of an additional, as yet unidentified factor playing a role in the negative regulation of TRP-1 in vivo, the evidence presented here suggests that Tbx2 most likely is the previously unidentified repressor of TRP-1 expression and as such is likely to represent the first example of transcriptional repression by a T-box family member.

Viral sequences enable efficient and tissue-specific expression of transgenes in Xenopus

Expression of transgenes within a single generation by direct DNA injection into vertebrate embryos has been plagued by inefficient and nonuniform gene expression. We report a novel strategy for efficient and stable expression of transgenes driven by both ubiquitous and tissue-specific promoters by direct DNA injection into developing Xenopus laevis embryos. This strategy involves flanking expression cassettes of interest with inverted terminal repeat sequences (ITRs) from adeno-associated virus. Our results suggest that the ITR strategy may be generally applicable to other systems, such as zebra fish and embryonic stem cells, and may enable tissue-specific expression of transgenes in problematic contexts.

Phenotypic rescue of the albino mutation in the medakafish (Oryzias latipes) by a mouse tyrosinase transgene

Mutations of the tyrosinase gene are one common cause of a similar phenotype in all vertebrates, known as albinism. In an attempt to contribute to an understanding of the genetic hierarchy governing the development of pigmentation, we have used a mouse tyrosinase minigene under the control of its 5.2 kb upstream promoter region to rescue two different albino mutations in the medakafish, Oryzias latipes. Around hatching stages an almost perfect phenocopy of the wildtype pigmentation was obtained. Subsequent ectopic melanin overproduction indicated a possible incompatibility of the heterologous mouse promoter for stable expression during the entire ontogenesis. Like in some tyrosinase transgenic mouse lines a strong variegation effect was observed. The transgene-mediated pigmentation phenotype was obtained up to the eighth offspring generation. The phenotypic effects of the tyrosinase transgene in different albino mutant strains places the i3-locus upstream and the b-locus downstream of the tyrosinase locus i1 in the genetic hierarchy leading to wildtype pigmentation.

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.

The regulation of tyrosinase gene transcription

Tyrosinase is one of the key enzymes essential for melanogenesis. The control of its activity rests in part at the level of transcriptional regulation. The 5' promoter regions of the human, mouse, chicken, quail, snapping turtle, and frog tyrosinase sequences have been isolated and the mechanisms regulating the activity of these sequences are beginning to be elucidated. This review provides an update on the following aspects of tyrosinase gene regulation: basal promoter elements that determine the site of transcription initiation for RNA polymerase II; the cis-acting elements and DNA-binding factors that mediate melanocyte-specific expression of the tyrosinase gene; promoter elements involved in the temporal control of tyrosinase gene expression; additional elements that may be required to achieve wild-type levels of gene expression; and specific elements that may be required for modulation of tyrosinase gene expression in response to humoral factors or external stimuli that are known to influence the amounts of melanin synthesized by fully differentiated melanocytes. The wild type expression of tyrosinase is the result of the interaction of many different factors and it is becoming evident that certain elements and factors play more than one role in this process.

Inherited somatic mosaicism caused by an intracisternal A particle insertion in the mouse tyrosinase gene

A recessive, fully penetrant mutation (c(m1OR)) at the mouse albino locus that results in coat-color mottling has been characterized at the molecular level. Restriction mapping and DNA sequencing analyses provide evidence that mutants carry a 5.4-kb intracisternal A particle (IAP) element insertion upstream of the tyrosinase (Tyr) promoter. Northern blot analysis and reverse transcription-PCR results show that the tyrosinase gene is expressed at much lower levels in mutant than in wild-type mice. The mutant Tyr gene still retains the tissue-specific expression pattern, and the Tyr transcript is not initiated from the IAP long terminal repeat promoter. We propose that the IAP insertion isolates the promoter of the tyrosinase gene from upstream cis-acting regulatory elements, leading to a substantially decreased level of Tyr gene expression in mutants.

Severe position effects imposed on a 1 kb mouse whey acidic protein gene promoter are overcome by heterologous matrix attachment regions

Matrix attachment regions (MARs) have been shown to participate in the insulation of transcription elements from surrounding chromatin in tissue culture cells and transgenic mice. A whey acidic protein (WAP) transgene containing 1 kb promoter sequence was active in mammary tissue from 1 out of 17 lines of mice, demonstrating that the transcription elements were highly susceptible to position effects. To test whether MARs could insulate this WAP gene promoter and thereby restore transcription, we ligated MARs from the chicken lysozyme gene to the WAP transgene. Seven of the nine lines generated exhibited WAP transgene activity, expression was confined to mammary tissue, and correct regulation was observed in three of the four lines analyzed. This study provides strong additional evidence that the MAR fragments from the chicken lysozyme gene have the capacity to insulate transgenes from severe position effects.

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.