Genetics, development, and malignancy of melanocytes

Photochemical reaction to increase melanogenesis using Buddleja officinalis and blue light-emitting diode irradiation in B16F10

Recently, the incidence of vitiligo has increased because of stresses induced by external environment. Ultraviolet (UV) light therapy is the most commonly used method of treating the disease; however, UV light therapy requires a long treatment period, and prolonged exposure to UV radiation has side effects. The purpose of the present study was to investigate the effects of natural products and LED irradiation (LED-IR) on the synthesis of melanin. It was not possible to effectively increase intracellular melanin production through individual applications of Buddleja officinalis (BO), which is a natural substance selected through screening, or blue light irradiation (Blue-IR). However, when used in combination, these two agents stimulated adenylyl cyclase (AC) and melanin production was induced in the stimulated cells via the CREB/MITF/TYR pathway. Furthermore, the combined treatment with BO and Blue-IR generated low levels of cellular reactive oxygen species (ROS) and induced p38 phosphorylation, which in turn activated MITF in ROS-stimulated synthetic melanocytes, resulting in the promotion of melanogenic pathways other than the CREB/MITF/TYR pathway. In addition, this treatment combination effected melanin transport. These results suggested that the combined therapies can be used to treat melanin-deficiency skin diseases such as vitiligo.

The air liquid-interface, a skin microenvironment, promotes growth of melanoma cells, but not their apoptosis and invasion, through activation of mitogen-activated protein kinase

The air-liquid interface (ALI) is a common microenvironment of the skin, but it is unknown whether the ALI affects melanoma cell behaviors. Using a collagen gel invasion assay, immunohistochemistry, and Western blots, here we show that melanoma cell proliferation in cultures with an ALI is higher than melanoma cell proliferation in submerged cultures. Bromodeoxyuridine (BrdU) uptake, an indicator of cell proliferation, of melanoma cells at the ALI was about 3 times that of submerged cells, while ALI and submerged melanoma cells had similar levels of single-stranded DNA (a marker of apoptosis). The ALI enhanced the expression of Raf-1, MEK-1 and pERK-1/2 components of the mitogen-activated protein kinase (MAPK) cascade, in cells more than the submerged condition did. The increases in BrdU uptake and pERK-1/2 expression promoted by ALI was abolished by the MEK inhibitor, PD-98059. ALI-treated and submerged melanoma cells did not infiltrate into the collagen gel, and they showed no significant difference in the expression of the invasion- and motility-related molecules, matrix metalloproteinase-1 and -9, laminin 5, and filamin A. Our data indicate that the ALI, a skin microenvironment, accelerates the growth, but not the apoptosis or invasion, of melanoma cells through MAPK activation.

Immunosuppression and melanocyte proliferation

Melanocytes are pigmented cells derived from the neural crest; their proliferation is restrained by immune system. The eruption of nevi after an immunosuppressive condition is a peculiar phenomenon indicating that the immune system may play a major role in limiting proliferation of melanocytes. In this review, we analyze the role of immunosuppressive regimens on melanocyte proliferation. In particular, we discuss the eruptive nevi phenomenon, which is determined by the inability of the immune system to inhibit melanocyte proliferation. These clinical observations indicate that the immune system has a pivotal role in restraining melanocyte proliferation. However, although the role of the immune system in the development of nonmelanoma skin cancer has been shown clearly in several studies involving organ transplant patients, the role of immunosuppression in melanoma genesis has not yet been established. Further investigations are required to establish the real immunogenicity of melanoma, particularly in the light of the dichotomy between the eruptive nevi phenomenon in immunosuppressed patients and the low incidence of melanoma in transplanted patients.

The genome and epigenome of malignant melanoma

Malignant melanoma originates in melanocytes, the pigment-producing cells of the skin and eye, and is one of the most deadly human cancers with no effective cure for metastatic disease. Like many other cancers, melanoma has both environmental and genetic components. For more than 20 years, the melanoma genome has been subject to extensive scrutiny, which has led to the identification of several genes that contribute to melanoma genesis and progression. Three molecular pathways have been found to be nearly invariably dysregulated in melanocytic tumors, including the RAS-RAF-MEK-ERK pathway (through mutation of BRAF, NRAS or KIT), the p16 INK4A-CDK4-RB pathway (through mutation of INK4A or CDK4) and the ARF-p53 pathway (through mutation of ARF or TP53). Less frequently targeted pathways include the PI3K-AKT pathway (through mutation of NRAS, PTEN or PIK3CA) and the canonical Wnt signaling pathway (through mutation of CTNNB1 or APC). Beyond the specific and well-characterized genetic events leading to activation of proto-oncogenes or inactivation of tumor suppressor genes in these pathways, systematic high-resolution genomic analysis of melanoma specimens has revealed recurrent DNA copy number aberrations as well as perturbations of DNA methylation patterns. Melanoma provides one of the best examples of how genomic analysis can lead to a better understanding of tumor biology. We review current knowledge of the genes involved in the development of melanoma and the molecular pathways in which these genes operate.

Melanocytes and the microphthalmia transcription factor network

The first mouse microphthalmia transcription factor (Mitf ) mutation was discovered over 60 years ago, and since then over 24 spontaneous and induced mutations have been identified at the locus. Mitf encodes a member of the Myc supergene family of basic helix-loop-helix zipper (bHLH-Zip) transcription factors. Like Myc, Mitf regulates gene expression by binding to DNA as a homodimer or as a heterodimer with another related family member, in the case of Mitf the Tfe3, Tfeb, and Tfec proteins. The study of Mitf has provided many insights into the biology of melanocytes and helped to explain how melanocyte-specific gene expression and signaling is regulated. The human homologue of MITF is mutated in patients with the pigmentary and deafness disorder Waardenburg Syndrome Type 2A (WS2A). The mouse Mitf mutations therefore serve as a model for the study of this human disease. Mutations and/or aberrant expression of several MITF family member genes have also been reported in human cancer, including melanoma (MITF), papillary renal cell carcinoma (TFE3, TFEB), and alveolar soft part sarcoma (TFE3). Genes in the MITF/TFE pathway may therefore also represent valuable therapeutic targets for the treatment of human cancer. Here we review recent developments in the analysis of Mitf function in vivo and in vitro and show how traditional genetics, modern forward genetics and in vitro biochemical analyses have combined to produce an intriguing story on the role and actions of a gene family in a living organism.

The Tomita collection of medaka pigmentation mutants as a resource for understanding neural crest cell development

All body pigment cells in vertebrates are derived from the neural crest. In fish the neural crest can generate up to six different types of pigment cells, as well as various non-pigmented derivatives. In mouse and zebrafish, extensive collections of pigmentation mutants have enabled dissection of many aspects of pigment cell development, including fate specification, survival, proliferation and differentiation. A collection of spontaneous mutations collected from wild medaka (Oryzias latipes) populations and maintained at Nagoya University includes more than 40 pigmentation mutations. The descriptions of their adult phenotypes have been previously published by Tomita and colleagues (summarised in Medaka (Killifish) Biology and Strains, 1975), but the embryonic phenotypes have not been systematically described. Here we examine these embryonic phenotypes, paying particular attention to the likely defect in pigment cell development in each, and comparing the spectrum of defects to those in the zebrafish and mouse collections. Many phenotypes parallel those of identified zebrafish mutants, although pigment cell death phenotypes are largely absent, presumably due to the different selective pressures under which the mutants were isolated. We have identified mutant phenotypes that may represent the Mitf/Kit pathway of melanophore specification and survival. We use in situ hybridisation with available markers to confirm a key prediction of this hypothesis. We also highlight a set of novel phenotypes not seen in the zebrafish collection. These mutants will be a valuable resource for pigment cell and neural crest studies and will strongly complement the mutant collections in other vertebrates.

Low incidence of iris pigmentation and eyelash changes in 2 randomized clinical trials with unoprostone isopropyl 0.15%

OBJECTIVE

To assess whether iris color and eyelash changes occur with the use of unoprostone for 2 years.

DESIGN

The 2 clinical trials described herein were prospective, randomized, double-masked, active-controlled, parallel group, multicenter studies.

PARTICIPANTS

A total of 1131 patients with primary open-angle glaucoma or ocular hypertension participated in 2 clinical trials and received either unoprostone isopropyl 0.15% (659), timolol maleate 0.5% (331), or betaxolol hydrochloride 0.5% (141), 1 drop per eye twice daily for up to 24 months.

METHODS

Color photographs (1:1 magnification) were taken of the iris and eyelid of each patient at baseline and at regular intervals thereafter through month 24 using a standardized camera system. Photography included 7 views of each eye plus a calibration photograph and a patient identification photograph, for a total of 16 photographs per patient per visit. Two independent (masked) readers subjectively compared baseline iris colors to subsequent visits. Side view photographs of the upper and lower eyelashes were used for the eyelash length analysis, with each having sufficient depth of field and a sufficient number of eyelashes in focus. Similarly, frontal eyelash views were used for the eyelash density analysis.

MAIN OUTCOME MEASURES

Changes from baseline in iris color and eyelash length and density within and between treatment groups.

RESULTS

Seven cases of iris color change (1.06%) were confirmed in patients treated with unoprostone for up to 24 months; no confirmed cases were reported in the timolol or betaxolol groups. In the unoprostone group, cases of iris color change were confirmed at months 12 (1 case), 18 (2 cases), and 24 (4 cases). No clinically relevant differences were observed among treatment groups for changes from baseline in eyelash length or density.

CONCLUSION

Although iris hyperpigmentation and abnormal eyelash changes may occur after treatment with unoprostone, the incidence of these events appears to be low in the 2-year clinical study.

The role of pigment cells in the brain of ascidian larva

The functions of melanin in the pigment cells, the ocellus and the otolith, of ascidian larvae were studied by their swimming behavior and cell morphology with and without 1-phenyl-2-thiourea (PTU), an inhibitor of vertebrate tyrosinase. Melanin formation in both the otolith and the ocellus of PTU-treated larvae at 12 hours of development was completely inhibited. These larvae were unable to swim because of abnormal tail development, but expression of rhodopsin in the outer segments of the photoreceptor was normal. In the PTU-treated larvae at 15 hours of development, melanin formation in the ocellus was inhibited, but that in the otolith seemed to be normal. The photic behavior of these larvae was normal, as was rhodopsin expression in the outer segments. However, the treated larvae lost upward swimming behavior. Synchrotron radiation X-ray fluorescence images showed that metallic elements of K, Ca, and Zn in the statocyte of larva were greatly decreased by PTU treatment, which may result in lowering the specific gravity of the pigment mass. SEM observations showed that the statocyte of Ciona intestinalis was supported by three parts, a foot-piece of the statocyte itself and two fibrous spring-like structures produced from protuberances. All three structures were synaptotagmin-positive. Movement of the statocyte would be detected by these three structures and thus would be responsible for the gravitational orientation.

Differential expression of versican isoforms is a component of the human melanoma cell differentiation process

Versican is a large chondroitin sulfate proteoglycan produced by human melanoma cell lines and malignant melanocytic lesions. In the present work, we have analyzed the expression of versican spliced variants V0, V1, V2 and V3 in human melanoma cell lines at several differentiation degrees. The isoform expression pattern depends on the degree of cell differentiation. Differentiated cell lines do not produce any of the versican isoforms as analyzed by Western blot, Northern blot and RT-PCR. All cell lines with an early or intermediate degree of differentiation (AX3, SK-mel-37, Rider, SK-mel-1.36-1-5 and SK-mel-3.44) expressed V0 and V1 transcripts, whereas V2 and V3 expression was shown only by the undifferentiated cell lines SK-mel-1.36-1-5 and Rider. Furthermore, we have analyzed the expression of versican isoforms in SK-mel-3.44 and SK-mel-1.36-1-5 cells induced to differentiate by TPA treatment. The expression of the large V0, V1 and V2 isoforms practically disappears in differentiated cells, whereas V3 remains detectable by RT-PCR analysis.

The color loci of mice--a genetic century

Color loci in mammals are those genetic loci in which mutations can affect pigmentation of the hair, skin, and/or eyes. In the mouse, over 800 phenotypic alleles are now known, at 127 identified color loci. As the number of color loci passed 100 only recently, we celebrate this 'century' with an overview of these loci, especially the 59 that have been cloned and sequenced. These fall into a number of functional groups representing melanocyte development and differentiation, melanosomal components, organelle biogenesis, organelle transport, control of pigment-type switching, and some systemic effects. A human ortholog has been identified in all cases, and the majority of these human genes are found to be loci for human disorders, often affecting other body systems as well as pigmentation. We expect that a significant number of color loci remain to be identified. Nonetheless, the large number known already provide a treasury of resources for reconstruction of the mechanisms, at the subcellular, cellular and tissue levels, that produce a functional pigmentary system and contribute to the normal development and functioning of many other organ systems. The mutant mice also provide valuable models for the study of human disease.

The genetics of malignant melanoma: lessons from mouse and man

Therapeutic resistance and proclivity for metastasis are hallmarks of malignant melanoma. Genetic, epidemiological and genomic investigations are uncovering the spectrum of stereotypical mutations that are associated with melanoma and how these mutations relate to risk factors such as ultraviolet exposure. The ability to validate the pathogenetic relevance of these mutations in the mouse, coupled with advances in rational drug design, has generated optimism for the development of effective prevention programmes, diagnostic measures and targeted therapeutics in the near future.

The Hermansky-Pudlak syndrome 1 (HPS1) and HPS4 proteins are components of two complexes, BLOC-3 and BLOC-4, involved in the biogenesis of lysosome-related organelles

Hermansky-Pudlak syndrome (HPS) is a genetic disease of lysosome, melanosome, and granule biogenesis. Mutations of six different loci have been associated with HPS in humans, the most frequent of which are mutations of the HPS1 and HPS4 genes. Here, we show that the HPS1 and HPS4 proteins are components of two novel protein complexes involved in biogenesis of melanosome and lysosome-related organelles: biogenesis of lysosome-related organelles complex-(BLOC) 3 and BLOC-4. The phenotypes of Hps1-mutant (pale-ear; ep) and Hps4-mutant (light-ear; le) mice and humans are very similar, and cells from ep and le mice exhibit similar abnormalities of melanosome morphology. HPS1 protein is absent from ep-mutant cells, and HPS4 from le-mutant cells, but le-mutant cells also lack HPS1 protein. HPS4 protein seems to be necessary for stabilization of HPS1, and the HPS1 and HPS4 proteins co-immunoprecipitate, indicating that they are in a complex. HPS1 and HPS4 do not interact directly in a yeast two-hybrid system, although HPS4 interacts with itself. In a partially purified vesicular/organellar fraction, HPS1 and HPS4 are both components of a complex with a molecular mass of approximately 500 kDa, termed BLOC-3. Within BLOC-3, HPS1 and HPS4 are components of a discrete approximately 200-kDa module termed BLOC-4. In the cytosol, HPS1 (but not HPS4) is part of yet another complex, termed BLOC-5. We propose that the BLOC-3 and BLOC-4 HPS1.HPS4 complexes play a central role in trafficking cargo proteins to newly formed cytoplasmic organelles.

Modeling gene-environment interactions in malignant melanoma

Many cancers are the pathological consequence of environmentally initiated disruptions to cellular genetic control mechanisms. For most cancers the relevant environmental carcinogens have not been identified, but one major exception is cutaneous malignant melanoma, for which the primary environmental agent is solar ultraviolet (UV) radiation. Hence, melanomagenesis represents a potential model of detrimental gene-environment interaction. Although the underlying genetic basis of melanoma is currently being elucidated, fundamental questions concerning UV and the mechanisms by which it operates remain unanswered. Significant progress has recently been made in creating UV-responsive, genetically tractable mouse models of melanoma that accurately recapitulate human disease. These models are providing novel insights into how the genome and environment interact in vivo.

The mouse organellar biogenesis mutant buff results from a mutation in Vps33a, a homologue of yeast vps33 and Drosophila carnation

In the mouse, more than 16 loci are associated with mutant phenotypes that include defective pigmentation, aberrant targeting of lysosomal enzymes, prolonged bleeding, and immunodeficiency, the result of defective biogenesis of cytoplasmic organelles: melanosomes, lysosomes, and various storage granules. Many of these mouse mutants are homologous to the human Hermansky-Pudlak syndrome (HPS), Chediak-Higashi syndrome, and Griscelli syndrome. We have mapped and positionally cloned one of these mouse loci, buff (bf), which has a mutant phenotype similar to that of human HPS. Mouse bf results from a mutation in Vps33a and thus is homologous to the yeast vacuolar protein-sorting mutant vps33 and Drosophila carnation (car). This is the first found defect of the class C vacuole/prevacuole-associated target soluble N-ethylmaleimide-sensitive factor attachment protein receptor (t-SNARE) complex in mammals and the first mammalian mutant found that is directly homologous to a vps mutation of yeast. VPS33A thus is a good candidate gene for a previously uncharacterized form of human HPS.

Progress in cutaneous cancer research

Cutaneous cancers represent a major public health concern due to the very high incidence, associated medical costs, substantial mortality, and cosmetic deformities associated with treatment. Considerable progress in basic research has provided new insights into the underlying genetic basis of the major human cutaneous cancers, malignant melanoma, basal cell carcinoma, and squamous cell carcinoma. In turn, these genetic insights have illuminated biochemical pathways that promise to provide new approaches to the prevention and treatment of cutaneous neoplasms. This review will detail the evolving genetic information and indicate how this information is being used to refine experimental models that serve to both define the biochemistry of cancer pathogenesis and test novel approaches to cancer therapy. Combined with preventive measures to reduce exposure to sunlight, these advances are likely to reduce this major public health burden in the coming decade.

Type I gamma-GT mRNA is expressed in B16 melanoma and levels correlate with pigmentation

Gamma-glutamyl transpeptidase (gamma-GT), an ectoenzyme involved mainly in glutathione metabolism, is expressed in B16 melanoma cells. B16 melanoma cells under continuous culture conditions show a phenotypic drift from melanotic to amelanotic and re-melanotic stages. We have investigated the regulation of gamma-GT in B16 melanoma cells under such different pigmentary conditions. High levels of gamma-GT messenger RNA (mRNA) and activity were detected in pigmented B16 melanoma cells, whereas in amelanotic B16 melanoma cells the levels were very low. Treatment with lactic acid, a known inhibitor of tyrosinase gene expression, also led to the down-regulation of gamma-GT mRNA and activity. Thus our results indicate that gamma-GT regulation depends on the pigmentation status in pigment cells. We have also assessed the levels of gamma-GT in normal murine melanocytes (melan-a cells). It was seen that melan-a cells express very low levels of gamma-GT. As gamma-GT is known to be regulated in a tissue-specific manner, and is expressed from as many as six promoters giving rise to six different types of mRNAs each having unique 5' ends, we have further investigated the type of gamma-GT mRNA expressed in B16 melanoma and melan-a cells. In this study, we have conclusively demonstrated that type I mRNA transcript of gamma-GT is expressed in B16 melanoma and melan-a cells.

Hermansky-Pudlak syndrome is caused by mutations in HPS4, the human homolog of the mouse light-ear gene

Hermansky-Pudlak syndrome (HPS) is a disorder of organelle biogenesis in which oculocutaneous albinism, bleeding and pulmonary fibrosis result from defects of melanosomes, platelet dense granules and lysosomes. HPS is common in Puerto Rico, where it is caused by mutations in the genes HPS1 and, less often, HPS3 (ref. 8). In contrast, only half of non-Puerto Rican individuals with HPS have mutations in HPS1 (ref. 9), and very few in HPS3 (ref. 10). In the mouse, more than 15 loci manifest mutant phenotypes similar to human HPS, including pale ear (ep), the mouse homolog of HPS1 (refs 13,14). Mouse ep has a phenotype identical to another mutant, light ear (le), which suggests that the human homolog of le is a possible human HPS locus. We have identified and found mutations of the human le homolog, HPS4, in a number of non-Puerto Rican individuals with HPS, establishing HPS4 as an important HPS locus in humans. In addition to their identical phenotypes, le and ep mutant mice have identical abnormalities of melanosomes, and in transfected melanoma cells the HPS4 and HPS1 proteins partially co-localize in vesicles of the cell body. In addition, the HPS1 protein is absent in tissues of le mutant mice. These results suggest that the HPS4 and HPS1 proteins may function in the same pathway of organelle biogenesis.

Mutant laboratory mice with abnormalities in pigmentation: annotated tables

Mammalian pigment cell research has recently entered a phase of significantly increased activity due largely to the exploitation of the many mutant mouse stocks that are coming on stream. Numerous transgenic, targeted mutagenesis (so-called 'knockouts'), conditional (so-called 'gene switch') and spontaneous mutant mice develop abnormal coat color phenotypes. The number of mice that exhibit such abnormalities is increasing exponentially as genetic engineering methods become routine. Since defined abnormalities in such mutant mice provide important clues to the as yet often poorly understood functional roles of many gene products, this overview includes a corresponding, annotated table of mutant mice with pigmentation alterations. These range from early developmental defects via a large array of coat color abnormalities to a melanoma metastasis model. This overview should provide helpful pointers to investigators who are looking for mouse models to explore or to compare functional activities of genes of interest and for comparing coat color phenotypes of spontaneous or genetically engineered mouse mutants with novel ones. Secondly, this review includes a table of mouse models of specific human diseases with genetically defined pigmentation abnormalities. In summary, this annotated table should serve as a useful reference for anyone interested in the molecular controls of pigmentation.

Agouti signaling protein and other factors modulating differentiation and proliferation of immortal melanoblasts

The melanocyte lineage potentially forms an attractive model system for studies in cell differentiation, developmental genetics, cell signaling, and melanoma, because differentiated cells produce the visible pigment melanin. Immortal lines of murine melanoblasts (melanocyte precursors) have been described previously, but induction of differentiation involved a complex culture system with keratinocyte feeder cells. Here we describe conditions for both growth and induced differentiation of the melanoblast line melb-a, without feeder cells, and analyze factors that directly control proliferation and differentiation of these pure melanoblasts. Several active factors are products of developmental and other coat color genes, including stem cell factor (SCF), melanocyte-stimulating hormone (alphaMSH), and agouti signaling protein (ASP), a natural antagonist at the MSH receptor (melanocortin 1 receptor, MC1R) encoded by the agouti gene. A stable analog of alphaMSH (NDP-MSH) stimulated differentiation and inhibited growth. ASP in excess inhibited both effects of NDP-MSH, that is, ASP could inhibit pigmentation and stimulate growth. These effects provide an explanation for the interactions in mice of melanocyte developmental mutations with yellow agouti and Mc1r alleles, and a role for embryonic expression patterns of ASP.

Theophylline administration markedly reduces hepatic and pulmonary implantation of B16-F10 melanoma cells in mice

Theophylline-treated B16-F10 melanoma cells show a lower experimental metastatic potential in vivo. To identify the possible mechanism(s) involved and on the basis of previous reports, we tested the induction of apoptosis in B16-F10 cells. Fluorescence activated cell sorter (FACS) analysis and p53 overexpression in theophylline-treated B16-F10 melanoma cells appeared to suggest enhanced cell death by apoptosis. The in vivo effects of orally administered theophylline in mice were investigated using different treatment schedules in mice that had undergone hepatic or pulmonary colonization with tumour cells. Mice received theophylline in their drinking water according to different protocols: (i) from 3 days before tumour cell inoculation until animal sacrifice ('early treatment'); (ii) from 3 days before until 3 days after tumour cell inoculation ('short treatment'); or (iii) from 3 days after tumour cell inoculation until animal sacrifice ('late treatment'). In the 'early treatment' group, the number of melanoma foci was reduced by 92.3% in the liver and 81.4% in the lung compared with control animals (P < 0.001). In the 'short treatment' group, there was an 80.2% and 72.2% reduction in liver and lung metastases, respectively (P < 0.001). In the 'late treatment' group, the inhibition of metastasis was 59.7% for liver and 45.3% for lung (P < 0.005). Survival studies showed that 50% of the 'early' theophylline-treated animals died 33.2 +/- 2.0 days after intrasplenic injection (control group: 23.1 1.8 days; P < 0.001) and 33.9 +/- 2.5 days after tail vein injection (control group: 24.1 +/- 1.4 days; P < 0.001). Taken together, these observations provide useful information for the potential clinical application of theophylline as a chemotherapeutic agent against malignant melanoma.

Lessons from melanocyte development for understanding the biological events in naevus and melanoma formation

Recent advances in mouse genetics have identified molecular changes that are critical for melanocyte maturation and differentiation. This review briefly summarizes the current knowledge of distinct steps in melanocyte development, and identifies for each step the most important molecules such as the growth factors stem cell factor and endothelin-3, with their respective receptors. Classical cadherins, i.e. E-cadherin, N-cadherin and P-cadherin, determine melanocyte positioning in the skin. During naevus and melanoma development, the two growth factor signalling pathways are downregulated, whereas cadherin expression shifts concomitantly with re-positioning of the naevus and melanoma cells in the skin. Loss of E-cadherin and gain of N-cadherin by melanoma cells has profound consequences for the regulatory cross-talk between various types of cells in the skin. Naevus and melanoma cells that do not express E-cadherin are resistant to control by keratinocytes and establish close communications with fibroblasts and endothelial cells. However, forced expression of E-cadherin in melanoma cells can reverse the malignant phenotype by re-establishing the control of keratinocytes over the melanoma cells. Even highly aggressive metastatic melanoma cells can be signalled to turn off the expression of genes associated with tumour invasion and metastasis, suggesting that this strategy could be utilized in the therapy of melanoma.

An orthologue of the kit-related gene fms is required for development of neural crest-derived xanthophores and a subpopulation of adult melanocytes in the zebrafish, Danio rerio

Developmental mechanisms underlying traits expressed in larval and adult vertebrates remain largely unknown. Pigment patterns of fishes provide an opportunity to identify genes and cell behaviors required for postembryonic morphogenesis and differentiation. In the zebrafish, Danio rerio, pigment patterns reflect the spatial arrangements of three classes of neural crest-derived pigment cells: black melanocytes, yellow xanthophores and silver iridophores. We show that the D. rerio pigment pattern mutant panther ablates xanthophores in embryos and adults and has defects in the development of the adult pattern of melanocyte stripes. We find that panther corresponds to an orthologue of the c-fms gene, which encodes a type III receptor tyrosine kinase and is the closest known homologue of the previously identified pigment pattern gene, kit. In mouse, fms is essential for the development of macrophage and osteoclast lineages and has not been implicated in neural crest or pigment cell development. In contrast, our analyses demonstrate that fms is expressed and required by D. rerio xanthophore precursors and that fms promotes the normal patterning of melanocyte death and migration during adult stripe formation. Finally, we show that fms is required for the appearance of a late developing, kit-independent subpopulation of adult melanocytes. These findings reveal an unexpected role for fms in pigment pattern development and demonstrate that parallel neural crest-derived pigment cell populations depend on the activities of two essentially paralogous genes, kit and fms.

Keratinocyte expression of transgenic hepatocyte growth factor affects melanocyte development, leading to dermal melanocytosis

Using the epidermis-specific cytokeratin 14 promoter to deliver HGF exclusively from epidermal keratinocytes, we have examined the potential of hepatocyte growth factor (HGF) secreted from the normal environment to control morphogenesis. The transgenic mice displayed a significant increase of the number of melanocytes and their precursors in embryos starting not later than 16.5 dpc, and then after birth an explosive increase of dermal melanocytes started within 1 week, and these melanocytes were maintained throughout the entire life of the mice. Thus, HGF acts as a paracrine agent to promote survival, proliferation and differentiation of melanocyte precursors in vivo, and eventually causes melanocytosis. Loss of E-cadherin expression in dermal melanocyte precursors suggests that HGF caused dermal localization of melanocytes and their precursors by down-regulation of E-cadherin molecules.

Multi-organellar disorders of pigmentation: intracellular traffic jams in mammals, flies and yeast

Several different mutant genes in humans, mice and Drosophila, most of which were identified initially on the basis of reduced pigmentation, have been associated with defects of multiple cytoplasmic organelles - melanosomes, lysosomes and granules. Recent discoveries show that several of these mutations directly affect components in the pathway of organelle-specific protein trafficking, and provide new insights into the relationships of these pathways in mammals, flies and yeast.

Genetic and epigenetic control of the proliferation and differentiation of mouse epidermal melanocytes in culture

Serum-free culture of epidermal cell suspensions from neonatal skin of mice of strain C57BL/10JHir (B10) showed that alpha-melanocyte-stimulating hormone (alpha-MSH) was involved in regulating the differentiation of melanocytes by inducing tyrosinase activity, melanosome formation, and dendritogenesis. Dibutyryl adenosine 3':5'-cyclic monophosphate (DBcAMP) similarly induced the differentiation of melanocytes. On the other hand, DBcAMP induced the proliferation of epidermal melanocytes in culture in the presence of keratinocytes. Basic fibroblast growth factor (bFGF) was also shown to stimulate the sustained proliferation of undifferentiated melanoblasts in the presence of DBcAMP and keratinocytes. These results suggest that the proliferation and differentiation of mouse epidermal melanoblasts and melanocytes in culture are regulated by the three factors; namely, cAMP, bFGF, and keratinocyte-derived factors. Moreover, serum-free primary culture of mouse epidermal melanocytes derived from B10 congenic mice, which carry various coat color genes, showed that the coat color genes were involved in regulating the proliferation and differentiation of mouse epidermal melanocytes by controlling the proliferative rate, melanosome formation and maturation, and melanosome distribution.

Impaired growth and differentiation of diploid but not immortal melanoblasts from endothelin receptor B mutant (piebald) mice

Endothelin 3 (Edn3) and its preferred receptor, endothelin receptor B (Ednrb), are implicated in development, especially that of two neural-crest-derived cell lineages: melanocytes and enteric ganglion cells. Mice and humans with a null mutation at either locus can show major deficiencies in both cell types: congenital white spotting and aganglionic megacolon (Hirschsprung disease in human). Numbers of early (migrating) embryonic melanoblasts are low in Ednrb(ls) mutant mice, while added Edn3 appears to promote the growth of melanocyte precursors in neural crest cultures. However, it is hard to assess cell differentiation in these mixed cultures, and it is not known whether Ednrb has any role in the postnatal melanocytic lineage. We have therefore studied primary cultures of neonatal melanoblasts homozygous for the piebald (Ednrb(s)) mutation. These mutant melanoblasts showed severe impairment of both net cell growth and differentiation compared to wild-type melanoblasts. They were also unresponsive to stimulation of growth by cholera toxin. We have established three immortal lines of melanoblasts and one of melanocytes homozygous for Ednrb(s). These immortal lines, however, had no detectable deficiency of growth or differentiation as judged by cell counts, induced pigmentation and immunocytochemistry for melanocytic markers. Consistent with this, neither Ednrb nor Edn3 mRNA was detected in 3/3 tested immortal lines of mouse melanoblasts and 5/5 lines of melanocytes, of various genotypes. We also report for the first time a method to grow immortal melanoblasts in pure culture, without feeder cells.

Transgene expression of steel factor in the basal layer of epidermis promotes survival, proliferation, differentiation and migration of melanocyte precursors

Mutations at the murine dominant white spotting (KitW) and steel (MgfSl) loci, encoding c-Kit receptor kinase and its ligand respectively, exert developmental defects on hematopoietic cells, melanocytes, germ cells and interstitial cells of Cajal. The expression patterns of steel factor (SLF) observed in the skin and gonads suggest that SLF mediates a migratory or a chemotactic signal for c-Kit-expressing stem cells (melanocyte precursors and primordial germ cells). By targeting expression of SLF to epidermal keratinocytes in mice, we observed extended distribution of melanocytes in a number of sites including oral epithelium and footpads where neither melanocytes nor their precursors are normally detected. In addition, enlarged pigmented spots of KitW and other spotting mutant mice were observed in the presence of the SLF transgene. These results provide direct evidence that SLF stimulates migration of melanocytes in vivo. We also present data suggesting that SLF does not simply support survival and proliferation of melanocytes but also promotes differentiation of these cells. Unexpectedly, melanocyte stem cells independent of the c-Kit signal were maintained in the skin of the SLF transgenic mice. After the elimination of c-Kit-dependent melanoblasts by function-blocking anti-c-Kit antibody, these stem cells continued to proliferate and differentiate into mature melanocytes. These melanoblasts are able to migrate to cover most of the epidermis after several months. The SLF transgenic mice described in this report will be useful in the study of melanocyte biology.

MSG1 (melanocyte-specific gene 1): mapping to chromosome Xq13.1, genomic organization, and promoter analysis

MSG1 (melanocyte-specific gene 1) is a recently isolated gene predominantly expressed in cultured normal melanocytes and pigmented melanoma cells. MSG1 encodes a 27-kDa nuclear protein that has strong intrinsic transcriptional transactivating activity. In this report, the human MSG1 gene was mapped to chromosome Xq13.1 using X chromosome-specific somatic cell hybrids, and the mouse Msg1 gene was mapped 1.9 +/- 1.3 cM proximal to Xist using an interspecific backcross panel. Both the human and the mouse MSG1 genes consist of three exons and two introns within 5 kb of genomic DNA, and their genomic structures are highly conserved. Southern blot analysis suggests the existence of MSG1 homologues in chicken, zebrafish, and Drosophila. A 2.0-kb fragment of the 5'-flanking region of the mouse Msg1 gene contains a TATA box and potential binding sites for several transcription factors including USF, Brn-3, Brn-2, TFE3, Oct-1, AP-2, and Spl. This promoter fragment activates transcription of a reporter gene in pigmented melanoma cells, but not in amelanotic melanoma cells or nonmelanocytic cells, indicating that Msg1 expression is at least partially regulated at the transcriptional level.

Regulation of expression of MSG1 melanocyte-specific nuclear protein in human melanocytes and melanoma cells

MSG1 is a nuclear protein and a possible transcriptional transactivator that is expressed strongly in melanocytes but very weakly, if at all, in most nonmelanocytic cells or adult mouse tissues. This strong expression of MSG1 in cultured normal human epidermal melanocytes was found to be dependent on both endothelin-1 and FGF-2. The phorbol ester TPA could be substituted for endothelin-1. The MSG1 mRNA transcripts were rapidly induced by either endothelin-1 or TPA. However, FGF-2 had no effects at the mRNA level, suggesting its contribution at the translational and/or posttranslational level(s). MSG1 (as well as its mRNA transcripts) was induced by TPA in human melanoma cells, which produce FGF-2 as an autocrine growth factor. Melanoma cells derived from primary tumors or tyrosinase-positive metastatic melanoma cells expressed MSG1 after TPA treatment, while tyrosinase-negative metastatic melanoma cells or nonmelanocytic cells did not. This TPA-induced MSG1 expression in melanoma cells correlated with the expression of the MSG1 mRNA transcripts and TPA-dependent transcriptional activation of the MSG1 promoter sequence, indicating its transcriptional regulation. In vivo, MSG1 protein was detected in human nevocytic nevus confined to the pigmented region, while MSG1 expression showed cell-level heterogeneity in pigmented melanoma tissues. These results demonstrate that MSG1 expression is regulated transcriptionally and posttranscriptionally by local growth factors as well as by the cellular status of differentiation.

Involvement of microphthalmia in the inhibition of melanocyte lineage differentiation and of melanogenesis by agouti signal protein

In mouse follicular melanocytes, production of eumelanins (brown-black pigments) and pheomelanins (yellow-brownish pigments) is under the control of two intercellular signaling molecules that exert opposite actions, alpha-melanocyte-stimulating hormone (alphaMSH) which preferentially increases the synthesis of eumelanins, and agouti signal protein (ASP) whose expression favors the production of hair containing pheomelanins. In this study, we report that ASP does not only affect mature melanocytes but can also inhibit the differentiation of melanoblasts. We show that both alphaMSH and forskolin promote the differentiation of murine melanoblasts into mature melanocytes and that ASP inhibits this process. We present evidence that the expression of a specific melanogenic transcription factor, microphthalmia, and its binding to an M box regulatory element, is inhibited by ASP. We also show that, in B16 murine melanoma cells, ASP inhibits alphaMSH-stimulated expression of tyrosinase, tyrosine-related proteins 1 and 2 through an inhibition of the transcription activity of their respective promoters. Further, ASP inhibits alphaMSH-induced expression of the microphthalmia gene and reduces the level of microphthalmia in the cells. Our data demonstrate that ASP can regulate both melanoblast differentiation and melanogenesis, pointing out the key role of microphthalmia in the control of these processes.

Mouse models of Hermansky Pudlak syndrome: a review

Hermansky Pudlak Syndrome (HPS) is a recessively inherited disease affecting the contents and/or the secretion of several related subcellular organelles including melanosomes, lysosomes, and platelet dense granules. It presents with disorders of pigmentation, prolonged bleeding, and ceroid deposition, often accompanied by severe fibrotic lung disease and colitis. In the mouse, the disorder is clearly multigenic, caused by at least 14 distinct mutations. Studies on the mouse mutants have defined the granule abnormalities of HPS and have shown that the disease is associated with a surprising variety of phenotypes affecting many tissues. This is an exciting time in HPS research because of the recent molecular identification of the gene causing a major form of human HPS and the expected identifications of several mouse HPS genes. Identifications of mouse HPS genes are expected to increase our understanding of intracellular vesicle trafficking, lead to discovery of new human HPS genes, and suggest diagnostic and therapeutic approaches toward the more severe clinical consequences of the disease.

recessive spotting: a linked locus that interacts with W/Kit but is not allelic

BACKGROUND

The murine coat-colour mutation recessive spotting (rs) maps very closely to the W/Kit locus, encoding the proto-oncoprotein Kit, the protein tyrosine kinase receptor for stem cell factor. Kit is important in the development of melanocytes, germ cells, interstitial cells of Cajal (ICC) and haemopoietic lineages, including mast cells. rs has never been genetically separated from Kit, and interacts with Kit mutations, suggesting that it is a recessive allele of Kit. Here we have tested this possibility. We have shown previously that diploid rs/rs melanocytes proliferated more slowly than did +/+ melanocytes, as did an immortal line of rs/rs melanocytes, melan-rs.

RESULTS

The Kit mRNA level in rs/rs melanocytes was indistinguishable from that of other melanocyte lines. The Kit cDNA sequence from rs/rs melanocytes and the kinase activity of Kit in rs/rs mast cells appeared to be normal. No deficiency of mast cells or ICC was observed in rs/rs mice. Moreover, following the overexpression of a normal Kit cDNA, proliferation of rs/rs melanocytes was retarded further, but that of +/+ melanocytes was increased, indicating an intracellular interaction between rs and Kit. Of other closely linked tyrosine kinase genes, melanocytes and melanoblasts did not express mRNA for Pdgfra, Flk-1 or Txk, but both expressed Tec, encoding a nonreceptor kinase that interacts with Kit.

CONCLUSIONS

rs is not a mutation in Kit, although we have confirmed that rs interacts with Kit. It seems unlikely that rs affects Pdgfra, Flk-1 or Txk, but Tec remains a candidate for rs.

Mutation analysis of patients with Hermansky-Pudlak syndrome: a frameshift hot spot in the HPS gene and apparent locus heterogeneity

Hermansky-Pudlak syndrome (HPS) is a rare, autosomal recessive disorder in which oculocutaneous albinism, bleeding, and lysosomal ceroid storage result from defects of multiple cytoplasmic organelles-melanosomes, platelet-dense granules, and lysosomes. As reported elsewhere, we mapped the human HPS gene to chromosome segment 10q23, positionally cloned the gene, and identified three pathologic mutations of the gene, in patients from Puerto Rico, Japan, and Europe. Here, we describe mutation analysis of 44 unrelated Puerto Rican and 24 unrelated non-Puerto Rican HPS patients. A 16-bp frameshift duplication, the result of an apparent founder effect, is nearly ubiquitous among Puerto Rican patients. A frameshift at codon 322 may be the most frequent HPS mutation in Europeans. We also describe six novel HPS mutations: a 5' splice-junction mutation of IVS5, three frameshifts, a nonsense mutation, and a one-codon in-frame deletion. These mutations define an apparent frameshift hot spot at codons 321-322. Overall, however, we detected mutations in the HPS gene in only about half of non-Puerto Rican patients, and we present evidence that suggests locus heterogeneity for HPS.

The murine misty mutation: phenotypic effects on melanocytes, platelets and brown fat

Although the recessive murine mutation misty (m) is well known, its phenotype has never been reported beyond brief descriptions of a dilution of coat color and white spotting of the belly and extremities, suggesting a developmental mutation. A report in abstract has also suggested effects on white fat and body weight. Here, we report effects of the homozygous misty mutation on an unusual combination of three cell types: melanocytes, platelets, and brown fat. Brown fat appeared to be completely absent from all expected locations in neonatal m/m mice. A prolonged bleeding time was observed; platelet count and platelet serotonin and ATP levels were normal, but the level of ADP in m/m platelets was low. Primary cultures and immortal lines of melanocytes from m/m mice showed several abnormalities. There was a marked deficiency in net proliferation, suggesting that the color dilution and spotting in vivo may result from reduced numbers of melanocytes and their precursors. m/m melanocytes were also hyperdendritic in morphology, overproduced melanin, and had deficient responses to the cAMP agonists cholera toxin and melanocyte-stimulating hormone, which normally promote melanin production. The misty gene product may be involved in adenine nucleotide metabolism or signaling.

Dorsoventral patterning of the vertebrate neural tube is conserved in a protochordate

The notochord and dorsal ectoderm induce dorsoventral compartmentalization of the vertebrate neural tube through the differential regulation of genes such as HNF-3beta, Pax3, Pax6 and snail. Here we analyze the expression of HNF-3beta and snail homologues in the ascidian, Ciona intestinalis, a member of the subphylum Urochordata, the earliest branch in the chordate phylum. A combination of in situ hybridization and promoter fusion analyses was used to demonstrate that the Ciona HNF-3beta homologue is expressed in the ventralmost ependymal cells of the neural tube, while the Ciona snail homologue is expressed at the junction between the invaginating neuroepithelium and dorsal ectoderm, similar to the patterns seen in vertebrates. These findings provide evidence that dorsoventral compartmentalization of the chordate neural tube is not an innovation of the vertebrates. We propose that precursors of the floor plate and neural crest were present in a common ancestor of both vertebrates and ascidians.

Comparative action spectrum for ultraviolet light killing of mouse melanocytes from different genetic coat color backgrounds

The photobiology of mouse melanocyte lines with different pigment genotypes was studied by measuring colony-forming ability after irradiation. The cell lines were wild-type black (melan-a) and the mutants brown (melan-b) and albino (melan-c). Four lamps emitting various UV wavelengths were used. These were germicidal (UVC, 200-280 nm), 82.3% output at 254 nm, TL01 (UVB, 280-320 nm), 64.2% at 310-311 nm, FS20, broadband with peak output at 312 nm and Alisun-S (UVA, 320-400 nm), broadband with peak output at 350-354 nm. Appropriate filtration reduced the contaminating UVC to nonlethal levels for the longer waverange lamps. Wild-type melan-a was resistant to UVC and UVA compared to the other two cell lines, but the differences were small. The melan-c cell line was more resistant to UVB and markedly more resistant to FS20 than the pigmented lines. With the exception of FS20 responses, melan-b was more sensitive than melan-a to killing by the various UV lamps. There were more pyrimidine dimers (cyclobutane dimers and 6-4 photoproducts) produced in melan-a than in melan-c cells by UVC, UVB and FS20 lamps. Unlike melan-c, melan-a and melan-b showed a strong free radical signal of melanin character with a detectable contribution of pheomelanin-like centers. The contribution of pheomelanin was higher in melan-b than in melan-a, while the total melanin content in these two cell lines was comparable. The abundant melanin granules of wild-type melan-a melanocytes were well melanized and ellipsoidal, whereas those of melan-b melanocytes tended to be spherical. In the albino line (melan-c) the melanocytes contained only early-stage melanosomes, all of which were devoid of melanin. The results indicate that pigment does not protect against direct effect DNA damage in the form of pyrimidine dimers nor does it necessarily protect against cell death. High pigment content is not very protective against killing by UVC and UVA, and it may photosensitize in UVB the very wavelength range that is of greatest concern with respect to the rising incidence in skin cancer, especially melanoma. It is clear from these studies that, in pigment cells, monochromatic results cannot predict polychromatic responses and that cell death from solar irradiations is a complex phenomenon that depends on more than DNA damage.

Mouse pale ear (ep) is homologous to human Hermansky-Pudlak syndrome and contains a rare 'AT-AC' intron

Hermansky-Pudlak syndrome (HPS) is a rare, often fatal, autosomal recessive disorder in which albinism, bleeding and lysosomal storage are associated with defects of diverse cytoplasmic organelles, including melanosomes, platelet dense granules and lysosomes. Similar multi-organellar defects occur in the Chediak-Higashi syndrome (CHS), as well as in a large number of different mouse mutants. The HPS gene is located in 10q23, and two genetically distinct mouse loci, pale ear (ep) and ruby-eye (ru), both with mutant phenotypes similar to human HPS, map close together in the homologous region of murine chromosome 19, suggesting that one of these loci might be homologous to human HPS. We recently identified the human HPS gene, which encodes a novel ubiquitously-expressed transmembrane protein of unknown function. Here, we describe characterization of the mouse Hps cDNA and genomic locus, and identification of pathologic Hps gene mutations in ep but not in ru mice, establishing mouse pale ear as an animal model for human HPS. The phenotype of homozygous ep mutant mice encompasses those of both HPS and CHS, suggesting that these disorders may be closely related. In addition, the mouse and human HPS genes both contain a rare 'AT-AC' intron, and comparison of the sequences of this intron in the mouse and human genes identified conserved sequences that suggest a possible role for pre-mRNA secondary structure in excision of this rare class of introns.

Ascidian tyrosinase gene: its unique structure and expression in the developing brain

Tadpole larvae of ascidians have two sensory pigment cells in the brain. One is the otolith cell that functions as a gravity receptor, the other pigment cell is part of a primitive photosensory structure termed the ocellus. These sensory cells, like vertebrate pigment cells, contain membrane-bounded melanin granules and are considered to reflect a crucial position in the evolutionary process of this cell type. To investigate the molecular changes accompanying the evolution of pigment cells, we have isolated from Halocynthia roretzi a gene encoding tyrosinase, a key enzyme in melanin biosynthesis. The cDNA has an open reading frame (ORF) of 596 amino acids, which is 36-39% identical in amino acid sequence to vertebrate tyrosinases. In addition, the sequence analysis of both cDNA and genomic clones reveals an unusual organization of the tyrosinase gene, an extraordinary 3' untranslated region of the transcripts with significant homology to the coding sequence, and a single short intron in the sequence encoding a cytoplasmic domain. Expression of the gene is detected first in two pigment precursor cells positioned in the neural plate of early neurulae, and later in two melanin-containing pigment cells within the brain of late tailbud embryos. Its expression pattern correlates well with the appearance of tyrosinase enzyme activity in the developing brain. These results provide the first description of pigment cell differentiation at the molecular level in the ascidian embryo, and also will contribute to a better understanding of the evolution of chordate pigment cells.

Positional cloning of a gene for Hermansky-Pudlak syndrome, a disorder of cytoplasmic organelles

Hermansky-Pudlak syndrome (HPS) is an often-fatal autosomal recessive disease in which albinism, bleeding, and lysosomal storage result from defects of diverse cytoplasmic organelles: melanosomes, platelet dense bodies, and lysosomes. HPS is the most common single-gene disorder in Puerto Rico, with an incidence of 1 in 1,800. We have identified the HPS gene by positional cloning, and found homozygous frameshifts in this gene in Puerto Rican, Swiss, Irish and Japanese HPS patients. The HPS polypeptide is a novel transmembrane protein that is likely to be a component of multiple cytoplasmic organelles and that is apparently crucial for their normal development and function. The different clinical phenotypes associated with the different HPS frameshifts we observed suggests that differentially truncated HPS polypeptides may have somewhat different consequences for subcellular function.

msg1, a novel melanocyte-specific gene, encodes a nuclear protein and is associated with pigmentation

Messenger RNA transcripts of the highly pigmented murine melanoma B16-F1 cells were compared with those from their weakly pigmented derivative B16-F10 cells by differential display. A novel gene called msg1 (melanocyte-specific gene) was found to be expressed at high levels in B16-F1 cells but at low levels in B16-F10 cells. Expression of msg1 was undetectable in the amelanotic K1735 murine melanoma cells. The pigmented murine melanocyte cell line melan-a expressed msg1, as did pigmented primary cultures of murine and human melanocytes; however, seven amelanotic or very weakly pigmented human melanoma cell lines were negative. Transformation of murine melanocytes by transfection with v-Ha-ras or Ela was accompanied by depigmentation and led to complete loss of msg1 expression. The normal tissue distribution of msg1 mRNA transcripts in adult mice was confined to melanocytes and testis. Murine msg1 and human MSG1 genes encode a predicted protein of 27 kDa with 75% overall amino acid identity and 96% identity within the C-terminal acidic domain of 54 amino acids. This C-terminal domain was conserved with 76% amino acid identity in another protein product of a novel human gene, MRG1 (msg1-related gene), isolated from normal human melanocyte cDNA by 5'-rapid amplification of cDNA ends based on the homology to msg1. The msg1 protein was localized to the melanocyte nucleus by immunofluorescence cytochemistry. We conclude that msg1 encodes a nuclear protein, is melanocyte-specific, and appears to be lost in depigmented melanoma 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.

Foetal human melanocytes: in situ detection, in vitro culture and differentiation characteristics at 6-11 weeks EGA

In vivo, melanocytes were detected in epidermis from human tissue of 6.5 weeks estimated gestinational age (EGA) and older. We have successfully established melanocyte monocultures from tissue of 9 to 10 weeks EGA. To our knowledge, this is the first report on physiology of human foetal melanocytes in monoculture. In culture, such melanocytes retained foetal characteristics. Proliferation rates noted were markedly higher (approximately 2.7-fold) when compared to those in cultures of neonatal melanocytes. Moreover, when analyzing cellular phenotypes by markers for cells of the melanocytic lineage, foetal cells isolated from tissue of 9 weeks EGA reproducibly showed expression of the high molecular weight (HMW) antigen and c-kit to an extent intermediate to that found in neonatal melanocytes and M14 melanoma cells. Such differential expression was not observed if cells were isolated from tissue of 10 weeks EGA, indicating that the foetal environment provides essential differentiation stimuli during the 10th week of gestation. Moreover, these results are supportive of the theory that malignant transformation involves a process of dedifferentiation. In all, human foetal melanocyte culture provides a useful model to investigate pigment cell differentiation.

Clonality of basal cell carcinoma--molecular analysis of an interesting case

Tumor cells represent a single clone of cells that have undergone a series of mutations in genomic DNA. This process, known as clonal evolution, is a distinguishing feature of cancer. The human androgen receptor gene (HUMARA; GenBank) contains a highly polymorphic cytosine-adenine-guanine trinucleotide repeat that can be used to determine clonality by depicting X chromosome inactivation patterns. Random X chromosome inactivation is consistent with polyclonality; nonrandom X chromosome inactivation indicates a clonal population of cells. Basal cell carcinoma (BCC) demonstrates an atypical growth pattern in that it grows slowly, rarely metastasizes, and is rarely lethal. Whether this tumor results from the accumulation of mutations in a single cell with subsequent clonal expansion or reflects a polyclonal response by a group of cells to a growth stimulus is unknown. To provide further insight into the molecular events characterizing BCCs, we determined the clonal origin of five modular BCCs from a female patient by analyzing X chromosome inactivation patterns at the HUMARA locus. All tumors demonstrated a nonrandom pattern of X chromosome inactivation, consistent with monoclonal proliferation. These findings provide strong genetic evidence that sporadic BCCs develop by clonal evolution and support the contention that a series of mutations in a single cell is responsible for the altered growth state seen in these transformed epithelial cells.

Genetics of Cutaneous Malignant Melanoma

An increase in the clinical significance of cutaneous malignant melanoma has paralleled a dramatic increase in the rate of death from this disease. The critical genetic changes associated with the genesis and progression of this disease are only beginning to be identified. This review highlights genetic changes in cutaneous melanoma and discusses the genetics of predisposition, cytogenetics, changes in proto-oncogenes and oncogenes, and evidence for the role of tumor suppressor genes in this malignancy. The viewpoint of this article is that malignant melanoma is a genetic disease.

A cloned, immortal line of murine melanoblasts inducible to differentiate to melanocytes

Cultures of differentiated melanocytes can readily be grown from the dissociated epidermis of neonatal mice, and immortal cell lines often develop from these. However, the first cells that grow and transiently dominate the cultures, while similar to melanocytes, are unpigmented. These have been shown to be precursors of melanocytes and may be termed melanoblasts. Under our previous standard culture conditions, involving the use of keratinocyte feeder cells, foetal calf serum, the phorbol ester 12-O-tetradecanoyl phorbol acetate (TPA) and cholera toxin, all the melanoblasts spontaneously differentiated to pigmented melanocytes within about 3 weeks. We now describe some factors affecting the proliferation and differentiation of diploid murine melanoblasts in the presence of serum. Murine stem cell factor/steel factor (SCF), basic fibroblast growth factor (bFGF) and murine leukaemia inhibitory factor/differentiation-inhibiting activity (LIF/DIA) all increased melanoblast numbers. SCF and LIF also slightly inhibited melanoblast differentiation, while cholera toxin and TPA promoted differentiation. Using some of these findings, and by regular replacement of keratinocyte or fibroblastoid feeder cells, we have established a clonal line of immortal murine melanoblasts, ‘melb-a’. These cells express tyrosinase-related protein-2 but not, in general, tyrosinase. They can be induced to differentiate irreversibly to functional melanocytes (also proliferative and immortal) by plating in the absence of feeder cells. Thus a new immortal melanocyte line, ‘melan-a2′, has also been produced.