Molecular and clinical aspects of melanocyte differentiation and functioning

The Rene Touraine Foundation for Dermatological Research was founded in 1991 by 12 pharmaceutical and cosmetic companies to ‘create the best possible products for the treatment and prevention of skin diseases’. As part of its mission, the foundation hosts an annual scientific symposium to promote a deeper understanding of basic dermatology and to foster collaboration and discussion between those working in clinical dermatology and the basic skin biology fields.

The Rene Touraine Foundation scientific symposia focuses on one skin cell type every year, over a 10-year cycle. This year’s meeting, held at the French Ministry of Education in Paris, France, was devoted to the biology of the melanocyte. The early session of the meeting concerned the basic biology of the melanocyte and the control of pigmentation with the later session focusing on melanocyte-associated pathologies, such as melanoma and pigmentary disorders. As an aid to those in training, the speakers were asked to spend some time reviewing the basics of their field before moving onto their own research.

The role of microphthalmia transcription factor in controlling melanocyte growth & pigmentation

In the first of two talks on the microphthalmia transcription factor (MITF), Colin Goding (Marie Curie Research Institute, Oxted, UK) gave a brief introduction on the wide variations in skin and hair pigmentation found in the human population. MITF is a key regulator of melanocyte survival during early development and is required subsequently for the regulation of nearly all of the genes associated with pigmentation. Indeed, many have suggested that MITF is the master controller of melanocyte behavior. MITF function is known to be extremely complex and it is regulated at the level of RNA by factors, such as Pax3, SOX10, CREB and Brn-2, as well as at the protein level by extracellular signal-regulated kinase 2, ribosomal S6 kinase, glycogen synthase kinase 3β and p38 mitogen-activated protein kinase. A number of post-translational modifications, such as ubiquitination, sumoylation and acetylation, also contribute to the fine tuning of MITF activity. Although these processes are highly complex, Colin Goding assured the audience that we are only a few years away from a complete understanding of the MITF ‘code’.

In the second part of his lecture, Colin Goding focused on the role of MITF in melanocyte proliferation. One of the paradoxical features of MITF is its ability to suppress cell growth through the upregulation of p21 and p27, while simultaneously being highly amplified in some melanomas in which it is apparently essential for proliferation. Through a series of elegant experiments, Goding’s group have demonstrated a role for MITF in regulating p27 protein stability and he suggested that MITF can function as a ‘proliferation rheostat’ in the melanocytic system, being required for both the proliferative and antiproliferative activity. The final part of the lecture detailed a novel role for MITF in melanoma cell invasion through the regulation of the balance of signaling between Rho Kinase (ROCK) and Diaphenous-1 (DIA-1).

The second lecture on the role of MITF in the pigmentation pathway was presented by Eirikur Steingrimsson (University of Reykjavik, Iceland), who began by reviewing the many MITF mutations associated with mouse coat color phenotypes. During the initial part of his lecture, Steingrimsson described the complex interaction between β-catenin and MITF in the initiation of gene transcription.

There is considerable evidence that the transcriptional activity of MITF is regulated by post-translational modifications, such as phosphorylation. Currently, Steingrimsson’s group are using bacterial artificial chromosome (BAC) rescue experiments to investigate the effects of various MITF mutations in vivo. Specifically, he demonstrated that the Ser73-Ala BAC rescued the MITF eye mutation and Ser409–Ala BAC rescued the coat/eye mutation. Steingrimsson was optimistic that these BAC rescue experiments will continue to yield valuable information regarding the in vivo roles of the many post-translational modification sites on MITF.

Surprise insight into human skin color from zebrafish

In his lecture, Keith Cheng (Pennsylvania State University, PA, USA) described some fascinating recent work demonstrating the role of SLC24A5 in determining human skin color. Until very recently, the genetics underlying the wide variety of human skin colors were largely unknown. Using the zebrafish model, Cheng and his team identified the gene responsible for the zebrafish golden mutation, which leads to a lightening of the normally dark stripes on the zebrafish. The gene identified with this phenotype has 69% homology with the human gene SLC24A5, which encodes a novel sodium/calcium exchanger. At present, the exact role of this novel ion exchanger in melanosome biogenesis and pigmentation is unknown.

Through a further series of studies, Cheng’s group demonstrated that people of European ethnic extraction harbor a common single nucleotide polymorphism (SNP) in SLC24A5, resulting in the characteristic lighter ‘white’ skin color. Interestingly, this SLC24A5 SNP was not found in people from any other ethnic group, such as those from Africa or Asia. Cheng suggested that the ancestral human skin color was likely to have been dark and that the lighter skin colorings are the result of acquired genetic mutations. At this stage, the identity of the mutation required for lighter skin color in Asian populations is unknown and will be the subject of further study.

Cheng concluded by discussing the genetic meaning of race and cautioned that throughout history the scientific study of skin pigmentation has been misused by both academics and politicians and has contributed to social inequality through racism. He reminded the audience that trivial genetic changes, such as the SLC24A5 SNP, can often make profound and unjustified social differences to people’s lives and that we should strive for a more egalitarian future.

Melanocyte senescence & melanoma

Cell senescence is an irreversible exit from the cell cycle after a limited number of cell divisions. The phenomena was first described in primary fibroblast cultures by Hayflick during the 1960s [1]. Cancer cells do not seem to undergo senescence and the escape from this process is thought to be a hallmark of cancer. Dorothy Bennett (St Georges Hospital Medical School, London, UK) began her lecture by describing the characteristics of senescence and its special relevance to melanoma. Some of the most compelling evidence of a role for senescence in melanoma comes from genetic studies showing that the cyclin-dependent kinase (CDK) inhibitor p16 is often lost in familial forms of melanoma. Benign melanocytic nevi produce high levels of p16 and stain for other markers of senescence, such as β-galactosidase. Therefore, it is suggested that senescence is one of the most important mechanisms of tumor suppression and that the development of melanoma from nevi may involve an escape from this process. In line with this hypothesis, Bennett presented evidence that melanomas activate telomerase, lose production of p16 and demonstrate increased p53 production. The precise role of this aberrant production of p53 in early melanoma lesions is unclear currently as it seems unlikely to be associated with apoptosis.

Molecular causes leading to defective melanosome transport in Griscelli syndrome

The final session of the afternoon described research into pigmentation defect diseases. de Saint Basile (Hopital Necker-Enfants Malades, Paris, France) described the rare autosomal recessive disorder Griscelli syndrome (GS), which is associated with a silver-gray sheen of the hair and skin depigmentation. Microscopic examination of hair from GS patients reveals the presence of large clusters of pigment in the hair shaft and abnormal melanosome distribution throughout the epidermis.

There are three distinct subtypes of GS (named GS 1–3), which are associated with distinct molecular pathologies. In the first type, GS1, hypopigmentation is associated with severe neurological defects and it occurs as a result of a myosin Va deficiency. The lack of myosin Va function prevents efficient melanomsome transport along actin fibers within the melanocyte. GS2 is also associated with an immunosuppressive disorder known as hemophagocytic syndrome and arises as a result of a deficiency in Rab27a. From a molecular perspective, Rab27a interacts closely with myosin Va to facilitate melanosome transport along the actin cytoskeleton. The final syndrome, GS3, is only associated with hypopigmentation and arises through a defect in melanophilin, a protein that links myosin Va and Rab27a. At present, there are no treatments for these diseases but a more thorough understanding of the molecular basis of these complaints should offer hope for the future.

Conclusion

The format of this meeting was unique within the dermatology field, focusing upon the behavior and pathologies of one skin cell type. From this perspective, it was interesting for those of us working in the more applied areas of melanoma research to become familiarized with the more basic areas of pigment cell biology. As an educational exercise, the meeting was a resounding success and a great opportunity for the various students and residents in attendance to keep updated with recent advances in the pigment cell biology field.