Molecular cloning of cDNA encoding a novel microphthalmia-associated transcription factor isoform with a distinct amino-terminus

Inherited causes of combined vision and hearing loss: clinical features and molecular genetics

Combined vision and hearing loss, also known as dual sensory impairment, can occur in several genetic conditions, including ciliopathies such as Usher and Bardet-Biedl syndrome, mitochondrial DNA disorders and systemic diseases, such as CHARGE, Stickler, Waardenburg, Alport and Alstrom syndrome. The retinal phenotype may point to the diagnosis of such disorders. Herein, we aim to provide a comprehensive review of the molecular genetics and clinical features of the most common non-chromosomal inherited disorders to cause dual sensory impairment.

Coordinated microRNA/mRNA Expression Profiles Reveal Unique Skin Color Regulatory Mechanisms in Chinese Giant Salamander (Andrias davidianus)

The Chinese giant salamander (Andrias davidianus) has been increasingly popular in the aquaculture market in China in recent years. In the breeding process of Andrias davidianus, we found that some albino individuals were extremely rare and could not be inherited stably, which severely limits their commercialization in the aquaculture market. In this study, we performed transcriptome and small RNA (sRNA) sequencing analyses in the skin samples of wild-type (WT) and albino (AL) Andrias davidianus. In total, among 5517 differentially expressed genes (DEGs), 2911 DEGs were down-regulated in AL, including almost all the key genes involved in melanin formation. A total of 25 miRNAs were differentially expressed in AL compared to WT, of which 17 were up-regulated. Through the integrated analysis, no intersection was found between the target genes of the differentially expressed miRNAs and the key genes for melanin formation. Gene Ontology (GO) and KEGG pathway analyses on DEGs showed that these genes involved multiple processes relevant to melanin synthesis and the key signal pathway MAPK. Interestingly, the transcription factors SOX10 and PAX3 and the Wnt signaling pathway that play a key role in other species were not included, while the other two transcription factors in the SOX family, SOX21 and SOX7, were included. After analyzing the key genes for melanin formation, it was interesting to note an alternative splicing form of the MITF in WT and a critical mutation of the SLC24A5 gene in AL, which might be the main reason for the skin color change of Andrias davidianus. The results contributed to understanding the molecular mechanism of skin pigmentation in Andrias davidianus and accelerating the acquisition process of individuals with specific body colors by genetic means.

The Regulation of MiTF/TFE Transcription Factors Across Model Organisms: from Brain Physiology to Implication for Neurodegeneration

The microphthalmia/transcription factor E (MiTF/TFE) transcription factors are responsible for the regulation of various key processes for the maintenance of brain function, including autophagy-lysosomal pathway, lipid catabolism, and mitochondrial homeostasis. Among them, autophagy is one of the most relevant pathways in this frame; it is evolutionary conserved and crucial for cellular homeostasis. The dysregulation of MiTF/TFE proteins was shown to be involved in the development and progression of neurodegenerative diseases. Thus, the characterization of their function is key in the understanding of the etiology of these diseases, with the potential to develop novel therapeutics targeted to MiTF/TFE proteins and to the autophagic process. The fact that these proteins are evolutionary conserved suggests that their function and dysfunction can be investigated in model organisms with a simpler nervous system than the mammalian one. Building not only on studies in mammalian models but also in complementary model organisms, in this review we discuss (1) the mechanistic regulation of MiTF/TFE transcription factors; (2) their roles in different regions of the central nervous system, in different cell types, and their involvement in the development of neurodegenerative diseases, including lysosomal storage disorders; (3) the overlap and the compensation that occur among the different members of the family; (4) the importance of the evolutionary conservation of these protein and the process they regulate, which allows their study in different model organisms; and (5) their possible role as therapeutic targets in neurodegeneration.

Characterization of a melanocyte progenitor population in human interfollicular epidermis

It is still unknown whether the human interfollicular epidermis harbors a reservoir of melanocyte precursor cells. Here, we clearly distinguish between three distinct types of melanocytes in human interfollicular epidermis: (1) cKit⁺CD90^-, (2) cKit⁺CD90⁺, and (3) cKit^-CD90⁺. Importantly, we identify the Kit tyrosine kinase receptor (cKit) as a marker expressed specifically in mature, melanin-producing melanocytes. Thus, both cKit⁺CD90^- and cKit⁺CD90⁺ cells represent polydendritic, pigmented mature melanocytes, whereas cKit^-CD90⁺ cells display bipolar, non-dendritic morphology with reduced melanin content. Additionally, using tissue-engineered pigmented dermo-epidermal skin substitutes (melDESSs), we reveal that the cKit expression also plays an important role during melanogenesis in melDESS in vivo. Interestingly, cKit^-CD90⁺ cells lack the expression of markers such as HMB45, TYR, and TRP1 in vitro and in vivo. However, they co-express neural-crest progenitor markers and demonstrate multilineage differentiation potential in vitro. Hence, we propose that cKit^-CD90⁺ cells constitute the precursor melanocyte reservoir in human interfollicular epidermis.

A Basic Domain-Derived Tripeptide Inhibits MITF Activity by Reducing its Binding to the Promoter of Target Genes

The keratinocytes in UV-irradiated skin produce and secrete α-melanocyte-stimulating hormone. α-Melanocyte-stimulating hormone upregulates the expression of MITF in melanocytes through the cAMP‒protein kinase A‒CREB signaling pathway. Thereafter, MITF induces the expression of melanogenic genes, including the tyrosinase gene TYR and TYRP-1 and TYRP-2 genes, which leads to the synthesis and accumulation of melanin. In this study, we examined whether MITF basic region-derived tripeptides can bind to the DNA-binding domain of MITF and inhibit MITF-induced melanogenesis through the inhibition of MITF‒DNA binding. MITF-KGR, a representative MITF-derived tripeptide, suppressed the transcriptional activity of MITF by disrupting its binding to the promoter region of the target genes, which resulted in the inhibition of skin epidermis thickness and melanin synthesis in vivo and in vitro. Our results indicate that MITF-KGR exerts an inhibitory effect on melanogenesis by targeting MITF.

The underestimated role of the microphthalmia-associated transcription factor (MiTF) in normal and pathological haematopoiesis

Haematopoiesis, the process by which a restrained population of stem cells terminally differentiates into specific types of blood cells, depends on the tightly regulated temporospatial activity of several transcription factors (TFs). The deregulation of their activity or expression is a main cause of pathological haematopoiesis, leading to bone marrow failure (BMF), anaemia and leukaemia. TFs can be induced and/or activated by different stimuli, to which they respond by regulating the expression of genes and gene networks. Most TFs are highly pleiotropic; i.e., they are capable of influencing two or more apparently unrelated phenotypic traits, and the action of a single TF in a specific setting often depends on its interaction with other TFs and signalling pathway components. The microphthalmia-associated TF (MiTF) is a prototype TF in multiple situations. MiTF has been described extensively as a key regulator of melanocyte and melanoma development because it acts mainly as an oncogene. Mitf-mutated mice show a plethora of pleiotropic phenotypes, such as microphthalmia, deafness, abnormal pigmentation, retinal degeneration, reduced mast cell numbers and osteopetrosis, revealing a greater requirement for MiTF activity in cells and tissue. A growing amount of evidence has led to the delineation of key roles for MiTF in haematopoiesis and/or in cells of haematopoietic origin, including haematopoietic stem cells, mast cells, NK cells, basophiles, B cells and osteoclasts. This review summarizes several roles of MiTF in cells of the haematopoietic system and how MiTFs can impact BM development.

User guide to MiT-TFE isoforms and post-translational modifications

The microphthalmia-associated transcription factor (MITF) is at the core of melanocyte and melanoma fate specification. The related factors TFEB and TFE3 have been shown to be instrumental for transcriptional regulation of genes involved in lysosome biogenesis and autophagy, cellular processes important for mediating nutrition signals and recycling of cellular materials, in many cell types. The MITF, TFEB, TFE3, and TFEC proteins are highly related. They share many structural and functional features and are targeted by the same signaling pathways. However, the existence of several isoforms of each factor and the increasing number of residues shown to be post-translationally modified by various signaling pathways poses a difficulty in indexing amino acid residues in different isoforms across the different proteins. Here, we provide a resource manual to cross-reference amino acids and post-translational modifications in all isoforms of the MiT-TFE family in humans, mice, and zebrafish and summarize the protein accession numbers for each isoform of these factors in the different genomic databases. This will facilitate future studies on the signaling pathways that regulate different isoforms of the MiT-TFE transcription factor family.

Non-BRAF Mutant Melanoma: Molecular Features and Therapeutical Implications

Melanoma is one of the most aggressive tumors of the skin, and its incidence is growing worldwide. Historically considered a drug resistant disease, since 2011 the therapeutic landscape of melanoma has radically changed. Indeed, the improved knowledge of the immune system and its interactions with the tumor, and the ever more thorough molecular characterization of the disease, has allowed the development of immunotherapy on the one hand, and molecular target therapies on the other. The increased availability of more performing technologies like Next-Generation Sequencing (NGS), and the availability of increasingly large genetic panels, allows the identification of several potential therapeutic targets. In light of this, numerous clinical and preclinical trials are ongoing, to identify new molecular targets. Here, we review the landscape of mutated non-BRAF skin melanoma, in light of recent data deriving from Whole-Exome Sequencing (WES) or Whole-Genome Sequencing (WGS) studies on melanoma cohorts for which information on the mutation rate of each gene was available, for a total of 10 NGS studies and 992 samples, focusing on available, or in experimentation, targeted therapies beyond those targeting mutated BRAF. Namely, we describe 33 established and candidate driver genes altered with frequency greater than 1.5%, and the current status of targeted therapy for each gene. Only 1.1% of the samples showed no coding mutations, whereas 30% showed at least one mutation in the RAS genes (mostly NRAS) and 70% showed mutations outside of the RAS genes, suggesting potential new roads for targeted therapy. Ongoing clinical trials are available for 33.3% of the most frequently altered genes.

A rare case of Waardenburg syndrome with unilateral hearing loss caused by nonsense variant c.772C>T (p.Arg259*) in the MITF gene in Yakut patient from the Eastern Siberia (Sakha Republic, Russia)

Waardenburg syndrome (WS) is an orphan genetic disease with autosomal dominant pattern of inheritance characterised by varying degrees of hearing loss accompanied by skin, hair and iris pigmentation abnormalities. Four types of WS differing in phenotypic characteristics are now described. We performed a Sanger sequencing of coding regions of genes PAX3, MITF, SOX10 and SNAI2 in the patient with WS from a Yakut family living in the Sakha Republic. No changes were found in the PAX3, SOX10 and SNAI2 coding regions while a previously reported heterozygous transition c.772C>T (p.Arg259*) in exon 8 of the MITF gene was found in this patient. This patient presents rare phenotype of WS type 2: congenital unilateral hearing loss, unilateral heterochromia of irises, and absence of skin/hair depigmentation and dystopia canthorum. Audiological variability in WS type 2, caused by the c.772C>T (p.Arg259*) variant in the MITF gene, outlines the importance of molecular analysis and careful genotype-phenotype comparisons in order to optimally inform patients about the risk of hearing loss. The results of this study confirm the association of pathogenic variants in the MITF gene with WS type 2 and expanded data on the variability of audiological features of the WS.

A homozygous MITF mutation leads to familial Waardenburg syndrome type 4

Waardenburg syndrome (WS) is a genetic disorder characterized by hearing loss and pigmentary abnormalities with variable penetrance. Though heterozygous mutations in MITF are a major cause for Waardenburg syndrome type 2 (WS2), homozygous mutations in this gene and the associated phenotype have been rarely characterized. In this study, we identified a novel p.R223H mutation in MITF in a Chinese Han family with variable WS features. Both parents carried a heterozygous p.R223H mutation. They had normal hearing, and premature greying of the hair is their only pigmentary abnormality. In contrast, their two children both carried a homozygous p.R223H mutation and had classic WS features including profound hearing loss, heterochromia irides and marked pigmentary abnormalities in hair and skin. Interestingly, the two affected children also have persistent chronic constipation since the neonatal period, symptoms suggestive of Waardenburg syndrome type 4 (WS4). Our study revealed a likely association between homozygous mutations in MITF and WS4, which implies a dosage effect for the underlying pathogenesis mechanism.

MITF-M regulates melanogenesis in mouse melanocytes

BACKGROUND

Although the impact of the microphthalamia-associated transcription factor (Mitf) signaling pathway on melanocytes progression has been extensively studied, the specific molecular mechanisms behind MITF-M-enhanced melanin production in melanocytes still need to be clarified.

METHODS

In this study, we analyzed the levels of Mitf-M in skin tissues of different coat mice in order to further reveal the relationship between Mitf-M and skin pigmentation. To address the function of Mitf-M on melanogenesis, we have used an overexpression system and combined morphological and biochemical methods to investigate its localization in different coat color mice and pigmentation-related genes' expression in mouse melanocytes.

RESULTS

The qRT-PCR assay and Western blotting analysis revealed that Mitf-M mRNA and protein were synthesized in all tested mice skin samples, with the highest expression level in brown skin, a moderate expression level in grey skin and the lowest expression level in black skin. Simultaneously, immunofluorescence staining revealed that MITF-M was mainly expressed in the hair follicle matrix and inner and outer root sheath in the skin tissues with different coat colors. Furthermore, overexpression of MITF-M led to increased melanin content and variable pigmentation-related gene expression.

CONCLUSION

These results directly demonstrate that MITF-M not only influences melanogenesis, but also determines the progression of melanosomal protein in mouse melanocytes.

Identification of polymorphisms in MITF and DCT genes and their associations with plumage colors in Asian duck breeds

OBJECTIVE

The aim of this study was to investigate the effect of single nucleotide polymorphisms (SNPs) of the melanogenesis associated transcription factor (MITF) and dopachrome tautomerase (DCT) genes on plumage coloration in Asian native duck breeds. MITF encodes a protein for microphthalmia-associated transcription factor, which regulates the development and function of melanocytes for pigmentation of skin, hair, and eyes. Among the tyrosinase-related family genes, DCT is a pigment cell-specific gene that plays important roles in the melanin synthesis pathway and the expression of skin, feather, and retina color.

METHODS

Five Asian duck varieties (black Korean native, white Korean native, commercial Peking, Nageswari, and Bangladeshi Deshi white ducks) were investigated to examine the polymorphisms associated with plumage colors. Among previously identified SNPs, three synonymous SNPs and one indel of MITF and nine SNPs in exon regions of DCT were genotyped. The allele frequencies for SNPs of the black and white plumage color populations were estimated and Fisher's exact test was conducted to assess the association between the allele frequencies of these two populations.

RESULTS

Two synonymous SNPs (c.114T>G and c.147T>C) and a 14-bp indel (GCTGCAAAC AGATG) in intron 7 of MITF were significantly associated with the black- and white-colored breeds (p<0.001). One non-synonymous SNP [c.938A>G (p.His313Arg)] in DCT, was highly significantly associated (p<0.001) and a synonymous SNP (c.753A>G) was significantly associated (p<0.05) with black and white color plumage in the studied duck populations.

CONCLUSION

The results of this study provide a basis for further investigations of the associations between polymorphisms and plumage color phenotypes in Asian duck breeds.

Microphthalmia-associated transcription factor ensures the elongation of axons and dendrites in the mouse frontal cortex

Long interspersed element-1 (LINE-1) is a mammalian transposable element, and its genomic insertion could cause neurological disorders in humans. Incidentally, LINE-1 is present in intron 3 of the microphthalmia-associated transcription factor (Mitf) gene of the black-eyed white mouse (Mitf^mi-bw allele). Mice homozygous for the Mitf^mi-bw allele show the white coat color with black eye and deafness. Here, we explored the functional consequences of the LINE-1 insertion in the Mitf gene using homozygous Mitf^mi-bw mice on the C3H background (C3H-bw mice) or on the C57BL/6 background (bw mice). The open-field test showed that C3H-bw mice moved more irregularly in an unfamiliar environment during the 20-min period, compared to wild-type mice, suggesting the altered emotionality. Moreover, C3H-bw mice showed the lower serum creatinine levels, which may reflect the creatine deficiency. In fact, morphologically abnormal neurons and astrocytes were detected in the frontal cortex of bw mice. The immunohistochemical analysis of bw mouse tissues showed the lower intensity for expression of guanidinoacetate methyltransferase, a key enzyme in creatine synthesis, in neurons of the frontal cortex and in glomeruli and renal tubules. Thus, Mitf may ensure the elongation of axons and dendrites by maintaining creatine synthesis in the frontal cortex.

A de novo silencer causes elimination of MITF-M expression and profound hearing loss in pigs

Background

Genesis of novel gene regulatory modules is largely responsible for morphological and functional evolution. De novo generation of novel cis-regulatory elements (CREs) is much rarer than genomic events that alter existing CREs such as transposition, promoter switching or co-option. Only one case of de novo generation has been reported to date, in fish and without involvement of phenotype alteration. Yet, this event likely occurs in other animals and helps drive genetic/phenotypic variation.

Results

Using a porcine model of spontaneous hearing loss not previously characterized we performed gene mapping and mutation screening to determine the genetic foundation of the phenotype. We identified a mutation in the non-regulatory region of the melanocyte-specific promoter of microphthalmia-associated transcription factor (MITF) gene that generated a novel silencer. The consequent elimination of expression of the MITF-M isoform led to early degeneration of the intermediate cells of the cochlear stria vascularis and profound hearing loss, as well as depigmentation, all of which resemble the typical phenotype of Waardenburg syndrome in humans. The mutation exclusively affected MITF-M and no other isoforms. The essential function of Mitf-m in hearing development was further validated using a knock-out mouse model.

Conclusions

Elimination of the MITF-M isoform alone is sufficient to cause deafness and depigmentation. To our knowledge, this study provides the first evidence of a de novo CRE in mammals that produces a systemic functional effect.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0273-2) contains supplementary material, which is available to authorized users.

Regional Fluctuation in the Functional Consequence of LINE-1 Insertion in the Mitf Gene: The Black Spotting Phenotype Arisen from the Mitfmi-bw Mouse Lacking Melanocytes

Microphthalmia-associated transcription factor (Mitf) is a key regulator for differentiation of melanoblasts, precursors to melanocytes. The mouse homozygous for the black-eyed white (Mitf^mi-bw) allele is characterized by the white-coat color and deafness with black eyes due to the lack of melanocytes. The Mitf^mi-bw allele carries LINE-1, a retrotransposable element, which results in the Mitf deficiency. Here, we have established the black spotting mouse that was spontaneously arisen from the homozygous Mitf^mi-bw mouse lacking melanocytes. The black spotting mouse shows multiple black patches on the white coat, with age-related graying. Importantly, each black patch also contains hair follicles lacking melanocytes, whereas the white-coat area completely lacks melanocytes. RT-PCR analyses of the pigmented patches confirmed that the LINE-1 insertion is retained in the Mitf gene of the black spotting mouse, thereby excluding the possibility of the somatic reversion of the Mitf^mi-bw allele. The immunohistochemical analysis revealed that the staining intensity for beta-catenin was noticeably lower in hair follicles lacking melanocytes of the homozygous Mitf^mi-bw mouse and the black spotting mouse, compared to the control mouse. In contrast, the staining intensity for beta-catenin and cyclin D1 was higher in keratinocytes of the black spotting mouse, compared to keratinocytes of the control mouse and the Mitf^mi-bw mouse. Moreover, the keratinocyte layer appears thicker in the Mitf^mi-bw mouse, with the overexpression of Ki-67, a marker for cell proliferation. We also show that the presumptive black spots are formed by embryonic day 15.5. Thus, the black spotting mouse provides the unique model to explore the molecular basis for the survival and death of developing melanoblasts and melanocyte stem cells in the epidermis. These results indicate that follicular melanocytes are responsible for maintaining the epidermal homeostasis; namely, the present study has provided evidence for the link between melanocyte development and the epidermal microenvironment.

Transcript variants and expression profiles analysis of Mitf gene in minipigs

Object

To identify transcript variants and expression patterns of porcine Mitf.

Materials and methods

A pairwise BLAST search at NCBI database was performed to deduce the structure of porcine Mitf gene. Subsequently, 5′ RACE and fluorescent quantitative RT-PCR were used to analyze the expression pattern of porcine Mitf in different tissues.

Results

Four transcript variants of porcine Mitf, MITF-A, MITF-H, MITF-M and MITF-SUS were identified, all sharing high homology with those in humans, except Mitf-SUS.

Conclusion

The sequence of porcine Mitf appear highly homologous to human MITF. However, only 4 transcript variants of porcine Mitf were identified in these minipigs, less than the 9 transcript variants in human MITF.

MITF in melanoma: mechanisms behind its expression and activity

MITF (microphthalmia-associated transcription factor) represents a melanocytic lineage-specific transcription factor whose role is profoundly extended in malignant melanoma. Over the last few years, the function of MITF has been tightly connected to plasticity of melanoma cells. MITF participates in executing diverse melanoma phenotypes defined by distinct gene expression profiles. Mutation-dependent alterations in MITF expression and activity have been found in a relatively small subset of melanomas. MITF activity is rather modulated by its upstream activators and suppressors operating on transcriptional, post-transcriptional and post-translational levels. These regulatory mechanisms also include epigenetic and microenvironmental signals. Several transcription factors and signaling pathways involved in the regulation of MITF expression and/or activity such as the Wnt/β-catenin pathway are broadly utilized by various types of tumors, whereas others, e.g., BRAF^V600E/ERK1/2 are more specific for melanoma. Furthermore, the MITF activity can be affected by the availability of transcriptional co-partners that are often redirected by MITF from their own canonical signaling pathways. In this review, we discuss the complexity of a multilevel regulation of MITF expression and activity that underlies distinct context-related phenotypes of melanoma and might explain diverse responses of melanoma patients to currently used therapeutics.

Alternative splicing of the sheep MITF gene: novel transcripts detectable in skin

Microphthalmia-associated transcription factor (MITF) is a basic helix-loop-helix leucine zipper (bHLH-LZ) transcription factor, which regulates the differentiation and development of melanocytes and pigment cell-specific transcription of the melanogenesis enzyme genes. Though multiple splice variants of MITF have been reported in humans, mice and other vertebrate species, in merino sheep (Ovis aries), MITF gene splicing has not yet been investigated until now. To investigate the sheep MITF isoforms, the full length mRNA/cDNAs from the skin of merino sheep were cloned, sequenced and characterized. Reverse transcriptase (RT)-PCR analysis and molecular prediction revealed two basic splice variants with (+) and without (-) an 18 bp insertion viz. CGTGTATTTTCCCCACAG, in the coding region (CDS) for the amino acids 'ACIFPT'. It was further confirmed by the complete nucleotide sequencing of splice junction covering intron-6 (2463 bp), wherein an 18bp intronic sequence is retained into the CDS of MITF (+) isoform. Further, full-length cDNA libraries were enriched by the method of 5' and 3' rapid amplification of cDNA ends (RACE-PCR). A total of seven sheep MITF splice variants, with distinct N-terminus sequences such as MITF-A, B, E, H, and M, the counterparts of human and mouse MITF, were identified by 5' RACE. The other two 5' RACE products were found to be novel splice variants of MITF and represented as 'MITF truncated form (Trn)-1, 2'. These alternative splice (AS) variants were illustrated using comparative genome analysis. By means of 3' RACE three different MITF 3' UTRs (625, 1083, 3167bp) were identified and characterized. We also demonstrated that the MITF gene expression determined at transcript level is mediated via an intron-6 splicing event. Here we summarize for the first time, the expression of seven MITF splice variants with three distinct 3' UTRs in the skin of merino sheep. Our data refine the structure of the MITF gene in sheep beyond what was previously known in humans, mice, dogs and other mammals.

PAX6 regulates melanogenesis in the retinal pigmented epithelium through feed-forward regulatory interactions with MITF

During organogenesis, PAX6 is required for establishment of various progenitor subtypes within the central nervous system, eye and pancreas. PAX6 expression is maintained in a variety of cell types within each organ, although its role in each lineage and how it acquires cell-specific activity remain elusive. Herein, we aimed to determine the roles and the hierarchical organization of the PAX6-dependent gene regulatory network during the differentiation of the retinal pigmented epithelium (RPE). Somatic mutagenesis of Pax6 in the differentiating RPE revealed that PAX6 functions in a feed-forward regulatory loop with MITF during onset of melanogenesis. PAX6 both controls the expression of an RPE isoform of Mitf and synergizes with MITF to activate expression of genes involved in pigment biogenesis. This study exemplifies how one kernel gene pivotal in organ formation accomplishes a lineage-specific role during terminal differentiation of a single lineage.

It is currently poorly understood how a single developmental transcription regulator controls early specification as well as a broad range of highly specialized differentiation schemes. PAX6 is one of the most extensively investigated factors in central nervous system development, yet its role in execution of lineage-specific programs remains mostly elusive. Here, we directly investigated the involvement of PAX6 in the differentiation of one lineage, the retinal pigmented epithelium (RPE), a neuroectodermal-derived tissue that is essential for retinal development and function. We revealed that PAX6 accomplishes its role through a unique regulatory interaction with the transcription factor MITF, a master regulator of the pigmentation program. During the differentiation of the RPE, PAX6 regulates the expression of an RPE-specific isoform of Mitf and importantly, at the same time, PAX6 functions together with MITF to directly activate the expression of downstream genes required for pigment biogenesis. These findings provide comprehensive insight into the gene hierarchy that controls RPE development: from a kernel gene (a term referring to the upper-most gene in the gene regulatory network) that is broadly expressed during CNS development through a lineage-specific transcription factor that together with the kernel gene creates cis-regulatory input that contributes to transcriptionally activate a battery of terminal differentiation genes.

The E-box binding factors Max/Mnt, MITF, and USF1 act coordinately with FoxO to regulate expression of proapoptotic and cell cycle control genes by phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase 3 signaling

Phosphatidylinositol (PI) 3-kinase/Akt signaling plays a critical role in cell proliferation and survival, partly by regulation of FoxO transcription factors. Previous work using global expression profiling indicated that inhibition of PI 3-kinase in proliferating cells led to induction of genes that promote cell cycle arrest and apoptosis. The upstream regulatory regions of these genes had binding sites not only for FoxO, but also for Myc/Max transcription factors. In the present study, we have addressed the role of Myc family members and related E-box-binding proteins in the regulation of these genes. Chromatin immunoprecipitations and RNA interference indicated that transcription was repressed by Max-Mnt-Sin3a-histone deacetylase complexes in proliferating cells. Inhibition of PI 3-kinase led to a loss of Max/Mnt binding and transcriptional induction by MITF and USF1, as well as FoxO. Both MITF and USF1 were activated by glycogen synthase kinase (GSK) 3, with GSK3 phosphorylation sites on USF1 identified as the previously described activating site threonine 153 as well as serine 186. siRNA against MITF as well as against FoxO3a protected cells from apoptosis following PI 3-kinase inhibition. These results define a novel E-box-regulated network that functions coordinately with FoxO to regulate transcription of apoptotic and cell cycle regulatory genes downstream of PI 3-kinase/Akt/GSK3 signaling.

Quantitative comparison of MiTF, Melan-A, HMB-45 and Mel-5 in solar lentigines and melanoma in situ

BACKGROUND

It is often challenging to reliably assess the number of lesional melanocytes in intraepidermal melanocytic proliferations involving sun-damaged skin. Therefore, dermatopathologists routinely use immunostains to help differentiate melanocytes from surrounding keratinocytes.

METHODS

Forty-three cases of solar lentigo or melanoma in situ (of the lentigo maligna type) were retrospectively chosen (20 melanomas in situ and 23 solar lentigo). Microphthalmia transcription factor (MiTF), HMB-45, Melan-A and Mel-5 immunostains were performed with an Azure blue counterstain, and the mean melanocyte counts were calculated within a 1-mm segment of epidermis.

RESULTS

In solar lentigines, the mean melanocyte counts were 27 (MiTF), 23 (HMB-45 and Mel-5) and 41 (Melan-A), as compared to hematoxylin and eosin (H&E) (25). In melanoma in situ, the mean melanocyte counts were 112 (MiTF), 149 (Melan-A), 111 (HMB-45) and 80 (Mel-5), as compared to H&E (109).

CONCLUSIONS

These results show that Melan-A significantly overestimates the density of melanocytes within dermatoheliotic skin. Compared to other tested stains, nuclear staining MiTF allowed greater distinction of melanocytes from keratinocytes with melanized cytoplasm. These findings indicate that MiTF is a superior marker for quantification of melanocytes in the evaluation of subtle intraepidermal melanocytic proliferations and in the differential diagnosis of solar lentigo.

Effect of the mutant microphthalmia-associated transcription factor found in Tietz syndrome on the in vitro development of mast cells

Mutations in microphthalmia-associated transcription factor (MITF) lead to Waardenburg syndrome type 2 (WS2), a dominantly inherited disorder involving hearing loss and pigment disturbances caused by a lack of melanocytes. On rare occasions, mutations in MITF lead to Tietz syndrome (TS), which is characterized by a severe WS2 phenotype. The MITF gene is the human homolog of the mouse microphthalmia (mi) gene in some families. Mi/mi mice show decreased numbers and an abnormal phenotype of mast cells (MC). In contrast, the number and phenotype of MC in WS2/TS patients who also have an alteration in their MITF gene are unclear. In this study, we identified a mutation in the MITF gene, delR217, which was equivalent to that found in mi/mi mice, in a case of TS. None of the MITF isoforms with the mutation were able to transactivate the tyrosinase gene promoter. In addition, mutant MITF-M showed dominant negative activity toward wild-type MITF-M, inhibiting its transactivation of the tyrosinase gene promoter. The patient's peripheral blood CD34 cells showed no differences with respect to total cell number or their expression levels of tryptase mRNA in a serum-deprived liquid culture system for 6 weeks when compared with normal control cells. These findings suggest that MITF does not play a critical role in MC development in humans.

A novel isoform of microphthalmia-associated transcription factor inhibits IL-8 gene expression in human cervical stromal cells

Cervical ripening during pregnancy is a profound change in cervix structure and function characterized by increases in the proinflammatory cytokine IL-8 and dissolution of the cervical extracellular matrix. Relatively little is known about the molecular mechanisms that underlie these events. Here, we report identification of a novel isoform of micropthalmia-associated transcription factor in human cervical stromal cells (MiTF-CX) that is down-regulated 12-fold during cervical ripening and that represses expression of IL-8. Ectopic expression of MiTF-CX in human cervical stromal cells resulted in substantial suppression of endogenous IL-8 mRNA and protein expression, whereas expression of dominant negative MiTF-CX mutants with impaired DNA binding resulted in dramatic increases in IL-8 production. Gel shift, reporter gene, and chromatin immunoprecipitation assays revealed one strong binding site (E-box (-397) CACATG(-391)) in the human IL-8 promoter that was crucial for mediating transcriptional repression by MiTF-CX. Moreover, we show that MiTF-CX expression in the cervix was itself positively autoregulated via two E-box motifs within a 2.1-kb promoter fragment. We therefore propose that maintenance of cervical competency during pregnancy is an active process maintained through suppression of IL-8 by the transcription factor MiTF-CX. During cervical ripening, loss of MiTF-CX would result in significant up-regulation of IL-8 mRNA and protein synthesis, thereby leading to recruitment and activation of leukocytes within the cervix and dissolution of the extracellular matrix.

Subcellular localization of Mitf in monocytic cells

Microphthalmia-associated transcription factor (Mitf) is a transcription factor that plays an important role in regulating the development of several cell lineages. The subcellular localization of Mitf is dynamic and is associated with its transcription activity. In this study, we examined factors that affect its subcellular localization in cells derived from the monocytic lineage since Mitf is present abundantly in these cells. We identified a domain encoded by Mitf exon 1B1b to be important for Mitf to commute between the cytoplasm and the nucleus. Deletion of this domain disrupts the shuttling of Mitf to the cytoplasm and results in its retention in the nucleus. M-CSF and RANKL both induce nuclear translocation of Mitf. We showed that Mitf nuclear transport is greatly influenced by ratio of M-CSF/Mitf protein expression. In addition, cell attachment to a solid surface also is needed for the nuclear transport of Mitf.

Melanoma: Stem cells, sun exposure and hallmarks for carcinogenesis, molecular concepts and future clinical implications

Background:

The classification and prognostic assessment of melanoma is currently based on morphologic and histopathologic biomarkers. Availability of an increasing number of molecular biomarkers provides the potential for redefining diagnostic and prognostic categories and utilizing pharmacogenomics for the treatment of patients. The aim of the present review is to provide a basis that will allow the construction–or reconstruction–of future melanoma research.

Methods:

We critically review the common medical databases (PubMed, EMBASE, Scopus and Cochrane CENTRAL) for studies reporting on molecular biomarkers for melanoma. Results are discussed along the hallmarks proposed for malignant transformation by Hanahan and Weinberg. We further discuss the genetic basis of melanoma with regard to the possible stem cell origin of melanoma cells and the role of sunlight in melanoma carcinogenesis.

Results:

Melanocyte precursors undergo several genome changes –UV-induced or not– which could be either mutations or epigenetic. These changes provide stem cells with abilities to self-invoke growth signals, to suppress antigrowth signals, to avoid apoptosis, to replicate without limit, to invade, proliferate and sustain angiogenesis. Melanocyte stem cells are able to progressively collect these changes in their genome. These new potential functions, drive melanocyte precursors to the epidermis were they proliferate and might cause benign nevi. In the epidermis, they are still capable of acquiring new traits via changes to their genome. With time, such changes could add up to transform a melanocyte precursor to a malignant melanoma stem cell.

Conclusions:

Melanoma cannot be considered a “black box” for researchers anymore. Current trends in the diagnosis and prognosis of melanoma are to individualize treatment based on molecular biomarkers. Pharmacogenomics constitute a promising field with regard to melanoma patients' treatment. Finally, development of novel monoclonal antibodies is expected to complement melanoma patient care while a number of investigational vaccines could find their way into everyday oncology practice.

Mitf induction by RANKL is critical for osteoclastogenesis

Microphthalmia-associated transcription factor (Mitf) regulates the development and function of several cell lineages, including osteoclasts. In this report, we identified a novel mechanism by which RANKL regulates osteoclastogenesis via induction of Mitf isoform E (Mitf-E). Both Mitf-A and Mitf-E are abundantly present in osteoclasts. Unlike Mitf-A, which is ubiquitously expressed and is present in similar amounts in macrophages and osteoclasts, Mitf-E is almost nondetectable in macrophages, but its expression is significantly up-regulated during osteoclastogenesis. In addition to their different expression profiles, the two isoforms are drastically different in their abilities to support osteoclastogenesis, despite sharing all known functional domains. Unlike Mitf-A, small amounts of Mitf-E are present in nuclear lysates unless chromatin is digested/sheared during the extraction. Based on these data, we propose a model in which Mitf-E is induced during osteoclastogenesis and is closely associated with chromatin to facilitate its interaction with target promoters; therefore, Mitf-E has a stronger osteoclastogenic activity. Mitf-A is a weaker osteoclastogenic factor, but activated Mitf-A alone is not sufficient to fully support osteoclastogenesis. Therefore, this receptor activator for nuclear factor-kappaB ligand (RANKL)-induced Mitf phenomenon seems to play an important role during osteoclastogenesis. Although the current theory indicates that Mitf and its binding partner Tfe3 are completely redundant in osteoclasts, using RNA interference, we demonstrated that Mitf has a distinct role from Tfe3. This study provides the first evidence that RANKL-induced Mitf is critical for osteoclastogenesis and Mitf is not completely redundant with Tfe3.

Mitf-Mdel, a novel melanocyte/melanoma-specific isoform of microphthalmia-associated transcription factor-M, as a candidate biomarker for melanoma

BACKGROUND

Melanoma incidence is on the rise and advanced melanoma carries an extremely poor prognosis. Treatment options, including chemotherapy and immunotherapy, are limited and offer low response rates and transient efficacy. Thus, identification of new melanocyte/melanoma antigens that serve as potential novel candidate biomarkers in melanoma is an important area for investigation.

METHODS

Full length MITF-M and its splice variant cDNA were cloned from human melanoma cell line 624 mel by reverse transcription polymerase chain reaction (RT-PCR). Expression was investigated using regular and quantitative RT-PCR in three normal melanocytes (NHEM), 31 melanoma cell lines, 21 frozen melanoma tissue samples, 18 blood samples (peripheral blood mononuclear cell; PBMC) from healthy donors and 12 non-melanoma cancer cell lines, including three breast, five glioma, one sarcoma, two kidney and one ovarian cancer cell lines.

RESULTS

A novel splice variant of MITF-M, which we named MITF-Mdel, was identified. The predicted MITF-Mdel protein contains two in frame deletions, 56- and 6- amino acid deletions in exon 2 (from V32 to E87) and exon 6 (from A187 to T192), respectively. MITF-Mdel was widely expressed in melanocytes, melanoma cell lines and tissues, but almost undetectable in non-melanoma cell lines or PBMC from healthy donors. Both isoforms were expressed significantly higher in melanoma tissues than in cell lines. Two of 31 melanoma cell lines expressed only one isoform or the other.

CONCLUSION

MITF-Mdel, a novel melanocyte/melanoma-specific isoform of MITF-M, may serve as a potential candidate biomarker for diagnostic and follow-up purposes in melanoma.

Main roads to melanoma

The characterization of the molecular mechanisms involved in development and progression of melanoma could be helpful to identify the molecular profiles underlying aggressiveness, clinical behavior, and response to therapy as well as to better classify the subsets of melanoma patients with different prognosis and/or clinical outcome. Actually, some aspects regarding the main molecular changes responsible for the onset as well as the progression of melanoma toward a more aggressive phenotype have been described. Genes and molecules which control either cell proliferation, apoptosis, or cell senescence have been implicated. Here we provided an overview of the main molecular changes underlying the pathogenesis of melanoma. All evidence clearly indicates the existence of a complex molecular machinery that provides checks and balances in normal melanocytes. Progression from normal melanocytes to malignant metastatic cells in melanoma patients is the result of a combination of down- or up-regulation of various effectors acting on different molecular pathways.

Serological reagents for the immunohistochemical analysis of melanoma metastases in sentinel lymph nodes

For the immunohistochemical analysis of melanoma, various serological reagents are available. Melanocyte differentiation markers are reactive with cells and tumors of melanocytic lineage. HMB45 to gp100 has been the most commonly used melanocyte differentiation marker. Recently it was complemented by reagents such as antibodies to Melan-A/MART-1 and tyrosinase. Other reagents, whose reactivity is not strictly confined to melanocyte differentiation antigens, are also commonly used. Among them, the most prominent is S100. Other reagents are D5 to MITF or PNL-2. The properties of these reagents are presented, and their usefulness as markers in the setting of metastatic melanoma in sentinel lymph nodes is discussed.

Gene expression is altered in piglet small intestine by weaning and dietary glutamine supplementation

Dietary supplementation of glutamine prevents intestinal dysfunction and atrophy in weanling piglets, but the underlying mechanism(s) are largely unknown. This study was conducted to test the hypothesis that weaning or glutamine may modulate expression of genes that are crucial for intestinal metabolism and function. In Expt. 1, we obtained small intestine from 28-d-old pigs weaned at 21 d of age and from age-matched suckling piglets. In Expt. 2, piglets were weaned at 21 d of age and then had free access to diets supplemented with 1% L-glutamine (wt:wt) or isonitrogenous L-alanine (control). At d 28, we collected small intestine for biochemical and morphological measurements and microarray analysis of gene expression using the Operon Porcine Genome Oligo set. Early weaning resulted in increased (52-346%) expression of genes related to oxidative stress and immune activation but decreased (35-77%) expression of genes related to macronutrient metabolism and cell proliferation in the gut. Dietary glutamine supplementation increased intestinal expression (120-124%) of genes that are necessary for cell growth and removal of oxidants, while reducing (34-75%) expression of genes that promote oxidative stress and immune activation. Functionally, the glutamine treatment enhanced intestinal oxidative-defense capacity (indicated by a 29% increase in glutathione concentration), prevented jejunal atrophy, and promoted small intestine growth (+12%) and body weight gain (+19%) in weaned piglets. These findings reveal coordinate alterations of gene expression in response to weaning and aid in providing molecular mechanisms for the beneficial effect of dietary glutamine supplementation to improve nutrition status in young mammals.

MITF-CM, a newly identified isoform of microphthalmia-associated transcription factor, is expressed in cultured mast cells

The microphthalmia-associated transcription factor (MITF) gene encodes a basic helix-loop-helix and leucin zipper protein. In this study, we identified a novel MITF isoform, MITF-CM, which possesses a unique amino terminus. Exon 1CM is located 84 kb upstream of the exon encoding the B1b domain. MITF-CM was expressed in the human mast cell line HMC-1, the human basophilic cell line KU812, and CB-derived mast cells cultured for 10 weeks as well as bone marrow mononuclear cells. Transient transfection of MITF-CM cDNA in COS-7 cells resulted in the expression of a 64-kDa protein, detected by Western blotting, and nuclear localization of the protein, detected by immunostaining. The transient cotransfection of a luciferase construct under the control of the tyrosinase promoter and MITF-CM cDNA increased luciferase activity threefold. In contrast, none of the MITF isoforms transactivated both the tryptase and chymase gene promoters, indicating differences in the gene transactivation system between humans and mice.

Evolutionary sequence comparison of the Mitf gene reveals novel conserved domains

The microphthalmia-associated transcription factor (MITF) is a member of the MYC family of basic helix-loop-helix leucine zipper transcription factors. The corresponding gene was initially discovered in the mouse based on mutations which affect the development of several different cell types, including melanocytes and retinal pigment epithelium cells. Subsequently, it was shown to be associated with deafness and hypo-pigmentation disorders in humans. More recently, the gene has been shown to be critical in melanoma formation and to play a role in melanocyte stem cell maintenance. Thus, the mouse Mitf gene represents an important model system for the study of human disease as well as an interesting model for the study of transcription factor function in the organism. Here we use the evolutionary relationship of Mitf genes from numerous distantly related species, including vertebrates and invertebrates, to identify novel conserved domains in the Mitf protein and regions of possible functional importance in the 3' untranslated region. We also characterize the nine different 5' exons of the Mitf gene and identify a new 5' exon in the Drosophila Mitf gene. Our analysis sheds new light on the conservation of the Mitf gene and protein and opens the door for further functional analysis.

Neuroendocrine functions of melanocytes: beyond the skin-deep melanin maker

The skin is armored with "dead cells", the stratum corneum, and is continuously exposed to external stressful environments, such as atmospheric oxygen, solar radiations, and thermal and chemical insults. Melanocytes of neural crest origin are located in the skin, eye, inner ear, and leptomeninges. Melanin pigment in the skin is produced by melanocytes under the influence of various endogenous factors, derived from neighboring keratinocytes and underlying fibroblasts. The differentiation and functions of melanocytes are regulated at multiple processes, including transcription, RNA editing, melanin synthesis, and the transport of melanosomes to keratinocytes. Impairment at each step causes the pigmentary disorders in humans, with the historical example of oculocutaneous albinism. Moreover, heterozygous mutations in the gene coding for microphthalmia-associated transcription factor, a key regulator for melanocyte development, are associated with Waardenburg syndrome type 2, an auditory-pigmentary disorder. Sun tanning, melasma, aging spots (lentigo senilis), hair graying, and melanoma are well-known melanocyte-related pathologies. Melanocytes therefore have attracted much attention of many ladies, makeup artists and molecular biologists. More recently, we have shown that lipocalin-type prostaglandin D synthase (L-PGDS) is expressed in melanocytes but not in other skin cell types. L-PGDS generates prostaglandin D2 and also functions as an inter-cellular carrier protein for lipophilic ligands, such as bilirubin and thyroid hormones. Thus, melanocytes may exert hitherto unknown functions through L-PGDS and prostaglandin D2. Here we update the neuroendocrine functions of melanocytes and discuss the possible involvement of melanocytes in the control of the central chemosensor that generates respiratory rhythm.

Therapeutic RNA interference of malignant melanoma by electrotransfer of small interfering RNA targeting Mitf

Microphthalmia-associated transcription factor (Mitf) is critically involved in melanin synthesis as well as differentiation of cells of the melanocytic lineage. Some earlier studies suggested that Mitf is also essential in the survival of melanoma cells, but this notion remains controversial. We synthesized short interfering RNA (siRNA) duplexes corresponding to the mitf sequence and transfected them into B16 melanoma. Lipid-mediated transfection in vitro of Mitf-specific siRNA resulted in specific downregulation of Mitf and of the tyrosinase that is a transcriptional target of Mitf. This treatment also remarkably reduced the viability of melanoma cells by inducing apoptosis. To examine the potential feasibility of RNAi therapy against melanoma, B16 cells were subcutaneously injected into syngenic mice and siRNA was transfected into the pre-established tumor by means of electroporation. The Mitf-specific siRNA drastically reduced outgrowth of subcutaneous melanoma, while nonspecific siRNA failed to affect tumor progression. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling-based analysis of tumor specimens demonstrated that the tumor cells transfected with Mitf-siRNA effectively underwent apoptosis in vivo. The present results indicate that Mitf plays important roles in melanoma survival. Intratumor electrotransfer of Mitf-specific siRNA may provide a powerful strategy for therapeutic intervention of malignant melanoma.

Functional Difference of the SOX10 Mutant Proteins Responsible for the Phenotypic Variability in Auditory-Pigmentary Disorders

Waardenburg syndrome (WS) is an inherited disorder, characterized by auditory-pigmentary abnormalities. SOX10 transcription factor and endothelin receptor type B (EDNRB) are responsible for WS type 4 (WS4), which also exhibits megacolon, while microphthalmia-associated transcription factor (MITF) is responsible for WS2, which is not associated with megacolon. Here, we investigated the functions of SOX10 mutant proteins using the target promoters, EDNRB and MITF. The SOX10 mutations chosen were E189X, Q377X, and 482ins6, which are associated with WS4, and S135T that is associated with Yemenite deaf-blind hypopigmentation syndrome (YDBS), which does not manifest megacolon. These SOX10 mutant proteins showed impaired transactivation activity on the MITF promoter. In contrast, E189X and Q377X proteins, each of which lacks its C-terminal portion, activated the EDNRB promoter, whereas no activation was detected with the SOX10 proteins mutated at the DNA-binding domain, 482ins6 and S135T. However, unlike 482ins6 protein, S135T protein synergistically activated EDNRB promoter with a transcription factor Sp1, indicating that Sp1 could compensate the impaired function of a SOX10 mutant protein. We suggest that the variability in transactivation ability of SOX10 mutant proteins may account for the different phenotypes between WS4 and YDBS and that Sp1 is a potential modifier gene of WS4.

MITF mediates cAMP-induced protein kinase C-beta expression in human melanocytes

The cAMP-dependent pathway up-regulates MITF (microphthalmia-associated transcription factor), important for key melanogenic proteins such as tyrosinase, TRP-1 (tyrosinase-related protein 1) and TRP-2. We asked whether MITF is also a key transcription factor for PKC-beta (protein kinase C-beta), required to phosphorylate otherwise inactive tyrosinase. When paired cultures of human melanocytes were treated with isobutylmethylxanthine, known to increase intracellular cAMP, both protein and mRNA levels of PKC-beta were induced by 24 h. To determine whether MITF modulates PKC-beta expression, paired cultures of human melanocytes were transfected with dn-MITF (dominant-negative MITF) or empty control vector. By immunoblotting, PKC-beta protein was reduced by 63+/-3.7% within 48 h. Co-transfection of an expression vector for MITF-M, the MITF isoform specific for pigment cells, or empty control vector with a full-length PKC-beta promoter-CAT (chloramphenicol acetyltransferase) reporter construct (PKC-beta/CAT) into Cos-7 cells showed >60-fold increase in CAT activity. Melanocytes abundantly also expressed MITF-A, as well as the MITF-B and MITF-H isoforms. However, in contrast with MITF-M, MITF-A failed to transactivate co-expressed PKC-beta/CAT or CAT constructs under the control of a full-length tyrosinase promoter. Together, these results demonstrate that MITF, specifically MITF-M, is a key transcription factor for PKC-beta, linking the PKC- and cAMP-dependent pathways in regulation of melanogenesis.

Regulation of melanoblast and retinal pigment epithelium development by Xenopus laevis Mitf

Mitf is a central regulator of pigment cell development that is essential for the normal development of the melanocyte and retinal pigment epithelium (RPE) lineages. To understand better the role of Mitf, we have used the Xenopus laevis experimental system to allow a rapid examination of the role of Mitf in vivo. Here, we report the function of XlMitfalpha-M on melanophore development and melanization compared with that of Slug that is expressed in neural crest cells. Overexpression of XlMitfalpha-M led to an increase in melanophores that was partly contributed by an increase in Slug-positive cells, indicating that XlMitfalpha-M is a key regulator of melanocyte/melanophore development and melanization. Moreover, overexpression of a dominant-negative form of XlMitfalpha led to a decrease in the number of melanophores and induced abnormal melanoblast migration. We also observed an induction of ectopic RPE and extended RPE by overexpression of XlMitfalpha-M and possible interactions between XlMitfalpha and several eye-related genes essential for normal eye development.

Lipocalin-type prostaglandin D synthase as a melanocyte marker regulated by MITF

Microphthalmia-associated transcription factor (MITF) is responsible for differentiation of melanocytes. A recessive MITF mutant, black-eyed white Mitf(mi-bw) mouse, is characterized by white coat color and deafness, due to the lack of melanocytes in the skin and inner ears. By cDNA microarray analysis, we have identified lipocalin-type prostaglandin D synthase (L-PGDS), whose mRNA is undetectable in the homozygous Mitf(mi-bw) skin. Immunohistochemical analysis of wild-type mice identified the specific expression of L-PGDS in follicular melanocytes. L-PGDS mRNA is expressed in B16 mouse melanoma cells, but undetectable in human melanoma cell lines. RNA interference analysis against MITF suggests that L-PGDS expression is dependent on MITF in B16 melanoma cells. Furthermore, we have provided evidence that MITF is involved in the melanocyte lineage-specific transcription of the mouse L-PGDS gene. Thus, L-PGDS represents a newly identified melanocyte marker. MITF may modulate the production of prostaglandin D(2) by activating the L-PGDS gene in melanocytes.

Genomic analysis of the Microphthalmia locus and identification of the MITF-J/Mitf-J isoform

The deafness-pigmentary disorder Waardenburg Syndrome Type 2 is caused by mutations in the human Microphthalmia-associated transcription factor (MITF) gene. Multiple related deafness-pigmentary disorders result from mutations in genes that regulate MITF expression or its activity. Similarly in mouse, homozygous mutations in the Mitf gene disrupt the development of melanocytes as well as retinal pigment epithelial (RPE) cells, osteoclasts, mast cells, and NK cells. Because abnormalities in Mitf/MITF function are associated with numerous inherited disorders of mouse and man, a detailed understanding of its gene structure is important for both diagnostic and structure/function analyses. While at least eight distinct isoforms of MITF/Mitf have been identified to date, each differing in their promoter and initial exon usage, the positions of these exons and their order within the locus have yet to be fully defined. In this study, we provide a detailed description of the MITF/Mitf locus, identify corresponding human and mouse isoforms, and utilize an informatics-based approach to identify a novel ninth MITF/Mitf isoform, MITF-J/Mitf-J, which we show is expressed in multiple cell types. The MITF/Mitf locus is over 200 kb in length, with strong but imperfect exon conservation between human and mouse. MITF/Mitf tissue expression data are presented from multiple datasets, including EST expression patterns and isoform-specific RT-PCR. The majority of isoforms were found to be broadly expressed, with the M- and Mc-isoforms being tissue-restricted to melanocytes and mast cells, respectively. Consequently, a detailed characterization of this complex locus may help to identify additional unknown deafness-pigmentary syndrome mutations in human kindred and permit a better understanding of tissue-regulated expression that likely underlies divergent biological functions of this factor across multiple cell types.

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.

Isolation and developmental expression of Mitf in Xenopus laevis

Mitf (gene for microphthalmia-associated transcription factor) encodes a transcription factor of the basic/helix-loop-helix/leucine-zipper family and is a key regulator during the development of two different types of melanin-producing cell lineages, namely neural crest-derived melanocytes/melanophores, and the retinal pigment epithelium (RPE) differentiated from the outer layer of the eye cup. Mitf-deficient mice show a lack of melanocytes and small eyes caused by abnormal RPE development. An interesting feature of Mitf is the existence of multiple isoforms with different amino termini and their functions in the development of these melanin-producing pigment cells. In this study, we isolated two Mitf homologues (XlMitfalpha and XlMitfbeta) and their isoforms from Xenopus laevis. Alignment analysis of the amino acid sequences of the N-termini suggests that these isoforms are homologues of mouse Mitf-M (expressed specifically in the melanocyte lineage) and Mitf-A (strongly expressed in the RPE, although this expression is ubiquitous). In Xenopus, XlMitfalpha is strongly expressed in the melanophore lineage (especially in premigratory melanoblasts) and the presumptive RPE and the epiphysis, in which melanin-producing cells differentiate in some vertebrates. Conservation of the Mitf isoforms expected to possess specific functions in the development of melanin-producing cells and of the expressions in such cell types in Xenopus suggest that XlMitf plays a central role in the development of melanin-producing cell lineages, and that, as in mice and humans, most of the signaling molecules or transcription factors implicated genetically in the development of melanin-producing cell lineages affect either Mitf expression or its function (Goding [2000] Genes Dev. 14:1712-1728).

The evolution of genes for pigmentation in African cichlid fishes

The cichlid fishes in the East African Great Lakes exhibit a great diversity of color patterns, presumably as adaptations to species-specific habitats and/or due to the action of sexual selection on color for species discrimination or female mate choice. To elucidate the mechanisms underlying such pigment pattern diversity, we cloned the cichlid homologs of tyrosinase, endothelin receptor b1, mitf, and Aim1 that previously had been cloned and characterized from pigmentation mutants of zebrafish and medaka fish. Gene sequence analysis among five cichlid species from the Great Lakes shows that the evolutionary rate of amino acid replacement in mitf is the highest of these four genes. We then compared the mitf amino acid replacement rates between species from the lacustrine and tilapiine/steatocranus lineages, and between Lake Malawi- and Victoria-haplochromine cichlids and Lake Tanganyika Lamprologini. We show that the evolutionary rate within the lacustrine lineage is twice that of the tilapiine/steatocranus lineage, but that rates for the Malawi-Victoria haplochromine and Lamprologini lineages are almost the same. These results suggest that the accelerated evolution of mitf might have occurred concomitantly with pigment pattern diversification in Great Lakes species, but not necessarily correlated with species under intense sexual selection on male mating color via female mate choice. Finally, we characterized a novel alternatively spliced variant of cichlid mitf that is similar to a mammalian mitf splice variant generated using alternate splice sites. We suggest that this new variant in cichlids, like that in mammals, encodes an MITF transcriptional factor having higher relative DNA binding affinity. These data provide a novel example of functional convergence in which a particular splice variant is independently generated via alternative splicing of a specific gene in different lineages.

Cloning and functional analysis of ascidian Mitf in vivo: insights into the origin of vertebrate pigment cells

The microphthalmia-associated transcription factor (Mitf) is a basic-helix-loop-helix-leucine zipper (bHLH-ZIP) transcription factor essential for the development and function of all melanin-producing pigment cells in vertebrates. To elucidate the evolutionary history of Mitf and the antiquity of its association with pigment cells, we have isolated and characterized HrMitf, a sole member of the Mitf-TFE bHLH-ZIP subfamily in the ascidian Halocynthia roretzi. Maternal HrMitf mRNA is detected in the fertilized egg and in the animal hemisphere from 4-cell stage through the gastrula stage. From the neurula through the early tailbud stage, HrMitf is preferentially expressed in the pigment-lineage cells that express the lineage-specific melanogenesis genes tyrosinase (HrTyr) and Tyrp. Overexpression of HrMitf induced ectopic expression of HrTyr enzyme activity in mesenchymal cells where the same enzyme activity was induced by overexpression of HrPax3/7, suggesting that a part(s) of the Pax3-Mitf-tyrosinase gene regulatory cascade seen in vertebrate melanocytes is operative during ascidian embryogenesis. We also show HrMitf and mouse Mitf-A, a Mitf isoform abundantly expressed in pigmented epithelial cells, share similar functional characteristics. These results suggest antiquity of the association of the Mitf-TFE subfamily with pigment cells and may support the idea that acquisition of multiple promoters (isoforms) by an ancestral Mitf gene has allowed the evolution of multiple pigment cell types.

Regulation of the MiTF/TFE bHLH-LZ transcription factors through restricted spatial expression and alternative splicing of functional domains

The MiTF/TFE (MiT) family of basic helix-loop-helix leucine zipper transcription factors is composed of four closely related members, MiTF, TFE3, TFEB and TFEC, which can bind target DNA both as homo- or heterodimers. Using real-time RT-PCR, we have analyzed the relative expression levels of the four members in a broad range of human tissues, and found that their ratio of expression is tissue-dependent. We found that, similar to the MiTF gene, the genes for TFEB and TFEC contain multiple alternative first exons with restricted and differential tissue distributions. Seven alternative 5' exons were identified in the TFEB gene, of which three displayed specific expression in placenta and brain, respectively. A novel TFEC transcript (TFEC-C) encodes an N-terminally truncated TFEC isoform lacking the acidic activation domain (AAD), and is exclusively expressed in kidney and small intestine. Furthermore, we observed that a considerable proportion of the TFEC transcripts splice out protein-coding exons, resulting in transcription factor isoforms lacking one or more functional domains, primarily the basic region and/or the AAD. These isoforms were always co-expressed with the intact transcription factors and may act as negative regulators of MiTF/TFE proteins. Our data reveal that multiple levels of regulation exist for the MiTF/TFE family of transcription factors, which indicates how these transcription factors may participate in various cellular processes in different tissues.

Transcriptional regulation of the melanoma prognostic marker melastatin (TRPM1) by MITF in melanocytes and melanoma

Determining the metastatic potential of intermediate thickness lesions remains a major challenge in the management of melanoma. Clinical studies have demonstrated that expression of melastatin/TRPM1 strongly predicts nonmetastatic propensity and correlates with improved outcome, leading to a national cooperative prospective study, which is ongoing currently. Similarly, the melanocytic markers MLANA/MART1 and MITF also have been shown to lose relative expression during melanoma progression. Recent studies have revealed that MITF, an essential transcription factor for melanocyte development, directly regulates expression of MLANA. This prompted examination of whether MITF also might transcriptionally regulate TRPM1 expression. The TRPM1 promoter contains multiple MITF consensus binding elements that were seen by chromatin immunoprecipitation to be occupied by endogenous MITF within melanoma cells. Endogenous TRPM1 expression responded strongly to MITF up- or down-regulation, as did TRPM1 promoter-driven reporters. In addition, MITF and TRPM1 mRNA levels were correlated tightly across a series of human melanoma cell lines. Mice homozygously mutated in MITF showed a dramatic decrease in TRPM1 expression. Finally, the slope of TRPM1 induction by MITF was particularly steep compared with other MITF target genes, suggesting it is a sensitive indicator of MITF expression and correspondingly of melanocytic differentiation. These studies identify MITF as a major transcriptional regulator of TRPM1 and suggest that its prognostic value may be linked to MITF-mediated regulation of cellular differentiation.