Eirikur Steingrimsson

Mutations at the mouse microphthalmia locus are associated with defects in a gene encoding a novel basic-helix-loop-helix-zipper protein

Mice with mutations at the microphthalmia (mi) locus have some or all of the following defects: loss of pigmentation, reduced eye size, failure of secondary bone resorption, reduced numbers of mast cells, and early onset of deafness. Using a transgenic insertional mutation at this locus, we have identified a gene whose expression is disrupted in transgenic animals. This gene encodes a novel member of the basic-helix-loop-helix-leucine zipper (bHLH-ZIP) protein family of transcription factors, is altered in mice carrying two independent mi alleles (mi and miws), and is expressed in the developing eye, ear, and skin, all anatomical sites affected by mi. The multiple spontaneous and induced mutations available at mi provide a unique biological resource for studying the role of a bHLH-ZIP protein in mammalian development.

Mitf and Tfe3, two members of the Mitf-Tfe family of bHLH-Zip transcription factors, have important but functionally redundant roles in osteoclast development

The Mitf-Tfe family of basic helix–loop–helix-leucine zipper (bHLH-Zip) transcription factors encodes four family members: Mitf, Tfe3, Tfeb, and Tfec. In vitro, each protein in the family can bind DNA as a homo- or heterodimer with other family members. Mutational studies in mice have shown that Mitf is essential for melanocyte and eye development, whereas Tfeb is required for placental vascularization. Here, we uncover a role for Tfe3 in osteoclast development, a role that is functionally redundant with Mitf. Although osteoclasts seem normal in Mitf or Tfe3 null mice, the combined loss of the two genes results in severe osteopetrosis. We also show that Tfec mutant mice are phenotypically normal, and that the Tfec mutation does not alter the phenotype of Mitf, Tfeb, or Tfe3 mutant mice. Surprisingly, our studies failed to identify any phenotypic overlap between the different Mitf–Tfe mutations. These results suggest that heterodimeric interactions are not essential for Mitf-Tfe function in contrast to other bHLH-Zip families like Myc/Max/Mad, where heterodimeric interactions seem to be essential.

Novel MITF targets identified using a two-step DNA microarray strategy

Malignant melanoma is a chemotherapy‐resistant cancer with high mortality. Recent advances in our understanding of the disease at the molecular level have indicated that it shares many characteristics with developmental precursors to melanocytes, the mature pigment‐producing cells of the skin and hair follicles. The development of melanocytes absolutely depends on the action of the microphthalmia‐associated transcription factor (MITF). MITF has been shown to regulate a broad variety of genes, whose functions range from pigment production to cell‐cycle regulation, migration and survival. However, the existing list of targets is not sufficient to explain the role of MITF in melanocyte development and melanoma progression. DNA microarray analysis of gene expression offers a straightforward approach to identify new target genes, but standard analytical procedures are susceptible to the generation of false positives and require additional experimental steps for validation. Here, we introduce a new strategy where two DNA microarray‐based approaches for identifying transcription factor targets are combined in a cross‐validation protocol designed to help control false‐positive generation. We use this two‐step approach to successfully re‐identify thirteen previously recorded targets of MITF‐mediated upregulation, as well as 71 novel targets. Many of these new targets have known relevance to pigmentation and melanoma biology, and further emphasize the critical role of MITF in these processes.

Mga, a dual-specificity transcription factor that interacts with Max and contains a T-domain DNA-binding motif.

The basic‐helix‐loop‐helix‐leucine zipper (bHLHZip) proteins Myc, Mad and Mnt are part of a transcription activation/repression system involved in the regulation of cell proliferation. The function of these proteins as transcription factors is mediated by heterodimerization with the small bHLHZip protein Max, which is required for their specific DNA binding to E‐box sequences. We have identified a novel Max‐interacting protein, Mga, which contains a Myc‐like bHLHZip motif, but otherwise shows no relationship with Myc or other Max‐interacting proteins. Like Myc, Mad and Mnt proteins, Mga requires heterodimerization with Max for binding to the preferred Myc–Max‐binding site CACGTG. In addition to the bHLHZip domain, Mga contains a second DNA‐binding domain: the T‐box or T‐domain. The T‐domain is a highly conserved DNA‐binding motif originally defined in Brachyury and characteristic of the Tbx family of transcription factors. Mga binds the preferred Brachyury‐binding sequence and represses transcription of reporter genes containing promoter‐proximal Brachyury‐binding sites. Surprisingly, Mga is converted to a transcription activator of both Myc–Max and Brachyury site‐containing reporters in a Max‐dependent manner. Our results suggest that Mga functions as a dual‐specificity transcription factor that regulates the expression of both Max‐network and T‐box family target genes.

The Microphthalmia-Associated Transcription Factor Mitf Interacts with β-Catenin To Determine Target Gene Expression

ABSTRACT Commitment to the melanocyte lineage is characterized by the onset of expression of the microphthalmia-associated transcription factor (Mitf). This transcription factor plays a fundamental role in melanocyte development and maintenance and seems to be crucial for the survival of malignant melanocytes. Furthermore, Mitf has been shown to be involved in cell cycle regulation and to play important functions in self-renewal and maintenance of melanocyte stem cells. Although little is known about how Mitf regulates these various processes, one possibility is that Mitf interacts with other regulators. Here we show that Mitf can interact directly with β-catenin, the key mediator of the canonical Wnt signaling pathway. The Wnt signaling pathway plays a critical role in melanocyte development and is intimately involved in triggering melanocyte stem cell proliferation. Significantly, constitutive activation of this pathway is a feature of a number of cancers including malignant melanoma. Here we show that Mitf can redirect β-catenin transcriptional activity away from canonical Wnt signaling-regulated genes toward Mitf-specific target promoters to activate transcription. Thus, by a feedback mechanism, Mitf can diversify the output of canonical Wnt signaling to enhance the repertoire of genes regulated by β-catenin. Our results reveal a novel mechanism by which Wnt signaling and β-catenin activate gene expression, with significant implications for our understanding of both melanocyte development and melanoma.

A detailed genome-wide reconstruction of mouse metabolism based on human Recon 1

BackgroundWell-curated and validated network reconstructions are extremely valuable tools in systems biology. Detailed metabolic reconstructions of mammals have recently emerged, including human reconstructions. They raise the question if the various successful applications of microbial reconstructions can be replicated in complex organisms.ResultsWe mapped the published, detailed reconstruction of human metabolism (Recon 1) to other mammals. By searching for genes homologous to Recon 1 genes within mammalian genomes, we were able to create draft metabolic reconstructions of five mammals, including the mouse. Each draft reconstruction was created in compartmentalized and non-compartmentalized version via two different approaches. Using gap-filling algorithms, we were able to produce all cellular components with three out of four versions of the mouse metabolic reconstruction. We finalized a functional model by iterative testing until it passed a predefined set of 260 validation tests. The reconstruction is the largest, most comprehensive mouse reconstruction to-date, accounting for 1,415 genes coding for 2,212 gene-associated reactions and 1,514 non-gene-associated reactions.We tested the mouse model for phenotype prediction capabilities. The majority of predicted essential genes were also essential in vivo. However, our non-tissue specific model was unable to predict gene essentiality for many of the metabolic genes shown to be essential in vivo. Our knockout simulation of the lipoprotein lipase gene correlated well with experimental results, suggesting that softer phenotypes can also be simulated.ConclusionsWe have created a high-quality mouse genome-scale metabolic reconstruction, iMM1415 (Mus Musculus, 1415 genes). We demonstrate that the mouse model can be used to perform phenotype simulations, similar to models of microbe metabolism. Since the mouse is an important experimental organism, this model should become an essential tool for studying metabolic phenotypes in mice, including outcomes from drug screening.

Melanocyte Stem Cell Maintenance and Hair Graying

Hair graying is an obvious sign of human aging, yet little was known about its causes. Two recent papers provide compelling evidence that hair graying is due to incomplete melanocyte stem cell maintenance and identify Pax3 and Mitf as key molecules that help regulate the balance between melanocyte stem cell maintenance and differentiation.

A polymorphism in IRF4 affects human pigmentation through a tyrosinase-dependent MITF/TFAP2A pathway.

Sequence polymorphisms linked to human diseases and phenotypes in genome-wide association studies often affect noncoding regions. A SNP within an intron of the gene encoding Interferon Regulatory Factor 4 (IRF4), a transcription factor with no known role in melanocyte biology, is strongly associated with sensitivity of skin to sun exposure, freckles, blue eyes, and brown hair color. Here, we demonstrate that this SNP lies within an enhancer of IRF4 transcription in melanocytes. The allele associated with this pigmentation phenotype impairs binding of the TFAP2A transcription factor that, together with the melanocyte master regulator MITF, regulates activity of the enhancer. Assays in zebrafish and mice reveal that IRF4 cooperates with MITF to activate expression of Tyrosinase (TYR), an essential enzyme in melanin synthesis. Our findings provide a clear example of a noncoding polymorphism that affects a phenotype by modulating a developmental gene regulatory network.

miR-148 Regulates Mitf in Melanoma Cells

The Microphthalmia associated transcription factor (Mitf) is an important regulator in melanocyte development and has been shown to be involved in melanoma progression. The current model for the role of Mitf in melanoma assumes that the total activity of the protein is tightly regulated in order to secure cell proliferation. Previous research has shown that regulation of Mitf is complex and involves regulation of expression, splicing, protein stability and post-translational modifications. Here we show that microRNAs (miRNAs) are also involved in regulating Mitf in melanoma cells. Sequence analysis revealed conserved binding sites for several miRNAs in the Mitf 3′UTR sequence. Furthermore, miR-148 was shown to affect Mitf mRNA expression in melanoma cells through a conserved binding site in the 3′UTR sequence of mouse and human Mitf. In addition we confirm the previously reported effects of miR-137 on Mitf. Other miRNAs, miR-27a, miR-32 and miR-124 which all have conserved binding sites in the Mitf 3′UTR sequence did not have effects on Mitf. Our data show that miR-148 and miR-137 present an additional level of regulating Mitf expression in melanocytes and melanoma cells. Loss of this regulation, either by mutations or by shortening of the 3′UTR sequence, is therefore a likely factor in melanoma formation and/or progression.

Mouse coat color mutations: From fancy mice to functional genomics

Mouse coat color mutations have a long history in biomedical research. The viable and visible phenotype of most coat color mutations has made the pigment cell, the melanocyte, an ideal system for genetic, molecular, and cellular analysis. Molecular cloning and analysis of many of the different coat color mutations have revealed the roles of a diverse range of genes, and today we know more about the pathways and proteins that regulate the development and function of pigment cells than we know about most other cell types in mammalian organisms. Coat color mutations have also provided novel insights into stem cell biology and human diseases, including melanoma. In the future, it will be important to build on this history and knowledge by taking advantage of the extensive repertoire of recently developed genome‐wide methodologies, available genomic information, and the powerful methods that have been developed for modifying the mouse genome to systematically dissect the development and function of this important cell type. The usefulness of coat color mutations has just begun to emerge. Developmental Dynamics 235:2401–2411, 2006.