A DNA insertional mutation results in microphthalmia in transgenic mice

MiT Family Transcriptional Factors in Immune Cell Functions

The microphthalmia-associated transcription factor family (MiT family) proteins are evolutionarily conserved transcription factors that perform many essential biological functions. In mammals, the MiT family consists of MITF (microphthalmia-associated transcription factor or melanocyte-inducing transcription factor), TFEB (transcription factor EB), TFE3 (transcription factor E3), and TFEC (transcription factor EC). These transcriptional factors belong to the basic helix-loop-helix-leucine zipper (bHLH-LZ) transcription factor family and bind the E-box DNA motifs in the promoter regions of target genes to enhance transcription. The best studied functions of MiT proteins include lysosome biogenesis and autophagy induction. In addition, they modulate cellular metabolism, mitochondria dynamics, and various stress responses. The control of nuclear localization via phosphorylation and dephosphorylation serves as the primary regulatory mechanism for MiT family proteins, and several kinases and phosphatases have been identified to directly determine the transcriptional activities of MiT proteins. In different immune cell types, each MiT family member is shown to play distinct or redundant roles and we expect that there is far more to learn about their functions and regulatory mechanisms in host defense and inflammatory responses.

Animal and cellular models of microphthalmia

Microphthalmia is a rare developmental eye disorder affecting 1 in 7000 births. It is defined as a small (axial length ⩾2 standard deviations below the age-adjusted mean) underdeveloped eye, caused by disruption of ocular development through genetic or environmental factors in the first trimester of pregnancy. Clinical phenotypic heterogeneity exists amongst patients with varying levels of severity, and associated ocular and systemic features. Up to 11% of blind children are reported to have microphthalmia, yet currently no treatments are available. By identifying the aetiology of microphthalmia and understanding how the mechanisms of eye development are disrupted, we can gain a better understanding of the pathogenesis. Animal models, mainly mouse, zebrafish and Xenopus, have provided extensive information on the genetic regulation of oculogenesis, and how perturbation of these pathways leads to microphthalmia. However, differences exist between species, hence cellular models, such as patient-derived induced pluripotent stem cell (iPSC) optic vesicles, are now being used to provide greater insights into the human disease process. Progress in 3D cellular modelling techniques has enhanced the ability of researchers to study interactions of different cell types during eye development. Through improved molecular knowledge of microphthalmia, preventative or postnatal therapies may be developed, together with establishing genotype-phenotype correlations in order to provide patients with the appropriate prognosis, multidisciplinary care and informed genetic counselling. This review summarises some key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.

Plain language summary

Animal and Cellular Models of the Eye Disorder, Microphthalmia (Small Eye) Microphthalmia, meaning a small, underdeveloped eye, is a rare disorder that children are born with. Genetic changes or variations in the environment during the first 3 months of pregnancy can disrupt early development of the eye, resulting in microphthalmia. Up to 11% of blind children have microphthalmia, yet currently no treatments are available. By understanding the genes necessary for eye development, we can determine how disruption by genetic changes or environmental factors can cause this condition. This helps us understand why microphthalmia occurs, and ensure patients are provided with the appropriate clinical care and genetic counselling advice. Additionally, by understanding the causes of microphthalmia, researchers can develop treatments to prevent or reduce the severity of this condition. Animal models, particularly mice, zebrafish and frogs, which can also develop small eyes due to the same genetic/environmental changes, have helped us understand the genes which are important for eye development and can cause birth eye defects when disrupted. Studying a patient's own cells grown in the laboratory can further help researchers understand how changes in genes affect their function. Both animal and cellular models can be used to develop and test new drugs, which could provide treatment options for patients living with microphthalmia. This review summarises the key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.

From Tank to Treatment: Modeling Melanoma in Zebrafish

Melanoma is the deadliest form of skin cancer and one of few cancers with a growing incidence. A thorough understanding of its pathogenesis is fundamental to developing new strategies to combat mortality and morbidity. Zebrafish—due in large part to their tractable genetics, conserved pathways, and optical properties—have emerged as an excellent system to model melanoma. Zebrafish have been used to study melanoma from a single tumor initiating cell, through metastasis, remission, and finally into relapse. In this review, we examine seminal zebrafish studies that have advanced our understanding of melanoma.

MITF-the first 25 years

In this review, Goding and Arnheiter present the current understanding of MITF's role and regulation in development and disease and highlight key areas where our knowledge of MITF regulation and function is limited.

All transcription factors are equal, but some are more equal than others. In the 25 yr since the gene encoding the microphthalmia-associated transcription factor (MITF) was first isolated, MITF has emerged as a key coordinator of many aspects of melanocyte and melanoma biology. Like all transcription factors, MITF binds to specific DNA sequences and up-regulates or down-regulates its target genes. What marks MITF as being remarkable among its peers is the sheer range of biological processes that it appears to coordinate. These include cell survival, differentiation, proliferation, invasion, senescence, metabolism, and DNA damage repair. In this article we present our current understanding of MITF's role and regulation in development and disease, as well as those of the MITF-related factors TFEB and TFE3, and highlight key areas where our knowledge of MITF regulation and function is limited.

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.

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.

Microphthalmia-associated transcription factor (MITF) locus lacks linkage to human vitiligo or osteopetrosis: an evaluation

The microphthalmia-associated transcription factor (MITF) locus has been mapped to human chromosome 3p12-p14.1, and encodes a basic helix-loop-helix zipper (bHLH-ZIP) protein homologous to a number of transcription factors. Numerous mutations at the mouse microphthalmia (mi) locus have been described, and all have reduced or absent pigmentation of the eyes, ears, and/or pelage, with some genotypes exhibiting small or absent eyes and osteopetrosis. The mivit/vit mutation at the mouse mi locus produces a postnatal depigmentation that resembles human vitiligo. The mice homozygous for this mi allele show a progressive loss of cutaneous, hair and ocular pigmentation with age. Vitiligo, an acquired depigmentary disorder, is characterized by patchy depigmentation of skin that generally begins around puberty and tends to become more progressive over time. There is suggestive evidence that human vitiligo may be inherited; however, the mode of inheritance is still debated and the pathogenesis is not clearly delineated. The human disorder osteopetrosis is characterized by a generalized net accumulation of skeletal mass and results from reduced osteoclast function in the bone. This is an inherited disorder and has been associated with mi in a mutant mouse. Therefore, the possible involvement of the MITF locus in the pathogenesis of either familial vitiligo or osteopetrosis was investigated. Linkage analysis was performed using microsatellite polymorphic markers D3S2465, D3S1261, and D3S1766 on genomic DNA from 26 families with vitiligo/osteopetrosis. D3S1261 is physically located at or near the MITF locus, while D3S2465 and D3S1766 are flanking the locus at about 17.5 cM genetic distance each side. Evidence from LOD score analysis surprisingly indicated that none of the families with vitiligo or osteopetrosis are linked to these short tandem repeat polymorphisms (STRPs). Thus, the human homolog (MITF) of the mouse mi gene, a good candidate gene at the phenotypic level, may not be involved in the pathogenesis of familial human vitiligo or osteopetrosis.

Microphthalmia-associated transcription factor interacts with PU.1 and c-Fos: determination of their subcellular localization

Marked osteopetrosis is observed only in mi/mi mutant mice although normal osteoclastgenesis is observed in other mutant mice including null mutants at the mi locus. Mutant microphthalmia-associated transcription factor (mi-MITF) has defective nuclear localization potential. We found normal MITF (+-MITF)-c-Fos and mi-MITF-c-Fos complexes in the cytoplasm by immunoblotting, and showed that PU.1 bound with both +-MITF and mi-MITF using an electrophoretic mobility shift assay. Furthermore, the nuclear localization of PU.1 and c-Fos was inhibited by over-expressed mi-MITF in WEHI-3 cells. These results indicate that mi-MITF expressing in osteoclasts specifically binds to c-Fos and PU.1 which are essential transcription factors of osteoclastgenesis and that mi-MITF blocks the nuclear localization of these other transcription factors, which may result in osteopetrosis in mi/mi mutant mice.

A big gene linked to small eyes encodes multiple Mitf isoforms: many promoters make light work

Among more than 80 different loci related to mouse coat color, microphthalmia-associated transcription factor (Mitf) encoded at the mouse microphthalmia locus is one of the most exciting molecules that regulates the development and survival of many cell types, including melanocyte, retinal pigment epithelium (RPE), and mast cells. Mitf and its human homolog MITF consist of at least three isoforms, referred to as Mitf-A/MITF-A, the heart-type Mitf-H/MITF-H, and the melanocyte lineage-specific Mitf-M/MITF-M, respectively. These isoforms differ in the amino-terminal domains but share a transactivation domain and a basic helix-loop-helix and leucine-zipper structure that is required for DNA binding and dimerization. MITF-M is exclusively expressed in melanocytes and melanoma cells, but not in other cell types, including RPE cells. In contrast, MITF-A mRNA is widely expressed in many cell types. These three isoform mRNAs are possibly generated by differential usage of the gene promoters and by alternative splicing. We predict that the entire MITF gene spans about 200 kb of DNA. Like MITF-M, MITF-A is able to activate the two melanogenesis gene promoters, tyrosinase and tyrosinase-related protein 1. These results suggest that melanogenesis may be regulated by different MITF isoforms in melanocyte and RPE. Possible implications of the multiplicity in Mitf/MITF isoforms are discussed.

Role of Mitf in differentiation and transdifferentiation of chicken pigmented epithelial cell

Mitf encodes a basic helix-loop-helix-leucine-zipper (bHLHzip) protein that is known to function in the development of melanocytes, pigmented epithelial cells (PECs), osteoclasts, and mast cells. In this paper, we report on the isolation, expression, and overexpression of the chicken Mitf and discuss the role of its protein product in the differentiation and transdifferentiation of PECs. Northern blotting showed that chicken Mitf is predominantly expressed in embryonic retinal pigmented epithelium (PE), but is expressed at low levels in other tissues. A 5' RACE analysis revealed differences in the 5' region Mitf nRNA in PE and other tissues. Immunological analysis revealed that Mitf, the protein encoded by Mitf, is first detected in the nuclei of the optic vesicle cells at embryonic stage 13 in a restricted region covered with mesenchymal cells. From stage 14 to 24, the specific staining is observable in the PE and precursor of the PE, the outer layer of the optic cup. In embryos at stages later than stage 29 the signals for Mitf in the future iris, ciliary body, and posterior retinal regions become faint. These results show that expression of Mitf starts at the optic vesicle stage at which no other marker genes for PECs such as mmp115 and tyrosinase are expressed. Dedifferentiation of cultured retinal PECs (rPECs) was induced by phenylthiourea and testicular hyaluronidase, bFGF, or TGF-beta. Mitf expression was inhibited by these factors and reactivated during redifferentiation of the dedifferentiated cells into rPECs, showing the correlation between Mitf expression and rPEC differentiation. Retrovirus-mediated overexpression of Mtif inhibited bFGF-induced dedifferentiation and transdifferentiation of rPECs to both lens and neural cells. These findings showed that downregulation of Mitf expression is essential for the transdifferentiation of rPEC. Mitf overexpression caused hyperpigmentation in cultured rPECs and suppressed the changes in gene expression induced by bFGF. Mitf overexpression promoted expression of mmp115 and tyrosinase in bFGF-treated rPECs suggesting a critical role for Mitf in rPEC differentiation. Mitf overexpression, however, did not promote expression of another rPEC-specific gene, pP344, in bFGF-treated rPECs. This result suggests the presence of other regulatory genes promoting rPEC differentiation. The expression patterns of pax6 and Mitf are complementary both in vivo in vitro. Overexpression of Mitf inhibited expression of pax6 in cultured rPECs. These observations suggest that Mitf regulates pax6 expression negatively.

Physical characterization of the chromosomal rearrangements that accompany the transgene insertion in the chakragati mouse mutant

The circling phenotype of the chakragati mouse is a result of a transgenic insertional mutation. The absence of the phenotype in mice heterozygous for the transgene insertion suggests that this is due to a loss of function of an endogenous gene. Efforts to identify this gene have led to a previous report that sequences flanking the transgene, D16Ros1 and D16Ros2, map 10 cM apart in wildtype mice. We present here physical mapping data indicating that the proximity of D16Ros1 and D16Ros2 in the ckr mouse is explained by a duplication of D16Ros2 and its insertion along with the transgene at D16Ros1. We further demonstrate that D16Ros1 sequences are also duplicated and that this duplication is also part of the insertion at the endogenous D16Ros1 locus.

Differentiation and transdifferentiation of the retinal pigment epithelium

The retinal pigment epithelium (RPE) lies between the retina and the choroid of the eye and plays a vital role in ocular metabolism. The RPE develops from the same sheet of neuroepithelium as the retina and the two derivatives become distinguished by different expression patterns of a number of transcription factors during embryonic development. As the RPE layer differentiates it expresses a set of unique molecules, many of which are restricted to certain regions of the cell. PRE cells undergo both a loss of polarity and a loss of expression of many of these cell type-specific molecules when placed in monolayer culture. The RPE of many species, including mammals, can be induced to transdifferentiate by growth factors such as basic fibroblast growth factor. Under the influence of such factors the RPE is triggered to alter expression of a wide array of molecules and to take on a retinal epithelium fate, from which differentiated retinal cell types including rod photoreceptors can be produced.

The recessive phenotype displayed by a dominant negative microphthalmia-associated transcription factor mutant is a result of impaired nucleation potential

In the DNA binding domain of microphthalmia-associated transcription factor (MITF), four mutations are reported: mi, Mi wh, mi ew, and mi or. MITFs encoded by the mi, Mi wh, mi ew, and Mi or mutant alleles (mi-MITF, Mi wh-MITF, Mi ew-MITF, and Mi or-MITF, respectively) interfered with the DNA binding of wild-type MITF, TFE3, and another basic helix-loop-helix leucine zipper protein in vitro. Polyclonal antibody against MITF was produced and used for investigating the subcellular localization of mutant MITFs. Immunocytochemistry and immunoblotting revealed that more than 99% of wild-type MITF and Mi wh-MITF located in nuclei of transfected NIH 3T3 and 293T cells. In contrast, mi-MITF predominantly located in the cytoplasm of cells transfected with the corresponding plasmid. When the immunoglobulin G (IgG)-conjugated peptides representing a part of the DNA binding domain containing mi and Mi wh mutations were microinjected into the cytoplasm of NRK49F cells, wild-type peptide and Mi wh-type peptide-IgG conjugate localized in nuclei but mi-type peptide-IgG conjugate was detectable only in the cytoplasm. It was also demonstrated that the nuclear translocation potential of Mi or-MITF was normal but that Mi ew-MITF was impaired as well as mi-MITF. In cotransfection assay, a strong dominant negative effect of Mi wh-MITF against wild-type MITF-dependent transactivation system on tyrosinase promoter was observed, but mi-MITF had a small effect. However, by the conjugation of simian virus 40 large-T-antigen-derived nuclear localization signal to mi-MITF, the dominant negative effect was enhanced. Furthermore, we demonstrated that the interaction between wild-type MITF and mi-MITF occurred in the cytoplasm and that mi-MITF had an inhibitory effect on nuclear localization potential of wild-type MITF.

Effect of the dpy-20 and rol-6 cotransformation markers on alpha-tubulin gene expression in C. elegans transformants

An alpha-1 tubulin::lacZ fusion gene was introduced into the germline of Caenorhabditis elegans, using either rol-6 or dpy-20 genomic DNA as a cotransformation marker. Distinct patterns in cellular specificity of the alpha-1 tubulin::lacZ fusion gene expression were observed, depending on the cotransformation marker used. For the rol-6 marker, the tubulin fusion gene was expressed in several neurons in the head and tail ganglia and a set of 38-39 ventral cord motor neurons along the body length of the animal during larval and adult development. In contrast, for the dpy-20 marker system, not only were fewer neurons stained in the head and tail region, but also the staining of ventral cord motor neurons was extremely reduced both in number and intensity. The dpy-20 marked-mediated suppression of the alpha-1 tubulin gene expression was observed both in the cis and trans configurations. Similar down-regulation in the ventral cord motor neurons was observed when the alpha-2 tubulin::lacZ fusion gene construct was tested in these experiments using the dpy-20 marker. In controls, where the tubulin fusion gene was directly injected to obtain transformants without any marker DNA, the cellular staining pattern was close to the fusion gene expression observed with the rol-6 marker DNA. These results underline the importance of the choice of transformation marker system in generation of the transgenic animals, and reveal a down-regulation of the alpha-tubulin fusion gene expression in the ventral cord motor neurons in transgenic animals when the dpy-20 gene was used as a cotransformation marker.

Transgenic models for eye malformations

Several lines of transgenic mice developing eye malformations have been described in the literature and appear to be of increasing interest for the study of eye teratology in humans, since gene expression and regulation can be studied in the developing animal. Transgenic applications are briefly described here and an overview of existing transgenic mouse models carrying different eye abnormalities is given according to the major diagnosis (e.g., cataract, microphthalmia, anterior segment dysgenesis, retinal dysplasia). Interestingly, many transgenic models exhibit pathological findings similar to those observed in human pediatric ophthalmology. Unfortunately, detailed embryological studies in transgenic mice bearing congenital eye malformations are not available for all lines. Thus, the importance of creating further transgenic models to study the function of morphogenes and growth factors in eye development is also discussed.

Molecular basis of mouse microphthalmia (mi) mutations helps explain their developmental and phenotypic consequences

Mutations in the mouse microphthalmia (mi) gene affect the development of a number of cell types including melanocytes, osteoclasts and mast cells. Recently, mutations in the human mi gene (MITF) were found in patients with Waardenburg Syndrome type 2 (WS2), a dominantly inherited syndrome associated with hearing loss and pigmentary disturbances. We have characterized the molecular defects associated with eight murine mi mutations, which vary in both their mode of inheritance and in the cell types they affect. These molecular data, combined with the extensive body of genetic data accumulated for murine mi, shed light on the phenotypic and developmental consequences of mi mutations and offer a mouse model for WS2.

Insertional mutagenesis in transgenic mice

Increasing numbers of transgenic mouse lines have resulted in several dozens of mutants created by insertional mutagenesis. The advantages of different vector systems and the problems associated with the analysis of mutations and the cloning of the affected genes are discussed in this review.

Insertional mutation of the hairless locus on mouse chromosome 14

Crosses between heterozygous transgenic mice from line 5053 produced offspring with progressive irreversible hair loss beginning at day 19. With increasing age, the skin of these animals became thicker and plicated in appearance. Histological analysis revealed the complete absence of normal hair follicles and numerous intradermic cystic structures, which enlarged with time and became filled with keratinaceous material. Test crosses demonstrated that the affected animals are homozygous for the transgene insertion. The clinical and histological phenotype of the new mutant closely resembles that of the rhino allele at the hairless locus on Chromosome (Chr) 14. Complementation tests and linkage analysis indicate that the transgene has interrupted the hairless locus. It has been demonstrated previously that mutation at the hr locus is accompanied by a variety of immune deficiencies. Many of the older affected transgenic mice developed an impetigo-like skin eruption which responded to antibiotic ointment and which may reflect impaired immune function. The transgenic allele, hrTgN5053Mm, will be useful for identification of the transcription unit of the hairless locus.