Microphthalmia-associated transcription factor

Microphthalmia-associated transcription factor
Available structures PDB Ortholog search: PDBe, RCSB List of PDB id codes 4C7N
Identifiers
Symbols	MITF ; CMM8; MI; WS2; WS2A; bHLHe32
External IDs	OMIM: 156845 MGI: 104554 HomoloGene: 4892 ChEMBL: 1741165 GeneCards: MITF Gene
Gene ontology Molecular function • RNA polymerase II core promoter proximal region sequence-specific DNA binding • RNA polymerase II core promoter sequence-specific DNA binding • transcriptional activator activity, RNA polymerase II core promoter proximal region sequence-specific binding • transcriptional repressor activity, RNA polymerase II transcription regulatory region sequence-specific binding • chromatin binding • transcription factor activity, RNA polymerase II distal enhancer sequence-specific binding • protein binding • protein dimerization activity Cellular component • nucleus • cytoplasm • protein complex Biological process • negative regulation of transcription from RNA polymerase II promoter • regulation of transcription, DNA-templated • transcription from RNA polymerase II promoter • protein complex assembly • positive regulation of gene expression • osteoclast differentiation • melanocyte differentiation • regulation of cell proliferation • camera-type eye development • canonical Wnt signaling pathway involved in negative regulation of apoptotic process • cell fate commitment • regulation of osteoclast differentiation • positive regulation of transcription, DNA-templated • positive regulation of transcription from RNA polymerase II promoter • bone remodeling • mast cell migration • positive regulation of DNA-templated transcription, initiation • regulation of RNA biosynthetic process Sources: Amigo / QuickGO
RNA expression pattern
More reference expression data
Orthologs
Species	Human	Mouse
Entrez	4286	17342
Ensembl	ENSG00000187098	ENSMUSG00000035158
UniProt	O75030	Q08874
RefSeq (mRNA)	NM_000248	NM_001113198
RefSeq (protein)	NP_000239	NP_001106669
Location (UCSC)	Chr 3: 69.74 – 69.97 Mb	Chr 6: 97.81 – 98.02 Mb
PubMed search	[1]	[2]

Microphthalmia-associated transcription factor also known as class E basic helix-loop-helix protein 32 or bHLHe32 is a protein that in humans is encoded by the MITF gene.

MITF is a basic helix-loop-helix leucine zipper transcription factor involved in lineage-specific pathway regulation of many types of cells including melanocytes, osteoclasts, and mast cells. The term “lineage specific,” as it relates to MITF, means genes or traits that are only found in a certain cell type. Therefore, MITF may be involved in the rewiring of signaling cascades that are specifically required for the survival and physiological function of their normal cell precursors.

MITF is the most characterized member of the MIT family. Its gene resides at the mi locus in mice, and its protumorogenic targets include factors involved in cell death, DNA replication, repair, mitosis, microRNA production, membrane trafficking, mitochondrial metabolism, and much more. Mutation of this gene results in deafness, bone loss, small eyes, and poorly pigmented eyes and skin. MITF is the most characterized member of the MIT family. In human subjects, because it is known that MITF controls the expression of various genes that are essential for normal melanin synthesis in melanocytes, mutations of MITF can lead to diseases such as Melanoma, Waardenburg syndrome, and Tietz syndrome. Its function is conserved across vertebrates, including in fishes such as zebrafish and Xiphophorus.

An understanding of MITF is necessary to understand how certain lineage-specific cancers and other diseases progress. In addition, current and future research can lead to potential avenues to target this transcription factor mechanism for cancer prevention.

Clinical significance

Mutations

As mentioned above, changes in MITF can result in serious health conditions. For example, mutations of MITF have been implicated in both Waardenburg syndrome and Tietz syndrome.

Waardenburg syndrome is a rare genetic disorder. It’s symptoms include deafness, minor defects, and abnormalities in pigmentation. Mutations in the MITF gene have been found in certain patients with Waardenburg syndrome, type II. Mutations that change the amino acid sequence of that result in an abnormally small MITF are found. These mutations disrupt dimer formation, and as a result cause insufficient development of melanocytes. The shortage of melanocytes causes some of the characteristic features of Waardenburg syndrome.

Tietz syndrome, first described in 1923, is a congenital disorder often characterized by deafness and leucism. Tietz is caused by a mutation in the MITF gene. The mutation in MITF deletes or changes a single amino acid base pair specifically in the base motif region of the MITF protein. The new MITF protein is unable to bind to DNA and melanocyte development and subsequently melanin production is altered. A reduced number of melanocytes can lead to hearing loss, and decreased melanin production can account for the light skin and hair color that make Tietz syndrome so noticeable.

Melanoma

Melanocytes are commonly known as cells that are responsible for producing the pigment melanin which gives coloration to the hair, skin, and nails. The exact mechanisms of how exactly melanocytes become cancerous are relatively unclear, but there is ongoing research to gain more information about the process. For example, it has been uncovered that the DNA of certain genes is often damaged in melanoma cells, most likely as a result of damage from UV radiation, and in turn increases the likelihood of developing melanoma. Specifically, it has been found that a large percentage of melanomas have mutations in the B-RAF gene which leads to melanoma by causing an MEK-ERK kinase cascade when activated. In addition to B-RAF, MITF is also known to play a crucial role in melanoma progression. Since it is a transcription factor that is involved in the regulation of genes related to invasiveness, migration, and metastasis, it can play a role in the progression of melanoma. Figure 1 shows the specific activators and targets of MITF that are related to the survival, migration, proliferation, invasion and metastasis of melanoma cells.

Target genes

MITF recognizes E-box (CAYRTG) and M-box (TCAYRTG or CAYRTGA) sequences in the promoter regions of target genes. Known target genes (confirmed by at least two independent sources) of this transcription factor include,

ACP5	BCL2	BEST1	BIRC7
CDK2	CLCN7	DCT	EDNRB
GPNMB	GPR143	MC1R	MLANA
OSTM1	RAB27A	SILV	SLC45A2
TBX2	TRPM1	TYR	TYRP1

Additional genes identified by a microarray study (which confirmed the above targets) include the following,

MBP	TNFRSF14	IRF4	RBM35A
PLA1A	APOLD1	KCNN2	INPP4B
CAPN3	LGALS3	GREB1	FRMD4B
SLC1A4	TBC1D16	GMPR	ASAH1
MICAL1	TMC6	ITPKB	SLC7A8

The LysRS-Ap₄A-MITF signaling pathway

The LysRS-Ap₄A-MITF signaling pathway was first discovered in mast cells, in which, the MAPK pathway is activated upon allergen stimulation. Lysyl-tRNA synthetase (LysRS), which normally resides in the multisynthetase complex with other tRNA sythetases, is phosphorylated on Serine 207 in a MAPK-dependent manner. This phosphorylation causes LysRS to change its conformation, detach from the complex and translocate into the nucleus, where it associates with the MITF-HINT1 inhibitory complex. The conformational change switches LysRS activity from aminoacylation of Lysine tRNA to diadenosine tetraphosphate (Ap₄A) production. Ap₄A binds to HINT1, which releases MITF from the inhibitory complex, allowing it to transcribe its target genes. Activation of the LysRS-Ap₄A-MITF signaling pathway by isoproterenol has been confirmed in cardiomyocytes, where MITF is a major regulator of cardiac growth and hypertrophy.

Interactions

Most transcription factors function in cooperation with other factors by protein-protein interactions. Association of MITF with other proteins is a critical step in the regulation of MITF-mediated transcriptional activity. Some commonly studied MITF interactions include those with MAZR, PIAS3, Tfe3, hUBC9, PKC1, and LEF1. Looking at the variety of structures gives insight into MITF's varied roles in the cell.

The Myc-associated zinc-finger protein related factor (MAZR) interacts with the Zip domain of MITF. When expressed together, both MAZR and MITF increase promoter activity of the mMCP-6 gene. MAZR and MITF together transactivate the mMCP-6 gene. MAZR also plays a role in the phenotypic expression of mast cells in association with MITF.

PIAS3 is a transcriptional inhibiter that acts by inhibiting STAT3's DNA binding activity. PIAS3 directly interacts with MITF, and STAT3 does not interfere with the interaction between PIAS3 and MITF. PIAS3 functions as a key molecule in suppressing the transcriptional activity of MITF. This is important when considering mast cell and melanocyte development.

MITF and TFE3 are both part of the basic helix-loop-helix-leucine zipper family of transcription factors. Each protein encoded by the family of transcription factors can bind DNA. MITF is necessary for melanocyte and eye development, and new research suggests that TFE3 is also required for osteoclast development, a function redundant of MITF. The combined loss of both genes results in severe osteopetrosis, pointing to an interaction between MITF and other members of its transcription factor family.

UBC9 is a ubiquitin conjugating enzyme whose proteins associates with MITF. Although hUBC9 is known to act preferentially with SENTRIN/SUMO1, an in vitro analysis demonstrated greater actual association with MITF. hUBC9 is a critical regulator of melanocyte differentiation. To do this, it targets MITF for proteasome degradation.

Protein kinase C-interacting protein 1 (PKC1) associates with MITF. Their association is reduced upon cell activation. When this happens MITF disengages from PKC1. PKC1 by itself, found in the cytosol and nucleus, has no known physiological function. It does, however, have the ability to suppress MITF transcriptional activity and can function as an in vivo negative regulator of MITF induced transcriptional activity.

The functional cooperation between MITF and the lymphoid enhancing factor (LEF-1) results in a synergistic transactivation of the dopachrome tautomerase gene promoter, which is an early melanoblast marker. LEF-1 is involved in the process of regulation by Wnt signaling. LEF-1 also cooperates with MITF-related proteins like TFE3. MITF is a modulator of LEF-1, and this regulation ensures efficient propagation of Wnt signals in many cells.

See also

• Microphthalmia

External links

• Microphthalmia-associated transcription factor at the US National Library of Medicine Medical Subject Headings (MeSH)