Evidence to suggest that expression of MITF induces melanocyte differentiation and haploinsufficiency of MITF causes Waardenburg syndrome type 2A.
MITF (microphthalmia-associated transcription factor) encodes a transcription factor with a basic-helix-loop-helix-leucine zipper (bHLH-Zip) motif. Ectopic expression of MITF is found to convert NIH/3T3 fibroblasts into cells with characteristics of melanocytes. MITF transfectants formed foci, which superficially resembled those induced by oncogenes, but did not exhibit malignant phenotypes. Instead, they contained dendritic cells that express melanogenic marker proteins such as tyrosinase and tyrosinase-related protein 1. Such properties were not observed in cells transfected with the closely related gene, TFE3. These findings indicated that MITF is involved in melanocyte differentiation. Two mutations (C760-->T and C895-->T) in MITF are found to be associated with individuals with Waardenburg syndrome type 2 (WS2). These mutations create stop codons in exon 7 and 8, respectively, and probably result in truncated proteins lacking HLH-Zip or Zip structure. To understand how these MITF mutations cause WS2 in heterozygotes, mutant MITF proteins were generated and used for DNA-binding and luciferase reporter assays. The mutated MITF proteins lose their DNA-binding activity and fail to transactivate the promoter of the tyrosinase gene. However, these mutated proteins do not appear to interfere with the activity of wild-type MITF protein in these assays, indicating that they do not show a dominant-negative effect. These findings suggest that the phenotypes of the two WS2 families are caused by loss-of-function mutations in one of the two MITF alleles, resulting in haploinsufficiency of the MITF protein, the transcription factor necessary for normal melanocyte differentiation.
Tachibana
M
M
Laboratory of Molecular Genetics, National Institute on Deafness and Other Communication Disorders, Rockville, Maryland 20850, USA. mtachiba@pop.nidcd.nih.gov
D000818:Animals; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D002454:Cell Differentiation; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D018257:Helix-Loop-Helix Motifs; D006801:Humans; D016350:Leucine Zippers; D008544:Melanocytes / Q000166:cytology*; D008562:Membrane Glycoproteins; D051739:Microphthalmia-Associated Transcription Factor; D014442:Monophenol Monooxygenase / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D010088:Oxidoreductases; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D012313:RNA; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*; D014162:Transfection; D014849:Waardenburg Syndrome / Q000235:genetics* / Q000378:metabolism
D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D004268:DNA-Binding Proteins; C497234:MITF protein, human; D008562:Membrane Glycoproteins; D051739:Microphthalmia-Associated Transcription Factor; D011506:Proteins; C069203:TFE3 protein, human; D014157:Transcription Factors; D012313:RNA; D010088:Oxidoreductases; C489799:TYRP1 protein, human; C064056:tyrosinase-related protein-1; D014442:Monophenol Monooxygenase