ABSTRACT
The human deafness-pigmentation syndromes, Waardenburg syndrome (WS) type 2a, and Tietz syndrome are characterized by profound deafness but only partial cutaneous pigmentary abnormalities. Both syndromes are caused by mutations in MITF. To illuminate differences between cutaneous and otic melanocytes in these syndromes, their development and survival in heterozygous Microphthalmia-White (Mitf(Mi-wh) /+) mice were studied and hearing function of these mice characterized. Mitf(Mi-wh) /+ mice have a profound hearing deficit, characterized by elevated auditory brainstem response thresholds, reduced distortion product otoacoustic emissions, absent endocochlear potential, loss of outer hair cells, and stria vascularis abnormalities. Mitf(Mi-wh) /+ embryos have fewer melanoblasts during embryonic development than their wild-type littermates. Although cochlear melanocytes are present at birth, they disappear from the Mitf(Mi-wh) /+ cochlea between P1 and P7. These findings may provide insight into the mechanism of melanocyte and hearing loss in human deafness-pigmentation syndromes such as WS and Tietz syndrome and illustrate differences between otic and follicular melanocytes.
Subject(s)
Albinism, Oculocutaneous/physiopathology , Deafness/physiopathology , Hearing/physiology , Heterozygote , Microphthalmia-Associated Transcription Factor/genetics , Waardenburg Syndrome/physiopathology , Action Potentials/physiology , Albinism, Oculocutaneous/genetics , Albinism, Oculocutaneous/pathology , Animals , Animals, Newborn , Deafness/genetics , Deafness/pathology , Disease Models, Animal , Embryo, Mammalian/metabolism , Embryo, Mammalian/pathology , Evoked Potentials, Auditory, Brain Stem/physiology , Hair Cells, Auditory, Outer/metabolism , Hair Cells, Auditory, Outer/pathology , Humans , Melanocytes/metabolism , Melanocytes/pathology , Mice , Mice, Inbred C57BL , Mice, Transgenic , Otoacoustic Emissions, Spontaneous/physiology , Stria Vascularis/metabolism , Stria Vascularis/pathology , Waardenburg Syndrome/genetics , Waardenburg Syndrome/pathologyABSTRACT
Polycomb group (PcG) proteins such as Enhancer of zeste homolog 2 (EZH2) are epigenetic transcriptional repressors that function through recognition and modification of histone methylation and chromatin structure. Targets of PcG include cell cycle regulatory proteins which govern cell cycle progression and cellular senescence. Senescence is a characteristic of melanocytic nevi, benign melanocytic proliferations that can be precursors of malignant melanoma. In this study, we report that EZH2, which we find absent in melanocytic nevi but expressed in many or most metastatic melanoma cells, functionally suppresses the senescent state in human melanoma cells. EZH2 depletion in melanoma cells inhibits cell proliferation, restores features of a cellular senescence phenotype, and inhibits growth of melanoma xenografts in vivo. p21/CDKN1A is activated upon EZH2 knockdown in a p53-independent manner and contributes substantially to cell cycle arrest and induction of a senescence phenotype. EZH2 depletion removes histone deacetylase 1 (HDAC1) from the CDKN1A transcriptional start site and downstream region, enhancing histone 3 acetylation globally and at CDKN1A. This results in recruitment of RNA polymerase II, leading to p21/CDKN1A activation. Depletion of EZH2 synergistically activates p21/CDKN1A expression in combination with the HDAC inhibitor trichostatin A. Since melanomas often retain wild-type p53 function activating p21, our findings describe a novel mechanism whereby EZH2 activation during tumor progression represses p21, leading to suppression of cellular senescence and enhanced tumorigenicity.
