Direct regulation of the Microphthalmia promoter by Sox10 links Waardenburg-Shah syndrome (WS4)-associated hypopigmentation and deafness to WS2

The transcription factor Sox10 is genetically linked with Waardenburg syndrome 4 (WS4) in humans and the Dominant megacolon (Dom) mouse model for this disease. The pigmentary defects observed in the Dom mouse and WS4 are reminiscent of those associated with mutations in the microphthalmia (Mitf) gene, which encodes a transcription factor essential for the development of the melanocyte lineage. We demonstrate here that wild type Sox10 directly binds and activates transcription of the MITF promoter, whereas a mutant form of the Sox10 protein genetically linked with WS4 acts as a dominant-negative repressor of MITF expression and can reduce endogenous MITF protein levels. The ability of Sox10 to activate transcription of the MITF promoter implicates Sox10 in the regulation of melanocyte development and provides a molecular basis for the hypopigmentation and deafness associated with WS4.

Regulation of the microphthalmia-associated transcription factor gene by the Waardenburg syndrome type 4 gene, SOX10

The absence of melanocytes from the cochlea and epidermis is responsible of deafness and hypopigmentation, two symptoms shared by the four Waardenburg syndrome (WS) subtypes. Microphthalmia-associated transcription factor (MITF) controls melanocyte survival and differentiation. Mutations, which impair MITF function or expression, result in an abnormal melanocyte development leading to the WS2. WS1 and WS3 are caused by mutation in the gene encoding the transcription factor Pax3, which regulates MITF expression. Recently, mutations in SOX10, a gene encoding a SRY-related transcription factor, have been reported in patients with WS4. However, the molecular basis of the defective melanocyte development in these patients remained to be elucidated. In the present report, we demonstrate that Sox10 is a strong activator of the MITF promoter, and we identify a Sox10 binding site between -264 and -266 of the MITF promoter. Finally, we show that three SOX10 mutations found in WS4 abolish the transcriptional activity of the resulting Sox10 proteins toward the MITF promoter. Taken together, our observations bring new and meaningful information concerning the molecular process that leads to a defective melanocyte development in WS4 patients with SOX10 mutations.

TFE3, a transcription factor homologous to microphthalmia, is a potential transcriptional activator of tyrosinase and TyrpI genes

Microphthalmia gene encodes a basic helix-loop-helix-leucine zipper (bHLH-Zip) transcription factor involved in the development of the melanocyte lineage and plays a key role in the transcriptional regulation of the melanogenic enzymes, tyrosinase and TyrpI. Recently, we have shown that Microphthalmia mediates the melanogenic effects elicited by alphaMSH that up-regulates the expression of tyrosinase through the activation of the cAMP pathway. Therefore, Microphthalmia appears as a principal gene in melanocyte development and functioning. Among the transcription factors of the bHLH-Zip family, TFE3 and TFEB show a remarkably elevated homology with Microphthalmia. These observations prompted us to investigate the role of TFE3 and TFEB in the regulation of tyrosinase and TyrpI gene transcription. We show in this report that overexpression of TFE3 stimulates the tyrosinase and TyrpI promoter activities, while TFEB acts only on the TyrpI promoter. TFE3 and TFEB elicit their effects mainly through the binding to Mbox (AGTCATGTGCT) and Ebox motifs (CATGTG) of tyrosinase and TyrpI promoters. In B16 melanoma cells, the high basal expression of TFE3 is down-regulated by forskolin and by alphaMSH. Interestingly, endogenous TFE3 cannot bind as homodimers to the Mbox, and we did not detect TFE3/Mi heterodimers. According to these data, TFE3 is clearly endowed with the capacity to regulate tyrosinase and TyrpI gene expression. However, TFE3 binding to the melanogenic gene promoters is hindered, thereby preventing its potential melanogenic action. In specific physiological or pathological conditions, the recovery of its binding function would make TFE3 an important element in melanogenesis regulation.