Sox proteins in melanocyte development and melanoma

Abstract

Over 10 years have passed since the first Sox gene was implicated in melanocyte development. Since then, we have discovered that SOX5, SOX9, SOX10 and SOX18 all participate as transcription factors that affect key melanocytic genes in both regulatory and modulatory fashions. Both SOX9 and SOX10 play major roles in the establishment and normal function of the melanocyte; SOX10 has been shown to heavily influence melanocyte development and SOX9 has been implicated in melanogenesis in the adult. Despite these advances, the precise cellular and molecular details of how these SOX proteins are regulated and interact during all stages of the melanocyte life cycle remain unknown. Improper regulation of SOX9 or SOX10 is also associated with cancerous transformation, and thus understanding the normal function of SOX proteins in the melanocyte will be key to revealing how these proteins contribute to melanoma.

Figure 1

Diagram of coding region mutations and polymorphisms of SOX10. Mutations are color-coded based upon phenotype, with developmental mutations listed above, melanoma mutations below, and polymorphisms at the bottom. Five additional cases of WS2 and WS4 that are associated with sizeable deletions encompassing the SOX10 coding region are not illustrated (Bondurand et al., 2007). In general, segregation of WS phenotypes and PCWH phenotypes are seen with the N-terminal and C-terminal halves of SOX10, respectively. Orthologous regions to published mouse coding region mutations are also indicated. Mutation nomenclature per (den Dunnen and Antonarakis, 2000).

Figure 2

Expression of SOX proteins during melanocyte development in mouse. At the trunk axial level, SOX9 and SOX10 are both detected in the dorsal neural tube and newly-emigrated neural crest cells beginning at E9.5. By E10.5, migratory neural crest cells no longer express SOX9 but retain expression of SOX10 as well as acquire positive staining for a number of melanoblast-specific markers. Between E10.5 and 12.5, Sox5 is also transiently detected in melanobasts. At E16.5, when melanoblasts are present in the dermis and epidermis, only SOX10 is present in these cells. During hair follicle morphogenesis, there appear to be three melanocyte subpopulations: 1) those that are present in the hair bulb and are SOX10−, 2) those that are present in the hair bulb and shaft of the hair follicle and are SOX10+, and 3) those that are in the hair bulge and are SOX10−. SOX9 expression in dermal and epidermal melanocytes at E16.5 and in the mouse hair follicle is currently unknown. Sox18 is expressed by mesenchymal cells that comprise the dermal papilla.

Figure 3

Cell autonomous transcriptional pathways involving Sox genes in melanocyte development, dermal melanocytes, and melanoma. Genes and pathways that have been identified in only one melanocyte stage to date are distinguished by color, as follows: melanoblast development only = blue (SOX5), dermal melanocytes only = brown (SOX9), melanoma only = purple (SOX9 and SOX10 regulation of Nestin). Figure generated using Biotapestry software (www.biotapestry.org).

Figure 4

A summary of published data on evolutionarily conserved regions that regulate Sox10 expression. These studies pair comparative genomic sequence analysis, to identify multiple-species conserved sequences (MCSs), with the use of spontaneous mutant mice and/or transgenic reporter mice. Using transgenics, Deal et al., 2006 showed that a 218kb BAC clone with Bgal inserted into the Sox10 locus confers normal Sox10 expression patterns. Three spontaneous deletions of these BAC transgenic mice (deletions A, B, and C) show reduced expression in various tissues, thus demonstrating some cell-specific functionality of the deleted conserved regions. However, all deletion lines maintain melanoblast expression, suggesting multiple regions contribute to Sox10 expression in melanocytes. Antonellis et al., 2006 showed that the spontaneous mouse mutant Sox10Hry, which displays reduced embryonic Sox10 expression and subsequent hypopigmentation and megacolon, has a deletion encompassing 3 regions of highly conserved sequence that are present within the 64.5 kb region upstream of Sox10. One of the 3 deleted conserved regions (in red) displays enhancer activity in melanocyte cell lines. Werner et al., 2007 generated transgenic mice harboring reporter constructs under the control of 7 conserved non-coding regions at the Sox10 genomic locus. None of the 7 regions confer melanoblast expression, although cell-specific expression does occur in other embryonic Sox10-expressing tissues. Antonellis et al., 2008 used zebrafish transgenesis to analyze 11 Sox10 MCSs; 8 originally identified in the 2006 study (numbered 2–9), along with 3 more spanning exon 1 and intron 1 (1, 1b, and 1c). Five MCSs (in red) target expression to developing melanocytes, and two of these (MCS4 and MCS7, marked with *) direct expression that coincides with most of the endogenous Sox10 expression patterns, in both zebrafish transgenics and in transgenic mice harboring LacZ under the control of each of these regions. Gray=deleted regions, Black=MCS, red=expression in melanocytes. MCS numbering from Antonellis et al., 2008.