Sox 7 and Sox 17 are strain-specific modifiers of the lymphangiogenic defects caused by Sox 18 dysfunction in mice

INTRODUCTION The expression of different phenotypes caused by the same mutation is commonly attributed to genetic modifiers (Nadeau, 2001). Modifier genes represent an important but poorly understood component of genetic regulatory circuitry, and attempts to identify and study genetic modifiers have become a priority in functional genomics. A number of modifier loci have been identified in humans and mice by polymorphism mapping, but in only a very few cases have the gene(s) responsible been pinpointed and their molecular functions characterized (for a review, see Rivera and Tessarollo, 2008). In mice, genetic background is well known to influence the phenotype resulting from gene inactivation by homologous recombination (gene knockout). Many examples exist in which a knockout phenotype may be mild or imperceptible on one genetic strain, but may reveal itself after the same mutation is bred onto a different background, typically C57BL/6 (Barthold, 2004). This phenomenon is usually attributed to strain-specific modifiers, but typically, the genes responsible remain unidentified, and hence little is known about how modifiers are involved at the mechanistic level in modulating the expressivity of the mutant phenotype. Sox18 encodes a transcription factor implicated in the development of hair follicles and blood and lymphatic vasculature. Like other members of the Sox family, SOX18 selectively binds the heptameric consensus DNA sequence, 5 -/T/TCAA/TG-3 , by virtue of a 79 amino acid high-mobility group (HMG) domain (Hosking et al., 1995; Hosking et al., 2001b). SOX18 also is able to activate transcription via a trans-activation domain C-terminal adjacent to the HMG domain (Hosking et al., 1995). Dominant and recessive mutations of SOX18 have been found to underlie the human hereditary syndrome hypotrichosis-lymphedematelangiectasia (HLT) (Irrthum et al., 2003). Physical features of this complex disorder are sparse hair, absence of eyebrows, telangiectasia in various sites and lymphedema, particularly manifest in the limbs. In the case of dominant HLT, nonsense mutations cause premature truncation of the SOX18 trans-activation domain. Recessive mutations cause amino acid substitutions that affect the DNA-binding HMG domain, and are therefore most likely loss-of-function mutations. We recently showed using cellular and in vivo genetic assays in mice that SOX18 is absolutely required for activating lymphatic Prox1 transcription and initiating lymphatic vascular development from blood endothelial cell precursors (Francois et al., 2008). Sox18 is a member of the Group F Sox subfamily, a group of three genes (Sox7, Sox17 and Sox18) encoding proteins with remarkably similar primary structure (Bowles et al., 2000). As is often the case with different members of the same Sox subfamily (Wegner, 1999), Sox7, Sox17 and Sox18 share some domains of expression that suggest functional overlap: for example, all three genes are known to be expressed in endothelial cells during vascular development (Young et al., 2006). These observations raise the possibility that SOX7 and/or SOX17 might interact with the SOX18 pathway in some developmental and disease contexts. In the present study, we sought to explore the role(s) that Sox7 and/or Sox17 might play in lymphatic vascular development. We found that although SOX7 and SOX17 are both able to activate the Prox1 promoter in vitro and in vivo, neither is normally active during lymphatic development, indicating that their relationship with Sox18 in this system is not one of simple redundancy. However, these genes were upregulated in the absence of Sox18 function, and specifically on a mixed background but not on C57BL/6, indicating that they act as strain-specific modifiers of the Sox18 lymphatic phenotype. Sox7 and Sox17 are strain-specific modifiers of the lymphangiogenic defects caused by Sox18 dysfunction in mice