In humans and mice, mutations in Hoxa13 cause malformation of limb and genitourinary (GU) regions. In males, one of the most common GU malformations associated with loss of Hoxa13 function is hypospadia,a condition defined by the poor growth and closure of the urethra and glans penis. By examining early signaling in the developing mouse genital tubercle,we show that Hoxa13 is essential for normal expression of Fgf8 and Bmp7 in the urethral plate epithelium. In Hoxa13GFP-mutant mice, hypospadias occur as a result of the combined loss of Fgf8 and Bmp7 expression in the urethral plate epithelium, as well as the ectopic expression of noggin(Nog) in the flanking mesenchyme. In vitro supplementation with Fgf8 restored proliferation in homozygous mutants to wild-type levels, suggesting that Fgf8 is sufficient to direct early proliferation of the developing genital tubercle. However, the closure defects of the distal urethra and glans can be attributed to a loss of apoptosis in the urethra,which is consistent with reduced Bmp7 expression in this region. Mice mutant for Hoxa13 also exhibit changes in androgen receptor expression, providing a developmental link between Hoxa13-associated hypospadias and those produced by antagonists to androgen signaling. Finally,a novel role for Hoxa13 in the vascularization of the glans penis is also identified.

Reciprocal epithelial-mesenchymal interactions shape site-specific development of skin. Here we show that site-specific HOX expression in fibroblasts is cell-autonomous and epigenetically maintained. The distal-specific gene HOXA13 is continually required to maintain the distal-specific transcriptional program in adult fibroblasts, including expression of WNT5A, a morphogen required for distal development. The ability of distal fibroblasts to induce epidermal keratin 9, a distal-specific gene, is abrogated by depletion of HOXA13, but rescued by addition of WNT5A. Thus, maintenance of appropriate HOX transcriptional program in adult fibroblasts may serve as a source of positional memory to differentially pattern the epithelia during homeostasis and regeneration.Supplemental material is available at http://www.genesdev.org.Received August 29, 2007; revised version accepted December 3, 2007. Epithelial tissues such as skin demonstrate remarkable anatomic diversity in their structure and function. For instance, while long terminal hairs decorate the scalp, palmo-plantar skin lacks hairs but possesses thickened epidermal barriers for mechanical stress. These anatomic differences lead to many body site-specific manifestations of diseases and guide their appropriate treatments (Bolognia et al. 2003). Because the epidermis is continually turned over, the apparent stability of sitespecific features raises the fundamental question of how positional identity is acquired and maintained in the skin. Classic heterotypic recombination experiments in chick suggested that a primary dermal signal may dictate the positional identity of epithelial differentiation (Dhouailly 1984). We and others have used a genomic approach to examine the anatomic diversity of gene expression of cell types in the dermis and identified fibroblasts as a cell type that demonstrates consistent and large-scale differences of the expression of cell-cell signaling proteins in a site-specific manner (Chang et al. 2002;Chi et al. 2003;Rinn et al. 2006). Systematic comparison of the global transcriptional programs of fibroblasts from anatomic sites that finely map the body suggest that fibroblasts are differentiated based on their position along three anatomic divisions: anterior-posterior, proximal-distal, and dermal-nondermal (Rinn et al. 2006). Moreover, primary adult fibroblasts retained many features of the embryonic pattern of expression of HOX genes (Chang et al. 2002;Rinn et al. 2006), a family of homeodomain transcription factors that act to specify positional identity during development. These data support the idea, first proposed by Chuong (1993), that a "HOX code" may dictate the positional identity of skin and thus influence site-specific epidermal differentiation. However, fundamental elements of this hypothesis, such as the stability of HOX expression, gene regulatory function, and inductive activities governed by HOX genes in adult fibroblasts remain poorly understood. Here we show that expression of HOXA13 is required and the transcriptio...

SUMMARY The combinatorial expression of Hox genes along the body axes is a major determinant of cell fate and plays a pivotal role in generating the animal body plan. Loss of HOXA13 and HOXD13 transcription factors (HOX13) leads to digit agenesis in mice, but how HOX13 proteins regulate transcriptional outcomes and confer identity to the distal-most limb cells has remained elusive. Here, we report on the genome-wide profiling of HOXA13 and HOXD13 in vivo binding and changes of the transcriptome and chromatin state in the transition from the early to the late-distal limb developmental program, as well as in Hoxa13−/−; Hoxd13−/−limbs. Our results show that proper termination of the early limb transcriptional program and activation of the late-distal limb program are coordinated by the dual action of HOX13 on cis-regulatory modules.

SummaryHOXA13 regulates the expression of bone morphogenetic proteins 2 and 7 to control distal limb morphogenesis