The Rhox genes

Abstract

Homeobox genes encode transcription factors that have crucial roles in embryogenesis. A recently discovered set of homeobox genes--the Rhox genes--are expressed during both embryogenesis and in adult reproductive tissues. The 33 known mouse Rhox genes are clustered together in a single region on the X chromosome, while likely descendents of the primodial Rhox cluster, Arx and Esx1, have moved to other positions on the X chromosome. Here, we summarize what is known about the regulation and function of Rhox cluster and Rhox-related genes during embryogenesis and gametogenesis. The founding member of the Rhox gene cluster--Rhox5 (previously known as Pem)--has been studied in the most depth and thus is the focus of this review. We also discuss the unusually rapid evolution of the Rhox gene cluster.

Figure 1

Organization of the Rhox cluster in rodents and humans. The syntenic region of the X chromosome containing the Rhox orthologs and conserved flanking genes is shown. The rodent Rhox gene subclusters are indicated by red (α), green (β), and blue (γ). Established orthologous genes are indicated by dotted lines. The orthologous relationship between human RHOX genes and rodent Rhox genes cannot be clearly assigned because of the rapid evolution of Rhox genes. The map positions shown are according to builds 37.1 (mouse), 3.4 (rat), and 37.1 (human).

Figure 2

Summary of mouse Rhox cluster gene expression and function. The relative expression of each Rhox gene within the indicated tissue or cell type is shown by the heat map, with highly expressed genes shown in bright red and modestly expressed genes in maroon. The expression of Rhox genes in ES cells was determined by northern blot analysis (Sasaki et al. 1991, Fan et al. 1999), qPCR analysis (Jackson et al. 2002, Fouse et al. 2008), and semi-quantitative PCR analysis (Oda et al. 2006). The expression of Rhox genes in the early embryo (blastocyst to E8.5) was determined by semi-quantitative PCR analysis (Takasaki et al. 2000, Jackson et al. 2002, Chazaud et al. 2006); while Rhox6 and Rhox9 cannot be detected in normal mouse embryos, they can be detected in Dnmt-mutant embryos deficient in DNA methylation by RT-PCR (33 cycles; Oda et al. 2006). The expression of Rhox genes in fetal gonads was determined by qPCR and in situ hybridization analyses (Daggag et al. 2008); these analyses demonstrated that all Rhox genes are predominantly expressed in primordial germ cells (PGCs), except for Rhox8, which was exclusively expressed in gonadal somatic cells (s). ‘m’ and ‘f’ indicate expression specifically in male and female PGCs respectively. The expression of Rhox1 to Rhox12 in placenta and adult testes, epididymides, and ovaries was determined using qPCR analysis and/or ribonuclease protection analysis (MacLean et al. 2005a). The expression of Rhox13 in ovary and testis was determined by northern and semi-quantitative RT-PCR analyses (Geyer & Eddy 2008); its expression in epididymis and placenta was determined by qPCR analysis (JA MacLean & MF Wilkinson 2007, unpublished observations). (*) Rhox9 mRNA was reported as highly expressed in epididymis, but is now known to be an alternatively spliced Rhox9 mRNA that generates an amino domain-truncated protein (JA MacLean & MF Wilkinson 2005, unpublished observations). In vitro and in vivo functional analyses of Rhox5 were performed by Fan et al. (1999) and MacLean et al. (2005a), respectively. In vitro and in vivo functional analyses of Rhox4 and Rhox9 were peformed by Takasaki et al. (2001) and Jackson et al. (2002), respectively.