The role of Rhox homeobox factors in tumorigenesis

Abstract

Homeobox genes encode transcription factors that have well-established roles in embryonic development. We recently discovered the Rhox genes, a new family of homeobox genes, which are selectively expressed in the developing embryo, postnatal and adult gonads, and accessory tissues associated with mammalian reproduction. The largest and best-characterized Rhox cluster is found in mouse. However, all mammals examined to date possess a set of Rhox genes that, while they may vary in number by species, appear relevant to reproduction and are located in the syntenic region of the X chromosome. Rhox5, the founding member of the family, was initially cloned from a screen to identify tumorigenic antigens from T-cell lymphomas, and was later found to be widely expressed in tumors from tissues of diverse origins that do not normally express the Rhox genes. This aberrant upregulation appears to be a general feature of many Rhox genes, but the implications of this misexpression remain largely uninvestigated. In this review, we will discuss the latest findings on the normal and abnormal roles of the Rhox genes and their potential contributions to the formation and progression of tumors.

Figure 1.

Phylogenetic Comparison of mouse homeodomain proteins. The 60 amino acid homeodomain portions of all described mouse homeobox genes were aligned and compared via an unrooted phylogenetic tree using the neighbor joining method. Branch lengths represent the extent of divergence. Homeobox genes with links to cancer are indicated by blue dots and genes expressed in the reproductive tract are indicated by red dots.

Figure 2.

Organization of the Rhox cluster in mammalian species. 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 (alpha), green (beta), and blue (gamma). Established orthologus genes are indicated by dotted lines. The orthologus 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), 3.1 (Dog), 6.1 (Cattle), and 37.1 (human).

Figure 3.

Expression of the mouse Rhox genes in tumor and immortalized cell lines. Quantitative real-time RT-PCR (qPCR) was used to determine the relative expression of each Rhox gene using primer pairs specific for each unique Rhox gene and pan-paralog-specific primers that amplify total Rhox2, Rhox3, and Rhox4. Data is presented as mean +/− SEM fold above background after normalization with Rpl19 mRNA to account for differences in cDNA loading. The cell lines examined were TEL (thymic epithelial cell line), WEHI122 (lymphoma), PCC4 (embryonic carcinoma), I-10 (Leydig cell line), PS-1 (prostate mesenchymal cell line), N4TG1 (neuroblastoma), TS3.5 (a trophoblast stem cell line), MME (mouse mammary epithelial), SL12.4 (T-cell lymphoma, the original source of Rhox5), F9 (embryo carcinoma), and NIH3T3 (fibroblast cell line).