The T-box gene family: emerging roles in development, stem cells and cancer

Abstract

The T-box family of transcription factors exhibits widespread involvement throughout development in all metazoans. T-box proteins are characterized by a DNA-binding motif known as the T-domain that binds DNA in a sequence-specific manner. In humans, mutations in many of the genes within the T-box family result in developmental syndromes, and there is increasing evidence to support a role for these factors in certain cancers. In addition, although early studies focused on the role of T-box factors in early embryogenesis, recent studies in mice have uncovered additional roles in unsuspected places, for example in adult stem cell populations. Here, I provide an overview of the key features of T-box transcription factors and highlight their roles and mechanisms of action during various stages of development and in stem/progenitor cell populations.

Keywords: T-box genes; Tbx; Transcription factors.

Fig. 1.

Phylogenetic tree of the T-box gene family in vertebrates. Colored circles on the right indicate involvement of particular genes in the areas discussed in this Primer; additional roles are indicated in Table 1. All of the genes are represented in human and mouse with the exception of Danio rerio (Dr) tbx16, which is present in birds and frogs (VegT) but not mammals, and Drtbx6. The Tbx6 subfamily is divergent, rendering relationships between members less clear and complicating the assignment of orthology, although analysis of non-sequence information such as the exon-intron structure of a gene or the composition of a gene's genomic neighbors can eliminate ambiguities. Thus, the zebrafish gene tbx24 (fused somites), rather than tbx6, is the ortholog of mammalian Tbx6, and it has been suggested that Drtbx6 should be renamed tbx26 (Ahn et al., 2012). There have also been some gene duplications, which are not indicated here, such as the duplication of T in Xenopus to form Xbra and Xbra3 (Hayata et al., 1999). Common synonyms are indicated after the solidus. The phylogenetic tree is based on and modified from Naiche et al. (2005) and Papaioannou and Goldin (2008).

Fig. 2.

Domain structure of T-box proteins and crystal structure of T-domains bound to DNA. (A) The domain structure of three example T-box proteins [T (Kispert, 1995), Tbx20a (Stennard et al., 2003) and Tbx5 (Zaragoza et al., 2004)] illustrating the location of the DNA-binding domain in the N-terminal portion and either a transcriptional activation domain or activation and repression domains in the C-terminal region. The domain involved in nuclear localization has not yet been determined for Tbx20. (B) Crystal structure of T-domains from Xenopus laevis T (Xbra) bound to a palindromic T-box binding element derived from the in vitro selected consensus sequence (Müller and Herrmann, 1997) and from human TBX5 bound to a natural T-box binding site from the ANF promoter (Stirnimann et al., 2010). Images are from the RCSB Protein Data Bank (www.rcsb.org): ID numbers 1XBR (Xbra) and 2X6V (TBX5).

Fig. 3.

Evolution of the T-box gene family. The subfamilies or classes of genes that have been identified in the different animal groups are indicated in the boxes. There has been a remarkable conservation of transcription factors between lineages that have been evolving independently since the last common ancestor to metazoans, and many of the T-box gene families have their origin at the base of the tree. However, diversification at the onset of metazoan evolution is evident. T, which is the most ancient T-box gene, is represented in unicellular organisms, as is Tbx7/8, a class not present in Bilateria. Sponges have a diverse set of T-box genes including several that have not been retained in Bilateria, whereas Tbx6 subfamily genes apparently arose in Bilateria. Note that this diagram represents one possible order of divergence of phyla (Degnan et al., 2009; Sebe-Pedros et al., 2013).

Fig. 4.

Expression domains of T-box genes during mouse pre-implantation and early post-implantation embryo development. Tbx3 is expressed in the inner cell mass of the pre-implantation embryo and later in the extraembryonic endoderm of the developing yolk sac. Mga is also expressed in the inner cell mass at pre-implantation stages and then in the epiblast of the post-implantation embryo. Eomes is expressed in the trophectoderm layer of the blastocyst and after implantation is expressed in the extraembryonic ectoderm and later the chorion, the visceral endoderm, including the anterior visceral endoderm, and the posterior proximal epiblast. T is expressed in the posterior epiblast and extraembryonic ectoderm and later in the core of the allantois, the primitive streak and node. Tbx6 overlaps T in the primitive streak but is not expressed in the node. Tbx4 is expressed in the allantois. AVE, anterior visceral endoderm; EPC, ectoplacental cone; ExE, extraembryonic ectoderm; ExEn, extraembryonic endoderm; ICM, inner cell mass; TE, trophectoderm; VE, visceral endoderm.

Fig. 5.

T-box gene expression during heart development. T-box genes are expressed in complex, overlapping patterns throughout heart development. The E8.25 heart is shown in a left lateral view of the whole embryo and in a frontal view of the isolated heart and second heart field. Hearts at later stages are shown as schematic transverse sections. Color coding indicates expression of individual genes or combinations of genes in different regions. AVC, atrioventricular canal; IFT, inflow tract; IVS, interventricular septum; LA, left atrium; LSH, left sinus horn; LV, left ventricle; OFT, outflow tract; RSH, right sinus horn; RV, right ventricle; V, ventricle; VV, venous valves. Reprinted from Greulich et al. (2011) with permission.

Fig. 6.

T-box gene expression during limb development. T-box gene expression is present at the earliest stages of limb outgrowth when Tbx4 and Tbx5 are expressed in the hindlimb and forelimb buds, respectively. Tbx2 and Tbx3 are first expressed in the posterior margin of the forelimb bud and then in the anterior and posterior margins of both limbs, where they are largely overlapping. In addition, Tbx3 is expressed in the apical ectodermal ridge and T is expressed in the mesoderm immediately subadjacent to the apical ectodermal ridge. Tbx15 and Tbx18 are largely overlapping in the core of the limbs, whereas Tbx1 is expressed in the developing muscle masses. Finally, Eomes is expressed in a small domain of mesoderm at the base of the developing fourth digit, although no function has yet been assigned to this area of expression (Hancock et al., 1999). E9.5 and E12.5 are left lateral views and E10.5 is a transverse section through the hindlimbs. For clarity, expression in areas other than the limbs is not shown. AER, apical ectodermal ridge; DA, dorsal aorta; FL, forelimb; G, gut; H, heart; HL, hindlimb; NC, notochord; NT, neural tube; OV, otic vesicle.

Fig. 7.

Roles of T-box genes during stem/progenitor cell self-renewal and differentiation. (A) The self-renewal of stem cell or progenitor cell populations is affected by several T-box transcription factors (highlighted), both in the maintenance of cell lines in vitro (left) and in cell populations within the embryo or adult in vivo (right). (B) Studies of stem or progenitor cells have shown that T-box transcription factors are also important for driving the differentiation of specific lineages (arrows), sometimes while inhibiting alternative differentiation pathways, both in vitro and in vivo. Several direct target genes involved in the differentiation pathways are indicated along the arrow path. ESC, embryonic stem cell; EpiSC, epiblast stem cell; ExEn, extraembryonic endoderm; FHF, first heart field; hESC, human embryonic stem cell; HF-SC, hair follicle stem cell; IPC, intermediate progenitor cell; iPSC, induced pluripotent stem cell; NSC, neural stem cell; SHF, second heart field; SSC, spermatogonial stem cell; TE, trophectoderm; TSC, trophoblast stem cell.