Corsi, A.K., Brodigan, T.M., Jorgensen, E.M. & Krause, M. Characterization of a dominant negative C. elegans Twist mutant protein with implications for human Saethre-Chotzen syndrome. Development 129, 2761−2772
Twist is a transcription factor that is required for mesodermal cell fates in all animals studied to date. Mutations of this locus in humans have been identified as the cause of the craniofacial disorder Saethre-Chotzen syndrome. The Caenorhabditis elegans Twist homolog is required for the development of a subset of the mesoderm. A semidominant allele of the gene that codes for CeTwist, hlh-8, has defects that occur earlier in the mesodermal lineage than a previously studied null allele of the gene. The semidominant allele has a charge change (E29K) in the basic DNA-binding domain of CeTwist. Surprisingly, the mutant protein retains DNA-binding activity as both a homodimer and a heterodimer with its partner E/Daughterless (CeE/DA). However, the mutant protein blocks the activation of the promoter of a target gene. Therefore, the mutant CeTwist may cause cellular defects as a dominant negative protein by binding to target promoters as a homo- or heterodimer and then blocking transcription. Similar phenotypes as those caused by the E29K mutation were observed when amino acid substitutions in the DNA-binding domain that are associated with the human Saethre-Chotzen syndrome were engineered into the C. elegans protein. These data suggest that Saethre-Chotzen syndrome may be caused, in some cases, by dominant negative proteins, rather than by haploinsufficiency of the locus.
"Additionally, the partial vm target gene activity in hlh-8(iΔ) may be due to the unique regulation of arg-1 and ceh-24 versus egl-15. Indeed, previous studies have shown animals that are heterozygous for a semi-dominant E29K mutation in the basic domain of CeTwist do not express egl-15 but do express arg-1 (Corsi et al, 2002) further emphasizing the unique response of the genes to the level of wild-type CeTwist. "
[Show abstract][Hide abstract] ABSTRACT: The temporospatial regulation of genes encoding transcription factors is important during development. The hlh-8 gene encodes the C. elegans mesodermal transcription factor CeTwist. Elements in the hlh-8 promoter restrict gene expression to predominantly undifferentiated cells of the M lineage. We have discovered that hlh-8 expression in differentiated mesodermal cells is controlled by two well-conserved E box elements in the large first intron. Additionally, we found that these elements are bound in vitro by CeTwist and its transcription factor partner, CeE/DA. The E box driven expression is eliminated or diminished in an hlh-8 null allele or in hlh-2 (CeE/DA) RNAi, respectively. Expression of hlh-8 is also diminished in animals harboring an hlh-8 intron deletion allele. Altogether, our results support a model in which hlh-8 is initially expressed in the undifferentiated M lineage cells via promoter elements and then the CeTwist activates its own expression further (autoregulation) in differentiated cells derived from the M lineage via the intron elements. This model provides a mechanism for how a transcription factor may regulate distinct target genes in cells both before and after initiating the differentiation program. The findings could also be relevant to understanding human Twist gene regulation, which is currently not well understood.
"In humans, loss of function heterozygous mutations in the twist gene are associated with an autosomal-dominant craniosynostosis disorder known as Saethre–Chotzen Syndrome (SCS) [Reardon and Winter, 1994; el Ghouzzi et al., 1997; Johnson et al., 1998]. Over 75 mutations have been identified in SCS patients, involving large deletions and mostly point mutations that cluster near the DNAbinding domain [Gripp et al., 2000; Corsi et al., 2002; Jabs, 2004]. SCS is characterized by premature fusion of cranial sutures, low frontal hairline, facial asymmetry, eyelid ptosis, and limb defects such as polydactyly, brachydactyly, and syndactyly [Howard et al., 1997]. "
[Show abstract][Hide abstract] ABSTRACT: Bone marrow-derived mesenchymal stem cells (MSC), are multipotent cells that give rise to multiple lineages including osteoblasts, adipocytes, muscle, and fibroblasts. MSCs are useful for clinical applications such as cell therapy because they can be isolated from an individual and expanded for use in tissue repair, as well as other therapeutic applications, without immune rejection. However, one of the key problems in the use of MSCs for these applications is the efficiency of these cells to engraft and fully regenerate damaged tissues. Therefore, to optimize this process, a comprehensive understanding of the key regulators of MSCs self-renewal and maintenance are critical to the success of future cell therapy as well as other clinical applications. The basic helix loop helix transcription factor, Twist, plays a master regulatory role in all of these processes and, therefore, a thorough understanding of the mechanistic insights in the role of Twist in lineage specification/differentiation and tumorigenesis is vital to the success of future clinical applications for the therapeutic use of MSCs. In this article, we highlight the basic mechanisms and signaling pathways that are important to MSC fate, maintenance, and differentiation, as well as the critical role that Twist plays in these processes. In addition, we review the known literature suggesting a critical role for Twist in the generation of cancer stem cells, as this information may contribute to a broader understanding of stem cell biology and stem-cell-based therapeutics.
"Twi proteins are conserved across species [mouse (Gitelman, 1997), chicken (Tavares et al., 2001), C. elegans (Corsi et al., 2000), and jellyfish (Spring et al., 2000)] and have been shown to dimerize with Da homologs (Connerney et al., 2006; Corsi et al., 2002; Spicer et al., 1996), suggesting that REP domain regulation of Twi activity is conserved. Similarly to flies, Mouse Twi1 (MTwi1) heterodimerizes with E proteins to compete with MyoD/E proteins for binding sites on myogenic enhancers (Spicer et al., 1996). "
[Show abstract][Hide abstract] ABSTRACT: Somatic myogenesis in Drosophila relies on the reiterative activity of the basic helix-loop-helix transcriptional regulator, Twist (Twi). How Twi directs multiple cell fate decisions over the course of mesoderm and muscle development is unclear. Previous work has shown that Twi is regulated by its dimerization partner: Twi homodimers activate genes necessary for somatic myogenesis, whereas Twi/Daughterless (Da) heterodimers lead to the repression of these genes. Here, we examine the nature of Twi/Da heterodimer repressive activity. Analysis of the Da protein structure revealed a Da repression (REP) domain, which is required for Twi/Da-mediated repression of myogenic genes, such as Dmef2, both in tissue culture and in vivo. This domain is crucial for the allocation of mesodermal cells to distinct fates, such as heart, gut and body wall muscle. By contrast, the REP domain is not required in vivo during later stages of myogenesis, even though Twi activity is required for muscles to achieve their final pattern and morphology. Taken together, we present evidence that the repressive activity of the Twi/Da dimer is dependent on the Da REP domain and that the activity of the REP domain is sensitive to tissue context and developmental timing.
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