The locations of the H-twist and H-dermo-1 genes are distinct on the human genome

Emerging role of Twist1 in fibrotic diseases

Epithelial–mesenchymal transition (EMT) is a pathological process that occurs in a variety of diseases, including organ fibrosis. Twist1, a basic helix–loop–helix transcription factor, is involved in EMT and plays significant roles in various fibrotic diseases. Suppression of the EMT process represents a promising approach for the treatment of fibrotic diseases. In this review, we discuss the roles and the underlying molecular mechanisms of Twist1 in fibrotic diseases, including those affecting kidney, lung, skin, oral submucosa and other tissues. We aim at providing new insight into the pathogenesis of various fibrotic diseases and facilitating the development of novel diagnostic and therapeutic methods for their treatment.

A twist of insight - the role of Twist-family bHLH factors in development

Members of the Twist-family of bHLH proteins play a pivotal role in a number of essential developmental programs. Twist-family bHLH proteins function by dimerizing with other bHLH members and binding to cis- regulatory elements, called E-boxes. While Twist-family members may simply exhibit a preference in terms of high-affinity binding partners, a complex, multilevel cascade of regulation creates a dynamic role for these bHLH proteins. We summarize in this review information on each Twist-family member concerning expression pattern, function, regulation, downstream targets, and interactions with other bHLH proteins. Additionally, we focus on the phospho-regulatory mechanisms that tightly control posttranslational modification of Twist-family member bHLH proteins.

Clinical and Genetic Analysis of Patients with Saethre-Chotzen Syndrome

BACKGROUND

Saethre-Chotzen syndrome is a craniosynostosis syndrome further characterized by distinctive facial and limb abnormalities. It shows complete penetrance and variable expressivity and has been linked to the TWIST gene on chromosome 7p21; more than 80 different intragenic mutations and, recently, large deletions have been detected in Saethre-Chotzen patients. The aim of this study was to genetically and phenotypically characterize patients with a clinical diagnosis of Saethre-Chotzen syndrome.

METHODS

Patients with a clinical diagnosis as well as those with a genetic diagnosis of Saethre-Chotzen syndrome (n = 34) were included in the study.

RESULTS

The study showed that the important features of Saethre-Chotzen syndrome are brachycephaly (occurring in 74 percent of patients), a broad, depressed nasal bridge (65 percent), a high forehead (56 percent), ptosis (53 percent), and prominent auricular crura (56 percent). Furthermore, using different molecular techniques, pathogenic mutations in the TWIST gene were identified in 71 percent of patients.

CONCLUSIONS

Patients with deletions of the TWIST gene did not differ from those with intragenic TWIST mutations in frequency or severity of craniofacial abnormalities. However, they did distinguish themselves by the presence of many additional anomalies and diseases and--most importantly--the high frequency of mental retardation, which was borderline significant. The authors conclude that when using stringent inclusion criteria for studies of Saethre-Chotzen syndrome, patients who have a pathogenic mutation of the TWIST gene should be excluded.

Human Dermo-1 has attributes similar to twist in early bone development

Basic helix-loop-helix (bHLH) transcription factors are implicated in cell lineage determination and differentiation. Dermo-1 encodes a bHLH transcription factor that shares extensive homology with another bHLH transcription factor, Twist. We have cloned and characterized human Dermo-1 from two different bone cytoplasmic DNA (cDNA) libraries. Dermo-1 mRNA and protein expression were examined in human embryo and adult tissue sections. Dermo-1 is expressed in a subset of mesodermally and ectodermally derived tissues. We further examined expression of Dermo-1/Twist in human tissues and cell lines. In addition, we observed Dermo-1 expression in response to basic fibroblast growth factor in osteoblastic cell lines. To evaluate the functionality of the human Dermo-1 transcription factor in osteoblast metabolism, we made stable osteoblastic cell lines that over- and underexpress human Dermo-1. These cell lines were analyzed and compared with previously published data of similar cell lines transfected with Twist. Our results demonstrate that Dermo-1 caused changes similar to Twist in the osteogenic properties of osteoblastic cells, such as morphology, bone marker gene expression, and biochemical response to cytokines. However, Dermo-1 expression also has unique effects in regulating the mechanism of proliferation, on alkaline phosphatase enzyme activity, and in temporal expression patterns. We speculate that expression of Twist and Dermo-1 maintains cells in an osteoprogenitor or preosteoblast-like state, respectively, and prevents premature or ectopic osteoblast differentiation. Therefore, Twist and Dermo-1 must be sequentially downregulated in order to initiate the cascade of events responsible for osteogenic cell differentiation. These results indicate that, during osteoblast development, Dermo-1 may inhibit osteoblast maturation and maintain cells in a preosteoblast phenotype by utilizing mechanisms similar but not identical to those utilized by Twist.

Identification of DERMO-1 as a member of helix-loop-helix type transcription factors expressed in osteoblastic cells

Several members of the basic helix-loop-helix (bHLH) type of transcription factors have now been reported, and almost every member of this class has been implicated in transcriptional regulation in cell type determination and differentiation. Previously, we reported that dominant negative HLH proteins are involved in osteoblastic phenotype expression, such as osteocalcin, and hence differentiation (Tamura and Noda [1994] J. Cell Biol. 126:773-782). In this work, we used degenerate PCR cloning in order to identify cDNA clones encoding bHLH proteins expressed in osteoblastic osteosarcoma ROS17/2.8 cells. Sequence analyses of the 47 clones revealed that 11 clones encoded products with a characteristic motif of the bHLH transcription factor family. Of these clones, sequences in the amplified region of seven clones were homologous to the mouse twist, and three clones were homologous to the mouse twist-related HLH protein, Dermo-1. To confirm Dermo-1 mRNA expression in osteoblastic cells, we performed reverse transcription polymerase chain reaction (RT-PCR) analysis using mRNA from ROS17/2.8 cells and MC3T3-E1 cells by Dermo-1 specific primers and Northern blot analysis. These analyses demonstrated that Dermo-1 mRNA was expressed in these osteoblast-like cell lines. Nucleotide sequence analysis of the partial rat Dermo-1 cDNA cloned from ROS17/2.8 library revealed that it has the highest degree of homology with the mouse Dermo-1 cDNA, and the partial amino acid sequence deduced from the obtained rat Dermo-1 was identical with the corresponding region of the mouse Dermo-1 amino acid sequence. To further examine the role of Dermo-1 in the regulation of osteoblastic differentiation, we examined mRNA levels of Dermo-1 and twist in C3H10T1/2 cells treated with recombinant human bone morphogenetic protein (rhBMP)-2. Using the RT-PCR method, the mRNA levels of Dermo-1 and twist were found to be decreased by the treatment with rhBMP-2 in C3H10T1/2 cells. We also observed that the mRNA level of Dermo-1 was decreased about fourfold by the treatment with rhBMP-2 in C3H10T1/2 cells by Northern blot analysis. Moreover, Dermo-1 mRNA was detected at lower levels in 21-day-old differentiated MC3T3-E1 cells compared with 3-day-old undifferentiated MC3T3-E1 cells. These results suggested that Dermo-1 could be involved in the osteoblastic differentiation in a negative manner.