Chapter Three - The Hand2 Gene Dosage Effect in Developmental Defects and Human Congenital Disorders
Introduction
Basic helix–loop–helix (bHLH) transcription factors have an evolutionarily conserved domain that comprises a short stretch of basic amino acids and two alpha helices separated by a loop motif (Murre et al., 1989). Phenotype analyses of mutant flies and mice revealed that the bHLH transcription factors play essential roles to regulate cell-fate determination and the development of several organs (Massari & Murre, 2000). To date, extensive searches for homologs of bHLH genes in higher vertebrate species, including mouse and human, have been carried out, and many homologs and novel bHLH transcription factors have been identified. From a functional aspect, they are divided into several groups, such as neurogenic differentiation (NeuroD), Hairy/enhancer-of-split (Hes), and myogenic differentiation antigen (MyoD) families, which act on neural differentiation, neural inhibition, and myogenic differentiation, respectively. On the other hand, based on their expression patterns, the bHLH transcription factors are categorized into two classes. One is the ubiquitously expressed Class A, which includes E12, E47, and HEB; and the other is tissue-specifically expressed Class B, which includes MyoD, Neurog1, and Mash1. The Class B proteins form homo- and/or hetero dimers with Class A or other Class B proteins through their HLH domains, and bind to genomic DNA through their basic amino acid stretch, which recognizes a consensus sequence, CANNTG, termed an E-box, which resides in cis-regulatory elements of downstream target genes (Murre et al., 1989, Wilson-Rawls et al., 2004).
Heart- and neural crest derivatives-expressed 2 (Hand2), also known as dHand, Hed, and Thing2, is a typical bHLH transcription factor. It belongs to the Twist family together with Hand1, which is also knows as eHand, Hxt, and Thing1 (Fig. 3.1). Hand2 forms a heterodimer with Twist1, which also belongs to the Twist family (Firulli et al., 2005). Haploinsufficiency of human TWIST1 resulting from mutation of the TWIST1 gene causes a congenital disorder, Saethre–Chotzen Syndrome (OMIM 101400), which manifests as craniofacial anomaly, facial asymmetry, premature closing of suture, polydactyly, and clinodactyly (el Ghouzzi et al., 1997, Howard et al., 1997, Jabs, 2001). Indeed, approximately 80% of Saethre–Chotzen Syndrome patients have a point mutation or deletion in TWIST1 (Gripp et al., 2000, Jabs, 2001, Johnson et al., 1998). Moreover, it was reported that mutations of the class A bHLH transcription factor, TCF12, which forms a dimer with Twist1, as does Hand2, is also responsible for Saethre–Chotzen Syndrome (Sharma et al., 2013).
Although, the Hand proteins form heterodimers with the Twist1 protein and play pivotal roles during the heart and limb development, human diseases caused by mutations in the Hand genes are largely unknown. Recently, a genetic study of a mouse mutant revealed that over dosage of Hand2 caused numerous developmental defects, and is responsible for several symptoms of a human chromosomal disorder named partial trisomy distal 4q (Tamura et al., 2013). In this review, we focus on the gene dosage effect of Hand2 on the cardiovascular, craniofacial, and limb development and its implication in human diseases.
Section snippets
Cloning of Hand Genes and Their Expression Patterns
Several research groups independently cloned the mouse Hand1 and Hand2 genes using the Yeast two-hybrid system with the E-protein as the bait, or by screening of homologous sequences in cDNA libraries using known sequences encoding the bHLH domain as probes (Cross et al., 1995, Cserjesi et al., 1995, Hollenberg et al., 1995, Srivastava et al., 1995). These studies revealed that the Hand1 and Hand2 proteins are members of the Hand subfamily, and belong to the Twist family, which include
Developmental Functions of Hand2
Developmental functions of the vertebrate Hand genes have been investigated by phenotype analyses of mice mutated in these genes. The first conventional KO mouse of Hand2 was generated soon after cloning of the gene (Srivastava et al., 1997). Hand2 comprises two exons, and this structure is evolutionally conserved from teleosts to mammals, including humans (Firulli, 2003). In the first KO mouse, a neomycin resistance cassette replaced a segment containing the two exons. This Hand2 KO mouse
Gene Dosage Effect of Hand2 in Mouse Embryogenesis
Hand2 forms a heterodimer with Twist1 (Firulli et al., 2005). Heterozygotes of the Twist1 KO mutant mouse exhibited an anterior ectopic expression of Shh, leading to excess digits in the anterior limb bud (Bourgeois et al., 1998, el Ghouzzi et al., 1997, Zhang et al., 2010). This suggests that Twist1 has an inhibitory effect on the Shh expression in the normal limb bud. The opposing actions of Hand2 and Twist1 in A–P patterning are controlled in a dose dependent manner. Polydactyly in
Disruption of Hand2 Dosage Causes Human Diseases
The human HAND2 gene is located on human chromosome 4, at 4q34.1 (UCSC Human Genome Browser; GRCh37/hg19). Duplication of the distal end of the long arm of human chromosome 4, which includes 4q34, leads to the congenital disorder, “Partial trisomy distal 4q” (denoted 4q +), whose typical phenotypes include growth retardation, psychomotor retardation, microcephaly, epicantic folds, high nasal bridge, short philtrum, micrognathia, low set/malformed ear, preaxial polydactyly, epilepsy, renal
Future Perspectives
It is intriguing that only expression level changes of Hand2, but not its ectopic expression, elicits several developmental defects, including preaxial polydactyly in mice and humans. Accumulating data have provided evidence that balanced expression of Hand2 and Twist1 is essential for proper limb development, and disruption of the expression balance between these two genes causes developmental defects.
Human 4q syndrome (4q −), which might be caused by lower levels of Hand2, shows several
Acknowledgments
This work was supported by the Japan Society for the Promotion of Science (12013551 to M. T.), and by the Ministry of Education, Culture, Sports, Science and Technology in Japan (17018033 to T. S.).
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2019, Life SciencesCitation Excerpt :However, it remains elusive that the role and the mechanism of miR-92b-3p in cardiac hypertrophy. Heart and neural crest derivatives-expressed 2 (HAND2) and HAND1, are typical basic Helix-Loop-Helix (bHLH) transcription factors, both of which belong to the Twist family [13]. HAND2 is one of the four cardiac transcription factors, GATA4, HAND2, MEF2C and T-box transcription factor 5 (TBX5), which can cooperatively reprogram non-myocytes and cardiac fibroblasts into beating cardiac-like myocytes for heart repair [14].
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2019, Journal of Hand SurgeryCitation Excerpt :In 4q34 deletion syndrome, it is the little finger that is most affected, consistent with HAND2 haploinsufficiency. Interestingly, overexpression of HAND2 in partial trisomy of distal 4q is associated with thumb duplications in line with a dosage-related extension of the anterior or radial AER boundary.39 In addition, point mutations that cause ectopic radial SHH and HAND2 expression extend the anterior AER boundary to generate preaxial polydactyly type II (PPD2).40–44
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