Cleft palate (CP) is a disease where there is an opening or split in the palate (the roof of the mouth). CP has been reported in 1 to 2 of every 1000 births worldwide and is the most frequent craniofacial birth defect in humans. CP phenotypes observed in humans include various subtypes of CP termed as complete CP, incomplete CP, and submucosal CP (a cleft occurs only in the muscles of the soft palate). Since the palate consists of both a bone-lined hard palate and a bone-free soft palate, incomplete CP includes the mildest forms of CP, which are soft palate cleft and bifid uvula. The soft palate mainly consists of muscle, and is crucial for proper speech, and swallowing. Since CP can contribute to facial deformities and problems (Fig. 1), such as difficulty with swallowing, nasal voice, dental malocclusion, and chronic ear infections, identifying the etiology of CP is important.

Fig. 1. Palatal malformations contribute to facial deformities and functions in humans.

Craniofacial deformities, including various degrees of CP and small mandible, are known to be associated with chromosome 22q11.2 deletion syndrome. It is the most frequent micro-deletion syndrome, affecting approximately 1 in 4000 live births. The T-box transcription factor gene,TBX1, is considered the causative gene for 22q11.2 deletion syndrome, which manifests as DiGeorge syndrome or velocardiofacial syndrome.TBX1mutations have been also identified in patients with nonsyndromic CP.Tbx1knockout mice (Tbx1^KOmice; deletion ofTbx1gene in mice) largely exhibit features of 22q11.2 deletion syndrome found in humans and present various types of the CP phenotype, indicating thatTBX1is a disease-causing gene responsible for phenotypically variable forms of nonsyndromic CP and 22q11.2 deletion syndrome. The phenotypic variation found in CP inTbx1^KOmice strongly suggests that multiple stochastic factors affect the palatal phenotype ofTbx1^KOmice.

In this study, we performed gene profiling of developing palatal shelves isolated from wild type andTbx1^KOmice embryos to examine the function of the TBX1 protein in palatogenesis. The deletion ofTbx1gene in mice results in the formation of different subtypes of CP and small mandible, similar to the phenotypes observed in humans. A small mandible leads to backward displacement of the tongue and can prevent the palatal shelves from closing, resulting in a CP. The palatal shelves are composed of epithelium and mesenchyme (Fig. 2). Gene ontology analysis indicated that genes associated with the development of the muscle, nervous system, biomineral tissue, and collagen biosynthesis were dysregulated inTbx1^KOpalatal shelves (Fig. 2). Interestingly, genes associated with human CP, specifically, myosin heavy chain 3 (Myh3), nebulin (Neb), and gamma-aminobutyric acid type A receptor beta1 subunit (Gabrb3) were downregulated inTbx1^KOpalatal shelves.

Fig. 2. Schematic showing the multiple factors affecting palatal phenotype in Tbx1 knockout mice.

These findings highlight a previously unknown role of TBX1 in palatal development and identify novel connections betweenTbx1and disease-causing genes of CP. We previously reported that the deletion ofTbx1results in the abnormal epithelial fusion between the palatal shelves and the mandible, which induces CP by inhibiting the elevation of the palatal shelves (Funato et al.,Hum Mol Genet, 2012). The various degrees of CP phenotypes ofTbx1^KOmice could be induced by the pathogenic adhesion-separation of the oral epithelium, together with compromised development of palatal shelves and small mandible (Fig. 2). The contributions of multiple factors may result in various CP phenotypes, including submucosal CP and soft palate cleft present in 22q11.2 deletion syndrome.

Noriko Funato
Department of Signal Gene Regulation, Tokyo Medical and Dental University (TMDU), Japan

Publication

Deletion of the T-box transcription factor gene, Tbx1, in mice induces differential expression of genes associated with cleft palate in humans.
Funato N, Yanagisawa H
Arch Oral Biol. 2018 Nov

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