Does Vitamin D play a role on Msx1 homeoprotein expression involving an endogenous antisense mRNA?

Msx1 homeobox gene, a member of Msx family, has been implicated in numerous organs. Its participation was established in different events, such as morphogenetic field determinism and epithelio-mesenchymal interactions. Most of Msx1 target organs are also known for their sensitivity to Vitamin D: such as bone, tooth germ, and hair follicle. Whereas, the expression of Msx2, another member of Msx family, has been shown to be controlled by Vitamin D, no information is available for Msx1. This study aims to analyze the potential relationships between Vitamin D and Msx1 through: (1) comparative analysis of Vitamin D receptor (VDR) and Msx1 protein expression, (2) investigation of Msx1 expression in VDR null mutant mice, and (3) study of Msx1 overexpression impact on osteocalcin VDR expression in immortalized MO6-G3 odontoblasts. Results show the existence of cross-talks between Vitamin D and Msx1 regulation pathways. In odontoblastic cells, Msx1 overexpression decrease VDR expression, whereas in rickets Msx1 sense transcript expression is decreased. These cross-talks may open a new window in the analysis of rickets mineralized tissues physiopathology. In Vitamin D null mutants, the study of the natural Msx1 antisense transcript which has been recently described should be informative.

[Genetic and experimental approach to bony craniofacial growth: the role of the divergent homeobox gene Msx1]. / Pour une approche génétique et expérimentale de la croissance squelettique cranio-faciale: étude du rôle de l'homéogène divergent Msx1.

Tooth agenesis and clef palate are associated to the mutation of the Msx1 homeobox genes, highlighting the pivotal role of homeobox genes during the initial development of the craniofacial skeleton. Msx1 also controls the terminal differentiation of mineralised tissues forming cells. Recently, a Msx1 antisense RNA has been identified which inhibits Msx1 protein expression in odontoblastic cells. In order to investigate the role of Msx1 gene and its antisense RNAs during the late developmental stages of the craniofacial bone formation, the expression pattern of Msx1 protein, sense and antisense transcripts and the aspects of bone growth have been studied in post-natal normal and Msx1 knock-in mutant mice. Msx1 protein was strongly expressed in preosteoblasts of specific bone sites such as the basal mandible. At the same bone sites, bone growth was impaired or markedly decreased in knock-in mice. The comparison between the various expression patterns of Msx1 protein, sense and antisense RNAs suggests that the site-specific action of Msx1 protein on bone growth and craniofacial morphogenesis and that Msx1 protein level could be controlled by the local ratio of Msx1 sense and antisense RNAs. Regarding our experimental data and hypothesis, a clinical study of patients with MSX1 mutation will be performed in order to better characterize the abnormalities of the craniofacial skeleton growth.