Myosin heavy chain expression in the human masseter muscle and vertical facial form.

Abstract Masseter muscle structure and function have been implicated in the development of extremes of vertical facial form (VFF), Long Face Syndrome (LFS) and Short Face Syndrome (SFS). The overall aims of this project were to investigate the relationship between masseter muscle structure and VFF and to establish whether certain structural features related to an adaptive response in the muscle. Masseter muscle biopsies were collected from 38 consenting adults and categorised into LFS (n=13), Normal VFF (n=15) and SFS (n=10), according to an objective cephalometric assessment developed using machine-learning techniques. The mRNA and protein expression of six myosin heavy chain (MyHC) isoforms were analysed quantitatively using Northern and Western analyses. Protein expression was analysed qualitatively using immunohistochemistry. The dental occlusion was analysed and the number of occlusal contacts was totalled. Statistical analysis of the data was performed using multivariate multilevel modelling (MLwiN). The promoter region of the perinatal MyHC gene was investigated using the pSEAP2 reporter vector transfected into C2C12 cells. The statistical modelling demonstrated that the most important determinant of masseter muscle structure was the occlusion. The VFF of a subject appeared to alter the adaptive response of the masseter muscle to changes in the occlusion, with the SFS individuals reacting differently from the LFS and Normal VFF individuals. The perinatal MyHC demonstrated a strong inverse relationship with the occlusion. The promoter region of the perinatal MyHC gene resided in a 2kb segment upstream of the start of the 5’ coding region. This region contained a consensus myogenin binding site. In conclusion, the occlusion is a major determinant of masseter muscle structure, although the VFF of an individual has a strong effect on the adaptive response of the muscle. The MyHC isoform most associated with the occlusion was the perinatal MyHC, one of the main activators of which is myogenin.Masseter muscle structure and function have been implicated in the development of extremes of vertical facial form (VFF), Long Face Syndrome (LFS) and Short Face Syndrome (SFS). The overall aims of this project were to investigate the relationship between masseter muscle structure and VFF and to establish whether certain structural features related to an adaptive response in the muscle. Masseter muscle biopsies were collected from 38 consenting adults and categorised into LFS (n=13), Normal VFF (n=15) and SFS (n=10), according to an objective cephalometric assessment developed using machine-learning techniques. The mRNA and protein expression of six myosin heavy chain (MyHC) isoforms were analysed quantitatively using Northern and Western analyses. Protein expression was analysed qualitatively using immunohistochemistry. The dental occlusion was analysed and the number of occlusal contacts was totalled. Statistical analysis of the data was performed using multivariate multilevel modelling (MLwiN). The promoter region of the perinatal MyHC gene was investigated using the pSEAP2 reporter vector transfected into C2C12 cells. The statistical modelling demonstrated that the most important determinant of masseter muscle structure was the occlusion. The VFF of a subject appeared to alter the adaptive response of the masseter muscle to changes in the occlusion, with the SFS individuals reacting differently from the LFS and Normal VFF individuals. The perinatal MyHC demonstrated a strong inverse relationship with the occlusion. The promoter region of the perinatal MyHC gene resided in a 2kb segment upstream of the start of the 5’ coding region. This region contained a consensus myogenin binding site. In conclusion, the occlusion is a major determinant of masseter muscle structure, although the VFF of an individual has a strong effect on the adaptive response of the muscle. The MyHC isoform most associated with the occlusion was the perinatal MyHC, one of the main activators of which is myogenin.