MASSETER MUSCLE MYOSIN 1C GENE EXPRESSION: RELATIONSHIPS TO GROWTH FACTORS AND INFLUENCE ON FIBER-TYPE AND SKELETAL GROWTH PATTERN

Abstract

The presence of dentofacial deformities in humans is prevalent, with distortions in jaw growth affecting about 20% of people worldwide. Least is known about the genetic etiology of malocclusions, so it is the purpose of this study to identify genetic factors which influence jaw growth and examine how their expression correlates with vertical and sagittal malocclusions. Myostatin (MSTN) is a negative growth regulator which functions to inhibit excessive growth of muscle. Mice in which this gene was absent exhibited increased muscle mass and altered skeletal form, indicating the role of genetic control on muscle mass and skeletal phenotype. IGF-1 is an anabolic growth factor which acts in coordination with growth hormone to promote myofiber regeneration and hypertrophy. A third gene of interest, myosin 1C is a class I myosin which functions to regulate glucose uptake via facilitated glucose transporter 4 (GLUT 4) in insulin and contraction stimulated pathways. Given its role in muscle metabolism in addition to its association to MYO1H, a paralogous protein which has been associated with Class III malocclusions, the goal of this study was to elucidate the possible role of MYO1C in mediating the metabolic effects of growth factors on fiber size and phenotype and subsequently skeletal form. The aims of this study are as follows: quantify MYO1C expression in masseter muscle from individuals of different occlusal groups; compare MYO1C expression to myosin heavy chain gene expression and fiber percent occupancy by sagittal and vertical malocclusion classes; compare expression of MSTN and IGF-1 to MYO1C to evaluate if a correlation exists; evaluate the expression of MYO1C and MYO1H to identify differences in proportions among malocclusion types. Human masseter muscle samples were provided by oral surgeons at the University of Lille, France from subjects undergoing bilateral sagittal split osteotomy surgery for treatment of malocclusion. Muscle RNA was isolated with TRIzolTM reagent, digested with DNase I, re-isolated with RNAqueous® and quantified in 42 samples by triplicate assays of TaqMan® real time PCR using RNA-to-CTTM 1-Step reagent and an Applied Biosystems Step One Plus instrument. A 25ng amount of skeletal muscle standard was selected as a reference calibrator and relative expression quantities of MYO1C were determined by the comparative threshold cycle (CT) method. The relative quantity (RQ) of expressed RNA is calculated from the CT value. Fiber type, area and percent occupancy had been determined previously for 39 of 42 masseter muscle samples used in this study. Expression of the genes for MyHC-I/β, IIA, IIX, perinatal (neonatal) and α (atrial) in another 39 of the 42 masseter muscle samples had also been previously quantified by RT-PCR for use in correlation analyses with MYO1C expression. Based on the results collected, the final conclusions were drawn: * MYO1C expression is greater in open and normal versus deep bites and Class III versus Class II malocclusions. The highest expression is seen with Class III open bites and the lowest with Class II normal malocclusions. * Class II deep and normal bites showed high correlation between MYO1C expression and atrial and neonatal/atrial MHC gene expression, which require increased MYO1C for oxidative metabolism. They exhibited a negative correlation to type I MHC gene expression and percent occupancy, as deep bites have fewer type I fibers. * Class III open bites had high correlation between MYO1C and neonatal MHC gene expression and low correlation to type II MHC gene expression due to increased percentage of high oxidative type I fibers in open bites and diminished type II fibers. * Correlations between MYO1C and hybrid I/II fiber percent occupancy was unpredictable by occlusal group due to transitional nature of fibers. * MYO1C expression is correlated to growth factor expression in Class III but not in Class II malocclusions, indicating its potential interactive role in masseter metabolism in the Class III group. * Class I myosins are highly expressed in Class III open bites. * Class II deep bites exhibited the lowest expression of MYO1H, indicating the masseters are less regulated by class I myosins. * MYO1H is closely linked with type II MHC gene expression, while MYO1C has a close association with types I and neonatal MHC gene expression. * An association exists between class I myosins and both type I and neonatal/atrial fiber percent occupancy. * A greater sample size of approximately 102 would permit an accurate test for significant differences in future studies.