Masseter muscle is one of the major muscles of mastication, and is comprised of actin and myosin myofibrils organized into sarcomeric contractile units. Structurally, sarcomeres are repeating portions of myofibrils between neighboring Z-lines (a.k.a. Z-disc, Z-band). The Z-line or Z-disc is composed of non-contractile proteins that provide mechanical stability to the sarcomere. One of the proteins of Z-disc is alpha-B-crystallin, a protein product of the gene CRYAB. Together with several other proteins of the Z-disc, CRYAB gene has been found to be up-regulated in Actn3 knock-out mice. In addition, CRYAB is suspected to be a pain mediator gene, having similar structure and function to CRYAA (alpha,A-crystallin) identified as one of the candidate genes from the Pain Research Panel, previously investigated in the Orofacial Pain: Prospective Evaluation and Risk Assessment (OPPERA) Study. Finally, in a microarray of global gene expression CRYAB was increased in subjects with facial asymmetry. We have examined CRYAB expression in masseter muscle of 64 orthognathic surgery patients to determine associations with skeletal malocclusions. Salivary DNA was genotyped for a single nucleotide polymorphism (SNP) for ACTN3 (rs1815739) and masseter muscle RNA isolated from an orthognathic surgery patient population. These genotyping and expression data have been used to identify differences in CRYAB expression in sub-groups of our patient population with Class II and III, normal, open and deep bite malocclusions who are null for ACTN3. In addition, we evaluated expression levels of CRYAB in patients with TMD-related myalgia. We found that relative quantities of CRYAB expression differed very significantly between sexes (p=0.005). ANOVA comparison between all subjects with and without TMD-myalgia indicated that males with TMD-myalgia had significantly greater (p<0.02) expression than other groups. An unpaired t-test showed that with TMD-related myalgia, CRYAB expression was significantly higher (p=0.03) in males than in females. ANOVA comparison between sexes with Class II and Class III malocclusions showed greater expression of CRYAB (p=0.005) in males with Class II. Expression was likewise greater in males with Class III malocclusion than in females with Class III (p<0.01). Among different age groups, subjects 25 years of age or younger had significantly (p value=0.025) increased expression of CRYAB gene. There were no significant differences for genotypes or facial asymmetry. / Oral Biology