Craniofacial and trunk skeletal muscles are derived from different progenitor populations during development. Trunk skeletal muscles contain mostly multinucleated myofibers that are formed through myoblast fusion. However, myoblast fusion in craniofacial muscles and its molecular regulation are not well understood. Recent studies revealed that genetic mutations in MYOMAKER and MYOMIXER fusogens in humans cause Carey-Fineman-Ziter Syndrome (CFZS), characterized by facial weakness and lower jaw deformity. Previous studies in zebrafish revealed that knockout of myomaker and myomixer resulted in deformed craniofacial formation. To establish the causal connection between loss of fusogen function and craniofacial deformities, we characterized myoblast fusion in zebrafish craniofacial muscles. Our results demonstrate that myomaker and myomixer are expressed in both slow and fast craniofacial muscles, and loss of these fusogens results in defects in craniofacial myoblast fusion. Interestingly, unlike trunk muscles of early embryos and larvae that show fast-fiber-specific fusogen expression and fusion while slow muscle fusion only occurs at 3 weeks post-fertilization, both slow and fast craniofacial muscles fuse as early as 3 days post-fertilization. Collectively, this study demonstrates that myomaker and myomixer are expressed in both slow and fast-twitch craniofacial muscles and are essential for myoblast fusion and the development of craniofacial muscles.

ObjectiveCongenital facial weakness (CFW) disorders are a heterogeneous group of rare conditions, that present at birth, with reduced facial movement, and mask-like facies. This study utilized a multimodality approach to examine the craniofacial and intraoral phenotypes among CFW disorders: Moebius syndrome (MBS), Hereditary Congenital Facial Palsy (HCFP), β-tubulin isotype 3 syndrome (CFEOM3A-TUBB3), Carey-Fineman-Ziter syndrome (CFZS), and a group of rarer disorders (Other).DesignProspective cohort study.Setting: Dental clinic.Participants: Sixty individuals (sex ratio 1:1, mean age 26.2 ± 17.5 years) with a diagnosis of CFW.Interventions: Deep clinical craniofacial and dental phenotyping, three-dimensional facial surface and cone-beam computed tomography scans, and cephalometric and geometric morphometric analyses.ResultsCFEOM3A-TUBB3, MBS, and CFZS groups had the highest prevalence of craniofacial anomalies; HCFP individuals were least affected. CFEOM3A-TUBB3 had a higher prevalence of short lower face (75%), poor oral hygiene (100%)/decay (75%), and Class II malocclusion (87.5%). Moebius syndrome was associated with lagophthalmos (90.9%), tongue fissures (72.4%), tight/small oral orifice (51.7%), and tongue fasciculations (50%). Carey-Fineman-Ziter syndrome had oblong facial shape (100%), downward lip commissures (100%), and abnormal hearing (60%). Moderate-severe decay/gingivitis correlated with restricted oral orifice, common among patients with facial animation/sling surgery. Morphologically, the CFW cohort had a relatively small craniofacial centroid size, anthropometric measurements, and distinct craniofacial shapes for each subtype.ConclusionsCongenital facial weakness can result in abnormal craniofacial development in addition to the loss of facial movement. Multimodality phenotypic characterization of CFW disorders elucidated key clinical findings and distinct craniofacial shape segregation among the different groups.

Congenital facial weakness (CFW) encompasses a heterogenous set of rare disorders presenting with decreased facial movement from birth, secondary to impaired function of the facial musculature. The aim of the present study is to provide an analysis of subject-reported oral health-related quality of life (OHRQoL) in congenital facial weakness (CFW) disorders. Forty-four subjects with CFW and age- and sex- matched controls were enrolled in an Institutional Review Board (IRB)-approved study. Demographic data, medical and surgical history, comprehensive oral examination, and the Oral Health Impact Profile (OHIP-14) were obtained. Compared to unaffected controls, subjects with CFW had higher OHIP-14 scores overall (mean ± SD: 13.11 ± 8.11 vs. 4.46 ± 4.98, p < 0.0001) and within five of seven oral health domains, indicating decreased OHRQoL. Although subjects with Moebius syndrome (MBS) were noted to have higher OHIP-14 scores than those with Hereditary Congenital Facial Paresis (HCFP), there was no significant correlation in OHIP-14 score to age, sex, or specific diagnosis. An increase in OHIP-14 scores in subjects was detected in those who had undergone reanimation surgery. In conclusion, subjects with CFW had poorer OHRQoL compared to controls, and subjects with MBS had poorer OHRQoL than subjects with HCFP. This study provides better understanding of oral health care needs and quality of life in a CFW cohort and suggests that guidelines for dental treatment are required.

Carey Fineman Ziter Syndrome (CFZS) is a rare autosomal recessive disease caused by mutations in the MYMK locus which encodes the protein, myomaker. Myomaker is essential for fusion and concurrent myonuclei donation of muscle progenitors during growth and development. Strikingly, in humans, MYMK mutations appear to prompt myofiber hypertrophy but paradoxically, induce generalised muscle weakness. As the underlying cellular mechanisms remain unexplored, the present study aimed to gain insights by combining myofiber deep-phenotyping and proteomic profiling. Hence, we isolated individual muscle fibers from CFZS patients and performed mechanical, 3D morphological and proteomic analyses. Myofibers from CFZS patients were ~ 4x larger than controls and possessed ~ 2x more myonuclei than those from healthy subjects, leading to disproportionally larger myonuclear domain volumes. These greater myonuclear domain sizes were accompanied by smaller intrinsic cellular force generating-capacities in myofibers from CFZS patients than in control muscle cells. Our complementary proteomic analyses indicated remodelling in 233 proteins particularly those associated with cellular respiration. Overall, our findings suggest that myomaker is somewhat functional in CFZS patients, but the associated nuclear accretion may ultimately lead to non-functional hypertrophy and altered energy-related mechanisms in CFZS patients. All of these are likely contributors of the muscle weakness experienced by CFZS patients.

Myocyte fusion is a pivotal process in the development and regeneration of skeletal muscle. Failure during fusion can lead to a range of developmental as well as pathological consequences. This review aims to comprehensively explore the intricate processes underlying myocyte fusion, from the molecular to tissue scale. We shed light on key players, such as the muscle-specific fusogens - Myomaker and Myomixer, in addition to some lesser studied molecules contributing to myocyte fusion. Conserved across vertebrates, Myomaker and Myomixer play a crucial role in driving the merger of plasma membranes of fusing myocytes, ensuring the formation of functional muscle syncytia. Our multiscale approach also delves into broader cell and tissue dynamics that orchestrate the timing and positioning of fusion events. In addition, we explore the relevance of muscle fusogens to human health and disease. Mutations in fusogen genes have been linked to congenital myopathies, providing unique insights into the molecular basis of muscle diseases. We conclude with a discussion on potential therapeutic avenues that may emerge from manipulating the myocyte fusion process to remediate skeletal muscle disorders.