STAC3D Against the Odds: Characterizing the Musculoskeletal Phenotype of STAC3 Gene Disorder

INTRODUCTION: STAC3 Disorder (STAC3D) is a rare genetic condition characterized by musculoskeletal abnormalities, myopathic facies, palatal anomalies, micrognathia, short stature, and an increased susceptibility to malignant hyperthermia. It was initially described in the Lumbee Native Americans of North Carolina, though it is now found globally. This study aims to characterize the musculoskeletal phenotype of STAC3D to aid in its diagnosis and clinical management. METHODS: A retrospective cohort study of STAC3D patients from 2014 to 2025 at a single institution was conducted. Patients diagnosed with STAC3D were included. Data collected included demographics, genetic testing, musculoskeletal conditions, and their management. Descriptive and comparative statistics were utilized, with rates reported as percentages and variables expressed as median and means with standard deviation. RESULTS: A total of 26 patients met the selection criteria and were included; 23 patients had musculoskeletal data available for analysis. The median age at diagnosis and time of data collection was 22 days and 4 years (25 d to 55 y), respectively (n=26). The median height and weight percentiles are 1%. In all, 83% (n=19/23) of patients had scoliosis, 48% (n=11/23) had hyperkyphosis, 61% (n=14/23) had joint contractures (8 upper extremity, 3 lower extremity, 3 unspecified), and 61% (n=14/23) had a congenital foot deformity (13 clubfoot deformities, 1 bilateral vertical talus). In all, 57% (n=13/23) of all STAC3D patients required at least one orthopaedic procedure, 78% (n=18/23) underwent at least one non-orthopaedic surgery, and 87% (n=20/23) had at least one general anesthesia event. CONCLUSION: STAC3D is associated with many musculoskeletal problems, including spinal and congenital foot deformities, joint contractures, and other less frequent pathologies. This study is the first to assess the rates of all musculoskeletal conditions in STAC3D patients. Our cohort represents the largest North American cohort and the second largest global cohort of STAC3D patients in the current literature. The results of our study aim to facilitate early diagnosis and expectation management of STAC3D. LEVEL OF EVIDENCE IV: This is a retrospective cohort study aimed at describing the musculoskeletal conditions associated with STAC3D.