Muscle ultrasonography in costello syndrome: unveiling new clinical insights of a complex muscular phenotype

Abstract Background Costello syndrome (CS) is a rare genetic disorder within the spectrum of RASopathies, caused by activating mutations in the HRAS gene, leading to constitutive dysregulation of the RAS/MAPK signalling pathway. Among its multisystemic manifestations, a distinctive musculoskeletal involvement is frequently observed with reduction in muscle force, pain of musculoskeletal origin and muscular hypotrophy. Moreover, abnormal histological findings with variability in size, atrophy, and prevalence of type 2-fibers have been detected on anecdotal muscular biopsies of affected individuals. Methods We conducted a monocentric study on 20 individuals (13 females, 7 males; median age 19 years) with molecularly confirmed CS diagnosis, recruited between December 2019 and December 2022. Muscle ultrasonography (US) was performed to study muscle architecture and detect fibroadipose infiltration (FAI), a key determinant of muscle quality and functional performance on lumbar paravertebral, quadriceps (vastus medialis, vastus lateralis, rectus femoris), and gastrocnemius muscles. FAI was graded according to the Heckmatt scale (I–IV). Nutritional and metabolic assessments were conducted including macronutrient intake (3 days diet recall) and resting energy expenditure (REE). To investigate the pathogenic contribution of HRAS dysregulation to skeletal muscle development, preclinical studies were performed in engineered mouse myoblasts expressing HRAS p.Gly12Ser and p.Gly13Cys variants. Results FAI was detected in 100% of participants in at least one muscle, most commonly in lumbar paravertebral muscles (85%) and vastus lateralis (70–75%); no grade IV involution cases occurred. FAI severity showed no association with age, physical activity, nutritional/biochemical parameters, or REE, but displayed district‑specific correlations with skeletal anomalies (tight Achilles tendon and coxa valga). In vitro, myoblasts expressing HRAS mutants exhibited increased proliferation, impaired myogenic differentiation/fusion, and lipid droplet accumulation with elevated cholesteryl esters compared with controls. Conclusions US shows potential as a non-invasive tool for assessing and monitoring muscular health in CS and related RASopathies, though further data are needed to support its use. The pattern supports HRAS‑mediated metabolic dysfunction as a primary driver of FAI, warranting multicenter longitudinal studies with standardized quantitative metrics integrated with functional testing in the view of future therapeutic trials.