Serum titin/creatinine ratio as a biomarker for discriminating disease severity in Duchenne and Becker muscular dystrophies.

Introduction: Duchenne/Becker muscular dystrophies (DMD/BMD) are inherited muscle diseases, collectively referred to as dystrophinopathy, which are characterized by progressive degeneration or loss. Although serum creatine kinase (CK) is a classical biomarker of DMD and BMD, it alone cannot clearly differentiate between severe DMD and milder BMD. Among potential biomarkers, the levels of the fragmented products of titin, a structural muscle protein, may directly reflect the degree of muscle loss in DMD and BMD. Therefore, this study measured the serum titin/creatinine (Cr) ratio and evaluated its discriminatory ability in patients with DMD and BMD. Methods: The patients with dystrophinopathy and healthy controls were included in this study. Patients were classified by the reading frame rule (out-of-frame, DMD; in-frame, BMD). Exceptional cases in which in-frame variants presented with severe symptoms or out-of-frame variants presented with mild symptoms were considered clinically DMD or clinically BMD, respectively. Serum titin levels were measured using enzyme-linked immunosorbent assay. Serum Cr levels measured on the same day were used to calculate serum titin/Cr ratios. Results: The DMD, BMD, and control groups included 89 patients (85 DMD and four clinically DMD; aged 3-32 years), 21 patients (16 BMD and five clinically BMD; aged 6-33 years), and five participants (aged 7-12 years), respectively. Although serum CK levels did not differ significantly between DMD and BMD, the serum titin/Cr ratio in DMD was approximately 10 times higher than that in BMD group ( Conclusion: Unlike serum CK, the serum titin/Cr ratio in patients with DMD was consistently higher than that in patients with BMD, regardless of age. Serum titin/Cr was shown to be a biomarker to discriminate clinical severity in patients with dystrophinopathy.