Background: Becker muscular dystrophy (BMD) is a rare and heterogeneous form of dystrophinopathy caused by expression of altered dystrophin proteins, as a consequence of in-frame genetic mutations. The majority of the BMD biomarker studies employ targeted approaches and focus on translating findings from Duchenne Muscular Dystrophy (DMD), a more severe disease form with clinical similarities but caused by out-of-frame mutations in the dystrophin gene. Importantly, DMD therapies assume that disease progression can be slowed by promoting the expression of truncated dystrophin comparable to what occurs in BMD patients. In this study, we explore similarities and differences in protein trajectories over time between BMD and DMD serum, and explore proteins related to motor function performance.

Methods: Serum samples collected from 34 BMD patients, in a prospective longitudinal 3-year study, and 19 DMD patients, were analyzed by using Data Independent Acquisition Tandem Mass Spectrometry (DIA-MS). Subsequent normalization, linear mixed effects model was employed to identify proteins associated with physical tests and dystrophin expression in skeletal muscle. Analysis was also performed to explore the discrepancy between DMD and BMD biomarker abundance trajectories over time.

Results: Linear mixed effects models identified 20 proteins with altered longitudinal signatures between DMD and BMD, including creatine kinase M-type (CKM) pyruvate kinase (PKM), fibrinogen gamma chain (FGG), lactate dehydrogenase B (LDHB) and alpha-2-macroglobulin (A2M). Furthermore, several proteins related to innate immune response were associated with motor function in BMD patients. In particular, A2M displayed an altered time-dependent decline in relation to dystrophin expression in the tibialis anterior muscle.

Conclusions: Our study revealed differences in the serum proteome between BMD and DMD, which comprises proteins involved in the immune response, extracellular matrix organization and hemostasis but not muscle leakage proteins significantly associated with disease progression in DMD. If further evaluated and validated, these biomarker candidates may offer means to monitor disease progression in BMD patients. A2M is of particular interest due to its association with dystrophin expression in BMD muscle and higher abundance in DMD patients in comparison to BMD. If validated, A2M could be used as a pharmacodynamic biomarker in therapeutic clinical trials aiming to restore dystrophin expression.