BACKGROUND: One of the hallmarks of injured skeletal muscle is the appearance of elevated skeletal muscle proteins in circulation. Human skeletal muscle generally consists of a mosaic of slow (type I) and fast (type IIa, IIx/d) fibers, defined by their myosin isoform expression. Limited evidence suggests that muscle injury in healthy volunteers (HV) results in the appearance of muscle biomarkers from fast but not slow fibers in circulation (1). We sought to understand if this is also the case in Becker and Duchenne muscular dystrophy (BMD, DMD) by measuring troponin I isoforms from fast and slow skeletal muscle fibers in blood samples from patients.
METHODS, SAMPLES AND DEMOGRAPHICS: An ELISA that selectively measures fast and slow skeletal troponin I (TNNI2 and TNNI1) was used to measure a cross-section of patient plasma samples from healthy volunteers (N=50) and affected individuals. All samples were obtained according to local ethics policies. Plasma and serum for DMD patients (N=132) was received from the Newcastle MRC Centre Biobank for Rare and Neuromuscular Diseases. BMD samples were from a biomarker study at Binghamton University – SUNY (BMD, n=52). All samples were obtained with the appropriate patient consent.
RESULTS: TNNI2 (fast muscle ) but not TNNI1 (slow muscle) TNNI is elevated in DMD and BMD. In DMD, TNNI2 and CK decrease with age but not TNNI1. In DMD, TNNI2 and CK decrease with loss of ambulation but not TNNI1.