Serum protein biomarker signature of Duchenne muscular dystrophy

Abstract

In contrast to invasive skeletal muscle biopsies and the associated complexity of tissue sampling techniques and potential detrimental side effects, the alternative application of liquid biopsy procedures has considerable advantages concerning minimal invasiveness, repeated sampling options, assay robustness and cost effectiveness. This article outlines the current status of serum biomarkers used for diagnosing and characterizing Duchenne muscular dystrophy (DMD), a primary muscle wasting disease of early childhood due to primary abnormalities in the extremely large DMD gene. Reviewed are important aspects of the discovery, characterization and diagnostic value of biofluid-based protein markers of dystrophinopathy. This includes an overview of traditional general skeletal muscle damage markers, such as creatine kinase, myoglobin and lactate dehydrogenase, which have been used for many decades in clinical applications to evaluate patients with muscular weakness. In addition, this article outlines the biochemical identification of novel biomarker candidates focusing on the usage of mass spectrometry-based proteomic surveys to establish comprehensive profiles of protein alterations in dystrophinopathy. Pathoproteomic serum markers of myonecrosis with great potential for improved patient screening, differential diagnosis, stage-specific prognosis and therapeutic monitoring include specific isoforms of muscle-derived cytosolic proteins, such as carbonic anhydrase isoform CA3 and fatty acid binding protein FABP3, as well as sarcomeric proteins, including specific isoforms of myosin light chain, myosin binding protein, troponin, and myomesin, in addition to peptide fragments derived from the giant protein titin. Biofluid-associated marker proteins of reactive myofibrosis include the extracellular matrix proteins fibronectin, osteopontin, collagen and matrix-metalloproteinases.

Figure 1.

Summary of biomarker categories used to evaluate skeletal muscle damage.

Figure 2.

Overview of major proteins that are actively or passively released from dystrophic skeletal muscle tissues. On the left is shown a flowchart of major steps that are involved in the molecular and cellular pathogenesis of dystrophinopathy ranging from the primary abnormality in the DMD gene to complex alterations in dystrophic skeletal muscles, including myonecrosis, chronic inflammation, fat substitution and reactive myofibrosis. On the right are listed major muscle-derived serum protein species that exhibit elevated levels in X-linked muscular dystrophy and have been routinely identified by biochemical and proteomic assays. The yellow boxes in the diagram highlight clinically approved protein markers that are frequently employed to evaluate skeletal muscle damage. Abbreviations used: ACT, actin; ALD, aldolase; ALT, alanine aminotransferase; AST, aspartate transaminase; c, cytosolic; CA3, carbonic anhydrase 3; CK, creatine kinase; COL, collagen; COMP, cartilage oligomeric matrix protein; ECM, extracellular matrix; ENO, enolase; FABP3, fatty acid binding protein 3; FN, fibronectin; HBDH, hydroxybutyrate dehydrogenase; LDH, lactate dehydrogenase; MB, myoglobin; MLC3, myosin light chain 3; MMP, matrix metalloproteinase; mt, mitochondrial; MYBP, myosin binding protein; MYOM3, myomesin 3; MyHC, myosin heavy chain; OPN, osteopontin; TIMP, tissue inhibitor of metalloproteinases; TnI, troponin subunit I; TTN, titin.

Figure 3.

Outline of the usefulness of serum protein biomarkers for the diagnosis, prognosis and therapeutic monitoring of dystrophinopathy.