P.56 Urine titin as a novel biomarker for Duchenne muscular dystrophy

Abstract

Duchenne muscular dystrophy (DMD) is a severe muscular disease caused by a frame-shift mutation in the DMD gene. DMD symptom onset occurs in early childhood (2-3 years old) and is progressive. Long-term clinical trials in a pediatric population are often required to evaluate novel therapies. Biomarkers are important tools to enable understanding disease progression and determining acute pharmacodynamic effect and how the treatment can correlate with clinical efficacy and the success rate of clinical trials. Blood Creatinine Kinase (CK) is widely used as muscle injury marker. However, levels can be confounded by the stress response elicited by blood sampling in pediatric patients. Urine samples are easier to collect and less invasive, therefore allowing for more frequent monitoring of analytes of interest over time. Titin is a muscle cell protein which contributes to muscle relaxation and contraction. Proteomic bioanalysis in urine samples revealed that fragmented titin is significantly elevated in patients with DMD. Here, we investigate whether urine titin can be used as a pharmacodynamic (PD) or progression biomarker for DMD. In mdx mice, a DMD model with a mutation on exon 23 of the <i>DMD</i> gene, we show that urine titin is significantly higher than in WT mice between 7 and 24 weeks of age. Furthermore, mdx mice treated with peptide-conjugated phosphorodiamidate morpholino oligomer (PPMO), a cell penetrating oligomer that triggers exon 23 skipping, demonstrated a dramatic decrease in both blood CK and urine titin. All PPMO-treated mice demonstrated recovery of dystrophin expression, suggesting that decreases in urine titin levels reflect rescue of dystrophin levels. A quantitative immunoassay was developed and demonstrated that urine titin in DMD patients is dramatically higher than in healthy volunteers, and declines slowly with age, even though urine creatinine levels do not change. The results from this study, along with previous data, suggest that urine titin may be a useful disease progression marker and PD marker for therapies designed to restore dystrophin levels.