A genome-wide association analysis of loss of ambulation in dystrophinopathy patients suggests multiple candidate modifiers of disease severity

The major determinant of disease severity in patients with severe Duchenne muscular dystrophy (DMD) or milder Becker muscular dystrophy (BMD) is whether their dystrophin gene (DMD) mutation disrupts the mRNA reading frame or allows expression of a partially functional protein. However, even in the complete absence of dystrophin, variability in disease severity is observed, and candidate gene studies have implicated several genes as possible modifiers. Our previous genome-wide association study (GWAS) for age at loss of ambulation (LOA) in DMD provided confirmation for the role of genetic modifiers of TGF-{beta} signaling in disease progression. Here we present the largest genome-wide search to date for loci influencing disease severity in DMD patients. Availability of subjects for such studies is still quite limited, leading to modest sample sizes, which present a challenge for GWAS design. We have therefore taken special steps to minimize heterogeneity within our dataset at the DMD locus itself, taking a novel and conservative approach to mutation classification to effectively exclude the possibility of residual dystrophin expression. We have also utilized statistical methods that are well adapted to smaller sample sizes, including the use of a novel linear regression-like residual for time to ambulatory loss and the application of evidential statistics for the GWAS approach. Based on the resulting sample size of N = 419 patients, we have identified multiple potential candidate genetic modifier loci. In a companion paper to this one, we use a systematic bioinformatic pipeline to implicate specific genes within these loci as potential DMD modifiers.