Clinical and Genetic Characteristics in Young, Glucocorticoid-Naive Boys With Duchenne Muscular Dystrophy

Association of DMD Gene Variant Classes With Motor Outcomes in a Drug Registration Clinical Trial Setting

Background and Objectives

Duchenne muscular dystrophy (DMD) is caused by pathogenic variants of the DMD gene, leading to the loss of dystrophin. Clinical variability in DMD can complicate interpretation of interventional data in clinical trials. One source of clinical variability is allelic heterogeneity (different pathogenic variants with different effects on dystrophin protein expression). We sought to determine whether gene variant classes in clinical trial participants potentially affect clinical trial data interpretation.

Methods

We analyzed 186 vamorolone trial participants with DMD (VBP15-002/003; VBP15-004) who were 4 to <7 years old and steroid-naïve at baseline. We stratified participants into gene variant classes by either variant location in the gene affecting different gene promoters (5' [Dp427-only] vs 3' [Dp427+other isoforms]) or residual dystrophin levels (null vs possible non-null [5' gene variants, exon 44 skippable, splice site]). We evaluated associations with baseline motor outcomes and treatment response (prednisone and vamorolone).

Results

Participants with variants in ex63 and downstream (null for Dp427+Dp140+Dp71 protein isoforms) showed poorer baseline motor outcomes for time to stand from supine velocity than those with variants in ex1-44 (Dp427 only). No significant baseline differences were found between likely null and possible non-null variants. Participants with only Dp427 involvement showed significantly better treatment response for the 6-minute walk distance. Most of the comparisons of baseline motor function and treatment response were similar between variant classes.

Discussion

The large variation in baseline motor function in young, steroid-naïve patients with DMD is only minimally explained by different gene variant classes. While there is strong literature support for 3' variants leading to a more severe motor and cognitive DMD phenotype, we found this variant class under-represented in our clinical trials. This suggests that they may fail inclusion criteria (failure to follow commands; poor motor function). Subgroup analyses in DMD clinical trials at a young age range based on gene variant class may not reveal significant differences and would be relatively noninformative.

Bone Health Determinants in Ambulant Prepubertal Boys With Duchenne Muscular Dystrophy Treated With Deflazacort: Findings From a 3-Year Study

INTRODUCTION/AIMS

Duchenne muscular dystrophy (DMD) is complicated by bone fragility. This study aimed to elucidate changes in bone mineral density (BMD) and body composition over time and to explore associations with adiposity measures in DMD.

METHODS

A three-year follow-up analysis was performed of total body (TB) and lumbar spine (LS) dual-energy x-ray absorptiometry (DXA) measurements, anthropometric measures, Tanner stage and bone turnover biomarkers assessments, and the incidence of fragility fractures in 26 ambulant prepubertal DMD patients treated with deflazacort (DFZ).

RESULTS

Age at baseline was 7.7 years (interquartile range: 6-9.2). The TB BMD Z-score declined over time and was negatively related to the TB fat mass percentage and fat mass index (p < 0.05), but not to body mass index (BMI) standard deviation score (SDS). In contrast LS bone mineral apparent density (BMAD) Z-score remained stable and normal. The cumulative incidence of fragility fractures was 19.2%; DMD boys with fractures displayed a 1.5-fold higher decline of TB BMD Z-score/year (p < 0.05) and a worse adiposity profile compared to fracture-free patients. No difference was found in DFZ dose or duration between the two groups.

DISCUSSION

We observed a high incidence of fragility fractures, and identified fat tissue as a potential detrimental factor for bone health, suggesting a need for monitoring in DMD patients with excessive adiposity. Fat mass measures assessed by DXA could help to identify those at risk, enabling targeted interventions for better bone health. The co-occurrence of multiple glucocorticoid side effects might characterize patients at higher risk of fractures.

Detecting early signs in Duchenne muscular dystrophy: comprehensive review and diagnostic implications

Despite the early onset of clinical signs suggestive of Duchenne muscular dystrophy (DMD), a diagnosis is often not made until four years of age or older, with a diagnostic delay of up to two years from the appearance of the first symptoms. As disease-modifying therapies for DMD become available that are ideally started early before irreversible muscle damage occurs, the importance of avoiding diagnostic delay increases. Shortening the time to a definite diagnosis in DMD allows timely genetic counseling and assessment of carrier status, initiation of multidisciplinary standard care, timely initiation of appropriate treatments, and precise genetic mutation characterization to assess suitability for access to drugs targeted at specific mutations while reducing the emotional and psychological family burden of the disease. This comprehensive literature review describes the early signs of impairment in DMD and highlights the bottlenecks related to the different diagnostic steps. In summary, the evidence suggests that the best mitigation strategy for improving the age at diagnosis is to increase awareness of the early symptoms of DMD and encourage early clinical screening with an inexpensive and sensitive serum creatine kinase test in all boys who present signs of developmental delay and specific motor test abnormality at routine pediatrician visits.