Background
Duchenne muscular dystrophy (DMD) is caused by DMD mutations leading to dystrophin loss. Full length Dp427 is the primary dystrophin isoform expressed in skeletal muscle and is also expressed in the central nervous system (CNS). Two shorter isoforms, Dp140 and Dp71, are highly expressed in the CNS. While a role for Dp140 and Dp71 on DMD CNS co-morbidities is well known, relationships between lack of Dp140 and Dp71 and DMD motor outcomes are not. We have conducted a series of investigations addressing this.
Methods
Functional outcome data from 387 DMD boys aged 4.0-15.4 years was subdivided by DMD mutation expected effect on isoform expression; Group 1 (Dp427 absent, Dp140/Dp71 present, n=201); group 2 (Dp427/Dp140 absent, Dp71 present, n=152); and group 3 (Dp427/Dp140/Dp71 absent, n=34). Relationships between isoform group and North Star ambulatory assessment (NSAA) scores, 10m walk/run and rise times were explored using regression analysis. We used Capillary Western immunoassay (Wes) analysis to study Dp427, Dp140 and Dp71 production in wild-type and DMD skeletal muscle and myogenic cultures. Grip strength was studied in wild-type, mdx (Dp427 absent, Dp140/Dp71 present), mdx52 (Dp427/Dp140 absent, Dp71 present) and DMD-null (lacking all isoforms) mice.
Results
In DMD boys, we found a strong association between isoform group and motor function. In DMD boys, mean NSAA scores at 5 years of age were 6.1 points lower in group 3 than group 1 (p<0.01) and 4.9 points lower in group 3 than group 2 (p=0.05). Mean peak NSAA scores were 4.0 points lower in group 3 than group 1 (p<0.01), 2.4 points lower in group 3 than group 2 (p=0.09) and 1.6 points lower in group 2 than group 1 (p=0.04).
Average grip strength in peak force at 3 months of age was higher in mdx than mdx52 mice (p=0.01).
Dp427, but not Dp71, was produced in normal skeletal muscle; low levels of Dp71 were detected in DMD skeletal muscle. High Dp71 levels were present in wild-type and DMD myogenic cultures.
Conclusions
DMD boys lacking Dp140 and Dp140/Dp71 displayed worse motor function with a cumulative effect of isoform loss. DMD mouse models lacking Dp427 and Dp140 had lower grip strength than those lacking Dp427 but not Dp140. Our results highlight the importance of considering the effects of dystrophin isoform loss on DMD motor impairment, with important implications for understanding the complex relationship between brain and muscle function in DMD and patient stratification for clinical trials.
Promoting the use of Motor Function Measure (MFM) as outcome measure in patients with Duchenne Muscular Dystrophy (DMD) treated by corticosteroids