Discussion

We describe a young man with marfanoid features and an initial erroneous diagnosis of limb-girdle muscle dystrophy 2a (or calpainopathy), based upon misleading results from the muscle biopsy's immunohistochemistry and Western blot analysis, and because both calpain and Marfan genes are located on chromosome 15q15–21.1. Calpainopathy can show wide phenotypic variation and muscle biopsy interpretation can be problematic due to technical factors (such as rapid degradation of calpain 3) or secondary calpain 3 deficiency associated with other types of muscular dystrophies.1 After a negative mutation screen of the CAPN3 gene, a genome-wide high-resolution SNP array identified an inframe deletion of exons 48–49 in the DMD gene, which secured a diagnosis of Becker's muscular dystrophy. This case highlights several important practical points that warrant brief discussion.

Becker's and Duchenne's muscular dystrophy are allelic disorders caused by mutations in the DMD gene. Duchenne's muscular dystrophy is a severe childhood-onset disease that is fatal in early adulthood. In Becker's muscular dystrophy, the onset is later, the phenotype milder and affected men often remain ambulant until at least the third decade. Some men with Becker's muscular dystrophy remain asymptomatic into late adulthood (reviewed in2).

The DMD gene is located on the X chromosome and comprises 79 exons. The encoded protein, dystrophin, localises to the inner muscle fibre sarcolemma, where it binds to several proteins, forming the dystrophin-associated protein complex.2 DMD gene mutations affect ∼1 in 3500 live male births and new mutations in the DMD gene are not infrequent. Mutations in the DMD gene most commonly comprise inframe deletions (∼65%)2 and 70% of deletion mutations are clustered in the distal region between exons 45 and 51.3 Importantly, mutations in this region are three times more likely to be new mutations arising later in embryonic development. This situation results in somatic mosaicism and therefore may occur as sporadic cases with a low risk of recurrence (∼4%).3

In Becker's muscular dystrophy, the deletions are usually inframe and predominantly between exons 45 and 55.2 Most people with exon 45–51 deletions manifest mild phenotypes or even remain without symptoms into late adulthood.2 Reports of cases harbouring the same mutation and manifesting varied phenotypes suggest additional influences, such as epigenetic factors.2 For instance, a person with the same exon deletion and similar age at presentation as our patient had cardiomyopathy as the only manifestation of dystrophinopathy.4 Although not all persons with Becker's dystrophy develop cardiac involvement, cardiac complications frequently occur and can vary from mild electrocardiographic abnormalities to dilated cardiomyopathy developing, on average, in mid-adulthood.2

Cramps and myalgia, often induced by exercise or cold, occur in a third of people with Becker's muscular dystrophy.2 As in our patient, teenagers with Becker's dystrophy frequently develop exercise-induced muscle cramps (often involving calves), myalgia or fatigue without weakness as their first symptoms.2 Exercise-induced muscle cramps may be due to abnormal neuronal nitric oxide (nNOS)-mediated regulation of muscle blood flow during exercise.2 Although exons 42–45 of DMD encode spectrin-like repeats that are necessary for securing nNOS to the sarcolemma—and thereby facilitating the regulation of vasodilation during muscular activity—persons with deletions distal to this domain may also experience myalgia or cramps, with or without myoglobinuria (reviewed in2).

Muscle biopsies in Becker's dystrophy can show dystrophin staining patterns that vary between 50% and 100% compared with controls.2 In addition, altered dystrophin expression (in dystrophinopathies) can cause substantial secondary reductions of many of the dystrophin-associated complex components including α-, β- and γ-sarcoglycan expression.2 To complicate matters further, muscle biopsies from cases with molecularly confirmed sarcoglycanopathies, either α-, β-, γ- or δ-sarcoglycanopathy, can show a secondary reduction in the expression levels of the remaining normal sarcoglycan proteins as well as dystrophin and β-dystroglycan.5 Muscle biopsy results therefore may not accurately predict genotype. Indeed, even in specialised laboratories, the quantified analysis of calpain 3 protein expression levels lack precision; for example, of 57 cases with CAPN3 mutations, Western blotting of muscle calpain 3 was accurate in 23%.1 Furthermore, altered/reduced expression of more than one protein suggests involvement of secondary mechanisms.5

Clinical pointers towards molecularly-confirmed calpainopathy—but absent in our patient—are scapular winging and weakness of elbow flexors.1 Typically in calpainopathy, the posterior leg muscles (hip extensors and knee flexors) as well as hip adductors are the most severely affected muscle groups.1 The person adopts a characteristic broad-based stance with locked knees (relatively preserved knee extensors) and a pronounced lumbar lordosis,1 which was absent in our patient. Joint contractures (ankle dorsiflexion, finger-, wrist- and elbow flexors) occur in 68% with calpainopathy and may be severe.1 In contrast to Becker's muscle dystrophy, calf pseudohypertrophy is uncommon and cardiac function, including ECG and echocardiography, is essentially normal in calpainopathy.1