Eosinophilic infiltration related to CAPN3 mutations: a pathophysiological component of primary calpainopathy?

To the Editor : Mutations in the gene encoding calpain-3 (CAPN3 ) cause the most prevalent form of autosomalrecessive limb girdle muscular dystrophy (LGMD), which is type 2A (LGMD2A OMIM#253600) (1), also referred to as calpainopathy. We previously reported six children with CAPN3 mutations that constitute a genetic cause of idiopathic eosinophilic myositis (EM) (2). This was subsequently confirmed in two adults (3), whereas eosinophilic infiltration is not known as a typical feature of LGMD2A. To further evaluate the relationship between calpain-3 mutations and EM/infiltration, we retrospectively studied five additional unrelated patients with idiopathic EM (one adult: PEM7 and four children: PEM8, PEM9, PEM10 and PEM11). Moreover, we evaluated eosinophil infiltration in muscle biopsies in 17 LGMD2A patients and report a case of lymphocytic myositis associated with CAPN3 mutations. Patient PEM7 presented with progressive proximal weakness and patients PEM8, PEM9, PEM10 and PEM11 presented with initial isolated increase in creatinine phosphokinase (CPK) levels in plasma. Circulating eosinophilia was identified at diagnosis in all children, but not in the adult. Repeated eosinophil counts, available for PEM10 and PEM11, showed intermittent hypereosinophilia between 9 and 12 years of age (oscillating between 601 and 1400/μl, with intermittent normal levels) and between 4 and 9 years of age (highest value 819/μl, with intermittent normal levels), respectively. For PEM10, mainly normal eosinophil counts were seen after he had developed muscle weakness at 12 years of age. After informed consent, we screened for CAPN3 mutations as described (2), based on the particular histopathological presentation. Retrospectively, a frozen muscle sample could only be obtained for PEM10, revealing complete absence of calpain-3. CAPN3 disease-causing mutations were identified either as a homozygote (PEM7 and PEM10) or compound heterozygote (PEM8, PEM9 and PEM11). Additionally, we identified two CAPN3 mutations in a 5-year-old boy (PEM12) who presented with proximal weakness of the lower limbs and a discrete focal lymphocytic infiltrate mainly composed of CD8+ T-lymphocytes, without dystrophic features, on muscle biopsy. Complete calpain-3 deficiency was found on routine immunoblot. All clinical and mutational data are compiled in Table 1. To further assess our previous hypothesis of EM being a possible histopathological manifestation of LGMD2A, we retrospectively analysed haematoxylin–eosin-stained cryosections from muscle samples of 17 patients with genetically confirmed, ‘typical’ LGMD2A (mean age of biopsy 29 years). This showed inflammatory changes, associated sparse presence of eosinophils, in 5 of the 17 patients (data not shown). The average disease duration at biopsy for patients presenting without or with eosinophils in the infiltrates was 13.9 years and 6 years, respectively. Our findings further confirm mutations in CAPN3 as a genetic cause of EM and highlight eosinophilic infiltration as an ‘early’ component (or event) of primary calpainopathy (4). Importantly, as in our previous report, inclusion criteria were based only on the particular histopathological presentation, without any identified aetiological factor (even after two muscle biopsies in patients PEM7 and PEM8). T-lymphocytes may be a key component in the eosinophilic infiltrative process (5) which together with macrophages are the main component of inflammatory lesions in the vicinity of damaged muscle fibres; they play a central role in the chemotaxis of eosinophils; and they express calpain-3. Interestingly, Baumeister et al. recently reported a case of EM caused by a homozygous gamma-sarcoglycan mutation (6),