P.15.6 Desminopathy or myotilinopathy? An integrated proteomics approach for diagnosis

Protein aggregation in skeletal muscle fibers is a hallmark of myofibrillar myopathies and relevant in pathogenesis. Our previous proteomic studies deciphered details of aggregate composition and revealed specific proteomic profiles in different MFM subtypes. The aim of this study was to validate if these profiles are helpful in differential diagnosis in a patient with new mutations in two different MFM genes. The index patient presented with adult-onset weakness affecting limb and respiratory muscles. Two novel and heterozygous nucleotide exchanges in exon 1 of DES and exon 2 of MYOT, both predicted missense mutations, were identified and MFM was proven on the muscle biopsy. Protein aggregates from abnormal fibers and control samples from normally looking muscle fibers were collected by laser microdissection and analyzed by a combination of mass spectrometry (LC-MS/MS) and spectral index calculation. Our proteomic approach detected 113 proteins that were over-represented in intramyoplasmic aggregates of the index patient with a ratio >1.8 compared to control sample. The proteomic profile was consistent with desminopathy: desmin, filamin C, XIRP2, N-Rap and α B-crystallin showed the highest spectral indices of over-represented proteins. Accumulation of the desmin binding partner desmuslin also pointed to a pathogenic desmin mutation. The myotilinopathy markers plectin and obscurin were not over-represented in aggregates of the index patient and spectral index and ratio of myotilin also argued against a MFM-causing MYOT mutation. The results of our study indicate that the index patient harbors a disease-causing desmin mutation and demonstrate that our combined laser microdissection and mass spectrometric approach is a helpful new tool in differential diagnostics of MFM patients. Subtype-specific proteomic profiles can contribute to evaluate the pathogenicity of new mutations in MFM genes.