Mitochondrial processes are impaired in hereditary inclusion body myopathy

Hereditary inclusion body myopathy (HIBM) is an adult onset, slowly progressive distal and proximal myopathy. Although the causing gene, GNE, encodes for a key enzyme in the biosynthesis of sialic acid, its primary function in HIBM remains unknown. To elucidate the pathological mechanisms leading from the mutated GNE to the HIBM phenotype, we attempted to identify and characterize early occurring downstream events by analyzing the genomic expression patterns of muscle specimens from 10 HIBM patients carrying the M712T Persian Jewish founder mutation and presenting mild histological changes, compared with 10 healthy matched control individuals, using GeneChip expression microarrays. When analyzing the expression profile data sets by the intersection of three statistic methods (Student's t-test, TNoM and Info score), we found that the HIBM-specific transcriptome consists of 374 differentially expressed genes. The specificity of the HIBM transcriptome was assessed by the minimal transcript overlap found between HIBM and the transcriptome of nine additional muscle disorders including adult onset limb girdle myopathies, inflammatory myopathies and early onset conditions. A strikingly high proportion (18.6%) of the overall differentially expressed mRNAs of known function were found to encode for proteins implicated in various mitochondrial processes, revealing mitochondria pathways dysregulation. Mitochondrial morphological analysis by video-rate confocal microscopy showed a high degree of mitochondrial branching in cells of HIBM patients. The subtle involvement of mitochondrial processes identified in HIBM reveals an unexpected facet of HIBM pathophysiology which could at least partially explain the slow evolution of this disorder and give new insights in the disease mechanism.