Sporadic inclusion body myositis: a continuing puzzle

There is now compelling evidence that sporadic inclusion body myositis (sIBM) is a muscle-specific autoimmune disease in which both T and B-cells play a part and in which both cytotoxic muscle fibre necrosis and degeneration occur. However the factors responsible for breakdown of immune tolerance and the nature of the target antigens expressed by muscle fibres remain unknown. Genetic factors are known to contribute to susceptibility, in particular MHC haplotyes which may influence antigenic presentation, and could also operate through genetic variations in muscle fibre constituents or immune effector mechanisms. Viral infection may act as a trigger mechanism, as in cases of HIV-associated sIBM. Our understanding of the mechanisms leading to the degenerative changes in muscle fibres is still incomplete. Protein misfolding and proteasomal dysfunction rather than defective transcriptional control is likely to underlie the abnormal accumulation of multiple proteins in the muscle fibre inclusions. However, aberrant transcription is thought to be the basis for the accumulation of potentially toxic mutant protein forms (e.g. UBB+1). The origin of the multiple clonally expanded somatic mtDNA mutations in COX-negative segments of muscle fibres remains uncertain but may be linked to the effects of oxidative stress. It is proposed that the disproportionate involvement of certain muscles in sIBM may be due to the existence of muscle group-specific transcriptomes which are differentially affected by the disease process and that the male predominance of the disease may indicate the influence of genes preferentially expressed in males. There is a need to develop better animal models of sIBM in which the relationship between the inflammatory and degenerative components of the disease as well as the gender difference in susceptibility and differential vulnerability of different muscle groups can be more critically investigated.