Hereditary complement (C9) deficiency associated with dermatomyositis

A 28-year-old Japanese woman with hereditary complement (C9) deficiency and dermatomyositis is reported. She had a 3-year history of facial erythema a nd a 1-month history of progressive muscle weakness. Clinical and laborator y findings were suggestive of dermatomyositis; muscle biopsy confirmed an i nflammatory myopathy. An unexpected finding, however, was the low titre of serum haemolytic complement (CH50). Treatment with prednisolone resulted in marked clinical improvement but did not affect the CH50 titre. Further inv estigation revealed a selective and total absence of the ninth complement c omponent (C9), with direct DNA sequence analysis revealing a non-sense muta tion at Arg(95) of the C9 gene. This case demonstrates that the muscle lesi ons of dermatomyositis can occur in the presence of a complement defect tha t would prevent the formation of the C5b-9 membrane attack complex.