Which autoantibody is commonly associated with both polymyositis and dermatomyositis?

• A. Anti-SRP
• B. Anti-Mi-2
• C. ANA
• D. Anti-Jo-1

Answer: (D)

Anti-Jo-1, the anti-histidyl-tRNA synthetase antibody, is the most commonly detected autoantibody in inflammatory myopathies and defines antisynthetase syndrome. This syndrome often includes features of both polymyositis and dermatomyositis, such as proximal muscle weakness and, in dermatomyositis, distinctive skin findings. Its presence ties the two conditions together because it signals an autoimmune process that can cause muscle inflammation and, in many patients, extra-muscular manifestations like interstitial lung disease, arthritis, mechanic’s hands, and Raynaud phenomenon.

Other antibodies fit different patterns. Anti-SRP is more linked to a necrotizing myopathy pattern with prominent weakness and less skin involvement. Anti-Mi-2 is strongly associated with dermatomyositis and its characteristic rash, but it is less commonly seen in polymyositis. ANA is a nonspecific marker that can be positive in many autoimmune diseases and doesn’t specifically point to PM or DM. So, the anti-Jo-1 antibody best accounts for the overlap between polymyositis and dermatomyositis.