P.11Clinicopathological difference between anti-SRP and anti-HMGCR myopathy

Abstract

Anti-signal recognition particle (SRP) and 3-hydroxy-3-methylglutaryl-coenzyme. A reductase (HMGCR) antibodies (Ab) are distinctive markers of immune-mediated necrotizing myopathy. However, the differences between patients with anti-SRP antibodies and those with anti-HMGCR have not been fully demonstrated. This study was aimed to determine the differences in clinical, radiological and pathological features between patients with anti-SRP Ab and anti-HMGCR Ab. Among Japanese patients with idiopathic inflammatory myopathies who underwent a muscle biopsy from January 1991 through February 2019, we extracted cases that were positive for anti-SRP or anti-HMGCR Ab. We reviewed clinical histories, MRI findings, muscle pathology, and treatment responses. Of 158 patients with IIMs other than inclusion body myositis, 16 (10%) were positive for anti-SRP Ab, and 10 (6.3%) were positive for anti-HMGCR Ab. The median CK level was significantly higher in patients with anti-HMGCR Ab (9,624 IU/L) than those with anti-SRP Ab (4,563 IU/L) (P=0.02). Trunk muscle weakness and interstitial lung disease tended to be more common in anti-SRP positive patients than in anti-HMGCR positive cases. Fat-saturated T2 weighted MRI in lower limb muscles revealed that semimembranosus muscle in hamstring was significantly more often damaged in anti-HMGCR patients (86%) than in the anti-SRP patients (25%) (P=0.02). Pathologically, 5 (31%) patients with anti-SRP Ab and 2 (20%) with anti-HMGCR Ab showed moderate or severe lymphocyte infiltration. Disease severity and treatment responses measured by modified Rankin scale were similar between both groups. In conclusion, we found that the serum CK level was significantly higher in patients with anti-HMGCR Ab than in those with anti-SRP Ab. In addition, MRI findings of inflammation or edema in semimembranosus muscle may be an important clue to anti-HMGCR myopathy. Pathological findings and treatment efficacy were similar in both groups.