03PA study of muscle biopsy cases positive for anti-cytosolic 5′-nucleotidase 1A antibodies

Anti-cytosolic 5′-nucleotidase 1A (anti-cN1A) antibodies are known to be related with inclusion body myositis (IBM). However, there are atypical patients whose clinical symptoms and pathological findings are different from typical cases of IBM. In addition, anti-cN1A antibodies are sometimes detected in serum of amyotrophic lateral sclerosis (ALS). Recently, autoantibodies associated with myositis, including anti-cN1A antibodies, can be detected by the Autoantibody Array Assay. In this study, we evaluated the characteristics of muscle biopsy cases positive for anti-cN1A antibodies. We evaluated clinicopathological characteristics of 32 patients from January 2023 to December 2024. They were examined by muscle biopsy and serological test by the Autoantibody Array Assay. Anti-cN1A antibodies were detected in 12 patients; 5 of the 12 patients were diagnosed with IBM. Antibody titers were 397.9±268.7 in the IBM group and 81.4±41.7 in the non-IBM group (p=0.011). Two patients in the non-IBM group had myositis-specific antibodies (anti-SRP or anti-HMGCR antibodies) and anti-SS-A antibodies detected simultaneously and were diagnosed as immune–mediated necrotizing myopathy (IMNM) clinicopathologically. Among 12 patients with anti-cN1A antibodies, five patients were positive for anti-cN1A antibodies alone, and two were diagnosed with IBM, two with seronegative IMNM complicated by malignancy, and one with ALS. Anti-cN1A antibodies are useful in the diagnosis of IBM, but anti-cN1A antibodies themselves have little pathological significance, and antibody levels may be elevated in correlation with the presence or absence of pathology.