Classic and clinically amyopathic dermatomyositis: Autoantibody positivity

Summary: Abstract Body: Dermatomyositis (DM) is subclassified as classic (CDM) and clinically amyopathic (CADM) and can involve myositis-associated (MAA) and myositis-specific (MSA) autoantibodies. Many MSA are associated with specific DM phenotypes, which helps in disease monitoring and prognostication. However, few studies have compared the positivity rates of all commonly tested MAA and MSA in CDM versus CADM. We performed a retrospective cross-sectional review of patients in the Penn DM database who had myositis autoantibody testing between April 2016 and October 2024. MAA included SSA, Pm-Scl, Ku, U1-RNP, U2-RNP, and U3-RNP. MSA included Jo-1, PL-7, PL-12, EJ, OJ, Mi-2, SRP, TIF-1γ, NXP-2, MDA5, and SAE. ANA status was also recorded. The relation between DM subtype and autoantibody positivity rate was assessed using chi-square and Fisher’s exact tests. Out of 320 patients, 198 (62%) were CDM and 122 (38%) CADM. MSA was positive in 47% (94/198) CDM vs. 40% (49/122) CADM, p = 0.2. Though typically seen in CADM, anti-MDA5 was comparably positive in CDM (8%) and CADM (10%), p = 0.49. Although anti-NXP2 was more common in CDM (10%) than in CADM (5%), the difference was not statistically significant (p = 0.2). Anti-TIF-1γ was the most common MSA in both subtypes (CDM 17%; CADM 22%). MAA was positive in 29% (48/168) CDM vs. 26% (26/100) CADM, p = 0.65. Anti-SSA was the most common MAA in both subtypes (CDM 21%; CADM 19%). ANA was positive in 63% (90/143) CDM vs. 49% (42/85) CADM, p = 0.045. Of the patients with negative myositis panels, 26/53 (49%) CDM and 13/36 (36%) CADM were ANA positive, p = 0.2. We found that MSA and MAA positivity rates did not differ significantly between CDM and CADM, whereas ANA was significantly more common in CDM. A sizeable portion of DM was ANA positive despite negative MSA and MAA, suggesting that there may be other DM-related autoantibodies not yet discovered. These data reiterate that DM management requires integrating clinical and lab findings and should not rely only on autoantibody status. X. Yang^{1, 2}, S. Chambers^{1, 2}, A. On^{1, 2}, H. Ali^{1, 2}, T. Hafshejani^{1, 2}, L. Gomes^{1, 2}, V. Werth^{1, 2} 1. Dermatology, U Penn, Philadelphia, PA, United States. 2. CMCVAMC, Philadelphia, PA, United States. Clinical Research: Epidemiology and Observational Research