IMPLICATIONS OF ANTI-CN1A SEROTYPE IN INCLUSION BODY MYOSITIS

Background Inclusion Body Myositis (IBM) pathogenesis is
incompletely understood. Anti-cytosolic 5-nucleotidase 1A
(cN1A/NT5c1A) autoantibodies have recently been identified in
∼1/3 of patients. We investigated the potential utility of
anti-cN1A testing in IBM.
Methods Data from four European IBM registries (UK,
Netherlands, France, Sweden) were pooled. Anti-cN1A serotyping
by ELISA was performed. Cases were stratified by
anti-cN1A serotype and associations investigated. Mortality
analysis was performed using Cox proportional hazards
regression.
Results Data from 311 IBM patients were analysed, 102
(32.8%) were anti-cN1A seropositive. At disease onset, fewer
seropositive patients had proximal arm weakness (8.3% vs.
23.2%, OR 0.30, 95% CI=0.11–0.73, p=0.004). A higher
proportion of seropositive patients had excess cytochrome
oxidase deficient fibres on muscle biopsy (86.9% vs. 71.8%,
OR 2.61, 95% CI=1.08–6.97, p=0.022). During follow-up,
70 patients died at a mean age of 77.8 years (SD=8.2).
Adjusted mortality risk was higher in the seropositive group
(HR 1.95, 95% CI=1.17–3.27, p=0.011)
Conclusions Anti-cN1A seropositivity was associated with an
increased mortality risk, suggesting a more severe disease
phenotype. Stratification of IBM by anti-cN1A serotype may
prove important in future treatment decisions i