Orbital pseudotumor can be a localized form of granulomatosis with polyangiitis as revealed by gene expression profiling

• Some types of orbital inflammation are distinguishable by gene expression profiles.
• Non-specific orbital inflammation can be subdivided by analysis of gene expression.
• Some NSOI gene expression profiles are indistinguishable from those of GPA.
• Limited GPA in the orbit requires gene expression profiling to be identified.

Biopsies and ANCA testing for limited forms of granulomatosis with polyangiitis (GPA) are frequently non-diagnostic. We characterized gene expression in GPA and other causes of orbital inflammation. We tested the hypothesis that a sub-set of patients with non-specific orbital inflammation (NSOI, also known as pseudotumor) mimics a limited form of GPA. Formalin-fixed, paraffin-embedded orbital biopsies were obtained from controls (n = 20) and patients with GPA (n = 6), NSOI (n = 25), sarcoidosis (n = 7), or thyroid eye disease (TED) (n = 20) and were divided into discovery and validation sets. Transcripts in the tissues were quantified using Affymetrix U133 Plus 2.0 microarrays.Distinct gene expression profiles for controls and subjects with GPA, TED, or sarcoidosis were evident by principal coordinate analyses. Compared with healthy controls, 285 probe sets had elevated signals in subjects with GPA and 1472 were decreased (> 1.5-fold difference, false discovery rate adjusted p < 0.05). The immunoglobulin family of genes had the most dramatic increase in expression. Although gene expression in GPA could be readily distinguished from gene expression in TED, sarcoidosis, or controls, a comparison of gene expression in GPA versus NSOI found no statistically significant differences.Thus, forms of orbital inflammation can be distinguished based on gene expression. NSOI/pseudotumor is heterogeneous but often may be an unrecognized, localized form of GPA.