Genes and Immunity (2006) 7, 583–591 & 2006 Nature Publishing Group All rights reserved 1466-4879/06 $30.00 www.nature.com/gene ORIGINAL ARTICLE A distinct inflammatory gene expression profile in patients with psoriatic arthritis AK Stoeckman1, EC Baechler1, WA Ortmann1, TW Behrens1, CJ Michet2 and EJ Peterson1 1Department of Medicine, University of Minnesota Medical School, Center for Immunology, University of Minnesota, Minneapolis, MN, USA and 2Division of Rheumatology, Mayo Clinic, Rochester, MN, USA Psoriatic arthritis (PsA) is a systemic inflammatory condition featuring polyarthritis associated with psoriasis. Apart from clinical indicators, few biomarkers exist to aid in the diagnosis and management of PsA. We hypothesized that whole blood gene expression profiling would provide new diagnostic markers and/or insights into pathogenesis of the disease. We compared whole blood gene expression profiles in PsA patients and in age-matched controls. We identified 310 differentially expressed genes, the majority of which are upregulated in PsA patients. The PsA expression profile does not significantly overlap with profiles derived from patients with rheumatoid arthritis or systemic lupus erythematosus. Logistic regression identified two lymphocyte-specific genes (zinc-finger protein 395 and phosphoinositide-3-kinase 2B) that discriminate PsA patients from normal controls. In addition, a highly coregulated cluster of overexpressed genes implicated in protein kinase A regulation strongly correlates with erythrocyte sedimentation rate. Other clusters of coregulated, yet suppressed genes in PsA patient blood include molecules involved in T-cell signaling. Finally, differentially expressed genes in PsA fall into diverse functional categories, but many downregulated genes belong to a CD40 signaling pathway. Together, the data suggest that gene expression profiles of PsA patient blood contain candidate novel disease markers and clues to pathogenesis. Genes and Immunity (2006) 7, 583–591. doi:10.1038/sj.gene.6364334; published online 14 September 2006 Keywords: microarray; gene expression; psoriatic arthritis; lymphocytes; cytokine signaling Introduction trials of TNF-a inhibitors in PsA have shown effective- ness in controlling skin manifestations and in alleviating Psoriatic arthritis (PsA) is a painful, often disabling musculoskeletal symptoms,6 a significant number inflammatory arthritis associated with psoriasis.1 Psor- of PsA patients fail to respond to TNF inhibition. iasis affects approximately 2% of the Caucasian popula- Given the expense and potential toxicity of anti-TNF tion, and arthritis occurs in about 11% of both male and agents, clinical predictors of response to treatment female patients with psoriasis.2 Despite considerable are urgently needed. heterogeneity in the presentation of arthropathy and the Recently, studies utilizing microarray technology to extent of skin disease, genetic studies support the notion survey gene expression profiles have generated new that PsA is a distinct disease entity with a strong hypotheses concerning the pathogenesis of complex heritable component.3 While a number of genetic loci autoimmune disorders.7 Analyses of differential gene have been associated with psoriasis and PsA,4 the expression in peripheral blood cells of patients with number of genes linked to both psoriasis and PsA is disorders such as rheumatoid arthritis (RA) and systemic limited.5 Few serologic or proteomic biomarkers exist to lupus erythematosus (SLE) have identified several gene aid in diagnosis or monitoring of disease activity. ‘signatures’, groups of genes that are differentially Histopathologic changes in psoriatic plaques include expressed in patients as compared to controls.8–10 In activation and expansion of keratinocytes, and accumu- some cases, differentially expressed genes correlate with lation of T cells, B cells, macrophages and neutrophils.1 particular disease manifestations, with disease activity Activation of both CD4 and CD8T cells has been scores, or with responsiveness to treatment regimens. implicated in the pathogenesis of the skin and joint These observations serve as ‘proof of principle’ that for phenotype. One of the major inflammatory molecules systemic inflammatory diseases in general, peripheral implicated in PsA pathogenesis is the cytokine tumor blood gene expression profiling can be applied to disease necrosis factor-alpha (TNF-a).1 Although recent clinical diagnosis and monitoring, and to the identification of novel therapeutic targets. The goal of this study was to examine the utility of Correspondence: Dr EJ Peterson, Department of Medicine, Uni- gene expression profiling as a hypothesis generating and versity of Minnesota, 6-122 Hasselmo Hall, 312 Church Street, diagnostic tool in PsA. The results suggest that PsA Minneapolis, MN 55455, USA. E-mail: [email protected] patients exhibit a peripheral blood gene expression Received 28 February 2006; revised 5 July 2006; accepted 10 July profile distinct from both healthy controls and from 2006; published online 14 September 2006 that observed in other systemic autoimmune states Gene-expression profiling in psoriatic arthritis AK Stoeckman et al 584 (e.g. the ‘interferon signature’ observed in SLE8,10,11 or the sets for any one gene, our focus was narrowed by ‘monocyte signature’ of RA9). Additionally, logistic removing genes that had an absolute difference between regression analysis identified two lymphocyte-specific patient and control mean intensity values of less than genes that most accurately discriminate patients from 500, and by excluding genes encoding hypothetical controls in this discovery sample. Highly coregulated proteins and non-descript mRNAs. Of approximately genes that correlate most closely with inflammation 12 070 genes expressed in whole blood, we identified include two members of the RAS oncogene family a group of 310 genes that were differentially expressed (RAB13 and RAB32) which are implicated in the in PsA patients (Figure 1). As we expected, all of the inhibition of protein kinase A (PKA) activity. A number patients clustered together by unsupervised hierarchical of differentially expressed genes observed in PsA clustering, while the controls clustered independently. blood are commensurate with previous observations Among the 310 differentially expressed genes, 236 were of disease dependency upon T-cell function, but also upregulated in patients vs controls, while 74 were suggest possible roles for genes involved in apoptosis, downregulated. These results suggest that peripheral cell adhesion, chemokine/cytokine signaling, G-protein blood gene expression profiles can distinguish PsA from coupled signaling and B-cell-mediated immunity. normal controls. Further, both the costimulatory molecule CD40 and its ligand are among a number of adaptive immune cell genes downregulated in PsA patients compared to controls. Results Whole blood gene expression profiles of PsA patients are distinct from profiles of age- and sex-matched controls We analyzed gene expression profiles in whole blood obtained from 16 PsA patients and 15 normal controls. Patient demographic and clinical data are summarized in Table 1. The following criteria were applied to generate a list of genes that were differentially expressed between PsA patients and normal controls: (a) P-value o0.00001 by an unpaired Student’s t-test for the difference in mean expression level between groups; (b) greater than a twofold change in the mean expression level between the two groups. Initially, we obtained a list of 578 differen- tially regulated transcripts. After filtering multiple probe Table 1 Demographic and clinical data for PsA patient subjects Gender Male n ¼ 7 Female n ¼ 9 Age 44 years712 (range ¼ 15–65 years) Disease duration 13 years712 (range ¼ 1–43 years) MD global disease activity score 4.772.7 (range 0–10) Patients with active psoriasis n ¼ 15 Swollen joints (number) 4.474.8 (range 0–15) Total white blood cell count ( Â 103/ml) 7.472.2 (range ¼ 4.2–12.8) Lymphocyte count 2.070.8 Monocyte count 0.7270.22 Peripheral mononuclear cell 4.571.8 Figure 1 Gene expression profiles of peripheral blood from 15 count (PMN) controls and 16 PsA patients. Hierarchical clustering of 310 Erythrocyte sedimentation rate 29.3733.5 (range ¼ 2–107) differentially expressed genes is shown. Each row indicates a single (mm/h) gene, and each column represents one subject. Before log2- transformation and clustering, individual data points were ex- Medications at study entry pressed as the ratio of intensity value to the mean of control Any DMARD n ¼ 10 intensity values. Expression value intensities are depicted according Methotrexate n ¼ 7 to the color-indicator key and range from À3 to 3 on a log2 scale. Prednisone n ¼ 1 Red indicates genes that are upregulated and green indicates genes Infliximab or Etanercept n ¼ 5 that are downregulated relative to mean expression levels. Vertical Azathioprine n ¼ 1 bars represent groups of genes that are highly coregulated and Sulfasalazine n ¼ 2 either correlate with erythrocyte sedimentation rate (A) or are implicated in adaptive immune cell function (B). Patient columns Values represent means7s.d. are indicated in red above the cluster. Genes and Immunity Gene-expression profiling in psoriatic arthritis AK Stoeckman et al 585 The PsA gene expression profile shows little overlap with gene TNFa and IL-1b are apical molecules in cytokine expression profiles of other inflammatory autoimmune diseases cascades that play roles in