Background  There is increased expression of type I interferon (IFN)-regulated proteins in the blood and target tissues of patients with cutaneous lupus erythematosus (CLE) and systemic lupus erythematosus (SLE). Patients with SLE have increased IFN-regulated gene expression pointing towards a possible underlying genetic defect.

Objectives  To determine expression levels of five type I IFN-regulated genes that are highly expressed in SLE in the peripheral blood of patients with CLE and to correlate the expression levels with cutaneous disease activity.

Methods  Peripheral blood was obtained from 10 healthy controls and 30 patients with CLE, including eight with concomitant SLE. Total RNA was extracted and reverse transcribed into complementary DNA. Gene expression levels were measured by real-time polymerase chain reaction. Gene expression was normalized to GAPDH, standardized to healthy controls and then summed to calculate an IFN score for each patient. Disease activity was assessed with the Cutaneous Lupus Area and Severity Index (CLASI).

Results  Patients with subacute CLE (SCLE) and discoid lupus erythematosus (DLE) had elevated IFN scores compared with healthy controls regardless of concomitant SLE (< 0·01 with SLE and < 0·05 without SLE). There was no difference between patients with tumid lupus erythematosus (TLE) and healthy controls. The IFN score correlated with CLASI scores (Spearman’s rho = 0·55, = 0·0017).

Conclusions  Patients with SCLE and DLE have an IFN signature, as seen in SLE. The level of gene expression correlates with cutaneous disease activity. These findings support a shared pathogenesis between SLE and some subtypes of CLE.