Lupus‐like disease and high interferon levels corresponding to trisomy of the type I interferon cluster on chromosome 9p

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Abstract
Objective Systemic lupus erythematosus (SLE) is associated with type I interferons (IFNs) and can be induced by IFNα treatment. This study looked for evidence of autoimmunity in a pedigree consisting of 4 family members with a balanced translocation 9;21 and 2 members with an unbalanced translocation resulting in trisomy of the short (p) arm and part of the long (q) arm of chromosome 9. These latter 2 subjects had 3 copies of the IFN gene cluster. Methods Subjects were evaluated clinically and serologically for autoimmune disease. Expression levels of IFNα4, IFNβ, the type I IFN–inducible gene Mx1, the type I IFN receptor, interleukin‐6, and tumor necrosis factor α were determined by real‐time polymerase chain reaction. Circulating plasmacytoid dendritic cells, the main IFN‐producing cells, were quantified by flow cytometry. Results Both subjects with trisomy of chromosome 9p had a lupus‐like syndrome with joint manifestations and antinuclear antibodies: one had anti‐RNP and antiphospholipid autoantibodies, and the other had anti–Ro 60. The 3 family members with a balanced translocation 9;21 had no clinical or serologic evidence of autoimmunity, similar to that in relatives who were unaffected by the chromosomal translocation. In the 2 subjects with trisomy of 9p, high levels of IFNα/β (comparable with those found in patients with SLE), increased signaling through the IFN receptor (as indicated by high Mx1 expression), and low levels of circulating plasmacytoid dendritic cells (as observed in patients with SLE) were evident. These abnormalities were not seen in individuals with a balanced translocation. Conclusion Trisomy of the type I IFN cluster of chromosome 9p was associated with lupus‐like autoimmunity and increased IFNα/β and IFN receptor signaling. The data support the idea that abnormal regulation of type I IFN production is involved in the pathogenesis of SLE.