Aberrant fumarate metabolism links interferon release in diffuse systemic sclerosis.

Background: Systemic Sclerosis (SSc) is an idiopathic rheumatic inflammatory disease that is characterised by inflammation and skin fibrosis. Type I interferon is significantly elevated in the disease.

Objective: The objective of this study is to determine the role of the TCA cycle metabolite fumarate in SSc.

Methods: CD14 + cells were isolated from 12 SSc patients and healthy controls. Fumarate hydratase and Interferon dependant genes were quantified by qPCR. In vitro inhibition of STING using a small molecule STING inhibitor and enforced mitophagy was induced in vitro and IFN-β release was quantified. VDAC1 inhibitor was used to determine the role of mt DNA release in IFN-β induction. In whole skin biopsies fumarate and succinate was quantified.

Results: Fumarate Hydratase is significantly reduced in SSc monocytes. Type I interferon is also elevated in monocytes from SSc donors compared to controls. The mitochondrial-specific stress marker GDF-15 was significantly elevated in SSc monocytes. Blockade of the cGAS-STING pathway chemically reduced interferon-β release and induced mitophagy also retarded release of the cytokine in response to LPS stimulation. Inhibition of VDAC1 mitigated IFN-β, as did the depletion of mitochondria in cells. Furthermore, the itaconate derivative 4-octyl itaconate reduced IFN-β induction in SSc monocytes, that was downstream of mitochondrial nucleic acid release. Fumarate, but not succinate was elevated in whole skin biopsies.

Conclusion: Fumarate metabolism links interferon release in SSc and may underlie the aberrant expression of interferon in SSc via cytosolic DNA released from mitochondria.