The cyclic GAMP pathway in SLE

Approximately 2/3 of SLE patients have a type I interferon (IFN) signature. Although IFN-a is implicated in SLE, it is also clear that lupus can occur in “interferonopathies” that are predominantly associated with elevation of IFN-b. Of note, TREX1 mutations occur in the interferonapathy called Aicardi Guitierre’s Syndrome (AGS) as well as in 1-2% of SLE patients. It was recently shown that impaired TREX1 degradation of DNA leads to DNA mediated activation EXCLUSIVELY of a newly described DNA sensor called cGAS resulting in the synthesis of the cyclic dinucleotide, cyclic GMP-AMP (cGAMP). cGAMP, in turn, activates STING leading to IRF3 translocation and the production of IFN-b. To determine whether the cGAS pathway contributes to IFN production in SLE, we quantified cGAS expression by QPCR and the cyclic dinucleotide product, cGAMP, by HPLC and mass spectrometry. We observed increases in cGAS and cGAMP expression in ~1/3 of SLE patients’ peripheral blood cells.
Since the cGAS pathway appears active in SLE patients and likely contributes to immune activation, we propose two specific aims:

Aim 1: What are the clinical correlates and the cell type of origin of cGAS and cGAMP in SLE?
We will quantify cGAS by QPCR and cGAMP by mass spectrometry and determine whether the expression levels of either cGAS enzyme mRNA or the cyclic dinucleotide product correlate with disease duration (early versus late) activity (SLEDAI), disease severity, target organ involvement or laboratory parameters of disease. We will determine which cells express cGAS and cGAMP by cell fractionation.
Significance: These studies will inform us of the clinical associations of this pathway in SLE. By comparing early onset with established SLE and the cell type of origin we will gain insight into likely pathogenetic mechanisms.

Aim 2: Improved inhibitors of cGAS.
We have identified inhibitors of cGAS belonging to the antimalarial class of drugs but these drugs work at uM concentrations. Based on in silico modeling and using straightforward chemical approaches, we will synthesize 3 drugs predicted to inhibit cGAS at nM concentrations. These drugs will be tested in a proof of concept model, TREX1 deficient mice, and survival as well as lupus-like pathology quantified. Assuming successful treatment, lupus mouse models exhibiting increased cGAMP will be treated with the lead compound.

Significance:
The advantage of this strategy is that it will specifically suppress the response of cGAS to DNA stimulation but leave other responses e.g. to viral DNA that interact through other pathways and the STING adaptor intact. The long term benefit would be to dampen the critical priming and adjuvant effects of IFN-b in SLE which, in turn, will reduce immune activation. At minimum, the new drugs could be used to treat the 2% of SLE patients with TREX1 mutations as well as patients with interferonopathies.