Metabolic and Transcriptomic Effects of Mediator Kinase Inhibition on the Interferon Response in Down Syndrome.

Down syndrome, also known as Trisomy 21, is a condition in humans resulting from full or partial triplication of chromosome 21. Affected individuals present with a constellation of symptoms that includes an increased risk of autoimmune disorders. Cells of patients with Down syndrome exhibit chronic, hyperactivated interferon (IFN) signaling, due in part to the presence of several interferon receptors on chromosome 21. Partial knockout of these receptors has been shown to increase cellular viability, indicating that treatments targeting the IFN response may ameliorate the comorbidities associated with Down syndrome. Among the regulators of the IFN response are the Mediator-associated kinases, CDK8 and CDK19. Studies using inhibitors such as cortistatin A, which targets the enzymatic activity of both kinases, have demonstrated improved outcomes in autoimmune disease models. However, the specific mechanisms and contributions of CDK8 vs. CDK19 remain unclear. In mouse and human cells, we and others have shown that the transcriptional response to IFN is different when CDK8/19 are inhibited as opposed to when CDK19 specifically is degraded, suggesting that each kinase regulates distinct sets of IFN-responsive genes through divergent mechanisms. While it seems likely that CDK8 acts through phosphorylation of proteins such as the transcription factor STAT1, CDK19 may instead serve as a physical scaffold. To begin to examine whether inhibition of the Mediator kinases is a viable strategy to treat Down syndrome-associated comorbidities, I assessed the metabolic effects of CDK8/19 inhibition in patient-derived lymphoblastoid cell lines from a pair of age- and sex-matched siblings, one with Down syndrome (T21) and one without (D21). Metabolomic analysis demonstrated a severely blunted response to IFNγ treatment in T21 cells as compared to D21 cells, and revealed genotype-specific changes in energy, nucleotide, and fatty acid metabolism. Interestingly, a subset of metabolites that were decreased or elevated in T21 cells treated with IFNγ alone showed no significant changes in cells also treated with cortistatin A, suggesting that Mediator kinase inhibition may reduce the responsiveness of T21 cells to IFNγ. Ongoing RNA-seq experiments are assessing the transcriptional response to IFN in D21 and T21 cells following CDK8/19 inhibition, with a focus on potential splicing defects and altered gene expression programs in T21. In sum, these experiments will address the under-explored question of how T21 alters metabolic and transcriptional responses to IFNγ and how these responses are controlled by the Mediator kinases. This work will also shed light on the therapeutic potential of Mediator kinase inhibitors, as a means to dampen hyper-activated IFN signaling in Down syndrome.