UC San Diego

Long interspersed nuclear element-1 (LINE-1) are endogenous, self-propagating genomic sequences that comprise ~17% of the human genome. Retrotransposons play a critical role in genomic and cellular instability, particularly in various forms of human disease. In the diseased state, there is mounting evidence that endogenous retroelements play a role in etiopathogenesis of inflammatory diseases. Mutations in methyl-CpG- binding protein 2 (MeCP2), widely regarded as a global methylator and transcriptional repressor, result in increased LINE-1 expression and activity. Patients with mutations in MeCP2 have subclinical inflammatory phenotypes and cytokine dysregulation. Although de novo LINE-1 activity seems to occur frequently in neurons, little is known about the contribution of this element in glial cells. Astrocytes with mutations in MECP2 are abnormal in several key functions, thus we sought to investigate astrocyte-mediated inflammation caused by MeCP2 loss-of-function mutations. We generated iPSC-derived astrocytes from MeCP2-KO and healthy controls to observe whether LINE-1, which we know to be upregulated, contributes to the inflammatory phenotype observed. We chronically treated our cells with reverse transcriptase inhibitors (RTi) to reduce endogenous LINE-1 activity. Interestingly, we found upregulation of neuroinflammatory- related genes, increased levels of the proinflammatory cytokine IL-6 as well as an increase in reactive oxygen species, extracellular glutamate and glutathione levels in MeCP2-mutant astrocyte cultures when compared to control cells. Remarkably, inhibition of LINE-1 with RTis improved most of these pathological phenotypes in mutated cells. We hope our work brings further attention to mobile genetic elements, as they contribute more to disease pathologies than previously thought.