Modeling of TREX1-Dependent Autoimmune Disease using Human Stem Cells Highlights L1 Accumulation as a Source of Neuroinflammation

Abstract

Summary Three-prime repair exonuclease I (TREX1) is an anti-viral enzyme that cleaves nucleic acids in the cytosol, preventing accumulation and a subsequent type-I interferon-associated inflammatory response. Autoimmune diseases, including Aicardi-Goutières syndrome (AGS) and systemic lupus erythematosus, can arise when TREX1 function is compromised. AGS is a neuroinflammatory disorder with severe and persistent intellectual and physical problems. Here, we generated a human AGS model that recapitulates disease-relevant phenotypes using pluripotent stem cells lacking TREX1. We observed abundant extrachromosomal DNA in TREX1-deficient neural cells, of which endogenous Long Interspersed Element-1 retrotransposons were a major source. TREX1-deficient neurons also exhibited increased apoptosis and formed three-dimensional cortical organoids of reduced size. TREX1-deficient astrocytes further contributed to the observed neurotoxicity through increased type-I interferon secretion. In this model, reverse transcriptase inhibitors rescued the neurotoxicity of AGS neurons and organoids, highlighting their potential utility in therapeutic regimens for AGS and related disorders.

Astrocytes, an active player in Aicardi–Goutières syndrome

“…Furthermore, conditioned media from TREX1 deficient astrocytes caused neuronal toxicity over time that was mitigated with IFN receptor blockers. Interestingly, human cortical organoids were generated using control and TREX1 deficient stem cells, which resulted in microencephaly‐like size reductions due to neuronal death, similar to what is observed in AGS individuals . Thus, TREX1 mutation in AGS leads to both a cell autonomous and non‐cell autonomous effect on neurons due to loss of L1 regulation, downstream activation of the cGAS‐STING pathway and elevated secretion of IFN signature.…”

RNase H2, mutated in Aicardi‐Goutières syndrome, promotes LINE‐1 retrotransposition

“…Two sources for such cytoplasmic nucleic acids have been proposed: DNA damage or retroelements. Notably, active LINE‐1s are expressed and highly active in the central nervous system (Muotri et al , ; Coufal et al , ) and strong experimental evidence suggests that TREX1, SAMHD1 and ADAR1 act as LINE‐1 restriction factors (Stetson et al , ; Zhao et al , ; Orecchini et al , ; Thomas et al , ). Although one study failed to detect elevated retrotransposition of L1s in the hippocampus of an AGS patient with SAMHD1 mutations (Upton et al , ), more recent work using TREX1‐deficient neural cells generated from human embryonic stem cells has strongly implicated accumulating LINE‐1‐derived single‐stranded DNAs (ssDNAs) in type I IFN production and neurotoxicity (Thomas et al , ), consistent with a role for active LINE‐1s in AGS pathophysiology (Garcia Perez & Alarcon‐Riquelme, ).…”

“…Notably, active LINE‐1s are expressed and highly active in the central nervous system (Muotri et al , ; Coufal et al , ) and strong experimental evidence suggests that TREX1, SAMHD1 and ADAR1 act as LINE‐1 restriction factors (Stetson et al , ; Zhao et al , ; Orecchini et al , ; Thomas et al , ). Although one study failed to detect elevated retrotransposition of L1s in the hippocampus of an AGS patient with SAMHD1 mutations (Upton et al , ), more recent work using TREX1‐deficient neural cells generated from human embryonic stem cells has strongly implicated accumulating LINE‐1‐derived single‐stranded DNAs (ssDNAs) in type I IFN production and neurotoxicity (Thomas et al , ), consistent with a role for active LINE‐1s in AGS pathophysiology (Garcia Perez & Alarcon‐Riquelme, ). This work also reinforces the concept that by‐products of active retrotransposition, rather than the accumulation of LINE‐1 insertions per se, may be the driver of AGS pathology (Upton et al , ), with TREX1 normally degrading LINE‐1 ssDNA retrotransposition intermediates (Stetson et al , ; Garcia Perez & Alarcon‐Riquelme, ; Thomas et al , ).…”

Treatments in Aicardi–Goutières syndrome

“…Retroelements are ‘remnants’ of ancient viruses that have become incorporated into the human genome, that now constitute up to 40% of our DNA. Such retroelements were first suggested as a driver of AGS in 2008, with recent papers adding weight to this hypothesis . Salvaging of the Trex1 null mouse phenotype with combination reverse transcriptase inhibitors (RTIs) has been reported, premised on a reduction of immunostimulatory DNA derived through an essential (reverse transcription: RNA>DNA) step in the lifecycle of certain endogenous retroelements (Fig.…”