A role for endogenous retroelements in Aicardi-Goutires syndrome Aicardi-Goutires syndrome (AGS) is a severe Mendelian inflammatory disorder that affects particularly the brain and frequently causes death in childhood. The disease is characterized by progressive encephalopathy, psychomotor regression, and lesions of the skin, together with increased levels of Type I interferon in the cerebrospinal fluid and serum, and induction of interferon-stimulated genes detectable in peripheral blood. Causative mutations have been identified in seven genes, TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, ADAR1 and IFIH1, with variation in the severity of their associated phenotypes. The protein products of these genes are involved in RNA metabolism or signaling, and have also been linked with suppression of infecting retroviruses and/or endogenous retrotransposons. This study is based on the hypothesis that functional loss of an AGS-associated gene alters the normal metabolism of retrotransposon RNA or its reverse-transcribed cDNA, triggering an interferon response and AGS. Retrotransposons are mobile DNA elements that duplicate themselves by a copy and paste mechanism using an RNA intermediate. Endogenous retroviruses (HERVs) comprise 8% of the human genome. Although no HERVs capable of replication have been identified, ongoing HERV expression has been implicated in several disease conditions, including multiple sclerosis, amyotrophic lateral sclerosis, and autoimmune rheumatic disease. LINE1 (L1) retrotransposons occupy 17% of human DNA, although it is believed that only about 100 remain competent for retrotransposition in any individual. L1 retrotransposition has also been responsible for the insertion of over a million non-autonomous Alu retrotransposons and thousands of processed pseudogenes. The cell has evolved defenses restricting retrotransposition, which are occasionally relaxed in certain somatic cell types, notably neuronal cells in the human brain. We predict relaxation of retroelement expression in AGS patients. Using RNA analyses, immunocytochemistry, and functional assays for reverse transcriptase activity, we will examine brain tissue, blood, sera, and derived cell lines from AGS patients to ascertain if retroelement RNA or protein expression is elevated in the disease state. We will determine if de novo L1 and Alu insertion numbers are increased in tissues of AGS patients using high-throughput sequencing protocols. The question of whether L1 expression can induce an interferon response will be addressed using a mouse model harboring an inducible L1 transgene. Finally, we will develop ELISA assays to ascertain if sera from AGS patients contain autoantibodies directed against retroelement proteins. This research should shed new light on a devastating childhood disease and suggest future strategies for treatment. Furthermore, new insights into the interaction of the intrinsic immune system of the cell and endogenous retroelements will be gained.
Aicardi-Goutires syndrome, an early-onset encephalopathy that typically causes severe intellectual and physical impairment and early death, has no known cure. It has been proposed that abnormal accumulation of nucleic acids from an as yet unknown endogenous source triggers an immune response and the disease pathology. This study will test the hypothesis that endogenous retroelements are that source: if our hypothesis gains support, reverse transcriptase therapies or antisense strategies targeting retroelement RNA may become treatment strategies for future investigation.
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|Li, Peng; Du, Juan; Goodier, John L et al. (2017) Aicardi-Goutières syndrome protein TREX1 suppresses L1 and maintains genome integrity through exonuclease-independent ORF1p depletion. Nucleic Acids Res 45:4619-4631|