Mutations in TREX1 have been linked to a spectrum of human autoimmune diseases including Aicardi-Goutières syndrome (AGS), familial chilblain lupus (FCL), systemic lupus erythematosus, and retinal vasculopathy and cerebral leukodystrophy. A common feature in these conditions is the frequent detection of antibodies to double-stranded DNA (dsDNA). TREX1 participates in a cell death process implicating this major 3 3 5 exonuclease in genomic DNA degradation to minimize potential immune activation by persistent self DNA. The TREX1 D200N and D18N dominant heterozygous mutations were identified in AGS and FCL, respectively. TREX1 enzymes containing the D200N and D18N mutations were compared using nicked dsDNA and single-stranded DNA (ssDNA) degradation assays. The TREX1WT/D200N and TREX1 WT/D18N heterodimers are completely deficient at degrading dsDNA and degrade ssDNA at an expected ϳ2-fold lower rate than TREX1 WT enzyme. Further, the D200N-and D18N-containing TREX1 homo-and heterodimers inhibit the dsDNA degradation activity of TREX1 WT enzyme, providing a likely explanation for the dominant phenotype of these TREX1 mutant alleles in AGS and FCL. By comparison, the TREX1 R114H homozygous mutation causes AGS and is found as a heterozygous mutation in systemic lupus erythematosus. The TREX1 R114H/R114H homodimer has dysfunctional dsDNA and ssDNA degradation activities and does not detectibly inhibit the TREX1 WT enzyme, whereas the TREX1heterodimer has a functional dsDNA degradation activity, supporting the recessive genetics of TREX1 R114H in AGS. The dysfunctional dsDNA degradation activities of these disease-related TREX1 mutants could account for persistent dsDNA from dying cells leading to an aberrant immune response in these clinically related disorders.The TREX1 gene encodes the major 3Ј 3 5Ј exonuclease detected in mammalian cells. TREX1 is a single open reading frame located on chromosome 3p21.31 and encodes a 314-amino acid polypeptide (1-3). TREX1 is closely related to TREX2 (located on chromosome Xq28) except TREX1 contains additional coding sequence for a C-terminal region not found in TREX2 (4). The N-terminal 242 amino acids of TREX1 contain the catalytically robust 3Ј 3 5Ј exonuclease that is active on ssDNA 2 and dsDNA (5) with the highest activity detected using a partial duplex DNA containing unpaired 3Ј nucleotides (1-3). The C-terminal 72 amino acids contain a transmembrane region that localizes TREX1 to the endoplasmic reticulum in the perinuclear space of cells (6). Activation of a cell death pathway and treatment of cells with DNA-damaging agents cause TREX1 to relocate to the nucleus where it acts on DNA 3Ј termini (6 -8).The structure of TREX1 catalytic domain with bound DNA reveals the protein-polynucleotide interactions that explain the preference for multiple unpaired 3Ј termini in TREX1 exonuclease action and highlights the extensive interface contacts in the stable dimeric enzyme (9). A short ssDNA of approximately four nucleotides is accommodated in the TREX1 active site, and the Arg-1...