The TREX1 Exonuclease R114H Mutation in Aicardi-Goutières Syndrome and Lupus Reveals Dimeric Structure Requirements for DNA Degradation Activity

Mutations in the TREX1 gene cause Aicardi-Goutières syndrome (AGS) and are linked to the autoimmune disease systemic lupus erythematosus. The TREX1 protein is a dimeric 3′ DNA exonuclease that degrades DNA to prevent inappropriate immune activation. One of the most common TREX1 mutations, R114H, causes AGS as a homozygous and compound heterozygous mutation and is found as a heterozygous mutation in systemic lupus erythematosus. The TREX1 proteins containing R114H and the insertion mutations aspartate at position 201 (D201ins) and alanine at position 124 (A124ins), found in compound heterozygous AGS with R114H, were prepared and the DNA degradation activities were tested. The homodimer TREX1R114H/R114H exhibits a 23-fold reduced single-stranded DNA (ssDNA) exonuclease activity relative to TREX1WT. The TREX1D201ins/D201ins and TREX1A124ins/A124ins exhibit more than 10,000-fold reduced ssDNA degradation activities. However, the TREX1R114H/D201ins and TREX1R114H/A124ins compound heterodimers exhibit activities 10-fold greater than the TREX1R114H/R114H homodimer during ssDNA and double-stranded DNA (dsDNA) degradation. These higher levels of activities measured in the TREX1R114H/D201ins and TREX1R114H/A124ins compound heterodimers are attributed to Arg-114 residues of TREX1D201ins and TREX1A124ins positioned at the dimer interface contributing to the active sites of the opposing TREX1R114H protomer. This interpretation is further supported by exonuclease activities measured for TREX1 enzymes containing R114A and R114K mutations. These biochemical data provide direct evidence for TREX1 residues in one protomer contributing to DNA degradation catalyzed in the opposing protomer and help to explain the dimeric TREX1 structure required for full catalytic competency. Background: Mutations in the TREX1 exonuclease gene cause a spectrum of autoimmune diseases. Results: The TREX1 Arg-114 residue acts across the stable dimer interface. Conclusion: TREX1 residues in one protomer contribute to DNA degradation catalyzed in the opposing protomer. Significance: These data help to explain the heterozygous disease condition.