Mice with a mutated form of RNase H2 found in patients with the neuroinflammatory Aicardi-Goutières Syndrome develop a lethal, cGAS–STING–dependent disease.

Gutless adenoviral vectors are devoid of all viral coding regions and display reduced cytotoxicity, diminished immunogenicity, and an increased coding capacity compared with early generation vectors. Using hemophilia A, a deficiency in clotting factor VIII (FVIII), as a model disease, we generated and evaluated a gutless vector encoding human FVIII. The FVIII gutless vector grew to high titer and was reproducibly scaled-up from vector seed lots. Extensive viral DNA analyses revealed no rearrangements of the vector genome. A quantitative PCR assay demonstrated helper virus contamination levels of <2%, with the best preparation containing 0.3% helper virus. We compared the gutless vector with an E1/E2a/E3-deficient (Av3) early generation vector encoding an identical FVIII expression cassette following intravenous administration to hemophilia A mice. Gutless vector-treated mice displayed 10-fold higher FVIII expression levels that were sustained for at least 9 months. In contrast, mice treated with the Av3 vector displayed FVIII levels below the limit of sensitivity of the assay at 3 months. Assessment of hepatotoxicity by measuring the serum levels of liver enzymes demonstrated that the gutless vector was significantly less toxic than the Av3 vector at time points later than 7 days. At the highest dose used, both vectors caused a transient 10-fold increase in liver enzymes 1 day after vector administration, suggesting that this increase was caused by direct toxicity of the input capsid proteins. These data demonstrate that the gutless vector displayed increased duration and levels of FVIII expression, and was significantly less toxic than an analogous early generation vector.

Encoding ribonuclease H1 (RNase H1) degrades RNA hybridized to DNA, and its function is essential for mitochondrial DNA maintenance in the developing mouse. Here we define the role of RNase H1 in mitochondrial DNA replication. Analysis of replicating mitochondrial DNA in embryonic fibroblasts lacking RNase H1 reveals retention of three primers in the major noncoding region (NCR) and one at the prominent lagging-strand initiation site termed Ori-L. Primer retention does not lead immediately to depletion, as the persistent RNA is fully incorporated in mitochondrial DNA. However, the retained primers present an obstacle to the mitochondrial DNA polymerase γ in subsequent rounds of replication and lead to the catastrophic generation of a double-strand break at the origin when the resulting gapped molecules are copied. Hence, the essential role of RNase H1 in mitochondrial DNA replication is the removal of primers at the origin of replication.H uman mitochondrial DNA (mtDNA) is most frequently organized as 16.5-kb circles of duplex DNA, whose two strands are called heavy (H) and light (L), based on their nucleotide composition. The human mitochondrial genome is gene dense, with only one substantial noncoding region (NCR) of approximately 1 kb (1). Based on electron microscopy (2), 5′ end mapping of DNA ends (3-5) and later 2D agarose gel electrophoresis (2D-AGE) studies (6, 7) it was inferred that replication of mammalian mtDNA is frequently unidirectional, commencing within the NCR.In mammalian mitochondria, initiation of DNA synthesis occurs preferentially at or near two specific sites. On the leading strand, initiation has been assigned to a prominent free 5′ end of DNA, designated as Ori-H, located at nucleotide position 16,034 on the map of the mouse mitochondrial genome. Ori-H lies downstream of the light-strand promoter (LSP), and RNA species spanning from LSP to approximately Ori-H have been detected in mammalian mitochondria, albeit not covalently linked with DNA (8). LSP has been robustly mapped both in vivo (9) and in vitro (10) and is assumed to be used by mitochondrial RNA polymerase (POLRMT) to synthesize polycistronic transcripts of the light strand and to generate much shorter products terminating in the NCR, which serve as primers for leading (H-)strand DNA synthesis. POLRMT is also implicated in the synthesis of a primer at a short spacer DNA amid a cluster of five tRNA genes (termed Ori-L) that is a major site of initiation of lagging-strand DNA synthesis (11,12). A role for encoding ribonuclease H1 (RNase H1) in generating, processing, or removing primers at Ori-H and Ori-L has been inferred but has not been experimentally tested.Other data indicate that the initiation of mtDNA replication is more complex, because the NCR contains a second putative origin of replication, termed cluster II, or Ori-b (7, 13). Moreover, other sites of second-strand DNA synthesis than Ori-L have been inferred both by atomic force microscopy (14) and neutral 2D agarose gel electrophoresis (7, 13). Mitochondrial DNA rep...

RNase H1 in mammalian cells is present in nuclei and mitochondria. Its absence in mitochondria results in embryonic lethality due to the failure to amplify mitochondrial DNA (mtDNA). Dual localization to mitochondria and nuclei results from differential translation initiation at two in-frame AUGs (M1 and M27) of a single mRNA. Here we show that expression levels of the two isoforms depend on the efficiency of translation initiation at each AUG codon and on the presence of a short upstream open reading frame (uORF) resulting in the mitochondrial isoform being about 10% as abundant as the nuclear form. Translation initiation at the M1 AUG is restricted by the uORF, while expression of the nuclear isoform requires reinitiation of ribosomes at the M27 AUG after termination of uORF translation or new initiation by ribosomes skipping the uORF and the M1 AUG. Such translational organization of RNase H1 allows tight control of expression of RNase H1 in mitochondria, where its excess or absence can lead to cell death, without affecting the expression of the nuclear RNase H1.