Val143 of human ribonuclease H2 is not critical for, but plays a role in determining catalytic activity and substrate specificity

Analysis of ribonucleotide content in the genomic DNA of ribonuclease H2 A subunit (RH2A)-knockout NIH3T3 cells after transient expression of wild-type RH2A or RH2A variants with an Aicardi-Goutières syndrome-causing mutation

Ribonuclease (RNase) H2 is involved in the removal of ribonucleotides embedded in genomic DNA. Eukaryotic RNase H2 is a heterotrimer consisting of the catalytic A subunit (RH2A) and the accessory B and C subunits. This study aimed to compare the cellular activities of wild-type ribonuclease (RNase) H2 and its variants with a mutation causing neuroinflammatory autoimmune disease, Aicardi-Goutières syndrome (AGS). We first analyzed cellular RNase H2 activity and ribonucleotide content in the genomic DNA of RH2A-knockout (KO) mouse fibroblast NIH3T3 cells after transfection with a transient expression plasmid encoding mouse wild-type RH2A. From four hours after transfection, the RNase H2 activity increased, and the amount of ribonucleotides decreased, as compared with the corresponding non-transfected RH2A-KO cells. This demonstrated the rapidness of ribonucleotide turnover in mammalian genomic DNA and the importance of continuous expression of RNase H2 to maintain the ribonucleotide amount low. Next, we expressed mouse RH2A variants with a mutation corresponding to a human AGS-causing mutation in RH2A-KO NIH3T3 cells. Neither increase in RNase H2 activity nor decrease in ribonucleotide amount were observed for G37S; however, both conditions were observed for N213I and R293H. This corresponded with our previous results on the activity of recombinant human RNase H2 variants.