Figure 6. Increased sensitivity to photodamage in ribonucleotide-containing DNA.

(A) Increased CPD formation in RNase H2–deficient fibroblasts (SLE1, SLE2, AGS1, and AGS2) compared with WT in response to 20 J/m² UVC, measured immediately or 7 and 24 hours after irradiation. Box plots depict interquartile range (box), mean (square), median (line) and SD (whisker) of ≥3 independent experiments per patient and for 5 WT cell lines. *P < 0.05, **P < 0.01, ***P < 0.001 versus WT, t test. (B and C) Dihedral angles between covalent C5-C6 bonds (B) and pyrimidine ring planes (C) of 2 consecutive thymidines of the unmodified (1NAJ) and the ribonucleotide-containing (2L7D) Dickerson dodecamer. Ribonucleotide substitution increased colinearity between C5-C6 covalent bonds and coplanarity between pyrimidine rings, enhancing the probability of a photoreactive dimerization. For each dodecamer, 5 structures deposited in PDB were compared. Mean and SD shown. ***P < 0.001 versus 1NAJ, t test. (D) CPD formation in genomic DNA from fibroblasts of individual RNase H2–deficient versus WT patients and from Rnaseh2b^–/– versus WT MEFs (n = 2 per group). Representative dot blot and mean intensities with SEM of 3 independent experiments are shown. *P < 0.05, **P < 0.01, ***P < 0.001 versus WT, t test. (E) CPD formation in double-stranded concatamers of chimeric DNA oligonucleotides containing 3 Dickerson dodecamer sequences in tandem. Replacement of a deoxyriboguanosine at position 4 of the dodecamer by a riboguanosine (R) is indicated.