RNaseH2 mediated Nucleotide Excision Repair and the cGas/STING system in Leukemia development
Insertion of RNA nucleotides into the genome occurs manifold during each cell division. Ribonucleotide excision repair (RER) by RNaseH2 has been found crucial to prevent severe DNA damage. If this fails, the cGAS/Sting system detects DNA debris inside cells and alerts the immune system. Cluster member Prof. Rayk Behrendt, together with colleagues from the Technical Universitiy Dresden and the University Hospital Heidelberg, now show that RNaseH2 acts as tumor suppressor in the hematopoietic system. The scientists also provide evidence, that the cGAS/STING system has no role in blood cell development or Leukemia. The results have recently been published in the journal Cancer Research.
To prevent tumor development, the genome is constantly surveilled for its integrity and repaired if needed. A major cause of DNA damage is the accidental insertion of RNA nucleotides. “A few years ago, we showed that the enzyme RNaseH2 is a previously unknown DNA repair enzyme in mammals” Prof. Rayk Behrendt from the Institute of Clinical Chemistry and Clinical Pharmacology at the University Hospital Bonn states. RNaseH2 was shown to remove individual RNA nucleotides from the genome that were inserted into the DNA double strand instead of DNA during DNA replication. “RNA is very reactive due to the free OH group at the 2' position of ribose, which is not present in DNA” Behrendt explains. “If the RNA building blocks are not removed from the DNA, the DNA molecule breaks.”
It is now clear that ribonucleotide excision repair (RER) is by far the most important post-replicative DNA repair mechanism in eukaryotic cells. More than 1 million RNA nucleotides are mistakenly incorporated into the genome per cell division. This also explains why a complete loss of RNaseH2 activity in humans and mice is incompatible with life.
Loss of DNA repair mechanism leads to leukemia in mouse model
The gene coding for RNaseH2 is located in a larger gene section, that is frequently deleted in leukemic cells. „Hence, we wondered whether the RNaseH2 gene might be a tumor suppressor for specific forms of leukemia“, Behrendt explains. To investigate this, the researchers developed a mouse model, in which RNaseH2 was inactivated specifically in the hematopoietic system. It was found that all mice that no longer had RNaseH2 activity in their blood cells developed lymphomas and leukemias. Tumor development was driven by the massive DNA damage and concomitant activation of other tumor suppressor genes like p53. This result clearly indicates, that RNAseH2 is crucial for the preservation of DNA integrity in the hematopoietic system.
Common intracellular alarm system plays no role in leukemia development
The cGAS/STING signaling pathway is part of the innate immune system. It serves the recognition of DNA inside of cells, which is a common indicator for viral infections. But also, insufficient DNA repair and consecutive tumor development goes along with the presence of DNA in the cytosol and can hence be recognized by cGAS/STING. Upon activation, the cGAS/STING pathway triggers an immune response, including the production of the antiviral cytokine type 1 interferon. Type I interferon can also help fight tumors by activating the immune system and arresting the cell cycle, preventing tumor growth. Hence, the cGAS/STING pathway currently is an important drug target. For example, specific activation of the pathway can make immunologically cold tumors visible to the immune system, which is utilized in cancer immunotherapy.
To test whether the cGAS/STING signaling pathway also plays a role for the development of spontaneous endogenous tumors, Behrendt and his team inactivated cGAS or STING in RNaseH2-deficient mice. “We were really surprised when we found that the loss of cGAS/STING had no effect on tumor development in mice with chronic DNA damage.” The scientists also tested whether detection of acutely induced DNA damage by irradiation or anti-cancer drugs through cGAS would affect the fitness of hematopoietic cells in vivo. “Again, in contrast to other groups, we found no effect of cGAS.” Behrendt explains. “These experiments also exclude previously propagated STING-independent functions of cGAS in our models.”
Overall, the study by Prof. Rayk Behrendt and his colleagues identifies RNaseH2 as a novel tumor suppressor in the hematopoietic system. While targeted activation of the cGAS/STING pathway remains a potent strategy to alert the immune system against tumors, their study demonstrates that both cGAS-induced cytokine production and STING-independent functions of cGAS, have no role in blood cell development or blood cancer.
Nicole Dressel et.al. (2023), Activation of the cGAS/STING axis in genome-damaged hematopoietic cells does not impact blood cell formation or leukemogenesis; Cancer Research, DOI: 10.1158/0008-5472.CAN-22-3860
Insititute for Clinical Chemistry and Clinical Pharmacology
University Hopital Bonn