Abstract Chronic myeloid leukemia (CML), caused by BCR::ABL1 fusion gene, is known to regulate disease progression by altering the expression of genes. However, the molecular mechanisms underlying these changes are largely unknown. In this study, we identified RNA Exonuclease 5 (REXO5/LOC81691) as a novel gene with elevated mRNA expression levels in chronic myeloid leukemia (CML) patients. Additionally, using the REXO5 knockout (KO) K562 cell lines, we revealed a novel role for REXO5 in the DNA damage response (DDR). Compared to wild-type (WT) cells, REXO5 KO cells showed an accumulation of R-loops and increased DNA damage. We demonstrated that REXO5 translocates to sites of DNA damage through its RNA recognition motif (RRM) and selectively binds to R loops. Interestingly, we identified that REXO5 regulates R-loop levels by degrading mRNA within R-loop using its exonuclease domain. REXO5 KO showed ATR-CHK1 activation. Collectively, we demonstrated that REXO5 plays a key role in the physiological control of R-loops using its exonuclease domain. These findings may provide novel insights into how REXO5 expression changes contribute to CML pathogenesis.

Professor Hongtae Kim and his research team in the Department of Biological Sciences at UNIST unveiled a novel role for REXO5 (RNA Exonuclease 5) in the DNA damage response (DDR) in collaboration with Professor Dong-Wook Kim from Eulji University and Professor Jae Jin Kim from Hallym University. Their findings suggest that REXO5 is linked to leukemia development through its role in increasing genomic instability.

CML arises from the rapid proliferation of abnormal blood cells driven by the BCR::ABL1 fusion gene, which encourages hematopoietic stem cells to produce blood cells uncontrollably. While targeted treatments like imatinib (Gleevec) have improved patient survival rates and offered potential cures, some patients face life-threatening challenges, including drug resistance or progression to the acute phase of the disease.

Figure 1. Schematic image, showing the R-loop resolution by REXO5 (left) and the inhibition of R-loop resolution by REXO5 mutation (right).

In their research, the team identified elevated levels of REXO5 expression in samples from 60 patients with CML, marking the first time the connection between REXO5 and the DNA damage response in leukemia has been established. Although REXO5 is known for its role in RNA processing, its involvement in CML progression was previously unrecognized.

The REXO5 protein has been shown to contribute to increased genomic instability through the formation of R-loop structures, where RNA attaches to damaged DNA. Insufficient levels of REXO5 lead to reduced DNA replication during the S-phase of the cell cycle, significantly impeding cell growth.

Moreover, REXO5 plays a crucial role in repairing damaged DNA by utilizing its RNA binding sites to interact with R-loop RNA strands and subsequently degrade them.

Professor Hongtae Kim emphasized the significance of this discovery, stating, "We have unveiled the key molecular mechanisms by which REXO5 contributes to blood cancer." He added, "This achievement lays the groundwork for developing a leukemia treatment strategy focused on the DNA damage response."

Professor Dong-Wook Kim noted the implications of this research, saying, "We have highlighted the potential for slowing or inhibiting the acute progression of CML through new therapeutic approaches that regulate the REXO5 protein."

The findings of this research have been published in the online version of Leukemia on July 30, 2024. This study was supported by the National Research Foundation of Korea (NRF) and the Ministry of Science and ICT (MSIT).

Journal Reference
Ye Jin Lee, Seo Yun Lee, Soomi Kim, et al., "REXO5 promotes genomic integrity through regulating R-loop using its exonuclease activity," Leukemia, (2024).