Differential Expression of Nuclear Body Component, Zc3h8, Disrupts DNA Double Strand Break Repair

Abstract

The processes of DNA damage recognition and repair are paramount to maintaining genome integrity in the cell. Failure to do so results in genetic instability and the accumulation of mutations. Double strand breaks have the greatest potential to introduce mutations because rearrangements, non‐homologous end joining, deletions, or insertions may result. Unrepaired or incorrectly repaired DNA double strand breaks can lead to cell death, apoptosis, or disease such as cancer. We previously demonstrated that the C3H1 zinc finger‐containing protein Zc3h8 (also called Fliz1) promotes oncogenic phenotypes when overexpressed in cells, while oncogenic phenotypes are suppressed when expression is reduced. In mice, Zc3h8 expression affected tumorigenesis and tumor size in vivo. Higher expression of Zc3h8 led to more frequent tumors and larger tumors in mice. In humans Zc3h8 is overexpressed in 2‐6% of cancers and is a statistically significant prognostic indicator for poor rates of survival. The Zc3h8 protein has been localized to nuclear bodies including promyelocytic leukemia (PML) and Cajal Bodies where key cellular processes including transcription, translation, post‐translational modifications, and RNA processing are regulated. PML Bodies, in particular, have a role in DNA damage response and repair since P53 is located in PML Bodies along with DAXX, ATRX, and CK2. Since Zc3h8 co‐localized with DNA damage response and repair proteins in PML bodies and promotes oncogenic properties in cells, we investigated if Zc3h8 expression alters how cells respond to DNA damage such as double strand breaks and how this damage is mitigated in cells. Differential expression of Zc3h8 affected mouse mammary tumor cell’s ability to repair double strand breaks caused by the topoisomerase II inhibitor etoposide as indicated by the presence of γH2AX, a marker for unrepaired double strand breaks. Using a Comet assay, we found extensive DNA fragmentation in cells with elevated Zc3h8 expression despite high rates of proliferation and migration. Furthermore, Zc3h8 increases cellular tolerance for DNA damage and alters apoptosis. These phenotypes promote further mutation and may indicate a molecular cause for highly aggressive and progressive cancers with elevated Zc3h8 expression. These conclusions suggest a DNA damage repair and response pathway that is altered by Zc3h8 expression levels can affect multiple aspects of cellular behavior and promote oncogenic phenotypes.