Cell death caused by perturbation of mitotic regulators is often attributed to mitotic failure or abnormal mitosis. Recently, researchers in the labs of Dr. ZHU Xueliang, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences and Dr. ZHENG Yixian, Carnegie Institution for Science, USA, jointly found that two interacting mitotic regulators, BuGZ and Bub3, have a role in pre-mRNA splicing during the interphase of the cell cycle. Perturbation of such a role contributes to cancer cell death.  

An earlier collaborative study by the two groups identifies BuGZ as a spindle matrix protein important for efficient and accurate mitosis through an interplay with the spindle checkpoint protein Bub3. Depletion of BuGZ by RNA interference leads to mitotic arrest followed by cancer cell death (Jiang et al., Dev Cell, 2014). Dr. WAN Yihan, a joint postdoctoral fellow, and her colleagues found that the depletion of BuGZ resulted in prominent DNA damage in multiple human cancer cell lines, which activated the p53 pathway and, together with the mitotic arrest, resulted in the cell death. Unexpectedly, they found that the DNA damage was due to accumulation of DNA-RNA hybrids (P-loops), intermediates of gene transcription and pre-mRNA processing. BuGZ associated with components of the spliceosome, a machinery in charge of mRNA splicing. The existence of massive mRNA splicing defects in the BuGZ-depleted cells further supported involvement of the protein in pre-mRNA splicing. 

Interestingly, although depletion of BuGZ in human foreskin fibroblasts also provoked P-loop accumulation, DNA damage, and p53 activation, the cells did not die. Rather, these cells showed decreased proliferation and premature senescence during a long term culture.   

As primary cells and cancer cells responded differently to the loss of BuGZ, they further explored whether this property could be used to treat cancer. They tested this with a simplified tumor model, in which the eyes of fruit flies were induced to overgrow through the expression of oncogenes. While the depletion of BuGZ by RNA interference had little effect on the eye development, it suppressed the oncogene-induced hyperplasia. Intriguingly, feeding the oncogene-expressing flies with a splicing inhibitor pladienolide B also suppressed the eye tumor formation.  

Depletion of Bub3 also induced similar splicing defects in both primary and cancer cells. Therefore, BuGZ and Bub3 are partners in pre-mRNA splicing as well.  

This study, published online on April 27, 2015 in The Journal of Cell Biology as a title of “Splicing function of mitotic regulators links R-loop–mediated DNA damage to tumor cell killing” was supported by grants from the Ministry of Science and Technology of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the National Institutes of Health. 

Fig. Depleting BuGZ or Bub3 by RNA interference resulted in accumulation of P-loops (A), nuclear DNA damage (B, arrowheads), and p53 activation (C-D). (Image by Prof. ZHU Xueliang`s group) 

CONTACT:
ZHU Xueliang, Principal Investigator
Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences
Shanghai 200031, China
Phone: +86-21-54921406
E-mail: xlzhu@sibcb.ac.cn