Abstract
Genome instability is the fundamental hallmark of malignant tumors. Tumor suppressors often play a role in maintaining genome stability. Our previous genetic screen identified inositol polyphosphate 4-phosphatase type B (INPP4B), primarily hydrolyzing phosphatidylinositol 3, 4-disphosphate, is a potential tumor suppressor in lung cancer cells. How INPP4B regulates the genome stability of lung cancer cells is unclear. Here we report knockout of INPP4B in lung adenocarcinoma A549 cells by Crispr-Cas9 gene editing leads to sensitization to ionizing radiation (IR), PARP inhibitor olaparib and impaired DNA homologous recombination repair. Re-introduction of a Crispr-Cas9 resistant INPP4B gene in the INPP4B knockout cells partially restored their resistance to IR, indicating loss of INPP4B protein is relevant to the increased IR sensitivity. Furthermore, we showed ectopic expressed INPP4B in A549 cells responds to IR irradiation by redistribution from cytoplasm to nucleus and endogenous INPP4B protein interacts with Rad50, a crucial MRN complex component for tethering DNA double-strand breaks. Loss of INPP4B protein results in decreased stability of Rad50 in vivo, suggesting an unanticipated role of tumor suppressor INPP4B in maintaining genome integrity via facilitating Rad50 mediated DNA double-strand break repair. Taken together, our findings support a dual role of INPP4B in suppression of tumorigenesis by safeguarding genome stability, as well as inhibiting of PI3K-Akt-mTOR signaling, and offer a new therapeutic strategy for personalized cancer treatment to patients with INPP4B defects or deficiency in the clinic.
Highlights
Genome instability is the fundamental hallmark of human malignancies[1], many gene mutations and large genomic alterations including deletions, translocations, loss of heterozygosity and amplifications are casual factors for tumorigenesis
Re-introduction of a Crispr-resistant-inositol polyphosphate 4-phosphatase type B (INPP4B) gene in the rescued cells partially restored the resistance to ionizing radiation (IR) irradiation, indicating the observed sensitization to IR is related to the loss of INPP4B expression (Fig. 2B)
Given the fact that olaparib only generates synthetic lethality through the combination of two molecular pathways[20,21], sensitization to olaparib hints an intrinsic DNA repair defect occurred in the INPP4B knockout A549 cells
Summary
Genome instability is the fundamental hallmark of human malignancies[1], many gene mutations and large genomic alterations including deletions, translocations, loss of heterozygosity and amplifications are casual factors for tumorigenesis. Sun et al Cell Death and Disease (2020)11:292 caretaker and directly participates in DNA damage repair process to prevent genome from alterations[3]. Accumulating evidence indicates that enzymes involved in the metabolism of phosphate inositol and a number of inositol polyphosphates directly involve in DNA damage repair and genome stability maintenance. Loss of PTEN, a well-known tumor suppressor and a lipid phosphatase antagonizing PIK3-Akt mTOR activation by dephosphorylating phosphoinositide-3, 4, 5-trisphosphate, plays a critical role in the maintenance of chromosomal stability through the physical interaction with centromeres and control of Rad[51] mediated doublestrand DNA repair[4,5]. Inositol hexakisphosphate kinase 1 (IP6K1) is required for homologous recombination (HR) repair in mammalian cells[6]. IP6K1 was found to convert IP6 into IP7 and release CRL4 and initiate the UV-elicited nucleotide excision repair (NER)[11]
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