Functional differences between wild-type and mutant-type BRCA1-associated protein 1 tumor suppressor against malignant mesothelioma cells

Our plain-English summary of Functional differences between wild-type and mutant-type BRCA1-associated protein 1 tumor suppressor against malignant mesothelioma cells, by S. Hakiri, H. Osada, F. Ishiguro, et al (Cancer Science, 2015).

The study

• Authors: S. Hakiri, H. Osada, F. Ishiguro, H. Murakami, Y. Murakami-Tonami, K. Yokoi, Y. Sekido
• Journal: Cancer Science
• Published: 2015;106(8):990-9
• Cited by: 32 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Malignant mesothelioma (MM) shows inactivation of the BRCA1-associated protein 1 (BAP1) gene. In this study, we found BAP1 mutations in 5 (26%) of the 19 cell lines that we established from Japanese MM patients, and examined functional differences between the WT and mutant BAP1. First, we studied the subcellular localization of BAP1, demonstrating that the WT primarily resides in the nucleus and that the mutant BAP1 is found in the cytoplasm of the cells. Transduction of the WT BAP1 vector into MM cells with homozygous deletion at the BAP1 3' side resulted in both inhibition of cell proliferation and anchorage-independent cell growth, whereas BAP1 mutants of a missense or C-terminal truncated form showed impaired growth inhibitory effects. Next, we studied how BAP1 is involved in MM cell survival after irradiation (IR), which causes DNA damage. After IR, we found that both WT and mutant BAP1 were similarly phosphorylated and phospho-BAP1 localized mainly in the nucleus. Interestingly, BRCA1 proteins were decreased in the MM cells with BAP1 deletion, and transduction of the mutants as well as WT BAP1 increased BRCA1 proteins, suggesting that BAP1 may promote DNA repair partly through stabilizing BRCA1. Furthermore, using the MM cells with BAP1 deletion, we found that WT BAP1, and even a missense mutant, conferred a higher survival rate after IR compared to the control vector. Our results suggested that, whereas WT BAP1 suppresses MM cell proliferation and restores cell survival after IR damage, some mutant BAP1 may also moderately retain these functions.

© 2015 The Authors. Cancer Science published by Wiley Publishing Asia Pty Ltd on behalf of Japanese Cancer Association.

In plain English

This laboratory study of cultured mesothelioma cells did not examine asbestos exposure. The authors tested 25 malignant mesothelioma cell lines for mutations in the BAP1 gene. Nineteen had been established by the authors from Japanese patients. Mutations were found in 5 of the 19 Japanese lines (26%) and in 7 of the 25 lines overall (28%). The authors then added normal or mutant BAP1 to a mesothelioma cell line missing part of the gene. Normal BAP1 sat mainly in the nucleus and cut cell growth by approximately 50% versus a control. The mutant forms showed reduced or no growth-slowing effect. Both normal and mutant BAP1 raised levels of BRCA1, a DNA-repair protein. After X-ray irradiation, normal BAP1 restored the cells' survival, which the authors described as comparable to an immortalized mesothelial cell line at 1 Gy. Mutant Y724X significantly improved survival versus control at 1 and 3 Gy; two other mutants did so only at 1 Gy. The authors concluded that different BAP1 mutations may keep different levels of function. They suggested that BAP1 mutation status be precisely determined when choosing treatment for patients with mesothelioma.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) study of mesothelioma cell lines: gene mutation analysis plus experiments adding normal (wild-type) or mutant BAP1 to cells
• Who was studied: 25 malignant mesothelioma cell lines. Nineteen were established by the authors from Japanese patients (Aichi Cancer Center, Japan), four were purchased from ATCC and two were gifts. One immortalized mesothelial cell line (MeT-5A) was also used. No patients were studied directly.
• Exposure: No asbestos exposure was studied. The experimental treatments were adding wild-type or mutant BAP1 to cells and X-ray irradiation to cause DNA damage.
• Main results:
  • BAP1 mutations were found in 26% (5 of 19) of the cell lines from Japanese patients, and in 7 of 25 (28%) of all cell lines tested.
  • When added to the BAP1-deleted cell line Y-MESO-25, wild-type BAP1 was found mainly in the nucleus. The mutant forms Y724X and F679LfsX37 showed impaired nuclear localization, and A95D showed a modest increase in the cytoplasm.
  • Wild-type BAP1 reduced growth of the Y-MESO-25 cells by approximately 50% compared with the control vector. The mutant BAP1 forms showed reduced or no inhibitory effect.
  • BRCA1 protein was decreased in more BAP1-mutant cell lines (4 [57%] of 7) than BAP1 wild-type cell lines (6 [35%] of 17). The difference was not statistically significant.
  • In the BAP1-deleted cells, adding wild-type BAP1 increased BRCA1 protein, and all three mutant BAP1 forms did as well.
  • After irradiation, BAP1-deleted cells with the control vector had significantly lower survival than MeT-5A cells. With wild-type BAP1 added, their survival rate was comparable to MeT-5A cells at 1 Gy of irradiation.
  • Mutant Y724X significantly restored survival after irradiation at both 1 and 3 Gy compared with the control. Mutants A95D and F679Lfs37X showed only marginal restoration, significant only at 1 Gy.
• The authors' conclusion: The authors concluded that different BAP1 mutations, as well as wild-type BAP1, may have different levels of growth-suppression and/or DNA-repair activity. They suggested that BAP1 mutation status must be precisely determined to select an optimal treatment strategy for individual patients with malignant mesothelioma.
• Limitations the authors state:
  • The authors stated that the precise mechanism by which mutant as well as wild-type BAP1 might be recruited to DNA damage sites after irradiation is yet to be determined.
  • The authors stated that the role of BAP1's deubiquitination activity in stabilizing BRCA1 needs to be determined in a future study.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer, Mesothelioma.

Medical terms in this study, explained in our Lexicon: Malignant Mesothelioma, BAP1, Homozygous Deletion.

Related studies

• Functional Genomic Screen in Mesothelioma Reveals that Loss of Function of BRCA1-Associated Protein 1 Induces Chemoresistance to Ribonucleotide Reductase Inhibition — A. Okonska, S. Bühler, et al. (2020)
• A Novel BRCA1-Associated Protein-1 Isoform Affects Response of Mesothelioma Cells to Drugs Impairing BRCA1-Mediated DNA Repair — R. Parrotta, A. Okonska, et al. (2017)
• Loss of BRCA1-associated protein 1 (BAP1) expression is rare in non-small cell lung cancer — D. Owen, B.S. Sheffield, et al. (2017)
• BAP1 (BRCA1-associated protein 1) is a highly specific marker for differentiating mesothelioma from reactive mesothelial proliferations — M. Cigognetti, S. Lonardi, et al. (2015)
• Diagnostic value of BRCA1-associated protein-1, glucose transporter-1 and desmin expression in the discrimination between reactive mesothelial proliferation and malignant mesothelioma in tissues and effusions — S. Önder, E. Özogul, et al. (2019)
• Different prognostic roles of tumor suppressor gene BAP1 in cancer: A systematic review with meta-analysis — C. Luchini, N. Veronese, et al. (2016)

Browse more research in Cancer Science, or return to medical research.