A Novel BRCA1-Associated Protein-1 Isoform Affects Response of Mesothelioma Cells to Drugs Impairing BRCA1-Mediated DNA Repair

Our plain-English summary of A Novel BRCA1-Associated Protein-1 Isoform Affects Response of Mesothelioma Cells to Drugs Impairing BRCA1-Mediated DNA Repair, by R. Parrotta, A. Okonska, M. Ronner, et al (Journal of Thoracic Oncology, 2017).

The study

• Authors: R. Parrotta, A. Okonska, M. Ronner, W. Weder, R. Stahel, L. Penengo, E. Felley-Bosco
• Journal: Journal of Thoracic Oncology
• Published: 2017;12(8):1309-1319
• Cited by: 52 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Introduction: BRCA1 associated protein1 (BAP1) is a tumor suppressor involved in multiple cellular processes such as transcriptional regulation, chromatin modification by deubiquitinating histone 2A, and DNA repair. BAP1 mutations are frequent in malignant pleural mesothelioma (MPM). Our aim was to functionally characterize a newly identified isoform of BAP1 and investigate the effects of its expression on drug sensitivity in MPM.

Methods: Expression of BAP1 isoforms was detected by quantitative polymerase chain reaction in MPM and normal mesothelium cell lines and tumor and nontumor samples. Histone H2A ubiquitination levels were analyzed by Western blot after acidic extraction of core histones. Subcellular localization of BAP1 isoforms was examined by immunofluorescence. MPM cell survival in response to poly(adenosine diphosphate-ribose) polymerase (PARP) and dual phosphoinositide 3-kinase (PI3K)-mammalian target of rapamycin (mTOR) inhibitors was analyzed by in vitro assays.

Results: We have identified a novel alternative splice isoform of BAP1 (BAP1Δ) that misses part of the catalytic domain. Cells transfected with BAP1Δ showed reduced deubiquitinating activity compared with full-length BAP1. The expression of BAP1Δ transcript is more abundant in nontumor than in tumor samples. MPM cell lines expressing more than 20% of BAP1Δ are more sensitive to olaparib (a PARP1 inhibitor) cytotoxicity, and this sensitivity is enhanced when olaparib treatment is combined with GDC0980 (a dual PI3K-mTOR inhibitor), which induces downregulation of BRCA1.

Conclusions: These observations suggest that BAP1Δ does regulate DNA damage response and influences drug sensitivity. It might therefore be relevant to investigate whether patients with high expression of BAP1Δ may be responsive to PARP/PI3K-mTOR inhibitors.

Copyright © 2017 International Association for the Study of Lung Cancer. Published by Elsevier Inc. All rights reserved.

In plain English

This Zürich laboratory study used mesothelioma cell lines, normal mesothelial cells and tissue samples, not asbestos exposure. The authors described BAP1∆, a shorter BAP1 transcript missing 54 base pairs within exon 9; its protein lacks 12 amino acids of the catalytic region. In transfected cells, full-length BAP1 reduced a chemical tag on histone H2A, but BAP1∆ did not. BAP1∆ was less abundant in tumors with normal (wild-type) BAP1 than in non-tumor samples. In cells made to express one form, BAP1∆'s nuclear accumulation was 53% (plus or minus 4%) lower than full-length BAP1's. Olaparib, a cancer drug, was 2 to 3 times more toxic to cells given BAP1∆ than to cells given full-length BAP1. Among tested cell lines, all with wild-type BAP1, those where BAP1∆ exceeded 20% of BAP1 expression (the lines' median) were more sensitive to olaparib. Adding GDC0980 produced a complete drop in ATP, a viability measure, in that group. The authors concluded that BAP1∆ appears to regulate the DNA damage response and to influence drug sensitivity. They said the mechanism needs further investigation.

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 using cell lines and human tissue samples
• Who was studied: Malignant pleural mesothelioma cell lines (including ZL55, ZL34, SPC111, MSTO-211H, ACC-Meso-1, Mero82), normal mesothelial cells SDM104, and HEK293 cells. Also 20 tumor RNA samples with wild-type BAP1 (16 epithelial, 3 biphasic, 1 sarcomatoid) and normal pleural tissue from patients having thoracic surgery unrelated to mesothelioma, at Zurich University Hospital.
• Exposure: No asbestos exposure was studied. Cells were treated with the PARP inhibitor olaparib alone or combined with GDC0980, a dual PI3K-mTOR inhibitor.
• Main results:
  • The authors identified a shorter BAP1 transcript variant (BAP1∆) missing 54 base pairs within exon 9 compared with full-length BAP1.
  • The BAP1∆ protein lacks 12 amino acids of the catalytic region and part of the BARD1 binding domain.
  • In HEK293 cells, full-length BAP1 reduced mono-ubiquitinated histone H2A, but no change was seen in cells expressing BAP1∆.
  • BAP1∆ was less abundant in tumors expressing wild-type BAP1 than in non-tumor samples.
  • In cell fractionation experiments on cells expressing a single isoform from transfected cDNA, nuclear accumulation of BAP1∆ was 53 plus or minus 4% lower than that of full-length BAP1.
  • Olaparib was 2 to 3 times more toxic to ZL55 mesothelioma cells transfected with BAP1∆ than to those transfected with full-length BAP1.
  • All the mesothelioma cell lines tested for the BAP1∆ ratio carried wild-type BAP1.
  • The cell lines were grouped by their BAP1∆-to-BAP1 expression ratio, split at 20%, which was the median ratio among the tested mesothelioma cell lines.
  • Cell lines with a high BAP1∆ ratio (above 20%) were more sensitive to olaparib, and showed a complete drop in ATP with olaparib plus GDC0980.
  • Spheroids from the normal mesothelium cell line SDM104 were resistant to olaparib. The combined treatment lowered their ATP content less than in mesothelioma cells.
• The authors' conclusion: The authors concluded that BAP1∆ appears to regulate the DNA damage response and to influence drug sensitivity.
• Limitations the authors state:
  • The authors stated that further investigation is needed to explain how BAP1∆ interferes with full-length BAP1.

Topics

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

Medical terms in this study, explained in our Lexicon: Mesothelioma, BAP1, Pleural Mesothelioma, Mesothelium.

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• Functional differences between wild-type and mutant-type BRCA1-associated protein 1 tumor suppressor against malignant mesothelioma cells — S. Hakiri, H. Osada, et al. (2015)
• Germline BARD1 variants predispose to mesothelioma by impairing DNA repair and calcium signaling — F. Novelli, Y. Yoshikawa, et al. (2024)
• Inhibition of autophagy sensitizes malignant pleural mesothelioma cells to dual PI3K/mTOR inhibitors — N. Echeverry, G. Ziltener, et al. (2015)
• Prevalence of BRCA-1 associated protein 1 germline mutation in sporadic malignant pleural mesothelioma cases — A. Rusch, G. Ziltener, et al. (2015)

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