Selective killing of tumors deficient in methylthioadenosine phosphorylase: a novel strategy

Our plain-English summary of Selective killing of tumors deficient in methylthioadenosine phosphorylase: a novel strategy, by M. Lubin, A. Lubin (PloS One, 2009).

The study

• Authors: M. Lubin, A. Lubin
• Journal: PloS One
• Published: 2009;4(5):e5735
• Cited by: 59 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: The gene for methylthioadenosine phosphorylase (MTAP) lies on 9p21, close to the gene CDKN2A that encodes the tumor suppressor proteins p16 and p14ARF. MTAP and CDKN2A are homozygously co-deleted, with a frequency of 35 to 70%, in lung and pancreatic cancer, glioblastoma, osteosarcoma, soft-tissue sarcoma, mesothelioma, and T-cell acute lymphoblastic leukemia. In normal cells, but not in tumor cells lacking MTAP, MTAP cleaves the natural substrate, 5'-deoxy-5'-methylthioadenosine (MTA), to adenine and 5-methylthioribose-1-phosphate (MTR-1-P), which are then converted to adenine nucleotides and methionine. This distinct difference between normal MTAP-positive cells and tumor MTAP-negative cells led to several proposals for therapy. We offer a novel strategy in which both MTA and a toxic adenine analog, such as 2,6-diaminopurine (DAP), 6-methylpurine (MeP), or 2-fluoroadenine (F-Ade), are administered. In MTAP-positive cells, abundant adenine, generated from supplied MTA, competitively blocks the conversion of an analog, by adenine phosphoribosyltransferase (APRT), to its active nucleotide form. In MTAP-negative tumor cells, the supplied MTA cannot generate adenine; hence conversion of the analog is not blocked.

Principal Findings: We show that this combination treatment--adenine analog plus MTA--kills MTAP-negative A549 lung tumor cells, while MTAP-positive human fibroblasts (HF) are protected. In co-cultures of the breast tumor cell line, MCF-7, and HF cells, MCF-7 is inhibited or killed, while HF cells proliferate robustly. 5-Fluorouracil (5-FU) and 6-thioguanine (6-TG) may also be used with our strategy. Though neither analog is activated by APRT, in MTAP-positive cells, adenine produced from supplied MTA blocks conversion of 5-FU and 6-TG to their toxic nucleotide forms by competing for 5-phosphoribosyl-1-pyrophosphate (PRPP). The combination of MTA with 5-FU or 6-TG, in the treatment of MTAP-negative tumors, may produce a significantly improved therapeutic index.

Conclusion: We describe a selective strategy to kill tumor cells lacking MTAP.

In plain English

This was a laboratory cell-culture study, not a study of people or of asbestos exposure. The authors tested a proposed treatment for tumors whose cells lack an enzyme called MTAP. Mesothelioma appeared only in the background, citing other studies, as a tumor that often lacks MTAP. No mesothelioma cells were tested. The authors used normal human dermal (skin) fibroblasts, which have MTAP, and lung (A549) and breast (MCF-7) cancer cell lines lacking it. Toxic adenine-like drugs strongly inhibited fibroblast growth, but adding MTA or 5'-deoxyadenosine almost entirely prevented this. The lung cancer cells were not protected. When 45,000 lung cancer cells were grown with 3,000 fibroblasts under combined treatment, dishes showed fewer than 200 cancer clones, while fibroblasts were protected. In mixed cultures, breast cancer cells were killed while fibroblasts kept growing. 5'-deoxyadenosine also protected MTAP-positive cells from the drugs 5-FU and 6-TG, though not at a higher 5-FU dose. The authors suggested the approach might work in the body and noted they tested only a few drugs.

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) cell-culture experiments, including co-cultures of normal and tumor cells
• Who was studied: Cell lines, not people. MTAP-positive cells: human dermal fibroblasts (HF) and the myeloblastic leukemia line ML-1. MTAP-negative cells: non-small cell lung carcinoma (A549, plus a ouabain-resistant version, A549-r) and breast carcinoma (MCF-7). No mesothelioma cells were studied.
• Exposure: No asbestos or fiber exposure was studied. Cells were treated with toxic purine or pyrimidine analogs: 2,6-diaminopurine (DAP), 6-methylpurine (MeP), 2-fluoroadenine (F-Ade), 5-fluorouracil (5-FU) and 6-thioguanine (6-TG). These were given with or without the protective MTAP substrates MTA or 5'-deoxyadenosine.
• Main results:
  • In MTAP-positive HF cells, the adenine analogs DAP, MeP and F-Ade strongly inhibited growth. Adding MTA or 5'-deoxyadenosine almost entirely prevented this inhibition.
  • MTAP-negative A549 lung tumor cells treated with MeP were not protected by adding MTA or 5'-deoxyadenosine.
  • In co-cultures of 45,000 A549-r and 3,000 HF cells, DAP alone reduced HF colony-forming cells to about 15% of the initial value. Adding 5'-deoxyadenosine increased HF viability about three-fold. In the same conditions, A549-r dishes that started with 45,000 viable cells showed fewer than 200 clones.
  • In co-cultures of HF and MCF-7 breast tumor cells, DAP alone killed both cell types. DAP plus MTA killed the MCF-7 cells while the HF cells continued to grow. After five further days of drug-free subculture, no MCF-7 clones were seen.
  • In MTAP-positive ML-1 and HF cells, 5-FU inhibited growth by about 50%. Adding 5'-deoxyadenosine doubled growth to about untreated levels. At a higher 5-FU concentration no protection could be shown. These 5-FU tests were reported for MTAP-positive cells only.
  • 5'-deoxyadenosine strongly protected HF cells from 6-TG, and also from the strongly inhibitory combination of 6-TG and DAP.
  • MTA alone modestly inhibited HF cell growth: to 85% of control at the lower concentration tested and 75% at the higher one, after 3 days.
• The authors' conclusion: The authors concluded that they had described a selective strategy to kill tumor cells lacking MTAP. They suggested that, if applied in the body, large numbers of normal host cells would not substantially interfere with selective tumor killing.
• Limitations the authors state:
  • The authors studied only a few toxic purine and pyrimidine analogs.
  • Of the many known MTAP substrates, the authors tested only MTA and 5'-deoxyadenosine.

Topics

Medical terms in this study, explained in our Lexicon: MTAP, CDKN2A, Sarcoma, Mesothelioma, Fibroblast, Fluorouracil (5-FU).

Related studies

• Increasing the therapeutic index of 5-fluorouracil and 6-thioguanine by targeting loss of MTAP in tumor cells — B. Tang, J.R. Testa, et al. (2012)
• Correlation of methylthioadenosine phosphorylase (MTAP) protein expression with MTAP and CDKN2A copy number in malignant pleural mesothelioma — D.B. Chapel, A.M. Dubuc, et al. (2021)
• Usefulness of methylthioadenosine phosphorylase and BRCA-associated protein 1 immunohistochemistry in the diagnosis of malignant mesothelioma in effusion cytology specimens — K.B. Berg, A.M. Churg, et al. (2020)
• The diagnostic value of immunohistochemically detected methylthioadenosine phosphorylase deficiency in malignant pleural mesotheliomas — Z.G. Zimling, A. Jørgensen, et al. (2012)
• Prevalence of S-methyl-5'-thioadenosine Phosphorylase (MTAP) Deficiency in Human Cancer: A Tissue Microarray Study on 13,067 Tumors From 149 Different Tumor Types — N. Gorbokon, N. Wößner, et al. (2024)
• Utility of Methylthioadenosine Phosphorylase Compared With BAP1 Immunohistochemistry, and CDKN2A and NF2 Fluorescence In Situ Hybridization in Separating Reactive Mesothelial Proliferations From Epithelioid Malignant Mesotheliomas — K.B. Berg, S. Dacic, et al. (2018)

Browse more research in PloS One, or return to medical research.