Leukemia cells lacking PTEN reveal a metabolic weakness that approved drugs can exploit

A new study published in EMBO Molecular Medicine reports that T-ALL cells lacking PTEN become unusually dependent on IMPDH, the rate-limiting enzyme in de novo guanylate nucleotide synthesis, for their growth and survival. When the researchers blocked IMPDH with clinically approved drugs, the PTEN-deficient leukemia cells died rapidly and selectively, while cells that still expressed PTEN were largely spared. In mouse models of the disease, the same inhibitors extended survival, offering a rationale for repurposing decades-old immunosuppressants as targeted leukemia therapy.

The finding grew out of a broader question about how cells with hyperactive mTORC1 signaling manage their nucleotide budgets. mTORC1, the mechanistic target of rapamycin complex 1, is a master regulator of anabolic growth that drives the synthesis of proteins, lipids and nucleic acids. In earlier work, the team had shown that tumors with uncontrolled mTORC1 activation caused by loss of the TSC complex develop a strong appetite for guanylate nucleotides, because mTORC1 pushes ribosomal RNA synthesis for ribosome biogenesis. Ribosomal RNA makes up roughly 80 to 85 percent of total cellular RNA in most mammalian cell types, so sustaining that output demands a steady supply of GTP. The new study asked whether mutations that activate mTORC1 from further upstream, which are far more common in cancer than TSC complex mutations, produce the same vulnerability.

To answer it, the researchers first mined The Cancer Dependency Map, a large CRISPR screening dataset. Leukemias and lymphomas scored as the most dependent on IMPDH2, with T-cell leukemia at the top of the list, and PTEN-mutant acute lymphoblastic leukemia cell lines were generally more dependent than PTEN-wild-type ones. The team then treated a panel of nine human T-ALL cell lines with mycophenolic acid, an FDA-approved IMPDH inhibitor, for three days. PTEN-deficient cells were consistently more sensitive than PTEN-expressing cells, and similar results appeared with two other IMPDH inhibitors, mizoribine and the recently discovered prodrug gliocidin. Immunoblotting confirmed that the PTEN-deficient lines carried robust, serum-independent activation of AKT and mTORC1, visible as phosphorylation of AKT, GSK-3, S6 kinase and 4E-BP1.

The cell-killing effect was striking. In three PTEN-deficient lines, including MOLT-3 cells, mycophenolic acid induced more than 98 percent cell death, with minimal effects on PTEN-wild-type TALL-1 and HPB-ALL cells. The death was apoptotic: caspase-3 and its substrate PARP were cleaved, and a pan-caspase inhibitor rescued the cells. Crucially, adding exogenous guanosine, which cells can convert to guanylate nucleotides through the salvage pathway without IMPDH, rescued proliferation, viability and apoptosis. That rescue experiment confirmed the drugs kill through guanylate depletion rather than an unrelated toxicity. The selectivity held in primary human patient-derived T-ALL cells, where IMPDH inhibition preferentially reduced viable cell numbers, triggered caspase cleavage and lowered the live-cell fraction in PTEN-deficient PDTALL-13 cells compared with PTEN-expressing PDTALL-10 cells.

Genetic experiments established that PTEN-loss itself, not some correlated feature, drives the sensitivity. Restoring PTEN expression in JURKAT cells with a doxycycline-inducible system dampened AKT/mTORC1 signaling and reduced caspase cleavage in response to the inhibitor. Conversely, deleting PTEN with CRISPR-Cas9 in PTEN-expressing Ba/F3 pro-B cells, or knocking it down in HPB-ALL cells, made those cells newly sensitive to IMPDH inhibition. Pharmacology pointed to the signaling axis responsible: the bi-steric mTORC1 inhibitor RMC-5552 and the allosteric AKT inhibitor MK-2206 both blocked the apoptosis triggered by mycophenolic acid in PTEN-deficient CCRF-CEM and MOLT-3 cells. In JURKAT cells, mTORC1 inhibition reduced drug-induced apoptosis while AKT inhibition did not, hinting that in some contexts factors beyond AKT contribute.

The mechanism unfolded in two stages. Within hours of IMPDH inhibition, the PTEN-deficient cells showed activating phosphorylation of ATR and its substrates CHK1 and RPA, canonical markers of DNA replication stress, along with rising Cyclin E levels and S-phase accumulation on flow cytometry. Guanosine supplementation reversed these effects, tying the stress to guanylate depletion. Later came the damage: the DNA damage marker gamma-H2AX rose, and ATM and CHK2 became phosphorylated, coinciding with apoptosis. Measuring RNA synthesis with 5-ethynyl-uridine labeling revealed that PTEN-deficient T-ALL cells had significantly elevated RNA synthesis rates compared with PTEN-expressing cells, and in PTEN-deficient Ba/F3 cells this elevation was abolished by the mTORC1 inhibitor RMC-5552, demonstrating that mTORC1 drives it. Mass spectrometry showed that six hours of treatment depleted GTP more strongly in PTEN-deficient cells, and the RNA polymerase I inhibitor CX-5461 partially rescued the replication stress, linking the vulnerability to the burden of ribosomal RNA production.

p53 emerged as a second determinant of sensitivity. In MOLT-3 cells, which have intact p53 function and showed the earliest and strongest responses, mycophenolic acid increased p53 and its target p21. Knocking down p53 with shRNA reduced the drug’s effect on viability, blocked the early caspase-3 and PARP cleavage, and suppressed the early wave of cell death, without changing the accumulation of replication stress. But the later cell death at 48 to 72 hours still occurred in p53-deficient cells, matching the timing of death in JURKAT and CCRF-CEM cells. The authors conclude that p53 enhances, but is not required for, IMPDH inhibitor-induced cell death, an encouraging nuance because intact p53 function is more common in acute lymphoblastic leukemias and pediatric cancers than in many other malignancies.

In vivo, the strategy translated into survival gains, though the choice of drug mattered. In a genetically engineered mouse model in which Pten deletion was combined with a constitutively active NOTCH1 mutant, an oncogenic lesion found in more than 60 percent of human T-ALL, daily mizoribine treatment beginning six days after transplantation significantly extended survival and delayed weight loss, even though vehicle-treated mice were only five days from succumbing when treatment started. Luciferase imaging showed a 60 percent reduction in leukemia burden and prevented thymic accumulation of leukemic cells. In mice bearing Pten-expressing T-ALL, mizoribine still extended lifespan, but less dramatically. Mycophenolate mofetil, the orally bioavailable FDA-approved prodrug of mycophenolic acid, did not robustly extend lifespan in the genetic model, though it modestly but significantly extended survival in a xenograft model of MOLT-3 cells and partially rescued splenomegaly without toxicity or weight loss. Gliocidin also extended survival in xenografts. Notably, MOLT-3 cells express low levels of adenosine kinase, the enzyme needed to activate mizoribine, which explains why mizoribine failed in that model while MMF worked.

The study also widened the relevant patient population beyond PTEN loss. Two PTEN-wild-type T-ALL lines, HSB-2 and KOPTK-1, proved as sensitive to mycophenolic acid as the PTEN-deficient cells, despite robust PTEN expression, because other mutations kept their mTORC1 signaling active even in serum-starved conditions. In HSB-2 cells, AKT inhibition blocked drug-induced caspase-3 cleavage much like mTORC1 inhibition, while in KOPTK-1 cells combined AKT and ERK inhibition was needed to suppress mTORC1 and the apoptotic response. These results indicate that mTORC1 activation through AKT and ERK signaling, driven by mutations upstream of the TSC complex, can confer the same drug sensitivity as PTEN loss, substantially broadening the potential reach of IMPDH inhibitors in leukemia and possibly other cancers.

The authors are careful about what the work does and does not show. All of the efficacy data come from cell lines, primary patient-derived samples and mouse models, and the inhibitors' immunosuppressive effects would need careful management in a leukemia setting. Mizoribine is used clinically in Asia as an immunosuppressant, while mycophenolic acid is standard in the US and Europe, and both have decades of safety data, which is precisely what makes repurposing attractive. Open questions include whether IMPDH inhibitors can be combined with existing chemotherapy or with agents that induce replication stress, whether PTEN loss sensitizes solid tumors such as prostate, endometrial and glioma cancers, and how differences in drug metabolism, including UDP-glucuronosyltransferase polymorphisms that alter mycophenolic acid pharmacokinetics, will shape responses. For now, the study delivers a mechanistically grounded case that a long-known immunosuppressant target sits at the center of a metabolic weakness in one of the harder-to-treat leukemia subtypes.

Subject of Research: Metabolic vulnerability of PTEN-deficient T-cell acute lymphoblastic leukemia to IMPDH inhibition

Article Title: PTEN-loss confers dependence on the guanylate synthesis enzyme IMPDH in T-cell acute lymphoblastic leukemia

Article References: Padder, R. A., Le Le, T., Hyde, E., Rubio, M. E. D., Chiles, E., Joshi, J. N., Singh, N., Akther, M., Sheshadri, N., Mulraney, K., Zong, W.-X., Su, X., Herranz, D., & Valvezan, A. J. (2026). PTEN-loss confers dependence on the guanylate synthesis enzyme IMPDH in T-cell acute lymphoblastic leukemia. EMBO Molecular Medicine. https://doi.org/10.1038/s44321-026-00533-2

Image Credits: AI Generated

DOI: 10.1038/s44321-026-00533-2

Keywords: T-cell acute lymphoblastic leukemia, PTEN, IMPDH, mTORC1, guanylate nucleotide synthesis, mycophenolic acid, mizoribine, DNA replication stress, p53, drug repurposing, AKT signaling, mouse models