For over seventy years, the chemotherapeutic agent 6-thioguanine (6-TG) has been a cornerstone in the treatment of leukemia, a group of blood cancers characterized by the uncontrolled proliferation of abnormal white blood cells. While its clinical efficacy is well-established, the precise molecular mechanisms dictating why some leukemic cells succumb to its cytotoxic effects while others exhibit remarkable resistance have remained an enduring scientific enigma. Now, a groundbreaking collaborative effort involving researchers from the CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, the University of Oxford, the Weizmann Institute of Science, and the University of Dundee has illuminated a previously unrecognized player in this complex cellular response: the protein NUDT5.
This pivotal discovery, published in a recent issue of Science, builds upon a prior investigation from the Kubicek and Huber laboratories. That earlier work had already hinted at an unconventional role for NUDT5, demonstrating that its cellular importance extended beyond its canonical enzymatic activity. Instead of merely acting as a catalyst to drive biochemical reactions, NUDT5 was found to function as a crucial molecular scaffold, instrumental in orchestrating the intricate network of cellular metabolism. This multifaceted functionality now appears to be directly linked to how cells interpret and react to the presence of 6-TG.
A Shift in Understanding: Beyond Enzymatic Activity
The prevailing paradigm for drug development often targets the enzymatic activity of proteins. Consequently, the initial hypothesis guiding this research posited that NUDT5’s influence on 6-TG’s efficacy would be mediated through its catalytic functions. However, the experimental results dramatically diverged from this expectation.
"We initially expected that NUDT5 would influence 6-TG through its enzymatic activity," explains co-first author Tuan-Anh Nguyen from CeMM. "Instead, we found that inhibiting the enzyme had little effect. What mattered was whether the protein itself was present." This statement underscores a significant conceptual shift: the physical presence of NUDT5, independent of its enzymatic catalysis, emerged as the critical determinant of cellular sensitivity to 6-TG.
Harnessing Targeted Protein Degradation to Unravel Hidden Functions
To rigorously test this hypothesis, the research team employed an innovative and increasingly powerful therapeutic strategy known as targeted protein degradation. Unlike conventional drug mechanisms that aim to block a protein’s enzymatic activity, targeted protein degradation actively induces the cell’s own machinery to dismantle and eliminate the protein entirely. This approach offers a unique advantage: it allows researchers to dissect the distinct biological roles a protein plays as an enzyme versus its functions as a structural or organizational component within the cell.
The development of this strategy was facilitated by a sophisticated cell-based platform, designed by Anne-Sophie Marques, a first author of the paper whose contributions at Oxford were integral to the findings. This platform was instrumental in accelerating the identification of molecules capable of degrading NUDT5. "We developed a cell-based platform to accelerate the discovery of NUDT5 degraders," stated Marques. "This platform helped guide the medicinal chemistry efforts that ultimately produced dNUDT5, our most active degrader."
A comprehensive medicinal chemistry program, spearheaded by the Huber laboratory at the University of Oxford, meticulously curated a collection of highly selective NUDT5 degraders. Crucially, this effort also yielded meticulously matched control compounds. These controls were designed to bind to NUDT5, mimicking the initial interaction of a degrader, but crucially, they did not trigger the protein’s degradation. This distinction was vital for isolating the effects of protein removal from those of mere binding.
The comparative analysis between these targeted degraders and conventional NUDT5 inhibitors yielded striking results. As predicted by the initial hypothesis, suppressing NUDT5’s enzymatic activity with traditional inhibitors had a negligible impact on how cells responded to 6-TG. In stark contrast, the complete removal of NUDT5 from the cells, achieved through the targeted degraders, conferred significant protection against the cytotoxic effects of the drug. These findings were further corroborated by independent genetic experiments, solidifying the conclusion that NUDT5’s physical presence, not its catalytic function, is paramount in dictating cellular sensitivity to 6-TG.
Professor Kilian Huber, from the Centre for Medicines Discovery at the University of Oxford and a co-corresponding author of the study, emphasized the power of this approach: "Chemical degraders give us a way to separate what a protein does as an enzyme from what it does as a physical presence in the cell," he remarked. "In this case, that distinction was decisive: removing NUDT5 revealed biology that conventional inhibitors missed."
A Hidden Function Beyond Catalysis: Implications for Thiopurine Therapy
The research strongly indicates that NUDT5 modulates sensitivity to thiopurine drugs, including 6-TG, through a mechanism that is entirely independent of its catalytic function. This implies that a sole focus on the biochemical reactions catalyzed by NUDT5 would be insufficient to comprehend its profound influence on 6-TG’s therapeutic impact.
"As the results came in, it became immediately clear that the dNUDT5 was protecting cells from 6-thioguanine toxicity in a dose-dependent manner. That was an incredibly exciting moment," shared Ludwig Bauer, another first author of the paper, conveying the palpable enthusiasm within the research team upon witnessing these unexpected findings.
Further intricate investigations unveiled a fascinating and potentially opposing relationship between NUDT5 and another protein, NUDT15. NUDT15 is already recognized within the medical community for its established role in influencing patient responses to thiopurine-based therapies. The current study suggests a complex interplay where these two proteins exert divergent effects on thiopurine sensitivity.
Specifically, the loss of NUDT15 function is known to render cells more susceptible to 6-TG. Conversely, the reduction or removal of NUDT5, as demonstrated in this research, appears to confer resistance to the treatment. This dynamic suggests a delicate balance, where the combined actions of NUDT5 and NUDT15, operating through distinct molecular pathways, can steer cellular fate towards either vulnerability or resilience in the face of thiopurine chemotherapy.
"Our results show that proteins can have important biological functions that are completely independent of their enzymatic activity," stated corresponding author Stefan Kubicek, Principal Investigator at CeMM. "By removing NUDT5 rather than simply inhibiting it, we were able to uncover a hidden layer of biology that helps determine how cells respond to a clinically important drug." This sentiment highlights the transformative potential of employing novel experimental strategies to probe biological systems.
A New Horizon in Understanding Leukemia Drug Response
While these findings do not immediately translate into new clinical treatments, they represent a significant leap forward in understanding the fundamental biology of leukemia drug response. By clearly demonstrating that NUDT5 exerts its influence on 6-TG through a non-catalytic mechanism, this research provides scientists with a crucial new lens through which to examine the variability in patient responses to thiopurine therapies. This deeper understanding could pave the way for personalized treatment strategies tailored to individual patient genetic profiles and protein expression levels.
Furthermore, the study serves as a compelling testament to the power of targeted protein degradation as a research tool. It showcases how this methodology can illuminate biological functions that might otherwise remain obscured when researchers rely solely on traditional enzyme inhibitors. This approach has broad implications for drug discovery and the fundamental exploration of cellular processes across various disease areas.
The collaborative nature of this research, spanning multiple leading academic institutions, underscores the global scientific community’s commitment to tackling complex medical challenges. The involvement of the European Research Council (ERC), the Austrian Science Fund (FWF), the Vienna Science and Technology Fund (WWTF), the Marie Skłodowska-Curie Actions Postdoctoral Fellowships program, the Innovative Medicines Initiative 2 Joint Undertaking (IMI2 JU), the Wellcome Trust, Merck Sharp & Dohme Corp., and Janssen Pharmaceutica NV highlights the significant investment and international cooperation dedicated to advancing biomedical knowledge. This multifaceted support network has been instrumental in enabling the rigorous investigation and groundbreaking discoveries presented in this study.
The implications of this research extend beyond the immediate context of 6-TG. It suggests that a broader re-evaluation of drug mechanisms of action might be warranted, particularly for drugs targeting proteins with known enzymatic functions. The possibility that the non-catalytic roles of many proteins could significantly impact drug efficacy and resistance opens up new avenues for therapeutic intervention and drug repurposing. Future research endeavors will likely focus on further delineating the precise structural and organizational functions of NUDT5 within the cell and exploring the potential of targeting this protein, or its interactions, for therapeutic benefit in leukemic patients.
