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Despite the efficacy of targeted treatments, multiple myeloma (MM) is still incurable, urging to identify novel vulnerabilities to design more effective therapies. Mitochondria are emerging therapeutic targets in oncology for their crucial role not only as cellular powerhouses, but also in signalling, redox homeostasis, initiation of apoptosis, production of metabolites, and supply of biosynthetic precursors. Owing to intensive immunoglobulin production, myeloma cells are heavily reliant on protein homeostasis and experience significant exposure to mitochondrial stressors. We hypothesized that myeloma cells depend on the prototypical mitochondrial stress-adaptive pathway, the mitochondrial unfolded protein response (UPRmt), for their fitness and survival. We tested its activation status and its manipulation as a possible tool against myeloma. We found that while a clear upregulation of the UPRmt signature is evident in MM, its regulation is independent of the master transcription factor ATF5, so far believed to mediate the mammalian UPRmt. One of the key players of the UPRmt and gatekeeper of mitochondrial homeostasis is ClpP, a resident mitochondrial protease suggested to maintain oxidative phosphorylation efficiency. Prompted by its distinctive expression in malignant plasma cells, we investigated the role of ClpP in multiple myeloma cells and tested it as a possible anti-myeloma target. We found that ClpP downregulation leads to disappearance of MM cells from culture due to apoptosis or cell cycle arrest. Surprisingly, toxicity extends to glycolytic cell lines and we demonstrated that ClpP knockdown has no effects on mitochondrial oxygen consumption by MM cells, thus unveiling an energy-independent vulnerability. By combining RNA-seq, proteomics, and metabolomics we identified unprecedented and unexpected cellular features regulated by ClpP, including protein translation both in the cytosol and in mitochondria, impairment of fatty acid metabolism with accumulation of acyl-carnitines and long chain unsaturated fatty acids, deregulation of the polyamine pathway with depletion of spermine, spermidine and putrescine. Intriguingly, we also detected a strong impact on interferon-regulated pathways, hinting at mitochondria and ClpP as possible tools to manipulate MM immunogenicity. Our data suggest that ClpP is vital to MM cells due to a novel non-bioenergetic function, and pave the way for its further evaluation as a therapeutic target. Despite the efficacy of targeted treatments, multiple myeloma (MM) is still incurable, urging to identify novel vulnerabilities to design more effective therapies. Mitochondria are emerging therapeutic targets in oncology for their crucial role not only as cellular powerhouses, but also in signalling, redox homeostasis, initiation of apoptosis, production of metabolites, and supply of biosynthetic precursors. Owing to intensive immunoglobulin production, myeloma cells are heavily reliant on protein homeostasis and experience significant exposure to mitochondrial stressors. We hypothesized that myeloma cells depend on the prototypical mitochondrial stress-adaptive pathway, the mitochondrial unfolded protein response (UPRmt), for their fitness and survival. We tested its activation status and its manipulation as a possible tool against myeloma. We found that while a clear upregulation of the UPRmt signature is evident in MM, its regulation is independent of the master transcription factor ATF5, so far believed to mediate the mammalian UPRmt. One of the key players of the UPRmt and gatekeeper of mitochondrial homeostasis is ClpP, a resident mitochondrial protease suggested to maintain oxidative phosphorylation efficiency. Prompted by its distinctive expression in malignant plasma cells, we investigated the role of ClpP in multiple myeloma cells and tested it as a possible anti-myeloma target. We found that ClpP downregulation leads to disappearance of MM cells from culture due to apoptosis or cell cycle arrest. Surprisingly, toxicity extends to glycolytic cell lines and we demonstrated that ClpP knockdown has no effects on mitochondrial oxygen consumption by MM cells, thus unveiling an energy-independent vulnerability. By combining RNA-seq, proteomics, and metabolomics we identified unprecedented and unexpected cellular features regulated by ClpP, including protein translation both in the cytosol and in mitochondria, impairment of fatty acid metabolism with accumulation of acyl-carnitines and long chain unsaturated fatty acids, deregulation of the polyamine pathway with depletion of spermine, spermidine and putrescine. Intriguingly, we also detected a strong impact on interferon-regulated pathways, hinting at mitochondria and ClpP as possible tools to manipulate MM immunogenicity. Our data suggest that ClpP is vital to MM cells due to a novel non-bioenergetic function, and pave the way for its further evaluation as a therapeutic target.

Assessment of minimal residual disease (MRD) is becoming a standard diagnostic tool for curable hematological malignancies such as chronic and acute myeloid leukemia. Multiple myeloma (MM) remains an incurable disease, as a major portion of patients even in complete response eventually relapse, suggesting that residual disease remains. Over the past decade, the treatment landscape of MM has radically changed with the introduction of new effective drugs and the availability of immunotherapy, including targeted antibodies and adoptive cell therapy. Therefore, conventional serological and morphological techniques have become suboptimal for the evaluation of depth of response. Recently, the International Myeloma Working Group (IMWG) introduced the definition of MRD negativity as the absence of clonal Plasma cells (PC) with a minimum sensitivity of −5 either by next-generation sequencing (NGS) using the LymphoSIGHT platform (Sequenta/Adaptative) or by next-generation flow cytometry (NGF) using EuroFlow approaches as the reference methods. While the definition of the LymphoSIGHT platform (Sequenta/Adaptive) as the standard method derives from its large use and validation in clinical studies on the prognostic value of NGS-based MRD, other commercially available options exist. Recently, the LymphoTrack assay has been evaluated in MM, demonstrating a sensitivity level of 10−5, hence qualifying as an alternative effective tool for MRD monitoring in MM. Here, we will review state-of-the-art methods for MRD assessment by NGS. We will summarize how MRD testing supports clinical trials as a useful tool in dynamic risk-adapted therapy. Finally, we will also discuss future promise and challenges of NGS-based MRD determination for clinical decision-making. In addition, we will present our real-life single-center experience with the commercially available NGS strategy LymphoTrack-MiSeq. Even with the limitation of a limited number of patients, our results confirm the LymphoTrack-MiSeq platform as a cost-effective, readily available, and standardized workflow with a sensitivity of 10−5. Our real-life data also confirm that achieving MRD negativity is an important prognostic factor in MM.

To the editor: Graft-versus-host disease (GVHD), both acute and chronic, is still a leading cause of nonrelapse mortality after transplantation.[1][1] High-dose, posttransplantation cyclophosphamide (PTCy) is an attractive approach for in vivo allodepletion across the HLA barrier in allogeneic