Cooperative pro-tumorigenic adaptation to oncogenic RAS through epithelial-to-mesenchymal plasticity.

In breast cancers, aberrant activation of the RAS/MAPK pathway is strongly associated with mesenchymal features and stemness traits, suggesting an interplay between this mitogenic signaling pathway and epithelial-to-mesenchymal plasticity (EMP). By using inducible models of human mammary epithelial cells, we demonstrate herein that the oncogenic activation of RAS promotes ZEB1-dependent EMP, which is necessary for malignant transformation. Notably, EMP is triggered by the secretion of pro-inflammatory cytokines from neighboring RAS-activated senescent cells, with a prominent role for IL-6 and IL-1α. Our data contrast with the common view of cellular senescence as a tumor-suppressive mechanism and EMP as a process promoting late stages of tumor progression in response to signals from the tumor microenvironment. We highlighted here a pro-tumorigenic cooperation of RAS-activated mammary epithelial cells, which leverages on oncogene-induced senescence and EMP to trigger cellular reprogramming and malignant transformation.

Spatial Transcriptomics Reveal Pitfalls and Opportunities for the Detection of Rare High-Plasticity Breast Cancer Subtypes.

Breast cancer is one of the most prominent types of cancers, in which therapeutic resistance is a major clinical concern. Specific subtypes, such as claudin-low and metaplastic breast carcinoma (MpBC), have been associated with high nongenetic plasticity, which can facilitate resistance. The similarities and differences between these orthogonal subtypes, identified by molecular and histopathological analyses, respectively, remain insufficiently characterized. Furthermore, adequate methods to identify high-plasticity tumors to better anticipate resistance are lacking. Here, we analyzed 11 triple-negative breast tumors, including 3 claudin-low and 4 MpBC, via high-resolution spatial transcriptomics. We combined pathological annotations and deconvolution approaches to precisely identify tumor spots, on which we performed signature enrichment, differential expression, and copy number analyses. We used The Cancer Genome Atlas and Cancer Cell Line Encyclopedia public databases for external validation of expression markers. By focusing our spatial transcriptomic analyses on tumor cells in MpBC samples, we bypassed the negative impact of stromal contamination and identified specific markers that are neither expressed in other breast cancer subtypes nor expressed in stromal cells. Three markers (BMPER, POPDC3, and SH3RF3) were validated in external expression databases encompassing bulk tumor material and stroma-free cell lines. We unveiled that existing bulk expression signatures of high-plasticity breast cancers are relevant in mesenchymal transdifferentiated compartments but can be hindered by abundant stromal cells in tumor samples, negatively impacting their clinical applicability. Spatial transcriptomic analyses constitute powerful tools to identify specific expression markers and could thus enhance diagnosis and clinical care of rare high-plasticity breast cancers.

Dissecting the Origin of Heterogeneity in Uterine and Ovarian Carcinosarcomas.

Gynecologic carcinosarcomas (CS) are biphasic neoplasms composed of carcinomatous (C) and sarcomatous (S) malignant components. Because of their rarity and histologic complexity, genetic and functional studies on CS are scarce and the mechanisms of initiation and development remain largely unknown. Whole-genome analysis of the C and S components reveals shared genomic alterations, thus emphasizing the clonal evolution of CS. Reconstructions of the evolutionary history of each tumor further reveal that C and S samples are composed of both ancestral cell populations and component-specific subclones, supporting a common origin followed by distinct evolutionary trajectories. However, while we do not find any recurrent genomic features associated with phenotypic divergence, transcriptomic and methylome analyses identify a common mechanism across the cohort, the epithelial-to-mesenchymal transition (EMT), suggesting a role for nongenetic factors in inflicting changes to cellular fate. Altogether, these data accredit the hypothesis that CS tumors are driven by both clonal evolution and transcriptomic reprogramming, essential for susceptibility to transdifferentiation upon encountering environmental cues, thus linking CS heterogeneity to genetic, transcriptomic, and epigenetic influences. Significance: We have provided a detailed characterization of the genomic landscape of CS and identified EMT as a common mechanism associated with phenotypic divergence, linking CS heterogeneity to genetic, transcriptomic, and epigenetic influences.

SMAD2/3 mediate oncogenic effects of TGF-ß in the absence of SMAD4.

TGF-ß signaling is involved in pancreatic ductal adenocarcinoma (PDAC) tumorigenesis, representing one of the four major pathways genetically altered in 100% of PDAC cases. TGF-ß exerts complex and pleiotropic effects in cancers, notably via the activation of SMAD pathways, predominantly SMAD2/3/4. Though SMAD2 and 3 are rarely mutated in cancers, SMAD4 is lost in about 50% of PDAC, and the role of SMAD2/3 in a SMAD4-null context remains understudied. We herein provide evidence of a SMAD2/3 oncogenic effect in response to TGF-ß1 in SMAD4-null human PDAC cancer cells. We report that inactivation of SMAD2/3 in SMAD4-negative PDAC cells compromises TGF-ß-driven collective migration mediated by FAK and Rho/Rac signaling. Moreover, RNA-sequencing analyses highlight a TGF-ß gene signature related to aggressiveness mediated by SMAD2/3 in the absence of SMAD4. Using a PDAC patient cohort, we reveal that SMAD4-negative tumors with high levels of phospho-SMAD2 are more aggressive and have a poorer prognosis. Thus, loss of SMAD4 tumor suppressive activity in PDAC leads to an oncogenic gain-of-function of SMAD2/3, and to the onset of associated deleterious effects.

Epithelial-to-mesenchymal transition promotes immune escape by inducing CD70 in non-small cell lung cancer.

INTRODUCTION: Epithelial-to-mesenchymal transition (EMT) is associated with tumor aggressiveness, drug resistance, and poor survival in non-small cell lung cancer (NSCLC) and other cancers. The identification of immune-checkpoint ligands (ICPLs) associated with NSCLCs that display a mesenchymal phenotype (mNSCLC) could help to define subgroups of patients who may benefit from treatment strategies using immunotherapy. METHODS: We evaluated ICPL expression in silico in 130 NSCLC cell lines. In vitro, CRISPR/Cas9-mediated knockdown and lentiviral expression were used to assess the impact of ZEB1 expression on CD70. Gene expression profiles of lung cancer samples from the TCGA (n = 1018) and a dataset from MD Anderson Cancer Center (n = 275) were analyzed. Independent validation was performed by immunohistochemistry and targeted-RNA sequencing in 154 NSCLC whole sections, including a large cohort of pulmonary sarcomatoid carcinomas (SC, n = 55). RESULTS: We uncover that the expression of CD70, a regulatory ligand from the tumor necrosis factor ligand family, is enriched in mNSCLC in vitro models. Mechanistically, the EMT-inducer ZEB1 impacted CD70 expression and fostered increased activity of the CD70 promoter. CD70 overexpression was also evidenced in mNSCLC patient tumor samples and was particularly enriched in SC, a lung cancer subtype associated with poor prognosis. In these tumors, CD70 expression was associated with decreased CD3+ and CD8+ T-cell infiltration and increased T-cell exhaustion markers. CONCLUSION: Our results provide evidence on the pivotal roles of CD70 and ZEB1 in immune escape in mNSCLC, suggesting that EMT might promote cancer progression and metastasis by not only increasing cancer cell plasticity but also reprogramming the immune response in the local tumor microenvironment.

Cellular Plasticity: A Route to Senescence Exit and Tumorigenesis.

Senescence is a dynamic, multistep program that results in permanent cell cycle arrest and is triggered by developmental or environmental, oncogenic or therapy-induced stress signals. Senescence is considered as a tumor suppressor mechanism that prevents the risk of neoplastic transformation by restricting the proliferation of damaged cells. Cells undergoing senescence sustain important morphological changes, chromatin remodeling and metabolic reprogramming, and secrete pro-inflammatory factors termed senescence-associated secretory phenotype (SASP). SASP activation is required for the clearance of senescent cells by innate immunity. Therefore, escape from senescence and the associated immune editing would be a prerequisite for tumor initiation and progression as well as therapeutic resistance. One of the possible mechanisms for overcoming senescence could be the acquisition of cellular plasticity resulting from the accumulation of genomic alterations and genetic and epigenetic reprogramming. The modified composition of the SASP produced by these reprogrammed cancer cells would create a permissive environment, allowing their immune evasion. Additionally, the SASP produced by cancer cells could enhance the cellular plasticity of neighboring cells, thus hindering their recognition by the immune system. Here, we propose a comprehensive review of the literature, highlighting the role of cellular plasticity in the pro-tumoral activity of senescence in normal cells and in the cancer context.

Opposite Roles for ZEB1 and TMEJ in the Regulation of Breast Cancer Genome Stability.

Breast cancer cells frequently acquire mutations in faithful DNA repair genes, as exemplified by BRCA-deficiency. Moreover, overexpression of an inaccurate DNA repair pathway may also be at the origin of the genetic instability arising during the course of cancer progression. The specific gain in expression of POLQ, encoding the error-prone DNA polymerase Theta (POLθ) involved in theta-mediated end joining (TMEJ), is associated with a characteristic mutational signature. To gain insight into the mechanistic regulation of POLQ expression, this review briefly presents recent findings on the regulation of POLQ in the claudin-low breast tumor subtype, specifically expressing transcription factors involved in epithelial-to-mesenchymal transition (EMT) such as ZEB1 and displaying a paucity in genomic abnormality.

EMT Transcription Factor ZEB1 Represses the Mutagenic POLθ-Mediated End-Joining Pathway in Breast Cancers.

A characteristic of cancer development is the acquisition of genomic instability, which results from the inaccurate repair of DNA damage. Among double-strand break repair mechanisms induced by oncogenic stress, the highly mutagenic theta-mediated end-joining (TMEJ) pathway, which requires DNA polymerase theta (POLθ) encoded by the POLQ gene, has been shown to be overexpressed in several human cancers. However, little is known regarding the regulatory mechanisms of TMEJ and the consequence of its dysregulation. In this study, we combined a bioinformatics approach exploring both Molecular Taxonomy of Breast Cancer International Consortium and The Cancer Genome Atlas databases with CRISPR/Cas9-mediated depletion of the zinc finger E-box binding homeobox 1 (ZEB1) in claudin-low tumor cells or forced expression of ZEB1 in basal-like tumor cells, two triple-negative breast cancer (TNBC) subtypes, to demonstrate that ZEB1 represses POLQ expression. ZEB1, a master epithelial-to-mesenchymal transition-inducing transcription factor, interacted directly with the POLQ promoter. Moreover, downregulation of POLQ by ZEB1 fostered micronuclei formation in TNBC tumor cell lines. Consequently, ZEB1 expression prevented TMEJ activity, with a major impact on genome integrity. In conclusion, we showed that ZEB1 directly inhibits the expression of POLQ and, therefore, TMEJ activity, controlling both stability and integrity of breast cancer cell genomes. SIGNIFICANCE: These findings uncover an original mechanism of TMEJ regulation, highlighting ZEB1 as a key player in genome stability during cancer progression via its repression of POLQ.See related commentary by Carvajal-Maldonado and Wood, p. 1441.

Zeb1 expression by tumor or stromal cells is associated with spatial distribution patterns of CD8+ tumor-infiltrating lymphocytes: a hypothesis-generating study on 113 triple negative breast cancers.

Spatial organization of tumor microenvironment (TME) may influence tumor response to immunomodulatory therapies. Zeb1 is a driver of epithelial-mesenchymal transition, with several roles in immune cell development, however its role in shaping of the immune TME is not fully explored. We conducted a pre-multiplex spatial analysis study to verify whether Zeb1 influences spatial distribution of tumor-infiltrating lymphocytes (TILs) in triple negative breast cancer (TNBC). We applied single and double immunohistochemistry to analyze spatial relationships between CD8+, FoxP3+ and CD20+ tumor-infiltrating lymphocytes (TILs) and the cells expressing Zeb1 in formalin-fixed, paraffin-embedded surgical specimens of 113 TNBCs. 15.5% of cases had Zeb1+ tumor cells and 72.8% of cases had stroma rich in Zeb1+ cells. Low density of intratumoral CD8+ TILs was observed in almost all TNBCs with high or moderate Zeb1+ expression in tumor cells (22/23 cases, 95.6%), and in 90.4% of TNBCs (75/83 cases) with stroma rich in Zeb1+ cells. On the other side, a majority of TNBCs with stroma rich in Zeb1+ cells had high density of stromal CD8+ TILs (55/83 cases, 66.3%). These associations were not observed between Zeb1-expressing cells and FoxP3+ or CD20+ TILs. This in situ analysis showed specific spatial relationship between tumor or stromal Zeb1+ cells and CD8+ TILs, which need to be validated in other cohorts. Zeb1 was highlighted both as a marker of tumor cell EMT and of tumor stroma richness in mesenchymal cells. Several hypotheses about causes of the observed relationship between Zeb1 and TILs are generated and the approaches to verify them discussed. Zeb1 is worth further investigation as a potential biomarker of intratumor immunosuppression of TNBC and of its response to immunotherapies.

Comprehensive characterization of claudin-low breast tumors reflects the impact of the cell-of-origin on cancer evolution.

Claudin-low breast cancers are aggressive tumors defined by the low expression of key components of cellular junctions, associated with mesenchymal and stemness features. Although they are generally considered as the most primitive breast malignancies, their histogenesis remains elusive. Here we show that this molecular subtype of breast cancers exhibits a significant diversity, comprising three main subgroups that emerge from unique evolutionary processes. Genetic, gene methylation and gene expression analyses reveal that two of the subgroups relate, respectively, to luminal breast cancers and basal-like breast cancers through the activation of an EMT process over the course of tumor progression. The third subgroup is closely related to normal human mammary stem cells. This unique subgroup of breast cancers shows a paucity of genomic aberrations and a low frequency of TP53 mutations, supporting the emerging notion that the intrinsic properties of the cell-of-origin constitute a major determinant of the genetic history of tumorigenesis.

Gestational trophoblastic neoplasms (GTNs) do not display epithelial-to-mesenchymal transition (EMT) features.

Although epithelial-to-mesenchymal transition (EMT) has been described in the development of complete hydatidiform moles and the invasion of the maternal decidua by trophoblasts during normal human placentation, its implication in gestational trophoblastic neoplasm (GTN) without villi is totally unknown. We studied the immunoexpression of EMT transcription factors (TWIST1, ZEB1, ZEB2), E-cadherin, and vimentin in 18 trophoblastic tumors and pseudo-tumors. Weak nuclear TWIST1 immunostaining was seen in 5% to 10% of all trophoblastic cells, without ZEB1 and ZEB2 nuclear staining. Trophoblastic cells did not express vimentin, and the expression of E-cadherin was maintained in all cases, indicating the absence of EMT features in GTN.

Role of epithelial-mesenchymal transition factors in the histogenesis of uterine carcinomas.

Several subtypes of high-grade endometrial carcinomas (ECs) contain an undifferentiated component of non-epithelial morphology, including undifferentiated and dedifferentiated carcinomas and carcinosarcomas (CSs). The mechanism by which an EC undergoes dedifferentiation has been the subject of much debate. The epithelial-mesenchymal transition (EMT) is one of the mechanisms implicated in the transdifferentiation of high-grade carcinomas. To improve our understanding of the role of EMT in these tumors, we studied a series of 89 carcinomas including 14 undifferentiated/dedifferentiated endometrial carcinomas (UECs/DECs), 49 CSs (21 endometrial, 29 tubo-ovarian and peritoneal), 17 endometrioid carcinomas (grade 1-3), and 9 high-grade serous carcinomas of the uterus, using a panel of antibodies targeting known epithelial markers (Pan-Keratin AE1/AE3 and E-cadherin), mesenchymal markers (N-cadherin), EMT transcription factors (TFs) (ZEB1, ZEB2, TWIST1), PAX8, estrogen receptors (ER), progesterone receptors (PR), and the p53 protein. At least one of the three EMT markers (more frequently ZEB1) was positive in the sarcomatous component of 98% (n = 48/49) of CSs and 98% (n = 13/14) of the undifferentiated component of UEC/DEC. In addition, 86% of sarcomatous areas of CSs and 79% of the undifferentiated component of UEC/DEC expressed all three EMT-TFs. The expression of these markers was associated with the loss of or reduction in epithelial markers (Pan-keratin, E-cadherin), PAX8, and hormone receptors. In contrast, none of the endometrioid and serous endometrial carcinomas expressed ZEB1, while 6% and 36% of endometrioid and 11% and 25% of serous carcinomas focally expressed ZEB2 and TWIST1, respectively. Although morphologically different, EMT appears to be implicated in the dedifferentiation in both CSs and UEC/DEC. Indeed, we speculate that the occurrence of EMT in a well differentiated endometrioid carcinoma may consecutively lead to a dedifferentiated and undifferentiated carcinoma, while in a type II carcinoma, it may result in a CS.

Cellular Pliancy and the Multistep Process of Tumorigenesis.

Completion of early stages of tumorigenesis relies on the dynamic interplay between the initiating oncogenic event and the cellular context. Here, we review recent findings indicating that each differentiation stage within a defined cellular lineage is associated with a unique susceptibility to malignant transformation when subjected to a specific oncogenic insult. This emerging notion, named cellular pliancy, provides a rationale for the short delay in the development of pediatric cancers of prenatal origin. It also highlights the critical role of cellular reprogramming in early steps of malignant transformation of adult differentiated cells and its impact on the natural history of tumorigenesis.

The cell-of-origin dictates the genomic landscape of breast cancers.

Aberrant cell proliferation induced by activated oncogenes triggers oxidative stress and uncontrolled DNA replication, promoting genomic instability. We recently reported that human mammary stem cells exhibit the unique capacity to withstand an oncogenic activation by dint of an anti-oxidant program driven by the ZEB1 transcription factor. This pre-emptive program prevents the onset of chromosomal instability, leading to the development of tumors with unique pathological features.

A stemness-related ZEB1-MSRB3 axis governs cellular pliancy and breast cancer genome stability.

Chromosomal instability (CIN), a feature of most adult neoplasms from their early stages onward, is a driver of tumorigenesis. However, several malignancy subtypes, including some triple-negative breast cancers, display a paucity of genomic aberrations, thus suggesting that tumor development may occur in the absence of CIN. Here we show that the differentiation status of normal human mammary epithelial cells dictates cell behavior after an oncogenic event and predetermines the genetic routes toward malignancy. Whereas oncogene induction in differentiated cells induces massive DNA damage, mammary stem cells are resistant, owing to a preemptive program driven by the transcription factor ZEB1 and the methionine sulfoxide reductase MSRB3. The prevention of oncogene-induced DNA damage precludes induction of the oncosuppressive p53-dependent DNA-damage response, thereby increasing stem cells' intrinsic susceptibility to malignant transformation. In accord with this model, a subclass of breast neoplasms exhibit unique pathological features, including high ZEB1 expression, a low frequency of TP53 mutations and low CIN.

Splicing factor ratio as an index of epithelial-mesenchymal transition and tumor aggressiveness in breast cancer.

Epithelial-to-mesenchymal transition (EMT) has been shown to be associated with tumor progression and metastasis. During this process in breast cancer, a crucial role is played by alternative splicing systems. To identify a new early prognostic marker of metastasis, we evaluated EMT-related gene expression in breast cell lines, and in primary tumor tissue from 31 patients with early breast cancer, focusing our attention on EMT-related splicing factors ESRP1, ESRP2 and RBFOX2. Results showed that the expression patterns of these genes were indicative of the onset of EMT in in-vitro models, but not in tissue samples. However, the ratio between ESRP1 or ESRP2 and RBFOX2 significantly decreased during EMT and positively correlated with the EMT-specific phenotype in cell models, representing a promising prognostic markers. Low ESRP1/RBFOX2 ratio value was associated with a higher risk of metastasis (p < 0.005) in early breast cancer patients, regardless other clinical features. A cut-off of ratio of 1.067 was determined by ROC curve analysis (AUC 0.8375; 95% CI 0.6963-0.9787). Our study show evidence that a decrease in this ratio correlates with cancer progression. The results provide a rationale for using ESRP1/RBFOX2 ratio as a new prognostic biomarker for the early prediction of metastatic potential in breast cancer.

A novel regulation of PD-1 ligands on mesenchymal stromal cells through MMP-mediated proteolytic cleavage.

Whether fibroblasts regulate immune response is a crucial issue in the modulation of inflammatory responses. Herein, we demonstrate that foreskin fibroblasts (FFs) potently inhibit CD3+ T cell proliferation through a mechanism involving early apoptosis of activated T cells. Using blocking antibodies, we demonstrate that the inhibition of T cell proliferation occurs through cell-to-cell interactions implicating PD-1 receptor expressed on T cells and its ligands, PD-L1 and PD-L2, on fibroblasts. Dual PD-1 ligand neutralization is required to abrogate (i) binding of the PD-1-Fc fusion protein, (ii) early apoptosis of T cells, and (iii) inhibition of T cell proliferation. Of utmost importance, we provide the first evidence that PD-1 ligand expression is regulated through proteolytic cleavage by endogenous matrix metalloproteinases (MMPs) without transcriptional alteration during culture-time. Using (i) different purified enzymatic activities, (ii) MMP-specific inhibitors, and (iii) recombinant human MMP-9 and MMP-13, we demonstrated that in contrast to CD80/CD86, PD-L1 was selectively cleaved by MMP-13, while PD-L2 was sensitive to broader MMP activities. Their cleavage by exogenous MMP-9 and MMP-13 with loss of PD-1 binding domain resulted in the reversion of apoptotic signals on mitogen-activated CD3+ T cells. We suggest that MMP-dependent cleavage of PD-1 ligands on fibroblasts may limit their immunosuppressive capacity and thus contribute to the exacerbation of inflammation in tissues. In contrast, carcinoma-associated fibroblasts appear PD-1 ligand-depleted through MMP activity that may impair physical deletion of exhausted defective memory T cells through apoptosis and facilitate their regulatory functions. These observations should be considered when using the powerful PD-1/PD-L1 blocking immunotherapies.

Deregulation of miR-183 and KIAA0101 in Aggressive and Malignant Pituitary Tumors.

Changes in microRNAs (miRNAs) expression in many types of cancer suggest that they may be involved in crucial steps during tumor progression. Indeed, miRNAs deregulation has been described in pituitary tumorigenesis, but few studies have described their role in pituitary tumor progression toward aggressiveness and malignancy. To assess the role of miRNAs within the hierarchical cascade of events in prolactin (PRL) tumors during progression, we used an integrative genomic approach to associate clinical-pathological features, global miRNA expression, and transcriptomic profiles of the same human tumors. We describe the specific down-regulation of one principal miRNA, miR-183, in the 8 aggressive (A, grade 2b) compared to the 18 non-aggressive (NA, grades 1a, 2a) PRL tumors. We demonstrate that it acts as an anti-proliferative gene by directly targeting KIAA0101, which is involved in cell cycle activation and inhibition of p53-p21-mediated cell cycle arrest. Moreover, we show that miR-183 and KIAA0101 expression significantly correlate with the main markers of pituitary tumors aggressiveness, Ki-67 and p53. These results confirm the activation of proliferation in aggressive and malignant PRL tumors compared to non-aggressive ones. Importantly, these data also demonstrate the ability of such an integrative genomic strategy, applied in the same human tumors, to identify the molecular mechanisms responsible for tumoral progression even from a small cohort of patients.

Dynamics of MBD2 deposition across methylated DNA regions during malignant transformation of human mammary epithelial cells.

DNA methylation is thought to induce transcriptional silencing through the combination of two mechanisms: the repulsion of transcriptional activators unable to bind their target sites when methylated, and the recruitment of transcriptional repressors with specific affinity for methylated DNA. The Methyl CpG Binding Domain proteins MeCP2, MBD1 and MBD2 belong to the latter category. Here, we present MBD2 ChIPseq data obtained from the endogenous MBD2 in an isogenic cellular model of oncogenic transformation of human mammary cells. In immortalized (HMEC-hTERT) or transformed (HMLER) cells, MBD2 was found in a large proportion of methylated regions and associated with transcriptional silencing. A redistribution of MBD2 on methylated DNA occurred during oncogenic transformation, frequently independently of local DNA methylation changes. Genes downregulated during HMEC-hTERT transformation preferentially gained MBD2 on their promoter. Furthermore, depletion of MBD2 induced an upregulation of MBD2-bound genes methylated at their promoter regions, in HMLER cells. Among the 3,160 genes downregulated in transformed cells, 380 genes were methylated at their promoter regions in both cell lines, specifically associated by MBD2 in HMLER cells, and upregulated upon MBD2 depletion in HMLER. The transcriptional MBD2-dependent downregulation occurring during oncogenic transformation was also observed in two additional models of mammary cell transformation. Thus, the dynamics of MBD2 deposition across methylated DNA regions was associated with the oncogenic transformation of human mammary cells.

Copper isotope effect in serum of cancer patients. A pilot study.

The isotope effect describes mass-dependent variations of natural isotope abundances for a particular element. In this pilot study, we measured the (65)Cu/(63)Cu ratios in the serums of 20 breast and 8 colorectal cancer patients, which correspond to, respectively, 90 and 49 samples taken at different times with molecular biomarker documentation. Copper isotope compositions were determined by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). When compared with the literature data from a control group of 50 healthy blood donors, abundances of Cu isotopes predict mortality in the colorectal cancer group with a probability p = 0.018. For the breast cancer patients and the group of control women the probability goes down to p = 0.0006 and the AUC under the ROC curve is 0.75. Most patients considered in this preliminary study and with serum δ(65)Cu lower than the threshold value of -0.35‰ (per mil) did not survive. As a marker, a drop in δ(65)Cu precedes molecular biomarkers by several months. The observed decrease of δ(65)Cu in the serum of cancer patients is assigned to the extensive oxidative chelation of copper by cytosolic lactate. The potential of Cu isotope variability as a new diagnostic tool for breast and colorectal cancer seems strong. Shifts in Cu isotope compositions fingerprint cytosolic Cu chelation by lactate mono- and bidentates. This simple scheme provides a straightforward explanation for isotopically light Cu in the serum and isotopically heavy Cu in cancer cells: Cu(+) escaping chelation by lactate and excreted into the blood stream is isotopically light. Low δ(65)Cu values in serum therefore reveal the strength of lactate production by the Warburg effect.