Biological characterization and clinical utility of circulating tumor cells in newly diagnosed multiple myeloma

Resumen

Multiple myeloma (MM) is a hematological neoplasm characterized by the clonal expansion of bone marrow (BM) plasma cells (PCs). Despite the evidence of its patchy medullar infiltration, invasive aspirates continue being the gold standard for patient characterization and stratification. Circulating tumor cells (CTCs) are detectable along the entire disease’s spectrum and are gaining interest as a more comprehensive and non-invasive alternative. However, how this circulating population could be involved in MM dissemination and progression remains uncertain. Here, we aimed to develop/standardize methodologies for quantification, isolation and genetic characterization of CTCs in order to study their biological and clinical relevance in comparison with its BM counterpart. Since genetic characterization is a key factor for predicting risk of progression and estimate survival, the first objective was to compare the genetic landscape of CTCs vs BM PCs. CTCs showed more than two-thirds of mutations present in medullar and extramedullar plasmacytomas as well as most of structural variants commonly used for prognostication. All patients were successfully studied using exome sequencing whereas the applicability dropped to 74% using arrays. This high concordance between CTCs and BM PCs was also observed at the transcriptomic level, either using single-cell or bulk approaches. Despite the low number of differentially expressed genes, these shed light into the mechanisms favoring BM egression and disease dissemination. Most differentially expressed genes displayed prognostic value and were associated with inflammatory and interferon response, hypoxia, epithelial mesenchymal transition and cell cycle. On clinical grounds, CTC levels correlated with advanced staging, high-risk cytogenetics and elevated levels of LDH, and outperformed classical BM assessments (by morphology or flow cytometry) in patients with newly-diagnosed, active MM. Patient stratification according to undetectable, low and high CTC numbers defined three different prognostic groups. Hence, the absence of CTCs identified a group with favorable outcome independently of depth of response or cytogenetic risk. Although achieving complete remission improved outcomes, it did not abrogate the poor prognosis of ≥0.24% CTCs. By contrast, undetectable measurable residual disease overcome the negative prognostic impact of high CTC levels. Because next-generation flow is broadly used, quantification, isolation and genetic characterization of CTCs may emerge as an optimal and standardized approach for non-invasive risk-stratification of MM patients. From a mutational and a transcriptomic point of view, we showed in the largest series ever studied, that CTCs are a reliable surrogate of intra and extramedullary genetic lesions. Additionally, we suggested a model for explaining BM egression in which (fully occupied) hypoxic BM niches together with a pro-inflammatory tumor microenvironment induce an arrest in proliferation of cancer, forcing their recirculation to seek for other BM niches to survive and continue growing. Finally, CTC enumeration surpassed current BM evaluations and helped to stratify patients regardless to depth of response and to identify unexpected outcomes in current clinical staging systems. In summary, CTC quantification was the most relevant prognostic factor in newly-diagnosed MM patients. Deep characterization of CTCs might unveil novel targets to overcome disease dissemination and could offer a less invasive and a broader approach for MM diagnosis and monitorization.