Characterization of Minimal Residual Disease

Minimal residual disease (MRD) defines persistence of minimal numbers (<10^-2-10^-6) of residual tumor cells after treatment. In recent years, evaluation of MRD has become more frequently used as a mean to assess the quality of response to therapy in multiple myeloma (MM), particularly among those patients who have reached complete remission (CR). At the same time, it has become one of the most relevant prognostic factors in MM, both among patients with standard-risk and those with high-risk cytogenetics. In parallel, the introduction of novel therapies has led to significantly higher CR rates, with also lower rates of MRD-positivity and lower MRD levels. Such improvement in response to therapy of MM has fostered the development of progressively more sensitive approaches that allow deeper evaluation of the quality of the response achieved. However, it is well-known that while most cases that show persistence of MRD after therapy will eventually relapse, some of these patients show persistence of MRD in the absence of disease recurrence. In turn, a significant fraction of MM patients with high-risk cytogenetics, despite reaching deep responses to therapy, show early relapse. Altogether, these findings point out the potential relevance of the biological features of MRD cells, in addition to the MRD levels, in determining long-term MRD control vs. disease recurrence. Therefore, understanding the biologic signature of MRD cells may provide important insight into the mechanisms involved in chemoresistance and the discovery of novel potential therapeutic targets. At present, information about the phenotypic and genetic/genomic features of the chemoresistant myeloma plasma cell (PC) clones remains limited; this is mainly due the minimal levels of residual tumor cells, particularly among the MRD+ patients identified at advanced stages of therapy. Characterization of the phenotypic and genetic profiles of MRD+ myeloma PC which are resistant to induction therapy vs. paired diagnostic myeloma PC from elderly patients treated with novel drugs in the GEM2010MAS65 clinical trial, unravel that therapy-induced clonal selection can be already identified at the MRD stage, after induction therapy. In these settings, chemoresistant myeloma PC showed a specific phenotypic signature that may result from the persistence of clones with unique cytogenetic alterations. Thus, MRD myeloma PC which persisted after induction therapy showed increased expression levels of integrins and adhesion molecules (e.g. CD11c, CD29, CD44, CD49d, CD49e, CD54 and CD138, suggesting that among the initial tumor bulk, the few chemoresistant cells are likely to be those with stronger adhesion properties. Such cells also showed overall different gene expression profiles, with de-regulated genes/pathways related to proteasome-inhibition chemoresistance (e.g.: genes encoding for proteasome subunits or endoplasmic reticulum proteins), and that may influence survival of MM patients. Comparison of both iFISH and copy number variation profiles between patient-paired diagnostic vs. MRD PC revealed different genetic profiles in a substantial percentage of cases, which may potentially be due to the acquisition of new alterations during therapy that render the cell more chemoresistant, and/or the emergence of ultra-chemoresistant MRD cells that represented a subclone of all PC present at diagnosis.