MRD Detection Testing to Study Clonal Selection and Drug Resistance

Minimal residual disease (MRD) in multiple myeloma (MM) reflects the persistence of chemoresistant clonal plasma cells (PCs) after treatment. The correlation between the presence of MRD and duration of remission and overall survival is now well established in MM.¹  At the 2015 ASH Annual Meeting, Dr. Hervé Avet-Loiseau and colleagues evaluated the presence of MRD in patients achieving a very good partial response or better after receiving lenalidomide, bortezomib, and dexamethasone induction therapy with or without autologous stem cell transplantation. Patients achieving an MRD-negative state by next-generation sequencing had significantly improved progression-free survival (PFS) when compared with those who did not.^2,3  Furthermore, Dr. Avet-Loiseau demonstrated that next-generation sequencing was more sensitive than flow cytometry at detecting MRD, with a sensitivity of 10^-6 versus 10^-4, respectively. The greater the sensitivity of the MRD detection instrument, the more powerful the prognosticator of PFS. As MRD monitoring is incorporated into an increasing number of study designs, the exploration of the biological features of the residual PCs detected by this evaluation becomes of increasing interest and importance.

Dr. Bruno Paiva and colleagues conducted a novel evaluation of the genetic and phenotypic properties of MRD in a small cohort of patients from the GEN2010MAS65 study using multidimensional flow cytometry. In this study, newly diagnosed, transplant-ineligible patients were treated with either nine cycles of bortezomib, melphalan, and prednisone (VMP), or alternating cycles of VMP and lenalidomide and low-dose dexamethasone. Of the 40 patients treated, MRD was characterized in 12 of the postinduction samples. The goal of their analysis was to compare genetic and phenotypic features of PCs from bone marrow aspirates collected before treatment to specimens obtained postinduction. To date, no biological studies have been performed in primary chemoresistant clonal PCs detected at MRD levels following frontline therapy.

Immunophenotypic protein expression profiles were generated from clonal PCs of paired samples and demonstrated upregulation of integrins (CD11a, CD11c, CD29, CD49d, CD49e), chemokine receptors (CXCR4), and adhesion molecules (CD44/CD54) in the MRD samples. This suggested that the MRD clonal PCs might represent a phenotypic subset of the whole diagnostic tumor population. To further explore this hypothesis, Dr. Paiva investigated copy number alterations (CNAs) of matched diagnostic versus MRD clonal PCs. Of the 12 sets examined, three showed identical copy number profiles. In the remaining nine, there were unique CNAs identified in at least one of the two PC populations. In some samples, CNAs detected at diagnosis were no longer present in MRD clonal PCs, whereas in others, a selected number of genetic alterations became apparent only at the MRD stage.

Gene expression profiling was performed on seven available samples and showed significant downregulation of genes related to protein processing in the endoplasmic reticulum, protein export, and N-glycan biosynthesis in the MRD subclone. Loss of ALCAM has been linked to aggressive phenotypes in a variety of diseases. In this cohort, there was a trend toward decreased expression in the chemoresistant PCs. This deregulated gene expression profiling may contribute to plasma cell survival after multidrug therapy.