ALBERT

Description: Background Although remarkable advances have been achieved in MM therapy, mainly thanks to the introduction of novel-agent-based regimens, the disease remains incurable. Neoplastic CD138+ plasma cells are the hallmark of MM: both their expansion in the bone marrow (BM) and the production of monoclonal immunoglobulin (Ig) are responsible for the clinical manifestation of the disease. However, the existence of a Myeloma Propagating Cells (MPCs) has been proposed as a major cause of MM drug-resistance, leading to relapse. Several studies support the hypothesis that MPCs are phenotypically close to memory B cells residing in the CD138- compartment; however, very little is known concerning their molecular characteristics. Here we present an extensive molecular characterization of clonotypic CD19+ B cells clones obtained from newly diagnosed MM patients (pts), in order to recognize biological pathways possibly explaining the malignant clone’s persistence. Methods CD138+ and CD138- cell fractions were collected from BM and peripheral blood (PBL) of 50 newly diagnosed MM pts. CD19+ B cell and CD27+ memory B cell populations were isolated from CD138- cell fraction. Clonogenic assays were performed by plating cell fractions obtained from RPMI-8226 and NCI-H929 cell lines. The molecular characterization included: IgH gene rearrangement Sanger sequencing; analysis of the whole spectrum of genomic aberrations and gene expression profiling, by Affymetrix 6.0 SNPs array and HG-U133 Plus 2.0 microarray, respectively. Results Clonogenic assays showed that CD138- cells, plated on conditioned media, were able to form colonies after two weeks of culture more efficiently than CD138+ cells. By VDJ gene rearrangement sequencing, a clonal relationship between the CD138+ clone and the memory B ones was confirmed. SNPs arrays showed that both BM and PBL CD138+ cell fractions carried exactly the same genomic macro-alterations. On the contrary, in the CD138-19+27+ cell fractions from BM and PBL any macro-alteration was detected, whereas several micro-alterations (median number per sample: 32 amplifications and 16 losses, range: 8-122 Kb, average markers per region: 50) unique of the memory B cells clone were highlighted. An enrichment analysis revealed the involvement of genes affected by losses (17 genes) in both DNA repair mechanisms and transcriptional regulation and the involvement of genes affected by gains (46 genes) in both the negative regulation of apoptosis and the angiogenesis. Interestingly, KRAS, WWOX and XIAP genes, renown to be involved in MM pathogenesis, are located in the amplified regions in the immature cells. Moreover, several LOH regions were described, which covered at least 106 tumor suppressor genes involved in MM and leukemia (including TP53, CDKN2C and RASSF1A). Transcriptome profiles analysis of the CD19+ cell fractions highlighted pathways suggesting a possible involvement of immature cells in MM pathogenesis. The gene expression profiles of 20 MM CD19+ cells samples (12 from PBL, 8 from BM) were compared both to their normal counterpart and to the mature CD138+ cell fractions. In particular, unsupervised analysis by hierarchical clustering discriminated the differential expression of 11480 and 11360 probes in the PBL and BM CD19+ clones, respectively (2; FDR=0,05; p