The Clinical Significance of a Novel microRNA Signature in Multiple Myeloma

Multiple myeloma (MM) is a hematologic malignancy characterized by infiltration of the bone marrow (BM) by malignant plasma cells. Multiple myeloma diagnosis is made by the presence of one or more of the CRAB criteria or one of the recently added biomarkers of malignancy. Smoldering MM (SMM) is a plasma cell dyscrasia preceding multiple myeloma, characterized by bone marrow infiltration of 10-60% and/or serum monoclonal protein ≥3g/dL or urinary monoclonal protein ≥500 mg per 24h, along with the absence of myeloma-defining events.

MicroRNAs (miRNA) are single-stranded, small non-coding RNA molecules (~21 nt) that regulate protein-coding gene expression at the post-transcriptional level, mainly through interactions with the 3′-untranslated region of target mRNAs. The results of such interactions can be mRNA degradation and/or translational repression, depending on the complementarity of the miRNA seed sequence with the mRNAs 3′-untranslated region. They can function as oncogenes or tumor suppressors, possessing a vital role in several aspects of all stages of tumorigenesis and cancer progression. In the present study, we have investigated the clinical value of a molecular signature consisting of 10 cancer-related miRNAs in MM: miR-15a, miR-16, miR-21, miR-221, miR-222, miR-25, miR-125, miR-155, miR-223, and miR-181a. These molecules were selected due to their well-documented role and clinical significance in numerous human malignancies. More specifically, miR-15a and miR-16 expression levels have been associated with chronic lymphocytic leukemia. The deletion 13q14, the most prevalent alteration in CLL, leads to the deletion of these miRNAs, which act on cell proliferation and in the process of apoptosis. miR-221 and miR-222 form a cluster that has been correlated with tumorigenesis and unfavorable prognosis in human malignancies, while miR-155 is a pro-inflammatory, oncogenic molecule, with a potential role in chronic lymphocytic leukemia.

Bone marrow aspiration samples were collected from 94 patients with MM and SMM, and CD138+ plasma cells were positively selected using magnetic beads coated with an anti-CD138 antibody. Total RNA was isolated using TRIZol. Thereafter, 200ng RNA of each sample were polyadenylated at the 3´ end and reversely transcribed. An in-house developed real-time quantitative PCR assay was conducted and the results were biostatistically analyzed. For the normalization of the expression levels of each miRNA, the mean expression of two small nucleolar RNAs (RNU43 and RNU48) was used as reference.

Seventy six out of the 94 BM aspiration samples were derived from MM patients and 18 of them from SMM patients, at the time of diagnosis. The MM patients were classified, according to the R-ISS staging system, as follows: 15 patients had stage I disease, 42 patients had stage II, and 19 patients stage III MM. Forty nine myeloma patients presented with osteolytic lesions at diagnosis.

The statistical analysis revealed significantly lower expression levels of miR-16 (p=0.036) and miR-155 (p=0.045) in CD138+ cells of MM patients, compared to those from SMM patients, highlighting their potential value to discriminate MM from SMM. Furthermore, miR-221 and miR-222 expression levels were negatively correlated with R-ISS; thus, miR-221 and miR-222 expression was significantly downregulated in MM patients with R-ISS stage III (p=0.004 and 0.034, respectively). This tendency reveals a potential favorable prognostic value of miR-221/222 cluster in MM. Next, miR-15a and miR-16 expression was shown to be associated with the presence of osteolytic lesions. In this regard, the expression levels of miR-15a (p=0.048) and miR-16 (p=0.047) were decreased in MM patients with bone disease, compared to those without bone disease. The observed decreased expression of these two miRNAs in symptomatic MM patients could constitute a predictive biomarker for the occurrence of bone disease and, hence, a putative predictive biomarker of SMM patients at high risk of evolution to symptomatic disease with bone lesions.

We conclude that miR-221/222 correlate with more favorable R-ISS stage, while miR-15a and miR-16 correlate with the presence of osteolytic disease in MM. This ongoing study will further reveal the possible prognostic significance of this 10 miRNAs signature studied, when response to therapy, progression-free and overall survival is available.