The RNA Binding Protein Musashi 2 Is up-Regulated in the Proliferative B-Cell Fraction of Chronic Lymphocytic Leukemia Clones

Chronic Lymphocytic Leukemia (CLL) is an incurable disease in which most of the tumor cells in the blood are arrested in G0/G1 stages of the cell cycle with only a minimal number displaying proliferative activity. In this regard, our group has found by gene expression profiling (GEP) that the proliferative fraction (PF) of CLL cells is enriched in the intraclonal subset marked by CXCR4^dim CD5^brite expression. Indeed, this subset differs by more than 1000 genes from the CXCR4^brite CD5^dim resting fraction (RF). The genes over-expressed in the PF relate to replication and migration as well as regulation of gene expression. One of these genes is Musashi 2 (MSI2). Of note, MSI2 is expressed at the highest levels in IGHV unmutated CLL (U-CLL) clones and their PFs.

Normally, MSI2 binds mRNA and blocks translation of proteins, playing an important role in post-transcriptional regulation. In addition, MSI2 has been linked to proliferation of normal and malignant stem cells, tumorigenesis, and poor prognosis. In CLL, high MSI2 mRNA expression has been identified in patients with worse prognosis. Nevertheless, nothing is known about the function of MSI2 in CLL cells. Therefore, we have begun to study the biological role of MSI2 in B-CLL cells and its possible association with B-cell proliferation and CLL disease progression.

Fist, we studied MSI2 protein expression by flow cytometry in CD19+ B cells from healthy donors (HD) and CD5+CD19+ cells from CLL patients, observing an up-regulation in CLL compared to HD. Also, we documented higher MSI2 expression in U-CLL compared to IGHV-mutated (M-CLL) CLL as well as HD. Within the leukemic clone, we observed that MSI2 expression was highest in the PF, lower in the intermediate (INT) fraction (defined as CXCR4^int CD5^int), and much lower in the RF (PF>INT>RF). The PF expressed 40% more MSI2 than the RF, suggesting that the highest amounts of MSI2 protein is in dividing and recently-divided cells of the clone.

Since CLL B-cell proliferation occurs in the microenvironment of lymphoid organs, presumably delivered by external signals, we tested whether such signals could stimulate MSI2 expression. After stimulating CLL cells with TLR9 agonist + IL15 + IL2 in vitro MSI2 protein was up-regulated form 0.3 to 2.5 fold. In addition, the increase in MSI2 protein was associated with an enhancement in Ki-67+ cells and in phosphorylation of MAPK/ERK and AKT signaling components, measured by flow cytometry. These results suggest that signals from the microenvironment that induce cell growth and proliferation lead to MSI2 synthesis in CLL B cells.

In order to study a possible association between MSI2 expression and cell division, we labeled CLL PBMCs with a dilutable cell tracer, CFSE, and then stimulated them in vitro with TLR9 agonist + IL15 + IL2. These studies indicated that MSI2 protein synthesis was increased in the activated cells and that MSI2 protein levels increased with each cell division. However, it was also clear that this increase was not directly associated to the extent of cell replication as CLL B cells from only 10% of the patients underwent 4 cycles of cell division. Since we observed an increase in MSI2 and Ki-67 expression after stimulation in all patients' clones but did not detect replication of CLL cells in all patients, we studied the effects of in vitro stimulation on cell cycle entry and completion and how this related to MSI2 expression. Experiments using propidium iodide to evaluate DNA content of PBMCs showed that in vitro stimulation increased the percentage of cells in S phase (5-25%) compared to control cells without activation (<5%), whereas only a small fraction of cells entered the M/G2 phases, with or without activation (<1% and <0.5%, respectably) suggesting that only a small portion of the cells completed the cell cycle and divided. Hence, MSI2 synthesis corresponds with DNA replication and not cell division, suggesting that MSI2 could be an important molecule involved in entry into and/or in the early phases of the cell cycle.

These results, and the facts that MSI2 plays an important role in post-transcriptional regulation and is associated with cell proliferation and poor prognosis in cancer, suggest that a better understanding of the role of MSI2 in CLL patients will provide clues to understanding the birth and growth of CLL B cells and to identifying and designing new therapeutic strategies for the disease.