Plasma cell regulatory pathways in WM. In recent studies, we have attempted to dissect the molecular mechanisms which prevent WM cells from fully differentiating into plasma cells. Ordinarily, B-cells mature in a defined manner passing through the mature B-cell stage to lymphoplasmacytic cells, and then onto mature plasma cells. Mature plasma cells make antibodies that serve to protect us against pathogens, and typically include the IgA and IgG antibodies.
Earlier B-cells, including lymphoplasmacytic cells make IgM. WM represents a disease where cells are held back from fully differentiating into mature plasma cells, and as a results patients make abundant amounts of IgM protein, but have have a paucity of IgA and IgG production. A series of genes normal controls the orderly differentiation of mature B-cells into plasma cells, and includes the master gene BLIMP-1, and its downstream mediator XBP-1. These genes coordinate a series of genes which allow B-cells to become plasma cells and undergo “heavy chain class switching” which permits the switching off of IgM production in favor of IgA and IgG production. We therefore evaluated the BLIMP1-XBP1 pathway given its significance in mediating differentiation of B-cells into plasma cells. The BLIMP1 pathway also holds particular significance given that deletions in one copy of 6q21, where BLIMP1 is located, are found in 30 to 50% of WM patients as reported by us and others. We thoroughly examined the BLIMP1-XBP1 pathway by semi-quantitative and real time RT PCR analysis complemented by extensive gene sequence analysis of BLIMP1, XBP-1, IRE1, PAX-5 and BCL-6. We showed the presence of Blimp1, Pax5, Xbp1 transcripts in all WM patients samples studied. Sequence analysis showed that BLIMP1 and PAX5 did not harbor any mutations. Interestingly, we identified variations for IRE1 and XBP1 spliced transcripts expression in patients with WM compared to healthy donors, with the majority of patients demonstrating elevated expression levels of IRE1 and XBP1 spliced transcripts versus healthy donors. Although we did not identify any mutations in either XBP1 or IRE1 to account for these events, we did observe a high incidence of germ line single nucleotide polymorphisms in IRE1 that potentially could modulate XBP1 expression. Further studies are necessary to delineate what changes if any may occur in protein expression of IRE1 and XBP1 as a consequence of such germline variants. The over expression of IRE1 and XBP1 could also reflect the relative over-abundance of more differentiated lymphoplasmacytic and plasma cells that form the WM clone versus mature B-cells from healthy donors which were used for these comparative studies. Lastly, the silencing of growth regulatory genes by epigenetic modifiers i.e. methylation, acetylation, summylation, as well micrRNA expression, may account for the divergent expression of IRE1 and XBP1 observed between WM patients and healthy donors. The results of these studies were recently published (Leleu et al, British Journal of Hematology 2008).
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