Abnormal Expression of the Plasma Cell Differentiation Factor X-Box Protein 1 (Xbp-1) in Waldenstrom’s Macroglobulinemia.

INTRODUCTION: Xbp-1 is a ubiquitously expressed basic leucine zipper protein which serves as key transcription factor for plasma cell differentiation and immunoglobulin production. XBP-1 mRNA undergoes splicing at an IRE-1a specific cleavage site to produce a spliced form (Xbp-1s) which due to a frame shift results in a more stable and potent transcription factor than the unspliced form (Xbp-1_us). Previous observations demonstrated that Xbp-1 was highly expressed in multiple myeloma (MM), though its expression in other B-cell disorders including Waldenstrom’s macroglobulinemia (WM) remains to be clarified.

METHODOLOGY: In this study, we investigated the different forms of Xbp-1 in CD19⁺ selected bone marrow (BM) cells from 61 patients with the consensus panel definition of WM using semi-quantitative PCR analysis. CD19⁺ selected BM cells from 6 healthy donors, and CD138⁺ selected BM cells from 4 MM patients were used as controls. All RT-PCR results were normalized to b-actin levels. Sequencing of Xbp-1 in CD19⁺ selected LPC was also performed for 20 WM patients.

RESULTS: In 9/61 (15%) WM patients, Xbp-1 transcripts were undetectable. Among WM patients expressing Xbp-1, as well as all MM patients evaluated, higher levels of XBP-1us were observed in comparison to normal healthy donors (p=0.001). Decreased XBP-1s transcript levels were also observed among WM and MM patients versus healthy donors, but did not reach statistical significance. Sequence analysis of Xbp1 in CD19⁺ selected BM LPC demonstrated variants 10/20 WM patients in exon 1 that are currently under investigation.

CONCLUSIONS: Abnormal expression of Xbp-1 is common in patients with WM, and includes loss of expression, and aberrations in splice patterns. The genetic basis for these findings is under investigation.