Involvement of Ets Factor Spi-B, E Protein E2-2, and Id2 in Waldenström's Macroglobulinemia.

Abstract 2946

Poster Board II-922

Waldenström's macroglobulinemia (WM) is an incurable B cell disorder with a lymphoplasmacytic infiltrate in the bone marrow (BM) and IgM monoclonal gammopathy. WM tumor cells show variable differentiation, ranging from mature B-cells to plasma cells, which likely results from failure to fully undergo differentiation. In this study, we analyzed the expression of several genes involved in B cell differentiation by real time RT-PCR, such as Ets factors, the basic helix-loop-helix (bHLH) E proteins, as well as the inhibitors of DNA binding (Id) proteins which antagonize E protein activity. Comparison of BM CD19+ B cells obtained from 13 WM patients with 6 age-matched healthy donors showed that expression of the Ets factor Spi-B was increased four-fold, while Id2 was decreased three-fold. However, transcript levels of E proteins were similar between the two groups. Transduction of Spi-B in BCWM.1 WM cells resulted in two-fold higher levels of Id2 and five-fold lower levels of E2-2 compared with control. Id2 transduced BCWM.1 cells expressed two-fold lower levels of E2-2 and Spi-B. Taken together, these results implicate that increased expression of Spi-B alone cannot suppress Id2 transcription in the absence of E2-2 activity. Interestingly, overexpressing Spi-B while concomitantly knocking down Id2 increased the expression of the XBP-1 splicing isoform 2.5-fold without changing levels of Blimp-1 and IRF4. Moreover, inhibition of Spi-B expression by RNA interference or forced expression of Id2 in transduced BCWM.1 cells induced a significant decrease of anti-apoptotic Bcl-2. Importantly, we also showed that Spi-B co-immunoprecipated with Blimp-1 in nuclear extracts. Collectively, these data suggest that the regulatory network of the Spi-B, E2-2, and Id2 plays an essential role in B cell differentiation as well as the pathogenesis of WM, and suggests that Spi-B overexpression may block WM cell differentiation by sequestration of Blimp-1 while promoting tumor cell survival though up-regulation of Bcl-2.