Expression of XIAP in Multiple Myeloma Patients

Introduction of novel drugs, in particular, proteasome inhibitors, for the treatment of Multiple Myeloma (MM) patients has significantly improved treatment response and overall survival. One of the effects of proteasome inhibition is down-regulation of the transcription factor NF kB that stimulates the expression of apoptosis inhibitors (IAPs). The expression of IAPs protects cells from the death due to temporary apoptotic stimuli. The overexpression of IAPs is one of the characteristics of malignant cells. A crucial gene of the IAPs family is XIAP which encodes a protein which contains not only the caspase 3 and 7 blocking domain BIR2, but also a unique caspase 9 inhibiting domain BIR3. Therefore, XIAP is able to block both apoptosis pathways: one that depends on external signals and the other that depends on mitochondrial activity. In addition, the RING domain of XIAP has an E3 ubiquitin ligase activity. The aim of our study was to investigate the XIAP expression in MM patients at diagnosis and during chemotherapy, especially with proteosome inhibitors. Our study included 25 primary MM patients; all of them have given informed consent. The median age was 48 years (range 31–62). IgG MM was diagnosed in 22 cases, IgA MM in 1 and Light Chain MM in 2. Initial treatment consisted of 3 cycles of VAD. If CR or PR were not achieved, the treatment was changed to bortezomib 1.3 mg/m² on days 1,4,8 and 11 and dexamethasone (dex) 40 mg daily on days 1–4 days (4–6 cycles). If CR or PR was attained, Stem Cell mobilization was performed with Cyclophosphamide 6 mg/m² +G-CSF. Melphalan at 200 mg/m² was given before auto-SCT. The XIAP expression level was analyzed before therapy (n=25), after VAD (n=12), after bortezomib (n=6) and after auto-SCT (n=4). XIAP expression was evaluated quantitatively by means of RQ-PCR using ABL gene expression for normalization. In primary MM patients the XIAP expression was found in 100%. The meaning of XIAP/ABL*100% varied in the range of 5 to 5382% (median 22%). 24% of MM patients demonstrated XIAP hyperexpression (XIAP/ABL*100%>40%). In the control group of healthy donors the XIAP expression level was not more 13%. We subdivided MM patient into two groups according to XIAP/ABL*100% meaning: I<40%, II>40%. The comparison of M-protein, beta-microglobulin and albumin levels did not reveal any difference between these two groups. However, in group II (with primary XIAP hyperexpression) we observed the decrease of XIAP expression paralled tumor reduction (from more then 40% to 5–20%). On the contrast, in group I the XIAP gene expression increased right after chemotherapy initiation to extremely high levels of 2425%. But, after high dose melphalan and auto-SCT, the XIAP level significantly decreased (22–157%) along with the attainment of CR or very good PR. The level of the XIAP gene expression was also evaluated after the bortezomib treatment. After 4–6 courses of bortesomib + dex in all 6 MM patients CR + PR were achieved, that correlated with XIAP reduction to 8–25%.

Conclusion: In MM patients at diagnosis, the level of the XIAP expression is high. The decrease of the XIAP expression correlates with the chemotherapy and proteasome inhibitor treatment efficicacy. XIAP expression comes to the normal values at the time of CR and PR achievement.