XBP1 Expression Is An Important Prognostic Factor for Newly Diagnosed Myeloma Patients.
Authors: Bagratuni Tina , Wu Ping , de Castro David Gonzalez , Davenport Emma L , Walker Brian A , Jenner Matthew W , Johnson David C , Morgan Gareth J. , Davies Faith E 
Published: April 2020

Abstract
The immunoglobulin production of myeloma plasma cells means they are dependent on the unfolded protein response process (UPR), which controls protein production and ensures its proper translation and folding. Knockout mouse studies have highlighted the importance of IRE1, a transducer of the UPR signal and XBP1 its downstream target, in B cell development. Gene expression studies show that XBP1 is highly expressed in myeloma plasma cells and it is consistently deregulated in clinical samples. XBP1s, the active moiety, is a known myeloma survival factor and IgVH-XBP1s transgenic mice develop myeloma. In this work we demonstrate that XBP1s levels correlate with survival of myeloma patients and that targeting it with small molecules may be a useful therapeutic strategy. We have re-sequenced IRE1 and determined the gene expression levels of IRE1, total XBP1, XBP1 unspliced (XBP1u), and XBP1 spliced (XBP1s) in a large series of myeloma patients (n=260), and correlated biological findings with clinical characteristics and patient outcome. To quantitatively assess the expression of XBP1s and XBP1u variants, we have designed a novel real time PCR method (Lux Technology, Invitrogen) capable of determining the ratio of these transcripts within a sample. These results show that total IRE1 and XBP1 are highly expressed in all patients. In addition all patients expressed higher levels of XBP1s, the active transcription factor which is anti-apoptotic, compared to the inactive precursor XBP1u. We correlated XBP1s/u gene expression levels with clinical parameters in 159 newly diagnosed patients. This group of patients contained 68 females and 91 males with a median age of 64 years (range 39–89), B2M median = 7.45mg/mL (range 1.1–30mg/mL), haemoglobin median = 10.2g/dL (range 5.7–14.7g/dL) and ISS status stage 1 = 21%, stage 2 = 36%, and stage 3 = 36%. The XBP1s/u ratio demonstrated a statistically significant effect on overall survival (OS) in patients with high B2M (B2M &gt;5), with a cut-off ratio of 2.8 for the XBP1s/u expression (assigned using the quartile method). In this group of patients those with higher XBP1s/u have shorter survival (28 months) compared to those with lower XBP1s/u (not reached, 50+ months). (p = 0.033). The cause for the high and variable XBP1s levels is uncertain but could be mediated by IRE activity. Sequence analysis of the kinase and endoribonuclease domain of IRE1 identified 3/50 patients with an inactivating mutation in the endoribonuclease domain (G923V): patients with this mutation had a lower ratio of XBP1s to XBP1u and a longer survival. In conclusion, this study is the first to demonstrate the clinical significance of high XBP1s levels in myeloma and shows that patients with high levels of XBP1s have a shorter overall survival. Coupled with our previous study highlighting the critical role of the UPR in myeloma and the report of the XBP1s transgenic mouse model, the importance of the IRE1-XBP1 axis myeloma is now established, and its role as a therapeutic target needs to be explored.