P597: XPO1 MUTATIONS IDENTIFY EARLY STAGE CLL CHARACTERIZED BY SHORTER TIME TO FIRST TREATMENT AND ENHANCED BCR SIGNALING

Background: The XPO1 gene, which codes for a nuclear exportin responsible for the partitioning of macromolecules essential for cell homeostasis, represents one of the chronic lymphocytic leukemia (CLL) driver genes. In cases of XPO1 mutations a negatively charged glutamic acid at position E571 is substituted with a positively charged lysine, thus promoting XPO1 interaction with proteins bearing a negatively charged nuclear export signals (NES). Most of newly diagnosed CLL patients do not require therapy initially and are managed with a watch and wait strategy. CLL is characterized by a high grade of molecular heterogeneity and the analysis of gene mutations may further improve the stratification of time to first treatment (TTFT). Aims: To evaluate the transcriptomic and the epigenomic profile of XPO1 mutant CLL and the correlation of XPO1 mutations with TTFT. Methods: CAPP-seq mutational analysis was performed targeting the most frequently mutated genes in CLL. Results were correlated with TTFT. CD19+/CD5+ tumoral cells from 8 XPO1 mutated patients were sorted for RNA-seq and ATAC-seq. Fifteen XPO1 wild type cases, matched for IGHV status, TP53 status and FISH karyotype, were analyzed for comparative purposes. Results: By principal component analysis of ATAC-seq, XPO1 mutated CLL showed a distinct epigenomic and transcriptomic profile compared to wild type cases. Chromatin regions more accessible in XPO1 mutated CLL were enriched in binding sites for transcription factors downstream to the B cell receptor (BCR), including NF-kB signaling, p38-JNK, RAS-RAF-MEK-ERK (Fig.1A). By RNA-seq, 236 genes were upregulated, and 296 genes were downregulated in XPO1 mutated CLL compared to wild type cases. Upregulated pathways included immediate early response, TGFβ and EGF-EGFR signaling, FOSB gene expression and MAPK regulation through DUSP. By combining epigenomic and transcriptomic data, MIR155HG, miR-155 host gene, and MYB, the transcription factor that positively regulates MIR155HG, were upregulated by RNA-seq and their promoters were more accessible by ATAC-seq. MYB induces miR-155 expression that, in turns, inhibits SHIP1 thus enhancing the BCR cascade. Consistently, an inverse correlation was found between MIR155HG and SHIP1 levels (Fig.1B). As XPO1 mutated CLL were characterized by a high proliferative behavior, we asked if XPO1 mutated CLL patients are characterized by a more aggressive clinical course in terms of TTFT. A training validation design was used for the analysis. In the training cohort (N=276 Rai 0/I patients), XPO1 mutations associated with shorter TTFT (at 10 years 0% in XPO1 mutated and 69.8% in wild type patients p < 0.0001) (Fig.1C). Superimposable results were observed in two independent cohorts composed of 286 Binet A patients (at 6 years 25.0% in XPO1 mutated and 61.3% in wild type patients, p value = 0.025) (Fig.1D) and 395 Rai 0 (at 7 years of 0% in XPO1 mutated patients and 73.4% in wild type patients, p value < 0.0001) (Fig.1E). In addition, XPO1 mutations maintained their prognostic value independently of IGHV status and of variables that build the prognostic models predicting TTFT in early stage CLL. Summary/Conclusion: These data suggest that XPO1 mutations, conceivably through increased miR-155 levels, may enhance BCR signaling leading to higher proliferation and shorter TTFT in early stage CLL.Keywords: Chronic lymphocytic leukemia, B cell chronic lymphocytic leukemia