DOWN9S Syndrome Acute Lymphoblastic LEUKEMIA: A HIGHLY Heterogeneous DISEASE DRIVEN by an Aberrant CRLF2/JAK2 Cooperation – A REPORT FROM the Ibfm-STUDY GROUP.

Abstract 11 Children with Down9s Syndrome (DS) have an increased risk for acute lymphoblastic leukemia (ALL). We have previously identified somatic mutations in R683 of JAK2 in 20% of DS-ALLs (Bercovich et al, Lancet 2008;372:1484). However the cooperating cytokine receptor and other molecular features have been obscured. Here we report that gene expression analysis reveals that DS-ALL is a highly heterogeneous disease not definable as unique ALL subtype. However, 62% (33 of 53) of the DS-ALL samples analyzed were characterized by aberrant expression of the type I cytokine receptor CRLF2 caused by two types of genomic aberrations. CRLF2 encodes one chain of the heterodimeric receptor of the TSLP cytokine involved in inflammation and lymphoid development and has been recently reported to be involved in translocations to the IgH locus or micro-deletions in 3% of ALLs in children without DS. We have confirmed its cell surface expression on primary DS-ALL cells by Flow cytometry analysis. Consisting with a possible role of CRLF2 as a cytokine receptor, all specimens with the R683 mutated JAK2 had high CRLF2 expression. We further show that CRLF2 and R683 mutated JAK2 cooperate in conferring cytokine independent growth of pro-B cells. Furthermore in 3 of 23 DS-ALL patients with wt. JAK2 we identified a novel activating somatic mutation in CRLF2 replacing the juxta-membrane F232 with Cystein. Mutated CRLF2 but not wild type CRLF2 conferred cytokine independent growth of BaF3 cells associated with phosphorylation of STAT5. Aberrant expression of CRLF2 was also associated with a younger age at diagnosis and worse prognosis. Intriguingly, bioinformatic analysis revealed that gene expression of DS-ALLs is enriched with DNA damage and BCL6 responsive genes suggesting the possibility that DS is associated with B-cell lymphocytic genomic instability leading to the genomic aberrations activating CRLF2. Thus DS confers increased risk for genetically highly diverse ALLs with frequent somatic anomaly in CRLF2 , whose higher expression further selects for the activating mutations in itself or in JAK2 (figure). Our data also suggests that the majority of DS children with ALL may benefit from therapy blocking the CRLF2/JAK2 pathway. Disclosures: No relevant conflicts of interest to declare.