CRLF2 and JAK2 Mutations: Occurrence and Function In Pre-B ALL Cell Lines

Abstract 5109

Thymic stromal lymphopoietin (TSLP), a cytokine produced by epithelial cells promotes early B-cell development and activates dendritic cells. It has recently been reported that a subset of B-cell precursor acute lymphoblastic leukemia (pre-B ALL) overexpresses the TSLP receptor CRLF2. CRLF2 overexpression is linked to translocations between sex chromosomes – localizing CRLF2 – and the immunoglobulin heavy chain locus on chromosome 14, or to an interstitial deletion on the gonosomes. Both events, translocation and deletion juxtapose CRLF2 to a different promoter (IgH or P2RY8). Performing quantitative real-time PCR we tested pre-B ALL and acute myeloid leukemia (AML) cell lines for overexpression of CRLF2. AML cell lines were included in the screening because we knew from an earlier TSLP project that the AML cell line MUTZ-3 is TSLP-responsive, and thus positive for the cytokine receptor. Three of 63 (5%) pre-B ALL cell lines tested (INC, MHH-CALL4, MUTZ-5) overexpressed CRLF2 mRNA. CRLF2-high cell lines carry a t(14;Y). With respect to the 58 AML cell lines tested: some expressed CRLF2 mRNA, but none of them rivalled the aforementioned pre-B cell lines. Pre-B ALL cell lines show the association between chromosomal CRLF2 aberrations and JAK2 pseudokinase domain mutations that has been described for primary pre-B ALL cells: cell lines MHH-CALL4 (JAK2I682F) and MUTZ-5 (JAK2R683G) and – newly described - also the CRLF2-high pre-B ALL cell line INC express a mutated version of Janus kinase 2 (JAK2R683G). We established a PCR based assay system that allowed for the rapid detection of the JAK2R683G mutation: none of the CRLF2-low or –negative pre-B ALL cell lines exhibited this mutation. All three CRLF2-high/JAK2mu cell lines showed high phosphorylation levels of the JAK2 downstream target STAT5. Inhibition of the JAK kinase led to dephosphorylation of STAT5. However, repression of 3H-thymidine uptake and induction of apoptosis by inhibition of the JAK2/STAT5 pathway was weaker in the JAK2mu pre-B ALL cell lines than in the JAK2V617F positive essential thrombocythemia-derived cell line SET-2. Provided that these results reflect the situation in primary cells, mutated JAK2 seems to be of lesser importance for growth and survival of pre-B ALL cells than for cells from myeloproliferative neoplasms. The CRLF2-high/JAK2mu cell lines INC, MHH-CALL4 and MUTZ-5 are promising model systems for the study of the roles of high-level CRLF2 expression and of JAK2 mutations in pre-B ALL.