Chimeric MEF2D and Dazap1 Fusion Proteins Are Created by a Variant t(1;19)(q23;p13.3) in Acute Lymphoblastic Leukemia (ALL).

The t(1;19)(q23;p13) is one of the most common chromosome translocations in ALL. In 90–95% of ALL cases with a t(1;19), the 19p13.3 gene E2A is fused to PBX1 located at 1q23, producing E2A-PBX1 chimeric proteins that possess transforming properties. The molecular abnormalities present in the 5–10% of ALL cases with a t(1;19) but no E2A-PBX1 fusion are unknown. TS-2 is an ALL cell line with a t(1;19)(q23;p13.3) but no E2A-PBX1 fusion. We used fluoresence in situ hybridization to localize the chromosome 19 breakpoint in TS-2 to a region approximately 400 kilobases telomeric to E2A and found that the t(1;19) in TS-2 fuses the 19p13 gene DAZAP1 (deleted in azoospermia associated protein 1) to the 1q23 gene MEF2D (myocte enhancer factor 2D). We cloned and sequenced the fusion genes and found they encode for reciprocal in-frame DAZAP1/MEF2D and MEF2D/DAZAP1 fusion transcripts, both of which are expressed in TS-2. MEF2D is a member of the MEF2 family of DNA binding proteins, which were originally characterized as muscle-specific transcription factors that regulated transcription of genes involved in myogenic differentiation. MEF2 proteins are now recognized to have more diverse functions: they are transcriptional effectors of mitogenic signaling pathways and inflammation, play critical roles in calcium-regulated signaling pathways that mediate survival of neurons and T-lympocytes, and participate in neuronal plasticity. DAZAP1 is a protein with novel RNA binding properties that is expressed most abundantly in testis and to a lesser extent in thymus. MEF2D-DAZAP1 includes the MEF2D MADS (MCM1, agamous, deficiens, and serum response factor) box and adjacent MEF2D domain that mediate sequence-specific DNA binding and protein-protein interactions, as well as one of two MEF2D transcriptional activation domains (TAD) fused to the C-terminus of DAZAP1. The DAZAP1-MEF2D chimera includes an intact first and truncated second RNA recognition motif from DAZAP1 joined to the C-terminus of MEF2D that includes its second TAD. We performed electrophoretic mobility shift assays using cognate and mutant MEF2D DNA recognition sites and found that MEF2D/DAZAP1 binds avidly and specifically to DNA in a manner indistinguishable from that of native MEF2D. We found that MEF2D/DAZAP1 activated transcription of a luciferase reporter gene under control of MEF2D recognition elements with substantially more potency than did wild type MEF2D. We also show that DAZAP1/MEF2D proteins bind RNA in a sequence specific manner analogous to that of wild type DAZAP1. MEF2D has been identified as a candidate oncogene involved in development of leukemia/lymphoma via murine retroviral insertional mutagenesis studies. Our data implicate MEF2D in human cancer and suggest that MEF2D/DAZAP1 and/or DAZAP1/MEF2D contributes to leukemogenesis by altering signaling pathways normally regulated by wild type MEF2D and DAZAP1.