Molecular analysis of the t(1;19) breakpoint cluster region in pre-B cell acute lymphoblastic leukemias.

The t(1;19) chromosomal translocation in acute lymphoblastic pre-B cell leukemias involves the gene E2A for helix-loop-helix (HLH) proteins E12 and E47, ubiquitous transcriptional proteins implicated in the regulation of various lymphoid and nonlymphoid genes. To characterize the molecular features of the t(1;19)(q23;p13) translocation, we molecularly cloned breakpoint DNA from t(1;19)-carrying pre-B cell leukemias. In all cases, breakpoints on chromosome 19 occurred within 2 kb of each other in a single intron of the E2A gene. This clustered arrangement resulted in specific truncation of the E2A gene and transcript, with loss of sequences encoding the basic DNA-binding and HLH dimerization motifs from the derivative 19 chromosome. In contrast, breakpoints on chromosome 1 were distributed over a large region and could not be linked to exonic sequences of the PBX1 gene, although identical chromosome 1 sequences are joined to E2A sequences in 1;19 fusion transcripts. These data show that the 1;19 translocation consistently results in exchange of 3' exons encoding the HLH motifs of E2A with DNA from chromosome 1 to form a fusion gene on the derivative 19 chromosome.