Structural organization of the bcr gene and its role in the Ph′ translocation

Abstract

The Philadelphia (Ph′) chromosome, an abnormal chromosome 22 (ref. 1), is one of the best-known examples of a specific human chromosomal abnormality strongly associated with one form of human leukaemia, chronic myelocytic leukaemia (CML). The finding² that a small region of chromosome 9 which includes the c-abl oncogene is translocated to chromosome 22 prompted studies to elucidate the molecular mechanisms involved in this disease. We have demonstrated previously that the chromosome 9 of one patient with CML contains a breakpoint 14 kilobases (kb) 5′ of the most 5′ v-abl-homologous exon³. These data suggest a role for c-abl in CML, a theory supported by the presence of an abnormally sized abl messenger RNA^4,5 and protein⁶ in the CML cell line K562. The region involved in the translocation on chromosome 22 has also been identified: all Ph′-positive patients examined to date have a breakpoint within a 5.8-kb region, for which we have proposed the name ‘breakpoint cluster region’ (bcr)⁷. To determine whether her contains protein-encoding regions, probes from bcr were tested for their ability to hybridize to complementary DNA sequences. A 0.6-kb HindIII/BamHl bcr restriction enzyme fragment proved suitable for isolating several cDNA clones from a human fibroblast cDNA library⁸. Using bcr cDNA sequences, we obtained data strongly suggesting the presence of a chimaeric bcr/abl mRNA in the leukaemic cells of Ph′-positive CML patients. The recent isolation of cDNA clones containing bcr and abl sequences confirms this finding¹². Because the bcr part of the chimaeric mRNA could be required to induce the transforming activity of the human c-abl oncogene, we have now initiated studies to characterize the normal ‘bcr gene’ and to determine the effect of a translocation within its coding domain. We demonstrate that as a result of the Ph′ translocation, a variable number of bcr exons are included in the chimaeric bcr/abl mRNA. The bcr gene sequences in this mRNA could be responsible for the transition of the abl cellular proto-oncogene into an oncogene.