Molecular analysis of breaks in BCL-1 proto-oncogene in B-cell lymphomas with abnormalities of 11q13

The t(11;14)(q13;q32) is a recurring translocation that occurs infrequently but non-randomly in B-cell chronic lymphocytic leukemia, B-cell non-Hodgkin's lymphoma, and multiple myeloma. The putative oncogene BCL-1 located at the chromosomal band 11q13 has been cloned previously from a B-cell CLL with t(11;14). We studied the molecular structure of the BCL-1 gene in eight B-cell NHL tumors that exhibited a break at 11q13. The cytogenetic changes in these tumors were t(11;14) in three, t(1;11;14)(q32;q13;q32) in two and dup(11)(pter→q23::11q13→ter) in three. The BCL-1 gene was found to have rearranged in two tumors. By Southern blot analysis of single and double digested DNA from placenta and from the tumors, we mapped the breakpoint in BCL-1 to a 0.5 kb Pst-I-HindIII restriction fragment which was approximately 2 kb away from the sites of previously mapped breakpoints. Sequential hybridization of Southern blots of these tumors with different immunoglobulin probes (for J region, Cu, Su) and BCL-1 probe identified co-migrating fragments with Su probe after BamHI restriction digestion. These results demonstrate that translocation breaks in the BCL-1 gene are not clustered in a short stretch of DNA at 11q13, and that the translocation related breaks in the immunoglobulin heavy chain can occur outside the joining region. The implications of these observations to the genesis of chromosomal translocations during B cell development are discussed.