ABSTRACT
The MLL gene at chromosome band 11q23 is specifically cleaved at a unique site within its breakpoint cluster region (bcr) during the higher order chromatin fragmentation associated with apoptosis. We now show that the same specific DNA cleavage event can be detected in an exogenous MLL bcr fragment that is integrated into the genome outside of its normal chromosomal context, as well as in an extrachromosomal episome containing an MLL bcr fragment. We also show that episomal or randomly integrated copies of the MLL bcr behave similar to the endogenous MLL bcr when tested in a scaffold-associated region (SAR) assay. Furthermore, an episomal murine MLL bcr introduced into human cells is cleaved at the same site as the endogenous murine MLL bcr; this episomal murine MLL bcr also functions as a SAR in human cells. We conclude that both nuclear DNA scaffold attachment as well as site-specific DNA cleavage can be directed by sequences contained within the MLL bcr, and that it is feasible to study these events using episomal shuttle vectors.
Subject(s)
DNA-Binding Proteins/genetics , Gene Rearrangement , Proto-Oncogenes , Transcription Factors , Animals , Apoptosis/genetics , Binding Sites/genetics , Binding Sites/physiology , Chromosome Breakage/genetics , DNA, Neoplasm/genetics , DNA, Neoplasm/metabolism , Genetic Vectors/genetics , Histone-Lysine N-Methyltransferase , Humans , Jurkat Cells , Mice , Myeloid-Lymphoid Leukemia Protein , Nuclear Matrix/genetics , Nuclear Matrix/physiology , Plasmids/genetics , Precursor Cell Lymphoblastic Leukemia-Lymphoma/genetics , Precursor Cell Lymphoblastic Leukemia-Lymphoma/pathology , Tumor Cells, CulturedABSTRACT
The t(12;21)(p13;q22) translocation, fusing the ETV6 and AML1 genes, is the most frequent chromosomal translocation associated with pediatric B-cell precursor acute lymphoblastic leukemia. Although the genomic organization of the ETV6 gene and a breakpoint cluster region (bcr) in ETV6 intron 5 has been described, mapping of AML1 breakpoints has been hampered because of the large, hitherto unknown size of AML1 intron 1. Here, we report the mapping of the AML1 gene between exons 1 and 3, cloning of ETV6-AML1 breakpoints from different patients, and localization of the AML1 breakpoints within AML1 intron 1. In contrast to the tightly clustered ETV6 breakpoints, the AML1 breakpoints were found to be dispersed throughout AML1 intron 1. Although nucleotide sequence analysis of the breakpoint junctions showed several 5/7 matches for the V(D)J consensus heptamer recognition sequence, these matches were present only on the ETV6 alleles and not on the AML1 alleles, making it unlikely that the translocations were mediated by a simple V(D)J recombination mistake. Interestingly, several breakpoints as well as a stable insertion polymorphism mapped close to a polymorphic, alternating purine-pyrimidine tract in the ETV6 gene, suggesting that this region may be prone to DNA recombination events such as insertions or translocations. Finally, the presence of an insertional polymorphism within the ETV6 bcr must be recognized to avoid incorrect genotype designation based on Southern blot analysis.
