Gene rearrangement and truncated mRNA in cell lines with 11q23 translocation

We have previously demonstrated that the breakpoints of t(11;19)(q23;p13) leukemias are within 360 kb of the CD3 gene. One of the phage clones, 6n, which was isolated from the yeast artificial chromosome clone yB22B2 containing CD3, was found to be within 60 kb of t(11;19) breakpoints. In this study, gene walking was conducted and two phage clones (lambda Hp8-3 and lambda Hp23-13) were isolated from a human placenta genomic library. Southern blot analysis with a genomic probe from lambda Hp8-3 detected gene rearrangements in t(4;11) and t(11;19) cell lines with BamHI digestion. Subsequently, using reiterated sequence-free probes from both ends of 6n that detected transcriptional units in various hematopoietic cells, we isolated cDNA clones. These cDNA clones were classified into two groups (designated MLL-a and MLL-b), which do not hybridize to each other. Northern blot analysis with MLL-a cDNA detected 15-, 14- and 12-kb mRNAs, while MLL-b detected the additional 9.7- and 5-kb mRNAs in peripheral blood lymphocytes. MLL-b cDNA detected a truncated form of 12.5-kb mRNA in t(4;11) cell lines and a truncated form of 10-kb or 9.2-kb mRNA in t(11;19) cell lines. MLL-a did not demonstrate a truncated form of mRNA, but the stronger 14-kb signal was noted in t(4;11) cells, while this signal was very weak in t(11;19) cells. By Southern blot analysis, MLL-b cDNA detected gene rearrangement in cell lines with t(4;11) and t(11;19), whereas MLL-a did not. Furthermore, chimeric cDNA clones were isolated from cDNA libraries of t(4;11) and t(11;19) cell lines with a MLL-b cDNA probe. These results indicate that the MLL-b cDNA is derived from the common target gene involved in 11q23 translocation with 4q21 or 19p13.