Duplicated regions of AF-4 intron 4 at t(4;11) translocation breakpoints.

AF-4 is a common partner gene of MLL. AF-4 breakpoints occur in introns, but most AF-4 introns are uncharacterized. We cloned AF-4 intron 4 and examined the frequency of breakpoints in this intron. The 5.8-kb intron is rich in repeat sequences and was the site of translocation in 3 of 17 leukemias with t(4;11). We cloned the der (11) and der (4) breakpoints and isolated the fusion transcripts in the cell line MV4-11 and in a de novo acute lymphoblastic leukemia (ALL). Both translocations joined MLL intron 6 and AF-4 intron 4. In MV4-11, 249 bases from AF-4 were present in both derivative chromosomes, indicating duplication. In the de novo ALL, duplication of 446 bases from MLL and AF-4 occurred. Reciprocal fusion transcripts were expressed. Intronic sequence of AF-4 is useful for molecular diagnosis of t(4;11). Duplicated intronic regions suggest staggered chromosomal breakage.