Molecular analysis of the genomic inversion and insertion of AF10 into MLL suggests a single-step event

The interstitial insertion of genetic material from one chromosome into another can achieve the type of gene-gene fusions more usually associated with chromosome translocations. An example of such an interstitial insertion, which has created an MLL-AF10 fusion in an acute myeloid leukaemia, has been analysed at the genomic level. The genomic fusion, which resulted in the juxtaposition of 3' AF10 sequence to 5' MLL sequence, was identified within MLL and AF10 intronic sequences. It was further established that the remaining 3' MLL sequence, from exon 6 onwards, was fused to novel sequence of unknown origin (named FM3 for fused to MLL 3'). The points of fusion of these 5' and 3' portions of MLL matched to adjacent nucleotides and lay between exons 5 and 6. The FM3 sequence was shown to be from chromosome arm 10p and located close to AF10 in a proximal position. It was subsequently demonstrated that in the leukaemia a third fusion existed between 5' AF10 and the FM3 sequence at a point immediately downstream from its fusion to MLL. It was therefore concluded that the MLL-AF10 gene fusion is the result of a simultaneous transposition of genetic material into the MLL gene and the joining of the remaining free ends on chromosome 10. This kind of event, characterised completely here for the first time, is a means to achieve a fusion when the genes involved lie in opposite orientations and results in three genomic junctions.