Novel Spliced MLL Fusions Have Been Identified Involving the MLL Partner Genes ELL, EPS15, MLLT3, and SEPT5.

Chromosomal rearrangements of the human MLL gene are a genetic hallmark for aggressive acute leukemias. More than 60 partner genes have been characterized at the molecular level until now. The observed recombination mechanisms between MLL and its many recombination partner genes include reciprocal chromosomal translocations, insertions, and intrachromosomal 11q deletions and inversion. “Spliced MLL fusions”, have recently been described as a novel mechanism to generate functional chimeric fusion transcripts for 50% of t(11;19)(q23;p13.3) leukemia patients where the MLL gene is fused to the MLLT1 (ENL) gene and for two single leukemia patients. One patient had a t(11;15) involving the ZFYVE19 gene and the other patient had a 11q interstitial deletion involving the DCPS gene. Within these cases the disrupted MLL gene is fused only on RNA level to a functional transcription unit located in the vicinity of the breakpoint. Here we describe eight novel “spliced MLL fusions” involving the MLL partner genes ELL (1x), EPS15 (3x), MLLT3 (1x), and SEPT5 (1x). For ELL, MLLT3 and SEPT5 the identified spliced fusion seems to be a single or rare event. However 1/3 of the t(1;11)(p32;q23) leukemia cases exhibit “spliced MLL fusions”. I.e. approximately 1/3 of all breakpoints are located upstream of the EPS15 gene. Further studies will help to investigate the remaining question whether these two groups of t(1;11)(p32;q23) leukemia patients have any significant difference in outcome. Interestingly, for the t(11;22)(q23;q11.2) leukemia the major identified fusion transcript has an additional 57 bp exon derived from the 2 kb area up-stream of the SEPT5 gene. Furthermore this patient has a T-ALL and thus differs from all other cases where the patients show myeloid markers when the MLL gene is fused to one of the five members of the septine family. This is leading to the question whether or not this 57 bp big exon can be made responsible for the lineage shift. With these results the number of genes involved in „spliced MLL fusions” has increased from three to seven. Supported in part by grants Ma 1876/7-1, /8-1 and /9-1 from the DFG, grant N1KR-S12T13 from the BMBF and grant 102362 from the Deutsche Krebshilfe.