Clonal expansion of a new MLL rearrangement in the absence of leukemia.

Clonal expansion of mixed-lineage leukemia (MLL) rearrangements is highly indicative for acute leukemia. 1-5 Here, we report the unusual finding of an MLL rearrangement in the absence of leukemia. A 5-year-old girl was diagnosed with acute myeloid leukemia (AML) characterized by a variant t(8;21) translocation. The patient achieved remission, and is now, 4 years later, still in ongoing remission. However, a follow-up control of the bone marrow 15 months after diagnosis revealed a new clone characterized by the MLL rearrangement t(11;11)(q13;q23). To exclude the possibility of a secondary AML, numerous bone marrow control samples were subsequently investigated over a 30-month period. During this period, the amount of MLL rearranged cells was decreasing from 50% to 20% as determined by karyotype and fluorescence in situ hybridization (FISH) analysis. Morphology was unsuspicious without any sign for a secondary malignancy in all samples. The breakpoint region of the MLL partner gene could be mapped to chromosome 11 at band q13 (Figure 1A). Using a newly developed restriction enzyme fragment length polymorphism– based cloning strategy, a new MLL partner gene was identified as ARHGEF17 (Figure 1B). 6 The ARHGEF17 protein is a Rho guanine nucleotide exchange factor involved in cellular signaling. 7 The fusion between the MLL and the ARHGEF17 genes occurred in introns 12 and 1, respectively. The reciprocal allele was isolated as well and fused ARHGEF17 intron 1 with MLL intron 12. The particular chimeric transcript leads to an in-frame fusion gene that expresses the corresponding fusion RNA (Figure 1C). To discriminate between a rearrangement that already existed at diagnosis at a low level from a secondary one, molecular analysis of the chromosomal breakpoint region was performed in the diagnostic material. However, detection of the MLL-ARHGEF17 fusion gene sequence by using specific primers failed, indicating the appearance of a secondary, potentially treatment-induced rearrangement. To further illuminate the biology of our observation, we precisely analyzed the morphology of those cells carrying the MLL-ARHGEF17 fusion gene 30 months after initial detection of the rearrangement (Figure 1D, Table 1). 8 The novel MLL rearrangement could only be detected in cells within the myeloid lineage. Remarkably, all cell types of the myeloid lineage, from promyelo-cytes to differentiated polymorphonuclear neutrophils, were affected by the novel fusion gene, highlighting 2 conclusions: first, the MLL-ARHGEF17 rearrangement is an early clonal event in myelopoiesis, and second, the chimeric protein does not lead to a block of the myeloid differentiation. Our …