Targeted Next-Generation Sequencing for Detecting MLL Gene Fusions in Leukemia

Abstract: Mixed Lineage Leukemia (MLL) gene rearrangements characterize approximately 70% of infant and 10% of adult and therapy-related leukemia. Conventional clinical diagnostics, including cytogenetics and fluorescence in situ hybridization (FISH) fail to detect MLL translocation partner genes (TPGs) in many patients. Long-Distance Inverse (LDI)-PCR, the 9gold standard9 technique that is used to characterize MLL breakpoints is laborious and requires a large input of genomic DNA (gDNA). To overcome the limitations of current techniques, a targeted Next-Generation Sequencing (NGS) approach that requires low RNA input was tested. Anchored Multiplex PCR based enrichment (AMP-E) was used to rapidly identify a broad range of MLL fusions in patient specimens. Libraries generated using Archer® FusionPlex® Heme and Myeloid panels were sequenced using the Illumina platform. Diagnostic specimens (n=39) from pediatric leukemia patients were tested with AMP-E and validated by LDI-PCR. In concordance with LDI-PCR, the AMP-E method successfully identified TPGs without prior knowledge. AMP-E identified 10 different MLL fusions in the 39 samples. Only two specimens were discordant; AMP-E successfully identified a MLL-MLLT1 fusion where LDI-PCR had failed to determine the breakpoint, whilst a MLL-MLLT3 fusion was not detected by AMP-E due to low expression of the fusion transcript. Sensitivity assays demonstrated that AMP-E can detect MLL-AFF1 in MV4-11 cell dilutions of 10-7 and transcripts down to 0.005 copies/ng. Implications: This study demonstrates a Next-Gen Sequencing methodology with improved sensitivity compared to current diagnostic methods for MLL-rearranged leukemia. Furthermore, this assay rapidly and reliably identifies MLL partner genes and patient-specific fusion sequences that could be used for monitoring minimal residual disease (MRD).