Identification of Mixed Lineage Leukemia Gene (MLL)/MLLT10 Fusion Transcripts by Reverse Transcription-PCR and Sequencing in a Case of AML With a FISH-Negative Cryptic MLL Rearrangement

Dear Editor The mixed lineage leukemia gene (MLL) on chromosome 11q23 is a frequent target of chromosomal translocations and rearrangements in childhood leukemia and adult therapy-related leukemia [1]. By using a commercially available dual-color FISH break-apart probe, almost all MLL rearrangements can be detected, including variant and cryptic forms [2-4]. We report a unique case of cryptic MLL rearrangement, in which the patient exhibited a karyotype that did not include t(10;11)(p12;q23) and FISH results did not reveal MLL break-apart signals; however, molecular analysis by reverse transcription (RT)-PCR revealed a 388-bp amplicon corresponding to the MLL/MLLT10 fusion transcript. A 17-yr-old male was admitted to our hospital for evaluation and management of multiple palpable neck lymphadenopathies. Blasts accounted for approximately 94% of cells on peripheral blood smear. On bone marrow (BM) aspiration smear, 90.5% of nucleated cells were blasts. Chromosomes were analyzed via GTG banding. Complex structural abnormalities were observed, and the karyotype was 46,XY,der(10)t(10;21)(p12;q21),der(12)del(12)(p11.2)add(12) (q24.2)[17]/46,XY[3] (Fig. 1). FISH analysis was performed according to the manufacturer’s instructions, including MLL (LSI Dual-Color, Break Apart Rearrangement Probe, Vysis, Downers Grove, IL, USA). No break-apart signals were observed for the MLL probe (Fig. 2). Multiplex RT-PCR was performed with HemaVision (DNA Technology, Aarhus, Denmark), according to the manufacturer’s instructions. Molecular analysis revealed an amplicon corresponding to the MLL/MLLT10 fusion transcript. RT-PCR analysis was performed with a combination of 5 ́ MLL and 3 ́ MLLT10 primers: MLL Forward 5 ́-GGAAGTCAAG CAAGCAGGTC-3 ́ and MLLT10 Reverse 5 ́-GCTGCCATTGATGAATTTG-3 ́. PCR products were subjected to direct sequencing by using the BigDye Terminator Cycle Sequencing Ready Reaction Kit (Applied Biosystems, Foster City, CA, USA) on ABI Prism 3130xl Genetic Analyzer (Applied Biosystems). RT-PCR revealed a 386-bp product ranging from the exon 8 region of the MLL gene to the exon 16 region of the MLLT10 gene, linked