Recurrent translocations involving the MLL gene on chromosome band 11q23 together with diverse partners have been reported in both acute myeloid leukemia (AML) and acute lymphocytic leukemia (ALL). 1 Most translocations occur in the 8.3 kb breakpoint cluster region (BCR) of the MLL gene, between exons 8 and 12. The same region is also affected by a second MLL alteration, a partial tandem duplication (PTD) repeating the 50 part of the gene (Figure 1). MLL PTD is associated with a poor prognosis, 2 but how mutated MLL contributes to leukemogenesis is not well understood. Continuous cell lines have often helped to understand better how gene alterations influence the fate of a cell. Since there has been no report of cell lines carrying MLL PTD, we set out to screen a panel of cell lines (66 AML, 73 ALL) for this mutation. The ALL-derived cell lines were included, although to our knowledge no ALL cases with MLL PTD have yet been described. However, the MLL BCR is affected by both translocations and PTD, and MLL translocations are common not only in AML but also in ALL. 1, 3 Therefore, we reasoned that ALL-derived cell lines might also carry the MLL PTD.

We performed reverse-transcriptase polymerase chain reaction (RT-PCR) analysis to detect expression of MLLwt and MLL PTD (Figures 1 and 2). MLLwt was shown to be widely expressed in acute leukemia-derived cell lines: 138/139 cell lines expressed MLLwt mRNA (Figure 2). ML-2 was the only cell line without detectable MLLwt transcripts (Figure 2). This cell line had previously been shown to carry at (6; 11) and to have lost the second MLL allele. 4 In four of the 139 cell lines, CTS, EOL-1 (and sister cell line EOL-3), MUTZ-11 and UG-3, the MLL PTD RT-PCR was clearly positive (Figure 2). Sequencing of the RT-PCR products identified unique MLL exon 9/exon 3 fusions in cell lines EOL-1, MUTZ-11 and UG-3. Cell line CTS expressed transcripts of an MLL exon 10/exon 3 fusion