Detection of unique ALL1 (MLL) fusion transcripts in normal human bone marrow and blood: distinct origin of normal versus leukemic ALL1 fusion transcripts.

The partial tandem duplication (PTD) of ALL1 (MLL) is one of the more common molecular abnormalities in adult de novo acute myeloid leukemia (AML) and carries a poor prognosis. The PTD of ALL1 is identified in leukemic blasts at the genomic level by ALL1 rearrangement upon Southern analysis and by genomic fusion following DNA PCR. The genomic defect encodes for a unique fusion transcript that is readily detected by reverse transcription-PCR (RT-PCR). To determine if the ALL1 fusion transcript is specific for leukemic blasts or instead can be found with any frequency in normal cells, we analyzed 52 bone marrow and 8 peripheral blood samples from 60 normal donors by nested RT-PCR. Ten of 60 samples (16%; 7 bone marrow and 3 peripheral blood) contained a unique transcript showing a fusion of two ALL1 exons that was consistent with the PTD of ALL1. However, a corresponding genomic rearrangement or a unique genomic fusion of ALL1 could not be demonstrated by Southern analysis or DNA PCR, respectively. Marked differences were observed in the size and sequence of the ALL1 fusion transcripts detected in normal donors, compared to those detected in leukemic patients with a PTD of ALL1. Moreover, although the ALL1 fusion transcripts seen in AML always maintain the open reading frame, the open reading frame was preserved in only 5 of 10 fusion transcripts from normal donors. Finally, in contrast to leukemic blasts with the PTD of ALL1, the fusion transcripts in normal cells could not be detected in the poly(A)+ RNA fraction by RT-PCR. In summary, the origin and the composition of the ALL1 fusion transcripts found in normal cells appear to be distinct from those found in the leukemic cells. The data accumulated thus far suggest that the ALL1 fusion product detected in normal tissue results from the process of differential mRNA splicing rather than true ALL1 gene rearrangement. These findings also suggest caution in the use of RT-PCR for detection of minimal residual disease in AML patients with the PTD of ALL1.