AIMS: The MLL gene on chromosome 11q23 is frequently disrupted by chromosomal translocations in association with haematological malignancies. Recently, a specific site within the 8.3 kb MLL break-point cluster region that is cleaved during the early stages of apoptosis has been identified. Because MLL gene rearrangements are used to identify patients with high risk leukaemia, it was the aim of this study to determine whether this DNA cleavage event could be triggered in diagnostic bone marrow samples solely through ex vivo incubation at room temperature. METHODS: Pretreatment bone marrow samples were collected from six paediatric leukaemia patients. Genomic DNA for Southern blot analysis of MLL gene rearrangements was isolated immediately after samples were obtained and compared to genomic DNA isolated after incubation of specimens for 24-60 hours at room temperature, simulating delays in processing that might occur when samples are delivered to reference laboratories. In addition, cryopreserved samples from 70 paediatric leukaemia patients were screened for evidence of site specific MLL cleavage. RESULTS: After ex vivo incubation of bone marrow samples, site specific MLL cleavage resulting in a pseudo-rearrangement of the MLL gene was detected in two of six patients. In addition, a third patient with a similar MLL pseudo-rearrangement in cryopreserved cells was identified. CONCLUSIONS: Pseudo-rearrangement of the MLL gene at chromosome 11q23 was caused by ex vivo incubation of bone marrow samples. This novel phenomenon, which could lead to misclassification of leukaemia patients, might also be of importance for genotype analysis by Southern blotting at other loci.
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