Comparison of metaphase and interphase FISH monitoring of minimal residual disease with MLL gene probe: case study of AML with t(9;11).

The place of FISH in the monitoring of minimal residual disease (MRD) is yet to be fully characterised. Routine bone marrow cytogenetics at diagnosis in a 22 year old patient with acute myeloid leukemia FAB type M5 detected a translocation t(9;11)(p22;q23). We report our investigations to assess residual levels of translocation using a FISH probe designed to detect a gene split by the translocation. We used MLL (Oncor), a probe which spans the MLL gene at 11q23, in both metaphase and interphase preparations. At diagnosis, metaphase FISH showed 3 distinct cell lines-normal with 2 signals, abnormal with 3 signals and abnormal with 2 signals, while interphase FISH showed only 2 cell lines, one with 2 signals (which could be normal or abnormal) and one with 3 signals (split MLL). Following treatment, with the patient in clinical remission, 7 further cytogenetic analyses and 2 further FISH analyses were compared. Our results suggest that monitoring of the t(9;11) by metaphase FISH is feasible and straightforward compared to cytogenetics but interphase FISH may be problematic.