A DNA probe combination for improved detection of MLL/11q23 breakpoints bydouble-color interphase-FISH in acute leukemias

Reciprocal translocations involving the MLL gene an chromosome band 11q23 h ave been observed in both acute myeloid leukemia (AML) and acute lymphoblas tic leukemia (ALL). In AML, identification of MLL breakpoints is an importa nt prognostic factor. Breakpoints are clustered in an 8 kb DNA fragment (bc r) and can be detected by Southern blotting or fluorescence in situ hybridi zation (FISH) analysis. Our objective in this study was to design a DNA pro be set that enables optimal detection of MLL rearrangements using interphas e FISH. Two PAC clones, 217A21 and 167K13, spanning the MLL gene with a min imal overlap in the bcr were isolated and labeled. Twenty-seven AML/ALL pat ients with cytogenetic 11q23 abnormalities, seven AML/ALL patients without. 11q23 abnormalities but MLL rearrangement by Southern blotting, and eight healthy donors were analyzed by FISH. We compared this double-color FISH an alysis with FISH using a YAC clone (yB22B2) and with Southern blotting. The PAC probe combination detects an MLL breakpoint in all cases with MLL rear rangement detected by Southern blotting except for cases with a partial tan dem duplication detected by reverse transcriptase-polymerase chain reaction (RT-PCR). FISH using the PAC probes also detected MLL breakpoints in four cases with MLL deletions telomeric to the breakpoint that could not be dete cted by the single probe yB22B2. This new probe set provides a reliable and rapid assay for che diagnosis of AML and ALL patients with MLL/11q23 break points. Genes Chromosomes Cancer 28:14-22, 2000. (C) 2000 Wiley-Liss, Inc.