Detection of unexpected clones of monosomy 7 in childhood acute lymphoblastic leukemia using fluorescence in situ hybridization.

The feasibility of a fluorescence in situ hybridization (FISH) technique for the detection of leukemic clones with masked chromosomal aberration in interphase nuclei was tested in childhood acute lymphoblastic leukemia (ALL). Twenty-one cases of ALL previously studied by classical metaphase cytogenetics were retrospectively analysed using a centromere-specific chromosome 7 probe. Five cases with karyotypic abnormalities of chromosome 7 (2 with trisomy 7, 2 with monosomy 7 and 1 with trisomy & tetrasomy 7) showed a correlation with FISH results, whereas in five other cases monosomy 7 was found in 12-43% of cells only by FISH. The unexpected detection of monosomy 7 in these latter ALL patients suggests that either these clones are quiescent or unable to enter mitosis in vitro. This suggests that FISH and metaphase cytogenetics must be combined whenever possible to obtain comprehensive karyotypic information.