FISH (fluorescent in situ hybridization): the second youth of cytogenetics.

he introduction of high resolution chromosome banding at the beginning of the eighties significantly improved the techniques for cancer cytogenetics. New criteria for classification and monitoring of leukemia and lymphoma have been established, and new insights into the understanding of the genetic bases of malignancies have been gained. l The limits of classical cytogenetics, however, are determined by the low number of metaphases available for analysis in some types of malignant hemopathies as well as by possible misinterpretations of banding in complex exchanges. Thus, much excitement was generated when the new techniques of molecular biology were introduced to investigate malignant hemo-pathies at the gene level. FlSH combines the resolution of molecular techniques with cytogenet-ics and allows the precise localization of nucleic acid sequences of interest on chromosomes or in interphase nuclei. 2 A single-stranded DNA probe hybridizes with a single-stranded sequence on a slide. Probes labeled with molecules such as biotin or digoxigenin may be detected after hybridization by using fluo-rochrome-conjugated molecules with green (fluorescein) and red (Texas red) coloration, respectively. DNA probes commonly available include centromeric Ͱ satellite probes, used in interphase analyses, 3 whole-chromosome specific probes, obtained from libraries of single chromosomes , which allow precise assignment of chromosome material (so-called chromosome painting), 4 locus-specific probes which detect restricted chromosome regions. 5 This last group of probes includes lambda phages, yeast artificial chromosome (YAC) and cosmids, which are vectors of DNA sequences of different sizes. In addition to DNA probes, RNA probes can localize transcription products. Furthermore morphology and immunophe-notyping may be combined with molecular cytogenetics in both metaphase and interphase. This technology may be applied on cryostat sections as well as on cytospin preparations, where metaphases are spread within an intact cytoplas-mic membrane. 6,7 Two recent papers in this journal show the usefulness of FISH in clinical hematology. Mancini et al. 8 studied a patient with a clinical picture suggestive of chronic myelogenous leukemia (CML), cytogenetically characterized by a t(9;21;22) with no clear involvement of chromosome 9. The dual color FISH technique performed by using specific painting probes for chromosomes 9, 21 and 22, and a bcr-abl translocation probe enabled them to confirm the diagnosis of CML by detecting bcr-abl rearrangement on chromosome 22q and involvement of chromosome 9 in a variant translocation t(9;21;22). The paper by Fugazza et al. 9 is an example of the association of interphase FISH with simultaneous evaluation of cell morphology. Results …