Molecular cytogenetic analysis of B-cell chronic lymphocytic leukemia

The genetic alterations underlying the pathogenesis of B-cell chronic lymphocytic leukemia (B-CLL) are difficult to assess. Cytogenetic studies are hindered by the low in vitro mitotic activity of the tumor cells and the limited resolution of chromosome banding. Molecular genetic analyses are hampered by nonclonal cells contained in the specimens and by the limited knowledge of candidate genes involved. As a complement to cytogenetic and molecular genetic techniques, fluorescence in situ hybridization (FISH) has proven powerful in the molecular cytogenetic analysis of B-CLL. FISH allows the detection of aberrations such as trisomies, deletions, and translocation breakpoints on the single cell level in dividing as well as non-dividing cells without the prerequisite of detailed physical maps or knowledge of involved genes. As detected by the interphase cytogenetic FISH approach, the most common chromosome abnormalities of B-CLL are deletions in band 13q14, followed by deletions in 11q22-q23, trisomy 12, deletions in 17p13, and deletions in 6q21. Abnormalities in 17p13 seem to involve the TP53 tumor suppressor gene, but as yet no candidate genes have been identified for the other frequent aberrations. Toward the identification of such genes by positional cloning, FISH can be applied for detailed aberration mapping at the molecular level. Furthermore, the accurate detection of chromosome aberrations in B-CLL by FISH provides a valid basis for the evaluation of their prognostic significance. Inactivation of TP53 in 17p13 and deletions in 11q22-q23 have already been shown to be among the most important independent prognostic factors. Genetic abnormalities may eventually provide biological parameters, allowing a risk assessment for individual patients at the time of diagnosis of this clinically heterogeneous disease.