A FISH COSMID COCKTAIL FOR DETECTION OF 13Q DELETIONS IN CHRONIC LYMPHOCYTIC-LEUKEMIA - COMPARISON WITH CYTOGENETICS AND SOUTHERN HYBRIDIZATION

The most frequent structural chromosome abnormality in chronic lymphoc ytic leukaemia (CLL) is deletion at chromosome 13q14. Studies with Sou thern blot hybridisation have revealed deletions in the region located telomeric of the retino-blastoma gene in more than 40% of cases. The highest frequency of homozygous deletions has been found at the D13S31 9 locus and it is likely that a new tumour suppressor gene is located close to this region. We have analysed deletions in the D13S319 region in 20 selected CLL patients using conventional cytogenetic analysis, fluorescence in situ hybridisation (FISH) and Southern blot hybridisat ion. FISH and Southern hybridisation are equally efficient in detectin g deleted clones in our study. However, FISH analysis indicate that su bclones with different numbers of alleles in the D13S319 region can ex ist simultaneously. The cytogenetic analyses confirm that clones with different chromosomal abnormalities can occur in patients with CLL and that 13q14 deletions can be limited to one of these subclones. Furthe rmore, the FISH analyses show that trisomy 12 and deletion of 13q14 ca n occur in the same cell clone. Finally, our study confirms that mitog en stimulation of peripheral blood cells from CLL patients before FISH analysis may result in a sharp increase in normal appearing cells, wh ich can hide leukaemic clones with deletions in the D13S319 region.