CYTOGENETICS OF CHRONIC MYELOGENOUS LEUKEMIA

The Philadelphia (Ph) chromosome is present in the leukaemic cells of most patients with chronic myelogenous leukaemia. Variant translocatio ns occur in 10% of patients but breakpoints on chromosomes 9 and 22 re main the same, so prognosis of these patients is unchanged. Clonal evo lution is infrequent in chronic phase and its significance depends on the specific chromosome involved, the number of metaphases affected an d the timing in the chronic phase. The majority of patients in blastic phase demonstrate clonal evolution; three specific abnormalities (+Ph , +8 and isochromosome 17q) are present in 70% of patients. Loss of th e Ph chromosome on therapy is associated with prolonged survival. For monitoring these events conventional G-band cytogenetics (CG) is essen tial at presentation to characterize the disease cytogenetically, whil e fluorescence in situ hybridization (FISH) on hypermetaphase preparat ions (hypermetaphase FISH (HMF)) is important for establishing the spe cific frequency of Ph+ cells. During treatment FISH on interphase cell s (I-FISH) can monitor the level of Ph+ cells in circulation, while CG may be used to identify any suspected clonal evolution. Where I-FISH is negative, HMF is essential to evaluate minimal residual disease.