Correlation of cytogenetics, BCR-ABL PCR studies and fluorescence in situ hybridisation (FISH) in adult acute lymphoblastic leukaemia

Background: Philadelphia positive (Ph+) acute lymphoblastic leukaemia (ALL) accounts for 11-29% of adult ALL. Reverse transcriptase polymerase chain reaction (RT-PCR) for the BCR-ABL fusion mRNA has identified patients with the fusion mRNA without cytogenetic evidence of the 9;22 translocation. The reason for discrepancies between cytogenetic and molecular diagnoses is unclear. Aim: Our aim was to study cases of ALL with discordant cytogenetic and RT-PCR results and identify any reasons for such discrepancies. Methods: Laboratory records were scanned for cases of ALL tested by both RT-PCR and cytogenetics and positive by either for the 9;22 translocation. Fluorescence in situ hybridisation (FISH) was used to study discordant results where a specimen was available. Results: We identified 15 patients with ALL who had both cytogenetic and RT-PCR studies for BCR-ABL. Seven had discordant results; five patients had positive RT-PCR studies with normal (four/five) or abnormal but Ph negative cytogenetics (one/five), and two were Ph+ but RT-PCR negative. FISH, using Vysis LSI bcr/abl translocation probes, showed fused signals in 12% interphase cells but not in metaphase cells in one specimen with normal cytogenetics, and 6% interphase cells in the Ph negative patient with abnormal cytogenetics. This second patient subsequently relapsed with a minor Ph+ cell line derived from the Ph negative line. Conclusions: These results confirmed the need for both cytogenetics and RT-PCR to identify Ph+ ALL. FISH did not show sub-microscopic rearrangements of BCR-ABL in normal metaphases. Failure to identify the Philadelphia chromosome cytogenetically appeared due rather to Ph+ cells failing to divide.