Highly sensitive fluorescence in situ hybridization method to detect double BCR/ABL fusion and monitor response to therapy in chronic myeloid leukemia

We investigated a new method using fluorescence in situ hybridization and DNA probes that span the common break-points of t(9;22)(q34;q11.2) and that detect double BCR/ABL fusion (D-FISH) in bone marrow cells with this translocation, one on the abnormal chromosome 9 and one on the Philadelphia chromosome (Ph chromosome). D-FISH patterns were abnormal in 30 of 30 specimens with classic, simple, complex, and masked Ph chromosomes. Based on 200 nuclei from each of 30 normal specimens, the mean percentage of false- positive cells was 0.25 ± 0.39. Thirty-seven specimens from 10 patients were studied before treatment and two or more times at 4-month intervals after treatment with interferon-α(2b) (IFN-α(2b)) with or without ara-C. Based on 200 nuclei, the results of D-FISH in these specimens correlated closely with quantitative cytogenetics and accurately quantified disease within a few percent. We studied 6,000 nuclei for each of six specimens, three normal and three from patients with chronic myeloid leukemia (CML) in cytogenetic remission. The normal cutoff for 6,000 nuclei was 0.079% and patients in cytogenetic remission had residual disease ranging from 7 (0.117%)to 53 (0.883%) Ph-positive nuclei. We conclude that D-FISH can detect the ph chromosome and its variant translocations and accurately quantify disease in CML at diagnosis and at all times after treatment, including cytogenetic remission.