Detection of residual leukemic cells in patients with acute promyelocytic leukemia by the fluorescence in situ hybridization method: potential for predicting relapse

The translocation between chromosomes 15 and 17, t(15;17)(q22-24;q11-21), is present in the bone marrow cells of most patients with acute promyelocytic leukemia (APL). Although conventional cytogenetic methods are useful for diagnosing this disease, difficulties are experienced in detecting residual disease among those patients who have achieved remission. In this study, we used the fluorescence in situ hybridization (FISH) method to attempt to detect residual leukemic cells in 10 APL patients in clinical remission. The duration of remission ranged from 2 to 93 months at the time of study. Multiple bone marrow samples were analyzed by FISH in most patients. In 6 patients, no cell with t(15;17) was found. These patients remain in complete remission at present (approximately 25 to 33 months since first studied by FISH). In 4 patients, low frequencies of cells with t(15;17) were observed in at least one bone marrow sample examined. All of these patients relapsed within 1 to 14 months. No cell with t(15;17) was identified by the conventional G-banding method in any sample. The FISH results correlated well with that of a two-round nested reverse transcription polymerase chain reaction assay that was performed on the same samples. Thus, our study suggests that FISH is potentially a useful tool for detecting residual APL cells and for identifying patients at high risk of relapse.

Detection of phosphorylated retTPC oncogene product in cytoplasm

The product of the retTPC oncogene, an activated form of the ret proto-oncogene found in human papillary thyroid carcinomas, was identified as a 57-kDa protein (p57retTPC) by Western blotting with a polyclonal antibody raised by an oligopeptide corresponding to the carboxy-terminal region of the ret proto-oncogene product. Subcellular fractionation experiments using NIH3T3 cell transformants induced by the retTPC cDNA showed that p57retTPC was localized in a soluble cytoplasmic fraction, whereas the ret proto-oncogene products expressed in a neuroblastoma cell line were present in a membrane fraction. Immunostaining also demonstrated that p57retTPC is localized in the cytoplasm. Immunoprecipitation with anti-phosphotyrosine antibody followed by Western blotting revealed that p57retTPC is constitutively phosphorylated, whereas the ret proto-oncogene products are not. These findings suggest that p57retTPC has an aberrant tyrosine kinase activity resulting in autophosphorylation associated with change in its location.