Chromosomal Abnormalities in the Early Stem Cell Population of CML Patients in Complete Cytogenetic Remission.

Clonal cytogenetic abnormalities, most commonly involving chromosomes 7 and 8, are detectable by conventional karyotyping in Ph-negative metaphases of some chronic myeloid leukemia (CML) patients with a major cytogenetic response (MCyR) to imatinib. It is unknown whether these abnormalities involve the primitive progenitor cell compartment, and whether their frequency in this compartment may exceed the frequency detected by karyotyping. To answer these questions we analyzed lineage-negative CD34+/CD38− and CD34+/CD38+ cells from CML patients in complete cytogenetic response (CCyR), using by fluorescent in situ hybridization (FISH) for chromosome 7 and 8 abnormalities and BCR-ABL.

Methods: Mononuclear cells (MNC) were selected from the bone marrow of patients with CCR by Ficoll-Hypaque density gradient centrifugation and enriched for lineage-negative cells using an immunomagnetic column. Lineage-negative cells were further sorted into CD34+/38− and CD34+/38+ cells by multicolor FACS. Interphase FISH analysis was performed using 7 LSI D7S522 Spectrum Orange / CEP7 Spectrum Green (chromosome 7), CEP8 Spectrum Aqua (chromosome 8) and LSI BCR/ABL +9q34 TriColor Dual Fusion Probe. Thus far, 5 CML patients with CCR (3 with a normal karyotype and 2 with trisomy 8) and 1 normal control have been analyzed.

Results: Of the three CML patients in CCR with normal karyotype, one had 9% deletion 7q (internal cut off of 0.5%) and the second 1.2% trisomy 8 (just over the internal cut off of 1%) cells in the CD-34+/38− population, while the CD-34+/38+ cell population did not show abnormalities. The third CCR patient had no abnormalities in the CD-34+/38− and CD-34+/38+ cell populations. Two CML patients in CCR with 45% trisomy 8 abnormal cells by conventional cytogenetics had 44% and 60% trisomy 8 positive cells in the CD34+/38+ population (two few CD34+/CD38− cells were available for analysis). No BCR-ABL signal was detected in any cell. In the healthy control, the CD34+/38+ cells were normal, but 4.1% of CD34+/38− showed a deletion of 7q.

Conclusion: Clonal chromosomal of chromosomes 7 and 8 in Ph-negative primitive hematopoietic progenitor cells may be more common than suggested by conventional karyotyping. A larger cohort of CML patients in CCR and healthy individuals is under study to determine if this phenomenon is indeed related to CML or occurs also in normals. Results will have implications for the interpretation of karyotypes in patients with hematologic malignancies.