Light microscopic detection of BCR-ABL transcripts after in-cell RT-PCR: fusion gene expression might correlate with clinical evolution of chronic myeloid leukemia

Expression of BCR/ABL and BCL-2 in myeloid progenitors leads to myeloid leukemias

Chronic myelogenous leukemia is a myeloproliferative disorder (MPD) that, over time, progresses to acute leukemia. Both processes are closely associated with the t(9;22) chromosomal translocation that creates the BCR/ABL fusion gene in hematopoietic stem cells (HSCs) and their progeny. Chronic myelogenous leukemia is therefore classified as an HSC disorder in which a clone of multipotent HSCs is likely to be malignantly transformed, although direct evidence for malignant t(9;22)+ HSCs is lacking. To test whether HSC malignancy is required, we generated hMRP8p210BCR/ABL transgenic mice in which expression of BCR/ABL is absent in HSCs and targeted exclusively to myeloid progenitors and their myelomonocytic progeny. Four of 13 BCR/ABL transgenic founders developed a chronic MPD, but only one progressed to blast crisis. To address whether additional oncogenic events are required for progression to acute disease, we crossed hMRP8p210BCR/ABL mice to apoptosis-resistant hMRP8BCL-2 mice. Of 18 double-transgenic animals, 9 developed acute myeloid leukemias that were transplantable to wild-type recipients. Taken together, these data indicate that a MPD can arise in mice without expression of BCR/ABL in HSCs and that additional mutations inhibiting programmed cell death may be critical in the transition of this disease to blast-crisis leukemia.

BCR-ABL down-regulates the DNA repair protein DNA-PKcs

This study demonstrates in both stable and inducible BCR-ABL-expressing hematopoietic cells a down-regulation of the major mammalian DNA repair protein DNA-PKcs by BCR-ABL. Similar results were found in BCR-ABL CD34(+) cells from patients with chronic myelogenous leukemia (CML). DNA-PKcs down-regulation is a proteasome-dependent degradation that requires tyrosine kinase activity and is associated with a marked DNA repair deficiency along with increased sensitivity to ionizing radiation. The conjunction of a major DNA repair deficiency and a resistance to apoptosis, both induced by BCR-ABL, provides a new mechanism to explain how secondary genetic alterations can accumulate in CML, eventually leading to blast crisis. The down-regulation of DNA-PKcs was reversible in CD34(+) CML cells suggesting that this approach might offer a novel and powerful therapeutic strategy in this disease, especially to delay the blast crisis. (Blood. 2001;97:2084-2090)

Loss of heterozygosity and heterogeneity of its appearance and persisting in the course of acute myeloid leukemia and myelodysplastic syndromes

Screening for loss of heterozygosity (LOH) of the panel of 18 highly polymorphic microsatellite markers, especially from the region 11p15, was carried out on 154 samples from 26 patients with acute myeloid leukemia and eight with myelodysplastic syndromes (MDS). LOH was detected at the majority (72%) of the loci tested: 47% of informative patients displayed LOH for at least one of the microsatellite locus from the region 11p15 and 23.5% of patients displayed LOH among the other markers tested within the study. A longitudinal follow-up of patients showed a remarkable heterogeneity of LOH appearance and its persistance during the course of the disease suggesting an intratumor clonal heterogeneity, or alternatively, presence of LOH in more than one cell clone. The data revealed two regions of high loss of one allele in 11p15.5, defined by markers D11S1363 and D11S1338, indicating that LOH at the subtelomeric region of the short arm of chromosome 11 is a much common event in hematological malignancies than it was previously reported.