Molecular mechanisms for the progression of chronic myeloid leukemia

Chronic myeloid leukaemia (CML) is caused by the BCR-ABL hybrid gene. The molecular mechanisms leading from chronic phase (CP) to blast crisis (BC) are not understood. However, both the presence and the levels of BCR-ABL seem to be important for CML progression. BCR-ABL is under the transcriptional control of BCR promoter. Here we focused on the gene expression control of BCR and BCR-ABL upon myeloid differentiation in healthy donors (HDs), CP and BC patients. As previously reported, BCR-ABL is downregulated during myeloid maturation in CP patients. A similar pattern was detected for BCR (but not for ABL) in CP-CML and in HD, thus suggesting that the two genes may be under a similar transcriptional control. In BC this mechanism is similarly impaired for both BCR-ABL and BCR. These data indicate the presence of an ‘in trans’ deregulated transcription of both BCR and BCR-ABL promoters, associated with CML progression. The results of the luciferase assay indicate that the region comprised between 420 and 900 bp from the coding ATG site is required to achieve a basal transcription level. Previous studies suggest that a putative SP1 binding site could have a role in the basal promoter activity. In fact, an almost complete absence of transcriptional activity was measured in delta1041 and delta1271 constructs, lacking both the main transcription start site and the putative SP1 binding region. We hypothesize that SP1 could be responsible for the basal promoter activity, present in the delta541 and in longer constructs. The ChIP assay confirmed the SP1-binding to the BCR promoter. The presence of 10 additional putative protein binding sites (PBSs), along the BCR promoter is also known from previous works. Six of these putative PBSs are localized in the region between −1443 to −1202 bp, which appears to be critical from in silico studies. In fact, only in presence of a 221 bp region upstream from delta241, a strong luciferase signal could be detected, suggesting that the promoter region between −1443 and −1202 bp from the coding ATG is indeed critical to achieve the highest level of expression.