Progressive de novo methylation at the bcr/abl locus in the course of chronic myelogenous leukemia

De novo methylation of CpG Islands is a rare event in mammalian cells. It has been observed In the course of developmental processes, such as X chromosome inactivation and genomic imprinting. The methylation of DNA, an Important factor in the epigenetic control of gene expression, may also be involved In tumorigenesis. After the t(9;22) chromosomal translocation and generation of the Philadelphia chromosome, the Initiating event In chronic myelogenousleemia (CML), most of the abl coding sequence is fused to the 5' region
of the bcr gene. Expression of the hybrid bcr-abl gene is, therefore, regulated by the bcr promoter. In most cases of CML, one of the two abl promoters (Pa) is nested within the bcr'-bl transcrptional unit and should be able to tr be the type Ia 6-kb normal abi mRNA from the Philadelphia chromosome. However, we have found that the 6-kb tapt is present only in CML cell lines coning a normal abl allele and that the apparent inactivation of the nested Pa promoter is associated with allele-specfic methylation. Furthermore, we have noticed that the Pa promoter is contaied within a CpG island and undergoes progressive de nova methylation in the course of the disease. This ls atts to by the fact that DNA samples from CML patients that are methylation-free at the time of diagnosis invariably become methylated in advanced CML. Since tumor progression in CML caunot always be inferred from the clinical presentation, a nt of de novo CpG methylation may prove to be of critical value in management of the disease. It could herald blastic transformation at a stage when bone marrow mnsplantation, the only potentially
curative therapeutic procedure in CML, is still effective.