METHYLATION OF THE MAJOR BREAKPOINT CLUSTER REGION (M-BCR) IN PHILADELPHIA-POSITIVE CML

It has previously been shown that a cluster of HpaII sites with the po tential to be methylated exist around exon b3 of the M-bcr region invo lved in the formation of the Philadelphia chromosome in chronic myeloi d leukemia (CML). The degree of hypermethylation of these sites can be directly correlated with the percentage of immature cells, whilst pro gressive hypomethylation occurs during the maturation of the granulocy te lineage. We have examined samples obtained from CML patients at dia gnosis, during chronic phase, and blast crisis to examine the degree o f methylation of this region in the non-rearranged BCR gene and the re arranged BCR-ABL gene. A low degree of methylation of the non-rearrang ed gene, similar to that observed in normal individuals, was observed in diagnosis and chronic phase samples. increased methylation was obse rved during blast crisis indicative of the presence of immature cells in the samples. In contrast, a significantly lower degree of methylati on was observed in the rearranged BCR-ABL gene at the onset of blast c risis. Divison of the samples into those patients who had lost exon b3 during the formation of the BCR/ABL gene and those that had retained exon b3 produced differing patterns of methylation during disease prog ression. The former group, who also expressed a b2-a2 mRNA, showed an increase in methylation of the non-rearranged BCR gene prior to and du ring blast crisis, with a inverse decrease in the methylation of the B CR/ABL gene. Those patients who had retained exon b3, and expressed a b3-a2 mRNA, showed no change in the extent of methylation of the BCR/A BL gene but did exhibit an increase in methylation of the BCR gene dur ing blast crisis. The consequence of the differing degree of methylati on during disease progression could affect, to some extent, the specif icity of protein binding or RNA expression.