Background: Gene silencing through aberrant CpG island methylation is the most extensively analyzed epigenetic event in human tumorigenesis and has huge diagnostic and prognostic potential. Methylation patterns are often very heterogeneous, however, presenting a serious challenge for the development of methylation assays for diagnostic purposes.
Methods: We used Pyrosequencing technology to determine the methylation status of 68 CpG sites in the CpG island of the CDKN2B gene [cyclin-dependent kinase inhibitor 2B (p15, inhibits CDK4)], frequently hypermethylated in myeloid malignancies, in a series of bone marrow samples from patients with myelodysplasia and myeloid leukemia (n = 82) and from 32 controls. A total of 7762 individual methylation sites were quantitatively evaluated. Precision and reproducibility of the quantification was evaluated with several overlapping primers.
Results: The use of optimized sequencing primers and the new Pyro Q-CpG software enabled precise and reproducible quantification with a single sequencing primer of up to 15 CpG sites distributed over approximately 100 bp. Extensive statistical analyses of the whole CpG island revealed for the first time disease-specific methylation patterns of the CDKN2B gene in myeloid malignancies and small regions of differential methylation with high discriminatory power that enabled differentiation of even low-grade myelodysplastic syndrome samples from the controls, a result that was confirmed in an independent group of 9 control and 36 patient samples.
Conclusion: The precise quantitative methylation mapping of whole CpG islands is now possible with Pyrosequencing software in combination with optimized sequencing primers. This method reveals disease-specific methylation patterns and enables the development of specific diagnostic assays.