Aging of Preleukemic Thymocytes Drives CpG Island Hypermethylation in T-Cell Acute Lymphoblastic Leukemia

Abstract Cancer cells display DNA hypermethylation at specific CpG islands in comparison with their normal healthy counterparts, but the mechanism that drives this so-called CpG island methylator phenotype (CIMP) remains poorly understood. Here, we show that CpG island methylation in human T-cell acute lymphoblastic leukemia (T-ALL) mainly occurs at promoters of Polycomb Repressor Complex 2 (PRC2) target genes that are not expressed in normal or malignant T cells and that display a reciprocal association with H3K27me3 binding. In addition, we reveal that this aberrant methylation profile reflects the epigenetic history of T-ALL and is established already in preleukemic, self-renewing thymocytes that precede T-ALL development. Finally, we unexpectedly uncover that this age-related CpG island hypermethylation signature in T-ALL is completely resistant to the FDA-approved hypomethylating agent decitabine. Altogether, we provide conceptual evidence for the involvement of a preleukemic phase characterized by self-renewing thymocytes in the pathogenesis of human T-ALL. Significance: We developed a DNA methylation signature that reveals the epigenetic history of thymocytes during T-cell transformation. This human signature was recapitulated by murine self-renewing preleukemic thymocytes that build an age-related CpG island hypermethylation phenotype, providing conceptual evidence for the involvement of a preleukemic thymic phase in human T-cell leukemia. This article is highlighted in the In This Issue feature, p. 215