Integrated Analysis of Global DNA Methylation and Transcription Reveals a Leukemia Subtype with Extreme Hypermethylation Associated with Silencing of Regulators of Differentiation

Acute myeloid leukemia (AML) represents a highly heterogeneous myeloid stem cell disorder classified based on various genetic defects. Besides genetic alterations, epigenetic changes are recognized as an additional mechanism contributing to leukemogenesis, but insight into the latter process remains minor. Using a combination of Methyl-CpG-Immunoprecipitation (MCIp-chip) and MALDI-TOF analysis of bisulfite-treated DNA in a cohort of 196 AML patients we previously demonstrated that (cyto)genetically defined AML subtypes, including CBFB-MYH11, AML-ETO, NPM1-mut, CEBPA-mut or IDH1/2-mut subtypes, express specific DNA-methylation profiles (Gebhard et al, Leukemia, 2018). A fraction of AML patients (5/196) displayed a unique abnormal hypermethylation profile that was completely distinct from any other AML subtype. These patients present immature leukemia (FAB M0, M1) with various chromosomal aberrations but very few mutations (e.g. no IDH1/2, KRAS, DNMT3A) that might explain the CpG island methylator phenotype (CIMP) phenotype. The CIMP patients showed high resemblance with a recently reported CEBPA methylated subgroup (Wouters et al, 2007 and Figueroa et al, 2009), which we confirmed by MCIp-chip and MALDI-TOF analysis.

To explore the whole range of epigenetic alterations in the CIMP-AML patients we performed in-depth global DNA methylation and gene expression analyses (MCIp-seq and RNA-seq) in 45 AML and 12 CIMP patients from both studies. Principle component analysis and t-distributed stochastic neighbor embedding (t-SNE) revealed that CIMP patients express a unique DNA-methylation and gene-expression signature that separated them from all other AMLs. We could discriminate promoter methylation from non-promoter methylation by selecting MCIp-seq peaks within 3kb around TSS. Promoter hypermethylation was highly associated with repression of genes (PCC = -0.053, p-value = 0.00075). Hypermethylation of non-promoter regions was more strongly associated with upregulation of genes (PCC = 0.046, p-value = 4.613e-06). Interestingly, differentially methylated regions also showed a positive association with myeloid lineage CTCF binding sites (27% vs 18% expected, p-value < 2.2e-16 in a chi-square test of independence). Methylation of CTCF sites causes loss of CTCF binding, which has been reported to disrupt boundaries between so-called topologically associated domains (TADs), allowing enhancers located in a particular TAD to become accessible to genes in adjacent TADs and affect their transcription. Whether this is the case is under investigation. In this study we particularly focused on the role of hypermethylation of promoters in CIMP-AMLs.

Promoters of many transcriptional regulators that are involved in the differentiation of myeloid lineages of which several are frequently mutated in AML were hypermethylated and repressed, including CEBPA, CEBPD, IRF8, GATA2, KLF4, MITF or MAFB. Notably, HMGA2, a critical regulator of myeloid progenitor expansion, exhibited the largest degree of CIMP promoter hypermethylation compared to the other AMLs, accompanied by a reduction in gene expression. Moreover, multiple members of the HOXB family and KLF1 (erythroid differentiation) were methylated and repressed as well.

In addition, these patients frequently showed hypermethylation of many chromatin factors (e.g. LMNA, CHD7 or TET2). Hypermethylation of the TET2 promoter could result in a loss of maintenance DNA demethylation and therefore successive hypermethylation at CpG islands. We carried out regulome-capture-bisulfite sequencing on CIMP-AMLs compared to other AML samples and normal blood cell controls and confirmed methylation of the same transcription and chromatin factor promoters.

We conclude that these leukemias represent very primitive HSCPs which are blocked in differentiation into multiple hematopoietic lineages, due to the absence of regulators of these lineages. Although the underlying cause for the extreme hypermethylation signature is still subject to ongoing studies, the consequence of promoter hypermethylation is silencing of key lineage regulators causing the differentiation arrest in these cells. We argue that these patients may particularly benefit from therapies that revert DNA methylation.