Molecular Alterations in Chronic Myelomonocytic Leukemia Monocytes: Transcriptional and Methylation Profiling

Chronic myelomonocytic leukemia (CMML) is a genetically heterogeneous hematopoietic stem cell disorder that combines features of a myelodysplastic syndrome and a myeloproliferative neoplasm and exhibits a strong bias towards older age. The prognosis of CMML is poor, with overall survival of less than 3 years in most studies, however recurrent somatic mutations explain only 15-24% of the clinical heterogeneity of CMML (Elena C. et al. Blood 128:1408-17, 2016). The extreme skewing of the CMML age distribution suggests that CMML reflects the malignant conversion of the myelomonocytic-biased differentiation characteristic of an aged hematopoietic system. We hypothesized that separating the contribution of the normal aging process from bona fide CMML-specific alterations will improve the molecular characterization and biological understanding of CMML. We decided to focus on monocytes as the phenotypic minimal common denominator of genetically heterogeneous diseases. CD14+ monocytes were sorted from the blood of untreated CMML patients (N=12, median age 77 years, range 61-90), age-matched healthy controls (old controls: N=12, median age 68 years, range 62-74) and young healthy controls (young controls: N=16, median age 29 years, range 24-44) and subjected to RNA sequencing and DNA methylation profiling. Differentially expressed genes in CMML monocytes compared to healthy controls were identified with DESeq2 using a 1% false discovery rate (FDR) and a fold-change cutoff set at >│2│ (Figure 1A). We identified the 2480 CMML-specific genes by subtracting all genes with significant differences in the young controls vs. old controls comparison from the CMML vs. old controls comparison. The top-25 most significantly upregulated genes (Figure 1B) included transcription factors, TNFα signaling genes, genes that regulate genomic stability, and genes involved in apoptosis. The most significantly downregulated transcripts were genes involved in response to DNA damage, RNA binding, monocyte differentiation and mediators of inflammatory process.

To link these observations to function, we imputed the 2480 CMML-specific differentially expressed genes into the ingenuity pathway analysis (IPA) application. This analysis uncovered significant enrichment of pathways involved in: mitotic roles of Polo-like kinase, G2/M DNA damage checkpoint regulation, lymphotoxin β receptor signaling, IL-6 signaling and ATM signaling (Figure 1C). DNA methylation profiling revealed 909 differentially methylated regions (DMRs) between CMML and age-matched controls, with most regions being hypermethylated in CMML monocytes. Of these, 37% of the DMRs were intronic, 22% were exonic, 14 % were in the promoter region (Figure 1D), 10% were downstream, 10% were upstream, the remainder were 3' and 5'-overlaps.

We also performed integrated analysis using the promoter DMRs and the gene expression profile to identify CMML-associated genes that are likely to be regulated by specific changes in methylation. We observed concomitant changes in CMML-specific mRNA transcripts and DNA methylation promoter regions in the CMML vs. old controls contrast for 10 genes (Figure 1E). AOAH, SERINC5, TAF3 and AHCYL1 were downregulated and hypermethylated; MS4A3, TNF, VCAM1, and IFT80, were upregulated and hypermethylated; TUBA1B was upregulated and hypomethylated and PITPNA was downregulated and hypomethylated.

Our study is the first to combine transcriptional and methylation profiling for molecular characterization of CMML monocytes. Conclusions: (i) age-related gene expression changes contribute significantly to the CMML transcriptome; (ii) the CMML-specific transcriptome is characterized by differential regulation of transcription factors, inflammatory response genes and anti-apoptotic pathway genes; (iii) differences in promoter methylation represent only a small proportion of overall differences in methylation, suggesting that intragenic or intronic methylation is a major contributor to the leukemic phenotype; (iv) age-related changes may be necessary, but are not sufficient to realize the CMML phenotype.

Figure 1.

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Figure 1.

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