The marrow microenvironment contributes to the pathogenesis of ineffective hematopoiesis in Myelodysplastic Syndromes (MDS). Since mutations and cytogenetic alterations seen in the hematopoietic compartment are generally not present in marrow stromal cells, we hypothesized that epigenetic alterations may be responsible for altered stroma function in MDS. To elucidate the epigenome of MDS microenvironment, we used the HELP (HpaII tiny fragment Enrichment by Ligation-mediated PCR) assay to study cytosine methylation patterns of 50,000 CpGs loci covering 14,000 promoters in primary stromal cells. Global DNA methylation of 6 MDS marrow-derived stroma was analyzed by HELP assay and compared to 3 healthy controls. MDS stroma showed aberrant hypermethylation compared with controls (3626 hypermethylated vs 306 hypomethylated loci in untreated MDS stromal cells) that preferentially occurred outside of CpG islands (Test of Proportions, P value <0.01) and involved important signaling pathways and binding sites for transcription factors (Sp1, NFY, MYB,). Aberrant genomic methylation was seen to be significantly enriched at chromosomal regions chr12q15, chr5q32, chr4q21, chr7q31, chr3q13, chr2p12 and chr8q24 when compared with the genomic distribution of all HpaII fragments from the HELP array (P value <0.05; MDSig Program). Comparison with stroma derived from 5-Azacytidine (5-Aza) treated MDS patients revealed abrogation of aberrant methylation in treated samples. Integrative expression analysis revealed that the WNT pathway was epigenetically dysregulated in MDS stroma, and the WNT antagonists FRZB and SFRP1 were aberrantly hypermethylated and underexpressed. These epigenetic changes were validated in a co-culture model of stroma (HS27) and leukemic cells (Kg1a) and analyzed at a single base pair resolution using next gen sequencing based HELP-Tagging assay. Epigenetic changes in MDS stroma involving the WNT antagonists FRZB/SFRP1 were also validated at the protein level by IHC in an independent set of MDS stroma samples .
After demonstrating aberrant hypermethylation in MDS stroma, we next examined the functional role of these alterations by treatment with DNMT inhibitors. 5-Aza treatment of MDS stroma enhanced hematopoietic activity and erythroid differentiation from co-cultured healthy CD34+ cells. Significantly increased erythroid differentiation was seen in CD34+ cells cocultured with aza pretreated MDS stroma as shown by their glycophorin A positivity.
Conclusions: These results reveal widespread aberrant epigenetic changes in the MDS marrow microenvironmentthat affect many important pathways. A recent report had shown that activation of beta-catenin in the bone marrow niche can trigger MDS/AML in vivo. Our results demonstrate that epigenetic silencing of WNT antagonists can potentially lead to activation of the WNT pathway in MDS stroma. Finally, we demonstrate that DNA methyl transferase inhibitors alter the epigenomic profiles of stromal cells and it raises the possibility that epigenetic modulation of the stroma by these drugs contributing to their therapeutic efficacy.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.