AbstractBackgroundMyelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML) are characterised by abnormal epigenetic repression and differentiation of bone marrow haematopoietic stem cells (HSCs). Drugs that reverse epigenetic repression, such as 5-azacytidine (5-AZA), induce haematological improvement in half of treated patients. Although the mechanisms underlying therapy success are not yet clear, induction of endogenous retroelements (EREs) has been hypothesised.MethodsUsing RNA sequencing (RNA-seq), we compared the transcription of EREs in bone marrow HSCs from a new cohort of MDS and chronic myelomonocytic leukaemia (CMML) patients before and after 5-AZA treatment with HSCs from healthy donors and AML patients. We further examined ERE transcription using the most comprehensive annotation of ERE-overlapping transcripts expressed in HSCs, generated here by de novo transcript assembly and supported by full-length RNA-seq.ResultsConsistent with prior reports, we found that treatment with 5-AZA increased the representation of ERE-derived RNA-seq reads in the transcriptome. However, such increases were comparable between treatment responses and failures. The extended view of HSC transcriptional diversity offered by de novo transcript assembly argued against 5-AZA-responsive EREs as determinants of the outcome of therapy. Instead, it uncovered pre-treatment expression and alternative splicing of developmentally regulated gene transcripts as predictors of the response of MDS and CMML patients to 5-AZA treatment.ConclusionsOur study identifies the developmentally regulated transcriptional signatures of protein-coding and non-coding genes, rather than EREs, as correlates of a favourable response of MDS and CMML patients to 5-AZA treatment and offers novel candidates for further evaluation.