Poster Session II: Myelodysplastic syndromes - Biology & translational research
Hypomethylating agents (HMA) such as azacytidine and decitabine are the mainstay of treatment for higher risk myelodysplastic syndromes (MDS). Being cytidine analogues, they are incorporated into DNA of highly proliferating cells leading to genome-wide decrease of methylation levels, whereas azacytidine is additionally incorporated into RNA. However, the precise mechanism underlying treatment success or failure remains incompletely understood. One possible mechanism of HMA action is through ‘viral mimicry' of transcriptionally repressed endogenous retroelements (EREs), which is thought to trigger innate immune pathways. According this hypothesis, HMA induce unphysiological levels of ERE transcription in transformed cells, which in turn generated double-stranded RNAs from complementary ERE transcripts, activating innate immune sensors.
Although support and a mechanistic basis for this hypothesis is provided from several in vitro studies, in vivo evidence from the clinical use of HMA is currently lacking. We sought to investigate, the transcriptional profile of EREs and its modulation by HMAs on primary MDS samples and the possible involvement of EREs in the HMA mode of action.
To explore the possible involvement of EREs in the HMA mode of action, we have compared the transcriptional profiles of CD34+ HSCs isolated from bone marrow samples of healthy donors (n = 9) and patients with acute myeloid leukemia (AML) (n = 9), chronic myelomonocytic leukemia-II (CMML-II, n = 9) or high-risk MDS (n = 11). Samples were obtained before, 15 days after the initiation of azacytidine and/or after cycle 6.
Our analysis revealed that ERE transcription is globally repressed in untreated MDS and CMML, in line with the epigenetic repression that characterizes these conditions. Treatment with azacytidine had a measureable effect in overall ERE transcription in HSCs from MDS and CMML patients, which by the 6th cycle was raised to levels equivalent to those seen in HSCs controls. Comparable results were also obtained following analysis of a publicly available dataset. However, the therapeutic response was not correlated with or predicted by ERE activity. Indeed, ERE transcriptional activation was frequently observed in azacytidine-treated patients who failed to respond to treatment, whereas it was frequently low in or absent from patients with complete remission. Only, few individual ERE loci differed in their activity between patients who responded or not to azacytidine treatment. Moreover, our analysis failed to detect induction of either interferon-inducible genes or interferon-inducible EREs, irrespective of treatment outcome.
Together, our current results do not support a role for transcriptional activation of EREs or for innate sensing of their nucleic acid products in the therapeutic response of MDS and CMML patients to azacytidine. Investigation of alternative potential mechanisms of azacytidine is therefore warranted.