Epigenetic dysfunctions, including DNA methylation alterations, play major roles in cancer initiation and progression. Although it is well established that gene promoter demethylation activates transcription, it remains unclear whether hypomethylation of repetitive heterochromatin similarly affects expression of non-coding RNA from these loci. Understanding how repetitive non-coding RNAs are transcriptionally regulated is important given that their established upregulation by the heat shock (HS) pathway suggests important functions in cellular response to stress, possibly by promoting heterochromatin reconstruction. We found that, although pericentromeric satellite 2 (Sat2) DNA hypomethylation is detected in a majority of cancer cell lines of various origins, DNA methylation loss does not constitutively hyperactivate Sat2 expression, and also does not facilitate Sat2 transcriptional induction upon heat shock. In melanoma tumor samples, our analysis revealed that the HS response, frequently upregulated in tumors, is probably the main determinant of Sat2 RNA expression in vivo. Next, we tested whether HS pathway hyperactivation may drive Sat2 demethylation. Strikingly, we found that both hyperthermia and hyperactivated RasV12 oncogene, another potent inducer of the HS pathway, reduced Sat2 methylation levels by up to 27% in human fibroblasts recovering from stress. Demethylation occurred locally on Sat2 repeats, resulting in a demethylation signature that was also detected in cancer cell lines with moderate genome-wide hypomethylation. We therefore propose that upregulation of Sat2 transcription in response to HS pathway hyperactivation during tumorigenesis may promote localized demethylation of the locus. This, in turn, may contribute to tumorigenesis, as demethylation of Sat2 was previously reported to favor chromosomal rearrangements.

Anthracyclines remain a cornerstone of induction chemotherapy for acute myeloid leukemia (AML). Refractory or relapsed disease due to chemotherapy resistance is a major obstacle in AML management. MicroRNAs (miRNAs) have been observed to be involved in chemoresistance. We previously observed that miR-15a-5p was overexpressed in a subgroup of chemoresistant cytogenetically normal AML patients compared with chemosensitive patients treated with daunorubicin and cytarabine. MiR-15a-5p overexpression in AML cells reduced apoptosis induced by both drugs in vitro. This study aimed to elucidate the mechanisms by which miR-15a-5p contributes to daunorubicin resistance. We showed that daunorubicin induced autophagy in myeloid cell lines. The inhibition of autophagy reduced cell sensitivity to daunorubicin. The overexpression of miR-15a-5p decreased daunorubicin-induced autophagy. Conversely, the downregulation of miR-15a-5p increased daunorubicin-induced autophagy. We found that miR-15a-5p targeted four genes involved in autophagy, namely ATG9a, ATG14, GABARAPL1 and SMPD1. Daunorubicin increased the expression of these four genes, and miR-15a-5p counteracted this regulation. Inhibition experiments with the four target genes showed the functional effect of miR-15a-5p on autophagy. In summary, our results indicated that miR-15a-5p induces chemoresistance in AML cells through the abrogation of daunorubicin-induced autophagy, suggesting that miR-15a-5p could be a promising therapeutic target for chemoresistant AML patients.

Tyrosine kinase inhibitors (TKI), such as imatinib, have revolutionized chronic myeloid leukemia (CML) treatment. Despite this success, TKI intolerance and resistance remain significant clinical challenges. A promising therapeutic approach is to simultaneously target the BCR::ABL1 oncogene and other oncogenic drivers. The polycistronic miR-17-92 cluster is known to contribute to CML development and progression, but the specific roles of miR-92a-1-5p within this cluster remain unclear. In this study, we assess the roles of this microRNA and evaluate the therapeutic potential of combining microRNA inhibition with imatinib to improve treatment outcome. Our results show that miR-92a-1-5p is downregulated by imatinib in myeloid cell lines harboring BCR::ABL1 and in CML patient samples. Inhibition of miR-92a-1-5p reduces proliferation and enhances imatinib-induced cell death, while its overexpression increases proliferation and counteracts the effects of imatinib on cell death. This decrease in proliferation caused by miR-92a-1-5p inhibition is rescued after simultaneous inhibition of two newly identified target genes: BNIP3L (NIX) and TP53INP1. We confirm that miR-92a-1-5p regulates proliferation and cell cycle by targeting TP53INP1 and decreases autophagy by targeting BNIP3L. Our data suggest that miR-92a-1-5p plays a role in CML progression, and its inhibition enhances imatinib anti-leukemic efficacy, making it a potential therapeutic target.