Chronic morphine treatment leads to a global DNA hypomethylation via active and passive demethylation mechanisms in mESCs

Epigenetic changes are essential for normal development and ageing, but there is still limited understanding of how environmental factors can cause epigenetic changes that leads to health problems or diseases. Morphine is known to pass through the placental barrier and impact normal embryo development by affecting the neural tube, frontal cortex and spinal cord development, and, as a consequence, delaying nervous system development. In fact, in-utero morphine exposure has shown alterations in anxiety-like behaviours, analgesic tolerance, synaptic plasticity and the neuronal structure of offspring. However, how morphine leads to abnormal neurogenesis and other physiological consequences during embryo development is still unknown. Considering that DNA methylation is a key epigenetic factor crucial for embryo development, our aim is to elucidate the role of methylation in response to morphine. Chronic morphine treatment (24h, 10μM) induces a global hypomethylation in mESC. WGBSeq identifies 16,808 sensitive to morphine which are involved in embryo development, signalling pathways, metabolism and/or gene expression, suggesting that morphine might impact methylation levels at developmental genes. Integrative analyses between WGBSeq and RNASeq identified Tet1 as morphine-sensitive gene. Morphine increased the gene expression of Tet1 , modifying the methylation levels at the promoter. On the other hand, RNASeq and qRT-PCR analyses revealed that Dnmt1 gene expression decreased after morphine treatment, without altering the methylation patter at its promoters. By MS/MS approaches confirms a decrease in DNA methylation after chronic morphine treatment, together with an increase in hydroxymethylation global levels in mESCs. In conclusion, morphine induces a global hypomethylation in mESC through different mechanisms that involves passive demethylation and a self-regulatory mechanism via active demethylation.