5-aza-2'-deoxycytidine-induced genome rearrangements are mediated by DNMT1

DNA damage in normally and prematurely aged mice

Steady-state levels of spontaneous DNA damage, the by-product of normal metabolism and environmental exposure, are controlled by DNA repair pathways. Incomplete repair or an age-related increase in damage production and/or decline in repair could lead to an accumulation of DNA damage, increasing mutation rate, affecting transcription, and/or activating programmed cell death or senescence. These consequences of DNA damage metabolism are highly conserved, and the accumulation of lesions in the DNA of the genome could therefore provide a universal cause of aging. An important corollary of this hypothesis is that defects in DNA repair cause both premature aging and accelerated DNA damage accumulation. While the former has been well-documented, the reliable quantification of the various lesions thought to accumulate in DNA during aging has been a challenge. Here, we quantified inhibition of long-distance PCR as a measure of DNA damage in liver and brain of both normal and prematurely aging, DNA repair defective mice. The results indicate a marginal, but statistically significant, increase in spontaneous DNA damage with age in normal mouse liver but not in brain. Increased levels of DNA damage were not observed in the DNA repair defective mice. We also show that oxidative lesions do not increase with age. These results indicate that neither normal nor premature aging is accompanied by a dramatic increase in DNA damage. This suggests that factors other than DNA damage per se, for example, cellular responses to DNA damage, are responsible for the aging phenotype in mice.

Acute hypersensitivity of pluripotent testicular cancer-derived embryonal carcinoma to low-dose 5-aza deoxycytidine is associated with global DNA Damage-associated p53 activation, anti-pluripotency and DNA demethylation

Human embryonal carcinoma (EC) cells are the stem cells of nonseminoma testicular germ cells tumors (TGCTs) and share remarkable similarities to human embryonic stem (ES) cells. In prior work we found that EC cells are hypersensitive to low nanomolar doses of 5-aza deoxycytidine (5-aza) and that this hypersensitivity partially depended on unusually high levels of the DNA methyltransferase, DNMT3B. We show here that low-dose 5-aza treatment results in DNA damage and induction of p53 in NT2/D1 cells. In addition, low-dose 5-aza results in global and gene specific promoter DNA hypomethylation. Low-dose 5-aza induces a p53 transcriptional signature distinct from that induced with cisplatin in NT2/D1 cells and also uniquely downregulates genes associated with pluripotency including NANOG, SOX2, GDF3 and Myc target genes. Changes in the p53 and pluripotency signatures with 5-aza were to a large extent dependent on high levels of DNMT3B. In contrast to the majority of p53 target genes upregulated by 5-aza that did not show DNA hypomethylation, several other genes induced with 5-aza had corresponding decreases in promoter methylation. These genes include RIN1, SOX15, GPER, and TLR4 and are novel candidate tumors suppressors in TGCTs. Our studies suggest that the hypersensitivity of NT2/D1 cells to low-dose 5-aza is multifactorial and involves the combined activation of p53 targets, repression of pluripotency genes, and activation of genes repressed by DNA methylation. Low-dose 5-aza therapy may be a general strategy to treat those tumors that are sustained by cells with embryonic stem-like properties.GEO NUMBER FOR THE MICROARRAY DATA: GSE42647.

Methylation mediated silencing of miR-23b expression and its role in glioma stem cells

MicroRNAs (miRNAs) are a class of small non-coding RNAs that negatively regulate gene expression at a post-transcriptional level. Some miRNAs harboring CGIs undergo methylation mediated silencing, a characteristic of many tumor suppressor genes. To identify such miRNAs in glioma stem cells (GSCs), we first showed that miR-23b is frequently methylated in GSCs but not in parallel U87 cells. Meanwhile, miR-23b expression was also markedly reduced in GSCs compared with matching U87 cells. Furthermore, treatment with 5-aza can increase miR-23b expression in GSCs. In addition, ectopic expression of miR-23b in GSCs induces cell cycle arrest and proliferation inhibition. Further analysis showed that miR-23b could enhance the sensitivity of U87 GSCs to TMZ. Our results suggest that miR-23b is epigenetically down-regulated and restoration of miR-23b can effectively suppress cell growth in GSCs.