Ascorbate induces ten-eleven translocation (Tet) methylcytosine dioxygenase-mediated generation of 5-hydroxymethylcytosine.

BACKGROUND: Tet methylcytosine dioxygenase converts 5-mC to 5-hmC in DNA. RESULTS: Ascorbate significantly and specifically enhances Tet-mediated generation of 5-hmC. CONCLUSION: Our findings suggest that ascorbate enhances 5-hmC generation, most likely by acting as a co-factor for Tet methylcytosine dioxygenase to generate 5-hmC. SIGNIFICANCE: The availability of ascorbate could have significant consequences for health and diseases by modulating the epigenetic control of genome activity. Ascorbate (vitamin C) is best known for its role in scurvy, in which the hydroxylation of collagen catalyzed by dioxygenases is incomplete due to ascorbate deficiency. Here, we report a novel function of ascorbate in the hydroxylation of 5-methylcytosine (5-mC) to 5-hydroxymethylcytosine (5-hmC) in DNA catalyzed by Tet (ten-eleven translocation) methylcytosine dioxygenase. The content of 5-hmC is extremely low in mouse embryonic fibroblasts cultured in ascorbate-free medium. Additions of ascorbate dose- and time-dependently enhance the generation of 5-hmC, without any effects on the expression of Tet genes. Treatment with another reducer glutathione (GSH) does not change the level of 5-hmC. Further, blocking ascorbate entry into cells by phloretin and knocking down Tet (Tet1, Tet2, and Tet3) expression by short interference RNAs (siRNA) significantly inhibit the effect of ascorbate on 5-hmC. These results suggest that ascorbate enhances 5-hmC generation, most likely by acting as a co-factor for Tet methylcytosine dioxygenase to hydroxylate 5-mC. Thus, we have uncovered a novel role for ascorbate in modulating the epigenetic control of genome activity.

Analysis of the indel at the ARMS2 3'UTR in age-related macular degeneration.

Controversy remains as to which gene at the chromosome 10q26 locus confers risk for age-related macular degeneration (AMD) and statistical genetic analysis is confounded by the strong linkage disequilibrium (LD) across the region. Functional analysis of related genetic variations could solve this puzzle. Recently, Fritsche et al. reported that AMD is associated with unstable ARMS2 transcripts possibly caused by a complex insertion/deletion (indel; consisting of a 443 bp deletion and an adjacent 54 bp insertion) in its 3'UTR (untranslated region). To validate this indel, we sequenced our samples. We found that this indel is even more complex and is composed of two side-by-side indels separated by 17 bp: (1) 9 bp deletion with 10 bp insertion; (2) 417 bp deletion with 27 bp insertion. The indel is significantly associated with the risk of AMD, but is also in strong LD with the non-synonymous single nucleotide polymorphism rs10490924 (A69S). We also found that ARMS2 is expressed not only in placenta and retina but also in multiple human tissues. Using quantitative PCR, we found no correlation between the indel and ARMS2 mRNA level in human retina and blood samples. The lack of functional effects of the 3'UTR indel, the amino acid substitution of rs10490924 (A69S), and strong LD between them suggest that A69S, not the indel, is the variant that confers risk of AMD. To our knowledge, it is the first time it has been shown that ARMS2 is widely expressed in human tissues. Conclusively, the indel at 3'UTR of ARMS2 actually contains two side-by-side indels. The indels are associated with risk of AMD, but not correlated with ARMS2 mRNA level.

AMD-associated variants at the chromosome 10q26 locus and the stability of ARMS2 transcripts.

PURPOSE: To analyze the effect of variants including age-related macular degeneration (AMD)-associated combinative insertion/deletion polymorphism (indel) at 3'UTR of ARMS2 and possibly associated R38X on the stability of ARMS2 transcripts. METHODS: ARMS2 transcription from minigene vectors carrying different alleles at variants R38X and the indel were assessed in mouse embryonic fibroblasts (MEFs). Dual luciferase assays were applied to evaluate the effect of the indel on gene expression. RT-PCR and quantitative RT-PCR (qRT-PCR) were used to measure the two ARMS2 transcripts (isoform A and isoform B) in MEFs and human retina-RPE-choroid samples (n = 83). RESULTS: Allele X at variant R38X decreased exogenous ARMS2 transcripts in MEFs compared to allele R. In contrast, the indel did not change the level of exogenous ARMS2 transcripts. After blocking transcription by actinomycin D, R38X appeared to accelerate the degradation of ARMS2 transcripts, while the indel did not obviously affect the stability of ARMS2 transcripts compared to the wild-type (WT) allele. Dual luciferase assays further indicated that the indel did not influence gene expression. Quantitative RT-PCR results showed that there was no significant difference in two ARMS2 transcript splice isoforms among retina-RPE-choroid samples carrying different genotypes at variants R38X and the indel. CONCLUSIONS: Variant R38X, not the indel, decreases the stability of ARMS2 transcripts in vitro. However, genotypes at R38X and the indel do not obviously affect the level of ARMS2 transcripts in retina-RPE-choroid samples. These results suggest that variants R38X and the indel are less likely to play a pathogenic role in AMD by changing the level of ARMS2 transcripts.

Localization of age-related macular degeneration-associated ARMS2 in cytosol, not mitochondria.

PURPOSE: To analyze the relationship between ARMS2 and HTRA1 in the association with age-related macular degeneration (AMD) in an independent case-control dataset and to investigate the subcellular localization of the ARMS2 protein in an in vitro system. METHODS: Two SNPs in ARMS2 and HTRA1 were genotyped in 685 cases and 269 controls by a genotyping assay. Allelic association was tested by a chi(2) test. A likelihood ratio test (LRT) of full versus reduced models was used to analyze the interaction between ARMS2 and smoking and HTRA1 and smoking, after adjustment for CFH and age. Immunofluorescence and immunoblot were applied to localize ARMS2 in retinal epithelial ARPE-19 cells and COS7 cell transfected by ARMS2 constructs. RESULTS: Both significantly associated SNP rs10490924 and rs11200638 (P < 0.0001) are in strong linkage disequilibrium (LD; D' = 0.97, r(2) = 0.93) that generates virtually identical association test and odds ratios. In separate logistic regression models, the interaction effect for both smoking with ARMS2 and with HTRA1 was not statistically significant. Immunofluorescence and immunoblot show that both endogenous and exogenous ARMS2 are mainly distributed in the cytosol, not the mitochondria. Compared with the wild-type, ARMS2 A69S is more likely to be associated with the cytoskeleton in COS7 cells. CONCLUSIONS: The significant associations in ARMS2 and HTRA1 are with polymorphisms in strong LD that confer virtually identical risks, preventing differentiation at the statistical level. ARMS2 was mainly distributed in the cytosol, not in the mitochondrial outer membrane as previously reported, suggesting that ARMS2 may not confer risk to AMD through the mitochondrial pathway.