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Circadian rhythm of covalent modifications in liver DNA
R G Nath1, S V Vulimiri, K Randerath
1Department of Pharmacology, Baylor College of Medicine, Houston, TX 77030.
Abstract:
32P-postlabeling analysis recently revealed that in addition to 5-methylcytosine, mammalian DNA contains covalently modified nucleotides of unknown structures and functions termed I-compounds whose levels increase with age. I-compound levels, in addition, depend on species, strain, sex, tissue, and diet and are generally lowered by carcinogen exposure. As shown here, levels of several non-polar I-compounds in liver DNA of untreated male C3H mice were elevated 2 to 8.5 times at 1800 h and 2400 h as compared to 0600 h and 1200 h, while polar I-compounds and persistent carcinogen-DNA adducts induced by safrole were unaffected by time of day. In liver DNA of male F-344 rats 4 non-polar I-compounds and 4 polar I-compounds showed significant circadian rhythm at 2000 h compared to 0800 h. This novel circadian variation of DNA structure implies mechanisms precisely regulating I-compound levels in vivo and may conceivably be linked to diurnal differences of DNA synthesis and gene expression.
Insights
Mammalian DNA contains I-compounds, modified nucleotides that vary with age and lifestyle. This study reveals a novel circadian rhythm in non-polar I-compound levels in mouse and rat liver DNA.
Area of Science:
- Biochemistry
- Molecular Biology
- Genetics
Background:
- Mammalian DNA contains 5-methylcytosine and other modified nucleotides known as I-compounds.
- I-compound levels are influenced by age, species, sex, tissue, diet, and carcinogen exposure.
- The structures and functions of I-compounds remain largely unknown.
Purpose of the Study:
- To investigate the potential diurnal variation of I-compound levels in mammalian DNA.
- To determine if I-compound levels exhibit a circadian rhythm.
- To explore the implications of circadian variations in DNA structure.
Main Methods:
- 32P-postlabeling analysis was used to quantify I-compounds in liver DNA.
- Experiments were conducted on untreated male C3H mice and male F-344 rats.
- DNA samples were collected at different times of day (e.g., 0600 h, 1200 h, 1800 h, 2400 h, 0800 h, 2000 h).
Main Results:
- Non-polar I-compounds in male C3H mouse liver DNA showed significant elevation at 1800 h and 2400 h compared to 0600 h and 1200 h.
- Polar I-compounds and safrole-induced DNA adducts were not affected by the time of day in mice.
- Four non-polar and four polar I-compounds in male F-344 rat liver DNA exhibited significant circadian rhythm between 0800 h and 2000 h.
Conclusions:
- A novel circadian variation in DNA structure, specifically in I-compound levels, has been identified.
- These findings suggest precise in vivo regulatory mechanisms for I-compound levels.
- The circadian variation may be linked to diurnal differences in DNA synthesis and gene expression.