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      Evidence that the Mouse Osteocalcin-Related Gene Does Not Encode Nephrocalcin

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          Background/Aims: The osteocalcin-related gene (ORG) is a mouse-specific member of the osteocalcin gene cluster predicted to encode a γ-carboxyglutamic acid-rich protein. ORG mRNA has been predicted to encode nephrocalcin and shown to be expressed in the kidney where it could serve as an important crystallization inhibitor. To determine whether ORG encodes mouse nephrocalcin, we investigated its in vivo and in vitro expression. Methods: We expressed fluorescent fusion ORG proteins in kidney cell lines and generated transgenic mice expressing enhanced green fluorescent protein under the control of the ORG promoter. Results: ORG mRNA was shown to be expressed in mouse kidneys and in a variety of other tissues. Fusion constructs transfected in opossum kidney cells demonstrated integrity of the open reading frame with the presence of a protein of the expected molecular weight. However, kidneys from transgenic mice carrying the enhanced green fluorescent protein gene under the control of the ORG promoter (5.8 kb fragment) demonstrated no expression of the transgene in kidneys or other tissues. Conclusion: We conclude that ORG, the third gene of the mouse osteocalcin gene cluster is silent and unlikely to play a major role in mouse renal physiology.

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          Cytosine methylation and the ecology of intragenomic parasites

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            Many human genes are transcribed from the antisense promoter of L1 retrotransposon.

            Human L1 retrotransposon has two transcription-regulatory regions: an internal or sense promoter driving transcription of the full-length L1, and an antisense promoter (ASP) driving transcription in the opposite direction into adjacent cellular sequences yielding chimeric transcripts. Both promoters are located in the 5'-untranslated region (5'-UTR) of L1. Chimeric transcripts derived from the L1 ASP are highly represented in expressed-sequence tag (EST) databases. Using a bioinformatics approach, we have characterized 10 chimeric ESTs (cESTs) derived from the EST division of GenBank. These cESTs contained 3' regions similar or identical to known cellular mRNA sequences. They were accurately spliced and preferentially expressed in tumor cell lines. Analysis of the hundreds of cESTs suggests that the L1 ASP-driven transcription is a common phenomenon not only for tumor cells but also for normal ones and may involve transcriptional interference or epigenetic control of different cellular genes.
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              Epigenetic regulation of an IAP retrotransposon in the aging mouse: progressive demethylation and de-silencing of the element by its repetitive induction.

              The recent insertion of a murine intracisternal A-particle (IAP) retrotransposon within one of the introns of a housekeeping gene, the circadian m.nocturnin gene, revealed a singular expression profile, both throughout the daytime and the mouse life span. Measurement of the levels of transcripts from this element by quantitative real-time RT-PCR, in organs of 1-24-month-old mice, disclosed that the inserted element--which is part of a large family of otherwise severely repressed mobile elements--becomes active upon aging, specifically in the liver where the m.nocturnin housekeeping gene is expressed in a circadian manner and induces a circadian expression of the IAP sequence. This age-dependent induction is cell-autonomous, as it persists in hepatocytes in primary culture. We further show, using methylation-sensitive enzymes, a correlation between the life-time kinetics of this process and a liver-specific demethylation of the IAP promoter. These results strongly support a model whereby the progressive demethylation and turning on of the IAP sequence is the sole result of the transient, daily activation-throughout the mouse life span--of its promoter. This phenomenon, which develops on a timescale of months to years in the aging mouse, might reveal a general epigenetic--and stochastic--process, which could account for a large series of events associated with cell and animal aging.

                Author and article information

                Nephron Exp Nephrol
                Cardiorenal Medicine
                S. Karger AG
                November 2006
                11 August 2006
                : 104
                : 4
                : e140-e146
                Centre de Recherche Guy-Bernier, Hôpital Maisonneuve-Rosemont, Montréal, Canada
                94965 Nephron Exp Nephrol 2006;104:e140–e146
                © 2006 S. Karger AG, Basel

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                Figures: 3, Tables: 1, References: 18, Pages: 1
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