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Telomerase activity is associated with an increase in DNA methylation at the proximal subtelomere and a reduction in telomeric transcription.

Ng LJ, Cropley JE, Pickett HA, Reddel RR, Suter CM - Nucleic Acids Res. (2009)

Bottom Line: We find that while ALT cells show highly heterogeneous patterns of subtelomeric methylation, subtelomeric regions in telomerase-positive cells invariably show denser methylation than normal cells, being almost completely methylated.When compared to matched normal and ALT cells, telomerase-positive cells also exhibit reduced levels of the telomeric repeat-containing-RNA (TERRA), whose transcription originates in the subtelomere.Our results are consistent with the notion that TERRA may inhibit telomerase: the heavy cytosine methylation we observe in telomerase-positive cells may reflect selection for TERRA silencing in order to facilitate telomerase activity at the telomere.

View Article: PubMed Central - PubMed

Affiliation: Victor Chang Cardiac Research Institute, Darlinghurst 2010, Australia.

ABSTRACT
Tumours and immortalized cells avoid telomere attrition by using either the ribonucleoprotein enzyme telomerase or a recombination-based alternative lengthening of telomeres (ALT) mechanism. Available evidence from mice suggests that the epigenetic state of the telomere may influence the mechanism of telomere maintenance, but this has not been directly tested in human cancer. Here we investigated cytosine methylation directly adjacent to the telomere as a marker of the telomere's epigenetic state in a panel of human cell lines. We find that while ALT cells show highly heterogeneous patterns of subtelomeric methylation, subtelomeric regions in telomerase-positive cells invariably show denser methylation than normal cells, being almost completely methylated. When compared to matched normal and ALT cells, telomerase-positive cells also exhibit reduced levels of the telomeric repeat-containing-RNA (TERRA), whose transcription originates in the subtelomere. Our results are consistent with the notion that TERRA may inhibit telomerase: the heavy cytosine methylation we observe in telomerase-positive cells may reflect selection for TERRA silencing in order to facilitate telomerase activity at the telomere. These data suggest that the epigenetic differences between telomerase-positive and ALT cells may underlie the mechanism of telomere maintenance in human tumorigenesis and highlight the broad reaching consequences of epigenetic dysregulation in cancer.

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Related in: MedlinePlus

Allelic methylation patterns at human subtelomeric regions in normal cells. (A) Subtelomeric regions of chromosomes 2p and 18p analysed by bisulphite sequencing. CpGs are represented by small vertical lines and PCR primers by arrows. (B) Bisulphite sequencing maps from normal human PBMC are shown below the corresponding region in (A). Within each map, horizontal lines show the methylation pattern of individual alleles: black squares represent methylated CpGs and white squares unmethylated CpGs. The average percent methylation for each individual is shown at the bottom right of each map. The gap in some alleles is due to a G/A polymorphism/mutation that results in loss of a CpG. C, cord blood; A, adult.
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Figure 1: Allelic methylation patterns at human subtelomeric regions in normal cells. (A) Subtelomeric regions of chromosomes 2p and 18p analysed by bisulphite sequencing. CpGs are represented by small vertical lines and PCR primers by arrows. (B) Bisulphite sequencing maps from normal human PBMC are shown below the corresponding region in (A). Within each map, horizontal lines show the methylation pattern of individual alleles: black squares represent methylated CpGs and white squares unmethylated CpGs. The average percent methylation for each individual is shown at the bottom right of each map. The gap in some alleles is due to a G/A polymorphism/mutation that results in loss of a CpG. C, cord blood; A, adult.

Mentions: We first sought to determine the normal levels and patterns of subtelomeric cytosine methylation in human somatic cells. Subtelomeric methylation has been shown to be a reliable surrogate marker for the epigenetic state of the telomere (17); although it is known that human cells are normally enriched for cytosine methylation at the subtelomere (14,15), the extent and patterns of methylation are not known. We analysed subtelomeric methylation by choosing CpG islands very close to the telomere (within 2 kb), in order to estimate as closely as possible the epigenetic state of the telomere itself. We identified subtelomeric CpG islands on chromosomes 2p, 4p and 18p (Figure 1A and Table S1) and analysed their methylation in peripheral blood mononuclear cells (PBMC) from four infants (cord blood) and four older adults (69–89 years). We found them to carry relatively dense cytosine methylation (Figure 1B and Table S2). We observed little inter-individual variation, with the average percentage methylation across all loci in all individuals being 81 ± 3%. Every allele examined showed methylation of most CpGs, with occasional unmethylated CpGs. There was no difference between levels of subtelomeric methylation in young and old individuals (P = 0.729). This finding suggests that the heterochromatic state of human telomeres is maintained despite the known telomere attrition that occurs with age, especially in the first few years of life (21). Thus, there may be an important role for telomeric heterochromatin in the maintenance of telomere function and integrity, independent of telomere length.Figure 1.


Telomerase activity is associated with an increase in DNA methylation at the proximal subtelomere and a reduction in telomeric transcription.

Ng LJ, Cropley JE, Pickett HA, Reddel RR, Suter CM - Nucleic Acids Res. (2009)

Allelic methylation patterns at human subtelomeric regions in normal cells. (A) Subtelomeric regions of chromosomes 2p and 18p analysed by bisulphite sequencing. CpGs are represented by small vertical lines and PCR primers by arrows. (B) Bisulphite sequencing maps from normal human PBMC are shown below the corresponding region in (A). Within each map, horizontal lines show the methylation pattern of individual alleles: black squares represent methylated CpGs and white squares unmethylated CpGs. The average percent methylation for each individual is shown at the bottom right of each map. The gap in some alleles is due to a G/A polymorphism/mutation that results in loss of a CpG. C, cord blood; A, adult.
© Copyright Policy - creative-commons
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC2651807&req=5

Figure 1: Allelic methylation patterns at human subtelomeric regions in normal cells. (A) Subtelomeric regions of chromosomes 2p and 18p analysed by bisulphite sequencing. CpGs are represented by small vertical lines and PCR primers by arrows. (B) Bisulphite sequencing maps from normal human PBMC are shown below the corresponding region in (A). Within each map, horizontal lines show the methylation pattern of individual alleles: black squares represent methylated CpGs and white squares unmethylated CpGs. The average percent methylation for each individual is shown at the bottom right of each map. The gap in some alleles is due to a G/A polymorphism/mutation that results in loss of a CpG. C, cord blood; A, adult.
Mentions: We first sought to determine the normal levels and patterns of subtelomeric cytosine methylation in human somatic cells. Subtelomeric methylation has been shown to be a reliable surrogate marker for the epigenetic state of the telomere (17); although it is known that human cells are normally enriched for cytosine methylation at the subtelomere (14,15), the extent and patterns of methylation are not known. We analysed subtelomeric methylation by choosing CpG islands very close to the telomere (within 2 kb), in order to estimate as closely as possible the epigenetic state of the telomere itself. We identified subtelomeric CpG islands on chromosomes 2p, 4p and 18p (Figure 1A and Table S1) and analysed their methylation in peripheral blood mononuclear cells (PBMC) from four infants (cord blood) and four older adults (69–89 years). We found them to carry relatively dense cytosine methylation (Figure 1B and Table S2). We observed little inter-individual variation, with the average percentage methylation across all loci in all individuals being 81 ± 3%. Every allele examined showed methylation of most CpGs, with occasional unmethylated CpGs. There was no difference between levels of subtelomeric methylation in young and old individuals (P = 0.729). This finding suggests that the heterochromatic state of human telomeres is maintained despite the known telomere attrition that occurs with age, especially in the first few years of life (21). Thus, there may be an important role for telomeric heterochromatin in the maintenance of telomere function and integrity, independent of telomere length.Figure 1.

Bottom Line: We find that while ALT cells show highly heterogeneous patterns of subtelomeric methylation, subtelomeric regions in telomerase-positive cells invariably show denser methylation than normal cells, being almost completely methylated.When compared to matched normal and ALT cells, telomerase-positive cells also exhibit reduced levels of the telomeric repeat-containing-RNA (TERRA), whose transcription originates in the subtelomere.Our results are consistent with the notion that TERRA may inhibit telomerase: the heavy cytosine methylation we observe in telomerase-positive cells may reflect selection for TERRA silencing in order to facilitate telomerase activity at the telomere.

View Article: PubMed Central - PubMed

Affiliation: Victor Chang Cardiac Research Institute, Darlinghurst 2010, Australia.

ABSTRACT
Tumours and immortalized cells avoid telomere attrition by using either the ribonucleoprotein enzyme telomerase or a recombination-based alternative lengthening of telomeres (ALT) mechanism. Available evidence from mice suggests that the epigenetic state of the telomere may influence the mechanism of telomere maintenance, but this has not been directly tested in human cancer. Here we investigated cytosine methylation directly adjacent to the telomere as a marker of the telomere's epigenetic state in a panel of human cell lines. We find that while ALT cells show highly heterogeneous patterns of subtelomeric methylation, subtelomeric regions in telomerase-positive cells invariably show denser methylation than normal cells, being almost completely methylated. When compared to matched normal and ALT cells, telomerase-positive cells also exhibit reduced levels of the telomeric repeat-containing-RNA (TERRA), whose transcription originates in the subtelomere. Our results are consistent with the notion that TERRA may inhibit telomerase: the heavy cytosine methylation we observe in telomerase-positive cells may reflect selection for TERRA silencing in order to facilitate telomerase activity at the telomere. These data suggest that the epigenetic differences between telomerase-positive and ALT cells may underlie the mechanism of telomere maintenance in human tumorigenesis and highlight the broad reaching consequences of epigenetic dysregulation in cancer.

Show MeSH
Related in: MedlinePlus