Article

Accelerated methylation of ribosomal RNA genes during the cellular senescence of Werner syndrome fibroblasts.

Laboratory of Molecular Genetics, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224, USA.
The FASEB Journal (impact factor: 5.71). 10/2000; 14(12):1715-24. pp.1715-24
Source: PubMed

ABSTRACT Ribosomal DNA (rDNA) metabolism has been implicated in cellular and organismal aging. The role of rDNA in premature and normal human aging was investigated by measuring rDNA gene copy number, the level of rDNA methylation, and rRNA expression during the in vitro senescence of primary fibroblasts from normal (young and old) donors and from Werner syndrome (WS) patients. In comparison to their normal counterparts, WS fibroblasts grew slowly and reached senescence after fewer doublings. The rDNA copy number did not change significantly throughout the life span of both normal and WS fibroblasts. However, in senescent WS and normal old fibroblasts, we detected rDNA species with unusually slow electrophoretic mobility. Cellular aging in Saccharomyces cerevisiae is accompanied by the formation and accumulation of rDNA circles. Our analysis revealed that the rDNA species observed in this study were longer, linear rDNA molecules attributable to the inhibition of ECO:RI cleavage by methylation. Furthermore, isoschizomeric restriction analysis confirmed that in vitro senescence of fibroblasts is accompanied by significant increases in cytosine methylation within rDNA genes. This increased methylation is maximal during the abbreviated life span of WS fibroblasts. Despite increased methylation of rDNA in senescent cells, the steady-state levels of 28S rRNA remained constant over the life span of both normal and WS fibroblasts.

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Keywords

abbreviated life span
 
cytosine methylation
 
increased methylation
 
linear rDNA molecules attributable
 
normal counterparts
 
normal human
 
normal old fibroblasts
 
primary fibroblasts
 
rDNA circles
 
rDNA copy number
 
rDNA gene copy number
 
rDNA genes
 
rDNA methylation
 
Ribosomal DNA
 
rRNA expression
 
Saccharomyces cerevisiae
 
senescent WS
 
significant increases
 
slow electrophoretic mobility
 
WS fibroblasts
 

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