Human Molecular Genetics Advance Access originally published online on June 16, 2008
Human Molecular Genetics 2008 17(18):2776-2789; doi:10.1093/hmg/ddn177
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Hypomethylation of subtelomeric regions in ICF syndrome is associated with abnormally short telomeres and enhanced transcription from telomeric regions
1 Department of Nephrology and Laboratory of Molecular Medicine, Rambam Medical Center and Rappaport Faculty of Medicine and Research Institute, Technion, Haifa 31096, Israel 2 U741 INSERM, 2 Place Jussieu, Tour 43-couloir 43–44, Paris 75251, France
* To whom correspondence should be addressed. Tel: +972 48295277; Fax: +972 48512380; Email: seligs{at}tx.technion.ac.il
Received May 15, 2008; Accepted June 14, 2008
Telomeres and adjacent subtelomeric regions are packaged as heterochromatin in many organisms. The heterochromatic features include DNA methylation, histones H3-Lys9 (Lysine 9) and H4-Lys20 (Lysine 20) methylation and heterochromatin protein1 alpha binding. Subtelomeric DNA is hypomethylated in human sperm and ova, and these regions are subjected to de novo methylation during development. In mice this activity is carried out by DNA methyltransferase 3b (Dnmt3b). Mutations in DNMT3B in humans lead to the autosomal-recessive ICF (immunodeficiency, centromeric region instability, facial anomalies) syndrome. Here we show that, in addition to several satellite and non-satellite repeats, the subtelomeric regions in lymphoblastoid and fibroblast cells of ICF patients are also hypomethylated to similar levels as in sperm. Furthermore, the telomeres are abnormally short in both the telomerase-positive and -negative cells, and many chromosome ends lack detectable telomere fluorescence in situ hybridization signals from either one or both sister-chromatids. In contrast to Dnmt3a/b–/– mouse embryonic stem cells, increased telomere sister-chromatid exchange was not observed in ICF cells. Hypomethylation of subtelomeric regions was associated in the ICF cells with advanced telomere replication timing and elevated levels of transcripts emanating from telomeric regions, known as TERRA (telomeric-repeat-containing RNA) or TelRNA. The current findings provide a mechanistic explanation for the abnormal telomeric phenotype observed in ICF syndrome and highlights the link between TERRA/TelRNA and structural telomeric integrity.
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