Quantitative fluorescence in situ hybridization (Q-FISH)

Probing the mitotic history and developmental stage of hematopoietic cells using single telomere length analysis (STELA)

In most human somatic cells, telomeres shorten as a function of DNA replication. Telomere length is therefore an indirect measure of the replicative history of cells. We measured the telomere lengths at XpYp chromosomes in purified human hematopoietic populations enriched for stem cells (Lin(-)CD34(+)CD38(-)Rho(-)) and successively more mature cells. The average telomere length showed expected length changes, pointing to the utility of this method for classifying novel differentiation markers. Interestingly, the frequency of abruptly shortened telomeres increased in terminally differentiated adult populations, suggesting that damage to telomeric DNA occurs or is not repaired upon hematopoietic differentiation. When Lin(-)CD34(+)CD38(-)Rho(-) cord blood cells were transplanted into immunodeficient mice, the telomeres of the most primitive regenerated human hematopoietic cells lost approximately 3 kb, indicative of more than 30 cell divisions. Further losses in differentiating cells were similar to those observed in pretransplantation cell populations. These results indicate extensive self-renewal divisions of human hematopoietic stem cells are the primary cause of telomere erosion upon transplantation rather than added cell divisions in downstream progenitors.

Telomere length in Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare premature aging disorder caused by mutations in the gene LMNA, which encodes the nuclear matrix protein lamin A. Previous research has shown that the average telomere length in fibroblasts from HGPS patients is shorter than in age-matched controls. How mutations in lamin A lead to shortened telomere lengths is not known nor is the contribution of individual chromosome ends to the low average length understood. To measure the telomere length of individual chromosomes, we used quantitative fluorescence in situ hybridization (Q-FISH). In agreement with previous studies, we found that the average telomere length in HPGS fibroblasts is greatly reduced; however, the telomere length at chromosome ends was variable. In contrast, the telomere length in hematopoietic cells which typically do not express lamin A, was within the normal range for three out of four HGPS patient samples. Our results suggest that mutant lamin A decreases telomere length via a direct effect and that expression of mutant LMNA is necessary for telomere loss in HGPS.

Hypomethylation of subtelomeric regions in ICF syndrome is associated with abnormally short telomeres and enhanced transcription from telomeric regions

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.