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Telomere length in Hutchinson-Gilford Progeria Syndrome
Mechanisms of Ageing and Development 130 (2009) 377–383
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Telomere length in Hutchinson-Gilford Progeria Syndrome Michelle L. Decker a,b, Elizabeth Chavez a, Irma Vulto a, Peter M. Lansdorp a,c,* a
Terry Fox Laboratory, BC Cancer Agency, Vancouver, BC, Canada Department of Medical Genetics, University of British Columbia, Vancouver, BC, Canada c Division of Hematology, Department of Medicine, University of British Columbia, Vancouver, BC, Canada b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 October 2008 Received in revised form 14 February 2009 Accepted 6 March 2009 Available online 20 March 2009
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. Crown Copyright ß 2009 Published by Elsevier Ireland Ltd. All rights reserved.
Keywords: Telomere length Hutchinson-Gilford Progeria Syndrome Aging Nuclear lamina
1. Introduction Hutchinson-Gilford Progeria Syndrome (HPGS) is a segmental premature aging disease which manifests in the first 2 years of life (Pollex and Hegele, 2004). Symptoms include postnatal growth restriction, loss of hair and subcutaneous fat, decreased joint mobility, and atherosclerosis (Hennekam, 2006). Patients die at a mean age of 12.6 years from progressive atherosclerosis of the coronary and cerebrovascular arteries leading to heart attacks and strokes (Baker et al., 1981; Hennekam, 2006). Cognitive development is normal and no increase in cancer incidence has been observed (Hennekam, 2006). In about 90% of cases, HGPS is caused by a C ! T mutation at nucleotide 1824 in exon 11 of the lamin A/C gene (LMNA) which activates a cryptic splice site in the mRNA (De Sandre-Giovannoli et al., 2003; Eriksson et al., 2003). This results in the translation of a protein with a 50 amino acid deletion near the C-terminus (Eriksson et al., 2003). The deletion includes a cleavage recognition site that is required for complete processing to the mature form of lamin A (Eriksson et al., 2003). The two classes of nuclear lamin proteins, A-type and B-type, are involved in many important nuclear functions including DNA replication, transcription, chromatin organization, nuclear shape
* Corresponding author at: Terry Fox Laboratory, BC Cancer Research Centre, 675 West 10th Avenue, Vancouver, BC, Canada V5Z 1L3. Tel.: +1 604 675 8135. E-mail address: [email protected] (P.M. Lansdorp).
and nuclear position in the cell (Zastrow et al., 2004). The two predominant A-type lamins, A and C, are expressed by alternative splicing of the LMNA mRNA (Zastrow et al., 2004). Lamin A and C are expressed in all differentiated tissues except for some hematopoietic lineages including CD20-positive B lymphocytes, CD3-positive T lymphocytes as well as neuroendocrine cells (Broers et al., 1997; Jansen et al., 1997). Lamin A interacts with many components of the nucleus including other lamins, lamin associated protein 2a (LAP2a), and actin as well as DNA and histones and potentially serves as a scaffolding network for multiprotein complexes (Zastrow et al., 2004). HGPS resembles aging at the cellular level as well. The mutant protein, often referred to as ‘progerin’, remains farnesylated at the C-terminus due to the lack of protein processing (Eriksson et al., 2003). This likely causes most of the cellular defects including lobulation of the nuclear membrane, diminished replication potential, slow DNA damage response and abnormal chromatin organization (Goldman et al., 2004; Liu et al., 2005). It has also observed that telomere length is shorter in HGPS fibroblasts when compared to age-matched controls (Allsopp et al., 1992). Telomeres are repetitive G-rich DNA sequences and associated binding proteins found at the ends of linear eukaryotic chromosomes. They are key in preventing genomic instability (Blackburn, 2001; de Lange, 2005). The telomere binding proteins aid in forming a protective structure which ‘caps’ chromosome ends and prevent their processing as double strand breaks (de Lange, 2005). Telomeres shorten with each cell division, in vivo and in vitro, due
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to the ‘end replication problem’ as well as sporadic losses following damage or replication errors (Lansdorp, 2005). Consequently, telomeres from elderly donors are much shorter that those from young donors. When telomeres reach a critical length, cells either stop dividing (senesce) or undergo apoptosis (Stewart and Weinberg, 2006). Limitations in the replicative potential of cells imposed by telomere shortening may restrict the proliferation of abnormal cells; however, progressive telomere attrition also contributes to the loss of cells and tissue function with age (Aubert and Lansdorp, 2008). In order to better understand the role of telomeres in HGPS, we examined the telomere length in cells from HGPS patients using two approaches. First, to determine how telomere lengths of individual chromosomes vary in fibroblasts of HGPS patients, we quantified telomere lengths using quantitative fluorescence in situ hybridization (Q-FISH) (Poon and Lansdorp, 2001). We show that telomere length is significantly shorter in HGPS cells; however no particular chromosome had consistently short or long telomeres. Second, to determine a causative role for lamin A in telomere shortening in HGPS patients, we used flow-FISH (Baerlocher et al., 2006) to examine telomere length in hematopoietic cells, which do not express lamin A (Baerlocher et al., 2006; Broers et al., 1997). We show that telomere length is in the normal range for three out of four HGPS patients examined. These results suggest that mutant lamin A is directly involved in the generation of short telomeres. 2. Materials and methods 2.1. Cell lines and patient samples The HGPS cell lines AG03513, AG06297 and AG11498 were obtained from the NIA Aging Cell Repository (Coriell Cell Repository, Camden, NJ). Cells were cultured in Dulbecco’s modified eagle medium (DMEM) containing 15% fetal calf serum, 200 mM glutamine, 100 U/ml penicillin and 100 mg/ml streptomycin at 37 8C in a 5% CO2 atmosphere incubator. At the time experiments were preformed, the HGPS cell lines were at the following population doublings: AG03513 – PD 17, AG06297 – PD 35 and AG11498 – PD 7. The six healthy control fibroblast cell lines that were used in the telomere length studies have been previously reported in (Martens et al., 1998). Blood samples were drawn from patients diagnosed with classical HGPS after informed consent from patients and their parents (Merideth et al., 2008). Blood was drawn in Heparin or EDTA tubes and shipped at room temperature. Upon arrival blood samples were frozen until analysis. Controls for Flow-FISH were 400 healthy persons ranging from birth to 100 years of ages (Yamaguchi et al., 2005). 2.2. Quantitative fluorescence in situ hybridization Q-FISH was preformed as previously described (Poon and Lansdorp, 2001). Briefly, metaphase cells were harvested, fixed with methanol–acetic acid then dropped onto slides. Slides were fixed with formaldehyde, treated with pepsin, and dehydrated with ethanol. The hybridization mix containing the Cy3-labeled (CCCTAA)3 peptide nucleic acid telomere probe was added to each slide which was then denatured at 80 8C for 2 min prior to incubation at room temperature for 1 h. Slides were washed, counterstained with DAPI then mounted using DABCO. Images were acquired and analyzed as described. 2.3. RNA extraction and RT-PCR RNA was extracted from BJ neonatal human foreskin fibroblasts, and T cells and granulocytes from control samples using the RNAeasy kit (Qiagen). A reversetranscriptase-polymerase-chain-reaction (RT-PCR) assay was performed with the isolated RNA to make complementary DNA (cDNA). The cDNA was amplified with either primers for LMNA (forward: CAAGGCATCTGCCAGCGG and reverse: TTTCTTTGGCTTCAAGCCCC) or b-actin (forward: AGAGATGGCCACGGCTGCTTC and reverse: GCATTTGCGGTGGACGATGGAG). PCR products were visualized on a 1.5% agarose gel by ethidium bromide staining. 2.4. Flow-FISH Details of the Flow-FISH method are as previously described (Baerlocher et al., 2006). Briefly, red blood cells from samples were lysed with NH4Cl. Leukocytes were denatured in formamide at 87 8C, hybridized with a fluorescein-conjugated (CCCTAA)3 peptide nucleic acid probe, and counterstained with LDS751 DNA dye. Analysis of fluorescence was performed on a FACSCalibur flow cytometer (BD Biosciences, San Jose, CA). The cell types analyzed included total leukocytes,
granulocytes, total lymphocytes, CD45RA-positive/CD20-negative naive T cells, CD45RA-negative memory T cells, CD20-positive B cells, and CD57-positive NK/NKT cells. Bovine thymocytes were used as an internal standard in every sample. Cellquest Pro software (BD Biosciences, CA, USA) was used to quantify the flow cytometry results; median telomere lengths were calculated using an automated Microsoft Excel calculator. 2.5. Statistical analysis HGPS samples were compared to control samples using an independent t-test, assuming unequal variance. A p-value of <0.005 was considered significant.
3. Results 3.1. Telomere length in HGPS fibroblasts To investigate the nature of telomere shortening in HGPS, QFISH was performed on metaphase chromosome spreads of three HGPS primary fibroblast cell lines derived from biopsies of three different patients (Fig. 1). The fibroblast cell lines from HGPS patients were grown using standard culture conditions. Cells from early passage cultures were arrested in metaphase and used for telomere length analysis. At least twelve metaphases were analyzed for each cell line. No significant chromosomal abnormalities such as fusions, translocations or aneuploidy were detected in the metaphases from HGPS patients (data not shown). The samples AG11498 and AG06297 have the common 1824 C ! T mutation; however, the mutation status of AG03513 is unknown although the patient had classical HGPS symptoms. Telomere length in AG06297 and AG03513 were significantly shorter than the control samples and AG11498 was around the same length as the control samples and is short for the age of the patient based on data from Allsopp et al., 1992 (Table 1) (Martens et al., 2000). While AG11498 was not shorter than control cells it should be noted that the average age of the control cell lines was 50 years which likely accounts for this. Telomere lengths were variable between different chromosome ends as well as between samples (Figs. 2 and 3). Signal free ends, representing telomeres that are too short to detect by Q-FISH, were observed in the HGPS samples but not in the normal fibroblast metaphases (Table 1) (Fig. 1, white asterisks). We found no evidence for consistent biased shortening of any particular chromosome end between samples, although all samples exhibited some extremely short chromosomes including chromosome 18q in AG06297 (Fig. 2 and Supplementary Figs. 1 and 2). The telomeres of chromosome 17p, which have been previously reported to be among the shortest (Britt-Compton et al., 2006; Martens et al., 1998), were below the mean telomere length in all HGPS samples however in AG11498 it was not statistically different from the mean telomere length for that sample (Fig. 3). In addition, the telomeres of chromosome 4q, often one of the longest telomeres and always above the mean in normal individuals (Martens et al., 1998), was only significantly different than the mean telomere length in AG03513. In the control fibroblast samples, the average telomere length of chromosome 18, which is gene poor, were above the total mean telomere length. The average telomere length of chromosome 19, which is gene-rich, was always found to be below the mean telomere length in the control samples. In the HGPS samples, the telomere length of chromosomes 18 and 19 was more similar to each other than they were in control samples. In some cases, such as chromosome 18q in sample AG06297, chromosomes were found to be on the opposite side of the mean from usual (Figs. 2 and 3). In summary, chromosome-specific differences in telomere length observed in cells from normal control individuals were not preserved in all HGPS cell lines. This suggests a much more random chromatin arrangement in the HGPS cells.
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Fig. 1. Quantitative-FISH analysis of individual telomeres in fibroblasts from 3 different patients with HGPS. The PNA probe specific for telomeric DNA is in shown in yellow while DAPI staining for DNA is shown in blue. Telomere ends with undetectable telomere signals are indicated using a white asterisk. (A) AG03513, (B) AG06297 and (C) AG11498.
3.2. Telomere length in HGPS hematopoietic cells To investigate if the expression of mutant lamin A is directly involved in the generation of short telomeres, we examined telomere lengths in hematopoietic cells, which reportedly either do not express LMNA or express it at much lower levels (Broers et al., 1997; Jansen et al., 1997). To confirm this, RT-PCR was performed on granulocyte and T cell mRNA samples and a fibroblast cell line (BJ) as a control (Fig. 4). The only sample that expressed LMNA at detectable levels was the control fibroblast cell line sample while the granulocyte and T cell samples had no band confirming the lack of LMNA expression in these hematopoietic cells (Fig. 4). Blood samples were obtained from four HGPS patients, all of whom have the common C ! T mutation at nucleotide 1824 and presented clinically with classical HGPS symptoms (personal communications, Dr. William A. Gahl). The median telomere
lengths of subsets of hematopoietic cells, including T cells, B cells, NK cells, and granulocytes were measured using flow-FISH. The average telomere length in all subsets of hematopoietic cells was consistent with the expected length for age-matched controls in three out of the four patient samples examined (Fig. 5). One sample, HGPS 3, had very low telomere length in all subsets of cells. This patient did not exhibit a more severe form of the disease compared to the other patients at this stage. The normal telomere lengths in three out of four patients suggest that the expression of progerin leads to the short telomeres observed in the fibroblast samples. 4. Discussion HGPS is a segmental aging disorder which resembles aging in specific tissues both physiologically and at the cellular level. The phenotype of short telomeres has been seen in other premature
Table 1 Mean telomere length and frequency of signal free ends in HGPS fibroblasts measured using Q-FISH. Sample Controls AG03513 AG06297 AG11498 a
Age (years) 49.7 13 8 13
a
p arm (kb SEM)
q arm (kb SEM)
Total (kb SEM)
Signal free ends
% signal free ends
– 3.18 0.05 3.60 0.04 6.57 0.06
– 3.46 0.05 3.40 0.04 6.34 0.06
6.17 0.03 3.32 0.05** 3.48 0.04** 6.45 0.06*
0 43 88 3
0 1.83 2.84 0.13
The control is the average of 6 normal fibroblast cell lines. p and q arm data values were not collected separately for the normal controls. p < 0.005 of mean telomere length between controls and HGPS samples calculated using an independent t-test assuming unequal variance. ** p < 0.0001 of mean telomere length between controls and HGPS samples calculated using an independent t-test assuming unequal variance. *
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Fig. 2. Telomere length is short in HGPS cell lines and highly variable between chromosomes. Box plot histograms of telomere fluorescence values in the HGPS fibroblast cell line AG06297. Telomere lengths of chromosomes from HGPS cells are highly variable. The horizontal line in the large box represents the 50th percentile; the upper and lower margins of the box are the 25th and 75th percentile; the small box inside represents the mean; the vertical lines above and below the box are the 5th and 95th percentile respectively; the ‘’ represents the 1st and 99th percentile; the ‘–’ represents the minimum and maximum values. (A) The p arm and (B) the q arm of the indicated chromosomes.
aging diseases including Werner Syndrome where telomere dysfunction has been proposed to cause genomic instability (Crabbe et al., 2007). Our results from telomere length analysis of fibroblasts from HGPS patients are consistent with the results described by Allsopp et al. (1992) (Table 1). By combining Q-FISH with karyotyping, the telomere length of individual chromosome ends was studied in the hope to gain insight into the mechanism of telomere shortening in HGPS. The length of individual chromosomes from HGPS patients did not always follow the established patterns of the control samples (Figs. 2 and 3 and Supplementary Figs. 1 and 2). On average, chromosome 17p has been reported to have the shortest telomeres, and in the HGPS samples it was also below the mean telomere length but not the absolute shortest (Britt-Compton et al., 2006; Martens et al., 1998). Chromosomes 18 and 19, which are relatively gene-poor and gene-rich respectively, have defined nuclear territories (Croft et al., 1999; Meaburn et al., 2007). Chromosome 18, typically located at the periphery of the nucleus in proliferating cells, is more internally localized in HGPS cells (Croft et al., 1999). Loss of peripheral heterochromatin may lead to this change in nuclear position and possibly alter the chromatin organization which could affect telomeres structure. In the HGPS fibroblasts, the telomere length in chromosomes 18 and 19 generally followed the pattern of the wild type cells (Fig. 3). However, the telomere lengths of these two chromosomes are
more similar. This taken with the chromosome 17p differences suggests that there may be differences in telomeric chromatin in HGPS. The median telomere length was measured in hematopoietic cells by flow FISH to determine if cells with reduced or no expression of LMNA also had short telomeres. In three out of the four samples, telomere length was within the normal expected range for the age of patient (Fig. 5). This implies that the expression of progerin is in general required for the generation of short telomeres. This puts into question previous results that found problems in hematopoietic cells from HGPS patients (De SandreGiovannoli et al., 2003). The lack of obvious telomere shortening in cells that do not normally express lamin A argues that the effect on telomere length in cells is direct rather than indirect. However one sample, HGPS 3, had very low telomere length in all subsets of hematopoietic cells (Fig. 5). This difference could be due to an indirect effect of the lamin A mutation (e.g. loss of cells supporting the hematopoietic stem cells) or due to normal variation in telomere length in the population. Because parental blood samples were not available for analysis the possibility that this particular patient inherited short telomeres cannot be excluded. Telomeres have been previously shown to attach to the nuclear matrix (de Lange, 1992). As part of the nuclear architecture dysfunction seen in HGPS, telomere structure and function may be
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Fig. 3. The pattern of telomere length in HGPS cell lines does not always follow that observed in normal cell lines. Average telomere length of individual chromosomes in three normal fibroblast samples (control 1, control 2, and control 3) and three HGPS fibroblast samples (AG03513, AG06297 and AG11498) measured by Q-FISH. Highlighted are chromosomes 17p (red), 4q (blue), 18p and 18q (green) and 19p and 19q (orange).
impaired. The lamina is important as a scaffold for multi-protein and chromatin containing complexes (Zastrow et al., 2004). Recently it has been shown that chromatin is divided into discrete domains of high or low lamin B binding (Guelen et al., 2008). The lamina-associated domains had specific characteristics including low gene density, low gene expression and enrichment of repressive chromatin marks. These complexes could have a role in the maintenance, repair and replication of telomeric DNA which could be disrupted in HGPS. This could lead to problems including an increase in replication errors, failure to repair, and accessibility of enzymes. This in turn could cause the generation of short telomeres in HGPS fibroblasts and symptoms of the disease in the patients. In a recent paper, Huang et al. (2008), it was demonstrated that over-expression of both progerin or wt lamin A caused a decrease in telomere length. This suggests a very active role for lamin A in the maintenance of telomere length. As this was an over-expression study, it is difficult to say if this observation is just merely due to cellular changes due to too much lamin A in general rather than a specific effect. Research by Han et al. (2008), showed a role for the tumour-suppressor ING1 in HGPS (Han et al., 2008). With its role in both apoptosis and chromatic structure it is possible that this also has a secondary effect leading to the short telomere phenotype observed (Han et al., 2008). Lamin A might have many effects on cell turn over as well as roles in adult stem cells through chromatin remodeling (Han et al., 2008), nuclear organization (Willis et al., 2008) and other factors.
Fig. 4. LMNA is not expressed in granulocytes (G), or T cells (T) but is expressed in the fibroblast cell line (BJ). b-actin was used as an internal control for the amount of cDNA in each PCR reaction.
Many mechanisms have been proposed to explain the cellular defects in HGPS. The simplest mechanism for decreased telomere length in HGPS would be a larger amount of telomeric DNA lost with each cell division (e.g. due to increased exonuclease processing). This was not observed by Allsopp et al. (1992) when telomere length was measured by TRF analysis however, it is possible that the sensitivity of this technique was insufficient to detect a small increase in telomere loss with each replication round. It would also not account for the variation observed. Similar to a previously proposed mechanism, an increase in apoptosis and/ or senescence in HGPS cells could cause a compensatory increase in cell division in remaining cells (Halaschek-Wiener and BrooksWilson, 2007). This likely has a partial role in the short telomere phenotype; however it does not explain the observed variability in telomere length, especially the ultra short telomeres (Table 1). A third mechanism is failure to repair DNA damage at telomeres, causing losses of variable amounts of DNA. This would explain both the high variability in telomere length between chromosomes and the increased frequency signal free ends (Table 1 and Fig. 1). Unrepaired DNA damage could also prevent the cells from completing cell division leading to apoptosis or senescence. The generation of short telomere ends in HGPS could be caused by a combination of direct effects on telomeric DNA as well as indirect effects related to increased cell turnover. Telomeres may have a critical role in the persistent DNA damage and early senescence in HGPS fibroblasts cells observed in culture (Jiang et al., 2008). It has been shown before that there is a deficiency in the DNA damage response in HGPS cells (Liu et al., 2005, 2006). We have also observed that there are many very small persistent 53BP1 foci in HGPS cells (unpublished data). These persistent foci could be short telomeres that are being recognized as sites of DNA damage which cannot be repaired since there is no telomerase expressed in fibroblasts. We suggest this DNA damage would signal cells to become senescent or undergo apoptosis thereby reducing the number of dividing cells and causing the remaining cells to have to divide more in order to make up the deficit. Cellular senescence in tissues leads to changes in gene
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Fig. 5. Telomere length in hematopoietic cells according to age in patients with HGPS and normal donors. Telomere length is normal in three of the HGPS patient samples measured. The vertical axis represents telomere length in kilobases. Lines in the figures indicate the 1st, 10th, 50th, 90th, and 99th percentiles determined from 400 normal control subjects. Symbols represent subjects: four patients with HGPS (coloured solid square), and 18 normal donors (yellow solid circles). (A) Lymphocytes and (B) granulocytes.
expression in the affected cells and has been demonstrated to alter the microenvironment around the cell (Campisi and d’Adda di Fagagna, 2007). Short telomeres have also previously been correlated with an increased risk of heart disease (Fuster and Andres, 2006). Senescence due to telomere length could contribute to, or exacerbate, the pathological changes in arteries that lead to heart attacks, strokes and eventually death in the patients. Human aging is a complex process with many different contributing factors that are not understood yet. The premature aging syndromes may give an insight into specific aspects of the aging process. Better appreciation of how this nuclear scaffolding protein can cause so many age-related symptoms will increase the understanding of aging in general could provide novel insight for treatment of HGPS and other age-related diseases as well as normal human aging. Acknowledgements We thank Drs. Wendy Introne and Melissa Merideth (National Human Genome Research Institute, Bethesda, Maryland) for providing blood samples of HGPS patients, Michael Schertzer for excellent technical assistance and Dr. Ester Falconer for through editing of the manuscript. MLD was supported by studentships from the Michael Smith Foundation for Health Research and the Canadian Institutes for Health Research. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.mad.2009.03.001. References Allsopp, R.C., Vaziri, H., Patterson, C., Goldstein, S., Younglai, E.V., Futcher, A.B., Greider, C.W., Harley, C.B., 1992. Telomere length predicts replicative capacity of human fibroblasts. Proc. Natl. Acad. Sci. U.S.A. 89, 10114–10118. Aubert, G., Lansdorp, P.M., 2008. Telomeres and aging. Physiol. Rev. 88, 557–579. Baerlocher, G.M., Vulto, I., de Jong, G., Lansdorp, P.M., 2006. Flow cytometry and FISH to measure the average length of telomeres (flow FISH). Nat. Protoc. 1, 2365–2376. Baker, P.B., Baba, N., Boesel, C.P., 1981. Cardiovascular abnormalities in progeria. Case report and review of the literature. Arch. Pathol. Lab. Med. 105, 384–386. Blackburn, E.H., 2001. Switching and signaling at the telomere. Cell 106, 661–673. Britt-Compton, B., Rowson, J., Locke, M., Mackenzie, I., Kipling, D., Baird, D.M., 2006. Structural stability and chromosome-specific telomere length is governed by cis-acting determinants in humans. Hum. Mol. Genet. 15, 725–733. Broers, J.L., Machiels, B.M., Kuijpers, H.J., Smedts, F., van den Kieboom, R., Raymond, Y., Ramaekers, F.C., 1997. A- and B-type lamins are differentially expressed in normal human tissues. Histochem. Cell Biol. 107, 505–517. Campisi, J., d’Adda di Fagagna, F., 2007. Cellular senescence: when bad things happen to good cells. Nat. Rev. Mol. Cell Biol. 8, 729–740. Crabbe, L., Jauch, A., Naeger, C.M., Holtgreve-Grez, H., Karlseder, J., 2007. Telomere dysfunction as a cause of genomic instability in Werner syndrome. Proc. Natl. Acad. Sci. U.S.A. 104, 2205–2210.
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M.L. Decker et al. / Mechanisms of Ageing and Development 130 (2009) 377–383 Merideth, M.A., Gordon, L.B., Clauss, S., Sachdev, V., Smith, A.C.M., Perry, M.B., Brewer, C.C., Zalewski, C., Kim, H.J., Solomon, B., Brooks, B.P., Gerber, L.H., Turner, M.L., Domingo, D.L., Hart, T.C., Graf, J., Reynolds, J.C., Gropman, A., Yanovski, J.A., Gerhard-Herman, M., Collins, M.D.F.S., Nabel, E.G., Cannon III, R.O., Gahl, W.A., Introne, W.J., 2008. Phenotype and course of Hutchinson-Gilford Progeria Syndrome. N. Engl. J. Med. 358, 592–604. Pollex, R.L., Hegele, R.A., 2004. Hutchinson-Gilford progeria syndrome. Clin. Genet. 66, 375–381. Poon, S.S., Lansdorp, P.M., 2001. Quantitative fluorescence in situ hybridization (QFISH). Curr. Protoc. Cell Biol. Chapter 18, Unit 18 4.
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Mechanisms of Ageing and Development journal homepage: www.elsevier.com/locate/mechagedev
Telomere length in Hutchinson-Gilford Progeria Syndrome Michelle L. Decker a,b, Elizabeth Chavez a, Irma Vulto a, Peter M. Lansdorp a,c,* a
Terry Fox Laboratory, BC Cancer Agency, Vancouver, BC, Canada Department of Medical Genetics, University of British Columbia, Vancouver, BC, Canada c Division of Hematology, Department of Medicine, University of British Columbia, Vancouver, BC, Canada b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 October 2008 Received in revised form 14 February 2009 Accepted 6 March 2009 Available online 20 March 2009
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. Crown Copyright ß 2009 Published by Elsevier Ireland Ltd. All rights reserved.
Keywords: Telomere length Hutchinson-Gilford Progeria Syndrome Aging Nuclear lamina
1. Introduction Hutchinson-Gilford Progeria Syndrome (HPGS) is a segmental premature aging disease which manifests in the first 2 years of life (Pollex and Hegele, 2004). Symptoms include postnatal growth restriction, loss of hair and subcutaneous fat, decreased joint mobility, and atherosclerosis (Hennekam, 2006). Patients die at a mean age of 12.6 years from progressive atherosclerosis of the coronary and cerebrovascular arteries leading to heart attacks and strokes (Baker et al., 1981; Hennekam, 2006). Cognitive development is normal and no increase in cancer incidence has been observed (Hennekam, 2006). In about 90% of cases, HGPS is caused by a C ! T mutation at nucleotide 1824 in exon 11 of the lamin A/C gene (LMNA) which activates a cryptic splice site in the mRNA (De Sandre-Giovannoli et al., 2003; Eriksson et al., 2003). This results in the translation of a protein with a 50 amino acid deletion near the C-terminus (Eriksson et al., 2003). The deletion includes a cleavage recognition site that is required for complete processing to the mature form of lamin A (Eriksson et al., 2003). The two classes of nuclear lamin proteins, A-type and B-type, are involved in many important nuclear functions including DNA replication, transcription, chromatin organization, nuclear shape
* Corresponding author at: Terry Fox Laboratory, BC Cancer Research Centre, 675 West 10th Avenue, Vancouver, BC, Canada V5Z 1L3. Tel.: +1 604 675 8135. E-mail address: [email protected] (P.M. Lansdorp).
and nuclear position in the cell (Zastrow et al., 2004). The two predominant A-type lamins, A and C, are expressed by alternative splicing of the LMNA mRNA (Zastrow et al., 2004). Lamin A and C are expressed in all differentiated tissues except for some hematopoietic lineages including CD20-positive B lymphocytes, CD3-positive T lymphocytes as well as neuroendocrine cells (Broers et al., 1997; Jansen et al., 1997). Lamin A interacts with many components of the nucleus including other lamins, lamin associated protein 2a (LAP2a), and actin as well as DNA and histones and potentially serves as a scaffolding network for multiprotein complexes (Zastrow et al., 2004). HGPS resembles aging at the cellular level as well. The mutant protein, often referred to as ‘progerin’, remains farnesylated at the C-terminus due to the lack of protein processing (Eriksson et al., 2003). This likely causes most of the cellular defects including lobulation of the nuclear membrane, diminished replication potential, slow DNA damage response and abnormal chromatin organization (Goldman et al., 2004; Liu et al., 2005). It has also observed that telomere length is shorter in HGPS fibroblasts when compared to age-matched controls (Allsopp et al., 1992). Telomeres are repetitive G-rich DNA sequences and associated binding proteins found at the ends of linear eukaryotic chromosomes. They are key in preventing genomic instability (Blackburn, 2001; de Lange, 2005). The telomere binding proteins aid in forming a protective structure which ‘caps’ chromosome ends and prevent their processing as double strand breaks (de Lange, 2005). Telomeres shorten with each cell division, in vivo and in vitro, due
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to the ‘end replication problem’ as well as sporadic losses following damage or replication errors (Lansdorp, 2005). Consequently, telomeres from elderly donors are much shorter that those from young donors. When telomeres reach a critical length, cells either stop dividing (senesce) or undergo apoptosis (Stewart and Weinberg, 2006). Limitations in the replicative potential of cells imposed by telomere shortening may restrict the proliferation of abnormal cells; however, progressive telomere attrition also contributes to the loss of cells and tissue function with age (Aubert and Lansdorp, 2008). In order to better understand the role of telomeres in HGPS, we examined the telomere length in cells from HGPS patients using two approaches. First, to determine how telomere lengths of individual chromosomes vary in fibroblasts of HGPS patients, we quantified telomere lengths using quantitative fluorescence in situ hybridization (Q-FISH) (Poon and Lansdorp, 2001). We show that telomere length is significantly shorter in HGPS cells; however no particular chromosome had consistently short or long telomeres. Second, to determine a causative role for lamin A in telomere shortening in HGPS patients, we used flow-FISH (Baerlocher et al., 2006) to examine telomere length in hematopoietic cells, which do not express lamin A (Baerlocher et al., 2006; Broers et al., 1997). We show that telomere length is in the normal range for three out of four HGPS patients examined. These results suggest that mutant lamin A is directly involved in the generation of short telomeres. 2. Materials and methods 2.1. Cell lines and patient samples The HGPS cell lines AG03513, AG06297 and AG11498 were obtained from the NIA Aging Cell Repository (Coriell Cell Repository, Camden, NJ). Cells were cultured in Dulbecco’s modified eagle medium (DMEM) containing 15% fetal calf serum, 200 mM glutamine, 100 U/ml penicillin and 100 mg/ml streptomycin at 37 8C in a 5% CO2 atmosphere incubator. At the time experiments were preformed, the HGPS cell lines were at the following population doublings: AG03513 – PD 17, AG06297 – PD 35 and AG11498 – PD 7. The six healthy control fibroblast cell lines that were used in the telomere length studies have been previously reported in (Martens et al., 1998). Blood samples were drawn from patients diagnosed with classical HGPS after informed consent from patients and their parents (Merideth et al., 2008). Blood was drawn in Heparin or EDTA tubes and shipped at room temperature. Upon arrival blood samples were frozen until analysis. Controls for Flow-FISH were 400 healthy persons ranging from birth to 100 years of ages (Yamaguchi et al., 2005). 2.2. Quantitative fluorescence in situ hybridization Q-FISH was preformed as previously described (Poon and Lansdorp, 2001). Briefly, metaphase cells were harvested, fixed with methanol–acetic acid then dropped onto slides. Slides were fixed with formaldehyde, treated with pepsin, and dehydrated with ethanol. The hybridization mix containing the Cy3-labeled (CCCTAA)3 peptide nucleic acid telomere probe was added to each slide which was then denatured at 80 8C for 2 min prior to incubation at room temperature for 1 h. Slides were washed, counterstained with DAPI then mounted using DABCO. Images were acquired and analyzed as described. 2.3. RNA extraction and RT-PCR RNA was extracted from BJ neonatal human foreskin fibroblasts, and T cells and granulocytes from control samples using the RNAeasy kit (Qiagen). A reversetranscriptase-polymerase-chain-reaction (RT-PCR) assay was performed with the isolated RNA to make complementary DNA (cDNA). The cDNA was amplified with either primers for LMNA (forward: CAAGGCATCTGCCAGCGG and reverse: TTTCTTTGGCTTCAAGCCCC) or b-actin (forward: AGAGATGGCCACGGCTGCTTC and reverse: GCATTTGCGGTGGACGATGGAG). PCR products were visualized on a 1.5% agarose gel by ethidium bromide staining. 2.4. Flow-FISH Details of the Flow-FISH method are as previously described (Baerlocher et al., 2006). Briefly, red blood cells from samples were lysed with NH4Cl. Leukocytes were denatured in formamide at 87 8C, hybridized with a fluorescein-conjugated (CCCTAA)3 peptide nucleic acid probe, and counterstained with LDS751 DNA dye. Analysis of fluorescence was performed on a FACSCalibur flow cytometer (BD Biosciences, San Jose, CA). The cell types analyzed included total leukocytes,
granulocytes, total lymphocytes, CD45RA-positive/CD20-negative naive T cells, CD45RA-negative memory T cells, CD20-positive B cells, and CD57-positive NK/NKT cells. Bovine thymocytes were used as an internal standard in every sample. Cellquest Pro software (BD Biosciences, CA, USA) was used to quantify the flow cytometry results; median telomere lengths were calculated using an automated Microsoft Excel calculator. 2.5. Statistical analysis HGPS samples were compared to control samples using an independent t-test, assuming unequal variance. A p-value of <0.005 was considered significant.
3. Results 3.1. Telomere length in HGPS fibroblasts To investigate the nature of telomere shortening in HGPS, QFISH was performed on metaphase chromosome spreads of three HGPS primary fibroblast cell lines derived from biopsies of three different patients (Fig. 1). The fibroblast cell lines from HGPS patients were grown using standard culture conditions. Cells from early passage cultures were arrested in metaphase and used for telomere length analysis. At least twelve metaphases were analyzed for each cell line. No significant chromosomal abnormalities such as fusions, translocations or aneuploidy were detected in the metaphases from HGPS patients (data not shown). The samples AG11498 and AG06297 have the common 1824 C ! T mutation; however, the mutation status of AG03513 is unknown although the patient had classical HGPS symptoms. Telomere length in AG06297 and AG03513 were significantly shorter than the control samples and AG11498 was around the same length as the control samples and is short for the age of the patient based on data from Allsopp et al., 1992 (Table 1) (Martens et al., 2000). While AG11498 was not shorter than control cells it should be noted that the average age of the control cell lines was 50 years which likely accounts for this. Telomere lengths were variable between different chromosome ends as well as between samples (Figs. 2 and 3). Signal free ends, representing telomeres that are too short to detect by Q-FISH, were observed in the HGPS samples but not in the normal fibroblast metaphases (Table 1) (Fig. 1, white asterisks). We found no evidence for consistent biased shortening of any particular chromosome end between samples, although all samples exhibited some extremely short chromosomes including chromosome 18q in AG06297 (Fig. 2 and Supplementary Figs. 1 and 2). The telomeres of chromosome 17p, which have been previously reported to be among the shortest (Britt-Compton et al., 2006; Martens et al., 1998), were below the mean telomere length in all HGPS samples however in AG11498 it was not statistically different from the mean telomere length for that sample (Fig. 3). In addition, the telomeres of chromosome 4q, often one of the longest telomeres and always above the mean in normal individuals (Martens et al., 1998), was only significantly different than the mean telomere length in AG03513. In the control fibroblast samples, the average telomere length of chromosome 18, which is gene poor, were above the total mean telomere length. The average telomere length of chromosome 19, which is gene-rich, was always found to be below the mean telomere length in the control samples. In the HGPS samples, the telomere length of chromosomes 18 and 19 was more similar to each other than they were in control samples. In some cases, such as chromosome 18q in sample AG06297, chromosomes were found to be on the opposite side of the mean from usual (Figs. 2 and 3). In summary, chromosome-specific differences in telomere length observed in cells from normal control individuals were not preserved in all HGPS cell lines. This suggests a much more random chromatin arrangement in the HGPS cells.
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Fig. 1. Quantitative-FISH analysis of individual telomeres in fibroblasts from 3 different patients with HGPS. The PNA probe specific for telomeric DNA is in shown in yellow while DAPI staining for DNA is shown in blue. Telomere ends with undetectable telomere signals are indicated using a white asterisk. (A) AG03513, (B) AG06297 and (C) AG11498.
3.2. Telomere length in HGPS hematopoietic cells To investigate if the expression of mutant lamin A is directly involved in the generation of short telomeres, we examined telomere lengths in hematopoietic cells, which reportedly either do not express LMNA or express it at much lower levels (Broers et al., 1997; Jansen et al., 1997). To confirm this, RT-PCR was performed on granulocyte and T cell mRNA samples and a fibroblast cell line (BJ) as a control (Fig. 4). The only sample that expressed LMNA at detectable levels was the control fibroblast cell line sample while the granulocyte and T cell samples had no band confirming the lack of LMNA expression in these hematopoietic cells (Fig. 4). Blood samples were obtained from four HGPS patients, all of whom have the common C ! T mutation at nucleotide 1824 and presented clinically with classical HGPS symptoms (personal communications, Dr. William A. Gahl). The median telomere
lengths of subsets of hematopoietic cells, including T cells, B cells, NK cells, and granulocytes were measured using flow-FISH. The average telomere length in all subsets of hematopoietic cells was consistent with the expected length for age-matched controls in three out of the four patient samples examined (Fig. 5). One sample, HGPS 3, had very low telomere length in all subsets of cells. This patient did not exhibit a more severe form of the disease compared to the other patients at this stage. The normal telomere lengths in three out of four patients suggest that the expression of progerin leads to the short telomeres observed in the fibroblast samples. 4. Discussion HGPS is a segmental aging disorder which resembles aging in specific tissues both physiologically and at the cellular level. The phenotype of short telomeres has been seen in other premature
Table 1 Mean telomere length and frequency of signal free ends in HGPS fibroblasts measured using Q-FISH. Sample Controls AG03513 AG06297 AG11498 a
Age (years) 49.7 13 8 13
a
p arm (kb SEM)
q arm (kb SEM)
Total (kb SEM)
Signal free ends
% signal free ends
– 3.18 0.05 3.60 0.04 6.57 0.06
– 3.46 0.05 3.40 0.04 6.34 0.06
6.17 0.03 3.32 0.05** 3.48 0.04** 6.45 0.06*
0 43 88 3
0 1.83 2.84 0.13
The control is the average of 6 normal fibroblast cell lines. p and q arm data values were not collected separately for the normal controls. p < 0.005 of mean telomere length between controls and HGPS samples calculated using an independent t-test assuming unequal variance. ** p < 0.0001 of mean telomere length between controls and HGPS samples calculated using an independent t-test assuming unequal variance. *
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Fig. 2. Telomere length is short in HGPS cell lines and highly variable between chromosomes. Box plot histograms of telomere fluorescence values in the HGPS fibroblast cell line AG06297. Telomere lengths of chromosomes from HGPS cells are highly variable. The horizontal line in the large box represents the 50th percentile; the upper and lower margins of the box are the 25th and 75th percentile; the small box inside represents the mean; the vertical lines above and below the box are the 5th and 95th percentile respectively; the ‘’ represents the 1st and 99th percentile; the ‘–’ represents the minimum and maximum values. (A) The p arm and (B) the q arm of the indicated chromosomes.
aging diseases including Werner Syndrome where telomere dysfunction has been proposed to cause genomic instability (Crabbe et al., 2007). Our results from telomere length analysis of fibroblasts from HGPS patients are consistent with the results described by Allsopp et al. (1992) (Table 1). By combining Q-FISH with karyotyping, the telomere length of individual chromosome ends was studied in the hope to gain insight into the mechanism of telomere shortening in HGPS. The length of individual chromosomes from HGPS patients did not always follow the established patterns of the control samples (Figs. 2 and 3 and Supplementary Figs. 1 and 2). On average, chromosome 17p has been reported to have the shortest telomeres, and in the HGPS samples it was also below the mean telomere length but not the absolute shortest (Britt-Compton et al., 2006; Martens et al., 1998). Chromosomes 18 and 19, which are relatively gene-poor and gene-rich respectively, have defined nuclear territories (Croft et al., 1999; Meaburn et al., 2007). Chromosome 18, typically located at the periphery of the nucleus in proliferating cells, is more internally localized in HGPS cells (Croft et al., 1999). Loss of peripheral heterochromatin may lead to this change in nuclear position and possibly alter the chromatin organization which could affect telomeres structure. In the HGPS fibroblasts, the telomere length in chromosomes 18 and 19 generally followed the pattern of the wild type cells (Fig. 3). However, the telomere lengths of these two chromosomes are
more similar. This taken with the chromosome 17p differences suggests that there may be differences in telomeric chromatin in HGPS. The median telomere length was measured in hematopoietic cells by flow FISH to determine if cells with reduced or no expression of LMNA also had short telomeres. In three out of the four samples, telomere length was within the normal expected range for the age of patient (Fig. 5). This implies that the expression of progerin is in general required for the generation of short telomeres. This puts into question previous results that found problems in hematopoietic cells from HGPS patients (De SandreGiovannoli et al., 2003). The lack of obvious telomere shortening in cells that do not normally express lamin A argues that the effect on telomere length in cells is direct rather than indirect. However one sample, HGPS 3, had very low telomere length in all subsets of hematopoietic cells (Fig. 5). This difference could be due to an indirect effect of the lamin A mutation (e.g. loss of cells supporting the hematopoietic stem cells) or due to normal variation in telomere length in the population. Because parental blood samples were not available for analysis the possibility that this particular patient inherited short telomeres cannot be excluded. Telomeres have been previously shown to attach to the nuclear matrix (de Lange, 1992). As part of the nuclear architecture dysfunction seen in HGPS, telomere structure and function may be
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Fig. 3. The pattern of telomere length in HGPS cell lines does not always follow that observed in normal cell lines. Average telomere length of individual chromosomes in three normal fibroblast samples (control 1, control 2, and control 3) and three HGPS fibroblast samples (AG03513, AG06297 and AG11498) measured by Q-FISH. Highlighted are chromosomes 17p (red), 4q (blue), 18p and 18q (green) and 19p and 19q (orange).
impaired. The lamina is important as a scaffold for multi-protein and chromatin containing complexes (Zastrow et al., 2004). Recently it has been shown that chromatin is divided into discrete domains of high or low lamin B binding (Guelen et al., 2008). The lamina-associated domains had specific characteristics including low gene density, low gene expression and enrichment of repressive chromatin marks. These complexes could have a role in the maintenance, repair and replication of telomeric DNA which could be disrupted in HGPS. This could lead to problems including an increase in replication errors, failure to repair, and accessibility of enzymes. This in turn could cause the generation of short telomeres in HGPS fibroblasts and symptoms of the disease in the patients. In a recent paper, Huang et al. (2008), it was demonstrated that over-expression of both progerin or wt lamin A caused a decrease in telomere length. This suggests a very active role for lamin A in the maintenance of telomere length. As this was an over-expression study, it is difficult to say if this observation is just merely due to cellular changes due to too much lamin A in general rather than a specific effect. Research by Han et al. (2008), showed a role for the tumour-suppressor ING1 in HGPS (Han et al., 2008). With its role in both apoptosis and chromatic structure it is possible that this also has a secondary effect leading to the short telomere phenotype observed (Han et al., 2008). Lamin A might have many effects on cell turn over as well as roles in adult stem cells through chromatin remodeling (Han et al., 2008), nuclear organization (Willis et al., 2008) and other factors.
Fig. 4. LMNA is not expressed in granulocytes (G), or T cells (T) but is expressed in the fibroblast cell line (BJ). b-actin was used as an internal control for the amount of cDNA in each PCR reaction.
Many mechanisms have been proposed to explain the cellular defects in HGPS. The simplest mechanism for decreased telomere length in HGPS would be a larger amount of telomeric DNA lost with each cell division (e.g. due to increased exonuclease processing). This was not observed by Allsopp et al. (1992) when telomere length was measured by TRF analysis however, it is possible that the sensitivity of this technique was insufficient to detect a small increase in telomere loss with each replication round. It would also not account for the variation observed. Similar to a previously proposed mechanism, an increase in apoptosis and/ or senescence in HGPS cells could cause a compensatory increase in cell division in remaining cells (Halaschek-Wiener and BrooksWilson, 2007). This likely has a partial role in the short telomere phenotype; however it does not explain the observed variability in telomere length, especially the ultra short telomeres (Table 1). A third mechanism is failure to repair DNA damage at telomeres, causing losses of variable amounts of DNA. This would explain both the high variability in telomere length between chromosomes and the increased frequency signal free ends (Table 1 and Fig. 1). Unrepaired DNA damage could also prevent the cells from completing cell division leading to apoptosis or senescence. The generation of short telomere ends in HGPS could be caused by a combination of direct effects on telomeric DNA as well as indirect effects related to increased cell turnover. Telomeres may have a critical role in the persistent DNA damage and early senescence in HGPS fibroblasts cells observed in culture (Jiang et al., 2008). It has been shown before that there is a deficiency in the DNA damage response in HGPS cells (Liu et al., 2005, 2006). We have also observed that there are many very small persistent 53BP1 foci in HGPS cells (unpublished data). These persistent foci could be short telomeres that are being recognized as sites of DNA damage which cannot be repaired since there is no telomerase expressed in fibroblasts. We suggest this DNA damage would signal cells to become senescent or undergo apoptosis thereby reducing the number of dividing cells and causing the remaining cells to have to divide more in order to make up the deficit. Cellular senescence in tissues leads to changes in gene
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Fig. 5. Telomere length in hematopoietic cells according to age in patients with HGPS and normal donors. Telomere length is normal in three of the HGPS patient samples measured. The vertical axis represents telomere length in kilobases. Lines in the figures indicate the 1st, 10th, 50th, 90th, and 99th percentiles determined from 400 normal control subjects. Symbols represent subjects: four patients with HGPS (coloured solid square), and 18 normal donors (yellow solid circles). (A) Lymphocytes and (B) granulocytes.
expression in the affected cells and has been demonstrated to alter the microenvironment around the cell (Campisi and d’Adda di Fagagna, 2007). Short telomeres have also previously been correlated with an increased risk of heart disease (Fuster and Andres, 2006). Senescence due to telomere length could contribute to, or exacerbate, the pathological changes in arteries that lead to heart attacks, strokes and eventually death in the patients. Human aging is a complex process with many different contributing factors that are not understood yet. The premature aging syndromes may give an insight into specific aspects of the aging process. Better appreciation of how this nuclear scaffolding protein can cause so many age-related symptoms will increase the understanding of aging in general could provide novel insight for treatment of HGPS and other age-related diseases as well as normal human aging. Acknowledgements We thank Drs. Wendy Introne and Melissa Merideth (National Human Genome Research Institute, Bethesda, Maryland) for providing blood samples of HGPS patients, Michael Schertzer for excellent technical assistance and Dr. Ester Falconer for through editing of the manuscript. MLD was supported by studentships from the Michael Smith Foundation for Health Research and the Canadian Institutes for Health Research. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.mad.2009.03.001. References Allsopp, R.C., Vaziri, H., Patterson, C., Goldstein, S., Younglai, E.V., Futcher, A.B., Greider, C.W., Harley, C.B., 1992. Telomere length predicts replicative capacity of human fibroblasts. Proc. Natl. Acad. Sci. U.S.A. 89, 10114–10118. Aubert, G., Lansdorp, P.M., 2008. Telomeres and aging. Physiol. Rev. 88, 557–579. Baerlocher, G.M., Vulto, I., de Jong, G., Lansdorp, P.M., 2006. Flow cytometry and FISH to measure the average length of telomeres (flow FISH). Nat. Protoc. 1, 2365–2376. Baker, P.B., Baba, N., Boesel, C.P., 1981. Cardiovascular abnormalities in progeria. Case report and review of the literature. Arch. Pathol. Lab. Med. 105, 384–386. Blackburn, E.H., 2001. Switching and signaling at the telomere. Cell 106, 661–673. Britt-Compton, B., Rowson, J., Locke, M., Mackenzie, I., Kipling, D., Baird, D.M., 2006. Structural stability and chromosome-specific telomere length is governed by cis-acting determinants in humans. Hum. Mol. Genet. 15, 725–733. Broers, J.L., Machiels, B.M., Kuijpers, H.J., Smedts, F., van den Kieboom, R., Raymond, Y., Ramaekers, F.C., 1997. A- and B-type lamins are differentially expressed in normal human tissues. Histochem. Cell Biol. 107, 505–517. Campisi, J., d’Adda di Fagagna, F., 2007. Cellular senescence: when bad things happen to good cells. Nat. Rev. Mol. Cell Biol. 8, 729–740. Crabbe, L., Jauch, A., Naeger, C.M., Holtgreve-Grez, H., Karlseder, J., 2007. Telomere dysfunction as a cause of genomic instability in Werner syndrome. Proc. Natl. Acad. Sci. U.S.A. 104, 2205–2210.
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