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1.  SLX4 Assembles a Telomere Maintenance Toolkit by Bridging Multiple Endonucleases with Telomeres 
Cell reports  2013;4(5):861-869.
SLX4 interacts with several endonucleases to resolve structural barriers in DNA metabolism. SLX4 also interacts with telomeric protein TRF2 in human cells. The molecular mechanism of these interactions at telomeres remains unknown. Here, we report the crystal structure of the TRF2-binding motif of SLX4 (SLX4TBM) in complex with the TRFH domain of TRF2 (TRF2TRFH) and map the interactions of SLX4 with endonucleases SLX1, XPF, and MUS81. TRF2 recognizes a unique HxLxP motif on SLX4 via the peptide-binding site in its TRFH domain. Telomeric localization of SLX4 and associated nucleases depend on the SLX4-endonuclease and SLX4-TRF2 interactions and the protein levels of SLX4 and TRF2. SLX4 assembles an endonuclease toolkit that negatively regulates telomere length via SLX1-catalyzed nucleolytic resolution of telomere DNA structures. We propose that the SLX4-TRF2 complex serves as a double-layer scaffold bridging multiple endonucleases with telomeres for recombination-based telomere maintenance.
PMCID: PMC4334113  PMID: 24012755
2.  Pot1a Prevents Telomere Dysfunction and ATM-Dependent Neuronal Loss 
The Journal of Neuroscience  2014;34(23):7836-7844.
Genome stability is essential for neural development and the prevention of neurological disease. Here we determined how DNA damage signaling from dysfunctional telomeres affects neurogenesis. We found that telomere uncapping by Pot1a inactivation resulted in an Atm-dependent loss of cerebellar interneurons and granule neuron precursors in the mouse nervous system. The activation of Atm by Pot1a loss occurred in an Atr-dependent manner, revealing an Atr to Atm signaling axis in the nervous system after telomere dysfunction. In contrast to telomere lesions, Brca2 inactivation in neural progenitors also led to ablation of cerebellar interneurons, but this did not require Atm. These data reveal that neural cell loss after DNA damage selectively engages Atm signaling, highlighting how specific DNA lesions can dictate neuropathology arising in human neurodegenerative syndromes.
PMCID: PMC4044246  PMID: 24899707
ATM; cerebellum; DNA damage; neural development; telomeres
3.  The mINO80 chromatin remodeling complex is required for efficient telomere replication and maintenance of genome stability 
Cell Research  2013;23(12):1396-1413.
The INO80 (inositol requiring mutant 80) chromatin remodeling complex plays important roles in transcriptional regulation and DNA replication and repair, and consists of several functional protein subunits, including the critical Ino80 ATPase catalytic subunit. While the function of INO80 has been studied in yeast and mammalian cell lines, we do not know how mIno80 contributes to the maintenance of genome stability to prevent cancer development in mice. Here, we use a conditional knockout approach to explore the cellular and organismal functions of mIno80. Deletion of mIno80 results in profound cellular proliferative defects and activation of p21-dependent cellular senescence. While mIno80 is required for efficient repair of DNA double strand breaks, its depletion did not impact upon the formation of γ-H2AX and 53BP1 DNA damage foci, or the activation of the ATM-CHK2-dependent DNA damage response. mIno80 deletion inhibited the generation of single-strand DNA, resulting in defects in homology-directed DNA repair (HDR) at telomeres. Fragile telomeres were prominent in mIno80Δ/Δ MEFs, suggesting that chromatin remodeling is required for efficient telomere replication. mIno80−/− mouse embryos die early during embryogenesis, while conditional deletion of mIno80 in adult mice results in weight loss and premature death. In a p53−/− tumor-prone background, mIno80 haploinsufficiency favored the development of sarcomas. Our studies suggest that the mIno80 chromatin remodeling complex plays important roles in telomere replication, HDR-mediated repair of dysfunctional telomeres, and maintenance of genome stability.
PMCID: PMC3847565  PMID: 23979016
DNA damage; telomere; cancer; genome instability; chromatin; replication
4.  Functional characterization of human CTC1 mutations reveals novel mechanisms responsible for the pathogenesis of the telomere disease Coats plus 
Aging cell  2013;12(6):1100-1109.
Coats plus is a rare recessive disorder characterized by intracranial calcifications, hematological abnormalities, and retinal vascular defects. This disease results from mutations in CTC1, a member of the CTC1–STN1–TEN1 (CST) complex critical for telomere replication. Telomeres are specialized DNA/protein structures essential for the maintenance of genome stability. Several patients with Coats plus display critically shortened telomeres, suggesting that telomere dysfunction plays an important role in disease pathogenesis. These patients inherit CTC1 mutations in a compound heterozygous manner, with one allele encoding a frameshift mutant and the other a missense mutant. How these mutations impact upon telomere function is unknown. We report here the first biochemical characterization of human CTC1 mutations. We found that all CTC1 frameshift mutations generated truncated or unstable protein products, none of which were able to form a complex with STN1–TEN1 on telomeres, resulting in progressive telomere shortening and formation of fused chromosomes. Missense mutations are able to form the CST complex at telomeres, but their expression levels are often repressed by the frameshift mutants. Our results also demonstrate for the first time that CTC1 mutations promote telomere dysfunction by decreasing the stability of STN1 to reduce its ability to interact with DNA Polα, thus highlighting a previously unknown mechanism to induce telomere dysfunction.
PMCID: PMC4083614  PMID: 23869908
aging; mouse models; telomeres
5.  Association Between Combined Interleukin-6 and C-Reactive Protein Levels and Pulmonary Function in Older Women: Results from the Women’s Health and Aging Studies I and II 
To determine whether combined higher interleukin-6 (IL-6) and C-reactive protein (CRP) levels are associated with lower pulmonary function levels in older women, accounting for chronic inflammatory diseases, physical function, and other factors associated with inflammation.
Cross-sectional study using data from two prospective cohorts.
Baltimore, Maryland.
Eight hundred forty disabled and 332 higher-functioning community-dwelling women aged 65 and older from the Women’s Health and Aging Studies (WHAS) I and II, respectively.
IL-6 and CRP, combined according to their tertile concentrations, and pulmonary function measures, assessed according to forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC).
In WHAS I and II, similar dose-response trends were observed between combined higher IL-6 and CRP levels and lower pulmonary function levels. In WHAS I (disabled women), the combined highest IL-6 and CRP levels were associated with the lowest levels of FEV1 (mean 137.0 mL, 95% confidence interval (CI) = 128.4–145.7 mL) and FVC (mean 191.7 mL, 95% CI = 180.4–202.9 mL). Similarly, in WHAS II (higher-functioning women), the combined highest IL-6 and CRP levels were associated with the lowest levels of FEV1 (mean 158.3 mL, 95% CI = 146.3–170.4 mL) and FVC (mean 224.2 mL, 95% CI = 209.9–238.5 mL).
Combined elevations in IL-6 and CRP were associated with the lowest pulmonary function levels in older women. These findings suggest that high IL-6 and CRP levels may be an indication of prevalent impaired pulmonary function. Future studies should determine whether measurement of IL-6 and CRP could enhance current methods of monitoring respiratory diseases beyond that provided by pulmonary function measures.
PMCID: PMC4050638  PMID: 21226682
pulmonary function; inflammation; older women
6.  Cancer chromosomes going to POT1 
Nature genetics  2013;45(5):473-475.
Alterations in the single-stranded telomere–binding protein POT1 have recently been identified in chronic lymphocytic leukemia. This discovery provides novel insights into how genomic instability induced by dysfunctional telomeres contributes to tumorigenesis.
PMCID: PMC4040961  PMID: 23619786
7.  Single strand DNA binding proteins 1 and 2 protect newly replicated telomeres 
Cell Research  2013;23(5):705-719.
Human single-strand (ss) DNA binding proteins 1 and 2 (hSSB1 and 2) are components of the hSSB1/2-INTS3-C9orf80 heterotrimeric protein complex shown to participate in DNA damage response and maintenance of genome stability. However, their roles at telomeres remain unknown. Here, we generated murine SSB1 conditional knockout mice and cells and found that mSSB1 plays a critical role in telomere end protection. Both mSSB1 and mSSB2 localize to a subset of telomeres and are required to repair TRF2-deficient telomeres. Deletion of mSSB1 resulted in increased chromatid-type fusions involving both leading- and lagging-strand telomeric DNA, suggesting that it is required for the protection of G-overhangs. mSSB1's interaction with INTS3 is required for its localization to damaged DNA. mSSB1 interacts with Pot1a, but not Pot1b, and its association with telomeric ssDNA requires Pot1a. mSSB1Δ/Δ mice die at birth with developmental abnormalities, while mice with the hypomorphic mSSB1F/F allele are born alive and display increased sensitivity to ionizing radiation (IR). Our results suggest that mSSB1 is required to maintain genome stability, and document a previously unrecognized role for mSSB1/2 in the protection of newly replicated leading- and lagging-strand telomeres.
PMCID: PMC3641597  PMID: 23459151
telomere; DNA damage and repair; cancer; apoptosis
8.  Effect of Co-existing Chronic Obstructive Pulmonary Disease and Cognitive Impairment on Health Outcomes in Older Adults 
To determine the extent to which the co-occurrence of chronic obstructive pulmonary disease (COPD) and cognitive impairment affect adverse health outcomes in older adults.
Multi-center longitudinal cohort study.
California, Pennsylvania, Maryland, and North Carolina.
Three thousand ninety-three community-dwelling adults aged ≥65 from the Cardiovascular Health Study. Four hundred thirty-one participants had chronic obstructive pulmonary disease (COPD) at study baseline.
Follow-up began at the second CHS visit and continued for three years. Spirometric criteria for airflow limitation served to establish COPD, using the Lambda-Mu-Sigma method, which accounts for age-related changes in lung function. Cognitive impairment was evaluated by the modified Mini Mental State Exam and claims data. Outcomes were respiratory-related and all-cause hospitalizations and death.
Participants with co-existing COPD and cognitive impairment had the highest rates of respiratory-related (adjusted hazard ratio [HR]=4.10, 95%CI=1.86–9.05) and all-cause hospitalizations (HR= 1.34, 95%CI=1.00–1.80) and death (HR=2.29, 95%CI=1.18–4.45). In particular, individuals with both conditions had a 48% higher rate of all-cause hospitalizations (adjusted synergy index [SI]=1.48, 95%CI=0.19–11.31) and nearly a three-fold higher rate of death (SI=2.74, 95%CI=0.43–17.32) than the sum of risks for each respective outcome associated with having COPD or cognitive impairment alone. However, tests for interaction were not statistically significant for the presence of synergism between both conditions relative to any of the outcomes. Therefore, we cannot conclude that the combined effect of COPD and cognitive impairment is greater than additive.
Co-existing COPD and cognitive impairment have an additive effect on respiratory-related and all-cause hospitalizations and death. Optimizing outcomes in older adults with COPD and cognitive impairment will require that we determine how to improve concurrent management of both conditions.
PMCID: PMC3470752  PMID: 23035917
Chronic obstructive pulmonary disease; cognitive impairment; health outcomes; hospitalizations; disability; death
9.  p16INK4a protects against dysfunctional telomere–induced ATR-dependent DNA damage responses  
The Journal of Clinical Investigation  2013;123(10):4489-4501.
Dysfunctional telomeres limit cellular proliferative capacity by activating the p53-p21– and p16INK4a-Rb–dependent DNA damage responses (DDRs). The p16INK4a tumor suppressor accumulates in aging tissues, is a biomarker for cellular senescence, and limits stem cell function in vivo. While the activation of a p53-dependent DDR by dysfunctional telomeres has been well documented in human cells and mouse models, the role for p16INK4a in response to telomere dysfunction remains unclear. Here, we generated protection of telomeres 1b p16–/– mice (Pot1bΔ/Δ;p16–/–) to address the function of p16INK4a in the setting of telomere dysfunction in vivo. We found that deletion of p16INK4a accelerated organ impairment and observed functional defects in highly proliferative organs, including the hematopoietic system, small intestine, and testes. Pot1bΔ/Δ;p16–/– hematopoietic cells exhibited increased telomere loss, increased chromosomal fusions, and telomere replication defects. p16INK4a deletion enhanced the activation of the ATR-dependent DDR in Pot1bΔ/Δ hematopoietic cells, leading to p53 stabilization, increased p21-dependent cell cycle arrest, and elevated p53-dependent apoptosis. In contrast to p16INK4a, deletion of p21 did not activate ATR, rescued proliferative defects in Pot1bΔ/Δ hematopoietic cells, and significantly increased organismal lifespan. Our results provide experimental evidence that p16INK4a exerts protective functions in proliferative cells bearing dysfunctional telomeres.
PMCID: PMC3784543  PMID: 24091330
10.  Cytogenetic analysis of telomere dysfunction 
Dysfunctional telomeres arising either through natural attrition due to telomerase deficiency or by the removal of telomere binding proteins are recognized as double stranded breaks (DSBs). Repair of DSBs is crucial for the maintenance of genome stability. In mammals, DSBs are repaired by either error prone nonhomologous end joining (NHEJ) or error free homologous recombination (HR) and can be visualized as chromosomal fusions.
PMCID: PMC3725757  PMID: 21461818
11.  The telomeric protein SNM1B/Apollo is required for normal cell proliferation and embryonic development 
Aging cell  2010;9(6):1047-1056.
Conserved metallo β-Lactamase and β-CASP (CPSF-Artemis-Snm1-Pso2) domain nuclease family member SNM1B/Apollo is a shelterin-associated protein that localizes to telomeres through its interaction with TRF2. To study its in vivo role, we generated a knockout of SNM1B/Apollo in a mouse model. Snm1B/Apollo homozygous null mice die at birth with developmental delay and defects in multiple organ systems. Cell proliferation defects were observed in Snm1B/Apollo mutant mouse embryonic fibroblasts (MEFs) owing to high levels of telomeric end-to-end fusions. Deficiency of the nonhomologous end-joining (NHEJ) factor Ku70, but not p53, rescued the developmental defects and lethality observed in Snm1B/Apollo mutant mice as well as the impaired proliferation of Snm1B/Apollo-deficient MEFs. These findings demonstrate that SNM1B/Apollo is required to protect telomeres against NHEJ-mediated repair, which results in genomic instability and the consequent multi-organ developmental failure. Although Snm1B/Apollo-deficient MEFs exhibited high levels of apoptosis, abrogation of p53-dependent programmed cell death did not rescue the multi-organ developmental failure in the mice.
PMCID: PMC3719988  PMID: 20854421
chromosome instability; Ku; mouse model; nonhomologous end-joining; SNM1B/Apollo; telomeres
12.  Probing the Telomere Damage Response 
Telomere dysfunctions, rendered through replicative attrition of telomeric DNA or due to the inhibition of shelterin components, are recognized as DNA double stranded breaks (DSBs) by the DNA damage repair (DDR) pathway. This leads to the activation of DNA damage checkpoint sensors including the Mre11-Rad50-Nbs1 (MRN) complex, γ-H2AX and 53BP1, the ATM and ATR signal transducing kinases and downstream effectors including Chk1, Chk2 and p53. Robust DNA damage response signals at dysfunctional telomeres, achieved by complete deletion of TRF2 or by expressing dominant negative mutant TPP1ΔRD, can be detected by their association with γ-H2AX and 53BP1 forming “telomere dysfunction induced foci (TIFs)”. Induction of TIFs at telomeres provides an opportunity to quantify the extent of telomere dysfunction and monitor the signaling pathways.
PMCID: PMC3690558  PMID: 21461819
DNA damage; Telomere dysfunction; Telomere induced foci; Telomere-FISH
13.  Dual roles of telomere dysfunction in initiation and suppression of tumorigenesis 
Experimental cell research  2008;314(9):1973-1979.
Human carcinomas arise through the acquisition of genetic changes that endow precursor cancer cells with a critical threshold of cancer-relevant genetic lesions. This complex genomic alterations confer upon precursor cancer cells the ability to grow indefinitely and to metastasize to distant sites. One important mechanism underlying a cell’s tumorigenic potential is the status of its telomere. Telomeres are G-rich simple repeat sequences that serve to prevent chromosomal ends from being recognized as DNA double-strand breaks (DSBs). Dysfunctional telomeres resemble DSBs, leading to the formation of dicentric chromosomes that fuel high degrees of genomic instability. In the setting of an intact p53 pathway, this instability promotes cellular senescence, a potent tumor suppressor mechanism. However, rare cells that stochastically lose p53 function emerge from this sea of genomic instability and progress towards cancer. In this review, we describe the use of mouse models to probe the impact of dysfunctional telomeres on tumor initiation and suppression.
PMCID: PMC3690559  PMID: 18448098
14.  A Conserved Motif within RAP1 Plays Diversified Roles in Telomere Protection and Regulation in Different Organisms 
Repressor activator protein 1 (RAP1) is the most highly conserved telomere protein. It is involved in protecting chromosome ends in fission yeast, promoting gene silencing in Saccharomyces cerevisiae while in Kluyveromyces lactis it is required to repress homology directed recombination (HDR) at telomeres. Since mammalian RAP1 requires TRF2 for stable expression, its role in telomere function has remained obscure. To understand how RAP1 plays such diverse functions at telomeres, we solved the crystal or solution structures of the C-terminal RCT domains of RAP1 from multiple organisms in complex with their respective protein-binding partners. Our comparative structural analysis establishes the RCT domain of RAP1 as an evolutionarily conserved protein-protein interaction module. In mammalian and fission yeast cells, this module interacts with TRF2 and Taz1, respectively, targeting RAP1 to chromosome ends for telomere end protection. While RAP1 repress NHEJ at fission yeast telomeres, at mammalian telomeres it is required to repress HDR. In contrast, S. cerevisiae RAP1 utilizes the RCT domain to recruit Sir3 to telomeres to mediate gene silencing. Together, our results reveal that depending on the organism, the evolutionarily conserved RAP1 RCT motif plays diverse functional roles at telomeres.
PMCID: PMC3688267  PMID: 21217703
15.  Telomere dysfunction and Tumor Suppression-the Senescence Connection 
Nature reviews. Cancer  2008;8(6):450-458.
Long lived organisms such as humans have evolved several intrinsic tumor suppressor mechanisms to combat the slew of oncogenic somatic mutations that constantly arise in proliferating stem cell compartments. One of these anti-cancer barriers is the telomere, a specialized nucleoprotein that caps the ends of eukaryotic chromosome. Impaired telomere function activates the canonical DNA damage response pathway that engages p53 to initiate apoptosis or replicative senescence. Here, we discuss how p53-dependent senescence induced by dysfunctional telomeres may be as potent as apoptosis in suppressing tumorigenesis in vivo.
PMCID: PMC3688269  PMID: 18500246
16.  Serum 25-Hydroxyvitamin D and Pulmonary Function in Older Disabled Community-Dwelling Women 
Recent studies have expanded the functions of vitamin D to a possible role in pulmonary function. Our objective was to examine the relationship between serum 25-hydroxyvitamin D (25[OH]D), serum parathyroid hormone, and pulmonary function in older women.
We examined the relationship of serum 25(OH)D and parathyroid hormone with pulmonary function (forced expiratory volume in one second [FEV1], forced vital capacity [FVC], and FEV1/FVC ratio) in a cross-sectional study of 646 moderately to severely disabled women, 65 years or more, living in the community in Baltimore, Maryland, who participated in the Women’s Health and Aging Study I.
Overall, median (25th, 75th percentile) serum 25-hydroxyvitamin D concentrations were 19.9 (14.7, 26.7) ng/mL. Serum 25(OH)D was positively associated with FEV1 (p = .03), FVC (p = .18), and FEV1/FVC (p = .04) in multivariable linear regression models adjusting for age, race, education, smoking, height, physical activity, cognition, interleukin-6, chronic diseases, and other potential confounders. In the same models, serum parathyroid hormone was not significantly associated with FEV1, FVC, or FEV1/FVC.
These findings support the idea that vitamin D deficiency is independently associated with poor pulmonary function in older disabled women.
PMCID: PMC3732158  PMID: 22156439
Aging; Lung function; Parathyroid hormone; Vitamin D; Women
17.  Defending the end zone: studying the players involved in protecting chromosome ends 
FEBS letters  2010;584(17):3773-3778.
The linear nature of eukaryotic chromosomes leaves natural ends susceptible to triggering DNA damage responses. Telomeres are specialized nucleoprotein structures that comprise the “end zone” of chromosomes. Besides having a specialized sequences and structures, there are six resident proteins at telomeres that play a prominent role in protecting chromosome ends. In the review, we will discuss this team of proteins called shelterin and how they are involved in regulating DNA damage signaling, repair and replication at telomeres.
PMCID: PMC3657741  PMID: 20579983
telomere; shelterin; DNA damage response; NHEJ
18.  The E3 ubiquitin ligase RNF8 stabilizes TPP1 to promote telomere end protection 
Nature structural & molecular biology  2011;18(12):1400-1407.
TPP1, a component of the mammalian shelterin complex, plays essential roles in telomere maintenance. It forms a heterodimer with POT1 to repress ATR-dependent DNA damage signaling at telomeres, and recruits telomerase to chromosome ends. Here we show that the E3 ubiquitin ligase RNF8 localizes to and promotes the accumulation of DNA damage proteins 53BP1 and γ-H2AX to uncapped telomeres. TPP1 is unstable in the absence of RNF8, resulting in telomere shortening and chromosome fusions via the alternative non-homologous end joining (A-NHEJ)-mediated DNA repair pathway. The RNF8 ubiquitin ligase RING domain is essential for TPP1 stability and retention at telomeres. RNF8 physically interacts with TPP1 to generate Ubc13-dependent K63 polyubiquitin chains that stabilizes TPP1 at telomeres. The conserved TPP1 lysine residue 233 is essential for RNF8-mediated TPP1 ubiquitylation and localization to telomeres. Our results demonstrate that TPP1 is a novel substrate for RNF8, and suggest a previously unrecognized role for RNF8 in telomere end protection. We propose a model in which engagement of classical vs. A-NHEJ repair pathways at dysfunctional telomeres is controlled by the ubiquitin ligase functions of RNF8.
PMCID: PMC3657743  PMID: 22101936
19.  Contribution of Multiple Chronic Conditions to Universal Health Outcomes 
To determine the relative effect of five chronic conditions on four representative universal health outcomes.
Cardiovascular Health Study.
Five thousand two hundred and ninety-eight community-living participants aged 65 and older.
Multiple regression and Cox models were used to determine the effect of heart failure (HF), chronic obstructive pulmonary disease (COPD), osteoarthritis, depression, and cognitive impairment on self-rated health, 12 basic and instrumental activities of daily living (ADLs and IADLs), six-item symptom burden scale, and death.
Each condition adversely affected self-rated health (P<.001) and ADLs and IADLs (P<.001). For example, persons with HF performed 0.70 ± 0.08 fewer ADLs and IADLs than those without; persons with depression and persons with cognitive impairment performed 0.59 ± 0.04 and 0.58 ± 0.06 fewer activities, respectively, than those without these conditions. Depression, HF, COPD, and osteoarthritis were associated with 1.18 ± 0.04, 0.40 ± 0.08, 0.40 ± 0.05, and 0.57 ± 0.03 more symptoms, respectively, in individuals with these conditions than in those without. HF (hazard ratio (HR) = 2.84, 95% confidence interval (CI) = 1.97–4.10), COPD (2.62, 95% CI = 1.94–3.53), cognitive impairment (2.05, 95% CI = 1.47–2.85), and depression (1.47, 95% CI = 1.08–2.01) were each associated with death within 2 years. Several paired combinations of conditions had synergistic effects on ADLs and IADLs. For example, individuals with HF plus depression performed 2.0 fewer activities than persons with neither condition, versus the 1.3 fewer activities expected from adding the effects of the two conditions together.
Universal health outcomes may provide a common metric for measuring the effects of multiple conditions and their treatments. The varying effects of the conditions across universal outcomes could inform care priorities.
PMCID: PMC3622699  PMID: 21883118
multiple chronic conditions; patient-reported outcomes; universal health outcomes
20.  RPA and POT1 
Cell Cycle  2012;11(4):652-657.
Telomere maintenance in cycling cells relies on both DNA replication and capping by the protein complex shelterin. Two single-stranded DNA (ssDNA)-binding proteins, replication protein A (RPA) and protection of telomere 1 (POT1) play critical roles in DNA replication and telomere capping, respectively. While RPA binds to ssDNA in a non-sequence-specific manner, POT1 specifically recognizes singlestranded TTAGGG telomeric repeats. Loss of POT1 leads to aberrant accumulation of RPA at telomeres and activation of the ataxia telangiectasia and Rad3-related kinase (ATR)-mediated checkpoint response, suggesting that POT1 antagonizes RPA binding to telomeric ssDNA. The requirement for both POT1 and RPA in telomere maintenance and the antagonism between the two proteins raises the important question of how they function in concert on telomeric ssDNA. Two interesting models were proposed by recent studies to explain the regulation of POT1 and RPA at telomeres. Here, we discuss how these models help unravel the coordination, and also the antagonism, between POT1 and RPA during the cell cycle.
PMCID: PMC3318101  PMID: 22373525
RPA; POT1; telomere; ATR; checkpoint
21.  Association Between Inflammatory-related Disease Burden and Frailty: Results From the Women’s Health and Aging Studies (WHAS) I and II 
Frailty is associated with a pro-inflammatory state, which has been characterized by elevated levels of systemic inflammatory biomarkers, but has not been related to the number of co-existing chronic diseases associated with inflammation. We sought to determine the extent to which a higher number of inflammatory-related diseases is associated with frailty and to identify the most common disease patterns associated with being frail in older adults. We performed binomial regression analyses to assess whether a higher count of inflammatory-related diseases increases the probability of frailty using data from the Women's Health and Aging Studies I and II, companion cohorts composed of 70–79-year-old community-dwelling older women in Baltimore, Maryland (n=620). An increase of one inflammatory-related disease was associated log-linearly with frailty (Prevalence Ratio (PR)=2.32, 95% Confidence Interval (CI)=1.85–2.92). After adjusting for age, race, education, and smoking status, the probability of frailty remained significant (PR=1.97, 95%CI=1.52–2.55). In the frail population, chronic kidney disease (CKD) and depressive symptoms (Prevalence=22.9%, 95%CI=14.2–34.8%); CVD and depressive symptoms (21.7%, 95%CI=13.2–33.5%); CKD and anemia (18.7%, 95%CI=11.1–29.7%); cardiovascular disease (CVD), CKD, and pulmonary disease (10.7%, 95%CI=5.2–21.0%); CKD, anemia, and depressive symptoms (8.7%, 95%CI=3.9–18.2%); and CVD, anemia, pulmonary disease, and depressive symptoms (5.0%, 95%CI=1.6–14.4%) were among the most frequent disease combinations. Their prevalence percentages were significantly higher in the frail versus non-frail women. A higher inflammatory-related disease count, perhaps reflecting a greater pro-inflammatory burden, increases the likelihood of frailty. Shared mechanisms among specific disease combinations may further contribute to this risk.
PMCID: PMC3197795  PMID: 21763008
comorbidity; inflammation; frailty
22.  An absence of both lamin B1 and lamin B2 in keratinocytes has no effect on cell proliferation or the development of skin and hair 
Human Molecular Genetics  2011;20(18):3537-3544.
Nuclear lamins are usually classified as A-type (lamins A and C) or B-type (lamins B1 and B2). A-type lamins have been implicated in multiple genetic diseases but are not required for cell growth or development. In contrast, B-type lamins have been considered essential in eukaryotic cells, with crucial roles in DNA replication and in the formation of the mitotic spindle. Knocking down the genes for B-type lamins (LMNB1, LMNB2) in HeLa cells has been reported to cause apoptosis. In the current study, we created conditional knockout alleles for mouse Lmnb1 and Lmnb2, with the goal of testing the hypothesis that B-type lamins are crucial for the growth and viability of mammalian cells in vivo. Using the keratin 14-Cre transgene, we bred mice lacking the expression of both Lmnb1 and Lmnb2 in skin keratinocytes (Lmnb1Δ/ΔLmnb2Δ/Δ). Lmnb1 and Lmnb2 transcripts were absent in keratinocytes of Lmnb1Δ/ΔLmnb2Δ/Δ mice, and lamin B1 and lamin B2 proteins were undetectable. But despite an absence of B-type lamins in keratinocytes, the skin and hair of Lmnb1Δ/ΔLmnb2Δ/Δ mice developed normally and were free of histological abnormalities, even in 2-year-old mice. After an intraperitoneal injection of bromodeoxyuridine (BrdU), similar numbers of BrdU-positive keratinocytes were observed in the skin of wild-type and Lmnb1Δ/ΔLmnb2Δ/Δ mice. Lmnb1Δ/ΔLmnb2Δ/Δ keratinocytes did not exhibit aneuploidy, and their growth rate was normal in culture. These studies challenge the concept that B-type lamins are essential for proliferation and vitality of eukaryotic cells.
PMCID: PMC3159554  PMID: 21659336
23.  Effect of chronic disease-related symptoms and impairments on universal health outcomes in older adults 
To determine the extent to which disease-related symptoms and impairments, which constitute measures of disease severity or targets of therapy, account for the associations between chronic diseases and universal health outcomes.
Cardiovascular Health Study (CHS) and Health ABC.
5,654 CHS, and 2,706 Health ABC, members.
Diseases included heart failure (HF), chronic obstructive pulmonary disease (COPD), osteoarthritis, and cognitive impairment. The universal health outcomes included self-rated health, basic and instrumental activities of daily living (BADLs-IADLs), and death. Disease-related symptoms/impairments included HF symptoms and ejection fraction (EF) for HF; Dyspnea Scale and FEV1 for COPD; joint pain for osteoarthritis, and executive function for cognitive impairment.
The diseases were associated with the universal health outcomes (p<0.001) except osteoarthritis with death (both cohorts) and cognitive impairment with self-rated health (Health ABC). Symptoms/impairments accounted for ≥30% of each disease’s effect on the universal health outcomes. In CHS, for example, HF, compared with no HF, was associated with one fewer (0.918) BADLs-IADL performed without difficulty; 27% of this effect was accounted for by HF symptoms, only 5% by EF. The hazard ratio for death with HF was 6.5 (95% CI, 4.7, 8.9) with 40% accounted for by EF and only 14% by HF symptoms.
Disease-related symptoms/impairments accounted for much of the significant associations between the 4 chronic diseases and the universal health outcomes. Results support considering universal health outcomes as common metrics across diseases in clinical decision-making, perhaps by targeting the disease-related symptoms/impairments that contribute most strongly to the effect of the disease on the universal health outcomes.
PMCID: PMC3287052  PMID: 21883120
chronic diseases; universal health outcomes; patient-reported outcomes; clinical decision-making
24.  Deficiencies in lamin B1 and lamin B2 cause neurodevelopmental defects and distinct nuclear shape abnormalities in neurons 
Molecular Biology of the Cell  2011;22(23):4683-4693.
Lamin B1 is essential for neuronal migration and progenitor proliferation during the development of the cerebral cortex. The observation of distinct phenotypes of Lmnb1- and Lmnb2-knockout mice and the differences in the nuclear morphology of cortical neurons in vivo suggest that lamin B1 and lamin B2 play distinct functions in the developing brain.
Neuronal migration is essential for the development of the mammalian brain. Here, we document severe defects in neuronal migration and reduced numbers of neurons in lamin B1–deficient mice. Lamin B1 deficiency resulted in striking abnormalities in the nuclear shape of cortical neurons; many neurons contained a solitary nuclear bleb and exhibited an asymmetric distribution of lamin B2. In contrast, lamin B2 deficiency led to increased numbers of neurons with elongated nuclei. We used conditional alleles for Lmnb1 and Lmnb2 to create forebrain-specific knockout mice. The forebrain-specific Lmnb1- and Lmnb2-knockout models had a small forebrain with disorganized layering of neurons and nuclear shape abnormalities, similar to abnormalities identified in the conventional knockout mice. A more severe phenotype, complete atrophy of the cortex, was observed in forebrain-specific Lmnb1/Lmnb2 double-knockout mice. This study demonstrates that both lamin B1 and lamin B2 are essential for brain development, with lamin B1 being required for the integrity of the nuclear lamina, and lamin B2 being important for resistance to nuclear elongation in neurons.
PMCID: PMC3226484  PMID: 21976703
25.  Absence of progeria-like disease phenotypes in knock-in mice expressing a non-farnesylated version of progerin 
Human Molecular Genetics  2010;20(3):436-444.
Hutchinson–Gilford progeria syndrome (HGPS) is caused by a mutant prelamin A, progerin, that terminates with a farnesylcysteine. HGPS knock-in mice (LmnaHG/+) develop severe progeria-like disease phenotypes. These phenotypes can be ameliorated with a protein farnesyltransferase inhibitor (FTI), suggesting that progerin's farnesyl lipid is important for disease pathogenesis and raising the possibility that FTIs could be useful for treating humans with HGPS. Subsequent studies showed that mice expressing non-farnesylated progerin (LmnanHG/+ mice, in which progerin's carboxyl-terminal –CSIM motif was changed to –SSIM) also develop severe progeria, raising doubts about whether any treatment targeting protein prenylation would be particularly effective. We suspected that those doubts might be premature and hypothesized that the persistent disease in LmnanHG/+ mice could be an unanticipated consequence of the cysteine-to-serine substitution that was used to eliminate farnesylation. To test this hypothesis, we generated a second knock-in allele yielding non-farnesylated progerin (LmnacsmHG) in which the carboxyl-terminal –CSIM motif was changed to –CSM. We then compared disease phenotypes in mice harboring the LmnanHG or LmnacsmHG allele. As expected, LmnanHG/+ and LmnanHG/nHG mice developed severe progeria-like disease phenotypes, including osteolytic lesions and rib fractures, osteoporosis, slow growth and reduced survival. In contrast, LmnacsmHG/+ and LmnacsmHG/csmHG mice exhibited no bone disease and displayed entirely normal body weights and survival. The frequencies of misshapen cell nuclei were lower in LmnacsmHG/+ and LmnacsmHG/csmHG fibroblasts. These studies show that the ability of non-farnesylated progerin to elicit disease depends on the carboxyl-terminal mutation used to eliminate protein prenylation.
PMCID: PMC3016906  PMID: 21088111

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