Reconstructing the in vivo dynamics of hematopoietic stem cells from telomere length distributions

Donor telomeres and their magnitude of shortening post-allogeneic haematopoietic cell transplant impact survival for patients with early-stage leukaemia or myelodysplastic syndrome

BACKGROUND

Donor selection is a key success factor in allogeneic haematopoietic cell transplant (HCT). We evaluated the potential impact of donor leucocyte telomere length (LTL) and LTL shortening in recipients at three-month post-HCT (LTL-3MS) on the two-year HCT outcomes.

METHODS

We identified a cohort of 384 HCT recipients for early-stage leukaemia or myelodysplastic syndrome in the Blood and Marrow Transplant Clinical Trial Network protocol#1202 with blood samples collected three-month post-HCT. Blood samples from respective donors were available at the Centre for International Blood and Marrow Transplant Research biorepository. We used Cox proportional hazards models for statistical analyses.

FINDINGS

A better two-year overall survival (OS) was associated with longer donor LTL (adjusted-hazard ratio [HR] = 0.60, 95% confidence interval [CI] = 0.37-0.96, for LTL ≥6.7 kb vs LTL< 6.7 kb, p = 0.03), and higher LTL-3MS (HR = 0.52, 95% CI = 0.34-0.80, for LTL-3MS ≥ 230 vs < 230 bp, p = 0.003). Longer donor LTL was associated with a lower risk of non-relapse mortality (NRM; HR = 0.48, p = 0.05), while higher LTL-3MS was associated with lower relapse risk (HR for relapse risk = 0.53, p = 0.008). The adjusted 2-year cumulative risk of all-cause mortality was reduced by about half for patients with both donor LTL ≥6.7 kb and LTL-3MS ≥ 230 bp vs patients with neither characteristic (21% vs 41%, respectively; p < 0.0001).

INTERPRETATION

Selection of donors with longer LTL may improve HCT outcomes. Limited LTL shortening in recipients post-HCT may guide relapse prediction.

FUNDING

The NCI intramural research program and NIH grant U01AG066529.

An incidental finding during a brain plasticity study: substantial telomere length shortening after COVID-19 lockdown in the older population

The detrimental effects of lockdowns have already been proven by numerous studies, mainly using psychometric measurements. Since telomere shortening is a driver of aging and aging-associated disorders, including cognitive decline, the telomere length in the older population has been investigated in the current study. Measurements were taken over a 6-month period just before and during the 6 months that included the first lockdown. The cohort of 55 persons aged 64 to 70 years was investigated in the context of a study focusing on neuroplasticity. Participants were recruited in Germany and Switzerland and characterized by psychometric measurements concerning neurocognition and neuroplasticity. Telomere lengths were measured by real-time PCR-based LTL measurement. We found an impressive and significant decline in telomere lengths in the period that included the lockdown (2.33 (± 0.1) at T1 vs. 1.35 (± 0.1) at T2), whereas it was stable in the phase before the lockdown in the same individuals (T0 was 2.25 (± 0.1 S.E.M.) vs. T1, 2.33 (± 0.1)). Correlation of the sudden decrease revealed no linkage to health issues or general physical activity but was in trend related to a decline in the WHOQOL-BREF Social Score referring to the social interaction of the study participants. Our data support, at a biological level, the results of clinical and psychosocial studies showing the detrimental effects of lockdowns.

Long-term lineage commitment in haematopoietic stem cell gene therapy

Haematopoietic stem cell (HSC) gene therapy (GT) may provide lifelong reconstitution of the haematopoietic system with gene-corrected cells1. However, the effects of underlying genetic diseases, replication stress and ageing on haematopoietic reconstitution and lineage specification remain unclear. In this study, we analysed haematopoietic reconstitution in 53 patients treated with lentiviral-HSC-GT for diverse conditions such as metachromatic leukodystrophy2,3 (MLD), Wiskott-Aldrich syndrome4,5 (WAS) and β-thalassaemia6 (β-Thal) over a follow-up period of up to 8 years, using vector integration sites as markers of clonal identity. We found that long-term haematopoietic reconstitution was supported by 770 to 35,000 active HSCs. Whereas 50% of transplanted clones demonstrated multi-lineage potential across all conditions, the remaining clones showed a disease-specific preferential lineage output and long-term commitment: myeloid for MLD, lymphoid for WAS and erythroid for β-Thal, particularly in adult patients. Our results indicate that HSC clonogenic activity, lineage output, long-term lineage commitment and rates of somatic mutations are influenced by the underlying disease, patient age at the time of therapy, the extent of genetic defect correction and the haematopoietic stress imposed by the inherited disease. This suggests that HSCs adapt to the pathological condition during haematopoietic reconstitution.

An in-silico approach to the dynamics of proliferation potential in stem cells and the study of different therapies in cases of ovarian dysfunction

A discrete mathematical model based on ordinary differential equations and the associated continuous model formed by a partial differential equation, which simulate the generational and temporal evolution of a stem cell population, are proposed. The model parameters are the maximum proliferation potential and the rates of mitosis, death events and telomerase activity. The mean proliferation potential at each point in time is suggested as an indicator of population aging. The model is applied on hematopoietic stem cells (HSCs), with different telomerase activity rates, in a range of variation of maximum proliferation potential in healthy individuals, to study the temporal evolution of aging. HSCs express telomerase, however not at levels that are sufficient for maintaining constant telomere length with aging [1,2]. Women with primary ovarian insufficiency (POI) are known to have low telomerase activity in granulosa cells and peripheral blood mononuclear cells [3]. Extrapolating this to hematopoietic stem cells, the mathematical model shows the differences in proliferation potential of the cell populations when telomerase expression is activated using sexual steroids, though the endogenous promoter or with gene therapy using exogenous, stronger promoters within the adeno-associated virus. In the first case, proliferation potential of cells from POI condition increases, but when adeno-associated viruses are used, the proliferation potential reaches the levels of healthy cell populations.

Modelling post-chemotherapy stem cell dynamics in the bone marrow niche of AML patients

Acute myeloid leukemia (AML) is a stem cell-driven malignancy of the blood forming (hematopoietic) system. Despite of high dose chemotherapy with toxic side effects, many patients eventually relapse. The "7+3 regimen", which consists of 7 days of cytarabine in combination with daunorubicin during the first 3 days, is a widely used therapy protocol. Since peripheral blood cells are easily accessible to longitudinal sampling, significant research efforts have been undertaken to characterize and reduce adverse effects on circulating blood cells. However, much less is known about the impact of the 7+3 regimen on human hematopoietic stem cells and their physiological micro-environments, the so-called stem cell niches. One reason for this is the technical inability to observe human stem cells in vivo and the discomfort related to bone marrow biopsies. To better understand the treatment effects on human stem cells, we consider a mechanistic mathematical model of the stem cell niche before, during and after chemotherapy. The model accounts for different maturation stages of leukemic and hematopoietic cells and considers key processes such as cell proliferation, self-renewal, differentiation and therapy-induced cell death. In the model, hematopoietic (HSCs) and leukemic stem cells (LSCs) compete for a joint niche and respond to both systemic and niche-derived signals. We relate the model to clinical trial data from literature which longitudinally quantifies the counts of hematopoietic stem like (CD34+CD38-ALDH+) cells at diagnosis and after therapy. The proposed model can capture the clinically observed interindividual heterogeneity and reproduce the non-monotonous dynamics of the hematopoietic stem like cells observed in relapsing patients. Our model allows to simulate different scenarios proposed in literature such as therapy-related impairment of the stem cell niche or niche-mediated resistance. Model simulations suggest that during the post-therapy phase a more than 10-fold increase of hematopoietic stem-like cell proliferation rates is required to recapitulate the measured cell dynamics in patients achieving complete remission. We fit the model to data of 7 individual patients and simulate variations of the treatment protocol. These simulations are in line with the clinical finding that G-CSF priming can improve the treatment outcome. Furthermore, our model suggests that a decline of HSC counts during remission might serve as an indication for salvage therapy in patients lacking MRD (minimal residual disease) markers.

The evolving hematopoietic niche during development

Mammalian hematopoietic stem cells (HSCs) emerge from the hemogenic endothelium in the major embryonic arteries. HSCs undergo a complex journey first migrating to the fetal liver (FL) and from there to the fetal bone marrow (FBM), where they mostly remain during adult life. In this process, a pool of adult HSCs is produced, which sustains lifelong hematopoiesis. Multiple cellular components support HSC maturation and expansion and modulate their response to environmental and developmental cues. While the adult HSC niche has been extensively studied over the last two decades, the niches present in the major embryonic arteries, FL, FBM and perinatal bone marrow (BM) are poorly described. Recent investigations highlight important differences among FL, FBM and adult BM niches and emphasize the important role that inflammation, microbiota and hormonal factors play regulating HSCs and their niches. We provide a review on our current understanding of these important cellular microenvironments across ontogeny. We mainly focused on mice, as the most widely used research model, and, when possible, include relevant insights from other vertebrates including birds, zebrafish, and human. Developing a comprehensive picture on these processes is critical to understand the earliest origins of childhood leukemia and to achieve multiple goals in regenerative medicine, such as mimicking HSC development in vitro to produce HSCs for broad transplantation purposes in leukemia, following chemotherapy, bone marrow failure, and in HSC-based gene therapy.

How mutation accumulation depends on the structure of the cell lineage tree

All the cells of a multicellular organism are the product of cell divisions that trace out a single binary tree, the so-called cell lineage tree. Because cell divisions are accompanied by replication errors, the shape of the cell lineage tree is a key determinant of how somatic evolution, which can potentially lead to cancer, proceeds. Carcinogenesis requires the accumulation of a certain number of driver mutations. By mapping the accumulation of mutations into a graph theoretical problem, we present an exact numerical method to calculate the probability of collecting a given number of mutations and show that for low mutation rates it can be approximated with a simple analytical formula, which depends only on the distribution of the lineage lengths, and is dominated by the longest lineages. Our results are crucial in understanding how natural selection can shape the cell lineage trees of multicellular organisms and curtail somatic evolution.

Telomere length is an epigenetic trait - Implications for the use of telomerase-deficient organisms to model human disease

Telomere length, unlike most genetic traits, is epigenetic, in the sense that it is not fully coded by the genome. Telomeres vary in length and randomly assort to the progeny leaving some individuals with longer and others with shorter telomeres. Telomerase activity counteracts this by extending telomeres in the germline and during embryogenesis but sizeable variances remain in telomere length. This effect is exacerbated by the absence of fully active telomerase. Telomerase heterozygous animals (tert+/-) have reduced telomerase activity and their telomeres fail to be elongated to wild-type average length, meaning that - with every generation - they decrease. After a given number of successive generations of telomerase-insufficient crosses, telomeres become critically short and cause organismal defects that, in humans, are known as telomere biology disorders. Importantly, these defects also occur in wild-type (tert+/+) animals derived from such tert+/- incrosses. Despite these tert+/+ animals being proficient for telomerase, they have shorter than average telomere length and, although milder, develop phenotypes that are similar to those of telomerase mutants. Here, we discuss the impact of this phenomenon on human pathologies associated with telomere length, provide a brief overview of telomere biology across species and propose specific measures for working with telomerase-deficient zebrafish.

Measures of genetic diversification in somatic tissues at bulk and single-cell resolution

Intra-tissue genetic heterogeneity is universal to both healthy and cancerous tissues. It emerges from the stochastic accumulation of somatic mutations throughout development and homeostasis. By combining population genetics theory and genomic information, genetic heterogeneity can be exploited to infer tissue organization and dynamics in vivo. However, many basic quantities, for example the dynamics of tissue-specific stem cells remain difficult to quantify precisely. Here, we show that single-cell and bulk sequencing data inform on different aspects of the underlying stochastic processes. Bulk-derived variant allele frequency spectra (VAF) show transitions from growing to constant stem cell populations with age in samples of healthy esophagus epithelium. Single-cell mutational burden distributions allow a sample size independent measure of mutation and proliferation rates. Mutation rates in adult hematopietic stem cells are higher compared to inferences during development, suggesting additional proliferation-independent effects. Furthermore, single-cell derived VAF spectra contain information on the number of tissue-specific stem cells. In hematopiesis, we find approximately 2 × 105 HSCs, if all stem cells divide symmetrically. However, the single-cell mutational burden distribution is over-dispersed compared to a model of Poisson distributed random mutations. A time-associated model of mutation accumulation with a constant rate alone cannot generate such a pattern. At least one additional source of stochasticity would be needed. Possible candidates for these processes may be occasional bursts of stem cell divisions, potentially in response to injury, or non-constant mutation rates either through environmental exposures or cell-intrinsic variation.

Hallmarks of stemness in mammalian tissues

All adult tissues experience wear and tear. Most tissues can compensate for cell loss through the activity of resident stem cells. Although the cellular maintenance strategies vary greatly between different adult (read: postnatal) tissues, the function of stem cells is best defined by their capacity to replace lost tissue through division. We discuss a set of six complementary hallmarks that are key enabling features of this basic function. These include longevity and self-renewal, multipotency, transplantability, plasticity, dependence on niche signals, and maintenance of genome integrity. We discuss these hallmarks in the context of some of the best-understood adult stem cell niches.

The "telomereless" erythrocytes and telomere-length dependent erythropoiesis

Approximately 25 trillion erythrocytes (red blood cells) circulate in the bloodstream of an adult human, surpassing the number of circulating leukocytes (white blood cells) by a factor of about 1000. Moreover, the erythrocyte turnover rate accounts for approximately 76% of the turnover rate of all circulating blood cells. This simple math shows that the hematopoietic system principally spends its telomere length-dependent replicative capacity on building and maintaining the erythrocyte blood pool. Erythropoiesis (red blood cell production) is thus the principal cause of telomere shortening with age in hematopoietic cells (HCs), a conclusion that holds significant implications for linking telomere length dynamics in HCs to health and lifespan of modern humans.

Early origins of health and disease risk: The case for investigating adverse exposures and biological aging in utero, across childhood, and into adolescence

In this article, we suggest that aging and development are two sides of the same coin, and that developing a comprehensive understanding of health and disease risk requires examining age-related processes occurring throughout the earliest years of life. Compared to other periods in life, during this early period of acute vulnerability, when children's biological and regulatory systems are developing, biological aging occurs most rapidly. We review theory and empirical research suggesting that processes of development and aging are intricately linked, and that early adversity may program biological parameters for accelerated aging and disease risk early in life, even though clinical signs of age-related disease onset may not be evident until many years later. Following from this, we make the case for widespread incorporation of biological aging constructs into child development research.

Shared and distinct genetic etiologies for different types of clonal hematopoiesis

Clonal hematopoiesis (CH)-age-related expansion of mutated hematopoietic clones-can differ in frequency and cellular fitness by CH type (e.g., mutations in driver genes (CHIP), gains/losses and copy-neutral loss of chromosomal segments (mCAs), and loss of sex chromosomes). Co-occurring CH raises questions as to their origin, selection, and impact. We integrate sequence and genotype array data in up to 482,378 UK Biobank participants to demonstrate shared genetic architecture across CH types. Our analysis suggests a cellular evolutionary trade-off between different types of CH, with LOY occurring at lower rates in individuals carrying mutations in established CHIP genes. We observed co-occurrence of CHIP and mCAs with overlap at TET2, DNMT3A, and JAK2, in which CHIP precedes mCA acquisition. Furthermore, individuals carrying overlapping CH had high risk of future lymphoid and myeloid malignancies. Finally, we leverage shared genetic architecture of CH traits to identify 15 novel loci associated with leukemia risk.

Predicting the number of lifetime divisions for hematopoietic stem cells from telomere length measurements

How many times does a typical hematopoietic stem cell (HSC) divide to maintain a daily production of over 1011 blood cells over a human lifetime? It has been predicted that relatively few, slowly dividing HSCs occupy the top of the hematopoietic hierarchy. However, tracking HSCs directly is extremely challenging due to their rarity. Here, we utilize previously published data documenting the loss of telomeric DNA repeats in granulocytes, to draw inferences about HSC division rates, the timing of major changes in those rates, as well as lifetime division totals. Our method uses segmented regression to identify the best candidate representations of the telomere length data. Our method predicts that, on average, an HSC divides 56 times over an 85-year lifespan (with lower and upper bounds of 36 and 120, respectively), with half of these divisions during the first 24 years of life.

HSC Niche Dynamics in Regeneration, Pre-malignancy, and Cancer: Insights From Mathematical Modeling

The hematopoietic stem cell (HSC) niche is a crucial driver of regeneration and malignancy. Its interaction with hematopoietic and malignant stem cells is highly complex and direct experimental observations are challenging. We here develop a mathematical model which helps relate processes in the niche to measurable changes of stem and non-stem cell counts. HSC attached to the niche are assumed to be quiescent. After detachment HSC become activated and divide or differentiate. To maintain their stemness, the progeny originating from division must reattach to the niche. We use mouse data from literature to parametrize the model. By combining mathematical analysis and computer simulations, we systematically investigate the impact of stem cell proliferation, differentiation, niche attachment, and detachment on clinically relevant scenarios. These include bone marrow transplantation, clonal competition, and eradication of malignant cells. According to our model, sampling of blood or bulk marrow provides only limited information about cellular interactions in the niche and the clonal composition of the stem cell population. Furthermore, we investigate how interference with processes in the stem cell niche could help to increase the effect of low-dose chemotherapy or to improve the homing of genetically engineered cells.

Telomere length dynamics measured by flow-FISH in patients with obesity undergoing bariatric surgery

Obesity has negative effects on comorbidities, health-related quality of life and survival. Telomere length (TL) changes after bariatric surgery have been reported, but the studies are contradictory, and analyses using state-of-the art techniques for TL measurement, such as flow-FISH, are sparse. We measured TL dynamics via flow-FISH in patients undergoing bariatric surgery and compared their TL with 105 healthy individuals. Patients with obesity who underwent bariatric surgery were included. Lymphocyte and granulocyte absolute and age-adjusted (aa) TL were analyzed by flow-FISH before (preoperative cohort, n = 45) and after surgery (follow-up cohort, n = 35) at month 5.5 ± 3.9 (mean ± standard deviation [SD]). The initial lymphocyte aaTL was significantly shorter (-0.37 kb ± 0.18 kb, P = 0.045) in patients with obesity, while the granulocyte aaTL was not different from that in the healthy comparison population (0.28 kb ± 0.17 kb, P = 0.11). The telomere dynamics after surgery showed an increase in mean TL in both lymphocytes and granulocytes of patients with a pronounced BMI loss of ≥ 10 kg/m². We did not find any association between TL increase after surgery and age, sex or the type of procedure selected for bariatric surgery. We confirmed that patients suffering from obesity have significantly shorter lymphocyte TL using flow-FISH. Along with and dependent on the degree of weight reduction after bariatric surgery, TL significantly increased in both lymphocytes and granulocytes after a mean of 5.5 months. Our results show that bariatric surgery affects not only body weight but also biomarkers of aging, such as TL.

The biology of hematopoietic stem cells and its clinical implications

Hematopoietic stem cells (HSCs) give rise to all types of blood cells and self-renew their own population. The regeneration potential of HSCs has already been successfully translated into clinical applications. However, recent studies on the biology of HSCs may further extend their clinical use in future. The roles of HSCs in native hematopoiesis and in transplantation settings may differ. Furthermore, the heterogenic pool of HSCs dynamically changes during aging. These changes also involve the complex interactions of HSCs with the bone marrow niche. Here, we review the opportunities and challenges of these findings to improve the clinical use of HSCs. We describe new methods of HSCs mobilization and conditioning for the transplantation of HSCs. Finally, we highlight the research findings that may lead to overcoming the current limitations of HSC transplantation and broaden the patient group that can benefit from the clinical potential of HSCs.

Why are cell populations maintained via multiple compartments?

We consider the maintenance of 'product' cell populations from 'progenitor' cells via a sequence of one or more cell types, or compartments, where each cell's fate is chosen stochastically. If there is only one compartment then large amplification, that is, a large ratio of product cells to progenitors comes with disadvantages. The product cell population is dominated by large families (cells descended from the same progenitor) and many generations separate, on average, product cells from progenitors. These disadvantages are avoided using suitably constructed sequences of compartments: the amplification factor of a sequence is the product of the amplification factors of each compartment, while the average number of generations is a sum over contributions from each compartment. Passing through multiple compartments is, in fact, an efficient way to maintain a product cell population from a small flux of progenitors, avoiding excessive clonality and minimizing the number of rounds of division en route. We use division, exit and death rates, estimated from measurements of single-positive thymocytes, to choose illustrative parameter values in the single-compartment case. We also consider a five-compartment model of thymocyte differentiation, from double-negative precursors to single-positive product cells.

Murine foetal liver supports limited detectable expansion of life-long haematopoietic progenitors

Current dogma asserts that the foetal liver (FL) is an expansion niche for recently specified haematopoietic stem cells (HSCs) during ontogeny. Indeed, between embryonic day of development (E)12.5 and E14.5, the number of transplantable HSCs in the murine FL expands from 50 to about 1,000. Here we used a non-invasive, multi-colour lineage tracing strategy to interrogate the embryonic expansion of murine haematopoietic progenitors destined to contribute to the adult HSC pool. Our data show that this pool of fated progenitors expands only two-fold during FL ontogeny. Although Histone2B-GFP retention in vivo experiments confirmed substantial proliferation of phenotypic FL-HSC between E12.5 and E14.5, paired-daughter cell assays revealed that many mid-gestation phenotypic FL-HSCs are biased to differentiate, rather than self-renew, relative to phenotypic neonatal and adult bone marrow HSCs. In total, these data support a model in which the FL-HSC pool fated to contribute to adult blood expands only modestly during ontogeny.

The bullwhip effect, T-cell telomeres, and SARS-CoV-2

Both myeloid cells, which contribute to innate immunity, and lymphoid cells, which dominate adaptive immunity, partake in defending against SARS-CoV-2. In response to the virus, the otherwise slow haematopoietic production supply chain quickly unleashes its preconfigured myeloid element, which largely resists a bullwhip-like effect. By contrast, the lymphoid element risks a bullwhip-like effect when it produces T cells and B cells that are specifically designed to clear the virus. As T-cell production is telomere-length dependent and telomeres shorten with age, older adults are at higher risk of a T-cell shortfall when contracting SARS-CoV-2 than are younger adults. A poorly calibrated adaptive immune response, stemming from a bullwhip-like effect, compounded by a T-cell deficit, might thus contribute to the propensity of people with inherently short T-cell telomeres to develop severe COVID-19. The immune systems of these individuals might also generate an inadequate T-cell response to anti-SARS-CoV-2 vaccination.

Specification of hematopoietic stem cells in mammalian embryos: a rare or frequent event?

Hematopoietic stem cells (HSCs) are the blood-forming stem cells thought to be responsible for supporting the blood system throughout life. Transplantability has long been the flagship assay used to define and characterize HSCs throughout ontogeny. However, it has recently become clear that many cells emerge during ontogeny that lack transplantability yet nevertheless are fated to ultimately contribute to the adult HSC pool. Here, we explore recent advances in understanding the numbers and kinetics of cells that emerge during development to support lifelong hematopoiesis; these advances are made possible by new technologies allowing interrogation of lifelong blood potential without embryo perturbation or transplantation. Illuminating the dynamics of these cells during normal development informs efforts to better understand the origins of hematologic disease and engineer HSCs from differentiating pluripotent stem cells.

Stem Cells, Hematopoiesis and Lineage Tracing: Transplantation-Centric Views and Beyond

An appropriate production of mature blood cells, or hematopoiesis, is essential for organismal health and homeostasis. In this developmental cascade, hematopoietic stem cells (HSCs) differentiate into intermediate progenitor types, that subsequently give rise to the many distinct blood cell lineages. Here, we describe tools and methods that permit for temporal and native clonal-level HSC lineage tracing in the mouse, and that can now be combined with emerging single-cell molecular analyses. We integrate new insights derived from such experimental paradigms with past knowledge, which has predominantly been derived from transplantation-based approaches. Finally, we outline current knowledge and novel strategies derived from studies aimed to trace human HSC-derived hematopoiesis.

Gain-of-function mutations in RPA1 cause a syndrome with short telomeres and somatic genetic rescue

Human telomere biology disorders (TBD)/short telomere syndromes (STS) are heterogeneous disorders caused by inherited loss-of-function mutations in telomere-associated genes. Here, we identify 3 germline heterozygous missense variants in the RPA1 gene in 4 unrelated probands presenting with short telomeres and varying clinical features of TBD/STS, including bone marrow failure, myelodysplastic syndrome, T- and B-cell lymphopenia, pulmonary fibrosis, or skin manifestations. All variants cluster to DNA-binding domain A of RPA1 protein. RPA1 is a single-strand DNA-binding protein required for DNA replication and repair and involved in telomere maintenance. We showed that RPA1E240K and RPA1V227A proteins exhibit increased binding to single-strand and telomeric DNA, implying a gain in DNA-binding function, whereas RPA1T270A has binding properties similar to wild-type protein. To study the mutational effect in a cellular system, CRISPR/Cas9 was used to knock-in the RPA1E240K mutation into healthy inducible pluripotent stem cells. This resulted in severe telomere shortening and impaired hematopoietic differentiation. Furthermore, in patients with RPA1E240K, we discovered somatic genetic rescue in hematopoietic cells due to an acquired truncating cis RPA1 mutation or a uniparental isodisomy 17p with loss of mutant allele, coinciding with stabilized blood counts. Using single-cell sequencing, the 2 somatic genetic rescue events were proven to be independently acquired in hematopoietic stem cells. In summary, we describe the first human disease caused by germline RPA1 variants in individuals with TBD/STS.

Regulation of Gene Expression by Telomere Position Effect

Many diseases that involve malignant tumors in the elderly affect the quality of human life; therefore, the relationship between aging and pathogenesis in geriatric diseases must be under-stood to develop appropriate treatments for these diseases. Recent reports have shown that epigenetic regulation caused by changes in the local chromatin structure plays an essential role in aging. This review provides an overview of the roles of telomere shortening on genomic structural changes during an age-dependent shift in gene expression. Telomere shortening is one of the most prominent events that is involved in cellular aging and it affects global gene expression through genome rearrangement. This review provides novel insights into the roles of telomere shortening in disease-affected cells during pathogenesis and suggests novel therapeutic approaches.

Telomere Shortening in Peripheral Leukocytes Is Associated With Poor Survival in Cancer Patients Treated With Immune Checkpoint Inhibitor Therapy

Background

Immune checkpoint inhibitor (ICI) therapy represents a new standard of care for an increasing number of malignancies. Nevertheless, response rates and outcome of ICI treatment vary between individuals and the identification of predictive markers or hints towards immune cell exhaustion during therapy has remained a major challenge. Leukocyte telomere length is an established predictive biomarker of replicative aging and cellular proliferative potential in various hematological diseases. However, its relevance in the context of ICI therapy has not been investigated to date. Here, we analyze the age-adapted delta telomere length (ΔTL) of peripheral leukocytes as a potential predictive and prognostic marker in patients undergoing ICI therapy.

Methods

Age-adapted delta telomere length (ΔTL) of 84 patients treated with ICIs for solid malignancies was measured via quantitative real-time PCR. ΔTL was correlated with outcome and clinical data.

Results

ΔTL was not significantly altered between patients with different tumor entities or tumor stages and did not predict tumor response to ICI therapy. However, ΔTLs at initiation of treatment were a prognostic marker for overall survival (OS). When using a calculated ideal cut-off value, the median OS in patients with shorter ΔTL was 5.7 months compared to 18.0 months in patients showing longer ΔTL. The prognostic role of age-adapted ΔTL was further confirmed by uni- and multivariate Cox-regression analyses.

Conclusion

In the present study, we demonstrate that shorter telomere lengths in peripheral blood leukocytes are associated with a significantly impaired outcome in patients receiving ICI therapy across different malignancies. We explain our findings by hypothesizing an older replicative age in peripheral leukocytes of patients with an impaired overall survival, reflected by a premature TL shortening. Whether this association is ICI-specific remains unknown. Further follow-up studies are needed to provide insights about the exact mechanism of how shortened telomeres eventually affect OS and could help guiding therapeutic decisions in future.

Epigenetic Clocks Are Not Accelerated in COVID-19 Patients

Age is a major risk factor for severe outcome of the 2019 coronavirus disease (COVID-19). In this study, we followed the hypothesis that particularly patients with accelerated epigenetic age are affected by severe outcomes of COVID-19. We investigated various DNA methylation datasets of blood samples with epigenetic aging signatures and performed targeted bisulfite amplicon sequencing. Overall, epigenetic clocks closely correlated with the chronological age of patients, either with or without acute respiratory distress syndrome. Furthermore, lymphocytes did not reveal significantly accelerated telomere attrition. Thus, these biomarkers cannot reliably predict higher risk for severe COVID-19 infection in elderly patients.

Characterization of tree shrew telomeres and telomerase

The use of tree shrews as experimental animals for biomedical research is a new practice. Several recent studies suggest that tree shrews are suitable for studying cancers, including breast cancer, glioblastoma, lung cancer, and hepatocellular carcinoma. However, the telomeres and the telomerase of tree shrews have not been studied to date. Here, we characterize telomeres and telomerase in tree shrews. The telomere length of tree shrews is approximately 23 kb, which is longer than that of primates and shorter than that of mice, and it is extended in breast tumor tissues according to Southern blot and flow-fluorescence in situ hybridization (FISH) analyses. Tree shrew spleen, bone marrow, testis, ovary, and uterus show high telomerase activities, which are increased in breast tumor tissues by telomeric repeat amplification protocol assays. The telomere length becomes shorter, and telomerase activity decreases with age. The tree shrew TERT and TERC are more highly similar to primates than to rodents. These findings lay a solid foundation for using tree shrews to study aging and cancers.

Telomere dynamics across the early life course: Findings from a longitudinal study in children

Telomeres are protective caps on chromosome ends that shorten with each cell division. Telomere length (TL) predicts the onset of cellular senescence and correlates with longevity and age-related disease risk. Previous research suggests that adults display fixed ranking and tracking of TL by age 20 years, supporting the importance of TL at birth and attrition during childhood. However, longitudinal research examining telomere dynamics during early life is sparse. Here, we used monochrome multiplex quantitative polymerase chain reaction to measure relative TL in leukocytes isolated from cord blood and child blood collected at ages 3, 5, 7, and 9 years among 224 minority children enrolled in a New York City-based birth cohort. We also measured maternal TL at delivery in a subset of 197 participants with a biobanked blood sample. TL decreased most rapidly in the first years of life (birth to 3 years), followed by a period of maintenance into the pre-puberty period. Mothers with longer telomeres gave birth to newborns with longer telomeres that remained longer across childhood, suggesting that the fixed ranking and tracking of TL observed among adults may extend to early childhood or even the prenatal period with a potential transgenerational basis. We did not find significant sex differences in the pattern of child TL change across development. These findings emphasize the need to understand factors and mechanisms that determine TL during early childhood.

The effect of pre-operative autologous blood donation on bone marrow hematopoietic functions in rabbits after hepatectomy

BACKGROUND

Pre-operative autologous blood donation (PABD) is one of the most widely distributed autologous blood donation means, which has positive effect on erythropoiesis. However, whether PABD can stimulate the bone marrow hematopoiesis after hepatectomy have not been reported.

METHODS

Totally 80 New Zealand rabbits were randomly divided into 4 groups that included control group, surgery group, hemodilutional autotransfusion (HA) group and PABD group. Automatic reticulocyte examination was performed to detect the content of reticulocyte and immature reticulocyte fractions (IRF). Flow cytometric analysis was employed to monitor the level of CD34+ cells and the cell cycle status. Southern blotting was conducted to determine the telomere length of CD34+ cells.

RESULTS

The content of high fluorescence reticulocytes (HFR) and IRF was decreased at 6 h and 24 h after autotransfusion. However, the level of CD34+ cells was upregulated after PABD. Cell cycle status analysis revealed that majority of the CD34+ cells in HA and PABD group were maintained in G0/G1 phase. The telomere length in HA and PABD group was shorten than that of control group and surgery group.

CONCLUSION

PABD could promote the bone marrow hematopoietic functions in rabbits after hepatectomy via stimulating proliferation of CD34+ cells and shortening the telomere length of CD34+ cells, but the content of HFR was not increased immediately because of the stuck of CD34+ cells in G0/G1 phase.

Cord blood telomere shortening associates with increased gestational age and birth weight in preterm neonates

Preterm birth is considered to be associated with premature cellular aging. To address this question, two hallmarks of aging were analyzed in cord blood cells, namely telomere length and age-associated DNA methylation. Cord blood samples from 35 preterm and 11 full-term neonates were enrolled in the present study. Furthermore, quantitative telomere fluorescence in situ hybridization and flow cytometry (flow-FISH) were applied to demonstrate that telomere shortening was strongly associated with advanced gestational age and increased birth weight (R²=0.267 for granulocytes and R²=0.307 for lymphocytes). The estimated rate of telomere attrition in newborns during gestation ranged from 126 base pairs (bp)/week and 186 bp/week for granulocytes and lymphocytes, respectively. In addition, neonates with longer telomeres at birth were characterized by increased weight gain during the first year of their life compared with that noted to neonates with shorter telomeres. By contrast, the epigenetic aging signature (EAS) revealed a negative correlation between epigenetic age and premature birth of unclear basis (R²=0.26). Pending prospective validation in a larger patient cohort, the present study suggested that telomere length may be a novel biomarker alone or in combination with traditional indicators for the prediction of weight development in preterm neonates.

Heterogeneity and 'memory' in stem cell populations

Modern single cell experiments have revealed unexpected heterogeneity in apparently functionally 'pure' cell populations. However, we are still lacking a conceptual framework to understand this heterogeneity. Here, we propose that cellular memories-changes in the molecular status of a cell in response to a stimulus, that modify the ability of the cell to respond to future stimuli-are an essential ingredient in any such theory. We illustrate this idea by considering a simple age-structured model of stem cell proliferation that takes account of mitotic memories. Using this model we argue that asynchronous mitosis generates heterogeneity that is central to stem cell population function. This model naturally explains why stem cell numbers increase through life, yet regenerative potency simultaneously declines.

Telomere Length and Hematological Disorders: A Review

Telomeres compose the end portions of human chromosomes, and their main function is to protect the genome. In hematological disorders, telomeres are shortened, predisposing to genetic instability that may cause DNA damage and chromosomal rearrangements, inducing a poor clinical outcome. Studies from 2010 to 2019 were compiled and experimental studies using samples of patients diagnosed with hematological malignancies that reported the size of the telomeres were described. Abnormal telomere shortening is described in cancer, but in hematological neoplasms, telomeres are still shortened even after telomerase reactivation. In this study, we compared the sizes of telomeres in leukemias, myelodysplastic syndrome and lymphomas, identifying that the smallest telomeres are present in patients at relapse. In conclusion, the experimental and clinical data analyzed in this review demonstrate that excessive telomere shortening is present in major hematological malignancies and its analysis and measurement is a crucial step in determining patient prognosis, predicting disease risk and assisting in the decision for targeted therapeutic strategies.

A systematic review on the effects of social discrimination on telomere length

Discrimination is unfair treatment against a certain group based on race, age, gender, sexual orientation, or other social identities. Discrimination is pervasive in society, elevates psychosocial stress, and is associated with negative mental and physical health outcomes. However, more research is needed to understand the biological mechanisms underlying discrimination-related health disparities. Telomere science may contribute to elucidate some of these aspects. Telomeres are protein-DNA complexes that shorten after cell division and are valuable markers of cellular aging. Short telomeres have been associated with the onset of age-related diseases. Evidence shows that chronic psychological stress may accelerate telomere shortening. Since discrimination can lead to psychological strain with cumulative impact on general health, we hypothesized that groups that report more discrimination show reduced telomere length (TL) as a consequence of psychosocial stress elevation. Through a systematic review of the literature we found 12 articles that met our criteria. Eligible studies measured racial, gender, unfair policing, and multiple forms of discrimination in association with TL. Our review showed mixed results, suggesting that there is weak evidence of a main association between discrimination and TL. However, discrimination may interact with several variables (such as depressive symptoms, acculturation, higher socioeconomic status, internalization of negative racial bias, and not discussing discrimination experiences with others) and contribute to shorten telomeres. Discrimination is a complex social construct composed of a vast sum of experiences, impressions, and contexts that in combination with other sources of stress may have an impact on TL. Telomeres may be a plausible pathway to investigate health discrepancies in discriminated groups in society, but more evidence is needed to investigate the potential harm of discrimination on cells.

Comparable Effects of the Androgen Derivatives Danazol, Oxymetholone and Nandrolone on Telomerase Activity in Human Primary Hematopoietic Cells from Patients with Dyskeratosis Congenita

Dyskeratosis congenita (DKC) is a rare inherited disease of impaired telomere maintenance that progressively leads to multi-organ failure, including the bone marrow. By enhancing telomerase activity, androgen derivatives (ADs) are a potential therapeutic option able to re-elongate previously shortened telomeres. Danazol, oxymetholone, and nandrolone are ADs most frequently used to treat DKC. However, no direct in vitro analyses comparing the efficacy of these ADs have been conducted so far. We therefore treated mononuclear cells derived from peripheral blood and bone marrow of four patients with mutations in telomerase reverse transcriptase (TERT, n = 1),in the telomerase RNA component (TERC, n = 2) and in dyskerin pseudouridine synthase 1 (DKC1, n = 1) and found no substantial differences in the activity of these three agents in patients with TERC/TERT mutations. All AD studied produced comparable improvements of proliferation rates as well as degrees of telomere elongation. Increased TERT expression levels were shown with danazol and oxymetholone. The beneficial effects of all ADs on proliferation of bone marrow progenitors could be reversed by tamoxifen, an estrogen antagonist abolishing estrogen receptor-mediated TERT expression, thereby underscoring the involvement of TERT in AD mechanism of action. In conclusion, no significant differences in the ability to functionally enhance telomerase activity could be observed for the three AD studied in vitro. Physicians therefore might choose treatment based on patients’ individual co-morbidities, e.g., pre-existing liver disease and expected side-effects.

Unraveling Hematopoiesis through the Lens of Genomics

Hematopoiesis has long served as a paradigm of stem cell biology and tissue homeostasis. In the past decade, the genomics revolution has ushered in powerful new methods for investigating the hematopoietic system that have provided transformative insights into its biology. As part of the advances in genomics, increasingly accurate deep sequencing and novel methods of cell tracking have revealed hematopoiesis to be more of a continuous and less of a discrete and punctuated process than originally envisioned. In part, this continuous nature of hematopoiesis is made possible by the emergent outcomes of vast, interconnected regulatory networks that influence cell fates and lineage commitment. It is also becoming clear how these mechanisms are modulated by genetic variation present throughout the population. This review describes how these recently uncovered complexities are reshaping our concept of tissue development and homeostasis while opening up a more comprehensive future understanding of hematopoiesis.

The Source and Dynamics of Adult Hematopoiesis: Insights from Lineage Tracing

The generation of all blood cell lineages (hematopoiesis) is sustained throughout the entire life span of adult mammals. Studies using cell transplantation identified the self-renewing, multipotent hematopoietic stem cells (HSCs) as the source of hematopoiesis in adoptive hosts and delineated a hierarchy of HSC-derived progenitors that ultimately yield mature blood cells. However, much less is known about adult hematopoiesis as it occurs in native hosts, i.e., without transplantation. Here we review recent advances in our understanding of native hematopoiesis, focusing in particular on the application of genetic lineage tracing in mice. The emerging evidence has established HSCs as the major source of native hematopoiesis, helped to define the kinetics of HSC differentiation, and begun exploring native hematopoiesis in stress conditions such as aging and inflammation. Major outstanding questions about native hematopoiesis still remain, such as its clonal composition, the nature of lineage commitment, and the dynamics of the process in humans.

Mathematical modelling of telomere length dynamics

Telomeres are repetitive DNA sequences located at the ends of chromosomes. During cell division, an incomplete copy of each chromosome's DNA is made, causing telomeres to shorten on successive generations. When a threshold length is reached replication ceases and the cell becomes 'senescent'. In this paper, we consider populations of telomeres and, from discrete models, we derive partial differential equations which describe how the distribution of telomere lengths evolves over many generations. We initially consider a population of cells each containing just a single telomere. We use continuum models to compare the effects of various mechanisms of telomere shortening and rates of cell division during normal ageing. For example, the rate (or probability) of cell replication may be fixed or it may decrease as the telomeres shorten. Furthermore, the length of telomere lost on each replication may be constant, or may decrease as the telomeres shorten. Where possible, explicit solutions for the evolution of the distribution of telomere lengths are presented. In other cases, expressions for the mean of the distribution are derived. We extend the models to describe cell populations in which each cell contains a distinct subpopulation of chromosomes. As for the simpler models, constant telomere shortening leads to a linear reduction in telomere length over time, whereas length-dependent shortening results in initially rapid telomere length reduction, slowing at later times. Our analysis also reveals that constant telomere loss leads to a Gaussian (normal) distribution of telomere lengths, whereas length-dependent loss leads to a log-normal distribution. We show that stochastic models, which include a replication probability, also lead to telomere length distributions which are skewed.

Clones assemble! The clonal complexity of blood during ontogeny and disease

Hematopoietic stem and progenitor cells (HSPCs) govern the daily expansion and turnover of billions of specialized blood cells. Given their clinical utility, much effort has been made toward understanding the dynamics of hematopoietic production from this pool of stem cells. An understanding of hematopoietic stem cell clonal dynamics during blood ontogeny could yield important insights into hematopoietic regulation, especially during aging and repeated exposure to hematopoietic stress-insults that may predispose individuals to the development of hematopoietic disease. Here, we review the current state of research regarding the clonal complexity of the hematopoietic system during embryogenesis, adulthood, and hematologic disease.

Reproductive history and blood cell telomere length

Telomeres are repetitive nucleotide sequences that protect against chromosomal shortening. They are replenished by telomerase, an enzyme that may be activated by estrogen. Women have longer telomeres than men; this difference might be due to estrogen exposure. We hypothesized that reproductive histories reflecting greater estrogen exposure will be associated with longer blood cell telomeres. Among women in the Sister Study (n= 1,048), we examined telomere length in relation to self-reported data on reproductive history. The difference between age at menarche and last menstrual period was used to approximate the reproductive period. Relative telomere length (rTL) was measured using qPCR. After adjustment, rTL decreased with longer reproductive period (β= -0.019, 95% CI: -0.04, -0.00, p= 0.03). Premenopausal women had shorter rTL than postmenopausal women (β= -0.051, 95% CI: -0.12, 0.01, p= 0.13). Longer breastfeeding duration was associated with longer rTL (β= 0.027, 95% CI: 0.01, 0.05, p=0.01); increasing parity was associated with shorter rTL (β = -0.016, 95% CI: -0.03, 0.00, p=0.07). Duration of exogenous hormone use was not associated with rTL. Reproductive histories reflecting greater endogenous estrogen exposure were associated with shorter rTL. Our findings suggest that longer telomeres in women are unlikely to be explained by greater estrogen exposure.

Comparison of flow-FISH and MM-qPCR telomere length assessment techniques for the screening of telomeropathies

Assessment of telomere length (TL) in peripheral blood leukocytes is part of the diagnostic algorithm applied to patients with acquired bone marrow failure syndromes (BMFSs) and dyskeratosis congenita (DKC). Monochrome multiplex-quantitative polymerase chain reaction (MM-qPCR) and fluorescence in situ hybridization (flow-FISH) are methodologies available for TL screening. Dependent on TL expressed in relation to percentiles of healthy controls, further genetic testing for inherited mutations in telomere maintenance genes is recommended. However, the correct threshold to trigger this genetic workup is still under debate. Here, we prospectively compared MM-qPCR and flow-FISH regarding their capacity for accurate identification of DKC patients. All patients (n = 105) underwent genetic testing by next-generation sequencing and in 16 patients, mutations in DKC-relevant genes were identified. Whole leukocyte TL of patients measured by MM-qPCR was found to be moderately correlated with lymphocyte TL measured by flow-FISH (r² = 0.34; P < 0.0001). The sensitivity of both methods was high, but the specificity of MM-qPCR (29%) was significantly lower compared with flow-FISH (58%). These results suggest that MM-qPCR of peripheral blood cells is inferior to flow-FISH for clinical routine screening for suspected DKC in adult patients with BMFS due to lower specificity and a higher rate of false-positive results.

Replicative cellular age distributions in compartmentalized tissues

The cellular age distribution of hierarchically organized tissues can reveal important insights into the dynamics of cell differentiation and self-renewal and associated cancer risks. Here, we examine the effect of progenitor compartments with varying differentiation and self-renewal capacities on the resulting observable distributions of replicative cellular ages. We find that strongly amplifying progenitor compartments, i.e. compartments with high self-renewal capacities, substantially broaden the age distributions which become skewed towards younger cells with a long tail of few old cells. For several of these strongly amplifying compartments, the age distribution becomes virtually independent of the influx from the stem cell compartment. By contrast, if tissues are organized into many downstream compartments with low self-renewal capacity, the shape of the replicative cell distribution in more differentiated compartments is dominated by stem cell dynamics with little added variation. In the limiting case of a strict binary differentiation tree without self-renewal, the shape of the output distribution becomes indistinguishable from that of the input distribution. Our results suggest that a comparison of cellular age distributions between healthy and cancerous tissues may inform about dynamical changes within the hierarchical tissue structure, i.e. an acquired increased self-renewal capacity in certain tumours. Furthermore, we compare our theoretical results to telomere length distributions in granulocyte populations of 10 healthy individuals across different ages, highlighting that our theoretical expectations agree with experimental observations.

In vivo dynamics of human hematopoietic stem cells: novel concepts and future directions

Unveiling the mechanisms and the cellular dynamics at the basis of human hematopoietic homeostasis has been a main focus for the scientific community since the discovery of a pool of multipotent hematopoietic stem cells (HSCs) capable of sustaining the hematopoietic output throughout life and after transplantation. Recently, new works shed light on the (1) differentiation paths, (2) size and replication rate of human HSC population at steady state, and (3) role of the distinct subpopulations comprising the hematopoietic stem and progenitor cell reservoir after transplantation. These papers exploited cutting-edge technologies, including vector integration site clonal tracking, spontaneous mutations, and deep transcriptome profiling. Here we discuss the latest updates in human hematopoietic system biology and in vivo dynamics, highlighting novel concepts and common findings deriving from different approaches and the future directions of these studies. Taken together, this information contributed to partially resolving the complexity of the in vivo HSC behavior and has major implications for HSC transplantation and gene therapy as well as for the development of future therapies.

Effects of continuous high-dose G-CSF administration on hematopoietic stem cell mobilization and telomere length in patients with amyotrophic lateral sclerosis - a pilot study

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease of complex and still poorly understood etiology. Loss of upper and lower motoneurons results in death within few years after diagnosis. Recent studies have proposed neuroprotective and disease-slowing effects of granulocyte-colony stimulating factor (G-CSF) treatment in ALS mouse models as well as humans. In this study, six ALS patients were monitored up to 3.5 years during continuous high-dose G-CSF administration. Repetitive analyses were performed including blood count parameters, CD34⁺ hematopoietic stem and progenitor cell (HSPC) and colony forming cell (CFC) counts, serum cytokine levels and leukocyte telomere length. We demonstrate that continuous G-CSF therapy was well tolerated and safe resulting in only mild adverse events during the observation period. However, no mobilization of CD34⁺ HSPC was detected as compared to baseline values. CFC mobilization was equally low and even a decrease of myeloid precursors was observed in some patients. Assessment of telomere length within ALS patients' leukocytes revealed that G-CSF did not significantly shorten telomeres, while those of ALS patients were shorter compared to age-matched healthy controls, irrespective of G-CSF treatment. During G-CSF stimulation, TNF-alpha, CRP, IL-16, sVCAM-1, sICAM-1, Tie-2 and VEGF were significantly increased in serum whereas MCP-1 levels decreased. In conclusion, our data show that continuous G-CSF treatment fails to increase circulating CD34⁺ HSPC in ALS patients. Cytokine profiles revealed G-CSF-mediated immunomodulatory and proteolytic effects. Interestingly, despite intense G-CSF stimulation, telomere length was not significantly shortened.

Telomere shortening correlates with leukemic stem cell burden at diagnosis of chronic myeloid leukemia

Telomere length (TL) in peripheral blood (PB) cells of patients with chronic myeloid leukemia (CML) has been shown to correlate with disease stage, prognostic scores, response to therapy, and disease progression. However, due to considerable genetic interindividual variability, TL varies substantially between individuals, limiting its use as a robust prognostic marker in individual patients. Here, we compared TL of BCR-ABL^-, nonleukemic CD34⁺CD38^- hematopoietic stem cells (HSC) in the bone marrow of CML patients at diagnosis to their individual BCR-ABL⁺ leukemic stem cell (LSC) counterparts. We observed significantly accelerated telomere shortening in LSC compared with nonleukemic HSC. Interestingly, the degree of LSC telomere shortening was found to correlate significantly with the leukemic clone size. To validate the diagnostic value of nonleukemic cells as internal controls and to rule out effects of tyrosine kinase inhibitor (TKI) treatment on these nontarget cells, we prospectively assessed TL in 134 PB samples collected in deep molecular remission after TKI treatment within the EURO-SKI study (NCT01596114). Here, no significant telomere shortening was observed in granulocytes compared with an age-adjusted control cohort. In conclusion, this study provides proof of principle for accelerated telomere shortening in LSC as opposed to HSC in CML patients at diagnosis. The fact that the degree of telomere shortening correlates with leukemic clone's size supports the use of TL in leukemic cells as a prognostic parameter pending prospective validation. TL in nonleukemic myeloid cells seems unaffected even by long-term TKI treatment arguing against a reduction of telomere-mediated replicative reserve in normal hematopoiesis under TKI treatment.

The individual's signature of telomere length distribution

Mean telomere length in human leukocyte DNA samples reflects the different lengths of telomeres at the ends of the 23 chromosomes and in an admixture of cells. However, only rudimentary information is available regarding the distribution of telomere lengths in all chromosomes and the different cell types in leukocyte samples. Understanding the configuration of leukocyte telomere length distribution (LTLD) could be helpful in capturing intrinsic elements that are not provided by the mean leukocyte telomere length (mLTL). The objective of this study was to analyse LTLD and its temporal variation in adults. Leukocyte samples were donated on two occasions (8 years apart) by 72 participants in the ADELAHYDE study. Telomere length was measured by Southern blotting of the terminal restriction fragments. Individuals with comparable mLTLs displayed different shapes of LTLDs. Inter-individual variation in LTLD shape was much larger than intra-individual variation in LTLD shape between baseline and follow-up leukocyte samples. These results show an important individual stability of LTLD shape over time indicating that each individual has a characteristic LTLD signature.

Modelling of the SDF-1/CXCR4 regulated in vivo homing of therapeutic mesenchymal stem/stromal cells in mice

Background

Mesenchymal stem/stromal cells (MSCs) are a promising tool for cell-based therapies in the treatment of tissue injury. The stromal cell-derived factor-1 (SDF-1)/CXC chemokine receptor 4 (CXCR4) axis plays a significant role in directing MSC homing to sites of injury. However in vivo MSC distribution following intravenous transplantation remains poorly understood, potentially hampering the precise prediction and evaluation of therapeutic efficacy.

Methods

A murine model of partial ischemia/reperfusion (I/R) is used to induce liver injury, increase the hepatic levels of SDF-1, and study in vivo MSC distribution. Hypoxia-preconditioning increases the expression of CXCR4 in human bone marrow-derived MSCs. Quantitative assays for human DNA using droplet digital PCR (ddPCR) allow us to examine the in vivo kinetics of intravenously infused human MSCs in mouse blood and liver. A mathematical model-based system is developed to characterize in vivo homing of human MSCs in mouse models with SDF-1 levels in liver and CXCR4 expression on the transfused MSCs. The model is calibrated to experimental data to provide novel estimates of relevant parameter values.

Results

Images of immunohistochemistry for SDF-1 in the mouse liver with I/R injury show a significantly higher SDF-1 level in the I/R injured liver than that in the control. Correspondingly, the ddPCR results illustrate a higher MSC concentration in the I/R injured liver than the normal liver. CXCR4 is overexpressed in hypoxia-preconditioned MSCs. An increased number of hypoxia-preconditioned MSCs in the I/R injured liver is observed from the ddPCR results. The model simulations align with the experimental data of control and hypoxia-preconditioned human MSC distribution in normal and injured mouse livers, and accurately predict the experimental outcomes with different MSC doses.

Discussion

The modelling results suggest that SDF-1 in organs is an effective in vivo attractant for MSCs through the SDF-1/CXCR4 axis and reveal the significance of the SDF-1/CXCR4 chemotaxis on in vivo homing of MSCs. This in vivo modelling approach allows qualitative characterization and prediction of the MSC homing to normal and injured organs on the basis of clinically accessible variables, such as the MSC dose and SDF-1 concentration in blood. This model could also be adapted to abnormal conditions and/or other types of circulating cells to predict in vivo homing patterns.

A stochastic model of myeloid cell lineages in hematopoiesis and pathway mutations in acute myeloid leukemia

A model for hematopoiesis is presented that explicitly includes the erythrocyte, granulocyte, and thrombocyte lineages and their common precursors. A small number of stem cells proliferate and differentiate through different compartments to produce the vast number of blood cells needed every day. Growth factors regulate the proliferation of cells dependent on the current demand. We provide a steady state analysis of the model and rough parameter estimates. Furthermore, we extend the model to include mutations that alter the replicative capacity of cells and introduce differentiation blocks. With these mutations the model develops signs of acute myeloid leukemia.