Stromal niche inflammation mediated by IL-1 signalling is a targetable driver of haematopoietic ageing.

Haematopoietic ageing is marked by a loss of regenerative capacity and skewed differentiation from haematopoietic stem cells (HSCs), leading to impaired blood production. Signals from the bone marrow niche tailor blood production, but the contribution of the old niche to haematopoietic ageing remains unclear. Here we characterize the inflammatory milieu that drives both niche and haematopoietic remodelling. We find decreased numbers and functionality of osteoprogenitors at the endosteum and expansion of central marrow LepR+ mesenchymal stromal cells associated with deterioration of the sinusoidal vasculature. Together, they create a degraded and inflamed old bone marrow niche. Niche inflammation in turn drives the chronic activation of emergency myelopoiesis pathways in old HSCs and multipotent progenitors, which promotes myeloid differentiation and hinders haematopoietic regeneration. Moreover, we show how production of interleukin-1ß (IL-1ß) by the damaged endosteum acts in trans to drive the proinflammatory nature of the central marrow, with damaging consequences for the old blood system. Notably, niche deterioration, HSC dysfunction and defective regeneration can all be ameliorated by blocking IL-1 signalling. Our results demonstrate that targeting IL-1 as a key mediator of niche inflammation is a tractable strategy to improve blood production during ageing.

TEAD Proteins Associate With DNA Repair Proteins to Facilitate Cellular Recovery From DNA Damage.

Transcriptional enhanced associate domain family members 1 to 4 (TEADs) are a family of four transcription factors and the major transcriptional effectors of the Hippo pathway. In order to activate transcription, TEADs rely on interactions with other proteins, such as the transcriptional effectors Yes-associated protein and transcriptional co-activator with PDZ-binding motif. Nuclear protein interactions involving TEADs influence the transcriptional regulation of genes involved in cell growth, tissue homeostasis, and tumorigenesis. Clearly, protein interactions for TEADs are functionally important, but the full repertoire of TEAD interaction partners remains unknown. Here, we employed an affinity purification mass spectrometry approach to identify nuclear interacting partners of TEADs. We performed affinity purification mass spectrometry experiment in parallel in two different cell types and compared a wildtype TEAD bait protein to a nuclear localization sequence mutant that does not localize to the nucleus. We quantified the results using SAINT analysis and found a significant enrichment of proteins linked to DNA damage including X-ray repair cross-complementing protein 5 (XRCC5), X-ray repair cross-complementing protein 6 (XRCC6), poly(ADP-ribose) polymerase 1 (PARP1), and Rap1-interacting factor 1 (RIF1). In cellular assays, we found that TEADs co-localize with DNA damage-induced nuclear foci marked by histone H2AX phosphorylated on S139 (γH2AX) and Rap1-interacting factor 1. We also found that depletion of TEAD proteins makes cells more susceptible to DNA damage by various agents and that depletion of TEADs promotes genomic instability. Additionally, depleting TEADs dampens the efficiency of DNA double-stranded break repair in reporter assays. Our results connect TEADs to DNA damage response processes, positioning DNA damage as an important avenue for further research of TEAD proteins.

Aged hematopoietic stem cells are refractory to bloodborne systemic rejuvenation interventions.

While young blood can restore many aged tissues, its effects on the aged blood system itself and old hematopoietic stem cells (HSCs) have not been determined. Here, we used transplantation, parabiosis, plasma transfer, exercise, calorie restriction, and aging mutant mice to understand the effects of age-regulated systemic factors on HSCs and their bone marrow (BM) niche. We found that neither exposure to young blood, nor long-term residence in young niches after parabiont separation, nor direct heterochronic transplantation had any observable rejuvenating effects on old HSCs. Likewise, exercise and calorie restriction did not improve old HSC function, nor old BM niches. Conversely, young HSCs were not affected by systemic pro-aging conditions, and HSC function was not impacted by mutations influencing organismal aging in established long-lived or progeroid genetic models. Therefore, the blood system that carries factors with either rejuvenating or pro-aging properties for many other tissues is itself refractory to those factors.

Replication stress is a potent driver of functional decline in ageing haematopoietic stem cells.

Haematopoietic stem cells (HSCs) self-renew for life, thereby making them one of the few blood cells that truly age. Paradoxically, although HSCs numerically expand with age, their functional activity declines over time, resulting in degraded blood production and impaired engraftment following transplantation. While many drivers of HSC ageing have been proposed, the reason why HSC function degrades with age remains unknown. Here we show that cycling old HSCs in mice have heightened levels of replication stress associated with cell cycle defects and chromosome gaps or breaks, which are due to decreased expression of mini-chromosome maintenance (MCM) helicase components and altered dynamics of DNA replication forks. Nonetheless, old HSCs survive replication unless confronted with a strong replication challenge, such as transplantation. Moreover, once old HSCs re-establish quiescence, residual replication stress on ribosomal DNA (rDNA) genes leads to the formation of nucleolar-associated γH2AX signals, which persist owing to ineffective H2AX dephosphorylation by mislocalized PP4c phosphatase rather than ongoing DNA damage. Persistent nucleolar γH2AX also acts as a histone modification marking the transcriptional silencing of rDNA genes and decreased ribosome biogenesis in quiescent old HSCs. Our results identify replication stress as a potent driver of functional decline in old HSCs, and highlight the MCM DNA helicase as a potential molecular target for rejuvenation therapies.

Resilient and resourceful: genome maintenance strategies in hematopoietic stem cells.

Blood homeostasis is maintained by a rare population of quiescent hematopoietic stem cells (HSCs) that self-renew and differentiate to give rise to all lineages of mature blood cells. In contrast to most other blood cells, HSCs are preserved throughout life, and the maintenance of their genomic integrity is therefore paramount to ensure normal blood production and to prevent leukemic transformation. HSCs are also one of the few blood cells that truly age and exhibit severe functional decline in old organisms, resulting in impaired blood homeostasis and increased risk for hematologic malignancies. In this review, we present the strategies used by HSCs to cope with the many genotoxic insults that they commonly encounter. We briefly describe the DNA-damaging insults that can affect HSC function and the mechanisms that are used by HSCs to prevent, survive, and repair DNA lesions. We also discuss an apparent paradox in HSC biology, in which the genome maintenance strategies used by HSCs to protect their function in fact render them vulnerable to the acquisition of damaging genetic aberrations.

Learning from a paradox: recent insights into Fanconi anaemia through studying mouse models.

Fanconi anaemia (FA) is a rare autosomal recessive or X-linked inherited disease characterised by an increased incidence of bone marrow failure (BMF), haematological malignancies and solid tumours. Cells from individuals with FA show a pronounced sensitivity to DNA interstrand crosslink (ICL)-inducing agents, which manifests as G2-M arrest, chromosomal aberrations and reduced cellular survival. To date, mutations in at least 15 different genes have been identified that cause FA; the products of all of these genes are thought to function together in the FA pathway, which is essential for ICL repair. Rapidly following the discovery of FA genes, mutant mice were generated to study the disease and the affected pathway. These mutant mice all show the characteristic cellular ICL-inducing agent sensitivity, but only partially recapitulate the developmental abnormalities, anaemia and cancer predisposition seen in individuals with FA. Therefore, the usefulness of modelling FA in mice has been questioned. In this Review, we argue that such scepticism is unjustified. We outline that haematopoietic defects and cancer predisposition are manifestations of FA gene defects in mice, albeit only in certain genetic backgrounds and under certain conditions. Most importantly, recent work has shown that developmental defects in FA mice also arise with concomitant inactivation of acetaldehyde metabolism, giving a strong clue about the nature of the endogenous lesion that must be repaired by the functional FA pathway. This body of work provides an excellent example of a paradox in FA research: that the dissimilarity, rather than the similarity, between mice and humans can provide insight into human disease. We expect that further study of mouse models of FA will help to uncover the mechanistic background of FA, ultimately leading to better treatment options for the disease.

Fancf-deficient mice are prone to develop ovarian tumours.

Fanconi anaemia (FA) is a rare recessive disorder marked by developmental abnormalities, bone marrow failure, and a high risk for the development of leukaemia and solid tumours. The inactivation of FA genes, in particular FANCF, has also been documented in sporadic tumours in non-FA patients. To study whether there is a causal relationship between FA pathway defects and tumour development, we have generated a mouse model with a targeted disruption of the FA core complex gene Fancf. Fancf-deficient mouse embryonic fibroblasts displayed a phenotype typical for FA cells: they showed an aberrant response to DNA cross-linking agents as manifested by G(2) arrest, chromosomal aberrations, reduced survival, and an inability to monoubiquitinate FANCD2. Fancf homozygous mice were viable, born following a normal Mendelian distribution, and showed no growth retardation or developmental abnormalities. The gonads of Fancf mutant mice functioned abnormally, showing compromised follicle development and spermatogenesis as has been observed in other FA mouse models and in FA patients. In a cohort of Fancf-deficient mice, we observed decreased overall survival and increased tumour incidence. Notably, in seven female mice, six ovarian tumours developed: five granulosa cell tumours and one luteoma. One mouse had developed tumours in both ovaries. High-resolution array comparative genomic hybridization (aCGH) on these tumours suggests that the increased incidence of ovarian tumours correlates with the infertility in Fancf-deficient mice and the genomic instability characteristic of FA pathway deficiency.

Fancm-deficient mice reveal unique features of Fanconi anemia complementation group M.

The Fanconi anemia (FA) core complex member FANCM remodels synthetic replication forks and recombination intermediates. Thus far, only one FA patient with FANCM mutations has been described, but the relevance of these mutations for the FA phenotype is uncertain. To provide further experimental access to the FA-M complementation group we have generated Fancm-deficient mice by deleting exon 2. FANCM deficiency caused hypogonadism in mice and hypersensitivity to cross-linking agents in mouse embryonic fibroblasts (MEFs), thus phenocopying other FA mouse models. However, Fancm(Delta2/Delta2) mice also showed unique features atypical for FA mice, including underrepresentation of female Fancm(Delta2/Delta2) mice and decreased overall and tumor-free survival. This increased cancer incidence may be correlated to the role of FANCM in the suppression of spontaneous sister chromatid exchanges as observed in MEFs. In addition, FANCM appeared to have a stimulatory rather than essential role in FANCD2 monoubiquitination. The FA-M mouse model presented here suggests that FANCM functions both inside and outside the FA core complex to maintain genome stability and to prevent tumorigenesis.

Impaired FANCD2 monoubiquitination and hypersensitivity to camptothecin uniquely characterize Fanconi anemia complementation group M.

FANCM is a component of the Fanconi anemia (FA) core complex and one FA patient (EUFA867) with biallelic mutations in FANCM has been described. Strikingly, we found that EUFA867 also carries biallelic mutations in FANCA. After correcting the FANCA defect in EUFA867 lymphoblasts, a "clean" FA-M cell line was generated. These cells were hypersensitive to mitomycin C, but unlike cells defective in other core complex members, FANCM(-/-) cells were proficient in monoubiquitinating FANCD2 and were sensitive to the topoisomerase inhibitor camptothecin, a feature shared only with the FA subtype D1 and N. In addition, FANCM(-/-) cells were sensitive to UV light. FANCM and a C-terminal deletion mutant rescued the cross-linker sensitivity of FANCM(-/-) cells, whereas a FANCM ATPase mutant did not. Because both mutants restored the formation of FANCD2 foci, we conclude that FANCM functions in an FA core complex-dependent and -independent manner.

Identification of the Fanconi anemia complementation group I gene, FANCI.

To identify the gene underlying Fanconi anemia (FA) complementation group I we studied informative FA-I families by a genome-wide linkage analysis, which resulted in 4 candidate regions together encompassing 351 genes. Candidates were selected via bioinformatics and data mining on the basis of their resemblance to other FA genes/proteins acting in the FA pathway, such as: degree of evolutionary conservation, presence of nuclear localization signals and pattern of tissue-dependent expression. We found a candidate, KIAA1794 on chromosome 15q25-26, to be mutated in 8 affected individuals previously assigned to complementation group I. Western blots of endogenous FANCI indicated that functionally active KIAA1794 protein is lacking in FA-I individuals. Knock-down of KIAA1794 expression by siRNA in HeLa cells caused excessive chromosomal breakage induced by mitomycin C, a hallmark of FA cells. Furthermore, phenotypic reversion of a patient-derived cell line was associated with a secondary genetic alteration at the KIAA1794 locus. These data add up to two conclusions. First, KIAA1794 is a FA gene. Second, this gene is identical to FANCI, since the patient cell lines found mutated in this study included the reference cell line for group I, EUFA592.