Jean-Philippe Defour

Telomere shortening is a hallmark of aging, yet telomere length (TL) varies considerably among individuals and is strongly influenced by inheritance. In mice, efficient mitochondrial function-characterized by low reactive oxygen species (ROS) production-is critical for telomere elongation during early embryogenesis. Since mitochondrial DNA (mtDNA) encodes several subunits of the electron transport chain, it may influence TL at birth by regulating mitochondrial function in utero . To explore the relationship between mtDNA and TL, we used a transmitochondrial cybrid approach, introducing mitochondria from donor platelets with varying telomere lengths into mtDNA-depleted cells. This revealed an inverse correlation between donor blood TL and mitochondrial ROS levels measured in the resulting cybrids. Under the specific conditions of cybrid formation, which involve a metabolic shift from glycolysis to oxidative phosphorylation, mtDNA variants associated with reduced complex I (CI) activity induced rapid telomere shortening, further implicating mitochondrial metabolism in TL regulation. Notably, this effect was prevented by antioxidant treatment and supplementation with NAD⁺ precursors, supporting a critical role for CI in sustaining PARP1 activity through maintenance of the NAD⁺/NADH balance to preserve telomere integrity under acute oxidative stress. Collectively, these findings suggest that specific mtDNA variants may contribute to the maintenance of long telomeres in humans by enhancing mitochondrial fitness, thereby highlighting potential therapeutic opportunities for mitochondrial replacement strategies. Significance Statement: Telomere length at birth influences aging trajectories and disease risk later in life, yet the mechanisms governing this trait remain incompletely understood. Using a transmitochondrial cybrid approach, we show that single-nucleotide variants in the mitochondrial genome of healthy donors directly affect mitochondrial metabolism and reactive oxygen species production. In addition, mitochondrial ROS levels measured in cybrids inversely correlate with blood cell telomere length in donors. Mitochondrial DNA variants associated with reduced complex I activity promote telomere shortening during an i n vitro metabolic shift toward oxidative phosphorylation, an effect that is reversed by antioxidant treatment or NAD⁺ precursor supplementation. Together, these findings establish a direct link between mitochondrial genetics, redox homeostasis, and telomere maintenance in human cells.

Dimerization of the thrombopoietin receptor (TpoR) is necessary for receptor activation and downstream signaling through activated Janus kinase 2. We have shown previously that different orientations of the transmembrane (TM) helices within a receptor dimer can lead to different signaling outputs. Here we addressed the structural basis of activation for receptor mutations S505N and W515K that induce myeloproliferative neoplasms. We show using in vivo bone marrow reconstitution experiments that ligand-independent activation of TpoR by TM asparagine (Asn) substitutions is proportional to the proximity of the mutation to the intracellular membrane surface. Solid-state NMR experiments on TM peptides indicate a progressive loss of helical structure in the juxtamembrane (JM) R/KWQFP motif with either proximity of Asn substitutions to the cytosolic boundary or mutation of W515 in the motif. Mutational studies in the TpoR cytosolic JM region show that the helical loss by itself can induce activation, but is localized to a maximum of 6 amino acids downstream of W515, the rest of the region until Box 1 requiring helicity for receptor function. The constitutive activation of TpoR mutants S505N and W515K can be inhibited by rotation of TM helices within the TpoR dimer, which also restores helicity around W515. Together these data allow us to develop a general model for activation and to explain the critical role of the JM W515 residue in the regulation of receptor activity. (Total manuscript without References and Figure Legends 6507 words, main text (without methods and materials):)

Background Evaluation of an individual's thrombin-generating capacity enables a global assessment of the coagulation cascade and is therefore thought to better reflect the clotting function of blood. However, the lack of standardization still hampers the use in routine clinical practice. Methods Nineteen healthy subjects were sampled once a week for 5 consecutive weeks. Thrombin generation assay (TGA) was performed in duplicate by calibrated automated thrombogram (CAT) on platelet poor plasma with and without thrombomodulin. After exclusion of outliers, a nested analysis of variance (ANOVA) was performed to evaluate the biological variability (BV) results. Analytical variation (CVA), within-individual variation (CVI), between-individual variation (CVG), index of individuality (II), and reference change value (RCV) were calculated. Results All parameters taken together, the CVA, CVI, and CVG without TM, ranged from 2.8% to 6.5%, from 4.1% to 13.3% and from 10.4% to 28.4%, respectively. For TG with TM, CVI and CVG were higher and ranged from 5.0% to 18.1% and from 14.9% to 35.3%, respectively. For endogenous thrombin potential (ETP), a CVI of 4.1% and CVG of 10.4% were obtained without addition of thrombomodulin (TM). With addition of TM, both CVI and CVG were higher: 14.0% and 34.8%, respectively. The II was low and the RCV ranged from 17.2% to 50.4%. Conclusion CAT parameters are highly individualized and population-based reference values could be called into question. The assessment of BV and RCV for thrombin generation assays could optimize interpretation of serial patient results and guide setting of analytical specification goals.

Mutations in the gene (MPL) encoding the human thrombopoietin receptor (TpoR) drive sporadic and familial essential thrombocythemia (ET). We identified two ET patients that harbor double mutations in cis in MPL, namely L498W-H499C and H499Y-S505N. Using biochemical and signaling assays along with partial saturation mutagenesis we show that L498W is an activating mutation potentiated by H499C, and that H499C/Y enhance the activity of the canonical S505N mutation. L498W and H499C can activate a truncated TpoR mutant, which lacks the extracellular domain, indicating these mutations act on the transmembrane (TM)-cytosolic domain. Using a protein complementation assay we show that L498W and H499C strongly drive dimerization of TpoR. Activation by tryptophan substitution is exquisitely specific for position 498. Using structure-guided mutagenesis we identify upstream amino acid W491 as a key residue required for activation by L498W or canonical activating mutations such as S505N and W515K, as well as by eltrombopag. Structural data point to a common dimerization and activation path for TpoR via its TM domain that is shared between the small molecule agonist eltrombopag and canonical and novel activating TpoR mutations that all depend on W491, a potentially accessible extracellular residue that could become a target for therapeutic intervention.

Introduction: The presence of high fluorescent cells (HF‐BF) on the Sysmex XN‐1000 hematology analyzers has gained interest regarding the prediction of malignant cells in body fluids, but lacks sensitivity. We aimed to increase this sensitivity by combining HF‐BF value, automated results, and clinical information.Methods: We evaluated a new workflow for the management of body fluids in the hematology laboratory, including the HF‐BF criterion and clinical information. In two laboratories, 1623 serous fluids were retrospectively analyzed on the XN‐1000 BF mode. All samples were morphologically screened for malignant cells. Optimal HF‐BF cutoffs were determined to predict their presence. Thereafter, the added value of clinical information was evaluated. Other reflex testing rules (eosinophilic count >5% and presence of the WBC Abnormal Scattergram flag) were also used to refine our workflow.Results: Optimal HF‐BF cutoffs in the two hematology centers were 108 and 45 cells/µL, yielding a sensitivity/specificity of 66.7/93.6% and 86.8/66.6% for malignant cell detection. When adding clinical information, sensitivity/specificity evolved to 100.0/68.9% and 100.0%/not determined. Of 104 samples containing malignant cells, 97 had positive clinical information; the remainder had a HF‐BF > cutoff.Conclusion: Combining clinical information and HF‐BF reached 100% sensitivity for malignant cell detection in body fluid analysis. Lack of robustness of the optimal HF‐BF cutoff deserves the use of local cutoffs. Rapid automated results reporting from the XN‐1000 BF mode are also feasible in clinical practice. Prospective evaluation of the workflow is needed before its implementation in clinical practice.

Haemophagocytic lymphohistiocytosis (HLH) is a life-threatening disorder characterized by hyperimmune response. The mortality is high despite progress being made in the diagnosis and treatment of the disease. This review aimed to update knowledge on adult HLH pathophysiology, identify the numerous causes and help clinicians make early diagnosis and initiate treatment. Using Embase, we searched relevant articles published from 1 January 2010 to 31 October 2019, with the MESH term ‘haemophagocytic lymphohistiocytosis; macrophagic activation syndrome, adult’. The mean age at presentation is about 50 years, with a male predominance. The most frequent disease associations are haematological diseases, viral or bacterial infections and autoimmune diseases. The pathophysiologic mechanism is probably the combination of inherited genetic mutations and extrinsic triggers. The mortality rate is 26.5–74.8%. HScore is more efficient than HLH-2004 criteria to identify HLH, with diagnostic sensitivity and specificity 90% and 79%, respectively. 18-fluorodeoxyglucose positron emission tomography–computed tomography is potentially useful for detecting underlying disease and the extent of secondary HLH. Disease-specific treatment should be given as soon as possible. Treatment with corticosteroids combined or not with etoposide is the mainstay of treatment. Monoclonal antibodies and JAK pathway inhibitors show promise of being effective. In adult HLH, infectious diseases, autoimmune disease and malignancy should be suspected so that disease-specific treatment can be given promptly. Treatment with corticosteroids combined or not with etoposide is the mainstay of treatment, but new therapies show promise of being effective.

Frameshifting mutations (−1/+2) of the calreticulin (CALR) gene are responsible for the development of essential thrombocythemia (ET) and primary myelofibrosis (PMF). The mutant CALR proteins activate the thrombopoietin receptor (TpoR) inducing cytokine-independent megakaryocyte progenitor proliferation. Here, we generated via CRISPR/Cas9 technology two knock-in mouse models that are heterozygous for a type-I murine Calr mutation. These mice exhibit an ET phenotype with elevated circulating platelets compared with wild-type controls, consistent with our previous results showing that murine CALR mutants activate TpoR. We also show that the mutant CALR proteins can be detected in plasma. The phenotype of Calr del52 is transplantable, and the Calr mutated hematopoietic cells have a slow-rising advantage over wild-type hematopoiesis. Importantly, a homozygous state of a type-1 Calr mutation is lethal at a late embryonic development stage, showing narrowed ventricular myocardium walls, similar to the murine Calr knockout phenotype, pointing to the C terminus of CALR as crucial for heart development.

Calreticulin (CALR) +1 frameshift mutations in exon 9 are prevalent in myeloproliferative neoplasms. Mutant CALRs possess a new C-terminal sequence rich in positively charged amino acids, leading to activation of the thrombopoietin receptor (TpoR/MPL). We show that the new sequence endows the mutant CALR with rogue chaperone activity, stabilizing a dimeric state and transporting TpoR and mutants thereof to the cell-surface in states that would not pass quality control, and this function is absolutely required for oncogenic transformation. Mutant CALRs determine traffic via the secretory pathway of partially immature TpoR, as they protect N117-linked glycans from further processing in the Golgi. A number of engineered or disease-associated TpoRs, such as TpoR/MPL R102P, which causes congenital thrombocytopenia are rescued for traffic and function by mutant CALRs, which can also overcome endoplasmic reticulum retention signals on TpoR. Besides requiring N-glycosylation of TpoR, mutant CALRs require a hydrophobic patch located in the extracellular domain of TpoR to induce TpoR thermal stability and initial intracellular activation, while full activation requires cell surface localization of TpoR. Thus, mutant CALRs are rogue chaperones for TpoR and traffic-defective TpoR mutants, a function required for the oncogenic effects. Key points CALR mutants rescue cell surface localization of traffic deficient TpoR including R102P that causes congenital thrombocytopenia. Oncogenic TpoR activation by CALR mutants requires interaction, stabilization and cell surface localization of the TpoR-CALR complex.

Neutropenia represents an important problem in patients with genetic deficiency in either the glucose-6-phosphate transporter of the endoplasmic reticulum (G6PT/SLC37A4) or G6PC3, an endoplasmic reticulum phosphatase homologous to glucose-6-phosphatase. While affected granulocytes show reduced glucose utilization, the underlying mechanism is unknown and causal therapies are lacking. Using a combination of enzymological, cell-culture, and in vivo approaches, we demonstrate that G6PT and G6PC3 collaborate to destroy 1,5-anhydroglucitol-6-phosphate (1,5AG6P), a close structural analog of glucose-6-phosphate and an inhibitor of low-K_M hexokinases, which catalyze the first step in glycolysis in most tissues. We show that 1,5AG6P is made by phosphorylation of 1,5-anhydroglucitol, a compound normally present in human plasma, by side activities of ADP-glucokinase and low-K_M hexokinases. Granulocytes from patients deficient in G6PC3 or G6PT accumulate 1,5AG6P to concentrations (∼3 mM) that strongly inhibit hexokinase activity. In a model of G6PC3-deficient mouse neutrophils, physiological concentrations of 1,5-anhydroglucitol caused massive accumulation of 1,5AG6P, a decrease in glucose utilization, and cell death. Treating G6PC3-deficient mice with an inhibitor of the kidney glucose transporter SGLT2 to lower their blood level of 1,5-anhydroglucitol restored a normal neutrophil count, while administration of 1,5-anhydroglucitol had the opposite effect. In conclusion, we show that the neutropenia in patients with G6PC3 or G6PT mutations is a metabolite-repair deficiency, caused by a failure to eliminate the nonclassical metabolite 1,5AG6P. Significance Neutropenia presents an important clinical problem in patients with G6PC3 or G6PT deficiency, yet why neutropenia occurs is unclear. We discovered that G6PC3 and G6PT collaborate to dephosphorylate a noncanonical metabolite (1,5-anhydroglucitol-6-phosphate; 1,5AG6P) which is produced when glucose-phosphorylating enzymes erroneously act on 1,5-anhydroglucitol, a food-derived polyol present in blood. In patients or mice with G6PC3 or G6PT deficiency, 1,5AG6P accumulates and inhibits the first step of glycolysis. This is particularly detrimental in neutrophils, since their energy metabolism depends almost entirely on glycolysis. Consistent with our findings, we observed that treatment with a 1,5-anhydroglucitol-lowering drug treats neutropenia in G6PC3-deficient mice. Our findings highlight that the elimination of noncanonical side products by metabolite-repair enzymes makes an important contribution to mammalian physiology.