La thrombopénie familiale avec prédisposition à la leucémie aiguë myéloïde (FPD/AML) est une pathologie rare caractérisée par une thrombocytopénie. La FPD/AML est causée par des mutations germinales dans le gène codant le facteur de transcription RUNX1. Ces mutations sont de type dominant négatif (DN), associées à un risque plus élevé de développer une leucémie, ou de type haploinsuffisance (HI) induisant une thrombocytopénie seule. Nous avons démontré une diminution presque complète de l’expression du répresseur transcriptionnel ZBTB1 dans les progéniteurs hématopoïétiques des patients porteurs de mutations DN. Le gène ZBTB1 pourrait être une cible directe de RUNX1, et pourrait contribuer à la dérégulation de la lymphopoïèse T conduisant à une prédisposition à la LAL-T.Nous avons identifié dans les cellules lymphocytaires murines le site de fixation de RUNX1 sur un enhanceur localisé à 270 kb en amont du promoteur de Zbtb1, et ce aux stades doubles négatifs pour les marqueurs CD4/CD8. Dans les stades plus matures (CD4+CD48+), cette fixation n’est pas observée. En utilisant des lignées lymphocytaires humaines représentant les différents stades doubles négatifs CD4/CD8 de la différenciation lymphocytaire, nous ne sommes pas arrivés à démontrer cette liaison suggérant qu’elle a lieu à un stade très précis et transitoire difficilement identifiable. Les souris KO Runx1 et KO Zbtb1 montrent un blocage de la lymphopoïèse T dès les stades les plus précoces de la maturation thymique. Nous voulions démontrer que la surexpression de Zbtb1 dans un contexte KO Runx1 aboutirait à un sauvetage du phénotype lymphocytaire. Pour cela, nous avons utilisé un modèle in vitro (culture des progéniteurs hématopoïétiques sur des lobes thymiques) et in vivo reposant sur la greffe de souris irradiées par des progéniteurs hématopoïétiques de souris KO Runx1 surexprimant Zbtb1. Le KO Runx1 incomplet et une quasi-absence de la prise de greffe dans les conditions de KO Runx1 ne nous ont pas permis de valider notre hypothèse. Cependant nous avons pu observer que ZBTB1 régule négativement le compartiment des cellules souches et la prise de greffe.Nous nous sommes aussi intéressés au phénotype mégacaryocytaire des souris KO Zbtb1. De manière intéressante, ces souris montrent in vivo un défaut du cycle cellulaire des mégacaryocytes, tandis que in vitro une diminution drastique de la différenciation mégacaryocytaire est observée suggérant ainsi une compensation du microenvironnement in vivo. De plus, nous avons montré une régulation négative directe de ZBTB1 par RUNX1 dans les mégacaryocytes humains. Dans la deuxième partie de ma thèse nous nous sommes intéressés au mécanisme d’induction de la leucémie chez un patient FPD/AML porteur de mutation de type DN (RUNX1R174Q), nous avons identifié une mutation additionnelle à une fréquence de 1% dans le gène TET2 ayant contribué à l’amplification du clone pré-leucémique. Actuellement nous étudions la coopération entre la mutation de RUNX1R174Q et le shTET2 in vivo en greffant des souris NSG avec des cellules progénitrices humaines CD34+ portant la mutation RUNX1 et le shTET2 qui mime la mutation perte de fonction de TET2P1962T observée chez le patient. Des résultats prometteurs montrent une prise de greffe primaire et secondaire plus importante dans les conditions RUNX1R174Q/shTET2 et shTET2 seul. Les expériences in vitro réalisées en parallèle montrent que la mutation de RUNX1R174Q induit des dommages à l’ADN alors que la diminution de l’expression de TET2 par shARN induit une prolifération augmentée des progéniteurs hématopoïétiques. L’addition des deux mutations pourrait ainsi conduire à l’acquisition de mutations additionnelles et à une transformation leucémique. / Familial platelet disorder with predisposition to acute myeloid leukaemia (FPD/AML) is a rare condition characterized by thrombocytopenia. FPD/AML is caused by germline mutations in the gene coding for the transcription factor RUNX1. These mutations are devided on dominant-negative (DN) mutations associated with a higher risk of developing leukaemia or haploinsufficiency (HI) mutations inducing thrombocytopenia alone.We have demonstrated an almost complete decrease in the expression of the transcriptional repressor ZBTB1 in hematopoietic progenitors of patients with DN-type mutations. ZBTB1 could be a direct target of RUNX1, and could contribute to deregulation of T lymphopoiesis, leading to a predisposition to T-ALL.In murine immature T lymphocytes (CD4-CD8- stages), we demonstrated a fixation of RUNX1 on an enhancer at 270 kb upstream of Zbtb1 promoter. This fixation is no longer observed in the more mature stages (CD4+CD8+). Using human lymphocyte cell lines representing the different CD4-CD8- differentiation stages, we have not been able to demonstrate this binding suggesting that it takes place at a very precise and transitory stage that is difficult to identify.The KO Runx1 and KO Zbtb1 mice show a blockade of T lymphopoiesis in the earliest stages of thymic maturation. We wanted to demonstrate that the overexpression of Zbtb1 in a KO Runx1 context would result at least in a partial rescue of the lymphocyte phenotype. For this we used an in vitro model (culture of hematopoietic progenitors on thymic lobes) and in vivo based on the grafting of irradiated mice with hematopoietic progenitors of KO Runx1 overexpressing Zbtb1. The incomplete KO Runx1 and the almost complete absence of engraftment in the KO Runx1 conditions did not allow us to validate our hypothesis. However, we observed that ZBTB1 negatively regulates the stem cell compartment and the engraftment capacity.We also studied the megakaryocytic phenotype of KO Zbtb1 mice. Interestingly, these mice show, in vivo, a megakaryocyte cell cycle defect; while in vitro a drastic decrease in megakaryocytic differentiation is observed suggesting an in vivo micro-environmental compensation. We also showed a direct negative regulation of ZBTB1 by RUNX1 in human megakaryoycytes.In the second part of my thesis, we investigated the mechanism of induction of leukemia in an FPD/AML patient with a DN-type mutation (RUNX1R174Q). We demonstrated an additional mutation at a frequency of 1% in TET2 gene, which contribute to the amplification of a preleucemic clone.Currently we are studying the cooperation between the RUNX1R174Q mutation and the shTET2 in vivo by grafting NSG mice with human CD34+ progenitor cells carrying RUNX1R174Q mutation and an shTET2, which mimics the loss of function of TET2 observed in the patient. Promising results show greater primary and secondary graft under RUNX1R174Q /shTET2 and shTET2 conditions. The in vitro experiments carried out, show that the mutation of RUNX1R174Q induces DNA damages, whereas the decrease in the expression of TET2 by shRNA induces an increased proliferation of hematopoietic progenitors. The addition of the two mutations could thus lead to the acquisition of additional mutations and to a leukemic transformation.

Les néoplasmes myéloprolifératifs (NMPs) classiques BCR-ABL négatifs regroupent la Polyglobulie de Vaquez, la Thrombocytémie Essentielle et la Myélofibrose Primaire. Ce sont des pathologies malignes clonales entraînées par la signalisation constitutive de la voie JAK2/STAT en raison de mutations somatiques acquises qui affectent trois gènes, JAK2, CALR et MPL. Il s’agit des mutations “motrices” de la maladie responsable du syndrome myéloprolifératif et du phénotype. Cependant CALR n’est pas une molécule de signalisation mais une chaperonne du réticulum endoplasmique. En utilisant des lignées dépendantes de facteurs de croissance soit murines (Ba/F3) soit humaines (UT-7), des cellules primaires de patients et des modèles murins nous avons montré que les mutants CALRdel52 et CALRins5 avaient acquis de nouvelles propriétés qui en font des molécules de signalisation en induisant: - une indépendance aux facteurs de croissance uniquement lorsque MPL, le récepteur de la thrombopoïétine est exprimé ; - une phosphorylation constitutive de JAK2, des STAT1, 3 et 5 et une activation faible des voies PI3K/AKT et ERK1/2, suggérant une activation de MPL/JAK2 par les mutants CALR différente de celle induite par JAK2V617F. De manière intéressante un mutant de la CALR ayant une délétion entière de l’exon 9 n’est pas transformant, suggérant que l’activité oncogénique est liée à la présence de la nouvelle séquence C-terminale. L’activation de JAK2 uniquement par MPL en présence des mutants CALR pourrait expliquer le phénotype mégacaryocytaire/plaquettaire de ces NMPs. Cette activation de MPL au contraire de celle exercée par JAK2V617F a lieu non seulement à la membrane mais aussi dans le cytoplasme.Les modèles murins ont montré que les mutants CALR étaient responsables de la maladie et que celle-ci était dépendante de MPL, validant les résultats obtenus sur les lignées.Nous avons également montré que contrairement à JAK2V617F, les mutants de la CALR induisent chez l’homme une dominance clonale très tôt au niveau du compartiment des cellules souches. L’ensemble de ces résultats contribue à une meilleure compréhension du rôle des mutations CALR dans les NMPs. La démonstration que les molécules mutées sont présentes à la surface cellulaire ouvre la voie à des immunothérapies ciblant le nouveau peptide C-terminal. / Classical BCR-ABL negative myeloproliferative neoplasms (MPNs) include three disorders: Polycythemia Vera, Essential Thrombocythemia and Primary Myelofibrosis. They are clonal malignant diseases driven by the constitutive JAK2/STAT signaling pathway due to acquired somatic mutations affecting three genes: JAK2, CALR and MPL. These are the "driver" mutations of the disease responsible of the myeloproliferation and of the disease phenotype. However, CALR is not a signaling molecule, but a chaperonne of the endoplasmic reticulum. Using murine (Ba/F3) and human (UT-7) cell lines dependent on growth factors and primary patient cells and mouse model, we have shown that the CALRdel52 and CALRins5 mutants have acquired new signaling properties and induce:- growth factor independence only when MPL, the thrombopoietin receptor, is expressed;- constitutive phosphorylation of JAK2, of STAT1, 3 and 5 and a low activation of the PI3K/AKT and ERK1/2 pathways, suggesting an activation of MPL/JAK2 by a different manner than JAK2V617F. Interestingly, a CALR mutant deleted for the entire exon 9 has not transformation properties suggesting that the oncogenic activity is related to the presence of the new C-terminal sequence. This JAK2 activation only by MPL in presence of CALR mutants could explain the megakaryocytic/platelet phenotype of these MPNs.The use of a mouse modeling using retroviral vectors and bone marrow transplantation has shown that CALRdel52 and ins5 were really the drivers of the disease and that in vivo the thrombocytosis was dependent of MPL validating the results obtained in vitro.In addition, we have shown that in human, CALR mutants induce a clonal dominance early in the stem cell compartment in ET. This is in sharp contrast with JAK2V617F in ET. Overall, these results contribute to a better comprehension of the role of CALR mutations in MPNs. Furthermore, the demonstration that the CALR mutants are expressed at the cell surface open the way to the development of new immunotherapy targetting the new C-terminus peptide.

Doctor of Philosophy (PhD) / Gene expression is known to be regulated at the level of transcription. Recently, however, there has been a growing realisation of the importance of gene regulation at the post-transcriptional level, namely at the level of pre-mRNA processing (5’ capping, splicing and polyadenylation), nuclear export, mRNA localisation and translation. Erythroid krüppel-like factor (Eklf) is the founding member of the Krüppel-like factor (Klf) family of transcription factors and plays an important role in erythropoiesis. In addition to its nuclear presence, Eklf was recently found to localise to the cytoplasm and this observation prompted us to examine whether this protein has a role as an RNA-binding protein, in addition to its well-characterised DNA-binding function. In this thesis we demonstrate that Eklf displays RNA-binding activity in an in vitro and in vivo context through the use of its classical zinc finger (ZF) domains. Furthermore, using two independent in vitro assays, we show that Eklf has a preference for A and U RNA homoribopolymers. These results represent the first description of RNA-binding by a member of the Klf family. We developed a dominant negative mutant of Eklf by expressing its ZF region in murine erythroleukaemia (MEL) cells. We used this to investigate the importance of this protein in haematopoietic lineage decisions by examining its effect on the multipotent K562 cell line. We provide evidence that Eklf appears to be critical not only for the promotion of erythropoiesis, but also for the inhibition of megakaryopoiesis.

Erythropoiesis, the process of producing mature erythrocytes from the haematopoietic stem cells (HSCs) that reside in the bone marrow, is tightly regulated at both the cell and molecular level by a well defined set of extracellular cytokine signals and intracellular transcription factors. Diseases affecting erythropoiesis are among the most commonly inherited conditions and result from disturbances to the cellular and molecular events that normally regulate this process. Erythroid Kruppel-like factor (EKLF/KLF1) is a transcription factor that is essential for erythropoiesis. EKLF is the founding member of the Kruppel-like factor family of transcription factors that bind to GC rich CACC-Box elements within gene promoters and activate transcription. The β-like globin genes are critical targets of EKLF through its binding at sites within the proximal promoters and the upstream locus control region (LCR) enhancer. Mice lacking EKLF die prior to birth by E16 with a phenotype that closely resembles the human disease thalassaemia. Thalassaemia is due to mutations in the α or β-globin genes, leading to globin chain imbalance, red cell destruction and ineffective erythropoiesis. However, restoration of expression of γ-globin (a β-like gene) failed to prevent embryonic lethality in EKLF knockout mice and suggested that additional target genes were critical to erythropoiesis. This thesis describes investigation into the transcriptional network of EKLF and an in depth analysis of previously uncharacterised phenotypes present in the EKLF knockout mouse. I have identified a suite of target genes for EKLF that include critical components of the cells cycle. I have also tested the hypothesis that EKLF is able to function in vivo as a tumour suppressor gene. Additionally, I report a role for EKLF in the determination of cell fate within the haematopoietic system and describe the development of a new approach to globally understanding erythroid transcription factor function. A previously performed microarray transcriptional profiling study provided a set of potential target genes for EKLF. I have expanded on this study by identifying that the cell cycle genes p18INK4c, and E2f2 are direct transcriptional targets of EKLF, where binding of EKLF occurs at the promoter and a novel intronic enhancer region, respectively. I have also described a previously undiscovered cell cycle phenotype of aberrant entry into S-phase in EKLF -/- erythroid cells that is directly related to abrogated expression of E2f2. The Kruppel-like factor family of genes have been implicated as players in the tumour process. By constructing a model for the loss of EKLF within HSCs in vivo, I have tested whether EKLF is functional as a tumour suppressor. The loss of EKLF in vivo was found to be insufficient to generate erythroleukaemia, however did result in erythroid hyperplasia, extramedullary haematopoieis and a mild macrocytic anaemia. In addition to regulation of erythropoiesis, EKLF performs a critical role in the lineage choice for a megakaryocyte-erythroid progenitor (MEP) between the megakaryocytic and erythroid lineages. This thesis describes that in the absence of EKLF, MEPs fail to commit properly to either lineage and proceed along a promiscuous pathway sharing the hallmarks of both megakaryocytes and erythroid cells. A detailed molecular mechanism for this phenotype remains undetermined, but is likely to involve interactions with the megakaryocyte transcription factor Fli1 and other members of the Kruppel-like factor family, such as BKLF (KLF3). While the transcriptional mechanisms that drive erythropoiesis have been slowly discovered, the development of chromatin immunoprecipitation (ChIP) assays and next generation DNA sequencing technology has presented the potential to rapidly enhance the progression of these studies. In this thesis I describe the development of ChIP-seq using Applied Biosystems SOLiD technology, an approach to rapidly identify binding sites for erythroid transcription factors in an unbiased genome wide approach. The work described in this thesis has expanded the transcriptional network of EKLF to include critical components of the cell cycle and has suggested many additional target genes from ChIP-seq requiring validation. As one of the major transcription factor players during erythropoiesis, EKLF performs many critical functions that include the regulation of the cell cycle, lineage selection and erythroid development. I suggest that current and future studies of EKLF function will influence our understanding of erythropoiesis and refine our understanding of human conditions such as thalassaemia and erythroleukaemia.

Doctor of Philosophy (PhD) / Gene expression is known to be regulated at the level of transcription. Recently, however, there has been a growing realisation of the importance of gene regulation at the post-transcriptional level, namely at the level of pre-mRNA processing (5’ capping, splicing and polyadenylation), nuclear export, mRNA localisation and translation. Erythroid krüppel-like factor (Eklf) is the founding member of the Krüppel-like factor (Klf) family of transcription factors and plays an important role in erythropoiesis. In addition to its nuclear presence, Eklf was recently found to localise to the cytoplasm and this observation prompted us to examine whether this protein has a role as an RNA-binding protein, in addition to its well-characterised DNA-binding function. In this thesis we demonstrate that Eklf displays RNA-binding activity in an in vitro and in vivo context through the use of its classical zinc finger (ZF) domains. Furthermore, using two independent in vitro assays, we show that Eklf has a preference for A and U RNA homoribopolymers. These results represent the first description of RNA-binding by a member of the Klf family. We developed a dominant negative mutant of Eklf by expressing its ZF region in murine erythroleukaemia (MEL) cells. We used this to investigate the importance of this protein in haematopoietic lineage decisions by examining its effect on the multipotent K562 cell line. We provide evidence that Eklf appears to be critical not only for the promotion of erythropoiesis, but also for the inhibition of megakaryopoiesis.

La protéine Bcl-xL fait partie de la famille des protéines anti-apoptotiques Bcl-2. Il a été montré que cette protéine avait un rôle majeur dans la formation des plaquettes chez la souris (mégacaryopoïèse). Une dérégulation de cette protéine pourrait aboutir à une altération de la mégacaryopoïèse et donner des pathologies humaines comme des thrombopénies chroniques. Une des causes de thrombopénies chroniques est le purpura thrombopénique immunologique (ou PTI), qui associe deux mécanismes physiopathologiques : une destruction auto-immune des plaquettes et une insuffisance de leur production par la moelle osseuse. Le PTI est un diagnostic d’exclusion par élimination de toutes les causes connues de thrombopénie. Au sein de notre cohorte de patients suivis en médecine interne pour cette maladie, nous avons identifié certains patients qui présentaient un profil non-immunologique, c’est-à-dire l’absence d’auto-immunité et une non réponse à tous les traitements immunomodulateurs, ou pas d’indication à un traitement compte tenu d’un taux de plaquettes suffisant. Nous montrons dans ce travail de thèse que Bcl-xL est nécessaire pour la survie du mégacaryocyte humain pendant toute la mégacaryopoïèse, à la différence de la souris. Par ailleurs, certains patients ont une altération intrinsèque de la formation des proplaquettes, et certains d’entre eux ont également une diminution de l’ARN messager et de la protéine Bcl-xL dans leurs plaquettes. Ces observations nouvelles suggèrent l’implication de Bcl-xL dans la physiopathologie de leur maladie et ouvrent la voie à l’identification d’une potentielle nouvelle cause de thrombopénie chronique. / The Bcl-xL protein is a member of Bcl-2 anti-apoptotic proteins. It has been shown in mouse that this protein had a major role in platelet production (megakaryopoiesis). Bcl-xL deregulation could lead to megakaryopoiesis impairement and explain some human diseases such as chronic thrombocytopenias. One cause of chronic thrombocytopenia is immune thrombocytopenia (ITP) that associates 2 pathophysiological mechanisms: an immune-mediated platelet destruction and an insufficient production from the bone marrow cells. ITP is a diagnosis of exclusion when all known causes of thrombocytopenia have been ruled out by diagnosis work-up. In ITP cohort of patients followed in our internal medicine department, we have identified some patients with a haematological profile of their disease, ie absence of overt features of auto-immunity, and absence of response to immunomudulatory treatments, or no indication to such treatment because of sufficient platelet count. We demonstrate in this study that Bcl-xL is necessary for megakaryocyte survival during all megakaryopoiesis, contrary to what was found in mouse. Moreover, some patients have an intrinsically impaired proplatelet formation, and some of them also have a decrease of Bcl-xL mRNA and protein in their platelets. These novel observations suggest that a deregulation of Bcl-xL is a possible cause of their disease and lead the way to the identification of a potentially new cause of chronic thrombocytopenia in human

Les plaquettes sanguines ont comme rôle principal d’arrêter les saignements. Elles sont produites dans la moelle osseuse par des mégacaryocytes (MK) qui proviennent de la différenciation des cellules souches hématopoïétiques (CSH). L’objectif de ma thèse a été d’identifier les éléments cellulaires du microenvironnement contrôlant la mégacaryopoïèse. Mon travail a permis d’identifier une population particulière de progéniteurs hépatiques du foie foetal capable de promouvoir in vitro les étapes précoces de la mégacaryopoïèse à partir de CSH humaines et murines (Brouard et al., 2017). Le rôle des cellules endothéliales (EC) dans les étapes tardives de maturation a été étudié après purification à partir de moelle humaine dans des expériences de co-culture avec des MK prédifférenciés. Mes résultats montrent que ces EC ont la propriété unique, par comparaison avec des EC d’autres tissus, de promouvoir la maturation des MK. Une analyse transcriptionnelle différentielle a permis d’identifier des effecteurs possibles ouvrant des pistes pour mieux comprendre les mécanismes de la mégacaryopoïèse et pour améliorer la production des plaquettes en culture. / The main role of platelets is to stop bleeding. They are produced in the bone marrow by megakaryocytes (MK) that are produced by the differentiation of hematopoietic stem cells (HSC). The objective of my thesis was to identify the cellular elements of the microenvironment controlling megakaryopoiesis. My work has identified a particular population of hepatic progenitors from the fetal liver capable of promoting in vitro the early stages of megakaryopoiesis from human and murine HSC (Brouard et al., 2017). The role of endothelial cells (EC), purified from human bone marrow, in late maturation stages was studied in co-culture experiments with predifferentiated MK. My results show that these EC have the unique property in comparison with EC from other tissues, of promoting the maturation of MK. A differential transcriptional analysis identified possible effectors that could lead to a better understanding of the mechanisms of megakaryopoiesis and improve platelet production in culture.

<p>Megakaryocyte cultures are a strong tool for the in vitro investigation of platelet production in platelet disorders. Peripheral blood derived hematopoietic progenitor cells (PB-HPCs) are the most accessible source of HPCs with high potential to produce mature megakaryocytes in vitro; however, they are present in low numbers making peripheral blood an inefficient source. Additionally, a megakaryocyte culture with an optimized thrombopoietin (TPO) concentration is required which can reliably allow the investigation of suppressive effects of antibodies/plasma from immune thrombocytopenia (ITP) patients. In this study, we developed a megakaryocyte culture with the utilization of human PB-HPCs in an efficient fashion resulting in the production of high purity megakaryocytes in a TPO-dependent manner.</p> <p>The mononuclear fraction was collected from 180 mL of peripheral whole blood and CD34+ cells were isolated by a positive selection yielding the average of 5.5 x 105 ± 2.5 x 105 CD34+ cells (n = 18). Using 96-well tissue-culture plates and seeding 10,000 CD34+ cells/well, the average of 13 experiments in triplicate can be set up utilizing isolated CD34+ in an efficient manner. Capitalizing on a TPO dose-dependent megakaryocyte production experiment, 20 ng/mL was established as the TPO concentration which resulted in the production of mature megakaryocytes without reaching the plateau in megakaryopoiesis response. On day 11 of culture, the expression of megakaryocytic lineage (CD41/61+) and maturation (CD41/61+CD42+) markers peaked at 90.65% and 76.10%. In conclusion, this culture system has broad application for investigation of platelet disorders and drug discovery which can be accessible to all researchers.</p> / Master of Science (MSc)

No description available.

L’objectif de cette thèse est d’étudier, à l’aide de modèles mathématiques, le mécanisme de régulation qui permet au corps de maintenir une quantité optimale de plaquettes sanguines. Le premier chapitre présente le contexte biologique et mathématique. Dans un second chapitre, un modèle pour la mégacaryopoïèse est introduit qui suppose une régulation ponctuelle par le nombre de plaquettes du taux de différentiation des cellules souches vers la lignée mégacaryocytaire et du nombre de plaquettes produites par mégacaryocyte. Nous montrons que la dynamique de ce modèle est régie par une équation différentielle à retard x'(t) = -?x(t)+f(x(t))g(x(t-t)), et nous obtenons ensuite de nouvelles conditions suffisantes pour la stabilité et l’oscillation des solutions de cette équation. Dans le troisième chapitre, nous analysons un second modèle pour la mégacaryopoïèse qui considère cette fois-ci une régulation opérée en continu uniquement via la vitesse de maturation des mégacaryoblastes. L’analyse de stabilité nécessite d’adapter un cadre pré-existant aux cas où le paramètre de bifurcation n’est pas le retard, et permet de montrer que l’augmentation du taux de mort des mégacaryoblastes conduit à l’apparition de solutions périodiques, en accord avec les observations cliniques de la thrombopénie cyclique amégacaryocytaire. Le dernier chapitre est consacré l’analyse de stabilité d’une équation différentielle à deux retards qui apparait notamment dans le cadre de la mégacaryopoïèse lorsque l’on considère que les plaquettes ont une durée de vie limitée / The object of this thesis is the study, using mathematical models, of the regulation mechanism maintaining an optimal quantity of blood platelets. The first chapter presents the biological and mathematical context of the thesis. In a second chapter, we introduce a model for megakaryopoiesis assuming a regulation by the platelet quantity of both the differentiation rate of stem cells to the platelet cell line and the amount of platelets produced by each megakaryocyte. We show that the dynamic of this model corresponds to a delay differential equation x'(t) = -?x(t) + f(x(t))g(x(t - t)), and we obtain for this equation new sufficient conditions for stability and for the oscillation of solutions. In a third chapter, we analyze a second model for megakaryopoiesis in which the regulation is continuous through the maturation speed of megakaryocyte progenitors. The stability analysis requires to adapt a pre-existing framework to problems where the bifurcation parameter is not the delay, and allows to show that increasing the death rate of megakaryocyte progenitors leads to the onset of periodic solutions, in agreement with clinical observation of amegakaryocytic cyclical thrombocytopenia. The last chapter covers a differential equation with two delays that appears among others in a model of platelet production which considers that platelet death can both age-independent and age-dependent