Congenital amegakaryocytic thrombocytopenia: a retrospective clinical analysis of 20 patients

CAMT in a female with developmental delay, facial malformations and central nervous system anomalies

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare disorder characterized by thrombocytopenia and absence or decline in the number of megakaryocytic precursors in the bone marrow. It is caused by mutations in the thrombopoietin receptor gene, c-mpl, involved in the proliferation and differentiation of megakaryocytes and platelets. The association between CAMT and central nervous system (CNS) anomalies has been reported in the literature, albeit not very frequently. Here we present a unique case where CAMT appeared associated to cerebellum agenesis, hypoplasia of the corpus callosum and brainstem, facial malformations, and developmental delay.

A novel nonsense mutation in the MPL gene in congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare autosomal recessive disorder characterized by thrombocytopenia from failure of megakaryopoiesis. CAMT is one of the bone marrow failure syndromes, and the disease progression may involve other lineages leading to pancytopenia. The genetic background of CAMT is mutations in the MPL gene encoding the thrombopoietin receptor. Here, we describe a Korean male with CAMT. Molecular genetic analyses by direct sequencing revealed that he was compound heterozygous for two nonsense mutations in MPL, Tyr63X (c.189C>A), and Arg357X (c.1069C>T), the latter being a novel mutation.

Megakaryopoiesis

The process of megakaryopoiesis and platelet production is complex, with the potential for regulation at multiple stages. Megakaryocytes are derived from the hematopoietic stem cell through successive lineage commitment steps, and they undergo a unique maturation process that includes polyploidization, development of an extensive internal demarcation membrane system, and finally formation of pro-platelet processes. Platelets are shed from these processes into vascular sinusoids within the bone marrow. Megakaryocyte differentiation is regulated both positively and negatively by transcription factors and cytokine signaling. Thrombopoietin (TPO) is the most important hematopoietic cytokine for platelet production. Clinically, acquired and inherited mutations affecting megakaryocytic transcription factors and thrombopoietin signaling have been identified in disorders of thrombocytopenia and thrombocytosis.

Outcomes after related and unrelated umbilical cord blood transplantation for hereditary bone marrow failure syndromes other than Fanconi anemia

BACKGROUND

Allogeneic stem cell transplantation is the only curative option for patients with hereditary bone marrow failure syndromes. Umbilical cord blood is an alternative source of stem cells for allogeneic transplantation.

DESIGN AND METHODS

This multicenter, retrospective study is based on data reported to the Eurocord Registry about patients with hereditary bone marrow failure syndrome who underwent umbilical cord blood transplantation.

RESULTS

Sixty-four patients with hereditary bone marrow failure syndromes were transplanted from related (n = 20) or unrelated donors (n = 44). Diagnoses were Diamond-Blackfan anemia (21 patients), congenital amegakaryocytic thrombocytopenia (16 patients), dyskeratosis congenita (8 patients), Shwachman-Diamond syndrome (2 patients), severe congenital neutropenia (16 patients) and unclassified (1 patient). In the group of patients who received grafts from related donors, all patients but one received an HLA-matched sibling transplant. The median number of total nucleated cells infused was 5 × 10⁷/kg. The cumulative incidence of neutrophil recovery at 60 days was 95%. Two patients had grade II-IV acute graft-versus-host disease, while the 2-year cumulative incidence of chronic graft-versus-host disease was 11%. The 3-year overall survival rate was 95%. In the group of patients who received grafts from unrelated donors, 86% had HLA-mismatched grafts and three received two umbilical cord blood units. The median number of total nucleated cells infused was 6.1 × 10⁷/kg. The cumulative incidence of neutrophil recovery at day 60 in this group was 55%. The 100-day cumulative incidence of grade II-IV acute graft-versus-host disease was 24%, while the 2-year cumulative incidence of chronic graft-versus-host disease was 53%. The 3-year overall survival rate was 61%; better overall survival was associated with age less than 5 years (P = 0.01) and 6.1 × 10⁷/kg or more total nucleated cells infused (P = 0.05).

CONCLUSIONS

In patients with hereditary bone marrow failure syndromes, related umbilical cord blood transplantation is associated with excellent outcomes while increasing cell dose and better HLA matching might provide better results in unrelated umbilical cord blood transplantation.

Pathophysiology and management of inherited bone marrow failure syndromes

The inherited marrow failure syndromes are a diverse set of genetic disorders characterized by hematopoietic aplasia and cancer predisposition. The clinical phenotypes are highly variable and much broader than previously recognized. The medical management of the inherited marrow failure syndromes differs from that of acquired aplastic anemia or malignancies arising in the general population. Diagnostic workup, molecular pathogenesis, and clinical treatment are reviewed.

F104S c-Mpl responds to a transmembrane domain-binding thrombopoietin receptor agonist: proof of concept that selected receptor mutations in congenital amegakaryocytic thrombocytopenia can be stimulated with alternative thrombopoietic agents

OBJECTIVE

To determine whether specific c-Mpl mutations might respond to thrombopoietin receptor agonists.

MATERIALS AND METHODS

We created cell line models of type II c-Mpl mutations identified in congenital amegakaryocytic thrombocytopenia. We selected F104S c-Mpl for further study because it exhibited surface expression of the receptor. We measured proliferation of cell lines expressing wild-type or F104S c-Mpl in response to thrombopoietin receptor agonists targeting the extracellular (m-AMP4) or transmembrane (LGD-4665) domains of the receptor by 1-methyltetrazole-5-thiol assay. We measured thrombopoietin binding to the mutant receptor using an in vitro thrombopoietin uptake assay and identified F104 as a potentially critical residue for the interaction between the receptor and its ligand by aligning thrombopoietin and erythropoietin receptors from multiple species.

RESULTS

Cells expressing F104S c-Mpl proliferated in response to LGD-4665, but not thrombopoietin or m-AMP4. Compared to thrombopoietin, LGD-4665 stimulates signaling with delayed kinetics in both wild-type and F104S c-Mpl-expressing cells. Although F104S c-Mpl is expressed on the cell surface in our BaF3 cell line model, the mutant receptor does not bind thrombopoietin. Comparison to the erythropoietin receptor suggests that F104 engages in hydrogen-bonding interactions that are critical for binding to thrombopoietin.

CONCLUSIONS

These findings suggest that a small subset of patients with congenital amegakaryocytic thrombocytopenia might respond to treatment with thrombopoietin receptor agonists, but that responsiveness will depend on the type of mutation and agonist used. We postulate that F104 is critical for thrombopoietin binding. The kinetics of signaling in response to a transmembrane domain-binding agonist are delayed in comparison to thrombopoietin.

Bone marrow transplantation for inherited bone marrow failure syndromes

The inherited bone marrow failure (BMF) syndromes are characterized by impaired hematopoiesis and cancer predisposition. Most inherited BMF syndromes are also associated with a range of congenital anomalies. Progress in improving the outcomes for children with inherited BMF syndromes has been limited by the rarity of these disorders, as well as disease-specific genetic, molecular, cellular, and clinical characteristics that increase the risks of complications associated with hematopoietic stem cell transplantation (HSCT). As a result, the ability to develop innovative transplant approaches to circumvent these problems has been limited. Recent progress has been made, as best evidenced in studies adding fludarabine to the preparative regimen for children undergoing unrelated donor HSCT for Fanconi anemia. The rarity of these diseases coupled with the far more likely incremental improvements that will result from ongoing research will require prospective international clinical trials to improve the outcome for these children.

Gene therapy of MPL deficiency: challenging balance between leukemia and pancytopenia

Signaling of the thrombopoietin (THPO) receptor MPL is critical for the maintenance of hematopoietic stem cells (HSCs) and megakaryocytic differentiation. Inherited loss-of-function mutations of MPL cause severe thrombocytopenia and aplastic anemia, a syndrome called congenital amegakaryocytic thrombocytopenia (CAMT). With the aim to assess the toxicity of retroviral expression of Mpl as a basis for further development of a gene therapy for this disorder, we expressed Mpl in a murine bone marrow transplantation (BMT) model. Treated mice developed a profound yet transient elevation of multilineage hematopoiesis, which showed morphologic features of a chronic myeloproliferative disorder (CMPD) with progressive pancytopenia. Ten percent of mice (3/27) developed erythroleukemia, associated with insertional activation of Sfpi1 and Fli1. The majority of transplanted mice developed a progressive pancytopenia with histopathological features of a myelodysplastic syndrome (MDS)-like disorder. To avoid these adverse reactions, improved retroviral vectors were designed that mediate reduced and more physiological Mpl expression. Self-inactivating gamma-retroviral vectors were constructed that expressed Mpl from the phosphoglycerate kinase (PGK) or the murine Mpl promoter. Mice that received BM cells expressing Mpl from the Mpl promoter were free of any previously observed adverse reactions.

Hematopoietic stem cell transplantation for bone marrow failure syndromes in children

Bone marrow failure (BMF) syndromes include a broad group of diseases of varying etiologies, in which hematopoeisis is abnormal or completely arrested in one or more cell lines. BMF can be an acquired aplastic anemia (AA) or can be congenital, as part of such syndromes as Fanconi anemia (FA), Diamond Blackfan anemia, and Schwachman Diamond syndrome (SDS). In this review, we first address the evolution and current status of bone marrow transplantation (BMT) in the pediatric population in the most common form of BMF, acquired AA. We then discuss pediatric BMT in some of the more common inherited BMF syndromes, with emphasis on FA, in which experience is greatest. It is important to consider the possibility of a congenital etiology in every child (and adult) with marrow failure, because identification of an associated syndrome provides insight into the likely natural history of the disease, as well as prognosis, treatment options for the patient and family, and long-term sequelae both of the disease itself and its treatment.

Congenital amegakaryocytic thrombocytopenia and thrombocytopenia with absent radii

Thrombocytopenia is a relatively common clinical problem in hospitalized neonates, and it is critical to distinguish infants who have rare congenital thrombocytopenias from those who have acquired disorders. Two well-described inherited thrombocytopenia syndromes that present in the newborn period are congenital amegakaryocytic thrombocytopenia (CAMT) and thrombocytopenia with absent radii (TAR). Although both are characterized by severe (< 50,000/microL) thrombocytopenia at birth, the molecular mechanisms underlying these disorders and their clinical presentations and courses are distinct. CAMT is an autosomal recessive disorder caused by mutations in the thrombopoietin (TPO) receptor c-Mpl. TAR is a syndrome of variable inheritance and unclear genetic etiology consisting of thrombocytopenia in association with bilateral absent radii and frequently additional congenital abnormalities. This article summarizes the current understanding of the pathophysiology and clinical course of CAMT and TAR.

Advances in the understanding of congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (MIM #604498) is an extremely rare inherited bone marrow failure syndrome, usually presenting as a severe thrombocytopenia at birth due to ineffective megakaryocytopoiesis and no characteristic physical anomalies. Usually the isolated thrombocytopenia progresses to pancytopenia during the first years of life. The only curative therapy to date is haematopoietic stem cell transplantation. Most of the cases of congenital amegakaryocytic thrombocytopenia are caused by defective expression or function of the thrombopoietin receptor due to homozygous or compound heterozygous mutations in the gene MPL. The essential roles of thrombopoietin as a lineage specific regulator of platelet production and as a regulator of haematopoietic stem cell function are reflected in the haematological defects seen in affected individuals.

Compound heterozygous c-Mpl mutations in a child with congenital amegakaryocytic thrombocytopenia: functional characterization and a review of the literature

OBJECTIVE

To genetically and functionally characterize mutations of c-Mpl that lead to thrombocytopenia in a child with congenital amegakaryocytic thrombocytopenia.

MATERIALS AND METHODS

We identified two c-Mpl mutations in a child with clinical features of congenital amegakaryocytic thrombocytopenia, one a previously described mutation in the extracellular domain (R102P) and the other a novel mutation leading to truncation of the receptor after the box 1 homology domain (541Stop). Cell line models were created to examine the ability of the mutant receptors to signal in response to thrombopoietin and thrombopoietin-like agonists.

RESULTS

Data from cell-line models indicate that c-Mpl R102P does not support significant signaling in response to thrombopoietin due to impaired trafficking of the mutant receptor to the cell surface. Alternative thrombopoietic agents do not circumvent this block to signaling, likely due to the inaccessibility of the receptor. In addition, previous data indicate that c-Mpl 541Stop does not support intracellular signaling due to the loss of critical intracellular domains.

CONCLUSIONS

This case demonstrates two different mechanisms by which c-Mpl mutations can impair thrombopoietin signaling, and suggests that mutations in the extracellular domain will not be rescued by c-Mpl agonists if they interfere with normal receptor expression.

Clinical approach to marrow failure

The treatment and medical management of aplastic anemia fundamentally differ between patients with inherited versus acquired marrow failure; however, the diagnosis of an inherited bone marrow failure syndrome is frequently obscure. Recent exciting advances in our understanding of the molecular pathophysiology of the inherited bone marrow failure syndromes have resulted in a profusion of new tests to aid in diagnosis. This in turn has raised questions regarding the appropriate choice of testing for the patient presenting with aplastic anemia. Important clues to the diagnosis of an inherited marrow failure syndrome may be gleaned from careful attention to the clinical history, physical exam, and laboratory workup.

An ENU-induced recessive mutation in Mpl leads to thrombocytopenia with overdominance

OBJECTIVE

The aim of this study was to identify and characterize the causative mutation in the thrombocytopenic mouse strain HLB219 that was generated at the Jackson Laboratory as part of a large-scale N-ethyl-N-nitrosourea mutagenesis screen.

MATERIALS AND METHODS

The HLB219 mutation was identified by interval mapping of F2 mice generated from intercross breeding of HLB219 to both BALB/cByJ (BALB) and 129/SvImJ (129/Sv). Mpl was identified as a candidate gene and sequenced. The mutation was characterized in vivo in mouse hematopoietic stem/progenitor cell assays and in cell culture by expression in Ba/F3 cells.

RESULTS

A novel mutation in the thrombopoietin (TPO) receptor Mpl in HLB219 mice caused a Cys-->Arg substitution at codon 40 in the extracellular region of the receptor. Mice homozygous for the Mpl(hlb219) mutation had an 80% decrease in the number of platelets in comparison to the wild-type C57BL/6J strain, low numbers of bone marrow megakaryocytes, high TPO levels, and decreased competitive repopulating ability, consistent with a loss-of-function mutation in the receptor. Mice heterozygous for Mpl(hlb219) however, showed an overdominance effect with a significant increase in platelet number. Functional analysis in vitro demonstrated that Ba/F3 cells expressing the mutant MPL(hlb219) protein failed to activate extracellular signal-regulated kinase and signal transducers and activators of transcription 5, but proliferated in the absence of TPO and required constitutive phosphorylation of RAC-alpha serine/threonine protein kinase (AKT) for cytokine-independent growth.

CONCLUSION

Thrombocytopenia in HLB219 mice is caused by a recessive mutation in Mpl that abrogates mitogen-activated protein kinase-extracellular signal regulated kinase and janus kinase-signal transducers and activators of transcription signaling.

Ataxia and pancytopenia caused by a mutation in TINF2

The syndrome of ataxia-pancytopenia is an autosomal dominant disorder characterized by cerebellar ataxia, peripheral neuropathies, pancytopenia and a predilection to myelodysplastic syndrome and acute myeloid leukemia. The genetic basis of this condition is unknown. We describe a child who presented with ataxia and pancytopenia and was found to have a heterozygous mutation, c.845G>A (Arg282His) in TINF2, a gene recently reported to be mutated in a subset of patients with autosomal dominant dyskeratosis congenita. We propose that some cases of ataxia-pancytopenia may be affected by DC.

Congenital amegakaryocytic thrombocytopenia: the diagnostic importance of combining pathology with molecular genetics

Congenital Amegakaryocytic Thrombocytopenia (CAMT) is a rare bone marrow failure syndrome that presents with isolated thrombocytopenia within the first year of life. Classic diagnostic bone marrow findings reveal absent or significantly decreased megakaryocytes with otherwise normal marrow cellularity. We present a newborn with thrombocytopenia whose initial bone marrow aspirate showed an appropriate number of megakaryocytes. CAMT was subsequently diagnosed after molecular testing demonstrated a mutation in the thrombopoietin receptor. The presence of a normal number of megakaryocytes on an initial bone marrow aspirate should not exclude CAMT from the differential diagnosis of thrombocytopenia within the first year of life.

Congenital amegakaryocytic thrombocytopenia-3 novel c-MPL mutations and their phenotypic correlations

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare bone marrow failure syndrome associated with thrombocytopenia and a tendency to progress to aplastic anemia. Mutations in the c-MPL gene encoding for thrombopoietin receptor have been identified in the majority of the patients. Previous studies suggest a genotype-phenotype correlation wherein the severity of the disease depends on the type of mutation present and residual thrombopoietin receptor activity. The present study describes the clinical and genetic findings on a series of 7 patients with CAMT, 3 of them siblings. The patients were homozygous for 5 mutations in the c-MPL gene, including 3 unique ones: c.212+5G>A, C76T, and G1162C. The clinical picture was variable; 1 patient who was homozygous for a nonsense mutation in exon 1 (C76T) developed infantile acute lymphoblastic leukemia, whereas patients who were homozygous for a splice-site mutation (c.212+5G>A) expressing both normal and mutated transcripts had a milder clinical course. As previously suggested, c-MPL mutation analysis in CAMT patients helps to predict the clinical course and to provide optimal therapy.

Mice rendered severely deficient in megakaryocytes through targeted gene deletion of the thrombopoietin receptor c-Mpl have a normal skeletal phenotype

To explore whether a functional relationship exists between megakaryocytes and the cellular processes responsible for bone formation, we examined if Mpl ( -/- ) mice, which are severely megakaryocyte-deficient through c-Mpl gene deletion, have an abnormal skeletal phenotype compared to Mpl ( +/- ) and wild-type littermates. We also analyzed whether the osteogenic response to high-dose estrogen treatment is altered in Mpl ( -/- ) mice. Megakaryocyte numbers and skeletal indices were compared between Mpl ( -/- ) mice and littermate Mpl ( +/- ) and wild-type 12-week-old mice (six per group). Dual-energy X-ray absorbtiometry of whole body, excised tibias, and femurs was performed. Histomorphometric analyses of the proximal metaphysis and mid-diaphysis were carried out on longitudinal and transverse sections, respectively. Histomorphometry was performed on the proximal tibial metaphysis of four Mpl ( -/- ) and four wild-type mice following high-dose estrogen treatment (0.5 mg/animal/week) for 4 weeks. Mpl ( -/- ) mice had 10% the megakaryocyte number of Mpl ( +/- ) and wild-type littermates. Bone mineral density values in Mpl ( -/- ) mice were identical to those in Mpl ( +/- ) and wild-type mice for whole body, femur, and tibia. Histomorphometric analysis demonstrated that cancellous and cortical tibial bone parameters were similar across all genotypes. The osteogenic response to estrogen treatment was indistinguishable between Mpl ( -/- )and wild-type mice. We found that mice severely deficient in megakaryocytes have a normal skeletal phenotype. Additionally, the deficiency did not diminish the osteogenic marrow response to high-dose estrogen treatment. These results represent the first in vivo evidence that severe megakaryocyte deficiency does not affect bone formation, suggesting that this process is not dependent on normal megakaryocyte number.

Hematopoietic growth factors for the treatment of inherited cytopenias

The clinical availability of recombinant hematopoietic growth factors was initially thought to be breakthrough in the treatment of bone marrow failure syndromes. However, in most disorders of hematopoeisis, the clinical use was rather disappointing. Only in congenital neutropenias (CNs) has the long-term administration of granulocyte colony-stimulating factor (G-CSF) led to a maintained increase in absolute neutrophil count (ANC) and a reduction of severe bacterial infections. In other disorders of hematopoiesis, the use of lineage-specific growth factors is either not possible due to mutations in the growth factor receptor or leads to a transient benefit only. Initial clinical trials with multilineage hematopoietic growth factors, such as stem cell factor (SCF; c-kit ligand) were discontinued due to adverse events. It is well known that bone marrow failure syndromes are pre-leukemic disorders. So far, there is no evidence for induction of leukemia by hematopoietic growth factors. However, it has been shown in patients with CN and Fanconi anemia that hematopoietic growth factors might induce preferential outgrowth of already transformed cells. Thus, it is strongly recommended to monitor patients for clonal aberrations prior to and during long-term treatment with hematopoietic growth factors.

Congenital Platelet Disorders

Congenital platelet disorders represent a rare group of diseases classified by either a qualitative or quantitative platelet defect. This article outlines the historical, clinical, laboratory, and genetic features of various inherited platelet disorders with attention given to updated information on disease classification, diagnosis, and genotypes. A separate discussion regarding management addresses the difficulty in treatment strategies, particularly in patients who develop alloimmunization to platelets.

Current diagnosis of inherited bone marrow failure syndromes

Prompt and accurate diagnosis is required for optimal treatment and genetic counseling of patients with inherited bone marrow failure syndromes (IBMFS). However, the diverse clinical picture of these syndromes and their rareness is often associated with diagnostic difficulties. Recently, an improved diagnostic approach is possible by the cloning of many of the causative genes. Fanconi anemia (FA) patients belong to at least 12 complementation groups, of which 11 genes have been cloned. An approach combining an induced chromosomal breakage test, detection of FANCD2-L by Western blot analysis, complementation group analysis, and detailed mutation analysis enables unraveling the causative mutation in the majority of patients. With the use of such strategies, genotype/phenotype correlations in FA are evolving. In dyskeratosis congenita mutations in DCK1, TERC, and TERT genes have been identified, but mutations have been found in less than half of these patients. In patients with Shwachman-Diamond syndrome, mutations in the SBDS gene were found in approximately 90% of patients. In Diamond-Blackfan anemia the RSP19 gene is mutated in 20-25% of patients. Heterozygote ELA2 mutations are found in 60-80% of severe congenital neutropenia patients. All patients with congenital amegakaryocytic thrombocytopenia have mutations in the thrombopoietin receptor gene c-Mpl.

Megakaryocyte development and platelet production

Megakaryocytopoiesis involves the commitment of haematopoietic stem cells, and the proliferation, maturation and terminal differentiation of the megakaryocytic progenitors. Circulating levels of thrombopoietin (TPO), the primary growth-factor for the megakaryocyte (MK) lineage, induce concentration-dependent proliferation and maturation of MK progenitors by binding to the c-Mpl receptor and signalling induction. Decreased platelet turnover rates results in increased concentration of free TPO, enabling the compensatory response of marrow MKs to increased platelet production. C-Mpl activity is orchestrated by a complex cascade of signalling molecules that induces the action of specific transcription factors to drive MK proliferation and maturation. Mature MKs form proplatelet projections that are fragmented into circulating particles. Newly developed thrombopoietic agents operating via c-Mpl receptor may prove useful in supporting platelet production in thrombocytopenic state. Herein, we review the regulation of megakaryocytopoiesis and platelet production in normal and disease state, and the new approaches to thrombopoietic therapy.

Acute immune thrombocytopenic purpura in infants: associated factors, clinical features, treatment and long-term outcome

The natural course of acute immune thrombocytopenic purpura (ITP) in infants is poorly described in the literature. A retrospective study of 17 consecutive patients <1 yr of age admitted and treated for acute ITP between 1996 and 2005 was conducted. We investigated their demographics, vaccination history, clinical features, laboratory examinations, response to treatment and long-term outcome. There were 11 male and six female infants. Their ages ranged from 24 d to 12 months with a median of 3 months. All infants presented with petechiae and/or ecchymoses. Fourteen cases had platelet counts below 20 x 10(9)/L at the time of admission. They all had good response to a single course of treatment (14/17) or multiple courses of treatment (3/17). None had progressed into chronic ITP. Seven infants had a causal relationship with immunization, five associated with hepatitis B, one diphtheria-pertussis-tetanus, one diphtheria-tetanus-acellular pertussis-inactivated poliovirus vaccine-conjugated Haemophilus influenza vaccines. These seven infants responded to treatment within 3-9 d after therapy with intravenous immunoglobulin, high-dose methylprednisolone or oral steroids. Re-boosters with vaccines revealed no recurrence of the disease in all of these seven patients. The study suggests that further immunization is not contraindicated in infants experiencing acute ITP associated with vaccines.

Inherited Thrombocytopenia: Congenital Amegakaryocytic Thrombocytopenia and Thrombocytopenia With Absent Radii

Thrombocytopenia in the newborn period can signify an inherited platelet disorder. Congenital amegakaryocytic thrombocytopenia (CAMT) and thrombocytopenia with absent radii (TAR) share features of isolated thrombocytopenia, reduced or absent marrow megakaryocytes, impaired responsiveness to thrombopoietin (TPO), and high plasma TPO levels. These disorders are most readily distinguished from each other by the finding of radial aplasia in TAR and the presence of c-MPL mutations in CAMT. In addition, their long-term outcomes are strikingly different: the development of trilineage marrow failure in CAMT in contrast to the general improvement of thrombocytopenia in TAR. The differential diagnosis for CAMT and TAR also includes other congenital disorders in which thrombocytopenia and radial abnormalities can be seen. In this article we will review our molecular and clinical understanding of these two inherited disorders of amegakaryocytosis.