Successful bone marrow transplantation in a patient with c-mpl-mutated congenital amegakaryocytic thrombocytopenia from a carrier donor

Unresolved issues in allogeneic hematopoietic cell transplantation for non-malignant diseases

Allogeneic hematopoietic cell transplantation (HCT) can be curative for a variety of non-malignant diseases (NMDs) as well as hematological malignancies. However, there are several fundamental differences between HCT for NMDs and hematological malignancies, which may necessitate the use of alternative HCT strategies. For example, these diseases differ in the intensity of conditioning regimen sufficient to improve disease. In addition, patients with NMDs are at higher risk of graft failure or mixed chimerism following HCT, and gain no or little survival benefit from graft-versus-host disease. Because more than 80% of patients with NMDs become long-term survivors, greater attention has been paid to late adverse effects and decreased of quality of life after HCT. This review addresses several unresolved issues in allogeneic HCT for patients with NMDs, such as (1) stem cell source, (2) conditioning regimen, (3) use of serotherapy or low-dose irradiation, and (4) therapeutic intervention for mixed chimerism. Resolving these issues may improve transplant outcomes in patients with NMDs.

Congenital amegakaryocytic thrombocytopenia: a case report of pediatric twins undergoing matched unrelated bone marrow transplantation

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare inherited disorder that presents with thrombocytopenia in infancy and evolves into bone marrow failure over time. Allogeneic hematopoietic stem cell transplant remains the only curative treatment option. We report our experience with identical twin sisters diagnosed with CAMT and treated successfully with matched unrelated donor bone marrow transplants. Before the transplant, 1 twin developed pancytopenia, whereas the other had a relatively benign clinical course. Choice of conditioning regimens was based on their pretransplant bone marrow cellularity and presence or absence of panyhypoplasia. Both twins tolerated the procedure well with no significant complications.

Congenital amegakaryocytic thrombocytopenia iPS cells exhibit defective MPL-mediated signaling

Congenital amegakaryocytic thrombocytopenia (CAMT) is caused by the loss of thrombopoietin receptor-mediated (MPL-mediated) signaling, which causes severe pancytopenia leading to bone marrow failure with onset of thrombocytopenia and anemia prior to leukopenia. Because Mpl(-/-) mice do not exhibit the human disease phenotype, we used an in vitro disease tracing system with induced pluripotent stem cells (iPSCs) derived from a CAMT patient (CAMT iPSCs) and normal iPSCs to investigate the role of MPL signaling in hematopoiesis. We found that MPL signaling is essential for maintenance of the CD34+ multipotent hematopoietic progenitor (MPP) population and development of the CD41+GPA+ megakaryocyte-erythrocyte progenitor (MEP) population, and its role in the fate decision leading differentiation toward megakaryopoiesis or erythropoiesis differs considerably between normal and CAMT cells. Surprisingly, complimentary transduction of MPL into normal or CAMT iPSCs using a retroviral vector showed that MPL overexpression promoted erythropoiesis in normal CD34+ hematopoietic progenitor cells (HPCs), but impaired erythropoiesis and increased aberrant megakaryocyte production in CAMT iPSC-derived CD34+ HPCs, reflecting a difference in the expression of the transcription factor FLI1. These results demonstrate that impaired transcriptional regulation of the MPL signaling that normally governs megakaryopoiesis and erythropoiesis underlies CAMT.

Congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is clinically characterized by thrombocytopenia presenting at birth in a child without congenital or skeletal malformations, reduced or absent bone marrow megakaryocytes, and eventual progression to bone marrow failure. Molecular studies in most cases confirm homozygous or compound heterozygous mutations in the thrombopoietin receptor c-Mpl. In addition to the clinical importance of recognizing this disorder, characterization of mutations identified in patients with CAMT has led to insights into thrombopoietin receptor structure and function. This review will summarize the diagnosis, pathophysiology, and management of CAMT.

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.

Unrelated bone marrow transplant for congenital amegakaryocytic thrombocytopenia: report of two cases and review of the literature

CAMT is a very rare cause of thrombocytopenia in infants. Most of the patients will progress to marrow failure. Allogeneic stem cell transplant remains the only curative therapy. We present two patients with CAMT who underwent an unrelated donor bone marrow transplant, one after developing marrow failure and another early in the course of the disease. Both patients tolerated the transplant with minimal toxicity and durable engraftment. We also present a comprehensive review of the literature for unrelated donor transplant for this condition.

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.

A review of inherited platelet disorders with guidelines for their management on behalf of the UKHCDO

The inherited platelet disorders are an uncommon cause of symptomatic bleeding. They may be difficult to diagnose (and are likely to be under-diagnosed) and pose problems in management. This review discusses the inherited platelet disorders summarising the current state of the art with respect to investigation and diagnosis and suggests how to manage bleeding manifestations with particular attention to surgical interventions and the management of pregnancy.

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.

Aplastic Anemia: Management of Pediatric Patients

Aplastic anemia is a term describing the common findings of pancytopenia and marrow hypoplasia arising from a variety of disease states, including acquired aplastic anemia and a variety of congenital marrow failure states. The management of children with these disorders has been confounded by difficulties of diagnosis. The availability of molecular testing has assisted in partial resolution of this problem but has raised new issues, such as the potential of genetic predisposition and the management of asymptomatic individuals with molecular markers. Longitudinal data from large cohort studies and disease registries are providing a rational basis for making more informed treatment decisions for children with these disorders. In particular, the ability to subset patients more accurately has improved triage of treatments. Approaches to hematopoietic stem cell transplantation (SCT), using both conventional and alternative donors, are changing rapidly, and the long-term sequelae of newer approaches are not entirely clear. Improved diagnosis and longer survival have fostered an understanding of the multidisciplinary approach necessary to manage both the underlying problems and the significant sequelae of treatment in both acquired and congenital disease.