Recombinant interleukin-6 in the treatment of congenital thrombocytopenia associated with absent radii

Tyrosyl-tRNA synthetase stimulates thrombopoietin-independent hematopoiesis accelerating recovery from thrombocytopenia

Aminoacyl-tRNA synthetases (aaRSs) catalyze aminoacylation of tRNAs in the first step of protein synthesis in the cytoplasm. However, in higher eukaryotes, they acquired additional functions beyond translation. In the present study, we show that an activated form of tyrosyl-tRNA synthetase (YRS^ACT) functions to enhance megakaryopoiesis and platelet production in vitro and in vivo. These findings were confirmed with human megakaryocytes differentiated from peripheral blood CD34⁺ hematopoietic stem cells and with human induced pluripotent stem (iPS) cells. The activity of YRS^ACT is independent of thrombopoietin (TPO), as evidenced by expansion of the megakaryocytes from iPS cell-derived hematopoietic stem cells from a patient deficient in TPO signaling. These findings demonstrate a previously unrecognized function of an aaRS which may have implications for therapeutic interventions.

New mechanisms behind blood cell formation continue to be uncovered, with therapeutic approaches for hematological diseases being of great interest. Here we report an enzyme in protein synthesis, known for cell-based activities beyond translation, is a factor inducing megakaryocyte-biased hematopoiesis, most likely under stress conditions. We show an activated form of tyrosyl-tRNA synthetase (YRS^ACT), prepared either by rationally designed mutagenesis or alternative splicing, induces expansion of a previously unrecognized high-ploidy Sca-1⁺ megakaryocyte population capable of accelerating platelet replenishment after depletion. Moreover, YRS^ACT targets monocytic cells to induce secretion of transacting cytokines that enhance megakaryocyte expansion stimulating the Toll-like receptor/MyD88 pathway. Platelet replenishment by YRS^ACT is independent of thrombopoietin (TPO), as evidenced by expansion of the megakaryocytes from induced pluripotent stem cell-derived hematopoietic stem cells from a patient deficient in TPO signaling. We suggest megakaryocyte-biased hematopoiesis induced by YRS^ACT offers new approaches for treating thrombocytopenia, boosting yields from cell-culture production of platelet concentrates for transfusion, and bridging therapy for hematopoietic stem cell transplantation.

Thrombocytopenia absent radius syndrome with Tetralogy of Fallot: a rare association

Thrombocytopenia absent radius (TAR) syndrome is a very rare and infrequently seen congenital disorder with an approximate frequency of 0.42/100,000 live births. It is associated with bilateral absence of radii, hypo-megakaryocytic thrombocytopenia, and presence of both thumbs. The other systems which are affected by TAR syndrome include skeletal, hematologic, and cardiac systems. Intracranial hemorrhages due to thrombocytopenia and cardiac disorders are a common association usually seen with this syndrome and are usual cause of death. We describe a 3-month-old infant who was diagnosed with TAR syndrome on the basis of clinical features (thrombocytopenia and bilateral absent radius bone and confirmed by genetic analysis). The patient was diagnosed to have Tetralogy of Fallot, for which the infant was managed with definitive repair and thrombocytopenia was managed with platelet transfusion. Infants with TAR syndrome should be assessed for other associated malformations of various systems and followed up regularly and parents should be counseled for associated expected complications in these patients. We report an infant with TAR syndrome with Tetralogy of Fallot, which has not been reported in medical literature until now and this is the first case of its type.

Two patterns of thrombopoietin signaling suggest no coupling between platelet production and thrombopoietin reactivity in thrombocytopenia-absent radii syndrome

BACKGROUND

Thrombocytopenia with absent radii syndrome is defined by bilateral radius aplasia and thrombocytopenia. Due to impaired thrombopoietin signaling there are only few bone marrow megakaryocytes and these are immature; the resulting platelet production defect improves somewhat over time. A microdeletion on chromosome 1q21 is present in all patients but is not sufficient to form thrombocytopenia with absent radii syndrome. We aimed to refine the signaling defect in this syndrome.

DESIGN AND METHODS

We report an extended study of 23 pediatric and adult patients suffering from thrombocytopenia with absent radii syndrome in order to scrutinize thrombopoietin signal transduction by immunoblotting and gel electrophoretic shift assays. In addition, platelet immunotyping and reactivity were analyzed by flow cytometry. Results were correlated with clinical data including age and platelet counts.

RESULTS

Two distinct signaling patterns were identified. Juvenile patients showed abrogated thrombopoietin signaling (pattern #1), which is restored in adults (pattern #2). Phosphorylated Jak2 was indicative of activation of STAT1, 3 and 5, Tyk2, ERK, and Akt, showing its pivotal role in distinct thrombopoietin-dependent pathways. Jak2 cDNA was not mutated and the thrombopoietin receptor was present on platelets. All platelets of patients expressed normal levels of CD41/61, CD49b, and CD49f receptors, while CD42a/b and CD29 were slightly reduced and the fibronectin receptor CD49e markedly reduced. Lysosomal granule release in response to thrombin receptor activating peptide was diminished.

CONCLUSIONS

We show a combined defect of platelet production and function in thrombocytopenia with absent radii syndrome. The rise in platelets that most patients have during the first years of life preceded the restored thrombopoietin signaling detected at a much later age, implying that these events are uncoupled and that an unknown factor mediates the improvement of platelet production.

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.

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.

Interleukin-6 and platelet protagonists in T lymphocyte and virological response

The present cross-sectional study evaluated the status and relationship of interleukin-6, a platelet growth factor, with platelet counts, viral load, CD4 counts, and antiretroviral treatment in 75 HIV-infected subjects with thrombocytopenia and 50 gender-, race-, age- and antiretroviral treatment-matched controls without thrombocytopenia. Mean IL-6 production was significantly higher in thrombocytopenic participants (13 432+/-8596) than in non-thrombocytopenic subjects (12 859+/-3538 pg/10(5) Lym). Univariate analyses indicated, however, that thrombocytopenic patients were more likely to have <3000 pg of IL-6 than non-thrombocytopenic patients (OR=7 95% CI 1.3-12; P=0.01). For additional analyses, participants were dichotomized above and below 3000 pg of IL-6. Despite similar age, gender, drug use and antiretroviral treatment, thrombocytopenic participants had lower CD4 counts (186.5+/-149 vs. 401+/-286, P=0.005) than non-thrombocytopenic subjects. Thrombocytopenic participants with elevated IL-6, with or without HAART, were more likely to have higher HIV-replication (496 273+/-210 416; 34 656+/-25 332) than thrombocytopenic individuals with low IL-6 levels (105 332+/-42 699; 19 015+/-14 296 P=0.05). Non-thrombocytopenic patients with high IL-6 levels exhibited the highest CD4s (466.7+/-333) and the lowest viral burden (63 094+/-53 300) of the groups. Two distinct categories of HIV-associated thrombocytopenia exist: one accompanied by low IL-6, and another with compensatory elevations of IL-6. In thrombocytopenic individuals, the latter was associated with the poorest immunological and virological responses.

Acute myelogenous leukemia in an adult with thrombocytopenia with absent radii syndrome

Acute myelogenous leukemia with t(8;21)(q22;q22) developed in a 42-yr-old woman having thrombocytopenia with absent radii syndrome (TARS). Standard induction and postremission therapies were safely administered. With each successive chemotherapy, the onset of platelet recovery was not delayed, but peak platelet counts were persistently suppressed. Nine months after achieving complete hematologic and cytogenetic remission, she remains severely thrombocytopenic (platelet count 6-12 x 109/L). She is, however, asymptomatic and transfusion independent. Of interest is the transient normalization of platelet count (rebound relative thrombocytosis). Our report and review of the literature suggests that TARS, contrary to previous belief, may be associated with increased risk of acute leukemia.