Some familial platelet disorders are associated with predisposition to leukemia, myelodysplastic syndrome (MDS) or dyserythropoietic anemia.1,2 We identified a family with autosomal dominant thrombocytopenia, high erythrocyte mean corpuscular volume (MCV) and two occurrences of B-cell precursor acute lymphoblastic leukemia (ALL). Whole exome sequencing identified a heterozygous single nucleotide change in ETV6 (Ets Variant Gene 6), c.641C>T, encoding a p.Pro214Leu substitution in the central domain, segregating with thrombocytopenia and elevated MCV. A screen of 23 families with similar phenotype found two with ETV6 mutations. One family had the p.Pro214Leu mutation and one individual with ALL. The other family had a c.1252A>G transition producing a p.Arg418Gly substitution in the DNA binding domain, with alternative splicing and exon-skipping. Functional characterization of these mutations showed aberrant cellular localization of mutant and endogenous ETV6, decreased transcriptional repression and altered megakaryocyte maturation. Our findings underscore a key role for ETV6 in platelet formation and leukemia predisposition.

High hepatic iron concentration (HIC) is associated with cardiac iron overload. However, simultaneous measurements of heart and liver iron often demonstrate no significant linear association. We postulated that slower rates of cardiac iron accumulation and clearance could reconcile these differences. To test this hypothesis, we examined the longitudinal evolution of cardiac and liver iron in 38 thalassemia major patients, using previously validated magnetic resonance imaging (MRI) techniques. On cross-sectional evaluation, cardiac iron was uncorrelated with liver iron, similar to previous studies. However, relative changes in heart and liver iron were compared with one another using a metric representing the temporal delay between them. Cardiac iron significantly lagged liver iron changes in almost half of the patients, implying a functional but delayed association. The degree of time lag correlated with initial HIC (r ‫؍‬ 0.47, P < .003) and initial cardiac R2* (r ‫؍‬ 0.57, P < .001), but not with patient age. Thus, longitudinal analysis confirms a lag in the loading and unloading of cardiac iron with respect to liver iron, and partially explains the weak cross-sectional association between these parameters. These data reconcile several prior studies and provide both mechanical and clinical insight into cardiac iron accumulation. (Blood. 2008; 112:2973-2978) IntroductionDespite availability of iron chelation, iron-mediated cardiac toxicity remains the leading cause of death in thalassemia major patients. 1 Cardiac dysfunction, whether detected by radionuclide angiography, echocardiography, or magnetic resonance imaging (MRI), is often a late finding and carries an ominous prognosis. 2,3 Although intense chelation can rescue many patients, depleting cardiac iron burden often takes years and mortality is high with incomplete compliance. 3 Thus, prevention of cardiac iron accumulation and dysfunction is imperative. Initial studies in this area examined hepatic iron concentration (HIC), as measured by liver biopsy, and serum ferritin levels as potential predictors of cardiac toxicity. [4][5][6] This hypothesis was logical because HIC is an excellent indicator of iron balance and total body iron stores. 6,7 These early studies concluded that elevated liver iron and serum ferritin trends raise prospective risk of cardiac dysfunction, implying a correlation between cardiac and liver iron deposition. [4][5][6] Based upon this work, treatment algorithms for iron removal therapy based primarily on HIC and ferritin levels 8,9 were developed with the goal of minimizing cardiac and endocrine toxicities.However, the use of HIC and ferritin to infer cardiac iron has been challenged by recent MRI studies. 10-13 MRI allows organ iron concentrations to be easily and noninvasively measured and has been validated on both animals and humans. [14][15][16] Crosssectional analysis has demonstrated poor correlation between HIC or ferritin and cardiac iron. [10][11][12]13 In addition, some patients develop cardiac deposition and symptom...

Megakaryocytes are hematopoietic cells, which are responsible for the production of blood platelets. The traditional view of megakaryopoiesis describes the cellular journey from hematopoietic stem cells, through a hierarchical series of progenitor cells, ultimately to a mature megakaryocyte. Once mature, the megakaryocyte then undergoes a terminal maturation process involving multiple rounds of endomitosis and cytoplasmic restructuring to allow platelet formation. However, recent studies have begun to redefine this hierarchy and shed new light on alternative routes by which hematopoietic stem cells are differentiated into megakaryocytes. In particular, the origin of megakaryocytes, including the existence and hierarchy of megakaryocyte progenitors, has been redefined, as new studies are suggesting that hematopoietic stem cells originate as megakaryocyte-primed and can bypass traditional lineage checkpoints. Overall, it is becoming evident that megakaryopoiesis does not only occur as a stepwise process, but is dynamic and adaptive to biological needs. In this review, we will reexamine the canonical dogmas of megakaryopoiesis and provide an updated framework for interpreting the roles of traditional pathways in the context of new megakaryocyte biology. Visual Overview— An online visual overview is available for this article.