Developmental differences in IFN signaling affect GATA1s-induced megakaryocyte hyperproliferation.

About 10% of Down syndrome (DS) infants are born with a transient myeloproliferative disorder (DS-TMD) that spontaneously resolves within the first few months of life. About 20%-30% of these infants subsequently develop acute megakaryoblastic leukemia (DS-AMKL). Somatic mutations leading to the exclusive production of a short GATA1 isoform (GATA1s) occur in all cases of DS-TMD and DS-AMKL. Mice engineered to exclusively produce GATA1s have marked megakaryocytic progenitor (MkP) hyperproliferation during early fetal liver (FL) hematopoiesis, but not during postnatal BM hematopoiesis, mirroring the spontaneous resolution of DS-TMD. The mechanisms that underlie these developmental stage-specific effects are incompletely understood. Here, we report a striking upregulation of type I IFN-responsive gene expression in prospectively isolated mouse BM- versus FL-derived MkPs. Exogenous IFN-α markedly reduced the hyperproliferation FL-derived MkPs of GATA1s mice in vitro. Conversely, deletion of the α/ß IFN receptor 1 (Ifnar1) gene or injection of neutralizing IFN-α/ß antibodies increased the proliferation of BM-derived MkPs of GATA1s mice beyond the initial postnatal period. We also found that these differences existed in human FL versus BM megakaryocytes and that primary DS-TMD cells expressed type I IFN-responsive genes. We propose that increased type I IFN signaling contributes to the developmental stage-specific effects of GATA1s and possibly the spontaneous resolution of DS-TMD.

Recurrent hypoglycemia due to growth hormone deficiency in an infant with Turner syndrome.

BACKGROUND: Growth hormone (GH) deficiency may occur in Turner syndrome (TS), but infantile hypoglycemia attributable to TS with GH deficiency has not been reported before. OBJECTIVES: We report a puzzling case of neonatal hypoglycemia due to GH deficiency in Turner syndrome. Array CGH was used to scrutinize the complex TS karyotype. METHODS: Standardized laboratory procedures. RESULTS: In a preterm (32 weeks) with prolonged and cholestatic jaundice, recurrent hypoglycemia occurred at the age of 1.5 months and was related to GH deficiency. There were no other endocrine or syndromic features. GH therapy was started at a usual dose of 25-30 µg/kg/day, but hypoglycemia recurred. Hepatopathy and hypogammaglobulinemia suggested X-recessive GH deficiency type 3 with non-random X-inactivation but resolved spontaneously. Nonetheless, a 45,X[75]/46,X,i(Xq)[21]/47,X,i(Xq)x2[4] TS karyotype was diagnosed with an apparent isochromosome fusion at the centromere. Upon this diagnosis, GH dose was doubled (50 µg/kg/day), and blood glucose was normalized consistently. In array CGH, the signal of Xp deviated more strongly than that of Xq, but the relation of the signals differed substantially from what the karyotype predicted. The isochromosome fusion point was relocated to Xp11.22, distal to a block of mental retardation genes that escape X-inactivation. CONCLUSIONS: i) TS with GH deficiency should be considered as a potential differential diagnosis of hypoglycemia in infants requiring higher doses of GH. ii) While array CGH may be erroneous in quantification of TS mosaicism, it is useful in precisely delineating isochromosomes and identifying genes on them that escape X-inactivation and thus possibly affect the TS phenotype.

Bradycardic and proarrhythmic properties of sinus node inhibitors.

Sinus node inhibitors reduce the heart rate presumably by blocking the pacemaker current If in the cardiac conduction system. This pacemaker current is carried by four hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels. We tested the potential subtype-specificity of the sinus node inhibitors cilobradine, ivabradine, and zatebradine using cloned HCN channels. All three substances blocked the slow inward current through human HCN1, HCN2, HCN3, and HCN4 channels. There was no subtype-specificity for the steady-state block, with mean IC50 values of 0.99, 2.25, and 1.96 microM for cilobradine, ivabradine, and zatebradine, respectively. Native If, recorded from mouse sinoatrial node cells, was slightly more efficiently blocked by cilobradine (IC50 value of 0.62 microM) than were the HCN currents. The block of I(f) in sinoatrial node cells resulted in slower and dysrhythmic spontaneous action potentials. The in vivo action of these blockers was analyzed using telemetric ECG recordings in mice. Each compound reduced the heart rate dose-dependently from 600 to 200 bpm with ED50 values of 1.2, 4.7, and 1.8 mg/kg for cilobradine, ivabradine, and zatebradine, respectively. beta-Adrenergic stimulation or forced physical activity only partly reversed this bradycardia. In addition to bradycardia, all three drugs induced increasing arrhythmia at concentrations greater than 5 mg/kg for cilobradine, greater than 10 mg/kg for zatebradine, or greater than 15 mg/kg for ivabradine. This dysrhythmic heart rate is characterized by periodic fluctuations of the duration between the T and P wave, resembling a form of sick sinus syndrome in humans. Hence, all available sinus node inhibitors possess an as-yet-unrecognized proarrhythmic potential.