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Objective-To genetically and functionally characterize mutations of c-Mpl, that lead to thrombocytopenia in a child with congenital amegakaryocytic thrombocytopenia (CAMT).Methods-We identified two c-Mpl mutations in a child with clinical features of CAMT, 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.

IMPORTANCEAlthough effective agents are available to prevent painful vaso-occlusive episodes of sickle cell disease (SCD), there are no disease-modifying therapies for ongoing painful vaso-occlusive episodes; treatment remains supportive. A previous phase 3 trial of poloxamer 188 reported shortened duration of painful vaso-occlusive episodes in SCD, particularly in children and participants treated with hydroxyurea.OBJECTIVE To reassess the efficacy of poloxamer 188 for vaso-occlusive episodes.DESIGN, SETTING, AND PARTICIPANTS Phase 3, randomized, double-blind, placebo-controlled, multicenter, international trial conducted from May 2013 to February 2016 that included 66 hospitals in 12 countries and 60 cities; 388 individuals with SCD (hemoglobin SS, SC, S-β 0 thalassemia, or S-β + thalassemia disease) aged 4 to 65 years with acute moderate to severe pain typical of painful vaso-occlusive episodes requiring hospitalization were included. INTERVENTIONS A 1-hour 100-mg/kg loading dose of poloxamer 188 intravenously followed by a 12-hour to 48-hour 30-mg/kg/h continuous infusion (n = 194) or placebo (n = 194).MAIN OUTCOMES AND MEASURES Time in hours from randomization to the last dose of parenteral opioids among all participants and among those younger than 16 years as a separate subgroup. RESULTSOf 437 participants assessed for eligibility, 388 were randomized (mean age, 15.2 years; 176 [45.4%] female), the primary outcome was available for 384 (99.0%), 15-day follow-up contacts were available for 357 (92.0%), and 30-day follow-up contacts were available for 368 (94.8%). There was no significant difference between the groups for the mean time to last dose of parenteral opioids (81.8 h for the poloxamer 188 group vs 77.8 h for the placebo group; difference, 4.0 h [95% CI, −7.8 to 15.7]; geometric mean ratio, 1.2 [95% CI, 1.0-1.5]; P = .09). Based on a significant interaction of age and treatment (P = .01), there was a treatment difference in time from randomization to last administration of parenteral opioids for participants younger than 16 years (88.7 h in the poloxamer 188 group vs 71.9 h in the placebo group; difference, 16.8 h [95% CI, 1.7-32.0]; geometric mean ratio, 1.4 [95% CI, 1.1-1.8]; P = .008). Adverse events that were more common in the poloxamer 188 group than the placebo group included hyperbilirubinemia (12.7% vs 5.2%); those more common in the placebo group included hypoxia (12.0% vs 5.3%).CONCLUSIONS AND RELEVANCE Among children and adults with SCD, poloxamer 188 did not significantly shorten time to last dose of parenteral opioids during vaso-occlusive episodes. These findings do not support the use of poloxamer 188 for vaso-occlusive episodes.