Thrombocytosis in preterm infants: a possible involvement of thrombopoietin receptor gene expression

Pathophysiology, classification, and complications of common asymptomatic thrombocytosis in newborn infants

We frequently encounter newborn infants with thrombocytosis in the neonatal intensive care unit. However, neonatal thrombocytosis is not yet fully understood. Thrombocytosis is more frequently identified in newborns and young infants, notably more often in those younger than 2 years than in older children or adults. The production of megakaryocytes (megakaryopoiesis) and platelets (thrombopoiesis) is mainly regulated by thrombopoietin (TPO). Increased TPO levels during infection or inflammation can stimulate megakaryopoiesis, resulting in thrombopoiesis. TPO concentrations are higher in newborn infants than in adults. Levels increase after birth, peak on the second day after birth, and start decreasing at 1 month of age. Initial platelet counts at birth increase with gestational age. Thus, preterm infants have lower initial platelet counts at birth than late-preterm or term infants. Postnatal thrombocytosis is more frequently observed in preterm infants than in term infants. A high TPO concentration and low TPO receptor expression on platelets leading to elevated plasma-free TPO, increased sensitivity of megakaryocyte precursor cells to TPO, a decreased red blood cell count, and immaturity of platelet regulation are speculated to induce thrombocytosis in preterm infants. Thrombocytosis in newborn infants is considered a reactive process (secondary thrombocytosis) following infection, acute/chronic inflammation, or anemia. Thrombocytosis in newborn infants is benign, resolves spontaneously, and, unlike in adults, is rarely associated with hemorrhagic and thromboembolic complications.

Asymptomatic late thrombocytosis is a common finding in very preterm infants even in the absence of erythropoietin treatment

Objectives

Thrombocytosis is more prevalent in pediatric than in adult patients and is associated with complications or worsened outcomes after vascular events. This study aimed to determine the prevalence of thrombocytosis in very preterm infants who had not received human recombinant erythropoietin treatment (rHuEPO) and its relationship with other hematological parameters and clinical complications.

Methods

We performed a retrospective study of hematological and clinical data of very preterm infants who were admitted to our unit in their first 48 hours of life and stayed for longer than 1 week.

Results

Thrombocytosis was prevalent (32.6% of patients) in very preterm infants (≤32 weeks of gestational age, n = 193) who had not received rHuEPO. The platelet count was positively correlated with calendar age. Infants with thrombocytosis were significantly more premature (28.0 ± 2.1 versus 29.6 ± 2.2 weeks) and had a lower birth weight (1036 ± 304 versus 1303 ± 304) than those without thrombocytosis. Thrombocytosis was associated with retinopathy of prematurity after adjusting for gestational age and comorbidities, but not with other prematurity-associated complications.

Conclusions

Late asymptomatic thrombocytosis is common in very preterm infants at approximately 1 month of postnatal age and it may be associated with retinopathy of prematurity.

The CBC: reference ranges for neonates

"Normal values" for blood parameters of neonates are generally unavailable, because blood is not usually drawn on healthy, normal neonates to establish normal ranges. Instead, "reference ranges" are used, consisting of the 5th to the 95th percentile values compiled from tests performed on neonatal patients with minimal pathology, under the premise that such ranges approximate normal values. In recent years, we have been seeking to establish reference ranges for various elements of the complete blood count (CBC) of neonates, using the large databases of Intermountain Healthcare, a health care system in the western United States. Establishing these reference ranges has been facilitated by using modern hematology analyzers and electronic data repositories of clinical and laboratory information. The present review brings together several of our recent reports, displaying reference ranges for elements of the CBC among neonates at various gestational and postnatal ages.

Platelet reference ranges for neonates, defined using data from over 47,000 patients in a multihospital healthcare system

OBJECTIVE

Identifying a platelet count as abnormal (thrombocytopenia or thrombocytosis) can facilitate recognizing various disease states. However, the published reference ranges for platelet counts in neonates may be imprecise, as they were generated from relatively small sample sizes and compiled before modern platelet enumeration methods.

STUDY DESIGN

We derived new neonatal reference ranges for platelet counts and mean platelet volume (MPV) measurements using electronic data accumulated during a recent 6-year period from a multihospital healthcare system.

RESULT

Platelet counts were obtained between the first and the 90th day after birth, from 47,291 neonates delivered at 22 to 42 weeks gestation. The first platelet counts obtained in the first 3 days of life, increased over the range of 22 to 42 weeks gestation. In those born < or =32 weeks gestation, the lower reference range (5th percentile) was 104,200 microl(-1), but it was 123,100 microl(-1) in late-preterm and -term neonates. Advancing postnatal age had a significant effect on platelet counts; during the first 9 weeks, the counts fit a sinusoidal pattern with two peaks; one at 2 to 3 weeks and a second at 6 to 7 weeks. The upper limit of expected counts (95th percentile) during these peaks were as high as 750,000 microl(-1).

CONCLUSION

The figures herein describe reference ranges for platelet counts and MPV determinations of neonates at various gestational ages during their first 90 days. Expected values differ substantially from the 150,000 microl(-1) to 450,000 microl(-1) range previously used to define neonatal thrombocytopenia and thrombocytosis. The new definitions will render the diagnoses of neonatal thrombocytopenia and thrombocytosis less commonly than when the old definitions were used, because the new ranges are wider than 150,000 microl(-1) to 450,000 microl(-1).

Incomplete restoration of Mpl expression in the mpl-/- mouse produces partial correction of the stem cell-repopulating defect and paradoxical thrombocytosis

Expression of Mpl is restricted to hematopoietic cells in the megakaryocyte lineage and to undifferentiated progenitors, where it initiates critical cell survival and proliferation signals after stimulation by its ligand, thrombopoietin (TPO). As a result, a deficiency in Mpl function in patients with congenital amegakaryocytic thrombocytopenia (CAMT) and in mpl(-/-) mice produces profound thrombocytopenia and a severe stem cell-repopulating defect. Gene therapy has the potential to correct the hematopoietic defects of CAMT by ectopic gene expression that restores normal Mpl receptor activity. We rescued the mpl(-/-) mouse with a transgenic vector expressing mpl from the promoter elements of the 2-kb region of DNA just proximal to the natural gene start site. Transgene rescued mice exhibit thrombocytosis but only partial correction of the stem cell defect. Furthermore, they show very low-level expression of Mpl on platelets and megakaryocytes, and the transgene-rescued megakaryocytes exhibit diminished TPO-dependent kinase phosphorylation and reduced platelet production in bone marrow chimeras. Thrombocytosis is an unexpected consequence of reduced Mpl expression and activity. However, impaired TPO homeostasis in the transgene-rescued mice produces elevated plasma TPO levels, which serves as an unchecked stimulus to drive the observed excessive megakaryocytopoiesis.

Circulating levels of cytokines during pregnancy: thrombopoietin is elevated in miscarriage

OBJECTIVE

To evaluate the hypothesis that cytokine levels are associated with miscarriage risk using serum samples collected before report of miscarriage.

DESIGN

A nested case-control study.

SETTING

Biospecimens from the multisite Collaborative Perinatal Project, University of Florida, laboratory assessment of interleukin (IL)-1 receptor antagonist, IL-1beta, IL-4, IL-6, interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, thrombopoietin (TPO), and granulocyte colony-stimulating factor (G-CSF).

PATIENT(S)

Cases of miscarriage (n = 439) were matched to controls (n = 373) by gestational age at sample collection.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Miscarriage.

RESULT(S)

Increased risk of miscarriage was associated with elevated TPO (adjusted odds ratio [OR] 1.16, 95% confidence interval [CI] 1.00-1.36) and decreased G-CSF (adjusted OR 0.78, 95% CI 0.64-0.95). When analysis was restricted to samples collected more than 35 days before miscarriage, the effect of G-CSF was not observed (adjusted OR 0.96, 95% CI 0.72-1.28), whereas increased risk related to higher TPO remained.

CONCLUSION(S)

Circulating levels of TPO may be associated with increased risk of miscarriage.