Trilineage Perturbation of Hematopoiesis In Neonates with Down Syndrome

Abstract 876

Children with Down syndrome (DS), under the age of 4 years, have a 150-fold excess risk of myeloid leukaemia (DS-ML), which arises in utero and is of erythro-megakaryocyte phenotype. Two known factors required for leukemic transformation- are: (i) presence of trisomy 21 (T21) in fetal liver (FL) hematopoietic cells and (ii) acquired N terminal-truncating mutation(s) of the transcription factor gene GATA1. Together, they lead to the preleukemic condition, Transient Abnormal Myelopoiesis (TAM). In humans, T21 provides the crucial initiating perturbation of hematopoiesis by inducing a marked expansion of FL megakaryocyte-erythroid progenitors (MEP) during the second trimester^[1]. Mouse models have suggested that effects of T21 are confined to FL and are lost post-natally^[2]. However, retrospective studies have reported blood counts abnormalities in some neonates with DS suggesting abnormal fetal hematopoiesis may persist beyond the second trimester into post-natal life.

To investigate this, blood counts and smears in the first week of life from the first 100 babies with DS enrolled in a prospective, longitudinal study have been compared with blood counts and smears from normal term cords (n=100) and normal term and preterm neonates in the first week of life (n=200).

11/100 neonates with DS had a diagnosis of Transient Abnormal Myelopoiesis (TAM) by WHO criteria and confirmed by GATA1 mutation; 22 (2 with TAM) were preterm- gestational age (GA)< 37 weeks). In neonates with DS, with or without TAM, Hb concentrations, hematocrits and RBC counts were higher than GA matched neonates without DS; 51 (57.3%) of babies with DS had a Hb concentration of >20 g/dl (vs 2% of non DS neonates); only 2 (2.2%) of DS neonates had a Hb <15 g/dl (range 14.5–25.4 g/dl; median 20.3 vs 16.0g/dl; p <0.001). All blood smears from neonates with DS showed macrocytosis, increased polychromasia and increased circulating erythroblasts compared to normal controls. Platelet counts were lower in neonates with DS either with TAM (median 125 x10⁹/L) or without TAM (median 144 x10⁹/L) vs normal controls (277 x10⁹/L) (p< 0.001). There were increased numbers of giant platelets, megakaryocyte fragments and abnormal platelet morphology in blood smears from all babies with TAM and 90% of DS neonates without TAM compared to 8% of non-DS neonates. Quantitative and qualitative abnormalities of erythropoiesis and thrombopoiesis in DS neonates were independent of GA, birthweight and cardiac disease.

Although total white cell, neutrophil, lymphocyte and eosinophil counts in DS neonates without TAM were not significantly different from normal term cord or GA-matched peripheral blood samples, monocyte (median 0.97 vs 0.7 × 10⁹/L) and basophil counts (median 0.2 vs 0 × 10⁹/L) were increased. Dysplastic features, (abnormal granulation and nuclear morphology) were seen in neutrophils, monocytes and basophils in ∼80% of DS neonates without TAM and in 11/11 with TAM; these findings were rare in term and preterm neonates without DS. Circulating blasts were seen on blood smears from all neonates with DS both with TAM (median 34%; range 15–54%) and without TAM (median 3%; range 0.5– 11%) but rarely in normal controls (median 0, range 0–8%). Many, but not all, blasts in neonates with DS without TAM were morphologically megakaryoblasts and indistinguishable from those in TAM.

Thus, abnormalities of hematopoiesis affecting all myeloid lineages are present in most, if not all, neonates with DS, regardless of whether or not they have TAM and GATA1 mutation(s). Consistent with this, we found that although cord blood from DS neonates without TAM had normal overall numbers of MEP, CMP and GMP compared to normal term cord, clonogenicity of megakaryocyte and erythroid progenitors cultured from flow sorted DS cord blood MEP and CMP was markedly increased compared to normal term cord blood (MEP 55 vs 18%; CMP 67 vs 10%; DS vs normal) whilst DS cord blood HSC and MPP populations gave rise to increased numbers of GM and myeloid blast colonies. These data show that in humans with DS, trilineage perturbation of fetal hematopoiesis persists into post-natal life, in contrast to mouse models.

1. Tunstall-Pedoe, O., et al., Abnormalities in the myeloid compartment in Down Syndrome fetal liver precede acquisition of GATA1 mutations. Blood, 2008. 112: 4507–11

2. Alford, K.A., et al., Perturbed hematopoiesis in the Tc1 mouse model of Down syndrome. Blood, 2010. 115: 2928-37