The role of the fetal microenvironment in abnormal fetal haematopoiesis in Down syndrome

Abstract

Children with Down syndrome (DS; trisomy 21, T21) have a 500-fold risk of developing acute megakaryoblastic leukaemia (AMKL) in early childhood compared to children without DS. DS-AMKL is preceded by a transient neonatal leukaemia, unique to DS, caused by acquired mutations in the GATA1 gene in fetal liver (FL) haematopoietic stem/progenitor cells (HSPC). Previous work has shown that T21 itself perturbs FL haematopoiesis prior to acquisition of GATA1 mutations providing a cellular substrate for leukaemic transformation. Similar changes are not reported in fetal bone marrow (FBM). The mechanism(s) that perturb FL haematopoiesis in DS and the role of the unique fetal microenvironment in DS are unknown. My project investigated the role of the environment in abnormal fetal haematopoiesis in DS using both an unbiased approach to evaluate the global gene expression by fetal stromal cells and a candidate pathway approach, investigating the specific role of the insulin-like growth factor (IGF) signalling pathway, a key regulator of cell proliferation previously linked to AMKL development in DS. To examine the role of the IGF system in fetal haematopoiesis, I measured expression of the IGF signalling proteins in stromal cells. In comparison with adult BM MSC, fetal MSC expressed marked differences in components of the IGF signalling pathway. In particular, IGF1 was selectively expressed by adult BM MSC whilst fetal MSC expressed IGF2 with very little IGF1. I next assessed the expression of IGF receptors (R) on FL haematopoietic progenitors. Both normal and DS FL CD34+ cells expressed high levels of IGF1R, confirming potential for IGF responsiveness, while IGF2R was slightly reduced in DS. IGF2 promoted the growth of DS FL megakaryocyte colonies in serum-free clonogenic assays and of megakaryocytes in liquid culture, implicating the IGF pathway in the perturbation of DS fetal haematopoiesis. To further investigate the DS fetal microenvironment, I derived mesenchymal stromal cells (MSCs) from normal and DS FL and FBM and performed gene expression profiling using microarray. Analysis identified marked differences between normal FL and FBM and between normal and DS populations. In particular, several genes which encode secreted proteins, including IGFBP1, showed differential expression in DS, suggesting that they may play a role in the abnormal haematopoiesis. Furthermore, there were remarkably few differences between DS FL and DS FBM MSC indicating strong similarities in the transcriptomes of these two microenvironments. Finally, I established a co-culture system to assess the effects of primary MSC on the differentiation of primary FL HSPC. Intriguingly, preliminary experiments indicate that DS MSC, but not normal FL MSC, promote erythroid differentiation of normal FL HSPC supporting the hypothesis that differences in the transcriptome of DS fetal liver MSC are functionally important. This finding provides strong evidence that the FL microenvironment is likely to play a significant role in the disruption of fetal liver haematopoiesis in DS.