The role of Dlk1 in haematopoiesis, leukaemia and angiogenesis

Abstract

Delta-Like Homologue 1 (DLK1), also known as Preadipocyte Factor 1 (PREF-1), is a non-canonical EGF-like NOTCH ligand. It is maternally imprinted at the Dlk1-Dio3 imprinted locus, and has been shown to regulate embryonic growth, lipid metabolism and skeletal development. However, the precise role of Dlk1 in haematopoiesis, leukaemia and angiogenesis, processes in which it has been previously implicated, is unknown. We generated Dlk1 knockout and conditional knockout mice and used a constitutive overexpression system via retroviral transduction to specifically study Dlk1 in these contexts.

Dlk1 knockout mice showed distinctive phenotype of increased perinatal mortality and growth retardation. In foetal livers, significant expression of Dlk1 was detected in the haematopoietic cells, with higher level of expression in the haematopoietic stem cells compared to lineage positive mature cells, with overall expression decreasing with embryonic age. Dlk1 knockout mice were not significantly different from wild type mice in the mature haematopoietic lineages, but serial competitive transplant assays demonstrated Dlk1 knockout bone marrow cells were inferior to controls in reconstituting lethally irradiated recipient mice in short term haematopoietic reconstitution assays, suggesting that Dlk1 knockout led to a defect in adult short term haematopoietic stem cells.

Despite frequent overexpression of DLK1 found in many human acute myeloid leukaemias, constitutive overexpression of Dlk1 did not lead to increase in acute leukaemia or death in reconstituted mice. However, Dlk1-overexpressing haematopoietic cells demonstrated competitive repopulation advantage compared to MIG-transduced controls.

Using a retinal model of angiogenesis, Dlk1 was found to be expressed by the pericytes rather than endothelium of newly developing blood vessels in postnatal murine pups, in contrast to the published data. Conditional knockout of Dlk1 in endothelial cells using the endothelial specific Tie2 Cre transgene did not lead to significant abnormality in postnatal retina, confirming that Dlk1 did not have a functional role in the retinal endothelium. These new findings add to our current knowledge of stem cell biology and leukaemia, and the role of Dlk1 in angiogenesis.