DNA polymerase epsilon (Pol ε), a component of the core replisome, is involved in DNA replication. Although genetic defects of Pol ε have been reported to cause immunodeficiency syndromes, its role in haematopoiesis remains unknown. Here, we identified compound heterozygous variants (p.[Asp1131fs];[Thr1891del]) in POLE, encoding Pol ε catalytic subunit A (POLE1), in siblings with a syndromic form of severe congenital transfusion-dependent anaemia. In contrast to Diamond-Blackfan anaemia, marked reticulocytopenia or marked erythroid hypoplasia was not found. Their bone marrow aspirates during infancy revealed erythroid dysplasia with strongly positive TP53 in immunostaining. Repetitive examinations demonstrated trilineage myelodysplasia within 2 years from birth. They had short stature and facial dysmorphism. HEK293 cell-based expression experiments and analyses of patient-derived induced pluripotent stem cells (iPSCs) disclosed a reduced mRNA level of Asp1131fs-POLE1 and defective nuclear translocation of Thr1891del-POLE1. Analysis of iPSCs showed compensatory mRNA upregulation of the other replisome components and increase of the TP53 protein, both suggesting dysfunction of the replisome. We created Pole-knockout medaka fish and found that heterozygous fishes were viable, but with decreased RBCs. Our observations expand the phenotypic spectrum of the Pol ε defect in humans, additionally providing unique evidence linking Pol ε to haematopoiesis.

We have identified a myeloid cell-specific enhancer in the 5' flanking region of the Xenopus tropicalis myeloperoxidase gene. Transgenic reporter analysis using Xenopus laevis revealed that the expression of GFP was detected in the tail fin macrophages of a swimming tadpole, and the distributions of the GFP-positive and XL-2 (a pan-marker for leukocytes)-positive cells were mostly overlapping. The GFP-positive cells in the liver of the transgenic tadpole were localized in the same areas where the myeloid cells were present. Isolation of leukocytes from the peripheral blood cells followed by flow cytometric analysis revealed that the GFP-positive fraction was specifically enriched in neutrophils with lobulated nuclei. Furthermore, the macrophages purified from the peritoneal cavity were also GFP-positive. In summary, a transgenic frog line in which the myeloid cells are labeled with GFP provides a useful tool to elucidate the physiological role of myeloid cells of multiple origins in the embryo.