The Methyltransferase Setd8 Is Essential for Erythroblast Survival and Maturation

Abstract

Erythropoiesis is a highly regulated process that generates enucleate red blood cells from committed erythroid progenitors. Chromatin condensation culminating in enucleation is a defining feature of this process. Setd8 is the sole enzyme that can mono-methylate histone H4, lysine 20 and is highly expressed in erythroblasts compared to most other cell types. Erythroid Setd8 deletion results in embryonic lethality from severe anemia due to impaired erythroblast survival and proliferation. Setd8 protein levels are also uniquely regulated in erythroblasts, suggesting a cell-type-specific role for Setd8 during terminal maturation. Consistent with this hypothesis, Setd8 Δ/Δ erythroblasts have profound defects in transcriptional repression, chromatin condensation, and heterochromatin accumulation. Together, these results suggest that Setd8, used by most cells to promote mitotic chromatin condensation, is an essential aspect of the transcriptional repression and chromatin condensation that are hallmarks of terminal erythroid maturation.

Keywords: H4K20me1; KMT5A; Setd8; chromatin condensation; erythropoiesis; heterochromatin.

Fig. 1. Setd8 is essential for mammalian erythropoiesis

(A) mRNA expression in various human cell types shown. Data from adapted from Biogps.org (Wu et al 2009) (B) Setd8 Δ/Δ and littermate control embryos. (C) Cytospins from Setd8 Δ/Δ and littermate control embryos. Blue arrow denotes maternal erythrocyte. Black arrow denotes enucleate fetal erythrocyte. (D) Benzidine counts from the fetal blood of Setd8 Δ/Δ and Setd8 Δ/+ littermate controls. (E) Quantification of CD71 and Ter119 staining from the fetal blood of E9.5 and E10.5 Setd8 Δ/Δ and Setd8 Δ/+ embryos. Representative flow cytometry plots shown in right panel. (F) Annexin V staining of Setd8 Δ/Δ and control erythroblasts at E9.5. Representative flow plots are shown in right panel. (G) Cell cycle analyses of Setd8 Δ/Δ and control erythroblasts by BrdU incorporation. Representative flow plots are shown in right panel. p-values determined by Students t-test. Error bars represent standard error of the mean.

Fig. 2. Setd8 deletion results in dysregulated gene expression and is not rescued by p53 deletion

(A) Heat map of gene expression in Setd8 Δ/Δ and littermate control erythroblasts. (B) Volcano plot, with differentially expressed genes in blue. Erythroid genes, identified by GO terms “erythrocyte maturation” and “definitive hematopoiesis,” are in red. (C) Ingenuity pathway analyses of the differentially expressed genes. (D) Gene set enrichment analyses of differentially expressed genes. (E) Photographs of embryos with genotype indicated. Bottom panel represents corresponding cytospin. (F) Benzidine counts from Setd8 Δ/Δ and control embryos, with varying p53 genotype.

Fig. 3. Setd8 expression in primary erythroblasts

(A) Setd8 protein levels in the indicated populations of maturing erythroblasts. (B) Setd8 and H4K20me1 levels in hematopoietic progenitors sorted into cell cycle fractions by DNA content. Representative histogram from cell sorting is shown in the right panel. (C) Setd8 and H4K20me1 levels in E10.5 primitive erythroblasts sorted into cell cycle fractions by DNA content. (D) Setd8 and H4K20me1 levels in definitive erythroblasts sorted into cell cycle fractions by DNA content. For all blots, histone H4 is used as a loading control. Where indicated, phospho histone H3 is used to confirm enrichment of the G2/M population and cyclin E2 is used to confirm enrichment of the S phase population.

Fig. 4. Setd8 Δ/Δ erythroblasts have a profound deficit in chromatin condensation and accumulation of heterochromatin

(A) Quantification of cell and nuclear area of Ter119+ Setd8 Δ/Δ, Setd8 Δ/+, and Setd8 +/+ erythroblasts by imaging flow cytometric analyses. Representative cell images are shown in the right panel. Error bars represent standard error of the mean. (B) Transmission electron microscopy (TEM) of Setd8 Δ/Δ and control erythroblasts at E9.5 and E11.5. Background cells in TEM images represent maternal erythrocytes used as carriers during preparation of embryonic cells for TEM. (C) Confocal microscopy with staining for H4K20me1, H3K9me2, and HP1. Top panels: cells from littermate control embryos. Bottom panels: cells from Setd8 Δ/Δ embryos. Quantification is shown in the right panel. Error bars represent SEM.