TRIM28 Is Essential For Erythroblast Differentiation In The Mouse

We previously identified TRIM28 (TIF1β, KAP-1) as a component of the mammalian DRED complex that prior studies had implicated as an embryonic/fetal β-type globin gene repressor. TRIM28 has been previously characterized as a transcriptional co-repressor thst interacts with heterochromatin protein 1 (HP1) and histone H3K9 methyltransferase SETDB1. We tested the contribution of TRIM28 to globin gene regulation and erythropoiesis by employing a mutant mouse that could be inducibly ablated for the Trim28 gene during the adult stage. The Mx1Cre transgene was activated by injecting poly(I:C) 5 times into these compound mutant mice every 48 hours. We hypothesized that TRIM28 might normally contribute to silencing of the embryonic and/or fetal globin genes and therefore that one might anticipate that the mouse εY- and/or βH1-globin genes (the structural homologues of human embryonic ε− and fetal γ-globin genes, respectively) would be de-repressed in the absence of TRIM28. To our surprise, the expression of εY- and βH1-globin mRNAs was not altered after induced genetic loss of Trim28. However, we discovered that its loss results in defective immature erythropoiesis in bone marrow and consequently to normocytic anemia. To address possible mechanisms that might be responsible for this observed defect in erythropoiesis, we analyzed staged, immature erythroid cells isolated from the Trim28 mutant mice by RNA-Seq and qRT-PCR. We observed diminished expression of multiple erythroid transcription factors and heme biosynthetic enzymes in the mutant. We also discovered increased expression of multiple apoptosis-related genes, and experimentally confirmed an increase in early apoptotic cells (Annexin V+ DAPI-) in the Trim28 mutant erythroblasts. We conclude that TRIM28 is vital for immature erythropoiesis in the adult bone marrow.