Loss of C/EBPα cell cycle control increases myeloid progenitor proliferation and transforms the neutrophil granulocyte lineage

Abstract

CCAAT/enhancer binding protein (C/EBP)α is a myeloid-specific transcription factor that couples lineage commitment to terminal differentiation and cell cycle arrest, and is found mutated in 9% of patients who have acute myeloid leukemia (AML). We previously showed that mutations which dissociate the ability of C/EBPα to block cell cycle progression through E2F inhibition from its function as a transcriptional activator impair the in vivo development of the neutrophil granulocyte and adipose lineages. We now show that such mutations increase the capacity of bone marrow (BM) myeloid progenitors to proliferate, and predispose mice to a granulocytic myeloproliferative disorder and transformation of the myeloid compartment of the BM. Both of these phenotypes were transplantable into lethally irradiated recipients. BM transformation was characterized by a block in granulocyte differentiation, accumulation of myeloblasts and promyelocytes, and expansion of myeloid progenitor populations—all characteristics of AML. Circulating myeloblasts and hepatic leukocyte infiltration were observed, but thrombocytopenia, anemia, and elevated leukocyte count—normally associated with AML—were absent. These results show that disrupting the cell cycle regulatory function of C/EBPα is sufficient to initiate AML-like transformation of the granulocytic lineage, but only partially the peripheral pathology of AML.

Mutation of C/EBPα predisposes to the development of myeloid leukemia in a retroviral insertional mutagenesis screen

“…by guest www.bloodjournal.org From myeloid progenitors in these mice, which in turn may lead to faster virus spreading even to other lineages. 31 However, contrary to what we would expect in this scenario, Cebpa BRM/BRM2 mice generally have fewer integrations than their control littermates.…”

“…30,31 F1 hybrid mice were obtained by intercrossing, and newborn pups were injected with 100 L culture supernatant (10 5 -10 6 infectious units) from SRS19-6-producing NIH3T3 fibroblasts (provided by Dr Hung Fan, University of California, Irvine 36 ). To adjust for any variation in virus titer, we sex-and litter-matched either a Cebpa ϩ/ϩ or a Cebpa ϩ/BRM2 animal with each Cebpa BRM2/BMR2 mouse analyzed.…”

“…31 We took advantage of the phenotypic Expressions levels were normalized to the level of ␤-actin mRNA. The expression level of each gene was further normalized to a sample consisting of pooled cDNA from 10 different tumors without retroviral integrations in the gene in question (p; all genotypes).…”

“…We have previously reported on the hematopoietic phenotypes of mice homozygous for the E2F repression-deficient Cebpa BRM2 allele, 30,31 which specifically abrogate the growth-inhibitory function of C/EBP␣. Young Cebpa BRM2/BRM2 mice initially suffer from neutropenia that over time progresses to a myeloproliferative syndrome or to a nonfatal AML-like syndrome with limited peripheral involvement.…”

Transcriptional dysregulation during myeloid transformation in AML

“…Nonconditional targeted disruption of CEBPA results in a selective block in early granulocyte maturation . Integrity of DNA binding and transactivation, as well as E2F interaction, resulting in downregulation of c-Myc and proliferation arrest is required for CEBPA-dependent granulocytic differentiation (Johansen et al, 2001;Porse et al, 2001Porse et al, , 2005. Conditional CEBPA deficiency in adult mice blocks the transition from common myeloid progenitors to granulocyte monocyte progenitors resulting in the accumulation of myeloid blasts in the bone marrow .…”