Growth Factor Independence 1 (Gfi-1) Plays a Role in Mediating Specific Granule Deficiency (SGD) in a Patient Lacking an Abnormality in the C/EBPε Gene.

Abstract

Neutrophil specific granule deficiency (SGD) is a rare congenital disorder marked by recurrent bacterial infections of the skin and respiratory system. Neutrophils from SGD patients lack secondary and tertiary granules and their content proteins and exhibit defects in chemotaxis and bactericidal activity. A mouse model deficient for the transcription factor CCAAT/enhancer binding protein epsilon (C/EBPε) manifests a similar phenotype as SGD patients and prompted examination of the human C/EBPε gene for mutation in this disease. Mutations in the C/EBPε gene have been identified by others in two patients with SGD, resulting in loss of gene expression. However, other patients with a similar disease phenotype have a normal C/EBPε coding sequence, suggesting that other genetic abnormalities in myelopoiesis can lead to SGD. Studies in our laboratory on one such patient lacking a C/EBPε mutation demonstrated an elevated level of the C/EBPε protein in the patient's peripheral blood neutrophils as compared to the level in normal control neutrophils. Microarray analysis of this patient's bone marrow compared with that of a normal control revealed, among other genes, elevated levels of the transcription factors C/EBPε and PU.1. As a consequence, several PU.1 target genes showed increased expression in the SGD patient bone marrow. This observation was confirmed by both real time PCR and western blot analysis. PU.1 is a hematopoietic-specific transcription factor belonging to the Ets family of DNA binding proteins and plays a critical role in B-cell, macrophage and late stage neutrophil development. Sequence analysis of the PU.1 gene from our SGD patient however, did not reveal any mutation in the coding region of the gene. Western blot analysis of nuclear extracts prepared from peripheral blood neutrophils from this patient did however reveal significantly decreased levels of the transcription factor Gfi-1 (Growth factor independent-1), although no mutation has been found thus far in the coding region of the Gfi-1 gene from the SGD patient. Gfi-1 belongs to a family of zinc finger containing transcriptional repressor oncoproteins. Mice lacking Gfi-1 were found not only to be neutropenic, but also demonstrated defects in neutrophil differentiation, including loss of neutrophil secondary and tertiary granule proteins, reminiscent of SGD. More recently, heterozygous germline mutations of Gfi-1 were shown to cause severe congenital neutropenia (SCN) in humans. It has been suggested that Gfi1 represses neutrophil elastase (Ela2), germline mutations within which are a major contributor to hereditary neutropenias. Our data suggest that decreased levels of Gfi1 in our SGD patient result in increased levels of PU.1 and C/EBPε; an effect consistent with observations first made in the neutrophils of Gfi-1-null mice. The increased PU.1 levels might then act to sequester C/EBPε protein via direct protein-protein interaction. This in turn could explain the loss of secondary granule protein gene expression in the SGD patient by inducing functional C/EBPε deficiency.