Analyses of genome wide association data, cytokines, and gene expression in African-Americans with benign ethnic neutropenia

Abstract

BackgroundBenign ethnic neutropenia (BEN) is a hematologic condition associated with people of African ancestry and specific Middle Eastern ethnic groups. Prior genetic association studies in large population showed that rs2814778 in Duffy Antigen Receptor for Chemokines (DARC) gene, specifically DARC null red cell phenotype, was associated with BEN. However, the mechanism of this red cell phenotype leading to low white cell count remained elusive.MethodsWe conducted an extreme phenotype design genome-wide association study (GWAS), analyzed ~16 million single nucleotide polymorphisms (SNP) in 1,178 African-Americans individuals from the Reasons for Geographic and Racial Differences in Stroke (REGARDS) study and replicated from 819 African-American participants in the Atherosclerosis Risk in Communities (ARIC) study. Conditional analyses on rs2814778 were performed to identify additional association signals on chromosome 1q22. In a separate cohort of healthy individuals with and without BEN, whole genome gene expression from peripheral blood neutrophils were analyzed for DARC.ResultsWe confirmed that rs2814778 in DARC was associated with BEN (p = 4.09×10−53). Conditioning on rs2814778 abolished other significant chromosome 1 associations. Inflammatory cytokines (IL-2, 6, and 10) in participants in the Howard University Family Study (HUFS) and Multi-Ethnic Study in Atherosclerosis (MESA) showed similar levels in individuals homozygous for the rs2814778 allele compared to others, indicating cytokine sink hypothesis played a minor role in leukocyte homeostasis. Gene expression in neutrophils of individuals with and without BEN was also similar except for low DARC expression in BEN, suggesting normal function. BEN neutrophils had slightly activated profiles in leukocyte migration and hematopoietic stem cell mobilization pathways (expression fold change <2).ConclusionsThese results in humans support the notion of DARC null erythroid progenitors preferentially differentiating to myeloid cells, leading to activated DARC null neutrophils egressing from circulation to the spleen, and causing relative neutropenia. Collectively, these human data sufficiently explained the mechanism DARC null red cell phenotype causing BEN and further provided a biologic basis that BEN is clinically benign.

Ethnic benign neutropenia: A phenomenon finds an explanation

“…However, it is unclear if this model reflects all features of the human ENP, including NP. Average serum IL‐8 and G‐CSF levels were found to be about 50% and 70% higher in African than in Caucasian volunteers; this finding was not confirmed in a recent study …”

“…Minor infections, such as the common cold, often accompanied by slight CRP rises suffice to increase the ANC above 1.5 G/L (JP, unpublished observations). Such phenomena might account for reported normal ANC in a third of ACKR1 ‐mutated individuals …”

“…Although the two conditions were now genetically linked and coexist in most subjects, it was not clear in which way low blood ANC in these individuals was coupled to the absence of the Duffy antigen on erythroid cells. Adding to the enigma was the fact that the Duffy antigen was not considered to be expressed on PMN in Duffy‐positive individuals.…”

“…However, in 2017 Duchene et al presented data showing that the rs2814778 SNP was in fact associated with an abnormal neutrophil phenotype, suggesting an enhanced efflux of PMN from blood to tissues and an increased expression of the CD16 molecule on PMN. Very recently, Charles et al added evidence that PMNs, in fact, express low levels of the Duffy antigen and show an activated migratory phenotype. Below, we describe the Duffy‐null and ENP histories.…”

“…There have been various speculations on how the lack of Duffy antigen, selectively expressed on erythroid cells, leads to NP. However, the role of the chemokine sink hypothesis as an explanation for ENP has been questioned because no signal was detected for such associations in a cytokine genome wide expression assay …”