Gene test interpretation: ACKR1 (Duffy blood group gene)

INTRODUCTION — The ACKR1 gene encodes the Duffy antigen receptor for chemokines (DARC), which is expressed on red blood cells (RBCs), endothelial cells, and other cell types.

Genetic variation in ACKR1 determines Duffy blood group status, which in turn impacts malaria resistance, hemolytic transfusion reactions, and hemolytic disease of the fetus and newborn (HDFN). Duffy antigen expression also affects baseline neutrophil counts.

Testing of ACKR1 may be performed as a component of a multi-gene panel for transfusion antigens ("RBC genotyping") to guide transfusions and evaluate hemolytic transfusion reactions. Targeted testing of ACKR1, especially for the c.-67T>C variant, may be performed to identify a genetic influence on baseline neutrophil counts. (See 'Neutrophils (effect of Duffy status)' below.)

This monograph discusses clinical implications of ACKR1 variants. Details of indications for testing and management are discussed separately [1]. (See 'UpToDate topics' below.)

BACKGROUND

Terminology

●ACKR1 and DARC – ACKR1 (figure 1) encodes the atypical chemokine receptor 1, also called Duffy antigen receptor for chemokines (DARC). DARC was the former name of the ACKR1 gene.

The DARC protein is a transmembrane receptor for chemoattractant cytokines (chemokines) of the CXC and CC families. (See "Transplantation immunobiology", section on 'Chemokines and chemokine receptors'.)

Other names for the protein include Duffy blood group, Fy glycoprotein, and CD234.

●Duffy blood group antigens – Duffy (Fy) blood group antigens (Fy^a and Fy^b) are genetically determined by a single nucleotide difference in the ACKR1 coding region that changes a single amino acid.

•Fy^a – Guanine at nucleotide 125 defines the FY*A haplotype, with glycine at amino acid 42.

•Fy^b – Adenine at nucleotide 125 defines FY*B haplotype, with aspartate at amino acid 42.

●Duffy-null phenotype – Duffy-null, also called Fy(a−b−), is characterized by lack of Duffy antigens on red blood cells (RBCs). (See "Red blood cell antigens and antibodies", section on 'Duffy antigens'.).

The Duffy-null phenotype is caused by a variant in the ACKR1 promoter that results in loss of expression of Duffy antigens on RBCs but does not affect expression on other cell types. (See 'Expression of ACKR1/DARC' below.)

●DANC – Duffy-null associated neutrophil count (DANC) is the normal range for neutrophil counts in individuals with Duffy-null RBCs. This is a genetically-determined baseline neutrophil count that is often lower than commonly-used laboratory reference ranges. This was formerly known as "benign ethnic neutropenia," but it is not a form of neutropenia or a risk factor for infection; rather, DANC represents normal human variation. (See 'Reference ranges for Duffy-null and non-Duffy-null' below.)

Expression of ACKR1/DARC

●RBCs

•Duffy blood group – The variants that determine Fy^a and Fy^b are autosomal co-dominant. (See "Red blood cell antigens and antibodies", section on 'Duffy antigens'.)

Presence of the variant at one allele is sufficient to confer the presence of that Duffy antigen. There are four main RBC phenotypes:

-Fy(a+b+)

-Fy(a+b−)

-Fy(a−b+)

-Fy(a−b−), also called Duffy-null

•Duffy-null – The presence or absence of Duffy antigens on RBCs depends on erythroid-specific transcription of ACKR1. Absence of Duffy antigens (Duffy-null) results from homozygosity for the ACKR1 promotor variant rs2814778-C, which contains cytosine instead of thymine at position –67 (also designated –46, depending on the numbering system), which prevents binding of the GATA1 transcription factor (figure 1) in RBC precursors [2]. The variant is designated c.-67T>C; homozygosity may be abbreviated C/C.

The c.-67T>C promoter variant is commonly linked to the coding region variant FY*B [3]. It prevents expression of the FY*B allele and is sometimes called FY*B^ES for "erythrocyte silent." The variant can rarely be associated with FY*A (FY*A^ES).

Duffy-null status is autosomal recessive. Only individuals who are homozygous for the c.-67T>C variant are Duffy-null. Other rare ACKR1 variants can also cause the Duffy-null phenotype but do not affect neutrophil count [2].

●Neutrophils – DARC (and Duffy antigens) are not expressed on neutrophils. However, the presence or absence of Duffy antigens on RBCs affects the baseline neutrophil count. (See 'Neutrophils (effect of Duffy status)' below.)

●Endothelial and epithelial cells – DARC is expressed on endothelial cells (capillary and postcapillary venules), epithelial cells (kidneys, collecting ducts, and lung alveoli), and Purkinje cells in the cerebellum.

Homozygosity for c.-67T>C does not affect expression of FY*B in non-RBC cell types. This is why Duffy-null individuals generally do not develop anti-Fy^b [2].

Geographic variation — Geographic mapping has been used to determine the distribution of ACKR1 alleles. The most common alleles in contemporary populations are [3]:

●Sub-Saharan Africa – FY*B^ES/FY*B^ES (producing the Duffy-null phenotype) in >95 percent  

●Americas – FY*A/FY*B in 31 percent

●Europe – FY*A/FY*B in 52 percent

●Asia – FY*A/FY*A in >80 percent

Worldwide distribution of the Duffy-null phenotype is illustrated in the figure (figure 2).

CLINICAL IMPLICATIONS

How to read the genetic test report — As with all genetic testing, confirm the correct person was tested, determine which gene(s) were evaluated, and verify that the test was performed in a Clinical Laboratory Improvement Amendments (CLIA)-certified laboratory or other nationally certified laboratory. These principles are summarized in the table (table 1).

Duffy blood group antigens — All red blood cells (RBCs) have surface antigens that determine blood type. When certain antigens are absent, exposure to these antigens via allogeneic blood (transfusion, pregnancy, sharing needles) can elicit an immune response. Alloantibodies against Duffy blood group antigens (anti-Fy) can cause clinically significant hemolysis (table 2).

Genetic testing and RBC serologic testing are considered comparable for determining Duffy antigen status for transfusion practice. Most transfusion medicine services will use serologic testing, although the genotype may be helpful in certain settings such as recent transfusion.

Hemolytic transfusion reactions — Hemolytic transfusion reactions can occur when an individual has an alloantibody against Fy^a or Fy^b and is transfused with RBCs that express that antigen. (See "Hemolytic transfusion reactions", section on 'Pathophysiology'.)

●Individuals who are Fy(a+b–) can make anti-Fy^b.

●Individuals who are Fy(a-b+) can make anti-Fy^a.

●Individuals who are Fy(a+b+) do not make anti-Fy^a or anti-Fy^b.

●Individuals who are Duffy-null typically can make anti-Fy^a but not anti-Fy^b, because they express the "b" antigen on other tissues. (See 'Expression of ACKR1/DARC' above.)

Routine pretransfusion testing will identify alloantibodies to Duffy antigens. If transfusion is needed, antigen-negative units will be provided. (See "Pretransfusion testing for red blood cell transfusion", section on 'Antibody screen'.)

Hemolytic disease of the fetus and newborn (HDFN) — If fetal RBCs express antigens inherited from the father that are recognized by the maternal immune system as foreign, alloimmunization can occur, and alloantibodies can cross the placenta and cause hemolysis in the fetus and newborn.

Anti-Fy^a can be associated with HDFN. During pregnancy, a maternal antibody screen that identifies anti-Fy^a will prompt testing of the father and enhanced monitoring for HDFN if appropriate. (See "Management of non-RhD red blood cell alloantibodies during pregnancy", section on 'Duffy' and "Management of non-RhD red blood cell alloantibodies during pregnancy", section on 'Antibody screening'.)

Malaria risk — Duffy antigen receptor for chemokines (DARC) serves as the only receptor on RBCs for Plasmodium vivax and the less common P. knowlesi [3]. Duffy-null individuals are protected from infection with these malarial species. P vivax and P knowlesi merozoites can attach to Fy(a–/b–) RBCs, but they cannot enter the RBC and eventually detach, leaving the cell markedly deformed. Risk for other malaria species is not affected. (See "Pathogenesis of malaria" and "Protection against malaria by variants in red blood cell (RBC) genes", section on 'Duffy blood group system'.)

Neutrophils (effect of Duffy status)

Absolute neutrophil count (ANC) — An individual's Duffy status (Duffy-null versus non-Duffy-null) affects the reference ranges for baseline neutrophil counts. Duffy-null status is a normal variant. It is not a disease, and it does not cause any clinical disorder (table 2).

The ANC is an absolute number of mature neutrophils and band forms per unit of blood and thus a more accurate measure of the quantity of circulating neutrophils than the percentage of neutrophils.

The mechanism by which Duffy status determines the normal range for the ANC remains unclear. Hypotheses include a role for RBC DARC as a sink for circulating chemokines, alterations in precursor cell differentiation in the bone marrow, and increased trafficking of neutrophils to other compartments (spleen, bone marrow, vasculature, or other tissues) in Duffy-null individuals [2,4,5]. DARC is not expressed on neutrophils. (See 'Expression of ACKR1/DARC' above.)

Reference ranges for Duffy-null and non-Duffy-null — Reference ranges for ANC differ for individuals with Duffy-null and non-Duffy-null phenotypes (reference ranges also differ for children and adults). Duffy-null-associated neutrophil count (DANC) refers to the ANC in individuals who are homozygous for the c.-67T>C promotor variant in ACKR1. (See 'Expression of ACKR1/DARC' above.)

If there is a question regarding the reference range for a particular individual, Duffy-null status can be ascertained by genotyping (preferred) or by RBC phenotyping using serologic testing. Serologic RBC phenotyping may have an important limitation in identifying DANC, as RBCs may type as Duffy-null if they are Fy(a–b–) or if they carry the rare variant Fy^x, which is not associated with DANC [6].

Geographic ancestry may be a reasonable marker of increased likelihood of a certain genotype (see 'Geographic variation' above), but race should not be used as a proxy for biological differences in neutrophil count [7,8]. (See "Overview of neutropenia in children and adolescents", section on 'Normal variants' and "Approach to the adult with unexplained neutropenia", section on 'Normal variants <1500/microL'.)

In a study of 120 individuals who self-identified as Black or African American seen for routine, nonurgent outpatient visits, 80 (67 percent) were Duffy-null and 40 (33 percent) were non-Duffy-null [9]. ANCs in the two groups were:

●Duffy-null – Median ANC 2820/microL, range 1080 to 5950/microL

●Non-Duffy-null – Median ANC 5005/microL, range 2360 to 10,500/microL

Nearly one-fourth of Duffy-null individuals had an ANC below the lower limit of published reference intervals; one-tenth had an ANC <1500/microL, which despite being a normal variant in these participants could be designated as neutropenia and could lead to unnecessary bone marrow examinations and inappropriate dose reduction or withholding of important medications. (See 'Drug dosing' below.)

A larger study reported similar findings, with an ANC <1000/microL in >5 percent of individuals with DANC [10].

Risk of infections — DARC has no impact on response to bacterial or common viral infections [11]. People with DANC have normal bone marrow cellularity and a robust neutrophil response to infection [9].

●Duffy-null individuals have resistance to P vivax and P knowlesi malaria. (See 'Malaria risk' above.)

●Some studies have suggested that Duffy-null individuals have an increased risk of HIV infection; others have suggested they have a survival advantage with HIV [2,12].

Drug dosing — One concern for individuals with DANC is that they will receive dose reductions or have doses held for important drugs based on their lower baseline neutrophil count; this may include critical medications for inflammatory conditions, sickle cell disease, and cancer.

It is important to consider the possibility of DANC and to avoid inadvertently withholding or underdosing medications, a practice that affects predominantly people with African ancestry [13]. Genotyping or phenotyping for DANC can reduce racial disparities in health care. (See "Overview of pharmacogenomics", section on 'Potential benefits of genotyping'.)

First-degree relatives — First-degree relatives of the tested individual may benefit from knowing their ACKR1 genotype or Duffy antigen status for purposes of determining whether their ANC is in the normal range and in determining the likelihood of HDFN during pregnancy. (See 'Clinical implications' above.)

Consultation with experts in genetics, hematology, or transfusion medicine may be helpful. (See 'Listings of experts' below.)

RESOURCES

UpToDate topics

●Duffy antigen – (See "Red blood cell antigens and antibodies", section on 'Duffy blood group system'.)

●Neutrophil count – (See "Overview of neutropenia in children and adolescents" and "Approach to the adult with unexplained neutropenia".)

●Transfusion reactions – (See "Hemolytic transfusion reactions".)

●Hemolytic disease of the fetus and newborn – (See "Management of non-RhD red blood cell alloantibodies during pregnancy".)

Listings of experts

●Genetic counselors – National Society of Genetic Counselors (NSGC)

●Clinical geneticists – American College of Medical Genetics and Genomics (ACMG)

●Hematologists – American Society of Hematology (ASH)

●Transfusion medicine – Association for the Advancement of Blood & Biotherapies (AABB)