Impact of Duffy-Null Phenotype on Neutrophil Counts and Clinical Outcomes in Patients with Sickle Cell Disease and without Hydroxyurea
Authors: Zheng Yan , Gossett Jeffery , Kang Guolian , Chen Pei-Lin , Takemoto Clifford M. 
Published: December 2024

Introduction
The Duffy blood group consists of two main antigens, Fya and Fyb, which are encoded by the atypical chemokine receptor 1 (ACKR1) gene. About 60-70% of Black individuals have the Duffy-null phenotype and do not express Fya and Fyb antigens on red cells. The Duffy-null phenotype is caused by homozygosity for a SNP (rs2814778; T&gt;C; CC) in the ACKR1 gene promoter, which disrupts the binding site for the GATA1 erythroid transcription factor and silences expression of the Duffy antigens on red cells. The Duffy null phenotype is extremely common in Blacks with lower range of absolute neutrophil counts (ANCs). The clinical impact of the Duffy-null phenotype on patients with sickle cell disease (SCD) has not been extensively studied. As hydroxyurea (HU) can affect the ANC, we examined the impact of the Duffy-null phenotype on clinical outcomes and ANC in pediatric patients with SCD who were not treated with HU.
Methods
Pediatric patients with SCD who were enrolled in the St. Jude Children's Research Hospital observational cohort study, the Sickle Cell Clinical Research and Intervention Program (SCCRIP; NCT02098863) were eligible. Patients who had available Duffy genotype from whole genome sequencing data were included. ANC at steady state was defined as count collected 30 days from acute care visits or hospitalization. We analyzed the data biannually from age 0 to 18 years old. The number of clinical events were extracted and counted for each period. The median value of ANCs was used if there were multiple measurements during one period. The observation period was censored at the hydroxyurea treatment initiation date if relevant, 18 years old or at last follow up for patients younger than 18 years old at data frozen date of 12/31/2021. The Wilcoxon rank sum test was used to compare ANCs and clinical outcomes between the Duffy-null and Duffy-positive phenotypes for each age period. False discovery rate adjusted p-values (= q-value) were calculated to account for multiple comparisons across nine age periods. Generalized linear mixed models with Gaussian and quasi-Poisson link functions were used to assess the associations of the Duffy-null phenotype with ANCs and clinical outcomes.
Results
A total of 492 patients with available Duffy genotypes and all were identified as Black. The median age of the patients was 16.5 years old (interquartile range (IQR): 12.7-21.0), and 50.4% (248 patients) of the patients were females. The hemoglobin (Hb) genotypes included HbSS/SBeta0-thalassemia (75.6%, 372), HbSC (19.7%, 97) and HbSBeta+-thalassemia and others (4.7%, 23). The Duffy null phenotype was found in 71% (348) of the patients. Among 29.1% (143) patients who did not receive HU treatment, we examined ANCs by age in patients with severe (HbSS/SBeta0-thalassemia) and mild (HbSC/ HbSBeta+-thalassemia and others) SCD separately. Patients with HbSS/SBeta0-thalassemia showed increased ANCs with age, from 3400 /mm3 (IQR: 2445-4400) at 0-2 years of age to 7100 /mm3 (IQR: 5700-10850) at 16-18 years of age (p&lt;0.001). Duffy-null patients had similar ANCs compared to Duffy-positive patients at all age ranges (q&gt; 0.05), which is consistent with the literature (Pincez et al, 2023). For patients with HbSC/ HbSBeta+-thalassemia, ANCs increased over time as well (p&lt; 0.001) but with a lower slope than patients with HbSS/SBeta0-thalassemia (p&lt; 0.001), from 3050 /mm3 (IQR: 2275-3875) at age of 0-2 years old to 4140 /mm3 (IQR: 3500-6650) at age of 16-18 years old. Notably, Duffy-null patients showed significantly lower ANCs than Duffy-positive patients at all age ranges examined (q&lt; 0.05) in this group. We also examined the impact of the Duffy-null phenotype on clinical outcomes and found that compared to Duffy-positive patients, Duffy-null patients had similar rates of acute care visits, hospitalization, vaso-occlusive crisis, acute chest syndrome, fever, invasive infections, and acute splenic sequestration regardless of the Hb genotypes.
Conclusion
In the absence of HU treatment, the Duffy-null phenotype was associated with low ANCs in patients with HbSC/ HbSBeta+-thalassemia at all ages during childhood, but did not affect ANCs in patients with HbSS/SBeta0-thalassemia. The Duffy-null phenotype was not associated with higher risks of SCD-associated complications in patients without HU treatment.