Sickle Cell Trait and Circulating Proteome

Sickle cell trait (hemoglobin AS) is the most prevalent hemoglobin carrier state in the world, found in over 3 million individuals in the United States and an estimated 300 million individuals worldwide.¹ Systemic erythrocyte sickling does not occur in sickle cell trait; therefore, sickle cell trait is not considered a sickling “disease.” On the other hand, overt systemic sickling does occur in homozygous hemoglobin SS (sickle cell anemia); therefore, children of parents with sickle cell trait who inherit sickle cell anemia are at higher risk for the development of and early mortality from end-organ damage. Acute complications of sickle cell anemia include infection, pain, and acute chest syndrome. Owing to the high incidence of emergency department visits and hospitalizations in patients with sickle cell anemia and the ability to ameliorate acute sickle cell complications with modifying therapies, patients with sickle cell anemia require frequent clinical care visits from experts in sickle cell disease. By contrast, individuals with sickle cell trait require counseling, often provided to parents, but do not require any follow-up with a hematologist.

Although systemic erythrocyte sickling and hemolysis does not occur in sickle cell trait, there is increasing evidence that local erythrocyte changes may occur in the areas of the body characterized by extreme hypoxia and acidosis, such as the renal medulla. Numerous large epidemiologic studies over the past decade have shown that adults with sickle cell trait are at a 1.5- to 2.0-fold risk of developing CKD and for progression to kidney failure.^2,3 However, not all individuals with sickle cell trait develop kidney disease; therefore, identification of risk modifiers and predictive markers is of particular interest.

In an attempt to elucidate potential pathways for kidney disease progression and to develop predictive risk modeling specific to patients with sickle cell trait, Cai et al. performed a 1:1 proteomic analysis of race and age-matched controls with and without sickle cell trait in the Women's Health Initiative (WHI), published in this issue of CJASN.⁴ Second, they attempted to replicate their findings within the Jackson Heart Study and Atherosclerosis Risk in Communities Study. Finally, they evaluated protein associated with the development of kidney failure in the 19 patients who progressed during the WHI study longitudinal follow-up period. The authors identified several potential circulating proteomes that are of great interest for future studies. An important question for the field of sickle cell trait is whether red blood cell sickling occurs in the renal medulla similar to patients with sickle cell anemia. If so, mechanistic and therapeutic approaches could be adopted that benefit both patient populations.

In the initial analysis within the WHI, this study identified three strong associations with CX3CL1, KIM-1, and HMOX1 that remained significant in the sensitivity analysis after eGFR adjustment. CX3CL1 has not been associated as a protein of interest in patients with sickle cell anemia; however, it recruits innate immune cells to the kidney. Although there is no consistent evidence that sickle cell trait is associated with increased inflammation, free heme and endothelial injury in sickle cell disease does contribute to disease biology.⁵ Whether similar inflammatory processes occur locally within the kidney of individuals with sickle cell trait is intriguing.

Urinary KIM-1 has been identified as a biomarker associated with both albuminuria and lower eGFR values in patients with sickle cell disease and murine sickle cell models.⁶ As KIM-1 is a biomarker of proximal tubular injury due to ischemia/reperfusion, the elevations of KIM proteins found in this study further confirm its inclusion in future research for kidney disease progression. In sickle cell, increase in KIM-1 is postulated to be associated with an increase in free heme exposure. However, the mechanism by which tubular injury leads to increased KIM-1 proteins in patients with sickle cell trait is poorly understood, and consideration for subclinical hemolysis remains relevant as postulated by the authors.

Finally, HMOX1 long tandem repeats has been associated with a lower eGFR and higher risk for AKI in patients with sickle cell disease.⁷ Heme-oxygenase-1 is induced during kidney injury and protects the kidney from injury associated with intravascular hemolysis. By degrading free heme, HO-1 reduces reactive oxygen species exposure and injury within the renal proximal tubular cells. In this study, the authors identified an association with HMOX1 and lower eGFR. Similar to KIM-1, it is critical that research continues to focus on clinical (sickle cell anemia) or subclinical (sickle cell trait) hemolysis within the kidney that portends earlier development of kidney disease.

A consistent concern for biomarker studies within the sickle cell research community is being able to identify a steady state level that reflects true baseline levels of biomarkers. Variability in acute hemolysis may alter daily levels of biomarkers associated with heme-mediated kidney injury, and the intermittent use of nephrotoxic medications may further increase this variability. Although patients with sickle cell trait may not experience daily overt anemia as compared with patients with sickle cell anemia, it is plausible that sickling of trait red blood cells in the hypoxic, hypertonic, acidic environment of the renal medulla leads to subclinical hemolysis as suggested in this study. Furthermore, we know that in extremes (heat, altitude, exercise), patients with sickle cell trait can develop acute sickling of red blood cells; acute organ injury, including AKI; and death. Therefore, it is plausible that patients with sickle cell trait also experience daily fluctuations of subclinical hemolysis in the renal medulla, which could increase the variability of daily circulating proteomes associated with heme-mediated kidney injury. Second, the use of single eGFR values is important to suggest the risk of CKD; however, it is important to ensure that low bias exists in eGFR equations. In patients with sickle cell anemia, intrapatient variability exists between eGFR and measured GFR. Therefore, it is important to extend the current findings in longitudinal studies with repeated analysis of circulating proteins and eGFR measurement to validate these findings and confirm an association between circulating proteins and progressive decline in eGFR. This exciting study should provide the basis for future longitudinal studies that can confirm mechanistic pathways and targets for novel therapeutics to minimize sickle cell trait–related progression to kidney failure.

Disclosures

J. Lebensburger reports consultancy for Agios, Bioproduct Laboratory, Editas, Forma Therapeutics, and Novartis and an advisory or leadership role as Director, Office of Faculty Development, UAB. The remaining author has nothing to disclose.

Funding

None.

Author Contributions

Conceptualization: Jeffrey Lebensburger, Rakhi Naik.

Writing – original draft: Jeffrey Lebensburger, Rakhi Naik.

Writing – review & editing: Jeffrey Lebensburger, Rakhi Naik.