Sickle Cell Genotype and Biomarkers of Endothelial Dysfunction Predict Early Kidney Disease in Patients with APOL1 High Risk Variants

Albuminuria is the earliest glomerular manifestation of sickle cell kidney disease. We previously demonstrated that about 25% of sickle cell patients with high risk APOL1 variants develop albuminuria prior to age 10 years. It has not been established whether inclusion of all sickle cell genotypes and biomarkers of kidney disease can identify the highest risk patients for early albuminuria. We hypothesized that participants with HbSC/SB+ thalassemia and high risk APOL1 variants would develop albuminuria earlier in life. Further, we hypothesized that participants with elevated markers of endothelial dysfunction (Endothelin-1, ET-1) and high risk APOL1 variants would develop albuminuria earlier in life.

Methods: Sickle Cell Clinical Research and Intervention Program (SCCRIP) is a longitudinal cohort study enrolling participants with sickle cell disease. Participants in SCCRIP can undergo genetic testing and banking of plasma for biomarker studies. We identified participants in SCCRIP with characterized APOL1 variants and hemoglobin genotypes. High risk variant APOL1 was defined as G1/G1, G2/G2, and G1/G2. A subset of 169 participants with HbSS/SB0 thalassemia with banked plasma samples were analyzed for ET-1, interferon-gamma, IL-17a, and sFLt-1. We categorized biomarker levels into three quantiles (lowest, middle, highest). Our primary outcome was the time to develop the first episode of albuminuria defined as ACR measurements ≥30mg/g with participants’ data censored at 24 years old due to sparse data. Multivariable Cox proportional regression analysis was used to associate genotype and APOL1 variants with time-to-event outcomes, adjusting for sex, hydroxyurea, transfusion, and 5 principal components (PCs). Proportional hazard assumptions were checked based on the scaled Schoenfeld residuals. Likelihood ratio tests were implemented to compare the model including both genotype and APOL1 variants with models that include each variable separately.

Results: We identified that 115 (35%) out of 324 SCCRIP participants with HbSS/SB0 thalassemia developed at least one episode of albuminuria (mean age: 13 years, interquartile range/IQR: 10-16). In contrast, 17 out of 98 (17%) participants with other sickle cell genotypes (SC, SB+ thalassemia, other) developed at least one episode of albuminuria (mean age: 14 years, IQR: 12-18). We identified a significant difference in the time to develop albuminuria among participants with and without high risk APOL1 and SCD genotype (p<0.0001) when adjusting for sex, hydroxyurea, transfusion, and 5 PCs. Participants with both HbSS/SB0 thalassemia and high risk APOL1 developed a first episode of albuminuria earliest in life (p ≤ 0.003). Participants with HB SS/SB0 thalassemia who did not have high risk APOL1 and participants with other SCD genotypes who did have a high risk APOL1 variant developed the first episode of albuminuria at similar time points (p = 0.26).

There were 169 participants with high risk APOL1 variants and banked plasma. For our biomarker analysis, after adjusting for sex, hydroxyurea, transfusion, and 5 PCs, we identified that high risk APOL1 (hazard ratio [HR] = 2.0: 95%CI:1.2-7.3) and ET-1 (HR = 1.6; 95% CI:1.1-2.4) were independently associated with the time to develop albuminuria. Furthermore, a model incorporating both elevated ET-1 levels and APOL1 high risk status better predicted the development of albuminuria than ET-1 alone (p =0 .03) or APOL1 high risk status alone (p = 0.01). We did not identify an association between the development of albuminuria with interferon-gamma (p = 0.5), IL-17a (p = 0.4), and sFLt-1 (p = 0.22).

Conclusion: Patients with HbSC/SB+ thalassemia and high risk APOL1 variants develop albuminuria at a similar rate as patients with HbSS but without high risk APOL1 variants. Therefore, a subset of participants with HbSC/SB+ thalassemia and high risk APOL1 should also undergo early screening for the development of kidney disease. Additionally, patients with elevated ET-1 developed albuminuria early in life. As SCD murine models have demonstrated the ET-1 blocker therapy improves kidney outcomes, this finding could represent both a diagnostic tool for identifying high risk patients and a possible novel therapeutic target to prevent albuminuria.