HMOX1 and Acute Kidney Injury in Sickle Cell Anemia

Acute kidney injury (AKI) is associated with tubulointerstitial fibrosis and nephron loss and may lead to an increased risk for subsequently developing chronic kidney disease (CKD). In adults with sickle cell anemia (SCA), high rates of CKD have been consistently observed, although the incidence and risk factors for AKI are less clear.

We evaluated the incidence of AKI, defined according to Kidney Disease Improving Global Outcomes (KDIGO) guidelines as a rise in serum creatinine by ≥0.3mg/dL within 48 hours or ≥1.5 times baseline within seven days, in 158 of 299 adult SCA patients enrolled in a longitudinal cohort from the University of Illinois at Chicago. These patients were selected based on the availability of genotyping for α-thalassemia, BCL11A rs1427407, APOL1 G1/G2, and the HMOX1 rs743811 and GT-repeat variants. Median values and interquartile range (IQR) are provided. With a median follow up time of 66 months (IQR, 51-74 months), 137 AKI events were observed in 63 (40%) SCA patients. AKI was most commonly observed in the following settings: acute chest syndrome (25%), an uncomplicated vaso-occlusive crisis (VOC)(24%), a VOC with pre-renal azotemia determined by a fractional excretion of sodium <1% or BUN-to-creatinine ratio >20:1 (14%), or a VOC with increased hemolysis, defined as an increase in serum LDH or indirect bilirubin level >1.5 times over the baseline value at the time of enrollment (12%).

Compared to individuals who did not develop AKI, SCA adults who developed an AKI event were older (AKI: median and IQR age of 35 (26-46) years, no AKI: 28 (23 - 26) years; P=0.01) and had a lower estimated glomerular filtration rate (eGFR) (AKI: median and IQR eGFR of 123 (88-150) mL/min/1.73m², no AKI: 141 (118-154) mL/min/1.73m²; P=0.02) by the Kruskal-Wallis test at the time of enrollment.

We evaluated the association of a panel of candidate gene variants with the risk of developing an AKI event. These included loci related to the degree of hemolysis (α-thalassemia, BCL11A rs1427407), to chronic kidney disease (APOL1 G1/G2 risk variants), and to heme metabolism (HMOX1) . Using a logistic regression model that adjusted for age and eGFR at the time of enrollment, the risk of an AKI event was associated with older age (10-year OR 2.6, 95%CI 1.4-4.8, P=0.002), HMOX1 rs743811 (OR 3.1, 95%CI 1.1-8.7, P=0.03), and long HMOX1 GT-repeats, defined as >25 repeats (OR 2.5, 95%CI 1.01-6.1, P=0.04).

Next, we assessed whether AKI is associated with a more rapid decline in eGFR and with CKD progression, defined as a 50% reduction in eGFR, on longitudinal follow up. Using a mixed effects model that adjusted for age and eGFR at the time of enrollment, the rate of eGFR decline was significantly greater in those with an AKI event (β = -0.51) vs. no AKI event (β = -0.16) (P=0.03). With a median follow up time of 66 months (IQR, 51-74 months), CKD progression was observed in 21% (13/61) of SCA patients with an AKI event versus 9% (8/88) without an AKI event. After adjusting for age and eGFR at the time of enrollment, the severity of an AKI event according to KDIGO guidelines (stage 1 if serum creatinine rises 1.5-1.9 times baseline, stage 2 if the rise is 2.0-2.9 times baseline, and stage 3 if the rise is ≥3 times baseline or ≥4.0 mg/dL or requires renal replacement therapy) was a risk factor for CKD progression (unadjusted HR 1.6, 95%CI 1.1-2.3, P=0.02; age- and eGFR-adjusted HR 1.6, 95%CI 1.1-2.5, P=0.03).

In conclusion, AKI is commonly observed in adults with sickle cell anemia and is associated with increasing age and the HMOX1 GT-repeat and rs743811 polymorphisms. Furthermore, AKI may be associated with a steeper decline in kidney function and more severe AKI events may be a risk factor for subsequent CKD progression in SCA. Future studies understanding the mechanisms, consequences of AKI on long-term kidney function, and therapies to prevent AKI in SCA are warranted.