Determinants Of Heme-Oxygenase-1 Upregulation In Patients With Sickle Cell Disease

Inflammation is a cardinal component of the pathogenesis of sickle cell disease (SCD). Increased plasma concentration of the inflammatory agonist hemin increases the odds of acute chest syndrome (ACS) in children with SCD (Adisa et al., Br. J Haematol, 2013). In addition, free hemin promotes the development of a lethal ACS-like disease in transgenic sickle mice (Ghosh et al., J Clin Invest, 2013). Hemin degradation is controlled by the rate-limiting enzyme heme oxygenase-1 (HO-1). Polymorphism of a (GT)n dinucleotide repeat in the HO-1 promoter, which enhances expression of the gene, is associated with lower rates of hospitalization for ACS in children. Over-expression of HO-1 reduces stasis in a mouse model of SCD vaso-occlusion. However, the role of plasma HO-1 in SCD patients is entirely unknown. In this study, we measured steady-state plasma HO-1 in two cohorts of patients. Cohort 1 in Atlanta (n=98) consisted of children with a mean age of 10.07±0.42 years (range 2-19 years) and cohort 2 from Accra (n=80) consisted of older patients (mean age 25.30±1.0 years, range 13-58 years). The mean plasma HO-1 of both cohorts was significantly higher compared to the mean value of age- and ethnic-matched individuals with normal adult Hb; Atlanta: 10.19±5.80 vs. 2.08± 1.16, p<0.0001 and Accra: 13.7±8.14 vs. 2.57± 0.82, p<0.0001. Plasma HO-1 varied by 25-fold in both cohorts and it correlated with the white blood cell count (Atlanta: r=0.3361, p<0.0001, Accra: r=0.25, p=0.02). Fifty-four percent (n=53) of subjects in the Atlanta cohort were on hydroxyurea. The mean plasma HO-1 of this subgroup was lower (8.1 ± 4.5) compared to the hydroxyurea naïve Accra cohort (p=<0.0001). Further studies of the Accra cohort revealed significant correlations between HO-1 and multiple markers of vascular inflammation; sICAM-1(r=0.2794, p=0.03, n=60), sE-selectin (r= 0.4209, p=0.0017, n=58) and sP-selectin (r=0.3855, p=0.0028, n=58). The number of the (GT)_n dinucleotide in the HO-1 promoter ranged 17 to 45; the distribution was trimodal with peaks at 23, 30 and 41 repeats. The overwhelming majority of patients had medium and large size alleles that are generally hypo-response to induction. Plasma HO-1 level correlated with the length of the (GT)n dinucleotide repeat (p=0.003, n=80). In a multivariable regression model, WBC, sICAM-1, sE-selectin and sP-selectin accounted for 13.4% of the total variance of plasma HO-1 level, and the (GT)n polymorphism accounted for 9.8%. In conclusion, the concentration of plasma HO-1 is generally raised among SCD patients at steady-state. However, a large proportion of patients have a relatively modest level that is probably inadequate to counter the severity of inflammation typical of SCD, due in part to a hypo-responsive HO-1 promoter. Therapeutic strategies that complement induction of the endogenous HO-1 gene may be critical to ameliorate inflammation in SCD.