From Stress to Sick(le) and Back Again-Oxidative/Antioxidant Mechanisms, Genetic Modulation, and Cerebrovascular Disease in Children with Sickle Cell Anemia

Abstract

Sickle cell anemia (SCA) is a genetic disease caused by the homozygosity of the HBB:c.20A>T mutation, which results in the production of hemoglobin S (HbS). In hypoxic conditions, HbS suffers autoxidation and polymerizes inside red blood cells, altering their morphology into a sickle shape, with increased rigidity and fragility. This triggers complex pathophysiological mechanisms, including inflammation, cell adhesion, oxidative stress, and vaso-occlusion, along with metabolic alterations and endocrine complications. SCA is phenotypically heterogeneous due to the modulation of both environmental and genetic factors. Pediatric cerebrovascular disease (CVD), namely ischemic stroke and silent cerebral infarctions, is one of the most impactful manifestations. In this review, we highlight the role of oxidative stress in the pathophysiology of pediatric CVD. Since oxidative stress is an interdependent mechanism in vasculopathy, occurring alongside (or as result of) endothelial dysfunction, cell adhesion, inflammation, chronic hemolysis, ischemia-reperfusion injury, and vaso-occlusion, a brief overview of the main mechanisms involved is included. Moreover, the genetic modulation of CVD in SCA is discussed. The knowledge of the intricate network of altered mechanisms in SCA, and how it is affected by different genetic factors, is fundamental for the identification of potential therapeutic targets, drug development, and patient-specific treatment alternatives.

Keywords: antioxidant mechanisms; cerebrovascular disease; genetic modulators; oxidative stress; sickle cell anemia; vasculopathy.

Figure 1

β-globin genotypes and the respective β-globin alterations in sickle cell disease. Partial HBB gene and protein sequences are shown. The β^S allele results from an A to T mutation in the 6th triplet of the HBB gene. This causes substitution of a glutamic acid residue to a valine residue in the 6th position of the mature β-globin chain and gives rise to the production of hemoglobin S (HbS). Mutation in both HBB alleles, whether in homozygosity or in compound heterozygosity, results in sickle cell disease (SCD). When the β^S allele is present in homozygosity sickle cell anemia (SCA), the most severe form of SCD, arises.

Figure 2

Hemoglobin and red blood cell changes resulting from the different genotypes in sickle cell disease. Heterozigosity for the normal β-globin and the β^S alleles underlies a condition called sickle cell trait, which is mostly an asymptomatic carrier state. Compound heterozygosity of β^S and other β allele mutation leads to HbS production. HbS has a lower O₂ affinity and tends to polymerize into rigid fibers inside red blood cells (RBC), under hypoxic conditions. This HbS polymerization leads to RBC sickling due to distortion, increased rigidity, and fragility. Initially, sickling is a reversible process occurring in cycles of oxygenation and deoxygenation. Increased and continuous oxy–deoxy cycles lead to irreversibly sickled RBCs, the hallmark of sickle cell disease.

Figure 3

Oxidative mechanisms in the pathobiology of sickle cell disease. Oxidative stress in SCD is not an isolated mechanism. Several pathobiological mechanisms unfold inside blood vessels, especially in those with lower oxygen pressure, like arterioles, capillaries, and post-capillary venules. Those mechanisms range from hemolysis to vaso-occlusion, culminate in ischemia, and ultimately, in tissue damage. Hemoglobin S (HbS) damages and causes membrane dysfunction on the sickle red blood cell (SSRBC) membrane, which leads to hemolysis. Oxidized membrane proteins expose phosphatidylserine. SSRBCs rupture and their content is released into the circulation through the intravascular hemolysis. This results in NO scavenging by cell-free Hb, enhanced by depletion of L-arginine, the nitric oxide synthases’ (NOS) substrate, and asymmetric dimethylarginine (ADMA) NOS inhibition. Reactive oxygen and nitrogen species (ROS and RNS, respectively) also deplete NO even further. The overall decrease in NO content elicits vasoconstriction which, together with endothelial proliferation, leads to vascular remodeling. Decreased NO and adenine dinucleotides levels lead to activation of platelets and blood clotting factors. Hemolysis also elicits activation of the innate immune system through heme release and other damage-associated molecular pattern (DAMP) molecules. Leukocytes are activated to release inflammatory cytokines which results in inflammation and activation of endothelial cells (EC). Enhanced circulating blood cells’ adhesion to each other promotes formation of multicellular aggregates. This blood cell adhesion, together with adhesion to the activated endothelium, strongly contributes to vaso-occlusion. While vasoconstriction increases blood flow velocity downstream from the constriction site, enhances shear stress, and further contributes to endothelial activation and dysfunction, vaso-occlusion causes flow blockage. The blockage ultimately results in (transient or permanent) ischemia and end-organ damage. Ischemic events are one of the main causes of cerebrovascular disease, namely silent cerebral infarction and stroke, in children with SCD. The interplay of all these mechanisms underlies the clinical manifestations of SCD, the severity of which may be modulated by variants in genes other than HBB. HbS: hemoglobin S; metHb: methemoglobin; NF-kB: nuclear factor kappa B.