Sickle cell disease is a classical monogenic disorder characterized by chronic hemolytic anemia, the degree to which is known to be influenced by specific mutations in the genes for the subunits of hemoglobin A.15,24-26
In the present study, common polymorphisms and mutations within and flanking the HBB, HBA1
, and HBA2
genes were analyzed for associations with pulmonary hypertension in SCD, where affected cases were identified by 2 validated non-invasive screening methods.5,7
In addition, the allelic spectrum of HBB
mutations present within an unselected SCD cohort is documented as a necessary initial step for future studies to identify other genetic modifiers for pulmonary hypertension susceptibility.27
Overall, SCD compound heterozygosity is associated with a lower prevalence of pulmonary hypertension.
The chromosome 11 analysis suggests that non-sickle HBB
mutations like Hb C that limit hemolytic rate in SCD are associated with a lower pulmonary hypertension risk, lending further support to the proposed hemolysis centered disease model of SCD pulmonary hypertension. These findings, together with prior studies of SC disease, indicate that the Hb C mutation is itself a significant genetic modifier of SCD. Specifically, Hb SC has a milder clinical course relative to homozygous SCD which is characterized by fewer vaso-occlusive events, a less intense hemolytic anemia, less frequent priapism, and a nearly normal average lifespan.2,6,28
However, some vaso-occlusive manifestations including retinopathy and osteonecrosis may be more frequent in SC disease.29,30
A similar protective association with pulmonary hypertension is also likely to be present in Sβ+
thalassemia. However, the statistical power for detecting this effect was below 80% and the high FPRP suggests a high likelihood of representing a false positive, which precludes a firm conclusion as to the validity of this suggested association.
Surprisingly, a higher hemolytic rate is not a distinguishing characteristic of pulmonary hypertension in Hb SC patients suggesting that this complication may be caused by etiologies unrelated to hemolysis. Perhaps intravascular hemolysis represents a lower fraction of total hemolysis in Hb SC than is the case in homozygous SCD. However, anemia, renal insufficiency, elevated alkaline phosphatase, and excess iron remain consistent distinguishing characteristics across all SCD pulmonary hypertension patients ().5,10
Prior studies have documented phenotypic associations between pulmonary hypertension, hemolysis, and priapism in patients who may have less frequent vaso-occlusive manifestations.2,5,31
In fact, the low 8% prevalence of Hb SC among pulmonary hypertension cases at NIH nearly matches that reported for Hb SC (11%) observed among those with priapism in the Cooperative Study of Sickle Cell Disease.2
Taken together, SC pulmonary hypertension may be due to more prominent relative contributions by other mechanisms for remodeling the pulmonary vasculature including activation of the endothelin 1 and prostocyclin/thromboxane A2
In contrast, the contributions of these two pathways in the severe hemolytic anemias like homozygous SCD and β thalassemia major occur in addition to alterations in the NO signaling pathway.33-35
Moreover, these observations justify further study of SCD compound heterozygotes and their less intense hemolytic anemia as a more sensitive population for elucidating non-hemolytic mechanisms that may also contribute to the development of secondary pulmonary hypertension.
Despite speculation about differences in underlying mechanisms, pulmonary hypertension remains a strong predictor of death for all SCD patients. In particular both the absence of prominent laboratory features of hemolysis and the poor survival for patients with high TRV's should prompt clinicians to perform a comprehensive evaluation to rule out other etiologies of pulmonary hypertension in Hb SC (e.g. acute pulmonary embolism, chronic thromboembolic pulmonary hypertension or HIV infection).33
In addition, the significantly poor survival for Hb SC patients with pulmonary hypertension highlights the need for including this important subgroup of SCD patients in therapeutic clinical trials. Indeed, the efficacy of hydroxyurea in Hb SC still remains to be established today because these patients are typically excluded from multicenter placebo controlled trials for a variety of reasons.36-40
Another important distinguishing characteristic of SCD pulmonary hypertension observed in this and other studies is advancing age.5
Age has been reported to be a risk factor for increased pulmonary pressures in other diseases including primary pulmonary hypertension and systemic sclerosis.5
In the general population, the risk of developing primary pulmonary hypertension is an age dependent phenomenon, where the prevalence of the disease increases over 10% between the ages of 34 and 64 based upon estimates derived from the National Health and Examination Survey (NHANES).41
While the true prevalence of pulmonary hypertension in asymptomatic individuals may have been overestimated by NHANES, these findings highlight the importance of recognizing age as a significant variable confounding epidemiologic studies, just as we have observed with this genetic analysis of globin mutations. Thus with respect to our protective association in Hb SC, it remains unclear as to whether pulmonary hypertension in SC disease merely represents the background prevalence of pulmonary hypertension in otherwise healthy, aging African Americans or whether there is a true increase in risk secondary to their underlying hemoglobinopathy.
The final notable observations from this study are that neither α-thalassemia nor β-globin locus haplotypes have a strong association with pulmonary hypertension. The absence of a strong effect for α-thalassemia in SCD pulmonary hypertension was unexpected in contrast to the findings with Hb SC, despite the increased statistical power from the presence of the α3.7
allele at nearly three times the frequency of Hb C. Still, the magnitude of α3.7
's effect may be sufficiently small to be below the threshold for detection for the statistical power in this study. Likewise, haplotypes of the β-globin locus have been used in the past to study both the variation in SCD severity and the evolutionary history of the βS
In particular, some of the variability in Hb F expression is attributable to functional alleles in the fetal hemoglobin genes inherited on specific βS
Fetal hemoglobin expression may be relevant to pulmonary hypertension, as this and other studies have observed significantly lower levels of Hb F in SCD pulmonary hypertension cases ().9,10
However, the haplotype analysis would predict that the strongest loci underlying this effect would be present outside the β-globin locus. Together, the absence of associations with co-incident α-thalassemia or β-globin haplotypes might be attributed to an insufficient hematologic effect to reach a critical threshold that alters either the degree of hemolysis or the frequency of acute vaso-occlusive events. It is also possible that the natural history of the previously observed associations between these markers and SCD complications like priapism and leg ulcers have been altered by modern interventional therapies (e.g. ACE inhibitors, hydroxyurea, etc.) that have become commonplace since the completion of the CSSCD.2,50
Overall, further study in larger, independent populations is warranted.
In conclusion, mutations in the β-globin subunit of hemoglobin, but not a common α-thalassemia mutation, are associated with protection from SCD pulmonary hypertension. Despite this association, pulmonary hypertension remains a risk factor for death in carriers of modifying β-globin alleles. We speculate that the diminished relative contributions of hemolysis to the development of pulmonary hypertension in SCD compound heterozygotes is the likely mechanism underlying these protective associations. Future studies may be helpful to determine if other genetic factors influence risk for developing non-hemolytic forms of pulmonary hypertension in SCD.