Plasma hepcidin of Ethiopian highlanders with steady-state hypoxia

To the editor:

Hepcidin impedes iron absorption and is suppressed when erythropoietic iron requirements are increased. Recent studies show that during acute exposure to high-altitude hypoxia, plasma hepcidin concentrations drop when iron demands for erythropoiesis and hemoglobin synthesis are sharply increased.^1,2  However, the effects of chronic exposure to high-altitude hypoxia with stable erythropoietic iron requirements have not been examined. We hypothesized that plasma hepcidin would not be suppressed in iron-replete individuals chronically adapted to high altitude.

People of Amhara and Oromo ethnicity have been living at high altitude in Ethiopia for more than 5000 years and about 500 years, respectively, and have been shown to differ from one another in hemoglobin and oxyhemoglobin percentage.³  Healthy volunteers from 3700 to 4000 m (high altitude) and 1200 to 1500 m (low altitude) were recruited, and they provided blood samples for analyses (see the supplemental Video(s)/Data Set(s) link at the top of the online article for genetic analysis methodology). The sample reported here had normal calculated body iron stores⁴  (Figure 1D) and did not have infection or inflammation, assessed with C-reactive protein levels and malarial plasmodium DNA.

In contrast to acutely exposed Europeans²  (see also the online supplement), high-altitude Amhara had higher plasma hepcidin, and high-altitude Oromo had similar hepcidin, compared with their respective lowland counterparts (Figure 1E). Furthermore, Amhara had higher plasma hepcidin and oxyhemoglobin percentage as well as lower hemoglobin and erythropoietin than Oromo at high altitudes (Figure 1A-C,E). Within Ethiopian subsamples, age, sex, BMI, erythropoietin, hemoglobin, oxyhemoglobin percentage, and transferrin receptor were not correlated with hepcidin (natural-log transformed for normality). Like Europeans at low altitude,⁵  serum ferritin (r = 0.35 to 0.77) and body iron stores (r = 0.39 to 0.85) correlated with ln(hepcidin) (all P < .05, except in the small sample of female low-altitude Oromo [n = 6]). An intronic SNP in GRAMD3 was associated with plasma hepcidin among Amhara at genome-wide significance (P = 4.94*10⁻⁸), accounting for 22% of the variation in covariate-adjusted hepcidin level. Allele A of rs7700582 increased hepcidin levels by 8.1 ng/mL only among Amhara, although allele frequency was similar in all 4 subsamples.

Hepcidin was not suppressed in Amhara or Oromo highland samples under steady-state hypoxia, likely because erythropoietic drive was stable.⁶  It is interesting to speculate that the higher plasma hepcidin of highlander Amhara, compared with Oromo, is due to lower iron demand indicated by lower hemoglobin and erythropoietin concentrations and higher body iron stores. Variants in GRAMD3 are associated with macular degeneration, a retinal disease that has been related to abnormalities in hepcidin and iron accumulation.^7,8  Another variant near GRAMD3 (rs1366100) has been associated with erythrocyte counts,⁹  consistent with the idea that this region plays a role in iron metabolism. Thus, the genetic results also support the idea that iron stores are primary regulators of hepcidin levels in hypoxic populations without increased erythropoietic drive. Previous work has shown that various highlander populations demonstrate different responses to hypoxia,¹⁰  which may also be the case with iron regulation.