Transferrin and Erythropoietin Increased Levels Correlate with Thrombosis at High Altitude

Living at high altitude involves great interest because of the challenging multiple adaptive responses to a hypoxic environment. Native Bolivian Andeans have lived at an altitude of 4000 m in The Andes mountains for about 14,000 years exposed to a low atmospheric pressure (453 mmHg) and low oxygen concentration (12,7%). It was described Andeans adaptation to high altitude is mainly related to cardiovascular issues (PMID: 29100088), with a low significantly genetic selection in HIF pathway components responsible of erythropoiesis regulation (PMID: 36980912), so that they have adapted to exist with increased hemoglobin concentrations. Thus, Andeans can be prevalent to undergone hematological and thromboembolic disorders.

Thrombotic events have been reported to be increased at high altitude, and this recently associated to increased transferrin (Tf) (PMID: 36040436), likewise to increased erythropoietin (Epo) (Amaru, RevMed 2022).

Increased Tf were described to be induced by both iron deficiency and hypoxia via HIF (PMID: 9242677). In addition, iron deficiency was postulated to inhibit the function of prolyl hydroxylase 2 (PHD2) in the hydroxylation of HIF-2, necessary for recognition and degradation by VHL (PMID:22304911). Under normal conditions, Tf is bound to fibrinogen at a molar ratio of 4:1, leading to plasma Tf sequestration by fibrinogen and leaving little Tf free in the circulation. While at hypoxia conditions, abnormally upregulated Tf potentiates thrombin, factor XIIa and inhibits antithrombin, inducing hypercoagulability (PMID:36040436). Similarly, increased HIF1a at high altitude increases Tf promoting the transferrin gene expression, which contains HIF-1α binding sites in its enhancer region (PMID:36844187). HIF is a DNA-binding transcription factor for activating expression of Epo gene (PMID:32561149).

Epo, in turn, is increased due to the hyperegulation of HIF as expected at low atmospheric pressure environments. Epo activates the hypoxia-inducible factor that leads to Epo synthesis to stimulate red blood cell production, which also increases the demand for iron to synthesize hemoglobin (PMID:21078592). This, added to a decreased iron and increased HIF 1a, gives rise to a hyperregulation of Tf. Epo has been described to have prothrombotic properties and be related to thrombotic events (PMID:10779449; Amaru, RevMed 2022).

In this sense, to further observe a correlation of transferrin and erythropoietin increased levels with the risk of thrombotic events at high altitude, we studied patients with erythrocytosis, anemia and polycythemia vera.

We analyzed clinical, lab tests and epidemiological data of 920 patients with Chronic Mountain Sickness erythrocytosis (CMS-e) (n=560), anemia (n=372), and Polycythemia Vera (PV) (n=20), all Bolivian Andeans born and residing at 4000 m.

Considering at this altitude normal hemoglobin concentrations vary from 14-17 g/dl in women and 15-18 g/dl in men, erythrocytosis condition comprises Hb levels >18 g/dl in women and >19 g/dl in men (Amaru et al, Rev Hematol Mex, 2016); similarly, it embraces Hb <12 g/dl for women and <13 g/dl for men in anemias (PMID: 23317073). Positive JAK ^F617V tests were corroborated in PV patients. Iron deficiency correlation considered serum ferritin <30 ug/L and MCV <80 fL (Clark et al, Nutrition, 2008). Eco-doppler study records confirmed the occurrence of thrombotic events.

Data analysis was performed through Excel 16.29.1 and SPSS program, Chi-squared, Fishers and Pearson tests were done.

We observed significative increased Tf levels in erythrocytosis, anemia, and PV patients mainly on those presenting iron levels deficiency. Epo was highly increased in erythrocytosis patients with iron deficiency. These results correlated with the incidence of thrombotic events in erythrocytosis (CMS-e) and anemia patients with iron deficiency. Age was also relevant in both groups. (Table 1). The increase of Epo levels correlated with thrombosis in erythrocytosis and PV patients with iron deficiency (Table 1).

Transferrin and Erythropoietin increased levels at high altitude correlated with thrombosis in Chronic Mountain Sickness erythrocytosis and iron deficiency anemia patients. However, Increased Tf did not correlate with thrombosis in PV patients at high altitude. Our data confirm that increased Tf and Epo have prothrombogenic properties in CMS-e, iron deficiency anemia patients at high altitude.