Thromboembolic and Vascular Complications in Patients with Secondary Erythrocytosis Caused By High Oxygen Affinity Variant Hb Coimbra

There are 99 hemoglobin (Hb) variants with increased oxygen affinity inducing compensatory erythrocytosis (Globin Gene Server Web Site: http://globin.cse.psu.edu). One of these mutations, named Hb Coimbra, was described in 1991 in Portugal by Tamagnini et al. (HBB c.300T>A, p.Asp100Glu). Limited amount of published clinical and laboratory data of this condition and the absence of specific guidelines for its management suggest it would be a very benign condition. Here we report clinical and laboratorial findings of carriers of Hb Coimbra with some unexpectedly more severe clinical presentations.

We collected peripheral blood venous samples from 10 patients (6 female), aged 31-72 years, upon signed informed consent. All of them were diagnosed with erythrocytosis, with hematocrits in the 46.9-78.1% range at the time of diagnosis, and were heterozygous for the Hb Coimbra mutation as confirmed by direct sequencing of the β-globin gene. The β-globin cluster haplotypes were determined by TaqMan SNP Genotyping Assay (rs7482144, rs968857, rs16911905) and other polymorphisms in the β-globin gene by direct sequencing (rs713040, rs10768683, rs7480526, rs7946748). We were able to identify a common haplotype for the Hb Coimbra allele, which strongly suggests a common origin despite the patients belonging to 5 different unrelated families.

Three patients had complications possibly associated with the disease. Patient 1, male, 64 years old, presented with pulmonary embolism in July 2012. Patient 2, male, 60 years old, suffered a left lower limb amputation due to complicated venous thromboembolism in 2010 before he was diagnosed with Hb Coimbra and, in January of 2015, he also developed right pulmonary artery thromboembolism. Patient 3, male, 72 years old, presented with left hemiparesis in 2016 due to intraparenchymatous hemorrhage in the right basal ganglia.

Differently from the other patients, elevated levels of serum erythropoietin were observed only in patients 1, 2, and 3 (81.1 mUI/mL, 423.5 mUI/mL, and 35.4 mUI/mL, respectively, reference range 2.6-18.5 mUI/mL). We speculate this abnormality could be associated with the therapeutic phlebotomy treatment performed in these cases.

Oxygen affinity was evaluated by determining the p50 in whole blood samples without Hb isolation. O₂ dissociation and saturation curves demonstrated that samples containing Hb Coimbra had considerably high affinity for oxygen in comparison to wild type (p50_deoxi and p50_oxi for Hb Coimbra were, 8.22 [SD 0.64] and 7.94 [SD 0.98], respectively, vs. wild type p50_deoxi = 15.56 [SD 0.50] and p50_oxi = 15.56 [SD 0.77]). Heme-heme cooperativity was also significantly reduced, tending to a non-cooperative event. Hill coefficients were n_{deoxi - Hb Coimbra}= 1.51 (SD 0.19) and n_{oxi - Hb Coimbra} = 1.48 (SD 0.32), n_{deoxi - wild type} = 2.52 (SD 0.22) and n_{oxi - wild type} = 2.41 (SD 0.32). In spite of the very high O₂ affinity of Hb Coimbra, no significant difference was observed between samples from patients with clinical complications and from the remaining Hb Coimbra carriers.

Alterations in the α1β2 interface, such as the one in Hb Coimbra, promote conformational changes in the structure of the hemoglobin, and tend to promote O₂ binding, reducing oxygen supply to peripheral tissues and resulting in compensatory erythrocytosis (Wajcman et al, 2005). Mutations that generate hemoglobin variants are very rare, but should be considered in the context of familial erythrocytosis or in cases in which more frequent hematological diseases such as polycythemia vera have been excluded.

In general, hemoglobin variants with high oxygen affinity manifest exclusively with erythrocytosis and a benign clinical course; however, our data suggest that this might not be the case in some patients, particularly in carriers of Hb Coimbra. Our experience supports that some patients may benefit from the prophylactic use of aspirin or therapeutic phlebotomy as attempts to reduce the incidence of thromboembolic or other vascular complications.

Taken together, our data underscore that larger and multicentric studies of the clinical complications secondary to high oxygen affinity hemoglobins are necessary to define guidelines for the optimal management of this type of erythrocytosis.