A combined top-down and bottom-up MS approach for the characterization of hemoglobin variants in Rhesus monkeys

Sickle cell disease is caused by one of the 1200 known hemoglobin variations. A single-point mutation β6(A3)Glu→Val leads to sickling of red blood cells, which in turn causes a lack of oxygen supply to tissue and organs. Although sickle cell disease is well understood, treatment options are currently underdeveloped. The only Food and Drug Administration-approved drug is hydroxyurea, an inducer of fetal γ-hemoglobin, which is known to have a higher oxygen affinity than adult hemoglobins and thus alleviates symptoms. In the search for better cures, Rhesus monkeys (Macaca mulatta) serve as models for monitoring success of induction of fetal γ-hemoglobins and with recent advances in proteomics, MS has become the leading technique to determine globin expression. Similar to humans, Rhesus monkeys possess hemoglobin variants that have not been sufficiently characterized to initiate such a study. Therefore, we developed a combined bottom-up and top-down approach to identify and characterize novel hemoglobin variants of the umbilical cord blood of Rhesus monkeys. A total of four different variants were studied: α, β, γ1 and γ2. A new α- and β-hemoglobin variant was identified, and the two previously hypothesized γ-hemoglobins were identified. In addition, glutathionylation of both γ-hemoglobin variants at their cysteines has been characterized. The combined approach outperformed either bottom-up or top-down alone and can be used for characterization of unknown hemoglobin variants and their PTMs.