New results of hemoglobin variant structure determinations by fast atom bombardment mass spectrometry

Detection and characterization of variant and modified structures of proteins in blood and tissues by mass spectrometry

Some variant proteins cause diseases, and some diseases result in increases of proteins with abnormally modified structures. The detection, characterization, and estimation of the relative amounts of protein variants and abnormally modified proteins are important for clinical diagnosis and for elucidation of the mechanisms of the pathogenesis of diseases. Analysis of the covalent structures of proteins using matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF-MS) and liquid chromatography-electrospray ionization MS (LC-ESI-MS), which had been developed by the early 1990s, have largely replaced analyses by conventional protein chemistry. Here, we review the detection and characterization of hemoglobin variants, HbA1c measurement, detection of carbohydrate-deficient transferrin, and identification of variants of transthyretin (TTR) and Cu/Zn-superoxide dismutase (SOD-1) using soft ionization MS. We also propose the diagnostic application of the signals of modified forms of TTR, that is, S-sulfonated TTR and S-homocysteinyl TTR. The relative peak height ratio of the abnormal/normal components gives valuable information about the instability of variants and enables the detection of unstable Hb subunits or thalassemia heterozygotes. We found unique modified structures of TTR that suggested changes in amyloid fibrils.

Some aspects of matrix-assisted laser desorption/ionization analysis of hemoglobin from whole human blood

Some aspects of the matrix-assisted laser desorption/ionization (MALDI) method in the analysis of hemoglobin from whole human blood are described. These aspects are: sensitivity of this method, fragmentation of hemoglobin ions, and influence of some impurities (sodium chloride NaCl, sodium sulfate Na2SO4, and potassium ferrocyanide K4Fe(CN)6) on the detection process. As a matrix, alpha-cyano-4-hydroxycinnamic acid (CCA) was used. The estimated maximum concentrations of tested salts for which no hemoglobin signal was observed were determined.

Abnormal hemoglobins: laboratory methods

Laboratory methods allowing the detection and characterization of hemoglobin variants are reviewed. Protein chemistry techniques such as isoelectrofocusing, electrophoreses under various experimental conditions, cation exchange and reversed phase high performance liquid chromatography, are the most frequently used for the detection of variants. When associated with a few additional data they may lead to a presumptive diagnosis. DNA studies are also developed in many laboratories. Final identification of a variant may be achieved either by molecular biology techniques or by protein sequence analysis in which mass spectrometry now occupies a key position.

Identification of Hb Villejuif [beta123(H1)Thr-->Ile] in Southern Italy

Hb Villejuif [beta123(H1)Thr-->Ile] is a silent and asymptomatic variant described in 1989 in an 87-year-old woman of French origin suffering from coincidental polycythemia vera. This paper reports the second observation of Hb Villejuif in three related subjects from Montesarchio, Southern Italy. All routine techniques for hemoglobin analysis yielded normal results with the exception of a slight increase in the Hb A2 value. The occurrence of a variant beta-globin was rapidly assessed by liquid chromatography mass spectrometric analysis and the abnormal chain purified by high performance liquid chromatography. The amino acid replacement Thr-->Ile at beta123 was determined by tandem electrospray mass spectrometric analysis of the tryptic digest of the variant beta chain. The corresponding DNA mutation was established as C-->T at the second position of codon 123 (ACC-->ATC) by polymerase chain reaction amplification techniques.

Hb Mont Saint Aignan [beta128(H6)Ala-->Pro]: a new unstable variant leading to chronic microcytic anemia

Hb Mont Saint-Aignan [beta128(H6)Ala-->Pro] is a mildly unstable variant, associated with hemolytic anemia, marked microcytosis and increased alpha/beta biosynthetic ratio (1.55 versus 1.1 +/- 0.1 in the control). The abnormal chain was isolated by selective precipitation with isopropanol and the structural modification determined by protein chemistry methods (reversed phase high performance liquid chromatography and mass spectrometry). Possible mechanisms underlying the beta(+)-thalassemia-like expression of this variant are discussed.

Two new variants with the same substitution at position beta122: Hb Bushey [beta122(GH5)Phe-->Leu] and Hb Casablanca [beta65(E9)lys-->Met; beta122(GH5)Phe-->Leu]

Hb Bushey, found in a Chinese baby and his father, is a new variant with a point mutation leading to the substitution Phe-->Leu at position beta122. Hb Casablanca, found in a family from Morocco, is a further example of a hemoglobin variant that carries two abnormalities in the same chain; the first is identical to that of Hb Bushey and the second to that of Hb J-Antakya [beta65 (E9)Lys-->Met]. Structural abnormalities of both Hbs were determined by protein chemistry methods including electrospray and tandem mass spectrometry. Their stability and oxygen binding properties were found to be identical to those of Hb A. Various mechanisms that may lead to two point mutations in the same chain are reviewed briefly.

Toward a simple, expedient, and complete analysis of human hemoglobin by MALDI-TOFMS

MALDI mass spectrometry is explored as a method for hemoglobin characterization. To simplify and expedite the analysis, hemoglobin is obtained without purification directly from whole human blood. The use of trypsinactivated bioreactive MALDI probes is evaluated as a means to further reduce the analysis time from hours to minutes. Moreover, variations of the MALDI matrix preparation facilitate detection of the problematic tryptic peptides alpha T12, alpha T13, and beta T12. The results reveal that MALDI-based methods are easily implemented, are rapid, and allow detection of traditionally elusive tryptic peptides.

HB Les Andelys [alpha83(F4)LEU-->PRO]: a new moderately unstable variant

Hb Les Andelys [alpha83(F4)Leu-->Pro] is a mildly unstable variant that was found during glycated hemoglobin measurement in a French family. In this hemoglobin molecule the affected site, in the alpha chain, and the amino acid substitution are identical to those of Hb Santa Ana, an unstable beta chain variant. The structural abnormality was demonstrated by protein chemistry methods, involving, in addition to the classical techniques, a selective precipitation of the abnormal hemoglobin by isopropanol and a mass spectrometry analysis of the alphaT-9 peptide following carboxypeptidase digestion. DNA sequencing demonstrated that the mutation was CTG-->CCG at codon 83 of the alpha2 gene.

Alteration of an intersubunit contact in hemoglobin variants: comparative study of modifications at position alpha 126 Asp (H9)

Four human hemoglobin variants have already been described at position alpha 126 (H9), which is normally occupied by an aspartate: Hb Montefiore (-->Tyr), Hb Tarrant (-->Asn), Hb Fukutomi (-->Val), Hb Sassari (-->His). An additional variant, Hb West One (alpha 126 (H9) Asp-->Gly) is herein described. Aspartate alpha 126 (H9) is involved in a set of hydrogen bonds and salt bridges located at the C-terminal portion of the alpha-chains and of the C-helix of the beta-chains, which are broken in the oxy conformer, providing one of the most important sources of the difference in free energy between the T- and R-state in hemoglobin. A comparative study of four of these alpha 126 Hb variants is presented. An identical degree of alteration of the oxygen binding properties (increased oxygen affinity and decreased cooperativity) was found in all cases, when measured under standard experimental conditions (pH 7.2, 0.1 M NaCl). In contrast, the effect of L345 (a derivative of bezafibrate, which is a specific alpha-chain binding effector) on oxygen binding to Hb differed from one variant to another. When a bulky Tyr or His residue occupied the alpha 126 (H9) position, little effect of L345 was observed. Conversely, when this position was occupied by a residue of smaller size (Gly or Asn), normal heterotropic effects were observed. Molecular graphic modelling indicates that two classes of three-dimensional structure modifications may occur.

Cation-exchange HPLC evaluated for presumptive identification of hemoglobin variants

A battery of relatively simple tests allows the presumptive identification of hemoglobin (Hb) variants, making unnecessary structural analysis by protein chemistry methods or DNA sequencing. The primary step in this strategy involves the use of a matrix of electrophoretic mobilities obtained under various experimental conditions. This leads to an unambiguous result in ∼90% of the cases. Additional tests are required to characterize with more confidence the remaining 10%. We describe here the use of cation-exchange HPLC on the Bio-Rad Variant automated analyzer with the “β Thalassemia Short” program. By comparing the elution time of 125 human Hb mutants, we found that some variants with almost identical pI values or produced by the same type of amino acid substitution displayed different elution times. We present several examples in which use of the HPLC profile helped establish the diagnosis.

Hb Uxbridge [beta 20 (B2)Val-->Gly]: a new variant with mild increase in oxygen affinity found during a neonatal screening program

Hb Uxbridge [beta 20(B2)Val-->Gly] was found in an English family during a neonatal hemoglobinopathy screening program. In both the child and the parent carrying this hemoglobin variant, the red cell parameters were normal. By isoelectrofocusing Hb Uxbridge appeared to have an isoelectric point slightly higher than Hb A but was silent on cellulose acetate and acid gel electrophoresis. Structural modifications affecting position beta 20(B2) have been demonstrated to be responsible for a high oxygen affinity and polycythemia in Hb Olympia (Val-->Met) and Hb Trölhattan (Val-->Glu). In the case of Hb Uxbridge, despite an alteration of the same site, the oxygen binding parameters of the patient's hemolysate showed only a mild (ca. 20%) increase in oxygen affinity.

Use of combined mass spectrometry methods for the characterization of a new variant of human hemoglobin: The double mutant hemoglobin villeparisis β77(EF1) His → Tyr, β 80 (EF4) Asn → Ser

Hemoglobin Villeparisis was found during a systematic measurement of glycated hemoglobin. Electrospray mass spectra of the globin indicate an apparently unchanged molecular weight within the error range (0.01%). The tryptic digest of the β chain shows a chromatographically abnormal βT-9 peptide. The mass-to-charge ratio value of its [M+H](+) ion, as measured by liquid secondary ionization mass spectrometry, is one mass unit lower than that of the normal βT-9. However, the electrospray mass spectrum of this peptide exhibits mainly a doubly charged ion, whereas the normal βT-9 gives a triply charged ion. None of the allowed single amino acid substitutions for a 1-u shift down (Glu → Gln, Asp → Asn, or Asn → Ile) can explain the suppression of one protonation site. This can be due only to the replacement of the internal histidine by a nonbasic residue. Thus at least two amino acid exchanges occur within the same peptide: one involves the internal histidine, and the sum of the mass shifts is -1 u. Consideration of the βT-9 sequence and taking account for the genetic code rules, the only possibility was (11)His → Tyr (+26 mass shift) associated with (14)Asn → Ser (-27 mass shift). This conclusion was consistent with the tandem mass spectrum of the [M+H](+) ion and was further confirmed by chemical microsequencing.

Germline mosaicism for an alanine to valine substitution at residue beta 140 in hemoglobin Puttelange, a new variant with high oxygen affinity

Hb Puttelange [beta 140(H18)Ala-->Val] was found as a de novo mutation in two siblings of a French family suffering from polycythemia. Both parents were phenotypically normal and exclusion of paternity has been ruled out by the study of several polymorphic markers located on different chromosomes. The structural modification of Hb Puttelange was established by reversed-phase HPLC analysis of the tryptic digest of the abnormal chain. The amino acid composition of an abnormal beta T14 peptide revealed that one of the four residues of Ala was replaced by a Val. Tandem mass spectrometry demonstrated that the substitution concerned position beta 140 (H18). This hemoglobin displays an increased oxygen affinity that is responsible for the polycythemia. De novo mutations, as demonstrated again in the case of this variant, have the highest probabilities of detection when they lead to pathological manifestations. They may result either from a somatic mutation in a very early stage of the embryological development of the propositus or may have a parental origin with occurrence of a germline mosaicism. The study of the beta-globin gene indicated that this case of Hb Puttelange probably arose from a mutation affecting a part of the germline of the father, therefore leading to a true recurrence risk.

Hb F-Sassari: a novel G gamma variant with a threonine residue at position gamma 75, characterized by mass spectrometric techniques

The cord blood sample of a Caucasian newborn contained about 40% of an abnormal fetal hemoglobin. The mutated gamma chain was isolated using reversed phase high performance liquid chromatography and characterized by means of electrospray and fast atom bombardment mass spectrometric techniques as a G gamma-globin variant with an Ile-->Thr substitution at position gamma 75. The variant chain shows the same structure as the previously described Hb F-Charlotte that was demonstrated to be an A gamma variant with an Ile-->Thr substitution at position gamma 75 and an additional Ala-->Gly substitution at gamma 136.

Hemoglobin variants in North Africa

The populations of Morocco, Algeria, and Tunisia are composed of different ethnic groups including Arabs, Berbers, Sub-Saharan Africans, Europeans, and Turks. Between 1981 and 1991, we studied more than 3,000 individuals from these North African countries. One-hundred and eighty-one carried one (or more) unusual hemoglobin variant(s) other than Hb S and Hb C which are the most frequent variants in these countries. Each of these 181 individuals was heterozygous for at least one of the 49 abnormal alpha or beta alleles identified by electrophoretic and/or structural studies, and some homozygotes were detected. A few mutants are common in North Africa: Hb O-Arab, Hb D-Punjab and Hb G-Philadelphia. Other mutants encountered in European or African populations are found in relatively few North African families. The observed polymorphisms in the populations of North Africa probably result largely from their complex ethnic origins.

Hb Villaverde [beta 89 (F5) Ser-->Thr]: the structural modification of an intrasubunit contact is responsible for a high oxygen affinity

Hb Villaverde [beta 89 (F5) Ser-->Thr], identified in a Spanish patient, is a new human hemoglobin variant, electrophoreticaly silent, responsible for a severe erythrocytosis. This abnormal hemoglobin displays a very high oxygen affinity and a markedly reduced cooperativity that is partly restored in the presence of IHP. Determination of the structural abnormality was achieved on a mixture of the normal and abnormal beta-chains. After isolation of the abnormal tryptic peptide by RP-HPLC, its sequence was determined by mass spectrometry. The structural abnormality disturbs the intrasubunit interaction between helices F and H and, thus, may weaken the C-terminal bonds of the deoxy conformation and the heme contacts of several hydrophobic residues. Hb Villaverde demonstrates that this intrasubunit contact between helices F and H is essential for the cohesion of the hemoglobin molecule.

Hb Cleveland or alpha 2 beta 2(93)(F9)Cys----Arg;121(GH4)Glu----Gln

Hb Cleveland is characterized by two amino acid substitutions, namely beta 121(GH4)Glu----Gln as in Hb D-Los Angeles and beta 93(F9)Cys----Arg as in Hb Okazaki, and shares with Hb Okazaki a decreased stability, an increase in oxygen affinity, and decreases in Bohr effect and heme-heme interaction. It is the 13th beta chain variant with two substitutions that has been described thus far.