Haemoglobin Madrid beta 115 (G17) alanine--proline: an unstable variant associated with haemolytic anaemia

Simulated pathogenic conformational switch regions matched well with the biochemical findings

Pathogenic conformational conversion is a general causation of many disease, such as transmissible spongiform encephalopathy (TSE) caused by misfolding of prion, sickle cell anemia, and etc. In such structural changes, misfolding occurs in regions important for the stability of native structure firstly. This destabilizes the normal conformation and leads to subsequent errors in folding pathway. Sites involved in the first stage can be deemed switch regions of the protein, and are vital for conformational conversion. Namely it could be a switch of disease at residue level. Here we report an algorithm that can identify such sites computationally with an accuracy of 93%, by calculating the probability of the native structure of a short segment jumping to a mistake one. Knowledge of such switch sites could be used to target clinical therapy, study physiological and pathologic mechanism of protein, and etc.

A new beta-chain haemoglobin variant with increased oxygen affinity: Hb Roma [beta115(g17)Ala-->Val]

BACKGROUND

Haemoglobin Roma [beta115(G17)Ala-->Val] is a new adult haemoglobin variant found in a patient presenting a mild hypochromia and microcytosis. We studied this previously uncharacterised variant in order to evaluate the effect on the structural and funcional properties of the Ala-->Val substitution at the alpha1beta1 interface.

METHODS AND RESULTS

The variant chain was identified by direct DNA sequencing of the beta-globin gene, which revealed a GCC-->GTC mutation in codon 115. This mutation was confirmed by mass spectrometric analysis of the tetramers and peptides. The oxygen-binding properties of the haemoglobin A/haemoglobin Roma mixture, in which the variant makes up 25% of the haemoglobins, showed a significant increase in oxygen affinity with respect to normal haemoglobin A, both in the absence and presence of 2,3-bisphosphoglycerate. The role of the betaG17 position, situated at the alpha(1)beta(1) interface, has been examined using computational models of haemoglobin Roma and other known betaG17 variants, in comparison with normal haemoglobin A.

CONCLUSIONS

This study suggests that the beta115(G17)Ala-->Val substitution at the alpha1beta1 interface is responsible for increased oxygen affinity and mild destabilisation of the haemoglobin Roma.

GENERAL SIGNIFICANCE

An amino acid substitution at the G17 position of the alpha1beta1 interface may result in stabilisation of the high affinity R-state of the haemoglobin molecule.

Dominant beta-thalassemia with hemoglobin Hradec Kralove: enhanced hemolysis in the spleen

We describe a 6-year-old girl and her mother with dominant beta-thalassemia due to hemoglobin Hradec Kralove (Hb HK). Both patients presented microcytic anemia, jaundice, splenomegaly, cholelithiasis, and recurrent hemolytic bouts. Osmotic resistance tests using saline and coiled planet centrifugation revealed the increased fragility of the red cell membrane. On the other hand, the glycerol lysing time was prolonged, and results of the isopropanol test were weakly positive. Despite mimicking the features of hereditary spherocytosis, the results of the genetic analyses verified the second reported family with Hb HK (codon 115, GCC [Ala] --> GAC [Asp]). Splenectomy was effective for the amelioration of hemolysis. Of 7 reported patients with Hb variants at beta-globin codon 115 (Hb Madrid and Hb HK), 5 underwent splenectomy. Because of the variable augmentation of extramedullary hemolysis in dominant beta-thalassemias, genotyping is necessary for determining the clinical indication of splenectomy.

Hb Madrid [beta115(G17)Ala-->Pro] in a Korean family with chronic hemolytic anemia

Hb Madrid, in which the alanine residue at beta115 (G17) is replaced by proline, results in a moderately severe hemolytic anemia due to the disruption of an alpha helical region and the weakening of an alpha1 beta1 contact (1,2). It was first discovered in a single Spanish patient, by protein structural analysis, whose parents did not carry the abnormality (1). The second observation of Hb Madrid was in an American Black teenager by DNA analysis, who had the family history of chronic hemolytic anemia, but none of family members were available for evaluation (3).

Hb Hradec Kralove (Hb HK) or alpha 2 beta 2 115(G17)Ala-->Asp, a severely unstable hemoglobin variant resulting in a dominant beta-thalassemia trait in a Czech family

We have identified through sequencing of amplified DNA a GCC-->GAC mutation in codon 115 of the beta-globin gene in a mother and daughter of a small Czech family. This base change was confirmed by hybridization with a 32P-labeled specific oligonucleotide probe and by gene mapping because it creates a new Ava II site. The mutation results in an Ala-->Asp replacement at beta 115(G17); this beta chain is severely unstable and could not be identified either as chain or as hemoglobin variant by isoelectrofocusing and various high performance liquid chromatography methods. Stability tests were mildly positive in freshly prepared lysates, but an unstable hemoglobin could not be detected in older lysates with these methods. Its presence results in a dominant type of beta-thalassemia in the two heterozygotes, with moderate anemia, reticulocytosis, nucleated red cells, target cells, and other red cell changes, Heinz body formation, and splenomegaly; the oldest of the two patients was splenectomized. Both subjects had a marked increase in fetal hemoglobin synthesis.

Hemoglobin saitama or beta 117 (G19) His leads to Pro, a new variant causing hemolytic disease

An electrophoretically silent unstable hemoglobin was found in a 23-year-old Japanese woman suffering from hemolytic anemia and jaundice. The unstable beta subunit was precipitated with p-chloromercuribenzoic acid and the globin was further purified by urea CM-52 column chromatography. Results of amino acid analysis, tryptic fingerprinting and manual Edman degradation of the abnormal beta T12B established a substitution of proline for histidine beta 117 (G19). This new abnormal hemoglobin was absent in other members of the family.

De novo mutations producing unstable Hbs or Hbs M. II. Direct estimates of minimum nucleotide mutation rates in man

Cases of unstable hemoglobin and hemoglobin M disease that have appeared as de novo mutants over a span of approximately 50 years were used to deriving minimal, direct estimates of mutation rates per nucleotide per generation in man. The estimates are based upon analysis of data related to 40 cases of unstable Hbs and 15 of Hbs M that arose in 13 countries. The estimated rate calculated using all de novo beta-gene mutants is 7.4 X 10(-9) per nucleotide per generation; that derived using de novo alpha-gene mutants is 10.0 X 10(-9). Subsequent calculations of mutation rates per alpha- and beta-chain gene and extrapolation of these rates to a hypothetical gene of 1000 nucleotides yield an estimated mutation rate of 8.6 X 10(-6) per 1000 nucleotides per generation. Even though some instances of false paternity may have biased these estimates in an upward direction, under-reporting of Hb M cases, and particularly of unstable hemoglobins, makes it likely that the cited values are minimal estimates of mutation rates at the molecular level.

Hemoglobin Dakar = Hb Grady: demonstration by a new approach to the analysis of the tryptic core region of the alpha chain and oxygen equilibrium properties

Preliminary studies have suggested that in Hb Dakar, histidine alpha112 was substituted by a glutamine. A re-investigation on this hemoglobin is presented in this report. A structural study has been performed using a new approach to analyse the tryptic core region of the human hemoglobin alpha chain. After tryptic digestion of the aminoethylated alpha chain, a secondary digestion of the tryptic core was carried out with chymotrypsin and with another protease, thermolysin. Analyses of the chymotryptic and thermolytic peptides indicated that the structure of Hb Dakar was identical to that of Hb Grady previously described by Huisman et al. who showed the insertion of three amino acid residues in position alpha115 or alpha118. The insertion, which was localized near two residues involved in the alpha1beta1 contact, did not produce a dissociation into dimers. Functional studies demonstrated a a slightly increased oxygen affinity, a lowered cooperativity and a normal Bohr effect. The low amount of the abnormal hemoglobin (8%) may in part be explained by a slight instability of the molecule.

Hemoglobin Willamette (alpha2beta2 51Pro replaced by Apg (D2)) a new abnormal human hemoglobin

A hemoglobin variant with the same electrophoretic mobility as hemoglobin S was found in three generations of a black family. No clinical symptoms or findings were present in subjects heterozygous for this mutant. Except for target forms of mature erythrocytes, they have no abnormal hematologic findings. Structural studies demonstrated a previously undescribed substitution, beta51 Pro replaced by Arg, in the abnormal fraction which accounts for about one-third of the total hemoglobin. This fraction is more unstable in vitro at 65 degrees than normal A hemoglobin. Both whole blood and purified abnormal hemoglobin have increased oxygen affinity and a slightly decreased Bohr effect.

Thalassemia intermedia caused by heterozygosity for both beta-thalassemia and hemoglobin Saki [beta 14 (A11) Leu replaced by Pro]

The syndrome thalassemia intermedia can be the clinical expression of heterozygosity for different tyes of thalassemia, beta-thalassemia and hereditary persistence of fetal hemoglobin, beta-thalassemia and Hb-Lepore, and in blacks it may even represent a true beta-thalassemia homozygote. This report describes thalassemia intermedia in a white male due to beta-thalassemia and an unstable hemoglobin. Chain-synthesis studies showed an excess of alpha-chain production over beta-chain production in the propositus and his mother but balanced chain synthesis in the clinically normal father, who is heterozygous for the unstable hemoglobin. The unstable hemoglobin was found to be beta14 (A11) Leu leads to Pro, which has previously been described in a clinically normal African woman, and named Hb-Saki. This hemoglobin is not distinguishable from Hb-A on routine electrophoresis at alkaline or acid pH and tests for unstable hemoglobins are necessary for its detection. The increasing list of such hemoglobin variants and previous cases of heterozygosity for beta-thalassemia and unstable hemoglobins are reviewed.

Hemoglobin Atlanta or alpha 2 beta 2 75 Leu-Pro (E19): an unstable variant found in several members of a Caucasian family

Hemoglobin Atlanta, alpha 2 beta 2 75 Leu-Pro (E19), has been found in several members of three generations of a Caucasian family living in metropolitan Atlanta. The abnormal hemoglobin is one of the nine unstable variants in which either a leucyl or an alanyl residue is replaced by a prolyl residue. These substitutions have been observed in the B, E, F, and G helixes of the beta chain and in the H helix of the alpha-chain. Hemoglobin Atlanta heterozygotes are mildly affected by the presence of this unstable hemoglobin.