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Molecular characterization and functional studies on Hb Kempsey, beta 99 (G-1) Asp----Asn, a high-oxygen affinity
G Ricco1, F Scaroina, M C Amprimo
1Ospedale Giovanni Bosco U.S.L. VI Torino, Italy.
Background:
Hb Kempsey (beta 99 Asp----Asn) is a high-oxygen affinity hemoglobin, never before reported in Italy, associated with secondary erythrocytosis. It has been found in the heterozygous state in two subjects from the same family originating from the Verona area in Northern Italy.
Methods:
The abnormal hemoglobin was studied both at the protein and at the DNA level. The amino acid substitution was identified by fingerprinting and amino acid analysis. The nucleotide replacement was investigated by means of polymerase chain reaction (PCR) of the beta gene and direct sequencing. Oxygen affinity and other functional parameters were assessed on whole blood and on the separate hemoglobin fractions.
Results:
These studies allowed us to establish the molecular substitution GAT (Asp)----AAT (Asn) at codon 99. Functional studies revealed a left-shifted and biphasic dissociation curve of the proposita, with a very low p50. The two carriers of this hemoglobinopathy have different degrees of polyglobulia, since iron deficiency in one of them reduces total and abnormal Hb.
Conclusions:
The compensatory mechanisms for tissue hypoxia are discussed with the conclusion that erythrocytosis has to be preserved in these patients to maintain adequate tissue oxygenation.