The synthesis and chemical heterogeneity of human fetal hemoglobin: overview and present concepts

Molecular aspects of embryonic hemoglobin function

In order to provide the appropriate level of oxygen transport to respiring tissues, we need to produce a molecular oxygen transporting system to supplement oxygen diffusion and solubility. This supplementation is provided by hemoglobin. The role of hemoglobin in providing oxygen transport from lung to tissues in the adult is well-documented and functional characteristics of the fetal hemoglobin, which provide placental oxygen exchange, are also well understood. However the characteristics of the three embryonic hemoglobins, which provide oxygen transport during the first three months of gestation, are not well recognized. This review seeks to describe the state of our understanding of the temporal control of the expression of these proteins and the oxygen binding characteristics of the individual protein molecules. The modulation of the oxygen binding properties of these proteins, by the various allosteric effectors, is described and the structural origins of these characteristics are probed.

HbF in the adult: could its composition discriminate normal from abnormal foetal globin gene expression?

The relative proportions of the two gamma chain species (G gamma and A gamma) have been reinvestigated in newborns, during the physiological switch from foetal to adult haemoglobin, and in adults with some persistent expression of HbF. In newborns, with about 80% HbF, the G gamma percentage was close to 70% while in adult RBC, with less than 0.5% HbF, the G gamma chain was almost non-detectable and may reflect the completion of the foetal to adult switch. Conversely, in adult patients with HbF above 0.6%, usually accompanying some degree of marrow stimulation, the relative ratio of G gamma varied between 40 and 60%, independently of HbF level. This ratio corresponds to what has been described in the literature as being the adult type of HbF. In all the cases where we found higher levels of G gamma, the results could be explained by the presence of a specific genetic background such as the Senegalese haplotype in sickle cell disease.

Hemoglobin C in association with hereditary persistence of fetal hemoglobin

Hemoglobin C and hereditary persistence of fetal hemoglobin (HPFH) are an uncommon combination of hemoglobinopathies. Several tests are needed to verify this condition, among them hemoglobin electrophoresis and Kleihauer-Betke staining of a peripheral blood smear. Family studies are useful in delineating the genetics of the hemoglobinopathy but could not be performed in our case. In more confusing cases or with an unusual subtype, more extensive testing may be required. HPFH, by itself, is without clinical manifestations. It may be confused with other conditions; therefore, its presence in patients with hematologic symptoms requires more precise definition of the hemoglobin abnormality.