Sickle cell Hb oarab disease - abstract

ABSTRACT

An infant presented with megaloblastic anaemia of folate deficiency complicating a haemolytic anaemia with hepatosplenomegaly. At first, the laboratory investigations indicated Hb SC disease but, as it is very unusual to see such severe clinical effects in this haemoglobinopathy, especially in infancy, further investigations were carried out which demonstrated a haemoglobin electrophoretically similar to Hb C but having certain distinguishing characteristics. This Hb was compared with Hb C and Hb E on various electrophoretic media. The tryptic peptide maps (prepared at the Institute for Anthropology, Leiden, Netherlands, by W. de Jong) showed an abnormality in beta Tp XIII and amino acid analysis further revealed that lysine had replaced the normal glutamic acid at the 121st residue in the beta chain. This substitution has been previously described for Hb O Arab. In the helical notation of Perutz this position, in the peptide chain, lies between the G and H helices, GH4. The glutamyl residue is invariant at this site in all human haemoglobin chains. Four mutations have been described at this site. One in the gamma chain, one in the alpha chain and two in the beta chain. The other one in the B chain is Hb D Punjab, and this also causes a more severe type of haemoglobinopathy when inherited together with Hb S. Another infant, presenting in a similar way, was found to have the same defect, Hb S with Hb O Arab. The mother of this second case has Hb O-Thalassaemia. A search was then carried out in patients who had previously been diagnosed as SC disease. Two further cases of Hb SO disease were found, making four families in all. The clinical features of Hb SO disease appear to be more like those of homozygous sickle cell desease than SC disease. There are more sickled cells in the blood, a lower haematocrit and haemoglobin level, a greater shift in oxygen dissociation curve and a shorter red cell survival. Hb O Arab is apparently not uncommon in Jamaica and all cases of suspected Hb SC disease who have a clinical course more like SS disease should be further investigated (AU)