Biblioteca Virtual en Salud

Of the 120 or so variants of human haemoglobin polypeptide chains so far described, nearly all are the result of the substitution of a single amino acid residue for another, or a deletion of one or more residues in the chain, caused by a point mutation in the codens of the chromosomal DNA. Hb H disease is also a genetically determined haemolytic anaemia, common in Orientals, but is caused by a reduced rate of synthesis of the O-chain of normal haemoglobin, resulting in an excess of á-chains (Hb H). A study of a Chinese family in Jamaica has revealed a slow moving haemoglobin in three children who have clinical haemoglobin H disease and in their fathers and siblings who have no haematological abnormality. The mothers have classical O-thalassaemia trait, as do some of the other siblings. Detailed biochemical analysis of this variant has shown that the O-chain is abnormal and is elongated by the addition of 31 residues to the 141 of the normal OA-chain. The extra residues are continuous with the normal C-terminal (O-141 Arg.) Biosynthetic studies, incorporating H(to 3rd power) leucine have indicated a low rate of synthesis for this abnormal chain. Interest lies in the nature of the genetic defect which could produce this abnormalty. While considering several hypotheses, the authors favour the theory that the genetic codon for 'end of chain' is the site of a mutation. This hypothesis is strengthened by finding Glutamine to be the next residue after the C-terminal Arginine. This extra piece of chain bears no resemblance to any known human globin chain sequence, so that it is unlikely that it has resulted from an unequal crossing over in a similar way to the Lepore haemoglobins. It may be that the messenge RNA for the normal OA-chain is longer than is required to direct synthesis of the chain. Further studies are in progress to try and define the nature of this genetic defect (AU)