Homozygous alpha thalassemia/Hb G Philadelphia

Structural and Functional Characterization of a New Double Variant Haemoglobin (HbG-Philadelphia/Duarte α(2)β(2))

WE REPORT THE FIRST CASE OF COSEGREGATION OF TWO HAEMOGLOBINS (HBS): HbG-Philadelphia [α68(E17)Asn → Lys] and HbDuarte [β62(E6)Ala → Pro]. The proband is a young patient heterozygous also for β°-thalassaemia. We detected exclusively two haemoglobin variants: HbDuarte and HbG-Philadelphia/Duarte. Functional study of the new double variant HbG-Philadelphia/Duarte exhibited an increase in oxygen affinity, with a slight decrease of cooperativity and Bohr effect. This functional behaviour is attributed to β62Ala → Pro instead of α68Asn → Lys substitution. Indeed, HbG-Philadelphia isolated in our laboratory from blood cells donor carrier for this variant is not affected by any functional modification, whereas purified Hb Duarte showed functional properties very similar to the double variant. NMR and MD simulation studies confirmed that the presence of Pro instead of Ala at the β62 position produces displacement of the E helix and modifications of the tertiary structure. The substitution α68(E17)Asn → Lys does not cause significant structural and dynamical modifications of the protein. A possible structure-based rational of substitution effects is suggested.

Hb G-Philadelphia, or [alpha 68(E17)Asn----Lys], in north Sardinia: detection by isoelectric focusing and identification by HPLC of tryptic peptides

11,129 individuals of the northern area of the island of Sardinia (7,717 newborns and 3,412 adults) were examined for the Hb G-Philadelphia variant [alpha 68(E17)Asn----Lys]. Hemolysates were analysed by isoelectric focusing, and the variant identified by reversed phase high performance liquid chromatography of tryptic peptides. A total of seven heterozygotes (1 in 1,589) were identified. This is probably the highest prevalence of this mutant so far described. Percentages of the variant (average 35.1 +/- 6.2%) were trimodally distributed with modes centering on 28, 36, and 42%, respectively. These values suggest that the G-Philadelphia allele occurs in Sardinians both on a single and on a double locus chromosome. The linkage with alpha-thalassemia may be the reason for the high frequency of the variant.

Hemoglobin I mutation encoded at both alpha-globin loci on the same chromosome: concerted evolution in the human genome

Genetic analysis of an individual expressing an unexpectedly high level of hemoglobin I, an alpha-globin structural mutant, reveals that the mutation is present at both the alpha 1- and the alpha 2-globin gene loci. Kindred analysis confirms that the two affected genes are located in cis. The most likely explanation for this finding is that a recent conversion event occurred within the human alpha-globin gene cluster.

Alpha-globin gene deletions associated with alpha A and alpha G Philadelphia in an Algerian family that includes two Hb G homozygotes

An Algerian family with a high degree of consanguinity and including two homozygotes for Hb-G Philadelphia is presented. Whether homozygotes or heterozygotes, all subjects displayed microcytosis (with various degrees of poikilocytosis) and a moderately depressed alpha-globin chain synthesis. Hb H and Heinz bodies were absent. DNA mapping revealed the presence of a 3.7 kb deletion resulting from the rightward type of recombination event between alpha 2 and alpha 1 genes on both the alpha A/ and the alpha G/ chromosomes. Such data indicate that the -alpha A/ and -alpha G/ haplotypes are involved and suggest that the -alpha G/ haplotype, which is very rare in Algeria, has an African Black origin. In subjects with genotype (-alpha A/-alpha G) or (-alpha G/-alpha G), the output of the remaining alpha genes is sufficiently high to avoid the appearance of Hb H. This situation contrasts with that reported in an Algerian patient, who had a (-alpha A/-alpha A) genotype but who was producing Hb H (Whitelaw et al. 1980). The data collected from this family suggest that the -alpha A/ haplotypes are heterogeneous in Algerians.