Quantitation of human gamma globin genes and gamma globin mRNA with purified gamma globin complementary DNA

gamma chain composition in five Italian newborns heterozygous for Hb F Malta G gamma-117 His leads to Arg

The percentage of Hb F Malta G gamma-177 His replaced by Arg and the gamma chain composition have been evaluated at birth and at different times after birth in five Italian newborns heterozygous for this variant. The percentage of Hb F Malta ranged at birth from 24% to 31% of the total Hb F, while the average G gamma/A gamma chain ratio was about 7/3, overlapping the values observed in normal newborns. T gamma chains were detected in three out of five newborns, with a percentage of about 10% of the total Hb F. After birth the Hb F Malta declined faster than the total Hb F; thus, the Hb F decrease during the first 45 d of life is mainly due to the switch-off of the G gamma locus containing the Hb F Malta gene.

Human haemoglobin genetics

The genes which direct the structure of human fetal and adult haemoglobin consist of a linked pair of alpha-chain loci on chromosome 16 and the G gamma-A gamma-delta-beta loci complex on chromosome 11. The delta-and beta-chain genes contain inserts similar to those of the mouse and rabbit globin genes. The structure of the various messenger RNAs transcribed from these loci is now worked out although the function of the non-coding regions is not known. The abnormal haemoglobin disorders and thalassaemias result from a variety of lesions at these loci which include single base substitutions, deletions of one or more bases or entire loci, insertions, frame-shifts, fusion genes caused by abnormal crossing over, chain termination mutations and ill-defined defects which lead to a reduced rate of transcription or abnormal structure of messenger RNA. Some progress has been made towards an understanding of the cellular mechanisms whereby the haemoglobin polymorphisms have been maintained. Very little is known about the regulatory mechanisms involved in the switch from fetal to adult haemoglobin production, although it is likely that certain specific areas of the gamma-delta-beta gene complex are involved in its control.

Restriction endonuclease mapping of the human gamma globin gene loci

The restriction endonuclease sites in and around the human gamma globin gene loci have been mapped using the gel blotting technique of Southern, in both normal DNA and DNA from an individual with hereditary persistence of fetal hemoglobin (HPFH). In normal DNA, the gamma genes are linked to the delta (and beta) globin genes, and the orientation of these genes with respect to transcription is (5') G gamma leads to A gamma leads to delta leads to beta (3'). The distance between the G gamma and A gamma genes is 3.5 kb and that between the A gamma and delta genes is 16 kb. In both normal DNA and HPFH DNA, the gamma genes are interrupted by an intervening sequence, approximately 1 kb in length that is situated between codon positions 99 and 121 of the coding sequence. In different DNA samples, there is polymorphism for the presence or absence of a Hind III site in the intervening sequence of either gamma golbin gene. In HPFH DNA, a deletion of at least 16 kb of DNA has been detected. This deletion starts at a point approximately 12.5 kb from the 3'-end of A gamma gene and extends through the delta and beta globin genes to a point at least 3 kb beyond the 3'-end of beta globin gene.

Defects in DNA and globin messenger RNA in homozygotes for hemoglobin Lepore

Globin messenger RNA (mRNA) isolated from three patients homozygous for hemoglobin Lepore is shown to have a marked reduction of the amount of beta-like globin mRNA (Lepore-globin mRNA sequences) compared with alpha-globin mRNA by molecular hybridization. The relative amounts of alpha- and Lepore mRNA are similar to the amounts of alpha- and Lepore globin synthesized in intact cells and by isolated mRNA in a cell-free system. It is also demonstrated that Lepore-globin mRNA can completely hybridize to full-length or nearly full-length beta-globin specific complementary DNA and protect it from nuclease digestion, indicating close homology between the delta-mRNA sequences present in Lepore mRNA and the beta-complementary-DNA probe. We have also quantitated the numbers of beta-like globin gene sequences in genomic Lepore DNA by molecular hybridization and demonstrated a reduction in their number consistent with the Lepore gene being a delta beta-gene fusion product.

Structure of the human fetal globin gene locus

We have derived a 'map' of restriction enzyme sites in and around the human gamma-globin genes. This has enabled us to show that there are two gamma-globin genes per haploid set, that the genes contain 'introns' within the same regions of DNA as the human beta and delta-globin genes, and that the genes are 3,500 base pairs apart. We conclude that the correct gene organisation of the human beta-like globin locus is GgammaAgammadeltabeta.

Relative stability of alpha- and beta-globin messenger RNAs in homozygous beta+ thalassemia

The relative concentrations of alpha-, beta-, and gamma-globin mRNA sequences were measured in bone marrow nuclear and cytoplasmic RNA and in RNA from peripheral blood reticulocytes of three patients with homozygous beta+ thalassemia. Our results suggest that the quantitative deficiency in beta-globin mRNA may arise because of abnormal metabolism of molecules containing beta mRNA sequences. Complementary DNAs specific for each of the globins were synthesized. Variable quantities of RNA were incubated to equilibrium with 3H-labeled alpha- and 32P-labeled beta- or gamma-enriched cDNA. We found for each of the patients that the alpha/beta mRNA sequence ratio was more nearly normal in the nuclear RNA than in either cytoplasmic or reticulocyte RNA. Conversely, gamma mRNA sequences were very low in the nucleus with an increase in the relative concentration in both cytoplasm and reticulocyte RNA. The thermal stability of nucleic acid duplexes formed between beta cDNA and nuclear RNA from one patient with beta+ thalassemia was equivalent to that of duplexes formed with normal nuclear RNA. Approximately equal amounts of thalassemic alpha and beta mRNA were retained by oligo(dT)-cellulose, indicating that the 3' poly(A) segment was present on both. Our results indicate that beta-globin mRNA, although grossly normal in structure, fails to accumulate in beta+ thalassemic erythroid cells in amounts equivalent to the mRNA for alpha-globin.