Expression of embryonic hemoglobin genes in alpha-thalassemic and in beta-duplication mice

Protein Conformational Heterogeneity as a Binding Catalyst: ESI-MS Study of Hemoglobin H Formation

Our previous studies of hemoglobin tetramer assembly in vitro suggested that the initial step in the oligomerization process, which ultimately dictates the high fidelity of the heterotetramer (alpha*beta*)2 assembly, is the binding of a flexible heme-free beta-globin chain to a highly ordered heme-bound alpha*-globin. In this work, we extend these studies to investigate formation of the homotetrameric hemoglobin H, whose formation in vivo is a well-documented clinical consequence of significant overexpression of beta-globin in alpha-thalassemic disorders. Upon reconstitution of the isolated beta-globin with excess heme, the predominant species in the ESI mass spectrum corresponds to the homotetramer beta*4, alongside homodimeric species and monomeric beta-globin chains in both apo and holo forms. The assembly process of the hemoglobin H homotetramer apparently follows a scenario similar to that of a normal heterodimeric hemoglobin (alpha*beta*)2 species, with the asymmetric binding event between compact and flexible polypeptide chains being the initial step. The extreme importance of large-scale chain dynamics and conformational heterogeneity for the protein assembly process is highlighted by the inability of highly structured alpha-globins to undergo ordered oligomerization to form dimers and tetramers as opposed to indiscriminate aggregation.

Deletions in the alpha-globin gene complex in alpha-thalassemic mice

Three induced, heritable mutations in the mouse cause alpha-thalassemias. The adult alpha-globin genes on each mutant chromosome are no longer expressed. Embryos heterozygous for one normal and any of the three mutant chromosomes also seem to be deficient in embryonic alpha-globin-like x-globin, suggesting that the x-globin gene is nearby and also inactivated. A normal genetic polymorphism for a specific EcoRI site in or around the mouse alpha-globin gene complex has been used here to show that each of the three mutated chromosomes has a deletion that includes the segment of a 12-kilobase EcoRI band which normally carries one of the two adult alpha-globin genes. The deletion of the comparable part of the second alpha-globin gene site is also inferred. Nonetheless, a 4.7-kilobase EcoRI segment which carries a characterized alpha-globin-like pseudogene is still present in each mutant. These mutations were recovered after triethylenemelamine or x-ray treatments.

Mouse alpha-globin genes and alpha-globin-like pseudogenes are not syntenic

A genetic polymorphism for a Bgl I endonuclease site near the alpha-globin-like pseudogene alpha-4 of C57BL/6 and C3H/HeN mice was used to show that alpha-4 was not affected by three independent mutations in which the adult globin genes alpha-1 and alpha-2 were deleted. These results indicated that alpha-4 might not be located adjacent to the adult alpha-globin genes on chromosome 11. Restriction endonuclease analysis of DNA of a primary clone of a Chinese hamster--mouse somatic cell hybrid that had lost mouse chromosomes 11 and 18 showed that this clone lacked the adult murine globin genes alpha-1 and alpha-2 but it did contain the alpha-globin-like pseudogenes alpha-3 and alpha-4. These results indicated that the adult alpha-globin genes and alpha-globin-like pseudogenes are not located on the same chromosome. Similar analyses of several other Chinese hamster--mouse somatic cell hybrids that had segregated other mouse chromosomes indicated that the alpha-globin-like pseudogenes alpha-3 and alpha-4 are located on mouse chromosomes 15 and 17, respectively. These data explain why alpha-3 and alpha-4 were not affected by the three independently induced deletion-type mutations that cause alpha-thalassemia in the mouse.

Three mouse models of human thalassemia

Three types of mice with globin gene mutations, called 352HB, 27HB, and Hbath-J, appear to be true animal models of human thalassemia. Expression of the alpha-globin genes in three stocks of mice, each one heterozygous for one of the alpha-globin mutations, was examined at the polypeptide, RNA, and DNA levels. alpha-Globin polypeptide chains, relative to beta-globin chains in heterozygous thalassemic mice, are present at approximately 80% of normal. The ratios of alpha-globin to beta-globin RNA sequences are also 75-80% of normal, exactly reflecting the alpha-globin to beta-globin chain ratios. In the case of mutant 352HB, at least one alpha-globin gene is deleted. Thalassemic mouse erythroid cells appear to compensate partially for the loss of half of their alpha-globin genes.