Unequal crossing-over: a common basis of single alpha-globin genes in Asians and American blacks with hemoglobin-H disease

The alpha-globin genes of five black Americans, two Chinese, and five Filipinos with HbH disease (an alpha-thalassemia state in which there is a single functional alpha gene) were analyzed by restriction endonuclease techniques. All subjects were found to have one chromosome 16, lacking both alpha genes, and another containing a single alpha gene (--/-alpha). Restriction endonuclease patterns of the DNA obtained from all 12 subjects were identical and compatible with unequal crossing-over as the mechanism of origin of the single alpha gene in these individuals.

Prenatal diagnosis of sickle cell anemia by restriction and endonuclease analysis: HindIII polymorphisms in gamma-globin genes extend test applicability

Polymorphism for a Hpa I restriction endonuclease site associated with about 60% of beta S genes in American Blacks allows exact prenatal diagnosis of sickle cell anemia by amniocentesis in 36% of couples at risk. In three families in whom exact diagnosis by Hpa I sites was impossible, we found analysis for the presence of polymorphic HindIII sites in the G gamma and A gamma intervening sequences would allow an exact prenatal diagnosis of sickle cell status in all three. In one of these families, the presence of an A gamma HindIII site in amniocyte DNA confirmed the diagnosis (sickle cell trait) made by synthetic studies using fetal erythrocytes obtained at fetoscopy. Studies of other Black families and individuals provide evidence for linkage disequilibrium in the G gamma-A gamma-delta-beta gene complex involving the four sites, G gamma HindIII, A gamma HindIII, beta S, and Hpa I, which span 33 kilobases (kb). Ten of 14 chromosomes bearing a beta S gene in a 7.6-kb Hpa I fragment contained a G gamma but not an A gamma HindIII site, whereas 16 of 16 chromosomes bearing a beta S gene in a 13-kb Hpa I fragment lacked both the G gamma and A gamma HindIII sites. Two-thirds of beta A-bearing chromosomes lacked both G gamma and A gamma sites, whereas one-third contained either the G gamma or both G gamma and A gamma sites. These data demonstrate that combined analysis of both Hpa I and HindIII polymorphisms and verification of their linkage phase should increase the fraction of couples for whom amniocentesis can provide an exact diagnosis of sickle cell status from 36% to greater than 80%.

A molecular basis for hemoglobin-H disease in American blacks

We have applied gene counting and restriction endonuclease mapping techniques to the study of two American black families in which there were one or more cases of HbH disease. We found deletions of three of the four normal alpha-globin genes in individuals with HbH disease. In two of these individuals, the chromosome containing the single alpha gene could have originated by crossing over between mispaired alpha genes, resulting in a deletion of about 4.2 kilobases (kb).

Characterization of 5'-terminal methylation of human alpha- and beta-globin mRNAs

After incubating peripheral blood of various anemic individuals with [methyl-3H]methionine, 5'-terminal cap structures and the sequential methylation of human globin mRNAs were determined. Human alpha- and beta-globin mRNAs have the same methylated nucleotides at their 5' ends as the alpha- and beta-globin mRNAs of mouse and rabbit: m7G(5')ppp(5')[m6Am/Am]pCmp. The order of methyl group addition to human globin mRNA is also similar to that of mouse globin mRNA in that methylation of the 5'-terminal guanosine and the ribose of adenosine occurs earlier than that of the base of adenosine and the ribose of cytidine. Thus, both the cap structures and the order of methylation are conserved in the globin mRNAs of these species, suggesting that variation in the structure of the 5' terminus of globin mRNAs may alter the function of these molecules.

Prenatal diagnosis of hemoglobinopathies

Techniques are currently available at a few specialized centers for the antenatal detection of hemoglobinopathies such as beta thalassemia and sickle cell anemia. The risks are not yet clearly defined, but it seems reasonable to suggest that about 90 per cent of families may obtain useful information about the genotype of their fetus. Obtaining fetal blood as well as analyzing the sample requires considerable experience and skill. These factors continue to limit the availability of this procedure.

Sequential methylation of globin mRNA in nucleated erythroid cells and reticulocytes of mice

The order of methylation of the 5'-terminus of globin mRNA of mice was studied by incubation of staged nucleated erythroid cells and peripheral reticulocytes with [methyl-3H] methionine. Methylation of the 5'-termini of alpha and beta- globin mRNAs in enucleated reticulocytes was demonstrated as follows: (a) [methyl-3H] incorporation into poly(A)+ RNA of reticulocytes co-migrated with the alpha- and beta- globin mRNAs on gel electrophoresis, and (b) following digestion of this RNA, radioactivity was localized to the four methyl sites at the 5'-capped structure of mouse globin mRNAs. However, this methylation is only 5 to 8% as efficient as in nucleated erythroid precursor cells, suggesting that most globin mRNA molecules are fully methylated prior to the reticulocyte stage. Incubations of early and late nucleated erythroid precursor cells and pulse-chase experiments with reticulocytes demonstrate that addition of the four 5'-terminal methyl groups follows an orderly sequence. In addition, the pulse-chase experiments suggest the turnover of the N7-methyl group on the 5'-terminal guanosine, but not of the other methyl groups in the 5'-terminus of globin mRNA. Thus, 5'-terminal methylation of globin mRNA is a nonrandom, dynamic process.

The 5'-terminal structures of murine alpha- and beta-globin messenger RNA

The 5'-terminal structures of murine alpha- and beta-globin mRNA were determined after incubating cells of the erythropoietic spleens of mice with [methyl-3H]methionine. Globin mRNA was obtained from total cellular RNA by oligo(dT)-cellulose chromatography followed by elution of mRNA from formamide gels after electrophoresis. The globin mRNA was then hydrolyzed with KOH or digested with a combination of RNase T2 and bacterial alkaline phosphatase, and 5'-terminal structures were isolated by DEAE-cellulose chromatography. The methylated nucleotides of these 5'-structures were determined following digestion with specific ribonucleases and bacterial alkaline phosphatase. Analyses of mRNA fractions enriched for either alpha- or beta-mRNA gave similar results. Our data indicate that murine alpha- and beta-globin mRNAs are identical through the first three nucleotides and that partial dimethylation exists at the second position: m7G(5')ppp(5') [m6Am/Am]pCmpNp.

Changing erythrocyte populations in juvenile chronic myelocytic leukemia: evidence for disordered regulation

Fetal and adult erythrocyte characteristics were studied serially in a 30-mo-old female with juvenile chronic myelocytic leukemia. On presentation the erythrocytes exhibited predominantly fetal characteristics as indicated by 69% hemoglobin F (HbF), 1.1% hemoglobin A2 (HbA2), absent I antigen, and fetal levels of the erythrocyte enzymes, carbonic anhydrase I and II, glucose-6-phosphate dehydrogenase, hexokinase, pyruvate kinase, and lactate dehydrogenase; 100% of the erythrocytes present contained HbF. However, Orskov-Jacobs-Stewart hemolysis demonstrated that at least one adult characteristic was present. Seven months later HbF was 17%; I antigen and carbonic anhydrase I had increased to adult levels. The number of cells containing HbF had decreased to 30%. Further studies indicated that at least three new populations of red cells were present after 7 mo which had not previously been detected. Two of these populations exhibited a mixture of both fetal and adult characteristics. Such findings suggested that an ongoing disturbance of regulatory mechanisms was responsible for the variable expression of fetal versus adult erythrocyte characteristics.

Tay-Sachs screening: social and psychological impact

Participants in two Tay-Sachs screening programs were generally satisifed with the organization of the tests and the results. There was no evidence of adverse impact on reproductive plans or interpersonal relations, and the respondents professed to believe in the value of screening. While the carriers discussed their condition freely with others and were no less favorable to the idea of screening than the noncarriers, about one-half of their number expressed discomfort in being told they were heterozygotes. These feelings were allayed by counseling, but there was evidence of some residual unease. It is suggested that this anxiety would be less prominent and more easily reduced if screening were done under conditions of ordinary primary medical care rather than outside the conventional system.

Tay-Sachs screening: motives for participating and knowledge of genetics and probability

A highly-educated, socially aware group of persons presented themselves for Tay-Sachs screening having learned about it mainly from friends, newspapers, radio, and television but not from physicians or rabbis. After learning that screening was possible and deciding that it is in principle a good idea, and after discussing it with relatives and friends but not with physicians and rabbis, they presented themselves for the test. Although the participants knew that Tay-Sachs is a serious disease and that Jews are vulnerable, few of them knew much about the genetics of the disease, its frequency, or the incidence of the carrier state. This experience of screening for Tay-Sachs carriers suggests the need for physicians to learn the relation of genetics to preventive medicine, and for the public to learn more about the biology of man.

Unequal accumulation of alpha- and beta-globin mRNA in erythropoietic mouse spleen

Relative amounts and rates of synthesis of alpha- and beta-globin mRNAs were determined during splenic erythropoiesis in mice. At times after injection of mice with phenylhydrazine, alpha- and beta-globin mRNAs were separated by gel electrophoresis and quantitated by densitometric scanning of stained gels. At 66 hr after injection, the ratio of beta to alpha mRNA is about 1.2. By 138 hr, total globin mRNA is 5-fold greater in spleen cells, and the beta to alpha mRNA ratio approaches 2. This ratio remains around 1 in reticulocytes throughout this period. Analyses of globin products directed by these mRNAs from spleen cells and reticulocytes in the ascites cell-free system reflect the beta to alpha mRNA ratio observed by electrophoresis. Relative rates of synthesis of globin mRNAs were estimated after incubation of spleen cells with either [3H] uridine or [3H] adenosine. Although synthesis of both mRNAs is maximal at 114 hr and then declines sharply, beta mRNA is synthesized at a greater rate than alpha mRNA at every developmental stage. In contrast to the excess accumulation of beta mRNA in spleen cells, synthesis of alpha- and beta-globin chains remains balanced throughout erythroid development. These data suggest that during erythropoiesis in this system, equal synthesis of alpha and beta globin involves regulation at both transcriptional and post-transcriptional levels.

Electrophoretic study of human ribosomal core proteins: evidence for selection against variation

Ten human ribosomal core proteins examined electrophoretically showed significantly less polymorphic variation than expected on the basis of other protein systems studied. This implies that selective forces may restrict variation at these loci. Variation in common multigenic traits must be determined by a combination of common alleles. If there is restriction of variation at certain types of loci, a limit is placed on the number of loci which can play a role in multigenic inheritance. Surveys for polymorphism in groups of proteins with similar structural or functional roles will further define these limits.