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Biomedical subjects

B G Forget

Publications and source records attributed to B G Forget.

At least 91 records · Page 5Linked to original sources
PubMed1989

The breakpoint of a large deletion causing hereditary persistence of fetal hemoglobin occurs within an erythroid DNA domain remote from the beta-globin gene cluster.

The DNA juxtaposed to the gamma-globin genes as a result of a large deletion associated with hereditary persistence of fetal hemoglobin (HPFH) was studied to define the role it may play in maintaining active expression of these genes in adult erythroid cells. The DNA located immediately 3' to the deletion breakpoint was found to function as an enhancer element in gene transfer experiments and to be specifically hypomethylated in normal erythroid cells of both fetal and adult origin. This DNA also contains a long open reading frame encoding a polypeptide chain 292 amino acids in length. Therefore, in this form of HPFH (HPFH-1), the continued expression of gamma-globin genes in adult life may result from the inclusion of these genes within a new chromosomal domain that is potentially transcriptionally active in adult erythroid cells. The 3' breakpoint of another large deletion causing delta beta thalassemia rather than HPFH was also identified. This deletion (Spanish G gamma A gamma (delta beta)(0) thalassemia) is nearly identical in size and location to that of HPFH-1, but extends an additional 8.5 to 9 kb in the 3' direction, and therefore results in loss of the sequences near the 3' breakpoint of HPFH-1. Thus, the presence of these sequences appears to be important for the expression of the HPFH phenotype.

Amino Acid Sequence↗
PubMed1989

Spectrin Tunis (Sp alpha I/78), an elliptocytogenic variant, is due to the CGG----TGG codon change (Arg----Trp) at position 35 of the alpha I domain.

Spectrin Tunis (Sp alpha I/78) is an alpha l domain variant that causes asymptomatic elliptocytosis in the heterozygote state. It is manifested by a reduction of spectrin dimer self-association and by the development of a major 78-Kd fragment at the expense of the alpha l 80-Kd fragment upon spectrin-limited digestion. Amino acid sequence analysis, following peptide transfer onto Immobilon membranes, showed that the 78-Kd fragment results from a sensitized cleavage after lysyl residue 10. Using a 13.5-kb genomic alpha-spectrin probe and the Xbal, Pvull, and Mspl polymorphic sites detected with this probe, we concluded that spectrin Tunis is associated with the + - + haplotype (in the above order). Twenty mer oligonucleotides, complementary to genomic segments from introns 2 and 3, respectively, were synthesized. We then performed DNA amplification and sequencing. In the two investigated carriers of spectrin Tunis, we found the C----T base substitution of the codon corresponding to position 35 of the alpha l domain (CGG----TGG; Arg----Trp). The mutation lies in the last part of an alpha helix that extends from residues 9 to 44 of partial repeat alpha 1' and is comparable with helix 3 of full repeats 1 to 5. The modified proteolytic site, located 25 amino acid residues upstream, occurs at the beginning of the helix.

Amino Acid Sequence↗
PubMed1989

Characterization of the calmodulin-binding site of nonerythroid alpha-spectrin. Recombinant protein and model peptide studies.

An important function of the mammalian nonerythroid alpha-spectrin chain (alpha-fodrin) that distinguishes it from the closely related erythroid isoform is its ability to bind calmodulin. By analysis of a series of deleted recombinant spectrin fusion proteins, we have identified a region in the nonerythroid alpha chain involved in calcium-dependent binding of calmodulin. The region is distinctive in that the sequence is absent from the homologous domain of the erythroid alpha chain and diverges from the normal internal repeat structure observed throughout other spectrins. In order to determine limits of this functional site, a synthetic peptide as small as 24 residues was shown to compete with either recombinant or brain alpha-spectrin in binding to calmodulin. The active peptide, which was derived from a segment between repeats 11 and 12, was composed of the following sequence: Lys-Thr-Ala-Ser-Pro-Trp-Lys-Ser-Ala-Arg-Leu-Met-Val-His-Thr-Val-Ala-Thr-Phe-Asn - Ser-Ile-Lys-Glu. Comparison of this sequence with functional sites in other diverse calcium-dependent calmodulin-binding proteins has revealed a structural motif common to all of these proteins, namely clusters of hydrophobic residues interspersed with basic residues. When folded into alpha-helical conformations, these binding sites are predicted to form amphipathic structures.

Amino Acid Sequence↗
PubMed1989

Sequence and exon-intron organization of the DNA encoding the alpha I domain of human spectrin. Application to the study of mutations causing hereditary elliptocytosis.

We have determined the exon-intron organization and the nucleotide sequence of the exons and their flanking intronic DNA in cloned genomic DNA that encodes the first 526 amino acids of the alpha I domain of the human red cell spectrin polypeptide chain. From the gene sequence we designed oligonucleotide primers to use in the polymerase chain reaction technique to amplify the appropriate exons in DNA from individuals with three variants of hereditary elliptocytosis characterized by the presence of abnormal alpha I spectrin peptides, 46-50 and 65-68 kD in size, in partial tryptic digests of spectrin. The alpha I/68-kD abnormality resulted from a duplication of leucine codon 148 in exon 4: TTG-CTG to TTG-TTG-CTG. The alpha I/50a defect was associated in different individuals with two separate single base changes in exon 6: CTG to CCG (leucine to proline) encoding residue 254, and TCC to CCC (serine to proline) encoding residue 255. In another individual with the alpha I/50a polypeptide defect, the nucleotide sequence encoding amino acid residues 221 through 264 was normal. The alpha I/50b abnormality resulted from a single base change of CAG (glutamine) to CCG (proline) encoding residue 465 in exon 11 in two unrelated individuals. In a third individual with alpha I/50b-kD hereditary elliptocytosis, the entire exon encoding residues 445 through 490 was normal. The relationship of the alpha I domain polypeptide structure to these mutations and the organization of the gene is discussed.

Amino Acid Sequence↗
PubMed1989

Molecular basis of Sp alpha I/65 hereditary elliptocytosis in North Africa: insertion of a TTG triplet between codons 147 and 149 in the alpha-spectrin gene from five unrelated families.

Hereditary elliptocytosis in North Africa is frequently associated with the alpha I/65 spectrin variant, characterized by an abnormal alpha I 65-kD instead of the normal alpha I 80-kD peptide following limited trypsin digestion of whole spectrin. A similar variant (although it yielded a 68-kD fragment) has been shown recently, in two black patients, to result from the insertion of a leucyl residue at position 148 (Marchesi et al: J Clin Invest 80:191, 1987). In order to determine if the underlying molecular defect was the same in North Africans and blacks (who originate from both sides of the Sahara Desert), we performed analysis directly at the DNA level. Starting from the DNA of an Algerian alpha I/65 heterozygote in whom the mutation was associated with identifiable RFLPs, we cloned and sequenced the alpha-spectrin gene region, which includes the mutation. We thus identified an extra leucine codon (TTG) between codons 147 and 149, the coding sequence becoming CAG TTG TTG CTG instead of CAG TTG CTG. We then used the polymerase chain reaction (PCR) method and dot-blot hybridization of the amplified DNA with mutant and normal allele-specific oligonucleotides to screen the DNA from four other unrelated North African subjects with Sp alpha I/65 hereditary elliptocytosis. In all families we studied, these subjects were heterozygous for the TTG insertion. These results demonstrate that Sp alpha I/65 hereditary elliptocytosis has the same molecular basis in North Africans and blacks.

Africa, Northern↗
PubMed1989

Beta-thalassemia due to two novel nucleotide substitutions in consensus acceptor splice sequences of the beta-globin gene.

We have identified two novel RNA-splicing mutations affecting a critical nucleotide (nt) in the acceptor consensus sequences at both the IVS-1/exon 2 and IVS-2/exon 3 junctions of the human beta-globin gene. Both mutations are single nt substitutions, T to G and C to A, at position -3 adjacent to the invariant AG dinucleotide. For the IVS-2/exon 3 mutation abnormal splicing into the cryptic splice site at IVS-2 nt 579 is documented. Identification of these two mutations provides further support for the importance of the location of specific nucleotides within the consensus sequences in splice site selection and RNA processing.

Base Sequence↗
PubMed1988

New amber mutation in a beta-thalassemic gene with nonmeasurable levels of mutant messenger RNA in vivo.

We have identified a beta-thalassemia gene that carries a novel nonsense mutation in a Chinese patient. This mutation, a G to T substitution at the first position of codon 43, changes the glutamic acid coding triplet (GAG) to a terminator codon (TAG). Based on oligonucleotide hybridization studies of 78 Chinese and Southeast Asian beta-thalassemia chromosomes, we estimate that this mutation accounts for a small minority of the beta-thalassemia mutations in that population. Study of the expression of this cloned gene in a transient expression system demonstrated a 65% decrease in levels of normally spliced mutant beta-globin mRNA. However, the study of reticulocyte RNA isolated from an individual heterozygous for this mutation demonstrated a total absence of this mutant mRNA in vivo. The basis for this big discrepancy between the level of accumulated mRNA in vivo and in vitro is probably the result of differences in the stabilities of the mutant mRNA in erythroid cells.

Amino Acid Sequence↗
PubMed1988

Analyses of linked beta-globin genes suggest that nondeletion forms of hereditary persistence of fetal hemoglobin are bona fide switching mutants.

The analysis of nondeletion forms of hereditary persistence of fetal hemoglobin (ndHPFH) has led to the identification of cis-acting elements, located in the promoter regions of the fetal genes, that appear to be involved in the process of fetal to adult hemoglobin switching. Individuals with these disorders demonstrate elevated levels of fetal hemoglobin and lowered levels of adult hemoglobin during adult life. This phenotype could be either the result of an abnormality in the switching process or the result of two independent mutations: one mutation increasing the level of fetal (gamma) gene expression and another mutation decreasing the level of adult (beta) globin gene expression. Here we demonstrate that the adult beta genes linked to two different forms of ndHPFH, G gamma beta + HPFH and Greek ndHPFH, produce normal levels of correctly processed mRNA in transient-expression systems. We also report that the nucleotide sequences of the beta genes are normal. These results indicate that these gamma gene promoter mutations are linked to functionally normal beta-globin genes and are consistent with the hypothesis that these mutations interfere with the normal switching process.

Chromosome Deletion↗
PubMed1988

Molecular cloning of the cDNA for human erythrocyte beta-spectrin.

Overlapping cDNA clones, totaling 3.3 kilobases (kb) in length, which encode over 50% of the human erythrocyte beta-spectrin subunit, were isolated by antibody screening of a lambda gt11 expression library constructed from human fetal liver mRNA. The amino acid sequence of the C-terminus of beta-spectrin was derived. The size of beta-spectrin mRNA in human erythroleukemia cells was found to be 7.5 kb. Erythrocyte beta-spectrin is encoded by a gene located on human chromosome 14, as determined by cDNA hybridization to human X mouse somatic cell hybrids.

Amino Acid Sequence↗
PubMed1987

Clinical and molecular correlations in the sickle/beta+-thalassemia syndrome.

Sickle/beta thalassemia is a sickling disorder of varying severity which results from compound heterozygosity for sickle cell trait and beta-thalassemia trait. Clinical and genetic studies have shown an inverse correlation between the level of hemoglobin A and the severity of the disease. It has been suggested that the level of hemoglobin A may be a function of the severity of the beta-thalassemia defect. In this study, we use molecular biological techniques to test this hypothesis. We show that the interaction of the mildest of the beta+-thalassemia genes with the sickle gene results in a high level of hemoglobin A. However, the interaction in this case resulted in a severe sickling disorder in the absence of significant anemia. We hypothesize that a mild beta+-thalassemia gene may have two opposite effects on the clinical course of sickle/beta+ thalassemia: (1) A high level of hemoglobin A which probably confers a favorable antisickling effect and (2) decreased hemolysis leading to increased numbers of total circulating red cells, thereby increasing the blood viscosity and the propensity for sickling. The inheritance of heterozygous alpha thalassemia 2 in conjunction with the mild beta+-thalassemia gene and sickle gene in this patient may have further enhanced the latter effect and resulted in a severe sickling disorder.

Adult↗