Molecular basis for nondeletion alpha-thalassemia in American blacks. Alpha 2(116GAG----UAG)

mRNA Analysis of Frameshift Mutations with Stop Codon in the Last Exon: The Case of Hemoglobins Campania [α1 cod95 (-C)] and Sciacca [α1 cod109 (-C)]

An insertion or deletion of a nucleotide (nt) in the penultimate or the last exon can result in a frameshift and premature termination codon (PTC), giving rise to an unstable protein variant, showing a dominant phenotype. We described two α-globin mutants created by the deletion of a nucleotide in the penultimate or the last exon of the α1-globin gene: the Hb Campania or α1 cod95 (−C), causing a frameshift resulting in a PTC at codon 102, and the Hb Sciacca or α1 cod109 (−C), causing a frameshift and formation of a PTC at codon 133. The carriers showed α-thalassemia alterations (mild microcytosis with normal Hb A2) and lacked hemoglobin variants. The 3D model indicated the α-chain variants’ instability, due to the severe structural alterations with impairment of the chaperone alpha-hemoglobin stabilizing protein (AHSP) interaction. The qualitative and semiquantitative analyses of the α1mRNA from the reticulocytes of carriers highlighted a reduction in the variant cDNAs that constituted 34% (Hb Campania) and 15% (Hb Sciacca) of the total α1-globin cDNA, respectively. We developed a workflow for the in silico analysis of mechanisms triggering no-go decay, and its results suggested that the reduction in the variant mRNA was likely due to no-go decay caused by the presence of a rare triplet, and, in the case of Hb Sciacca, also by the mRNA’s secondary structure variation. It would be interesting to correlate the phenotype with the quantity of other frameshift mRNA variants, but very few data concerning α- and β-globin variants are available.

A Novel Mutation at HBA1: c.349G>T Causing α-Thalassemia in a Chinese Family

α-Thalassemia (α-thal) is one of the most common genetic diseases in Southern China. Although more than 300 α-thal mutations have been reported in the world, the mutation spectrum is still not comprehensive. In this study, a novel mutation (HBA1: c.349G>T) in a newborn (proband) was first found by next-generation sequencing (NGS). Subsequently, hematological analysis and thalassemia genetic testing were performed for the family members. The results showed that both the proband and her mother were heterozygotes for this novel mutation and presented abnormal hematological indices. Based on the features observed in clinical practice, this novel mutation was considered as a type of α-thal variation.

Novel nonsense mutation in the α1-globin gene [HBA1:C.49A>T] is responsible for non-deletion α-thalassemia

BACKGROUND

In the α-thalassemia one of the less frequent mechanisms is the nonsense mutations, which generate the substitution of a triplet that encodes an amino acid for a stop codon and, therefore, protein synthesis stops prematurely. At present, 9 mutations of this type have been documented, 6 that affect the HBA2 gene and 3 that affect the HBA1 gene.

OBJECTIVES

We present a new mutation in CD16 of the HBA1 gene, where the change AAG>TAG generates a stop codon.

METHODS

A 48-year-old woman from Madrid, was studied because she had maintained microcytosis without iron deficiency. Hb A2 and Hb F levels were measured by ion exchange HPLC (VARIANT II). Hemoglobin was studied by capillary zone electrophoresis and ion exchange HPLC (short program of β-thalassemia). Molecular characterization was performed by automatic sequencing of alpha globin genes.

RESULTS

The propositus presented no abnormal hemoglobins and Hb A2 and Hb F levels were within normal limits. The molecular characterization identified the new transversion mutation HBA1: c.49 A>T, which resulted in an amino acid change of Lys > Stop at codon 16 of exon 1 in the state heterozygous [α₁16 (A14) Lys>Stop; HBA1: c.49A>T].

CONCLUSION

In this new nonsense mutation, short genetic products may suffer nonsense-mediated degradation, whereas the abnormal protein will be eliminated through the proteolytic pathway mediated by ubiquitin. Regardless, the phenotype is mild. The most severe end of the clinical spectrum will probably occur when a mutation is inherited together with a mutation that results in suppression of two genes (-/αα^T or -α/-α^T).

Molecular characterization of Hb Hamilton Hill (HBA2: c.388delC), a novel HBA2 variant generating a premature termination codon and truncated HBA2 chain

In recent years, the identification of α-thalassemias caused by nondeletional mutations has increased significantly due to the advancement of sensitive molecular genetics tools. We report clinical and experimental data for a novel frameshift mutation caused by a single base deletion at position 388 in exon 3 of the α2-globin gene (HBA2: c.388delC; Hb Hamilton Hill), resulting in the phenotype of α-thalassemia (α-thal). Hb Hamilton Hill was identified in an adult female of unknown ethnicity investigated for unexplained microcytosis. Direct DNA sequencing of the HBA2 gene revealed a heterozygous mutation, HBA2: c.388delC, and the molecular effect of this mutation was assessed experimentally using our previously described in vitro model. The experimental analysis involved transfection of a human bladder carcinoma (5637) cell line with expression vectors carrying either HBA2-wild type (HBA2-WT) or HBA2: c.388delC followed by total RNA purification and cDNA synthesis. Both wild type and mutant gene expression was studied and compared at the transcriptional and translational levels using quantitative real time polymerase chain reaction (qReTi-PCR) and immunofluorochemistry (IFC), respectively. Our experimental data showed a significant reduction by 25.0% (p = 0.04) in the transcriptional activity generated from HBA2: c.388delC compared to HBA2-WT. As a result of this base deletion, a frameshift in the open reading frame generates a premature termination codon (PTC) at codon 132 of exon 3 resulting in the formation of a truncated α-globin chain. The truncated α-globin chain, observed by the IFC technique, is most likely unstable and undergoes a rapid turnover resulting in the thalassemic phenotype.

α-thalassemia trait caused by frameshift mutations in exon 2 of the α2-globin gene: HBA2:c.131delT and HBA2:c.143delA

We describe two frameshift mutations associated with an α-thalassemia (α-thal) phenotype, identified in three unrelated individuals investigated for persistent microcytosis. The first mutation, HBA2:c.131delT, is located in codon 43, and the second, HBA2:c.143delA, is located in codon 47. Both are due to single base pair deletions that cause a frameshift and a premature termination codon (PTC) at positions 48/49. The presence of a PTC at this position has been documented to result in nonsense mediated mRNA decay that would account for the thalassemic phenotype.

Hb Lynwood [α107(G14) (-T) (α2) HBA2:c.323delT)] in conjunction with the α(3.7) deletion produces a moderately severe α-thalassemia phenotype

We describe a novel frameshift mutation associated with an α-thalassemia (α-thal) phenotype in a patient of Sudanese origin investigated for persistent microcytosis. In addition to the α(3.7) deletion, a novel mutation on the α2 gene was detected: HBA2:c.323delT. This mutation causes a frameshift at codon 107 of the α2 gene. The result is a disturbed amino acid sequence for the following 24 amino acids, and a premature termination codon at position 132.

A Novel α‐Thalassemia Nonsense Mutation in Codon 23 of the α2‐Globin Gene (GAG→TAG) in a Tunisian Family

Herein we describe a novel alpha-thalassemia (thal) point mutation in the alpha2-globin gene, found in a 3-year-old Tunisian girl who had Hb Bart's (gamma4) at birth, later on presenting with moderate anemia, microcytosis and hypochromia. She had a normal Hb A2 level and no abnormal hemoglobin (Hb) fraction. After excluding most of the common Mediterranean mutations, the alpha2-globin gene was sequenced and found to have a point mutation in the heterozygous state that creates a premature stop signal for translation (GAG-->TAG or Glu-->Term) at codon 23. The same mutation was also found in the mother in the heterozygous state, while the father had a normal sequence. The presence of the mutation was also confirmed by nucleotide sequencing of the opposite strand. Since the mutation creates a restriction site for the BfaI enzyme, a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP)-based assay was established for screening purposes.

A targeted apolipoprotein B-38.9-producing mutation causes fatty livers in mice due to the reduced ability of apolipoprotein B-38.9 to transport triglycerides

Nonphysiological truncations of apolipoprotein (apo) B-100 cause familial hypobetalipoproteinemia (FHBL) in humans and mice. An elucidation of the mechanisms underlying the FHBL phenotypes may provide valuable information on the metabolism of apo B-containing lipoproteins and the structure-function relationship of apo B. To generate a faithful mouse model of human FHBL, a subtle mutation was introduced into the mouse apo B gene by targeting embryonic stem cells using homologous recombination followed by removal of the selection marker gene by Cre-loxP-mediated site-specific recombination. The engineered mice bear a premature stop codon at residue 1767 and a 42-base pair loxP inserted into intron 24 of the apo B gene, thus closely resembling the apo B-38.9-producing mutation in humans. Apo B-38.9 was the sole apo B protein in homozygote (apob(38.9/38.9)) plasma. In heterozygotes (apob(+/)(38. 9)), apo B-100 and apo B-48 were reduced by 75 and 40%, respectively, and apo B-38.9 represented 20% of total circulating apo B. Hepatic apo B-38.9 mRNA levels were reduced by 40%. In cultured apob(+/)(38. 9) hepatocytes, apo B-100 was produced in trace quantities, and the synthesis rate of apo B-38.9 relative to apo B-48 was reduced by 40%. However, almost equimolar amounts of apo B-38.9 and apo B-48 were secreted into the media. Pulse-chase studies revealed that apo B-38. 9 was secreted at a faster rate and more efficiently than apoB-48. Nevertheless, both apob(+/)(38.9) and apob(38.9/38.9) mice had reduced hepatic triglyceride secretion rates and fatty livers. Thus, low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the FHBL phenotypes caused by the apo B-38.9 mutation. Rather, a reduced capacity of apo B-38.9 for triglyceride transport may account for the fatty livers in these mice.

Mutation pattern in the Bruton's tyrosine kinase gene in 26 unrelated patients with X-linked agammaglobulinemia

Mutation pattern was characterized in the Bruton's tyrosine kinase gene (BTK) in 26 patients with X-linked agammaglobulinemia, the first described immunoglobulin deficiency, and was related to BTK expression. A total of 24 different mutations were identified. Most BTK mutations were found to result in premature termination of the translation product. Mutations were detected in most BTK exons with a predominance of frameshift and nonsense mutations in the 5' end of the gene and missense mutations in its 3' part, corresponding to the catalytic domain of the enzyme. Nonsense and frameshift mutations were associated with diminished levels of BTK mRNA expression, except for a frameshift mutation in exon 17 and two nonsense mutations in exon 2, indicating that these cases are not confined to penultimate exons. One amino acid substitution (R28H) was found in the pleckstrin homology domain's residue, which is mutated in mice bearing the X-linked immunodeficiency phenotype; another substitution (R307G) was identified in the src homology domain 2. All remaining amino acid substitutions were found in the catalytic domain of Btk.

Regulation of the apolipoprotein B in heterozygous hypobetalipoproteinemic knock-out mice expressing truncated apoB, B81. Low production and enhanced clearance of apoB cause low levels of apoB

Low levels of cholesterol are protective against development of coronary artery disease. Heterozygous hypobetalipoproteinemic individuals expressing truncated apolipoprotein (apo)B as a result of mutation in the apob gene have low levels of cholesterol and apoB in their plasma. To study the molecular mechanism of low levels of apoB in these individuals, we employed a previously reported knock out mouse model generated by targeted modification of the apob gene. The heterozygous, apoB-100/B-81, mice express full length and truncated apoB, B-81, and have 20 and 35% lower levels of total cholesterol and apoB, respectively, when compared to WT (apoB-100/B-100) mice. The majority of the truncated apoB, B-81, fractionated in the VLDL- density range. The mechanism of low levels of apoB in B-100/B-81 mice was examined. Total hepatic apoB mRNA levels decreased by 15%, primarily due to lower levels of apoB-81 mRNA. Since apoB mRNA transcription rates were similar in B-100/B-100 and B-100/B-81 mice, low levels of mutant apoB-81 mRNA occurred by enhanced degradation of apoB mRNA transcript containing premature translational stop codon. ApoB synthesis measured on isolated hepatocytes decreased in B-100/B-81 mice by 35%, while apoB-48, apoE, and apoAI syntheses remained unchanged. Metabolic studies using whole animal showed a 32% decrease in triglyceride secretion rates, consistent with the apoB secretion rates. Inhibition of receptor-mediated clearance of apoB-81-containing particles resulted in greater relative accumulation of apoB-81 in plasma than apoB-100, suggesting enhanced clearance of apoB-81-containing particles. These results demonstrate that low levels of apoB in heterozygous hypobetalipoproteinemic mice occurs by low rates of apoB secretion, and increased clearance of truncated apoB. Similar mechanisms appear to contribute to low levels of apoB in hypobetalipoproteinemic humans.

Alpha-thalassaemia

alpha-Thalassaemias are genetic defects extremely frequent in some populations and are characterized by the decrease or complete suppression of alpha-globin polypeptide chains. The gene cluster, which codes for and controls the production of these polypeptides, maps near the telomere of the short arm of chromosome 16, within a G + C rich and early-replicating DNA region. The genes expressed during the embryonic (zeta) or fetal and adult stage (alpha 2 and alpha 1) can be modified by point mutations which affect either the processing-translation of mRNA or make the polypeptide chains extremely unstable. Much more frequent are the deletions of variable size (from approximately 3 to more than 100 kb) which remove one or both alpha genes in cis or even the whole gene cluster. Deletions of a single gene are the result of unequal pairing during meiosis, followed by reciprocal recombination. These unequal cross-overs, which produce also alpha gene triplications and quadruplications, are made possible by the high degree of homology of the two alpha genes and of their flanking sequences. Other deletions involving one or more genes are due to recombinations which have taken place within non-homologous regions (illegitimate recombinations) or in DNA segments whose homology is limited to very short sequences. Particularly interesting are the deletions which eliminate large DNA areas 5' of zeta or of both alpha genes. These deletions do not include the structural genes but, nevertheless, suppress completely their expression. Larger deletions involving the tip of the short arm of chromosome 16 by truncation, interstitial deletions or translocations result in the contiguous gene syndrome ATR-16. In this complex syndrome alpha-thalassaemia is accompanied by mental retardation and variable dismorphic features. The study of mutations of the 5' upstream flanking region has led to the discovery of a DNA sequence, localized 40 kb upstream of the zeta-globin gene, which controls the expression of the alpha genes (alpha major regulatory element or HS-40). In the acquired variant of haemoglobin H (HbH) disease found in rare individuals with myelodysplastic disorders and in the X-linked mental retardation associated with alpha-thalassaemia, a profound reduction or absence of alpha gene expression has been observed, which is not accompanied by structural alterations of the coding or controlling regions of the alpha gene complex. Most probably the acquired alpha-thalassaemia is due to the lack of soluble activators (or presence of repressors) which act in trans and affect the expression of the homologous clusters and are coded by genes not (closely) linked to the alpha genes. The ATR-X syndrome results from mutations of the XH2 gene, located on the X chromosome (Xq13.3) and coding for a transacting factor which regulates gene expression. The interaction of the different alpha-thalassaemia determinants results in three phenotypes: the alpha-thalassaemic trait, clinically silent and presenting only limited alterations of haematological parameters, HbH disease, characterized by the development of a haemolytic anaemia of variable degree, and the (lethal) Hb Bart's hydrops fetalis syndrome. The diagnosis of alpha-thalassaemia due to deletions is implemented by the electrophoretic analysis of genomic DNA digested with restriction enzymes and hybridized with specific molecular probes. Recently polymerase chain reaction (PCR) based strategies have replaced the Southern blotting methodology. The straightforward identification of point mutations is carried out by the specific amplification of the alpha 2 or alpha 1 gene by PCR followed by the localization and identification of the mutation with a variety of screening systems (denaturing gradient gel electrophoresis (DGGE), single strand conformation polymorphisms (SSCP)) and direct sequencing.

mRNA and protein expression of p53 mutations in human bladder cancer cell lines

We investigated mRNA and protein expression in p53 gene mutations in four human bladder cancer cell lines using polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) and Northern blot and Western blot analyses. The following mutations were identified in three of the four cell lines: a missense transversion at codon 110, a missense transition at codon 250 and a non-sense transversion at codon 126. These mutations were located outside previously identified hot spot codons and have rarely been reported in bladder cancer tissues or other neoplasms. Positive intranuclear p53 immunostaining in neoplastic cells in the two missense mutations and the premature stop codon in the non-sense mutation suggested the presence of structural and functional alterations in the p53 protein. Northern and Western blot analyses revealed either an intense or a weak p53 mRNA band together with an intense p53 protein band in the missense mutations, but no p53 mRNA or protein band in the non-sense mutation. A weak p53 mRNA band, but no distinct p53 protein band was observed in the cell line without a mutation and in normal control bladder cells. Our findings suggest that regulation of p53 expression in these cell lines differs at the post-transcriptional and/or post-translational level between the wildtype and the mutant p53 genes and also among different mutant p53 genes. The three cell lines with mutations were derived from high-grade carcinomas; the cell line without mutation was derived from a low-grade carcinoma.

Nonsense mutation R1162X of the cystic fibrosis transmembrane conductance regulator gene does not reduce messenger RNA expression in nasal epithelial tissue

Cystic fibrosis (CF) patients bearing the premature translation termination mutation (nonsense mutation) W1282X present severe pulmonary and pancreatic disease, whereas patients carrying other nonsense mutations such as G542X, R553X, S1255X, R1162X, and W1316X show a severe pancreatic but mild pulmonary illness. CF gene expression was found absent in respiratory tissues with mutations R553X and W1316X, which led to the hypothesis that the absence of the gene product in the lung is more favorable than the presence of an altered one. We asked whether or not all the nonsense mutations characterized by mild pulmonary disease phenotypes do present the absence of CF gene expression. We therefore investigated gene expression at the mRNA level in respiratory cells obtained from nasal polyps from a patient homozygous for the R1162X mutation. Gene expression was studied by amplification with polymerase chain reaction of segments of the CF transmembrane conductance regulator cDNA that was obtained by reverse transcription of RNA. Semiquantitative analysis was performed by Northern analysis. By comparing the data obtained from polyps deriving from non-CF subjects and a CF patient homozygous for dF508 mutation, it is shown that no reduction of CF gene expression is evident in R1162X respiratory tissue. We conclude that CF nonsense mutations have heterogeneous mechanisms of gene expression.

alpha-Thalassaemia

The large number of naturally occurring mutants of this well-characterized locus provides an excellent opportunity for elucidating the relationship between its structure and function. Comparisons of what has been learned about the alpha-globin locus with complementary observations on the beta-globin locus, provide a strategy for understanding the co-ordinate regulation of eukaryotic gene expression. From a practical point of view it is important to remember that millions of individuals throughout the world are carriers of alpha-thalassaemia and every year many thousands of pregnancies are at risk of producing children with the severe alpha-thalassaemia syndromes. The data summarized here provide the basis for accurately predicting the genotype in such cases and thus enabling appropriate prenatal testing. However, because this is a genetic disease that predominantly affects individuals from countries with limited health resources, simpler and cheaper methods of screening and diagnosis will have to be developed before this information has a significant impact on the attendant morbidity and mortality (see Chapter 9, this volume).

Human gene mutations affecting RNA processing and translation

Gene mutations affecting mRNA processing and translation are not common causes of human genetic disease. Their analysis has nevertheless provided important insights into the basic biochemical mechanisms underlying mRNA transcription and translation.

Independent mutations of the human CD3-epsilon gene resulting in a T cell receptor/CD3 complex immunodeficiency

The T-cell receptor (TCR) is composed of two glycoproteins (alpha and beta or gamma and delta) associated with four invariant polypeptides (CD3-gamma, delta, epsilon and zeta). The majority of TCR/CD3 complexes contain six polypeptide chains, and although there is some flexibility in the complex subunit stoichiometry the CD3-epsilon chain is central to CD3 core assembly and full complex formation. We have described previously defective expression of the TCR/CD3 complex in an immunodeficient child. We now report that two independent CD3-epsilon gene mutations present in the parents have segregated in the patient, leading to defective CD3-epsilon chain synthesis and preventing normal association and membrane expression of the TCR/CD3 complex.

Studies on the in vitro and in vivo expression of a dysfunctional alpha-globin gene

In a black family with members having alpha-thalassemia and hemoglobin H (HbH) disease, a deletion of an AG dinucleotide at the 3' end of exon 1 near the junction with intron 1 was shown previously to produce a dysfunctional alpha-thalassemia gene with a reading frame-shift and a nonsense codon (Safaya S, Rieder RF: J Biol Chem 263:4328-4332, 1988). We have found that the same mutation is responsible for alpha-thalassemia and HbH disease in a second unrelated black family (Bellevue R, Dosik H, Rieder RF: Br J Haematol 41:193-202, 1979). Despite the loss of two nucleotides from the consensus sequence at the 5' splice donor site of intron 1, studies employing an in vitro plasmid-based expression system indicated that the mutant alpha-globin mRNA was spliced normally and expressed in amounts equal to normal alpha-globin mRNA in COS-7 cells. The correct processing of the mRNA in these studies is probably due to the presence of a tandem repeat of the affected AG dinucleotide. However, in reticulocytes from subjects bearing the mutant gene, we were unable to detect any of the abnormal mRNA. These findings suggest that there is accelerated post-transcriptional loss of mRNA bearing a premature terminator codon.

Effects of alpha-thalassemia and sickle polymerization tendency on the urine-concentrating defect of individuals with sickle cell trait

A defect in urine concentrating ability occurs in individuals with sickle cell trait (HbAS). This may result from intracellular polymerization of sickle hemoglobin (HbS) in erythrocytes, leading to microvascular occlusion, in the vasa recta of the renal medulla. To test the hypothesis that the severity of the concentrating defect is related to the percentage of sickle hemoglobin present in erythrocytes, urinary concentrating ability was examined after overnight water deprivation, and intranasal desmopressin acetate (dDAVP) in 27 individuals with HbAS. The HbAS individuals were separated into those who had a normal alpha-globin genotype (alpha alpha/alpha alpha), and those who were either heterozygous (-alpha/alpha alpha) or homozygous (-alpha/-alpha) for gene-deletion alpha-thalassemia, because alpha-thalassemia modulates the HbS concentration in HbAS. The urinary concentrating ability was less in the alpha alpha/alpha alpha genotype than in the -alpha/alpha alpha or -alpha/-alpha genotypes (P less than 0.05). After dDAVP, the urine osmolality was greater in patients with the -alpha/-alpha genotype than with the -alpha/alpha alpha genotype (882 +/- 37 vs. 672 +/- 38 mOsm/kg H2O) (P less than 0.05); patients with the -alpha/alpha alpha genotype had greater concentrating ability than individuals with a normal alpha-globin gene arrangement. There was an inverse linear correlation between urinary osmolality after dDAVP and the percentage HbS in all patients studied (r = -0.654; P less than 0.05). A linear correlation also existed for urine concentrating ability and the calculated polymerization tendencies for an oxygen saturation of 0.4 and O (r = -0.62 and 0.69, respectively). We conclude that the severity of hyposthenuria in HbAS is heterogeneous. It is determined by the amount of HbS polymer, that in turn is dependent upon the percentage HbS, which is itself related to the alpha-globin genotype.

Nonsense mutations affect C1 inhibitor messenger RNA levels in patients with type I hereditary angioneurotic edema

Members of two unrelated families with type I hereditary angioneurotic edema (HANE) were found to have elevated levels of C1 inhibitor (C1INH) mRNA. DNA sequence analysis of PCR-amplified monocyte C1INH mRNA revealed normal and mutant transcripts, as expected in this disorder that occurs in heterozygous individuals. Single base mutations near the 3' end of the coding sequence were identified in affected members of each family. One mutation consisted of insertion of an adenosine at position 1304 which created a premature termination codon (TAA), whereas the second consisted of deletion of the thymidine at position 1298 which created a premature termination codon (TGA) 23 nucleotides downstream. These mutations are approximately 250 nucleotides upstream of the natural termination codon. Nuclear run-off experiments in one kindred revealed no difference in transcription rates of the C1INH gene between the patients and normals. C1INH mRNA half-life experiments were not technically feasible because of the prolonged half-life of the normal transcript. Dideoxynucleotide primer extension experiments allowed the differentiation of the normal and mutant transcripts. These studies showed that the mutant transcript was not decreased relative to the normal, and this therefore was at least partially responsible for the C1INH mRNA elevation. This elevation may be due to the decreased catabolism of the mutant transcript.

Isolation and characterization of the translation product of a beta-globin gene nonsense mutation (beta 121 GAA----TAA)

The beta o-thalassaemia gene of an individual who was a mixed heterozygote for this allele (GAA to TAA in codon 121) and beta(+)-thalassaemia (IVS-1 position 110 G to A) was examined to determine if the beta o-thalassaemia allele directed the synthesis of any detectable protein product. This beta o-thalassaemia allele was of particular interest, because it is the only example of a premature chain termination codon in the third exon of the beta-globin gene that produces thalassaemia. A very small amount of an abnormal protein was found in the red blood cells of the proband and was purified by preparative column chromatography. This abnormal protein was digested with trypsin, the peptides were separated by reverse phase high performance liquid chromatography (HPLC), and the amino acid content of each peptide was determined. All of the soluble beta-globin peptides were found except for T-13, T-14 and T-15 (residues 121-146), indicating the presence of a truncated protein that corresponded to the translation product of the beta 121 Glu----Term mRNA. This truncated globin was estimated to comprise between 0.05% and 0.1% of the total non-alpha-globin protein. These results may explain the phenotype of inclusion body beta-thalassaemia in heterozygotes, which is atypical of heterozygous beta o-thalassaemia.

Clinical features and molecular analysis of the alpha thalassemia/mental retardation syndromes. II. Cases without detectable abnormality of the alpha globin complex

We have identified five unrelated patients, all of north European origin, who have hemoglobin H (Hb H) disease and profound mental handicap. Surprisingly, detailed molecular analysis of the alpha globin complex is normal in these subjects. Clinically, they present with a rather uniform constellation of abnormalities, notably severe mental handicap, microcephaly, relative hypertelorism, unusual facies and genital anomalies. Hematologically, their Hb H disease has subtly but distinctly milder properties than the recognized Mendelian forms of the disease. These common features suggest that these five "nondeletion" patients have a similar underlying mutation, quite distinct from the 16p13.3 deletion associated with alpha thalassemia and mild to moderate mental retardation described in the accompanying paper. We speculate that the locus of this underlying mutation is not closely linked to the alpha globin complex and may encode a trans-acting factor involved in the normal regulation of alpha globin expression.

A nonsense mutation in the apolipoprotein C-IIPadova gene in a patient with apolipoprotein C-II deficiency

The apo C-II gene from a patient with apo C-II deficiency has been sequenced after amplification by the polymerase chain reaction. A substitution of an adenosine for a guanosine at position 3002 in exon 3 of the patient's gene was identified by sequence analysis. This mutation leads to the introduction of a premature termination codon (TAA) at a position corresponding to amino acid 37 of mature apo C-II and to the formation of a new Rsa I restriction enzyme site not present in the normal apo C-II gene. Amplification of DNA from family members by the polymerase chain reaction and digestion with Rsa I established that the patient is a true homozygote for the mutation. Analysis of the patient's plasma by two-dimensional gel electrophoresis and immunoblotting detected an apo C-II that exhibited abnormal electrophoretic mobility. We propose that the C to A substitution in the apo C-IIPadova gene is the primary genetic defect that leads to premature termination and the synthesis of a truncated 36 amino acid apo C-II that is unable to activate lipoprotein lipase.

Thalassemia: molecular pathology and management

Recent advances in molecular biology have allowed us to develop an almost complete picture of the molecular pathology of the thalassemia syndromes. The different classes of mutations that are responsible for the thalassemia syndromes will be discussed along with the special insights they have provided into the controls of eukaryotic gene expression. While management of these disorders has not kept pace with our understanding of their cause, there have been notable advances in treatment. Perhaps even more exciting is what the future holds, as the continued march of molecular biology is melded with novel approaches to the definitive treatment of thalassemias.

Nonsense mutations in the human beta-globin gene affect mRNA metabolism

A number of premature translation termination mutations (nonsense mutations) have been described in the human alpha- and beta-globin genes. Studies on mRNA isolated from patients with beta zero-thalassemia have shown that for both the beta-17 and the beta-39 mutations less than normal levels of beta-globin mRNA accumulate in peripheral blood cells. (The codon at which the mutation occurs designates the name of the mutation; there are 146 codons in human beta-globin mRNA.) In vitro studies using the cloned beta-39 gene have reproduced this effect in a heterologous transfection system and have suggested that the defect resides in intranuclear metabolism. We have asked if this phenomenon of decreased mRNA accumulation is a general property of nonsense mutations and if the effect depends on the location or the type of mutation. Toward this end, we have studied the effect of five nonsense mutations and two missense mutations on the expression of human beta-globin mRNA in a heterologous transfection system. In all cases studied, the presence of a translation termination codon correlates with a decrease in the steady-state level of mRNA. The data suggest that the metabolism of a mammalian mRNA is affected by the presence of a mutation that affects translation.

The different types of alpha-thalassemia-2: genetic aspects

The alpha-thal-2 haplotype is the most common cause of alpha-thal and is found in a great many of the world's populations. It is most commonly due to the deletion of a single alpha-globin structural gene, and the deletion of a single alpha locus using gene mapping methods provides objective diagnostic evidence for this haplotype. Seven varieties of nondeletion syndromes have also been described, but these seem to be responsible for a small minority of alpha-thal in all populations studied. Detailed structural analyses of the deletional types have revealed that they were caused by a wide variety of DNA recombinant events, the racial difference of which remain poorly understood.

An initiation codon mutation (AUG----GUG) of the human alpha 1-globin gene. Structural characterization and evidence for a mild thalassemic phenotype

alpha-globin is encoded by two adjacent genes, alpha 1 and alpha 2. Recent evidence suggests that these genes are not equally expressed and that the alpha 2-globin gene encodes the majority of alpha-globin. This finding would predict that a thalassemic mutation of the alpha 2-globin gene would result in a more severe loss of alpha-chain synthesis than a similar mutation in the alpha 1-globin gene. In a previous study we described a nondeletion alpha-thalassemia defect in the alpha 2-globin gene resulting from an AUG----ACG initiation codon mutation. In the present study we describe a different initiation codon mutation, AUG----GUG, present in the alpha 1-globin gene. The alpha 1- and alpha 2-globin gene initiation codon mutations result in similarly lowered levels of encoded mRNA. Despite the similarity of these two mutations, the alpha 2 mutant results in a more severe loss of alpha-globin synthesis and a more severe clinical alpha-thalassemia phenotype than the corresponding alpha 1-globin gene mutation. This difference reflects the dominant role of alpha 2-globin gene in overall alpha-globin synthesis.