Molecular basis of beta thalassemia in south China. Strategy for DNA analysis

CRISPR/Cas-based gene editing in therapeutic strategies for beta-thalassemia

Beta-thalassemia (β-thalassemia) is an autosomal recessive disorder caused by point mutations, insertions, and deletions in the HBB gene cluster, resulting in the underproduction of β-globin chains. The most severe type may demonstrate complications including massive hepatosplenomegaly, bone deformities, and severe growth retardation in children. Treatments for β-thalassemia include blood transfusion, splenectomy, and allogeneic hematopoietic stem cell transplantation (HSCT). However, long-term blood transfusions require regular iron removal therapy. For allogeneic HSCT, human lymphocyte antigen (HLA)-matched donors are rarely available, and acute graft-versus-host disease (GVHD) may occur after the transplantation. Thus, these conventional treatments are facing significant challenges. In recent years, with the advent and advancement of CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 (CRISPR-associated protein 9) gene editing technology, precise genome editing has achieved encouraging successes in basic and clinical studies for treating various genetic disorders, including β-thalassemia. Target gene-edited autogeneic HSCT helps patients avoid graft rejection and GVHD, making it a promising curative therapy for transfusion-dependent β-thalassemia (TDT). In this review, we introduce the development and mechanisms of CRISPR/Cas9. Recent advances on feasible strategies of CRISPR/Cas9 targeting three globin genes (HBB, HBG, and HBA) and targeting cell selections for β-thalassemia therapy are highlighted. Current CRISPR-based clinical trials in the treatment of β-thalassemia are summarized, which are focused on γ-globin reactivation and fetal hemoglobin reproduction in hematopoietic stem cells. Lastly, the applications of other promising CRISPR-based technologies, such as base editing and prime editing, in treating β-thalassemia and the limitations of the CRISPR/Cas system in therapeutic applications are discussed.

U7 snRNA: A tool for gene therapy

Most U-rich small nuclear ribonucleoproteins (snRNPs) are complexes that mediate the splicing of pre-mRNAs. U7 snRNP is an exception in that it is not involved in splicing but is a key factor in the unique 3' end processing of replication-dependent histone mRNAs. However, by introducing controlled changes in the U7 snRNA histone binding sequence and in the Sm motif, it can be used as an effective tool for gene therapy. The modified U7 snRNP (U7 Sm OPT) is thus not involved in the processing of replication-dependent histone pre-mRNA but targets splicing by inducing efficient skipping or inclusion of selected exons. U7 Sm OPT is of therapeutic importance in diseases that are an outcome of splicing defects, such as myotonic dystrophy, Duchenne muscular dystrophy, amyotrophic lateral sclerosis, β-thalassemia, HIV-1 infection and spinal muscular atrophy. The benefits of using U7 Sm OPT for gene therapy are its compact size, ability to accumulate in the nucleus without causing any toxic effects in the cells, and no immunoreactivity. The risk of transgene misregulation by using U7 Sm OPT is also low because it is involved in correcting the expression of an endogenous gene controlled by its own regulatory elements. Altogether, using U7 Sm OPT as a tool in gene therapy can ensure lifelong treatment, whereas an oligonucleotide or other drug/compound would require repeated administration. It would thus be strategic to harness these unique properties of U7 snRNP and deploy it as a tool in gene therapy.

Analysis of genotype distribution of thalassemia and G6PD deficiency among Hakka population in Meizhou city of Guangdong Province

Objective

The aim of the study was to explore genotype distribution thalassemia and G6PD deficiency in Meizhou city, China.

Methods

A total of 16 158 individuals were involved in thalassemia genetic testing. A total of 605 subjects were screened for common Chinese G6PD mutations by gene chip analysis. Genotypes and allele frequencies were analyzed.

Results

A total of 5463 cases carried thalassemia mutations were identified, including 3585 cases, 1701 cases, and 177 cases with α‐, β‐, and α + β‐thalassemia mutations, respectively. ‐‐^SEA (65.12%), ‐α^3.7 (19.05%), and ‐α^4.2 (8.05%) deletion were the main mutations of α‐thalassemia, while IVS‐II‐654(C → T) (40.39%), CD41‐42(‐TCTT) (32.72%), ‐28(A → G) (10.11%), and CD17(A → T) (9.32%) mutations were the principal mutations of β‐thalassemia in Meizhou. There were significant differences in allele frequencies in some counties. Genetic testing for G6PD deficiency, six mutation sites, and one polymorphism were detected in our study. A total of 198 alleles with the mutation were detected among 805 alleles (24.6%). G6PD Canton (c.1376 G → T) (45.96%), G6PD Kaiping (c.1388 G → A) (39.39%), and G6PD Gaohe (c.95 A → G) (9.09%) account for 94.44% mutations, followed by G6PD Chinese‐5 (c.1024 C → T) (4.04%), G6PD Viangchan (c.871G → A) (1.01%), and G6PD Maewo (c.1360 C → T) (0.51%). There were some differences of the distribution of G6PD mutations among eight counties in Meizhou.

Conclusions

The ‐‐^SEA, ‐α^3.7, and ‐α^4.2 deletion were the main mutations of α‐thalassemia, while IVS‐II‐654(C → T), CD41‐42(‐TCTT), ‐28(A → G), and CD17(A → T) mutations were the principal mutations of β‐thalassemia in Meizhou. G6PD c.1376 G → T, c.1388 G → A, and c.95 A → G were the main mutations of G6PD deficiency. There were some differences of the distribution of thalassemia and G6PD mutations among eight counties in Meizhou.

Efficient gene correction of an aberrant splice site in β-thalassaemia iPSCs by CRISPR/Cas9 and single-strand oligodeoxynucleotides

β‐thalassaemia is a prevalent hereditary haematological disease caused by mutations in the human haemoglobin β (HBB) gene. Among them, the HBB IVS2‐654 (C > T) mutation, which is in the intron, creates an aberrant splicing site. Bone marrow transplantation for curing β‐thalassaemia is limited due to the lack of matched donors. The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR‐associated protein 9 (Cas9), as a widely used tool for gene editing, is able to target specific sequence and create double‐strand break (DSB), which can be combined with the single‐stranded oligodeoxynucleotide (ssODN) to correct mutations. In this study, according to two different strategies, the HBB IVS2‐654 mutation was seamlessly corrected in iPSCs by CRISPR/Cas9 system and ssODN. To reduce the occurrence of secondary cleavage, a more efficient strategy was adopted. The corrected iPSCs kept pluripotency and genome stability. Moreover, they could differentiate normally. Through CRISPR/Cas9 system and ssODN, our study provides improved strategies for gene correction of β‐Thalassaemia, and the expression of the HBB gene can be restored, which can be used for gene therapy in the future.

Molecular analysis of hemoglobinopathies in a large ethnic Hakka population in southern China

Thalassemia is an inherited autosomal recessive disorder with microcytic hypochromic anemia resulting from reduced or absent synthesis of 1 or more of the globin chains of hemoglobin. This study provided the insight into prevalence and molecular characterization of thalassemia in Hakka population. 14,524 unrelated subjects were included in our study from January 2015 to November 2017. All the subjects were detected by hematological analysis, hemoglobin electrophoresis analysis, and molecular diagnosis (gap-polymerase chain reaction and flow-through hybridization technology). Data analysis was used to compare allele frequencies between the Hakka populations. Seven thousand four hundred twenty-two cases of microcytosis were found. The percentage of microcytosis in Meizhou, Ganzhou, and Heyuan was 50.91% (6738/13,236), 51.27% (445/868), and 56.90% (239/420), respectively. A total of 5516 mutant chromosomes were identified, including 3775 α-thalassemia and 1741 β-thalassemia. --/αα was the most common α-thalassemia genotype, followed by -α/αα and -α/αα, accounted for 84.92% of α-thalassemia genotypes. Twelve kinds of mutations and 26 genotypes in β-thalassemia were found. IVS-II-654(C→T), CD41-42(-TCTT), -28(A→G), and CD17(A→T) alleles accounted for 92.65% of these mutations. IVS-II-654/N, CD41-42/N, -28/N, CD17/N genotypes accounted for 91.53% of β-thalassemia genotypes. 27 fetuses with at-risk pregnancies were subjected to prenatal diagnosis. Five fetuses were Bart's hydrops syndrome and 2 fetuses with β-thalassemia major. There were some differences in molecular characterization of thalassemia among Hakka people in different areas of southern China. Our results enriched the related information of thalassemia in the region, which provided valuable references for the prevention and control of thalassemia.

A Novel -72 (T→A) β-Promoter Mutation Causing Slightly Elevated HbA2 in a Vietnamese Heterozygote

We report a novel β⁺-thalassemia mutation found in a Vietnamese family. The molecular defect T→A lies at -72 of the β-globin gene promoter, within the conserved CCAAT box. The index case was a 5-year-old child having red blood cells indices close to normal and slightly increased level of HbA₂ (3.96%). The expression of the mutated β allele was inferred by luciferase reporter assay in K562 cells. The β -72 determinant is the eighth β-thalassemic mutation identified in Vietnam and it was not previously reported in any population. The absence of homozygous or compound heterozygous states did not allow us to precisely predict either its clinical impact or its relevance in management programs. Our results further underline the importance of identifying and characterizing new or rare β⁺-thalassemic alleles in carrier screening and prenatal diagnosis.

Enhancement of β-Globin Gene Expression in Thalassemic IVS2-654 Induced Pluripotent Stem Cell-Derived Erythroid Cells by Modified U7 snRNA

The therapeutic use of patient‐specific induced pluripotent stem cells (iPSCs) is emerging as a potential treatment of β‐thalassemia. Ideally, patient‐specific iPSCs would be genetically corrected by various approaches to treat β‐thalassemia including lentiviral gene transfer, lentivirus‐delivered shRNA, and gene editing. These corrected iPSCs would be subsequently differentiated into hematopoietic stem cells and transplanted back into the same patient. In this article, we present a proof of principle study for disease modeling and screening using iPSCs to test the potential use of the modified U7 small nuclear (sn) RNA to correct a splice defect in IVS2‐654 β‐thalassemia. In this case, the aberration results from a mutation in the human β‐globin intron 2 causing an aberrant splicing of β‐globin pre‐mRNA and preventing synthesis of functional β‐globin protein. The iPSCs (derived from mesenchymal stromal cells from a patient with IVS2‐654 β‐thalassemia/hemoglobin (Hb) E) were transduced with a lentivirus carrying a modified U7 snRNA targeting an IVS2‐654 β‐globin pre‐mRNA in order to restore the correct splicing. Erythroblasts differentiated from the transduced iPSCs expressed high level of correctly spliced β‐globin mRNA suggesting that the modified U7 snRNA was expressed and mediated splicing correction of IVS2‐654 β‐globin pre‐mRNA in these cells. Moreover, a less active apoptosis cascade process was observed in the corrected cells at transcription level. This study demonstrated the potential use of a genetically modified U7 snRNA with patient‐specific iPSCs for the partial restoration of the aberrant splicing process of β‐thalassemia. Stem Cells Translational Medicine2017;6:1059–1069

Both TALENs and CRISPR/Cas9 directly target the HBB IVS2-654 (C > T) mutation in β-thalassemia-derived iPSCs

β-Thalassemia is one of the most common genetic blood diseases and is caused by either point mutations or deletions in the β-globin (HBB) gene. The generation of patient-specific induced pluripotent stem cells (iPSCs) and subsequent correction of the disease-causing mutations may be a potential therapeutic strategy for this disease. Due to the low efficiency of typical homologous recombination, endonucleases, including TALENs and CRISPR/Cas9, have been widely used to enhance the gene correction efficiency in patient-derived iPSCs. Here, we designed TALENs and CRISPR/Cas9 to directly target the intron2 mutation site IVS2-654 in the globin gene. We observed different frequencies of double-strand breaks (DSBs) at IVS2-654 loci using TALENs and CRISPR/Cas9, and TALENs mediated a higher homologous gene targeting efficiency compared to CRISPR/Cas9 when combined with the piggyBac transposon donor. In addition, more obvious off-target events were observed for CRISPR/Cas9 compared to TALENs. Finally, TALENs-corrected iPSC clones were selected for erythroblast differentiation using the OP9 co-culture system and detected relatively higher transcription of HBB than the uncorrected cells. This comparison of using TALENs or CRISPR/Cas9 to correct specific HBB mutations in patient-derived iPSCs will guide future applications of TALENs- or CRISPR/Cas9-based gene therapies in monogenic diseases.

Genetic heterogeneity of the β-globin gene in various geographic populations of Yunnan in southwestern China

OBJECTIVES

The aim of this study was to investigate the geographic distribution of β-globin gene mutations in different ethnic groups in Yunnan province.

METHODS

From 2004 to 2014, 1,441 subjects with hemoglobin disorders, identified by PCR-reverse dot blot and DNA sequencing, were studied according to ethnicity and geographic origin. Haplotypes were examined among 41 unrelated thalassemia chromosomes.

RESULTS

Eighteen β-thalassemia mutations and seven hemoglobin variants were identified for 1,616 alleles in 22 different ethnic groups from all 16 prefecture-level divisions of Yunnan. The prevalence of β-thalassemia was heterogeneous and regionally specific. CD 41-42 (-TCTT) was the most prevalent mutation in the populations of northeastern Yunnan. CD 17 (A>T) was the most common mutation in the populations of southeastern Yunnan, especially for the Zhuang minority, whereas Hb E (CD 26, G>A) was the most prevalent mutation in populations of southwestern Yunnan, especially for the Dai minority. Among the seven types of haplotypes identified, CD 17 (A>T) was mainly linked to haplotype VII (+ - - - - - +) and IVS-II-654 (C>T) was only linked to haplotype I (+ - - - - + +).

CONCLUSION

Our data underline the heterogeneity of β-globin gene mutations in Yunnan. This distribution of β-globin mutations in the geographic regions and ethnic populations provided a detailed ethnic basis and evolutionary view of humans in southern China, which will be beneficial for genetic counseling and prevention strategies.

Prenatal Diagnosis of β-Thalassemia by Reverse Dot-Blot Hybridization in Southern China

Beta-thalassemia (thal) is the most common genetic disease and is widely distributed in southern China. Prenatal diagnosis is needed to prevent the birth of thalassemic offspring in couples at-risk. This can be performed in the first or second trimester of pregnancy by DNA analysis using polymerase chain reaction (PCR). As there are more than 30 mutations causing beta-thal in Chinese, the point mutation detection by reverse dot-blot for common mutations together with direct DNA sequencing was developed for prenatal diagnosis. Using reverse dot-blot, we were able to offer complete diagnosis in 315 (99.4%) of 317 pregnancies. Only two fetuses needed the DNA sequencing technique for diagnosis. Of the 319 at-risk fetuses, 82 (25.7%) were found to be normal, 143 (44.8%) to be heterozygous for beta-thal and 94 (29.5%) to be affected with beta-thal. Therefore, the combination of reverse dot-blot with direct DNA sequencing can perform prenatal diagnosis by DNA analysis in almost all cases at- risk of beta-thal in southern China.

Amelioration of beta654-thalassemia in mouse model with the knockdown of aberrantly spliced beta-globin mRNA

Large amounts of aberrantly spliced mRNA from the beta(654) allele was present in erythroid cells, which might impair the erythropoiesis. A therapeutic strategy for beta-thalassemia was explored by knocking down the aberrantly spliced mRNA of beta-globin. Lentiviral vector with siRNA fragment targets on the specific portion of beta(654)-globin aberrantly spliced pre-mRNA was constructed. In HeLa beta(654) cells, the siRNA vector could reduce approximately 60% of aberrantly spliced mRNA, which was assessed by RT-PCR and qRT-PCR. Furthermore, a disease model of beta(654) thalassemia mice with lentiviral-mediated siRNA was produced by subzonal injection (named Hbetai-Hbb(th-4)/Hbb(+) transgenic mice). Our results showed that the hemotological parameters were improved in Hbetai-Hbb(th-4)/Hbb(+) transgenic mice. This study provides a potential way for beta(654)-thalassemia therapy by knocking down the aberrantly spliced beta-globin mRNA, whilst supporting that the aberrantly spliced beta-globin mRNA may aggravate the disease.

Evidence of gene conversion in the evolutionary process of the codon 41/42 (-CTTT) mutation causing beta-thalassemia in southern China

The 4-bp deletion (-CTTT) at codon 41/42 (CD41/42) of the human beta-globin gene represents one of the most common beta-thalassemia mutations in East Asia and Southeast Asia, which is historically afflicted with endemic malaria, thus hypothetically evolving under natural selection by malaria infection. To understand the evolutionary process of generating the beta(CD41/42) allele and its maintenance, including the effect of natural selection on the pattern of linkage disequilibrium (LD), we sequenced a 15.933-kb region spanning 20.693 kb of the beta-globin cluster surrounding the 4-bp deletion using a sample from a Chinese population consisting of 24 normal individuals and 16 heterozygotes for the deletion. Forty-nine polymorphic sites were found. Analysis of the data, using a variety of methods including formal population genetics analysis and visual approaches, suggests that the spread of the CD41/42 (-CTTT) deletion is most likely mediated by interallelic gene conversion, although independent deletions in different lineages are also possible. The neutrality test resulted in a significant positive Tajima's D for the beta-globin locus, which is consistent with the existence of balancing selection. This suggests that the 4-bp deletion that occurred at this locus may be an event that is subject to natural selection, due to malaria, which leads to the heterozygote advantage, spread widely with further help by conversion and migration. The evolutionary process of this mutant through gene conversion that could conceivably take place between the 4-bp deletion and the normal sequence in the respective region is discussed in detail.

Carrier screening for alpha- and beta-thalassemia in pregnancy: the results of an 11-year prospective program in Guangzhou Maternal and Neonatal hospital

OBJECTIVES

To evaluate the first prospective screening program in China for control of alpha and beta-thalassemia in the population of pregnant couples.

METHODS

During the period between January 1993 and December 2003, a hospital-based preventive program was conducted at the biggest birth center in Guangzhou, with 1/17 of all deliveries in this city referred annually by use of conventional heterozygote screening strategy in combination with the system of regular healthcare examination in pregnancy.

RESULTS

The screened records included 49 221 pregnant women, and 4503 husbands of the pregnant women showed positive on the screening test. Of the at-risk couples, there were 198 for alpha-thal (4.4%) and 83 for beta-thal (1.8%), respectively. Genetic counseling was offered to all at-risk couples and a successful prenatal diagnosis was performed for 269 out of 281 (95.7%) for alpha- or beta-thal major, with the remaining 12 couples refusing to accept prenatal diagnosis. Out of 187 pregnancies at risk for homozygous alpha0-thal and 82 at risk for beta-thal major, 51 hydrops fetalis with Hb Bart's and 18 beta-thal major were identified. All pregnancies with affected fetuses were voluntarily terminated, leading to a marked reduction of severe alpha- and beta-thal births at this hospital since the program has been launched.

CONCLUSIONS

Our hospital-based program proved to be highly effective in reducing severe thals in pregnant populations.

The prevalence and spectrum of alpha and beta thalassaemia in Guangdong Province: implications for the future health burden and population screening

AIM

Thalassaemia is a good candidate disease for control by preventive genetic programmes in developing countries. Accurate population frequency data are needed for planning the control of thalassaemia in the high risk Guangdong Province of southern China.

METHODS

In total, 13397 consecutive samples from five geographical areas of Guangdong Province were analysed for both haematological and molecular parameters.

RESULTS

There was a high prevalence of carriers of alpha thalassaemia (8.53%), beta thalassaemia (2.54%), and both alpha and beta thalassaemia (0.26%). Overall, 11.07% of the population in this area were heterozygous carriers of alpha and beta thalassaemia. The mutation spectrum of alpha and beta thalassaemia and its constitution were fully described in this area. This study reports the true prevalence of silent alpha thalassaemia in the southern China population for the first time. In addition, two novel mutations that give rise to alpha thalassaemia, one deletion resulting in beta thalassaemia, and a rare deletion (--(THAI) allele) previously unreported in mainland China were detected. The frequency of the most common mutation, the Southeast Asian type of deletion (--(SEA), accounting for 48.54% of all alpha thalassaemias) was similar to the total of two alpha(+) thalassaemia deletions (-alpha(3.7) and -alpha(4.2), accounting for 47.49% of alpha thalassaemia).

CONCLUSION

Both alpha and beta thalassaemia are widely distributed in Guangdong Province of China. The knowledge gained in this study will enable the projected number of pregnancies at risk to be estimated and a screening strategy for control of thalassaemia to be designed in this area.

Molecular analysis of beta-thalassemia in South Vietnam

In Vietnam, the carrier rate for beta-thalassemia varies from 1.5% to 25% depending on the ethnic groups of the population. The molecular basis of beta-thalassemia in South Vietnam was studied in 50 unrelated beta-thalassemia patients. Of these, 31 had beta-thalassemia/Hb E, 18 were homozygous for beta-thalassemia, and 1 carried the beta-thalassemia trait. The majority of the patients were Kinh, four were Chinese, and two were Kinh-Chinese. All had severe anemia and received blood transfusions regularly, every 1-3 months. Hepatosplenomegaly was found in all patients, and splenectomy had been done in six patients. Normal alpha-globin genotype (alpha alpha/alpha alpha) was found in all subjects. Reverse dot-blot hybridization using oligonucleotide probes specific for Southeast Asian mutations can detect beta-thalassemia in 60 chromosomes in addition to 31 chromosomes with beta(E) mutation. Excluding the beta(E) gene, six previously reported Thai and Chinese beta-thalassemia mutations were found, including codons 41/42 (-TCTT) 35.3%, codon 17 (A-->T) 25.0%, -28 (A-->G) 7.3%, codons 71/72 (+A) 7.3%, IVS-II nt 654 (C-->T) 7.3%, and IVS-I nt 1 (G-->T) 6.0%. The Vietnamese frameshift mutation at codon 95 (+A) was detected by ARMS in seven chromosomes (10.3%). DNA polymorphism of the beta-globin gene cluster carrying the codon 95 mutation was - + - - - - - + for (G)gamma/XmnI, epsilon/HincII, (G)gamma/HindIII, (A)gamma/HindIII, psi beta/HincII, 3' psi beta/HincII, beta/AvaII, and 3'beta/BamHI, respectively. The remaining mutation detected by the gap PCR was a large deletion known as the Chinese (G)gamma((A)gamma delta beta)(0)-thalassemia. The two most common mutations were the frameshift at codons 41/42 (-TCTT) and the nonsense mutation in codon 17 (A-->T). Thus beta-thalassemia mutations in South Vietnam is similar to the previous report from the North, although at different frequencies. This result will help to establish a center for prenatal diagnosis and for prevention and control of thalassemia in Vietnam.

Clinical and hematological features of codon 17, A-T mutation of beta-thalassemia in Thai patients

Forty-one patients with codon 17, A-T mutation of beta-thalassemia, which is commonly found in Thailand, were studied to determine whether it is possible to predict phenotypic severity from genetic factors. The clinical phenotype of homozygotes for codon 17, A-T and compound heterozygotes for codon 17, A-T and beta+-thalassemia may be used to predict a severe phenotype with TM. However, the clinical phenotype of compound heterozygotes for codon 17, A-T and beta+-thalassemia or Hb E were variable and could not be accurately predicted. The association of alpha-thalassemia2 and milder disease was and was not evident in patients with codon 17, A-T and Hb E. The association between Hb CS gene or the presence of XmnI-Ggamma polymorphism and a mild clinical phenotype is not apparent, indicating the involvement of other ameliorating determinants or genetic modifications.

Unusually Severe Heterozygous β-Thalassemia: Evidence for an Interacting Gene Affecting Globin Translation

A common β-thalassemia mutation in Asian populations is the C → T substitution at position 654 of intron 2, which leads to the activation of two cryptic splicing sites and the incorporation of 73 extra nucleotides into the mutant mRNA. Like most β-thalassemia mutations, it normally exhibits recessive inheritance. We investigated the unusually severe phenotype in two heterozygotes for this mutation, father and son, who had thalassemia intermedia and an apparent dominant mode of inheritance. An increased level of aberrantly spliced transcript in the reticulocytes of the probands compared with asymptomatic β654heterozygotes led us to investigate the production and processing of β654 RNA. We showed that large amounts of the aberrant β654 transcript were detectable in erythroblasts from one of the asymptomatic cases. The translation product of this mRNA was not detectable in vivo, and we were unable to demonstrate the translation of the mutant mRNA in a cell-free translation system. Although the reticulocyte :β mRNA ratios in the two probands were within the range observed in the asymptomatic heterozygotes, globin chain biosynthesis studies showed that the probands had considerably greater :β chain imbalance. These results imply that the more severe phenotype may be due to a second defect, possibly unlinked to the β-globin cluster, that acts at the translational or posttranslational level.

© 1998 by The American Society of Hematology.

Unusually Severe Heterozygous β-Thalassemia: Evidence for an Interacting Gene Affecting Globin Translation

A common β-thalassemia mutation in Asian populations is the C → T substitution at position 654 of intron 2, which leads to the activation of two cryptic splicing sites and the incorporation of 73 extra nucleotides into the mutant mRNA. Like most β-thalassemia mutations, it normally exhibits recessive inheritance. We investigated the unusually severe phenotype in two heterozygotes for this mutation, father and son, who had thalassemia intermedia and an apparent dominant mode of inheritance. An increased level of aberrantly spliced transcript in the reticulocytes of the probands compared with asymptomatic β654heterozygotes led us to investigate the production and processing of β654 RNA. We showed that large amounts of the aberrant β654 transcript were detectable in erythroblasts from one of the asymptomatic cases. The translation product of this mRNA was not detectable in vivo, and we were unable to demonstrate the translation of the mutant mRNA in a cell-free translation system. Although the reticulocyte :β mRNA ratios in the two probands were within the range observed in the asymptomatic heterozygotes, globin chain biosynthesis studies showed that the probands had considerably greater :β chain imbalance. These results imply that the more severe phenotype may be due to a second defect, possibly unlinked to the β-globin cluster, that acts at the translational or posttranslational level.

© 1998 by The American Society of Hematology.

A Common Human β Globin Splicing Mutation Modeled in Mice

The βIVS-2-654 C→T mutation accounts for approximately 20% of β thalassemia mutations in southern China; it causes aberrant RNA splicing and leads to β0 thalassemia. To provide an animal model for testing therapies for correcting splicing defects, we have used the “plug and socket” method of gene targeting in murine embryonic stem cells to replace the two (cis) murine adult β globin genes with a single copy of the human βIVS-2-654 gene. No homozygous mice survive postnatally. Heterozygous mice carrying this mutant gene produce reduced amounts of the mouse β globin chains and no human β globin, and have a moderate form of β thalassemia. The heterozygotes show the same aberrant splicing as their human counterparts and provide an animal model for testing therapies to correct splicing defects at either the RNA or DNA level.

A Common Human β Globin Splicing Mutation Modeled in Mice

The βIVS-2-654 C→T mutation accounts for approximately 20% of β thalassemia mutations in southern China; it causes aberrant RNA splicing and leads to β0 thalassemia. To provide an animal model for testing therapies for correcting splicing defects, we have used the “plug and socket” method of gene targeting in murine embryonic stem cells to replace the two (cis) murine adult β globin genes with a single copy of the human βIVS-2-654 gene. No homozygous mice survive postnatally. Heterozygous mice carrying this mutant gene produce reduced amounts of the mouse β globin chains and no human β globin, and have a moderate form of β thalassemia. The heterozygotes show the same aberrant splicing as their human counterparts and provide an animal model for testing therapies to correct splicing defects at either the RNA or DNA level.

The population genetics of the haemoglobinopathies

The haemoglobinopathies are the commonest single-gene disorders known, almost certainly because of the protection they provide against malaria, as attested by a number of observations. The geographical distributions of malaria and haemoglobinopathies largely overlap, and microepidemiological surveys confirm the close relationship between them. For two of the commonest disorders, haemoglobin S and alpha(+)-thalassaemia, there is also good clinical evidence for protection against malaria morbidity. However, not all the evidence appears to support this view. In some parts of the world malaria and haemoglobinopathies are not, and never have been, coexistent. It is also difficult to explain why the majority of haemoglobinopathies appear to be recent mutations and are regionally specific. Here we argue that these apparent inconsistencies in the malaria hypothesis are the result of processes such as genetic drift and migration and of demographic changes that have occurred during the past 10,000 years. When these factors are taken into account, selection by malaria remains the force responsible for the prevalence of the haemoglobinopathies.

Prevalence and genotypes of alpha- and beta-thalassemia carriers in Hong Kong -- implications for population screening

BACKGROUND

The thalassemias are common in southern China. We determined the prevalence of heterozygous carriers of these genetic disorders in Hong Kong and assessed the feasibility of a community-based screening program.

METHODS

An educational and screening program for the thalassemias was carried out in three high schools with a total of 2420 students. Seventy-five percent of the students agreed to undergo screening, which consisted of blood counts, hemoglobin electrophoresis, serum ferritin measurements, and DNA analyses.

RESULTS

Of the 1800 blood samples tested, 150 (8.3 percent) had microcytosis (mean corpuscular volume, <80 microm3). Ninety students (5.0 percent) were carriers of alpha-thalassemia, of whom 81 (4.5 percent) were carriers of the Southeast Asian type of deletion, in which both alpha-globin genes on the same chromosome 16 are deleted. Sixty-one students (3.4 percent) were carriers of either beta-thalassemia or the mutation coding for hemoglobin E. Six students were carriers of both alpha- and beta-thalassemias. On the basis of these figures, the estimated numbers of pregnancies in Hong Kong in which the fetus is at risk for homozygous alpha-thalassemia and beta-thalassemia major or intermedia are 145 and 80 per year, respectively. In Hong Kong the actual numbers of women referred for prenatal diagnoses of these disorders are approximately 95 and 40 per year, respectively.

CONCLUSIONS

Despite the availability of hospital-based screening and prenatal diagnosis for many years in Hong Kong, many women carrying fetuses at risk for thalassemia are not referred for genetic counseling. A community-based program of education, screening, and counseling is needed in Hong Kong and southern China.

Molecular characterization of beta-thalassemia in Taiwan and the identification of two new mutations

Polymerase chain reaction-based techniques were used to study the molecular defects of 480 unrelated beta-thalassemia heterozygotes in Taiwan. Analysis of artificially created restriction sites and gap-polymerase chain reaction were performed to detect four common mutations, i.e. IVS-II-654 (C-->T), codons 41/42 (-TCTT), codon 17 (A-->T), -28 (A-->G), and a deletional form of delta beta-thalassemia in the Chinese population. In cases with negative or ambiguous results with the aforementioned methods, direct DNA cycle sequencing using either S35-dATP or a fluorescent dye terminator, was carried out to determine the defects. A total of 14 different mutations have been found in this series. The IVS-II-654 mutation was the most common (39.6%), followed by the codons 41/42 mutation (37.9%). The four common genotypes accounted for 92.3% of defects. Two new mutations were detected: codon 31 (-C) and codons 40/41 (+T). Both defects resulted in a frameshift and a premature terminator, the former at codon 60, the latter at codon 43. Although we have studied our cases extensively, the molecular defects in seven alleles are still unknown.

Analysis of beta-thalassemia mutations in the United Arab Emirates provides evidence for recurrent origin of the IVSI nt 5 (G-C) mutation

Beta-thalassemia mutations were characterized in a sample of 70 patients from United Arab Emirates (U.A.E.), resulting in an enlargement of the spectrum of types found in the country. The complete association between the most common IVS I nt 5 (G-C) mutation and a specific haplotype reveals an independent origin of this mutation in U.A.E.

RNA transcripts of the beta-thalassaemia allele IVS-2-654 C-->T: a small amount of normally processed beta-globin mRNA is still produced from the mutant gene

IVS-2-654 C-->T is a common Chinese beta-thalassaemia mutation. Previous studies report that this mutation resulted in the formation of an abnormally spliced mRNA and the absence of detectable normal beta-globin mRNA, hence the mutation was considered to cause beta o-thalassaemia. We recently used the method of PCR amplified cDNA copies of circulating erythroid cell mRNA to analyse the mutant gene transcripts and found that this IVS-2-654 mutation does not abolish normal RNA processing entirely, but that a significant amount (over 15%) of normally processed beta-globin mRNA is produced. Microglobin chain biosynthetic analysis using the HPLC method showed that beta-globin chain was also present in the blood of patients with IVS-2-654 C-->T mutation. Accordingly, this mutant allele leads to a beta (+)-thalassaemia. Further, the methodology described in this paper provides a new approach towards the detection of RNA transcripts of beta-thalassaemia alleles as well as the study of gene expression in beta-thalassaemia and other genetic diseases.

Beta-thalassemia mutations in Singapore--a strategy for prenatal diagnosis

The strategy for early prenatal diagnosis of beta-thalassemia in Singapore by direct detection of the mutant beta-globin gene requires the spectrum of mutations producing the disorder in this population to be characterized. We analyzed 134 beta-thalassemia alleles from Singapore by specific oligonucleotide hybridization after DNA amplification, using a nonradioactive enhanced chemiluminescence detection system. The mutations were identified in 90% of the alleles using five oligonucleotide probes for the following mutations: codons 41/42 (deletion-TCTT), IVS II nt 654 (C-->T), codon 17 (A-->T), IVS I nt 5 (G-->C), and -28 TATA box (A-->G). Together with the strategy of direct sequencing, a total of 97% of the mutations were identified. In the Chinese subpopulation, 97% of the mutations were detected by the oligonucleotide probes. Using just four oligonucleotide probes would identify 96% of the mutations, and 76% of the mutations were accounted for by codon 41/42 (-TCTT) and IVS II nt 654 (C-->T) mutations. Thus in this subpopulation early prenatal diagnosis would be possible in virtually all the affected families.

Direct genotyping and prenatal diagnosis of beta-thalassemia in Chinese by polymerase chain reaction mediated restriction fragment length polymorphism method

The molecular basis of beta-thalassemia is predominantly point mutations in the beta-globin gene. Frameshift 41-42 (-CTTT), IVS-2 position 654 (C-->T) mutation, nonsense codon 17 (A-->T), TATA box position -28 (A-->G) mutation and frameshift 71-72 (+A) account for more than 95% of beta-thalassemia alleles in the population of South China. We have developed a polymerase chain reaction (PCR)-mediated restriction fragment length polymorphism (RFLP) method for the identification of these alleles. In this method, artificial mispairing bases in PCR-amplified products were created to distinguish normal from mutant alleles on the basis of RFLPs. The size of the five PCR-amplified DNA fragments that may potentially contain the above five types of mutations is 93 or 89 bp (codons 41-42), 221 bp (IVS-2 nt 654), 110 bp (codon 17), 123 bp (TATA box nt -28), and 97 or 98 bp (codons 71-72). After these fragments were digested with Hinc II, Mae III, Nhe I, EcoR I, and Dde I, respectively, the allele-specific RFLPs produced were analyzed by gel electrophoresis. DNA samples of 24 patients with the above five types of beta-thalassemia were investigated with the present method and allele-specific oligonucleotide (ASO) probing simultaneously. We used this method in the prenatal diagnosis of 14 Chinese families for beta-thalassemia. The results obtained by the present method correspond well with those by the ASO probe test.

The population genetics of the haemoglobinopathies

The haemoglobinopathies are the commonest single gene disorders known, and are so common in some regions of the world that the majority of the population carries at least one genetic abnormality affecting the structure or synthesis of the haemoglobin molecule. The prevalence of the common haemoglobinopathies (the alpha- and beta-thalassaemias, HbS, HbC and HbE) is almost certainly a result of the protection they provide against malaria, as the epidemiological evidence reviewed in this chapter shows. World-wide, the distributions of malaria and the common haemoglobinopathies largely overlap, and micro-epidemiological surveys have confirmed the close relationship between the disorders. However, there are complications to this picture which appear to undermine the malaria hypothesis. First, in some areas, malaria and haemoglobinopathies are not coincident. Second, the malaria hypothesis does not easily explain why no two regions of the world have the same haemoglobinopathy or combination of haemoglobinopathies. The majority of mutations have arisen only once and are regionally specific. By using molecular characterization of mutations and the analysis of haplotypes on haemoglobinopathy-bearing chromosomes it is possible to show how a combination of selection by malaria, genetic drift and population movements can explain the first complication. In order to explain the second, we have argued that malaria selection has operated relatively recently on human populations (within the last 5000 years). The present distribution is then seen as the result of selection elevating sporadic mutations in local populations. In the absence of sufficient gene flow to spread all mutations to all populations, the consequence is a patchwork distribution of haemoglobinopathies. Given time, we would expect the mutations that protect and do not compromise the health of their carriers to become widely disseminated, but it is likely that human intervention will alter this process of natural selection.

Why are some genetic diseases common? Distinguishing selection from other processes by molecular analysis of globin gene variants

Various processes (selection, mutation, migration and genetic drift) are known to determine the frequency of genetic disease in human populations, but so far it has proved almost impossible to decide to what extent each is responsible for the presence of a particular genetic disease. The techniques of gene and haplotype analysis offer new hope in addressing this issue, and we review relevant studies of three haemoglobinopathies: sickle cell anaemia, and alpha and beta thalassaemia. We show how for each disease it is possible to recognize a pattern of regionally specific mutations, found in association with one or a few haplotypes, that is best explained as the result of selection; other patterns are due to population migration and genetic drift. However, we caution that such conclusions can be drawn in special circumstances only. In the case of the haemoglobinopathies it is possible because a selective agent (malaria) was already suspected, and the investigations could be carried out in relatively genetically homogenous populations whose migratory histories are known. Moreover, some data reviewed here suggest that gene conversion and the haplotype composition of a population may affect the frequency of a mutation, making interpretation of gene frequencies difficult on the basis of standard population genetics theory. Hence attempts to use the same approaches with other genetic diseases are likely to be frustrated by a lack of suitably untrammelled populations and by difficulties accounting for poorly understood genetic processes. We conclude that although this combination of molecular and population genetics is successful when applied to the study of haemoglobinopathies, it may not be so easy to apply it to the study of other genetic diseases.

Molecular characterisation of Vietnamese HPFH

A novel 30 kb deletion of the beta-globin gene cluster associated with the phenotype of hereditary persistence of fetal hemoglobin (HPFH) is described in two unrelated individuals of Vietnamese background. The Vietnamese G gamma A gamma HPFH deletion has a unique 5' breakpoint 3.5 kb downstream of the delta-globin gene. The 3' breakpoint lies approximately 8 kb upstream from the HPFH-3 breakpoint (Henthorn et al., 1986) and in the region of the 3' breakpoints of HPFH-4 (Saglio et al., 1986), German and Belgian G gamma+ (A gamma delta beta)zero-thalassemias (Anagnou et al., 1988; Losekoot et al., 1991). Characterisation of the 3' breakpoint in the present study has enabled more precise localisation of other deletion breakpoints at this locus. Further evidence is provided that the 3' breakpoint region contains functionally important sequences and that the juxtaposition of these sequences to the gamma-globin genes is a significant factor in the increased fetal hemoglobin levels.

Molecular basis and haematological characterization of beta-thalassaemia major in Taiwan, with a mutation of IVS-1 3' end TAG-->GAG in a Chinese patient

We studied 41 patients with beta-thalassaemia major and their parents by using a combination of polymerase chain reaction (PCR) amplification, slot-blot hybridization of allele-specific oligonucleotide (ASO), and direct genomic sequencing. Eight different point mutations were characterized. C to T substitution at nucleotide (nt) 654 of intervening sequences (IVS) 2, accounting for 46.3% of mutant beta-globin genes, is the most common mutation in Taiwan, followed by frameshift codons 41/42 with four nucleotides (TCTT) deletion for 31.7%, A to G substitution at position -28 of promotor area for 8.5%, A to T substitution at codon 17 for 6.1%, frameshift codons 27/28 (insertion of C) for 2.4%, G to T substitution at nucleotide 1 of IVS-1 for 2.4%, frameshift codons 71/72 (insertion of A) and IVS-1 3 end TAG-->GAG for 1.2%. The former four mutations showed no obvious difference between two major ethnic groups in Taiwan. As to mutations in each individual of beta-thalassaemia major, the incidence of compound heterozygotes of two different mutations is much higher than homozygotes of single mutation, 78.0% v 22.0%. Compound heterozygotes of C to T substitution at nt 654 of IVS-2 and frameshift codons 41/42 with four nucleotides deletion is the most common pattern of beta-thalassaemia mutations in each individual (41.5%). The results are somewhat different from other documented reports concerning the mutations of beta-thalassaemia in southern China. This is the first report of mutation of IVS-1 3' end TAG-->GAG which causes consensus change in Chinese people. Patients with heterozygotes of beta zero and -28 beta(+)-thalassaemia mutations would have a greater delay in initial transfusion in comparison to patients with homozygotes of both beta zero-thalassaemia mutation, but their initial clinical manifestation might be aggravated when combined with a glucose-6-phosphate dehydrogenase (G-6-PD) deficiency and an insult such as exposure to infection and certain drugs.

Molecular basis of beta thalassemia in the south of Thailand

A total of 103 beta thalassemia genes from 78 children (45 with Hb E/beta thalassemia, 8 with beta thalassemia heterozygotes, and 25 with homozygous beta thalassemia) were analyzed using dot-blot hybridization of the polymerase chain reaction-amplified DNA and direct DNA sequencing. Nine mutations were characterized in 98/103 (95%) of beta thalassemia alleles, of which six (a 4 bp deletion in codons 41-42, a G-C transition at position 5 of IVS-1, A-G transition at codon 19, an A-T transition at codon 17, an A-G transition at position -28 upstream of the beta globin gene, a G-T transition at position 1 of IVS-1), accounted for 92%. The spectrum of beta thalassemia mutations in Chinese Thai is similar to that reported among the Chinese from other parts of the world. The distribution of beta thalassemia mutations in Muslim Thai is similar to that reported among Malaysians. The most common beta thalassemia mutation in Thai and Chinese Thai patients is the frameshift mutation at codons 41-42, in comparison with the Muslim Thai in whom the G-C transition at position 5 of the IVS-1 mutation predominates. The heterogeneity of molecular defects causing beta thalassemia should aid in the planning of a prenatal diagnosis program for beta thalassemia in the South of Thailand.

Identification of five rare mutations including a novel frameshift mutation causing beta zero-thalassemia in Thai patients with beta zero-thalassemia/hemoglobin E disease

6 out of 14 uncharacterized beta-thalassemia alleles from 187 Thai beta-thalassemia/HbE patients were identified by direct sequencing of DNA amplified by polymerase chain reaction. A novel mutation occurring from an insertion of adenosine in codon 95, which results in a shift of the reading frame with terminator at the new codon 101, was detected in one patient. In addition, two frameshift mutations not previously reported among the Thai population were also detected in 3 patients: one with a deletion of thymidine in codon 15 and two with an insertion of cytidine in codons 27/28. A frameshift mutation that occurred from a cytidine deletion in codon 41 was also found in one patient in this study. The remaining case was an amber mutation, GAG-TAG, in codon 43 in exon 2 of the beta-globin gene. These mutations bring the number of mutations known to be present in the Thai population to a total of 20, 15 of which were detected in beta-thalassemia/HbE patients.

The spectrum of β thalassaemia in Burma

The molecular defects causing beta thalassaemia have been analysed in 85 unrelated Burmese patients. The patients included 14 with homozygous beta thalassaemia, 70 with HbE/beta thalassaemia and one with HbS/beta thalassaemia. Using a combination of allele-specific oligoprobe hybridization and direct sequencing of genomic DNA amplified by the polymerase chain reaction, 95/99 of the beta-thalassaemia alleles have been characterized. Six mutations have been identified of which three, the G-T at IVS-1 position 1, the G-C at IVS-1 position 5 and the deletion of TCTT in codons 41/42, accounted for 85% of the alleles. Despite the diversity of ethnic groups in Burma, the number of beta-thalassaemia alleles in Burma is relatively small. Thus, diagnosis of the majority of the beta thalassaemias would be possible using a limited number of oligonucleotide probes.

Beta-thalassemia major resulting from a compound heterozygosity for the beta-globin gene mutation: further evidence for multiple origin and migration of the thalassemia gene

We describe in a Japanese family beta zero-thalassemia resulting from a compound heterozygosity for a beta-globin gene mutation. One mutation is a C-to-T transition at IVS-2 nucleotide position 654 on the background of Mediterranean haplotype IX. Another mutation is a G-to-A transition at IVS-2 nucleotide position 1, associated with a novel haplotype XI. The occurrence of these mutations on various chromosomal backgrounds provides strong evidence for an interplay of gene migration, interallelic gene conversion, and multiple origins of the same mutation.

A new frameshift beta zero-thalassemia mutation (codons 27-28 +C) found in a Chinese family

A new beta zero-thalassemia mutation, a frameshift mutation with an insertion of a single cytosine nucleotide in codon 27-28, is described. The propositus, who is compound heterozygous for this mutation and the IVSII-654 C----T beta zero-thalassemia mutation, has the phenotype of severe beta-thalassemia major.

Molecular basis of HbE-beta-thalassemia and the origin of HbE in northeast Thailand: identification of one novel mutation using amplified DNA from buffy coat specimens

Amplification of DNA via polymerase chain reaction directly from a small amount of a buffy coat fraction was used to study the molecular basis of HbE-beta-thalassemia in the northeastern Thai population. Eight different mutations including the new one causing a beta o-thalassemia phenotype were detected. This novel mutation is an amber mutation at codon 26, which occurs at the same position as that of HbE; the most common hemoglobin variant in Southeast Asian countries. A pitfall in detection of the HbE mutation by restriction enzyme analysis was pointed out and differential diagnosis of the HbE mutation and the novel one by using allele specific oligonucleotide probes were described. Analysis of polymorphic restriction sites in the beta-globin gene cluster containing the beta E gene revealed two previously undescribed haplotypes in the Southeast Asian populations, which provide evidence for the multiple origins of beta E gene in Southeast Asian populations.

DNA polymorphism and globin chain analysis in the prenatal diagnosis of beta-thalassaemia major in Taiwan

Thirty-six pregnancies in 25 families at risk of beta-thalassaemia major received prenatal diagnosis. Chorionic villus sampling or amniocentesis was done in 35 pregnancies to obtain fetal cells for DNA linkage study, for which Southern blotting and DNA hybridization were used to detect seven restriction fragment length polymorphisms (RFLPs) within the beta-globin gene cluster: epsilon-HincII, G gamma-HindIII, A gamma-HindIII, phi beta-HincII, 3' phi beta-HincII, beta-AvaII, and 3' beta-BamHI. beta-Thalassaemia major was diagnosed in seven and excluded in 22 pregnancies. In the remaining six cases, beta-thalassaemia major could not be excluded. In these six pregnancies and another one with late booking, ultrasound-guided cordocentesis was performed at the 22nd to 27th week of gestation. Globin chain composition was determined with urea-acetic acid-Triton X-100-12 per cent polyacrylamide gel electrophoresis. beta-Thalassaemia major was diagnosed in two fetuses and excluded in the other five. Eleven fetuses (in which beta-thalassaemia major was excluded) have been delivered and are healthy at more than 5 months old. DNA linkage analysis coupled with globin chain electrophoresis provides an effective way for prenatal diagnosis of beta-thalassaemia major, although these methods are being replaced by more direct detection techniques using oligonucleotide probes.

Molecular heterogeneity of beta-thalassaemia in the Japanese: identification of two novel mutations

Five unrelated Japanese beta-thalassaemia genes, from one homozygote and four heterozygotes, have been systematically characterized using DNA polymorphism analysis, polymerase chain reaction, dot-blot hybridization and direct sequencing of amplified genomic DNA. Four different molecular defects were observed on three different beta-globin gene frameworks. One of these, the A----G mutation in the TATA box, a previously described mutation, was detected by dot-blot hybridization in one homozygote and one heterozygote with the beta-globin gene of framework 2. The second mutation is a C----T substitution at position 654 of IVS-2, the mutation commonly found in Chinese, which was associated with the framework 1 gene. Another two mutations, both associated with framework 3 genes, are novel ones; an amber mutation in codon 90 (GAG to TAG) and a frameshift (+G) insertion in codon 54, both of which cause a beta 0-thalassaemia phenotype by premature termination of the beta-globin chain synthesis.

Detection of specific DNA sequences by fluorescence amplification: a color complementation assay

We have developed a color complementation assay that allows rapid screening of specific genomic DNA sequences. It is based on the simultaneous amplification of two or more DNA segments with fluorescent oligonucleotide primers such that the generation of a color, or combination of colors, can be visualized and used for diagnosis. Color complementation assay obviates the need for gel electrophoresis and has been applied to the detection of a large and small gene deletion, a chromosomal translocation, an infectious agent, and a single-base substitution. DNA amplification with fluorescent oligonucleotide primers has also been used to multiplex and discriminate five different amplified DNA loci simultaneously. Each primer set is conjugated to a different dye, and the fluorescence of each dye respective to its amplified DNA locus is scored on a fluorometer. This method is valuable for DNA diagnostics of genetic, acquired, and infectious diseases, as well as in DNA forensics. It also lends itself to complete automation.

Molecular basis of beta-thalassemia in Thailand: analysis of beta-thalassemia mutations using the polymerase chain reaction

beta-Thalassemia mutations in 71 chromosomes of Thai patients from the northeast, the middle and the south of the country were investigated using dot blot hybridization of PCR (polymerase chain reaction)-amplified DNA with allele-specific oligonucleotide probes. Eight different known molecular defects were detected, at different frequencies. There was an amber mutation in codon 17, a C-T transversion at position 654 of IVS-2, a frameshift mutation between codons 71 and 72, an A-G transition at nucleotide -28 within the TATA box (known as Chinese mutations), a G-T transversion at position 1 of IVS-1 (an Indian mutation), a 4 bp deletion in codons 41/42 and a G-C transversion at position 5 of IVS-1 (described as both Chinese and Indian mutations) and a Thai original mutation, an ochre mutation in codon 35. Analysis of the three unknown alleles by DNA sequencing of the cloned DNA fragment amplified by PCR revealed an A-G substitution at the second position of the codon for amino acid 19 (AAC-AGC). The analytic approach used in the present study and the characteristic distribution of mutations in each region of Thailand will prove useful for setting up a prenatal diagnosis program.

Prenatal diagnosis of inherited hemoglobinopathies

This paper reviews the methodology available to make prenatal diagnosis of inherited hemoglobinopathies by DNA analysis and the strategy to be used for the large scale application of this procedure to high-risk populations. The most straightforward approach for prenatal diagnosis is nowadays based on the analysis of DNA enzymatically amplified by the polymerase chain reaction (PCR). The mutations, produced by gross structural rearrangement of the DNA and those affecting a restriction recognition site, are directly detected by visualization following ethidium bromide staining of the electrophoretic pattern resulting from enzymatic digestion of amplified DNA. The remaining ones are detected by dot blot analysis with allelic specific oligonucleotide probes. Because in each population a limited number of specific beta-thalassemia mutations are prevalent, prenatal diagnosis by DNA analysis may be carried out by a population-specific strategy based on the amplification of those regions of the beta-globin genes containing the mutations most frequently occurring in each population followed by dot blot analysis with allelic specific oligonucleotide probes. This approach has the great advantage of being very simple, because radioactive probes are not necessary, very rapid, the results being obtained within 24 hours from sampling and very sensitive, only a limited amount of DNA in the order of 50 ng being necessary.

Prenatal diagnosis of beta-thalassaemia in Mediterranean populations by dot blot analysis with DNA amplification and allele specific oligonucleotide probes

In this study, we describe a simple strategy to detect beta-thalassaemia mutations in prospective parents and to make prenatal diagnosis in pregnancies at risk in the Mediterranean population. Screening of prospective parents is carried out by dot blot analysis on enzymatically amplified DNA with a set of oligonucleotide probes complementary to the most common mutations in this population. Prenatal diagnosis is accomplished by the same procedure on enzymatically amplified amniocyte or trophoblast DNA. The main advantages of this procedure are the simplicity, sensitivity (0.05 micrograms of DNA), and rapidity (12-24 h). Further simplification is obtained by amplification of the DNA from crude amniotic cell lysate. The very low amount of fetal material necessary for this analysis eliminates the need to culture amniotic fluid cells and may decrease the fetal loss rate associated with trophoblast sampling. The number of specific DNA sequences obtained by the amplification procedure allowed us to use non-radioactive labelled oligonucleotide probes, which have several advantages compared to radioactive probes.