John F. Prior

Single-Tube Multiplex-PCR Screen for Anti-3.7 and Anti-4.2 α-Globin Gene Triplications

The coexistence of α-globin gene triplication (ααα) is an important modulator of the severity of β-thalassemia trait or β-thalassemia intermedia, exacerbating the phenotypic severity of β-thalassemia by causing more globin chain imbalance (1)(2). Typically, the inheritance of a single β-thalassemia allele is associated with mild anemia and hypochromic microcytic red cells. Compared with simple β-heterozygotes, co-inheritance of triplicated or quadruplicated α-globin genes in β-heterozygotes often leads to more significant anemia, splenomegaly, more pronounced red cell abnormalities, the presence of circulating normoblasts, higher hemoglobin F concentrations, and even the presence of inclusion bodies in erythroblasts (3)(4). Because the α- and β-globin gene clusters are physically unlinked and segregate independently, β-thalassemia carriers who also carry triplicated or quadruplicated α-globin genes have a 25% risk of having a similarly affected offspring, although their partners may be entirely normal. Triplicated α-globin genes appear to be ubiquitous and have been found in most populations (2). They result from misalignment and unequal crossover between the homologous X-, Y-, and Z-box segments of the α-globin gene cluster during meiosis (Fig. 1A⇓ ). Generally, two types of triplicated alleles can be generated from an unequal crossover, αααanti3.7 and αααanti4.2. If the crossover occurs between the homologous Z2 and Z1 boxes, also referred to as a “rightward crossover”, this produces a −α3.7 single-gene deletion allele and the reciprocal αααanti3.7 triplicated allele. However, if the crossover occurs between the X2 and X1 boxes (a “leftward crossover”), a −α4.2 single-gene deletion allele and the reciprocal αααanti4.2 triplicated allele are generated (5). A Sri Lankan study of individuals with severe to moderate β-thalassemia revealed a 2% frequency of α-globin gene triplications (6), whereas a preliminary study in Hong Kong suggests that the frequency …

Heterozygosity for the IVS-I-5(G→C) mutation with a G→A change at codon 18 (Val→Met; Hb Baden) in cis and a T→G mutation at codon 126 (Val→Gly; Hb Djonburi) in trans resulting in a thalassemia intermedia

We have analyzed the hemoglobins of a young German patient with beta-thalassemia intermedia and of his immediate family and included in these studies an evaluation of possible nucleotide changes in the beta-globin genes through sequencing of amplified DNA. One chromosome of the propositus and one of his fathers carried the GTG-->GGG mutation at codon 126 leading to the synthesis of Hb Dhonburi or alpha 2 beta (2)126(H4)Val-->Gly; this variant is slightly unstable and is associated with mild thalassemic features. His second chromosome and one of his mothers had the common IVS-I-5 (G-->C) mutation that leads to a rather severe beta(+)-thalassemia and the GTG-->ATG mutation at codon 18, resulting in the replacement of a valine residue by a methionine residue. This newly discovered beta-chain variant, named Hb Baden, was present for only 2-3% in both the patient and his mother. This low amount results from a decreased splicing of RNA at the donor splice-site of the first intron that is nearly completely deactivated by the IVS-I-5 (G-->C) thalassemic mutation. The chromosome with the codon 18 (GTG-->ATG) and the IVS-I-5 (G-->C) mutations has thus far been found only in this German family; analysis of 51 chromosomes from patients with the IVS-I-5 (G-->C) mutation living in different countries failed to detect the codon 18 (GTG-->ATG) change.

Thalassemia in Western Australia: 11 novel deletions characterized by Multiplex Ligation-dependent Probe Amplification

The number of immigrants in Western Australia from many different areas where hemoglobinopathies are endemic has increased dramatically since the 1970s. Therefore, many different thalassemia mutations have been introduced in the country, which add a technological diagnostic problem to the serious burden of hemoglobinopathy management and to public health care. Recently, we have developed a rapid and simple technique based on Multiplex Ligation-dependent Probe Amplification to detect deletions causing alpha-and beta-thalassemia, deltabeta-thalassemia and Hereditary Persistence of Fetal Hemoglobin. A screening for (unknown) deletions was performed in a cohort of patients of different ethnic backgrounds preselected for their thalassemia phenotype, in which common deletions and point mutations were excluded. Out of 37 cases suspected to carry a deletion, 27 were found to carry 17 different deletion types of which 6 causing alpha-thalassemia and 5 causing beta-thalassemia were novel. For 3 of the deletions, we have been able to characterize the exact breakpoint sequences by long-range PCR and direct sequencing. These results show that MLPA is a suitable technology to detect unknown and uncommon deletions. These could represent a diagnostic problem when offering prevention to couples at risk presenting with unclear phenotypes and might result in a serious fetal problem when the deletion involves embryonic genes.

The Prevalence and Molecular Basis of Hemoglobinopathies in Cambodia

Blood counts, hemoglobin (Hb) high performance liquid chromatography (HPLC), and DNA analyses were performed on 260 children, aged 5 months to 16 years, at Siem Reap to assess the prevalence of thalassemia and other hemoglobinopathies in regional Cambodia. Hemoglobinopathies were present in 134 children (51.5%) with 20 abnormal genotypes identified. α-Thalassemia (thal) (35.4%) was the most prevalent disorder and the –α3.7 gene deletion was the most common α-globin gene abnormality. The − −SEA deletion and nondeletional forms of α-thal, Hb Constant Spring [Hb CS, α142, Term→Gln, TAA→CAA (α2)], Hb Paksé [α142, Term→Tyr, TAA→TAT (α2)] and triplicated α genes, were also present but at low frequencies. Hb E [β26(B8)Glu→Lys, GAG→AAG] (28.8%) was the most common β-globin gene abnormality, whilst β-thal was only detected in two children (0.8% of cases). Although hemoglobinopathies were common, the majority of abnormalities detected (heterozygous −α3.7 and Hb E) were not clinically significant. On the basis of these findings, and with the majority of abnormalities being mild, it seems improbable that thalassemia represents a major health burden in this region of Cambodia.

Two New α1-Globin Gene Point Mutations: Hb Nedlands (HBA1:c.86C>T) [α28(B9)Ala→Val] and Hb Queens Park (HBA1:c.98T>A) [α32(B13)Met→Lys]

We report two new point mutations of the α1-globin gene found in a Greek and a Burmese patient, both living in Western Australia. The patients were initially selected for their microcytic hypochromic parameters as belonging to a group suspected for uncommon (deletion) defects. Gap-polymerase chain reaction (gap-PCR) and multiplex ligation-dependent probe amplification (MLPA) technologies were applied, and in those cases not showing deletions, direct sequencing was performed. We have found 1) HBA1:c.86C>T, Hb Nedlands [α28(B9)Ala→Val] which, based on the red cell indices and phenotype prediction scores, is presumed to be clinically silent, and 2) HBA1:c.98T>A, Hb Queens Park [α32(B13)Met→Lys] which seems to be associated with a mild α-thalassemia (α-thal) phenotype. The phenotype/genotype correlation is briefly described.

A Moderately Severe α-Thalassemia Condition Resulting From a Combination of the α2 Polyadenylation Signal (AATAAA→AATA– –) Mutation and a 3.7 Kb α Gene Deletion in an Australian Family

We have recently studied a family with a rare combination of two abnormal α-globin genes. The combination of a two-base (AA) deletion in the α2 polyadenylation signal (poly A) (AATAAA→AATA– –) and a 3.7 kb α gene deletion, found in two children, resulted in a moderately severe thalassemic condition. Both parents and three siblings were tested and the hematological condition and molecular findings are presented. The father was born in India with Portuguese and British ancestry; the mother is of Dutch ancestry. All three siblings were born in Australia.

A Community Profile of Alpha Thalassaemia in Western Australia

Objective: To investigate the current prevalence of α-thalassaemia in the population of Western Australia, which has received substantial immigration from South-East Asia during the last 30 years. Method: Over a 1-year period commencing July 2002, α-thalassaemia DNA testing was performed on 920 blood samples received from the Migrant Health Service, referring doctors or pathology laboratories in Western Australia. Molecular testing for α-thalassaemia was performed on extracted DNA for single and double α-globin gene deletions and mutations by PCR. Results: An α-globin gene abnormality was detected in 35.4% (326/920) of samples. There were 177 cases (50.6%) with a single gene deletion α+-thalassaemia, most commonly –3.7 kb, and 102 cases (31.2%) with double α-gene deletions (α⁰-thalassaemia), including 7 cases of HbH disease. Conclusion: Overall, the findings amount to 1.7 new cases of α-thalassaemia per 10,000 population in the 12-month period and demonstrate that α-thalassaemia is an increasingly common disorder in the Western Australian population. This has important implications for community outreach programmes, genetic counselling and the screening of at-risk populations.

Hemoglobinopathies in the Christmas Island Population

Christmas Island is a remote Australian territory 2,400 km north of Perth. Health care is administered from Perth. The population is predominantly Chinese, with some Malay, Indian and European. As hemoglobinopathies are known to be common amongst these ethnic groups, a study was performed to determine their prevalence and significance in the Christmas Island population. Three‐hundred and sixty‐four individuals (adults and children) were tested. All subjects were assessed by full blood count, α‐globin multiplex polymerase chain reaction (PCR) and PCR testing for Hb Constant Spring [α142, Term→Gln, TAA→CAA (α2)]. Microcytic patients (MCV < 80 fL) were further investigated by high performance liquid chromatography (HPLC) and serum ferritin was determined. Where present, β‐thalassemia (thal) mutations were characterised by PCR. Thirty‐four subjects (9.3%) were microcytic and of these five were iron deficient. The remainder were heterozygous for a hemoglobinopathy, giving a 9.1% incidence of hemoglobinopathies in Christmas Islanders. α‐Thalassemia was identified in 23 subjects, seven of whom were heterozygous for α− 3.7; the remaining 16 were heterozygous for the – –SEA deletion. One case of heterozygous δβ‐thal and one case of heterozygous Hb E [β26(B8)Glu→Lys] was detected. Of the eight subjects heterozygous for β‐thal, at least five mutations are represented, indicating a diverse and heterogeneous origin for this population