Wendy N. Erber

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 …

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.

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.

Genetics and Population Health

There has been a widespread belief that genetic disorders are of little importance inlow income countries, an opinion that is perhaps understandable given the often dauntingprevalence of infectious diseases and nutritional problems faced by these populations.The failure to recognize genetic disorders as significant contributors to the overall diseaseprofile of low income countries is, however, a serious error. For example, an estimated7.6 million children are born per year with a severe congenital or genetic disorder (Alwanand Modell 2003), and one in seven of the world’s population are carriers of a haemoglobindisorder (WHO 2002). In both cases a large majority of those affected are resident inlow income countries, which currently comprise over 80% of the global population(PRB 2004). There is also convincing preliminary evidence that some sub-populations aregenetically predisposed to contract serious infectious diseases, including tuberculosis andleprosy (Pitchappan 2002). Further, because of the requirement to treat -thalassaemiamajor individuals with regular blood transfusions, in 2003 Thalassaemia InternationalFederation estimated that 30–80% of all cases worldwide were infected with hepatitis Band/or hepatitis C, a testament to the lack of adequate blood screening facilitiesin low income countries. Thus the contribution of genetic disorders to global populationdisease profiles, directly and indirectly, is of major significance.The aim of the conference Genetics and Population Health, from which the presentcollection of papers was selected, was to review the current global prevalenceand distribution patterns of genetic disease. The 17 invited papers are presented in fivesections:(1) Haemoglobin disorders as a paradigm of genetic disease;(2) Population genetic structure as affected by migration, marriage preference andsubdivision;(3) Medical and community genetics in countries with different resource bases andreligious influences;

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