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Keywords:

  • polyadenylation site mutation;
  • β-globin gene;
  • Chinese;
  • thalassaemia intermedia

Polyadenylation, the addition of around 200 adenosine residues at the 3′-end of mRNA 18–20 bp downstream from the AAUAAA signal, is believed to play a role in mRNA stability and its transport out of the nucleus. For the β-globin gene, four nucleotide substitutions (Orkin et al, 1985; Jankovic et al, 1990; Rund et al, 1992) and two small deletions (Ghanem et al, 1992; Rund et al, 1992) of the conserved AATAAA sequence at the 3′-UTR that affect the poly A signal have been described. Point mutations decrease the efficiency of the cleavage polyadenylation process but do not abolish it completely (Orkin et al, 1985; Rund et al, 1992), showing that the AATAAA sequence is not an absolute requirement for correct cleavage of the transcript. It follows therefore that subjects heterozygous for point mutations of poly A signal show a mild β+-thalassaemia carrier phenotype with normal haemoglobin (Hb), a moderately reduced mean corpuscular volume (MCV) of 70–80 fl, and borderline raised Hb A2 levels of 3·5–4·0% (Jankovic et al, 1990; Altay et al, 1991). We describe a novel poly A (A[RIGHTWARDS ARROW]C); AATAAA[RIGHTWARDS ARROW]CATAAA mutation of the β-globin gene in a Chinese family.

Haematological and genotypic findings of the family are given in Table I. The patient was found to be anaemic with a Hb level of 7·4 g/dl at 4 years of age during hospitalization for an upper respiratory tract infection. Physical examination showed hepatosplenomegaly (liver 3 cm and spleen 1 cm below costal margin respectively). Hb analysis showed 23·4% Hb F and 5·4% Hb A2. The patient was first transfused at the age of 5 years, when the Hb level fell to 5·8 g/dl, and subsequent transfusion frequency averaged four times a year, with a pretransfusion Hb level of around 6·5 g/dl. Regular blood transfusion was started from the age of 7 years as a result of symptomatic anaemia, and iron chelation therapy was instituted in the same year. The patient attained normal growth and was in early puberty at the time of writing, and there was no hepatomegaly and only a palpable spleen tip. Latest investigations showed Hb 9·1 g/dl (pretransfusion), ferritin 1915 μg/l (normal range for male: 6·8–50·5 μg/l) and bilirubin level 36 µmol/l (normal range: 7–19 µmol/l). A splenectomy was contemplated for the patient to alleviate the transfusion requirement.

Table I.  Haematological and genotypic findings of the family members in this report.
SubjectSex/Age (years)Hb (g/dl)MCV (fl)MCH (pg)HbA2 (%)HbF (%)HbH bodies α-Genotype β-Genotype
  • *

    On regular transfusion.

  • Hb, haemoglobin; MCV, mean corpuscular volume; MCH, mean cell Hb. Reference ranges: Hb, 13·0–18·0 g/dl (male), 11·5–16·5 g/dl (female); MCV, 80–97 fl; MCH, 27–32 pg; Hb A2, 2·3–3·0%; Hb F, < 0·9%.

FatherMale/3515·480·526·33·4< 0·3Negativeζζαα/ζζααβPoly–A(A[RIGHTWARDS ARROW]C); AATAAA[RIGHTWARDS ARROW]CATAAAA
MotherFemale/4110·059·017·34·90·5Negativeζζαα/ζζααβ41/42(–CTTT)A
Patient*Male/139·178·425·13·25·0Negativeζζαα/ζζααβPoly–A(A[RIGHTWARDS ARROW]C); AATAAA[RIGHTWARDS ARROW]CATAAA41/42(–CTTT)

Genotyping showed that the patient was heterozygous for codons 41/42 (–CTTT) β0-thalassaemia allele and was negative for other common β-globin gene mutations in the Chinese on molecular screening. Direct sequencing of the β-globin gene showed a novel poly A (A[RIGHTWARDS ARROW]C); AATAAA[RIGHTWARDS ARROW]CATAAA mutation, indicating that the patient was compound heterozygous for poly A signal and β0-thalassaemia mutations. He had normal complement of the α-globin genes. The poly A signal mutation was inherited from his father who was a native of the Guangdong province.

This poly A signal mutation is the second mild β+-thalassaemia allele identified in the Chinese population, in addition to the nt +8 (C[RIGHTWARDS ARROW]T) mutation at the 5′-UTR, as previously described (Ma et al, 2000). The normal MCV and borderline raised Hb A2 levels in the heterozygote state renders a phenotypic diagnosis uncertain. The implication for genetic counselling was that these mutations would have to be specifically screened for by molecular techniques in partners of known β-thalassaemia carriers in our locality in order to predict for the risk of β-thalassaemia in the fetus. Finally, our patient and those reported previously (Altay et al, 1991) suggests that a point mutation in the poly A signal of the β-globin gene results in β-thalassaemia intermedia of moderate severity when co-inherited with β0-thalassaemia allele. Although the child in the present report continues to be transfused at regular intervals, the frequency is only four times per year in contrast to the more frequent intervals needed for β-thalassaemia major, and the presentation was later than usual.

References

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  2. References
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  • Ghanem, N., Girodon, E., Vidaud, M., Martin, J., Fanen, P., Plassa, F., Goossens, M. (1992) A comprehensive scanning method for rapid detection of β-globin gene mutations and polymorphisms. Human Mutation, 1, 229239.
  • Jankovic, L., Efremov, G.D., Petkov, G., Kattamis, C., George, E., Yang, K.-G., Stoming, T.A., Huisman, T.H.J. (1990) Two novel polyadenylation mutations leading to β+-thalassaemia. British Journal of Haematology, 75, 122126.
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