Codon 24 (TAT>TAG) and codon 32 (ATG>AGG) (Hb Rotterdam): two novel alpha2 gene mutations associated with mild alpha-thalassemia found in the same family after newborn screening

A novel mutation Hb jiangnan[β3(NA3) Leu→Lys, HBB:c.10-_11delinsAA] causing elevated Hb A2 level

OBJECTIVES

To report the hematolgocial and molecular features of a nove β-globin variant in a Chinese fimaly.

METHODS

The proband was a 19-year-old Chinese man whose Hb analysis by HPLC for thalassemia revealed an abnormal peak. Hb analysis was performed by HPLC and CE. Gap-PCR and PCR-reverse dot blot hybridization (PCR-RDB) were used to detect the common mutations in Chinese population. DNA sequencing was used to determine the Hb variant.

RESULTS

The Hb variant and Hb A can be separated but co-elutes with Hb F by the CE method. However, the variant can be separated from Hb A₀, Hb F, and Hb A₂ using HPLC. DNA sequencing showed a mutation of codon 3 in the β-globin gene. His wife's HPLC revealed a high value of Hb A₂, which proved to be the Hb E using PCR-RDB.

CONCLUSION

It was the first report of the mutation, so we named it Hb Jiangnan according to the place of residence of the proband. It can be separated by HPLC but not CE. Hb Jiangnan can cause an increased level of Hb A_2.

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).

Severe α-thalassemia intermedia due to a compound heterozygosity for the highly unstable Hb Adana (HBA2: c.179G>A) and a novel codon 24 (HBA2: c.75T>A) mutation

HBA2

We report a novel mutation at codon 24 of the α2-globin gene (HBA2: c.75T > A) found in a Sundanese family. This novel mutation was detected during prenatal diagnosis. The couple already had a 7-year-old boy who exhibited clinically severe α-thalassemia intermedia (α-TI), and he was found to be a compound heterozygote for the novel mutation at codon 24 and the previously described Hb Adana (HBA2: c.179G > A) at codon 59 of the α2-globin gene. The father was a carrier of the novel point mutation and showed normal hemoglobin (Hb) and a low mean corpuscular volume (MCV) and mean corpuscular Hb (MCH) value.

Nondeletional Hb Queens Park [α32(B13)Met→Lys]/Hb H (β4) disease

A rare nondeletional α-thalassemia-2 (α-thal-2) allele was identified in a Thai boy with Hb H (β4) disease. The proband has α-thal-1 (- -(SEA) type) together with a non productive Hb Queens Park (HBA1:c.98T>A) [α32(B13)Met→Lys] α1-globin variant. No abnormal hemoglobin (Hb) fraction was detected by high performance liquid chromatography (HPLC). The clinical effect of this mutation in the proband was comparable to that of deletional α-thal-2 present in Hb H disease.

Relationship between neonatal screening results by HPLC and the number of α-thalassaemia gene mutations; consequences for the cut-off value

Objectives

To evaluate the relationship between FAST peak percentage by adapted Bio-Rad Vnbs analysis using the valley-to-valley integration and genotypes with the aim to improve differentiation between severe α-thalassaemia forms (HbH disease) and the milder disease types.

Method

DNA analysis for α-thalassaemia was performed on 91 dried blood spot samples presenting normal and elevated FAST peak levels, selected during three years of Dutch national newborn screening.

Results

Significant differences were found between samples with and without α-thalassaemia mutations, regardless of the genetic profiles. No significant difference was demonstrated between HPLC in -α/αα and -α/-α, between -α/-α and –/αα and between –/αα and –/-α genotypes.

Conclusion

This study confirms that the percentage HbBart's, as depicted by the FAST peak, is only a relative indication for the number of α genes affected in α-thalassaemia. Based on the data obtained using the modified Bio-Rad Vnbs software, we adopted a cut-off value of 22.5% to discriminate between possible severe α-thalassaemia or HbH disease and other α-thalassaemia phenotypes. Retrospectively, if this cut-off value was utilized during this initial three-year period of neonatal screening, the positive predictive value would have been 0.030 instead of 0.014.