Identification of an extensive zeta-alpha globin gene deletion in a Chinese individual

Molecular characterization of a novel 27.6-kb deletion causing α(+) thalassemia in a Chinese family

Over 80% of the α-thalassemia cases in southern China are caused by large deletions involving the α-globin gene cluster on chromosome 16p13.3. Here, we characterized a novel 27.6-kb deletion on the α-globin gene cluster in a Chinese family. Its breakpoints were detected to lie between coordinates 9079 and 36718 of the α-globin gene cluster (NG_000006.1), with a total of 27,640 nucleotides deleted. It was designated as -α (27.6) deletion. The proband is a compound heterozygote of --(SEA) and -α (27.6) and he displayed very mild hemoglobin H disease phenotype with Hb 7.9-9.3 g/dl. Phenotypic analysis on heterozygote of this deletion revealed it as α(+) mutation. It leads to a very mild phenotype as adult heterozygotes have normal hematological parameters with the values at the lower border of the normal range. RT-PCR analysis showed that the α-globin mRNA level of the heterozygotes was decreased when compared with that of normal people.

Alpha-thalassaemia

Alpha-thalassaemia is inherited as an autosomal recessive disorder characterised by a microcytic hypochromic anaemia, and a clinical phenotype varying from almost asymptomatic to a lethal haemolytic anaemia.

It is probably the most common monogenic gene disorder in the world and is especially frequent in Mediterranean countries, South-East Asia, Africa, the Middle East and in the Indian subcontinent. During the last few decades the incidence of alpha thalassaemia in North-European countries and Northern America has increased because of demographic changes. Compound heterozygotes and some homozygotes have a moderate to severe form of alpha thalassaemia called HbH disease. Hb Bart's hydrops foetalis is a lethal form in which no alpha-globin is synthesized. Alpha thalassaemia most frequently results from deletion of one or both alpha genes from the chromosome and can be classified according to its genotype/phenotype correlation. The normal complement of four functional alpha-globin genes may be decreased by 1, 2, 3 or all 4 copies of the genes, explaining the clinical variation and increasing severity of the disease. All affected individuals have a variable degree of anaemia (low Hb), reduced mean corpuscular haemoglobin (MCH/pg), reduced mean corpuscular volume (MCV/fl) and a normal/slightly reduced level of HbA₂. Molecular analysis is usually required to confirm the haematological observations (especially in silent alpha-thalassaemia and alpha-thalassaemia trait). The predominant features in HbH disease are anaemia with variable amounts of HbH (0.8-40%). The type of mutation influences the clinical severity of HbH disease. The distinguishing features of the haemoglobin Bart's hydrops foetalis syndrome are the presence of Hb Bart's and the total absence of HbF. The mode of transmission of alpha thalassaemia is autosomal recessive. Genetic counselling is offered to couples at risk for HbH disease or haemoglobin Bart's Hydrops Foetalis Syndrome. Carriers of alpha⁺- or alpha⁰-thalassaemia alleles generally do not need treatment. HbH patients may require intermittent transfusion therapy especially during intercurrent illness. Most pregnancies in which the foetus is known to have the haemoglobin Bart's hydrops foetalis syndrome are terminated due to the increased risk of both maternal and foetal morbidity.

Alpha0 thalassaemia as a result of a novel 11.1 kb deletion eliminating both of the duplicated alpha globin genes

AIMS

To characterise a novel 11.1 kb deletion that eliminated both of the duplicated alpha globin genes, giving rise to a typical alpha0 thalassaemia phenotype in four carriers from a Chinese family.

METHODS

Haematological investigations were carried out on all family members. The seven common forms of alpha thalassaemia were screened for by the polymerase chain reaction (PCR) and Southern blotting was used to analyse the alpha globin gene cluster. DNA sequence analysis of the entire alpha1 and alpha1 globin gene region was carried out and reverse transcription (RT)-PCR was used to investigate the transcription levels of the alpha and beta globin genes.

RESULTS

The breakpoints were found to lie between coordinates 31695-31724 and 42846-42867 of the alpha globin gene cluster (NG_000006), with a total of about 11,135 nucleotides deleted. These sequences are involved in (CA)n repeats, suggesting a homologous recombination event. RT-PCR analysis gave a transcription level of the alpha globin gene in heterozygotes comparable with that of SEA deletion heterozygotes, confirming no output of alpha globin from the linked pair of alpha globin genes. The heterozygosity for this novel deletion was confirmed by PCR diagnosis in all four carriers from this family.

CONCLUSIONS

This rare mutation constitutes an additional heterogeneous defect causing alpha thalassaemia in the Chinese population.

Universal newborn screening for Hb H disease in California

Newborn screening is an accepted public health measure to ensure that appropriate health care is provided in a timely manner to infants with hereditary/metabolic disorders. Alpha-thalassemia is a common hemoglobin (Hb) disorder, and causes Hb H (beta4) disease, and usually fatal homozygous alpha(0)-thalassemia, also known as Hb Bart's (gamma4) hydrops fetalis syndrome. In 1996, the State of California began to investigate the feasibility of universal newborn screening for Hb H disease. Initial screening was done on blood samples obtained by heel pricks from newborns, and stored as dried blood spots on filter paper. Hb Bart's levels were measured as fast-moving Hb by automated high-performance liquid chromatography (HPLC) identical to that currently used in newborn screening for sickle cell disease. Subsequent confirmation of Hb H disease was done by DNA-based diagnostics for alpha-globin genotyping. A criterion of 25% or more Hb Bart's as determined by HPLC detects most, if not all cases of Hb H disease, and few cases of alpha-thalassemia trait. From January, 1998, through June, 2000, 89 newborns were found to have Hb H disease. The overall prevalence for Hb H disease among all newborns in California is approximately 1 per 15,000. Implementation of this program to existing newborn hemoglobinopathy screening in populations with significant proportions of southeast Asians is recommended. The correct diagnosis would allow affected infants to be properly cared for, and would also raise awareness for the prevention of homozygous alpha(0)-thalassemia or Hb Bart's hydrops fetalis syndrome.

Monosomy for the most telomeric, gene-rich region of the short arm of human chromosome 16 causes minimal phenotypic effects

We have examined the phenotypic effects of 21 independent deletions from the fully sequenced and annotated 356 kb telomeric region of the short arm of chromosome 16 (16p13.3). Fifteen genes contained within this region have been highly conserved throughout evolution and encode proteins involved in important housekeeping functions, synthesis of haemoglobin, signalling pathways and critical developmental pathways. Although a priori many of these genes would be considered candidates for critical haploinsufficient genes, none of the deletions within the 356 kb interval cause any discernible phenotype other than alpha thalassaemia whether inherited via the maternal or paternal line. These findings contrast with previous observations on patients with larger (> 1 Mb) deletions from the 16p telomere and therefore address the mechanisms by which monosomy gives rise to human genetic disease.

Alpha-thalassaemia

alpha-Thalassaemias are genetic defects extremely frequent in some populations and are characterized by the decrease or complete suppression of alpha-globin polypeptide chains. The gene cluster, which codes for and controls the production of these polypeptides, maps near the telomere of the short arm of chromosome 16, within a G + C rich and early-replicating DNA region. The genes expressed during the embryonic (zeta) or fetal and adult stage (alpha 2 and alpha 1) can be modified by point mutations which affect either the processing-translation of mRNA or make the polypeptide chains extremely unstable. Much more frequent are the deletions of variable size (from approximately 3 to more than 100 kb) which remove one or both alpha genes in cis or even the whole gene cluster. Deletions of a single gene are the result of unequal pairing during meiosis, followed by reciprocal recombination. These unequal cross-overs, which produce also alpha gene triplications and quadruplications, are made possible by the high degree of homology of the two alpha genes and of their flanking sequences. Other deletions involving one or more genes are due to recombinations which have taken place within non-homologous regions (illegitimate recombinations) or in DNA segments whose homology is limited to very short sequences. Particularly interesting are the deletions which eliminate large DNA areas 5' of zeta or of both alpha genes. These deletions do not include the structural genes but, nevertheless, suppress completely their expression. Larger deletions involving the tip of the short arm of chromosome 16 by truncation, interstitial deletions or translocations result in the contiguous gene syndrome ATR-16. In this complex syndrome alpha-thalassaemia is accompanied by mental retardation and variable dismorphic features. The study of mutations of the 5' upstream flanking region has led to the discovery of a DNA sequence, localized 40 kb upstream of the zeta-globin gene, which controls the expression of the alpha genes (alpha major regulatory element or HS-40). In the acquired variant of haemoglobin H (HbH) disease found in rare individuals with myelodysplastic disorders and in the X-linked mental retardation associated with alpha-thalassaemia, a profound reduction or absence of alpha gene expression has been observed, which is not accompanied by structural alterations of the coding or controlling regions of the alpha gene complex. Most probably the acquired alpha-thalassaemia is due to the lack of soluble activators (or presence of repressors) which act in trans and affect the expression of the homologous clusters and are coded by genes not (closely) linked to the alpha genes. The ATR-X syndrome results from mutations of the XH2 gene, located on the X chromosome (Xq13.3) and coding for a transacting factor which regulates gene expression. The interaction of the different alpha-thalassaemia determinants results in three phenotypes: the alpha-thalassaemic trait, clinically silent and presenting only limited alterations of haematological parameters, HbH disease, characterized by the development of a haemolytic anaemia of variable degree, and the (lethal) Hb Bart's hydrops fetalis syndrome. The diagnosis of alpha-thalassaemia due to deletions is implemented by the electrophoretic analysis of genomic DNA digested with restriction enzymes and hybridized with specific molecular probes. Recently polymerase chain reaction (PCR) based strategies have replaced the Southern blotting methodology. The straightforward identification of point mutations is carried out by the specific amplification of the alpha 2 or alpha 1 gene by PCR followed by the localization and identification of the mutation with a variety of screening systems (denaturing gradient gel electrophoresis (DGGE), single strand conformation polymorphisms (SSCP)) and direct sequencing.

Characterization of a new alpha-thalassemia-1 mutation in a Spanish family

A novel alpha-thalassemia-1 deletion of 14-15.4 kb that removes the alpha-2, alpha-1, theta-1 genes and pseudo-alpha-1 genes, has been detected in a father and 2 of his children from northern Spain.

Detection of the (--SEA) double alpha-globin gene deletion by a simple immunologic assay for embryonic zeta-globin chains

Homozygous alpha-thalassemia [alpha-thal-1], with loss of all four alpha-globin genes, causes lethal hydrops fetalis. The most common mutation producing this syndrome is the Southeast Asian (--SEA) double alpha-globin gene deletion. Erythrocytes from adults heterozygous for the (--SEA) deletion have minute amounts of embryonic zeta-globin chains detectable by anti-zeta-globin monoclonal antibodies. Among 225 peripheral blood samples tested by a simple anti-zeta-immunobinding tetrazolium dye test, 81 were positive and 144 were negative. The majority of subjects were of Filipino, Chinese, or Laotian ancestry. All 81 positive samples were confirmed by Bam HI digests and a zeta-cDNA probe to have the (--SEA) mutation. The (--SEA) double alpha-deletion was the only abnormality in 58. In the others, it was combined with alpha-globin or beta-globin mutations, or coincidental iron deficiency. Four other samples from (--SEA) heterozygotes were negative by this immunologic assay. Anti-zeta negative samples included 78 deletions of the total alpha-globin region, (--Tot), 23 single alpha-globin deletions, and a variety of beta-globin mutations; 16 normocytic samples with normal alpha-genes were also negative. Ten anti-zeta positive and 25 anti-zeta negative samples had benign triplicated zeta-globin genes. In this population, the sensitivity of this test was 95%; and specificity for the (--SEA) mutation was 100%. Anti-zeta immunobinding testing provides rapid, simple, and reliable screening for the (--SEA) double alpha-globin deletion, although it does not detect the (--Tot) total alpha-deletions.

alpha-Thalassaemia

The large number of naturally occurring mutants of this well-characterized locus provides an excellent opportunity for elucidating the relationship between its structure and function. Comparisons of what has been learned about the alpha-globin locus with complementary observations on the beta-globin locus, provide a strategy for understanding the co-ordinate regulation of eukaryotic gene expression. From a practical point of view it is important to remember that millions of individuals throughout the world are carriers of alpha-thalassaemia and every year many thousands of pregnancies are at risk of producing children with the severe alpha-thalassaemia syndromes. The data summarized here provide the basis for accurately predicting the genotype in such cases and thus enabling appropriate prenatal testing. However, because this is a genetic disease that predominantly affects individuals from countries with limited health resources, simpler and cheaper methods of screening and diagnosis will have to be developed before this information has a significant impact on the attendant morbidity and mortality (see Chapter 9, this volume).