Haemoglobin Tunis-Bizerte: a new alpha 1 globin 129 Leu-->Pro unstable variant with thalassaemic phenotype

Exome sequencing for diagnosis of congenital hemolytic anemia

Background

Congenital hemolytic anemia constitutes a heterogeneous group of rare genetic disorders of red blood cells. Diagnosis is based on clinical data, family history and phenotypic testing, genetic analyses being usually performed as a late step. In this study, we explored 40 patients with congenital hemolytic anemia by whole exome sequencing: 20 patients with hereditary spherocytosis and 20 patients with unexplained hemolysis.

Results

A probable genetic cause of disease was identified in 82.5% of the patients (33/40): 100% of those with suspected hereditary spherocytosis (20/20) and 65% of those with unexplained hemolysis (13/20). We found that several patients carried genetic variations in more than one gene (3/20 in the hereditary spherocytosis group, 6/13 fully elucidated patients in the unexplained hemolysis group), giving a more accurate picture of the genetic complexity of congenital hemolytic anemia. In addition, whole exome sequencing allowed us to identify genetic variants in non-congenital hemolytic anemia genes that explained part of the phenotype in 3 patients.

Conclusion

The rapid development of next generation sequencing has rendered the genetic study of these diseases much easier and cheaper. Whole exome sequencing in congenital hemolytic anemia could provide a more precise and quicker diagnosis, improve patients’ healthcare and probably has to be democratized notably for complex cases.

Molecular characterization of Hb Hamilton Hill (HBA2: c.388delC), a novel HBA2 variant generating a premature termination codon and truncated HBA2 chain

In recent years, the identification of α-thalassemias caused by nondeletional mutations has increased significantly due to the advancement of sensitive molecular genetics tools. We report clinical and experimental data for a novel frameshift mutation caused by a single base deletion at position 388 in exon 3 of the α2-globin gene (HBA2: c.388delC; Hb Hamilton Hill), resulting in the phenotype of α-thalassemia (α-thal). Hb Hamilton Hill was identified in an adult female of unknown ethnicity investigated for unexplained microcytosis. Direct DNA sequencing of the HBA2 gene revealed a heterozygous mutation, HBA2: c.388delC, and the molecular effect of this mutation was assessed experimentally using our previously described in vitro model. The experimental analysis involved transfection of a human bladder carcinoma (5637) cell line with expression vectors carrying either HBA2-wild type (HBA2-WT) or HBA2: c.388delC followed by total RNA purification and cDNA synthesis. Both wild type and mutant gene expression was studied and compared at the transcriptional and translational levels using quantitative real time polymerase chain reaction (qReTi-PCR) and immunofluorochemistry (IFC), respectively. Our experimental data showed a significant reduction by 25.0% (p = 0.04) in the transcriptional activity generated from HBA2: c.388delC compared to HBA2-WT. As a result of this base deletion, a frameshift in the open reading frame generates a premature termination codon (PTC) at codon 132 of exon 3 resulting in the formation of a truncated α-globin chain. The truncated α-globin chain, observed by the IFC technique, is most likely unstable and undergoes a rapid turnover resulting in the thalassemic phenotype.

First Description in Tunisia of a Point Mutation at Codon 119 (CCT→TCT) in the α1-Globin Gene: Hb Groene Hart in Association with the − α3.7Deletion

Herein we describe the case of a Tunisian girl who presented with 3% Hb Bart's (gamma4) at birth. At the age of 3 years, she showed microcytosis and hypochromia in the absence of iron deficiency. The first step of molecular analysis was to test for the common Mediterranean mutations and the classical -alpha3.7 deletion was found in the heterozygous state. Since this finding could not explain the level of Hb Bart's at birth, or the hypochromia and microcytosis, all the alpha-globin genes were sequenced. This revealed a rare point mutation at codon 119 (CCT-->TCT) in the alpha1-globin gene, identified for the first time in Tunisia, and which has previously been described as an unstable hemoglobin (Hb) variant named Hb Groene Hart [alpha119(H2)Pro-->Ser (alpha1)]. Here the -alpha3.7/alpha(alpha)119(CCT-->TCT) genotype is responsible for the alpha-thalassemia (thal) trait phenotype.

Molecular spectrum of alpha-thalassemia in Tunisia: epidemiology and detection at birth

We present the characterization of the molecular spectrum and frequency data of alpha-thal (thal) defects in Tunisia, and an evaluation of the efficacy and limitations of Hb Bart's (gamma4) measurement for the screening of alpha-thal at birth. Cord blood samples were collected from two different areas: the northeast of the country, an area where Hb H (beta4) disease frequently occurs, and Tunis, the capital city, representative of the average Tunisian population. From the first group, 110 samples with Hb Bart's and/or microcytosis at birth were selected from 1270 randomly collected samples. Two additional population samples, one from the same northeastern region (n = 90), the other from Tunis (n = 104) were collected randomly. Nine common deletional alpha-thal defects and nondeletional mutations were screened. In the northeastern samples, selected for the presence of Hb Bart's and microcytosis, the -alpha3.7 deletion was the most common defect (4.5% allele frequency) followed by a polyadenylation (poly A) signal mutation (1.8%), the five nucleotide (nt) deletion and the -alpha4.2 deletion (both 0.9%). The African polymorphism (G-->TCGGCCC at position 7238 and T-->G at 7174) was found with an allele frequency of 11% in the selected northeastern samples. In the random population samples, the overall alpha-thal allele frequency was 4% in the northeast region, against 2% in the average Tunisian population. The +14 (G-->C) polymorphism in the 5'UTR (untranslated region) of the alpha2 gene and the African polymorphism in the second intron of the same gene, were found in 3.5% of the alleles. No alpha0-thal alleles were found among the 304 blood samples studied at the DNA level during this survey.

Hb Dartmouth [alpha66(E15)Leu-->Pro (alpha2) (CTG-->CCG)]: a novel alpha2-globin gene mutation associated with severe neonatal anemia when inherited in trans with Southeast Asian alpha-thalassemia-1

We report a novel mutation at alpha66(E15)Leu-->Pro (alpha2) (CTG-->CCG), that we have named Hb Dartmouth for the medical center at which the patients were cared for, in monozygotic twins who also inherited the Southeast Asian alpha-thalassemia-1 deletion. The mother, of Khmer ancestry, is heterozygous for alpha-thalassemia-1. The father, who is of Scottish-Irish ancestry, is a silent carrier of the codon 66 mutation. The twins had severe neonatal anemia requiring transfusion.

An alpha-thalassemic hemoglobinopathy: homozygosity for the HB Agrinio alpha 2-globin chain variant

This report describes the first case of homozygosity for the Hb Agrinio [alpha 29(B10)Leu-->Pro] alpha 2-globin gene variant (codon 29, CTG-->CCG) in a Greek patient. At 12 months of age, the proband presented with a marked hypochromic, microcytic anemia, a very low level of Hb H (< 2.5%), rare Hb H inclusions, and a balanced alpha/non-alpha biosynthesis ratio. The mother had hematological findings and globin biosynthesis consistent with heterozygous beta-thalassemia, but paradoxically, red cell morphology demonstrated very rare Hb H inclusions. The father had mild microcytosis and hypochromia. Analysis of alpha- and beta-globin genotypes demonstrated that the patient was homozygous for the highly unstable Hb Agrinio variant, caused by a T-->C mutation in codon 29 of the alpha 2-globin gene. At the age of 13 years, the proband had a clinical phenotype compatible with mild thalassemia intermedia with moderate anemia (Hb 7-8 g/dL), normal growth and development, slight splenomegaly, and minimal bone changes, while Hb H and inclusion bodies were not detected.

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.

Investigation of microcytosis: a comprehensive approach

Microcytosis is a highly prevalent finding during blood examination. This study investigates the causes of microcytosis (defined as mean corpuscular volume (MCV) < 82 fl) in 466 patients referred to our laboratory for suspected hemoglobinopathy. The following data were obtained: Hb, MCV, serum iron, transferrin, ferritin, HbA2, HbF, isoelectric focusing of the Hb, gene mapping of chromosome 16 with Xba I and Bgl II and hybridization with an alpha- and a zeta-probe, inflammatory status. Results show that iron deficiency remains the first cause of microcytosis (35.2% of our patients), even in a selected population such as ours. Deletional alpha-thalassemia, probably the most frequent hemoglobinopathy throughout the world, represents the second most frequent cause of microcytosis (31.1%), followed by beta-thalassemia heterozygous state (18.9%). Of our patients, 1.3% had microcytosis due to the presence of an abnormal hemoglobin (HbC, Hb S/C, HbE). Three cases (0.6%) had other possible causes of microcytosis. Of the remaining 60 cases, 28 had an inflammatory state. Finally, 32 cases (6.9%) remain unexplained; taking into consideration the origin of these cases, their hematological parameters and their family history, we postulate that these cases are at high risk for non-deletional alpha-thalassemia.

Hb Sun Prairie: diagnostic pitfalls in thalassemic hemoglobinopathies

We report an Asian Indian family in which two daughters have Hb Sun Prairie, a known unstable alpha 2-globin variant [codon 130, GCT-->CCT; alpha 2 130(H13)Ala-->Pro beta 2]. While the homozygous probands have chronic hemolysis-the same phenotype as previously reported, the heterozygous parents are asymptomatic with a thalassemia carrier phenotype, distinct from the chronic hemolytic state previously described in a heterozygote. Unlike the earlier cases in which family studies were not available, this family clearly exhibits autosomal recessive inheritance, unusual amongst variants within the same region of helix H. Globin chain biosynthesis ratios initially suggested a beta-thalassemic hemoglobinopathy-this was excluded by normal sequence analysis of both beta-globin genes. This case report further illustrates the complexity of phenotypes in the thalassemic hemoglobinopathies. It also demonstrates inversion of the alpha/beta-globin chain biosynthesis ratio, a phenomenon which had been noted in other alpha-globin variants and can be a confounding factor in the investigation of thalassemic hemoglobinopathies.