Functional analysis of three new alpha-thalassemia deletions involving MCS-R2 reveals the presence of an additional enhancer element in the 5' boundary region.

We report three novel deletions involving the Multispecies Conserved Sequences (MCS) R2, also known as the Major Regulative Element (MRE), in patients showing the α-thalassemia phenotype. The three new rearrangements showed peculiar positions of the breakpoints. 1) The (αα)ES is a telomeric 110 kb deletion ending inside the MCS-R3 element. 2) The (αα)FG, 984 bp-long, ends 51 bp upstream to MCS-R2; both are associated with a severe α-thalassemia phenotype. 3) The (αα)CT, 5058 bp-long starts at position +93 of MCS-R2 and is the only one associated to a mild α-thalassemia phenotype. To understand the specific role of different segments of the MCS-R2 element and of its boundary regions we carried out transcriptional and expression analysis. Transcriptional analysis of patients' reticulocytes showed that (αα)ES was unable to produce α2-globin mRNA, while a high level of expression of the α2-globin genes (56%) was detected in (αα)CT deletion, characterized by the presence of the first 93 bp of MCS-R2. Expression analysis of constructs containing breakpoints and boundary regions of the deletions (αα)CT and (αα)FG, showed comparable activity both for MCS-R2 and the boundary region (-682/-8). Considering that the (αα)CT deletion, almost entirely removing MCS-R2, has a less severe phenotype than the (αα)FG α0thalassemia deletion, removing both MCS-R2 almost entirely and an upstream 679 bp, we infer for the first time that an enhancer element must exist in this region that helps to increase the expression of the α-globin genes. The genotype-phenotype relationship of other previously published MCS-R2 deletions strengthened our hypothesis.

HbA2-Partinico or delta(A2)Pro-->Thr, a new genetic variation in the delta-globin gene in cis to the beta(+) thal IVS-I-110 G>A, and the heterogeneity of delta-globin alleles in double heterozygotes for beta- and delta-globin gene defects.

The study of the alleles of the delta-globin gene is relevant to the prevention of beta-thalassemia homozygosis; in fact, the increase of the HbA2 is an invaluable hematological marker of the beta-thalassemia heterozygosis and the double heterozygosis for alleles of delta- and beta-globin genes can cause the decrease of the HbA2 up to normal or borderline values. We carried out the characterization of alleles of the delta- and beta-globin genes, restriction fragment length polymorphism (RFLP) haplotype background, and hematologic phenotype in 23 double heterozygotes belonging to 18 unrelated families. A wide heterogeneity of the delta-globin alleles was detected; seven known alleles in trans to the beta-globin gene defects were revealed in 17 out of 18 families, while a new allele in cis to a beta-thalassemia allele was detected in one family. Moreover, the relative frequency of the delta-mutants was quite different from that found among heterozygotes. The new allele delta-cod 5 CCT>ACT, in cis to the allele beta(+) thal IVS-I-110 G>A, was found in five carriers of a Sicilian family. The new variant delta5(A2)Pro-->Thr, named HbA2-Partinico upon the origin of the family, was detected with high-performance liquid chromatography; it overlapped the HbA2 peak which was partially split. The double in cis heterozygotes had increased percentage of normal and variant HbA2 of comparable size. The variant originated most likely from a new mutational event because it was associated with RFLP haplotype I, commonly found with the beta(+) thal IVS-I-110 G>A, even if crossing over or gene conversion cannot be excluded.

Genotype-phenotype relationship of the δ-thalassemia and Hb A(2) variants: observation of 52 genotypes.

The increase of Hb A(2) (α2δ2) beyond the upper limit [2.0-2.2/3.3-3.4% of the total hemoglobin (Hb)] is an invaluable tool in the hematological screening of ß-thalassemia (ß-thal) carriers. Factors decreasing Hb A(2) percentages can hinder correct diagnosis. In order to analyze the genotype-phenotype relationship, we characterized δ-, ß- and α-globin genotypes in 190 families where the probands had Hb A(2) values of ≤2.0% or were ß-thal heterozygotes with normal Hb A(2) levels. Hb A(2) was measured with cation exchange high performance liquid chromatography (HPLC). Mutations were detected with allele-specific methods or DNA sequencing; two multiplex-ARMS (amplification refractory mutation system) assays were set up. The molecular basis underlying the decrease in Hb A(2) was extremely heterogeneous. Nineteen δ-globin alleles (Hb A(2)-S.N. Garganico was new) were detected; their interaction with α- or ß-globin alleles (10 and eight, respectively) led us to observe 52 genotypes in 261 carriers. The type of δ-globin mutations, the relative genotypes, the interaction with α(0)-thal traits, are the most important factors in decreasing the Hb A(2) percentage. These results are extremely useful in addressing the molecular diagnosis of hemoglobinopathies and thalassemias.

Effect of Mutations on mRNA and Globin Stability: The Cases of Hb Bernalda/Groene Hart and Hb Southern Italy.

We identified two unstable variants in the third exon of α-globin genes: Hb Bernalda/Groene Hart (HBA1:c.358C>T), and Hb Caserta (HBA2:c.79G>A) in cis to Hb Sun Prairie (HBA2:c.391G>C), also named Hb Southern Italy. These mutations occurred in the H helix of the α-globin that is involved in heme contacting, specific recognition of α-hemoglobin-stabilizing protein (AHSP), and α1ß1 interactions. The carriers showed α-thalassemia phenotype, but one also jaundice and cholelithiasis. Molecular identification of clusters of families in Southern Italy encouraged molecular characterization of mRNA, globin chain analyses, molecular modeling studies, and comparison with globin variants to understand the mechanisms causing the α-thalassemia phenotype. A normal amount of Hb Bernalda/Groene Hart mRNA were found, and molecular modeling highlighted additional H bonds with AHSP. For Hb Southern Italy, showing an unexpected α/ß biosynthetic ratio typical of the ß-thalassemia type, two different molecular mechanisms were shown: Reduction of the variant mRNA, likely due to the No-Go Decay for the presence of unused triplet ACG at cod 26, and protein instability due to the impairment of AHSP interaction. The UDP glucuronosyltransferase 1A (UGT1A1) genotyping was conclusive in the case of jaundice and cholelithiasis. Multiple approaches are needed to properly identify the mechanisms leading to unstable variants and the effect of a mutation.

Molecular evidences of single mutational events followed by recurrent crossing-overs in the common delta-globin alleles in the Mediterranean area.

The human delta-globin gene (HBD) is one of the beta-like globin genes expressed in adults. In the Mediterranean countries the carriers of delta-thalassemia defects or Hb A2-variants are >1% and about 40/70 known alleles have been found in families with this ethnic origin. The scope of this study was to investigate the variability of the gene and of the chromosomal background in order to highlight the origin and spreading of the delta-globin gene alleles in the Mediterranean area. We carried out the characterization of the delta-globin gene alleles and of RFLP-haplotypes, SNPs and one microsatellite associated with them in 231 carriers originating principally from East Sicily. Seventeen alleles were identified, of which five were new. The chromosomes associated with mutated alleles from unrelated carriers were 158; the allele Hb A2-Yialousa accounted for about 75% of relative frequency, Hb A2-Mitsero for about 8%. The alleles were associated with RFLP 5'-haplotypes "- - - -" or "+ - + +", prevalent in the Mediterranean area, except Hb A2-Mitsero associated with the 5'-haplotype "Benin" "- - - +" and the Hb A2' associated with "+ - - +", both of African origin. Each allele showed linkage with one haplotype with these exceptions. The Hb A2-Yialousa showed heterogeneity of the 5'-haplotype in 2/58 chromosomes; the Hb A2-Mitsero showed SNPs and (A)gamma-microsatellite typical of a "Benin" haplotype found associated with the Hb C and Hb S chromosomes; the Hb A2-Yialousa (14/58 chromosomes), Hb A2-Mitsero, Hb A2-Pylos, Hb A2-Fitzroy showed heterogeneity in the 3'-haplotypes and beta-globin gene SNPs. The Hb A2-Coburg was found associated with the haplotype "+ - + +/+ +" different from that already reported "- - - -/+ -". With the exception of this last allele, the linkage of each mutation with a core of RFLPs or SNPs around or inside the delta-globin locus suggested the unicentric origin of the mutations followed by recurrent recombination events causing the chromosomal background heterogeneity.

Hb Gun Hill: a further de novo observation.

Here we report the third observation (the second de novo) of unstable Hb Gun Hill or [b91(F7)-95(FG2)Leu-His-Cys-Asp-Lys-->0]. The two-year old male carrier showed low mean corpuscular hemoglobin (MCH) and mean hemoglobim concentration (MCHC), 8.5% fetal hemoglobin and trade mark 30% variant hemoglobin. Mild hemolytic symptoms were detected seven years later. DNA sequencing and functional studies of mRNA and globin chains were performed.

Hb Bronte or alpha93(FG5)Val-->Gly: a new unstable variant of the alpha2-globin gene, associated with a mild alpha(+)-thalassemia phenotype.

We report a new unstable variant identified in three carriers of a family from East Sicily; it was named Hb Bronte after the place from which the family originated. DNA sequencing from nucleotides -181 to +894 (alpha1) and to +884 (alpha2) revealed a GTG-->GGG substitution at codon 93 of the alpha2-globin gene. The MCV and MCH values were at the lower end of the normal range in the carriers. On cation exchange high performance liquid chromatography (HPLC), the Hb A2 level was apparently increased to around 6%, and a small abnormal peak (0.3-0.4%) was detected after Hb A2. Two abnormal bands were detected by cellulose acetate electrophoresis: a major band (about 3-4%) migrated between Hb A and Hb F; a minor band (<1%) migrated between Hb A2 and carbonic anhydrase. Normal values of Hb A2 were detected by DEAE microchromatography. On reversed phase HPLC the variant chain was not detected, and most likely it was eluted with the alpha chain peak. The isopropanol stability test was very slightly positive in the carriers. Hemolytic symptoms were absent with the exception of indirect bilirubin, which was at high borderline in 2/3 carriers. In biosynthesis in vitro, the specific activity of the alpha chains was much higher than that of the beta-globin chains, and the alpha/beta biosynthetic ratio in the mother and proband was of the beta-thalassemia (thal) type (2.24 and 2.54, respectively). Time course experiments showed that the increase of the 3H-specific activity of the peak containing normal and variant alpha chains was not linear and was much higher than that of beta chains; moreover, the alpha/beta biosynthetic ratio varied during the 2 hours incubation.

Hb G-San Josè variant levels correlate with alpha-thalassemia genotypes.

Hb G-San Josè or beta7(A4)Glu-->Gly has been reported in Southern Italian or Mexican families. We have studied four families from Sicily and Campania, Southern Italy. In six carriers, the hemoglobin variant level ranged from 32 to 38%. In four double heterozygotes for Hb G-San Josè and alpha-thalassemia the variant level showed a strong correlation with the alpha-thalassemia genotype. In fact, the variant level was 15% when interacting with the - (alpha)20.5/alphaalpha, 19.6% with the alphaalpha/alphaPoly Aalpha, and 24.8% with alphaalpha/alpha(-5) ntalpha genotypes. In two double heterozygotes for Hb G-San Josè and beta+ -IVS-I-6 (T-->C) the hemoglobin variant level was 67%. These data show that the reduced synthesis of alpha chains causes drastic reduction of probability to form Hb G-San Josè in favor of the formation of Hb A. Moreover, this reduction, (i) correlates with the type of alpha-thalassemia genotype and with the degree of the alpha chain deficiency, and (ii) is, most probably, more marked than the degree of alpha chain reduction. The minor affinity of the beta chain variant for the alpha chains associated with the reduced synthesis of the alpha chains is probably the principal cause of the variant hemoglobin reduction. Moreover, the rapid removal of the abnormal chains by proteolytic enzymes must have an essential role in order to reduce the chain variant pool. These conclusions are in agreement with the results obtained in reticulocyte and in vitro recombination experiments.