Alpha And Beta Thalassemia Autosomal Recessive Disease Biology Essay

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Alpha thalassemia is an autosomal recessive disease in which the formation of alpha globin protein of fetal and adult hemoglobin is partially or completely suppressed. It is characterized by microcytic hypochromic anemia, and a large spectrum of clinical phenotype varying from a silent trait to a very severe anemia that is fatal in uterus or soon after birth. [108, 109]

Alpha thalassemia is prevalent in Mediterranean, South East Asian, African, Middle Eastern and Indian populations. [110-115] Alpha thalassemia is mostly caused from deletional mutations that remove one or all of the alpha globin; however point mutations less commonly induce alpha thalassemia. The normal phenotype for Alpha globin in a diploid cell is represented as /. When one of the Alpha globin genes is removed or deactivated (-/); the -thalassemia syndrome is known as silent carrier state. When 2 of the Alpha globin genes are removed or deactivated (-/-) or (--/); the -thalassemia syndrome is known as -thalassemia trait. When 3 of the Alpha globin genes are removed or deactivated (--/-); the -thalassemia syndrome is known as hemoglobin-H disease. Finally, when all of the Alpha globin genes are deleted or inactivated (--/--); the -thalassemia is known as Hb Barts Hydrops fetalis syndrome. [116] (Fig.1.14).

The silent carrier state is also known as +-thalassemia, and the -thalassemia trait is also known as 0-thalassemia. This nomenclature has been replaced by mild (-thalassemia2) and severe (-thalassemia1). [68]

Structure of alpha-globin gene locus present on chromosome 16. The oval shape represents the telomere and the boxes represent the genes in the region. The vertical bars represent the regulatory sequences of alpha globin genes. "K" means kilobases. The alpha gene locus is enlarged where the traditional names and the HGVS names of the genes are shown above and below respectively. Different classes and phenotypes of alpha thalassemia are also shown. [40]

1.10.1. Hematologic Indices:

Alpha thalassemia is expected initially on routine erythrocyte count. As shown in Table 1.11 and Table.1.12, all affected people have unstable degree of hemoglobin, such as reduced mean corpuscular hemoglobin (MCH/pg), reduced mean corpuscular volume (MCV/fl) and normal to reduced level of HbA2. [117, 118]

Indices of Erythrocytes in Alpha-Thalassemic Adults. [119, 120]

Alpha thalassemia mostly results from the deletion of one (-) or two (--) Alpha genes. Non deletional thalassemia is due to point mutations in the critical regions of 2 (T) or 1 (T) genes. Very rarely, -thalassemia may be caused by deletion of MCS-R regulatory elements (()T). MCS-R regulatory elements (Multispecies Conserved Sequences or MCS-R1, MCS-R2, MCS-R3, and MCS-R4) are located far upstream of the Alpha-globin gene, they regulate the expression of 1 and 2 genes. [108]

Gene deletions causing alpha thalassemia:

Deletions of large region of DNA are the major cause of alpha thalassemia.

+-thalassemia (-thalassemia2) due to deletions:

+-thalassemia mainly caused from the deletion of one gene. In this case, small size of DNA is removed and one Alpha globin gene is left functional (Fig.1.16). It has been observed that there are 2 types of alpha thalassemia2. One involving a rightward deletion, 3.7 kb deletion, and one involving a leftward deletion, 4.2 kb deletion of DNA [68, 121-124] (Fig.1.15).

Normally, the 4 kb -globin duplication unit contains 3 highly conserved homology blocks (X, Y, and Z). Therefore, misaligned reciprocal recombination between the two Alpha globin genes may cause deletion of one of the two genes. [68]

-3.7 deletions are caused by reciprocal recombination between 2 homologous Z fragments forming a chromosome with only one active gene. According to the different areas of crossover, -3.7 deletions are divided into three haplotypes; (-3.7 I, -3.7 II, and -3.7 III). However,-4.2 deletion is caused by reciprocal recombination between mispaired X-boxes. [125, 126]

0-thalassemia (-thalassemia1) due to deletions:

0-thalassemia usually results from deletions which eliminate 2 and 1 genes (Fig.1.17). The size of the DNA removed range from 20 to 30 kb. Therefore, no -globin chain is produced from this abnormal chromosome. [68, 127]

Many deletions remove  and the -genes. Heterozygotes develop normally, also homozygotes could survive the early gestation stages because neither embryonic (22) nor foetal (22) hemoglobins could be made. 0-thalassemia is rarely due to regulatory regions deletions leaving the -globin gene intact [68, 108] (Fig.1.18)

Deletional mutations causing +-thalassemia. The colored boxes represent the homologous duplication units X, Y and Z in which alpha genes are inserted. During meiosis, when mis-paired Z boxes are crossed over, -3.7 and anti3.7 chromosomes are formed. However, when X boxes are misaligned, -4.2 and anti4.2 are formed.[108]

Point mutations that cause alpha thalassemia:

-thalassemia may result from point mutation that give rise to reduced or no production of -globin from the abnormal gene. These mutations include single nucleotide substitutions and small deletions or insertions that involve the canonical sequences controlling gene expression such as coding sequences, non-translated regulatory or signaling regions and critical areas whose integrity is necessary for a correct mRNA processing (Table.1.13). Therefore, many mutations have been determined that affect mRNA processing and translation, and therefore affect alpha-globin stability. [68,108]

1.10.3. Clinical Description:

The clinical manifestation differs among the 4 genotypes; (-/), (--/) or (-/-), (- -/-) and (- -/- -). Clinical feature severity is related to the molecular basis of the disease. The homozygosity or compound heterozygosity for 0 -thalassemia or the varieties of +- thalassemia determinants and their interactions with normal haplotype account for continuous range of clinical severity ranging from death in fetal period to mild trait. The order of increasing severity of the alpha thalassemia haplotypes is: 21T<2-<-1<2T1<-1T<- -. Patients with non deletional types of mutations are much severely affected than those with deletional mutations. [108, 109]

-thalassemia trait:

These patients are essentially asymptomatic and diagnosis is established during regular health check. [108, 109]

Hemoglobin H disease:

Hemoglobin H disease is characterized by readily detectable amounts of hemoglobin. Underproduction of -globin chains (less than 30%) give rise to excess beta-globin chains which forms 4 tatramers in adult life. Hb-H disease is due to the interaction of an 0 with + determinant. This is divided in to 2 types:

The first type include patients with compound heterozygosity for 0 -thalassemia (- -) and +-thalassemia (-) have 3 deleted -globin genes, and the second type include patients with compound heterozygosity between 0-thalassemia (- -) and thalassemia caused by point mutations (T) or (T) leading to minor changes, have 3 nonfunctional -globin genes (- -,T) or (- - /T). [128, 129]

Patients with Hb-H disease have a large spectrum of phenotypes that ranges from asymptomatic to severe and it is dependent to transfusion. [109] Most of patients suffer from microcytic hypochromic anemia, splenomegaly, mild jaundices, children may have growth retardation. Other complications include gall stones, leg ulcers, aplastic or hypoplastic crises, changes in skeleton, development and metabolism due to ineffective erythropoiesis, delayed pneumatization of sinuses, prominent frontal bossing, overgrowth of maxillaes, shortened limbs, folic acid deficiency and acute hemolytic episodes due to drugs and infections. [130]

Hb Barts Hydrops Fetalis Syndrome:

Hb Barts syndrome is the most severe clinical condition because it affects individuals with no functional -globin genes (- -/- -). Underproduction of -globin chains forms 4 tetramers called Hb Barts in foetal life. About 25% of the fetuses die in utero between 28 and 38 weeks of gestation and the rest at delivery or soon after birth. These infants are oedemateous because of heart failure, pallor, massive enlargement of spleen and liver, brain growth retardation, skeletal distortions and massive enlargement of placenta. [109, 131]

1.10.4. Molecular genetic testing:

Alpha thalassemia can be detected by using various techniques depending on the mutation that causes it. When alpha thalassemia is caused by deletion of one or two alpha globin genes it can be detected by polymerase chain reaction (PCR) based methods using specific primers that flank the relational breakpoints. Also Southern blotting is used to detect less common deletions.

However, when alpha thalassemia is caused by point mutations, it can be detected by Amplification Refractory Mutation System (ARMS), sequencing, or by restriction enzyme digestion. If the mutation causes elongated protein chains, then ARMS is used indirectly detect the mutation and sequencing is used to detect the mutation directly. [132, 133, 134, 135, 136, 137]

1.10.5. Alpha thalassemia in Lebanon:

There is no study that describes the molecular basis of alpha thalassemia in Lebanon.

1.11. Hemoglobin C:

Hemoglobin C is an autosomal recessive disease characterized by mild normochromic anemia. Hemoglobin C results from the substitution of glutamate by lysine at the sixth amino acid position of the beta globin gene. In erythrocytes, the hemoglobin produced precipitates and crystalizes leading to reduction in erythrocyte deformability and an increase in blood viscosity. These crystal-containing cells are removed by the spleen. Hemoglobin C may be present in many forms: heterozygous state, homozygous state, and a variety of compound heterozygous states. When HbC is in the heterozygous state (A/C); it is called hemoglobin C trait. When HbC is in the homozygous state (C/C); it is called hemoglobin C disease. Also HbC can be present in a compound heterozygous state such as HbC/-thalassemia and Sickle cell/HbC. Hemoglobin C is present in West Africa, Togo and Benin, North Africa, and Southern Europe. [138, 139, 140]

Hemoglobin C Trait:

Patients that have hemoglobin C trait are totally asymptomatic. The hemoglobin level is normal, the mean cell hemoglobin concentration is higher than normal. HbA represents more than 50% of hemoglobin. Patients also have mild hemolytic anemia and an abnormal quantity of target cells. [138]

Hemoglobin C Disease:

Patients with HbC disease have mild to moderate anemia, splenomegaly, cholelithiasis, folate deficiency, musculoskeletal pain, retinopathy, dental infarction and gall stones. [141, 142]