**1. What we need to know about β-thalassemia**

Thalassemia is a hereditary, autosomal recessive blood disorder due to partial or complete deficiency in the synthesis of or β-globin chains (β-thalassemia) or α-globin chains (α-thalassemia) that compose the major adult hemoglobin resulting in chronic hemolytic state.

β-Thalassemia is caused by mutations in the HBB gene resulting in decrease of the production of β chain leading to excessive accumulation of unpaired α chains that aggregate and precipitates along the red cell membranes, causing their damage and resulting in intravascular hemolysis. Also, premature destruction of erythroid precursors results in intramedullary death and ineffective erythropoiesis and a short lifespan of mature RBCs in the circulation. Frequent RBC transfusion is the main supportive therapy, leading to excessive accumulation of iron (iron overload), a condition that is exacerbated by excessive hemolysis and the increased iron reabsorption secondary to ineffective erythropoiesis.

Excessive iron is toxic and catalyzes the generation of reactive oxygen species, which in excess are toxic, causing damage to numerous body organs such as the heart and liver as well as the endocrine system. Herein, we represent an overview on thalassemia regarding the underlying pathophysiology of the disease, clinical presentations, and potential therapeutic modalities for the amelioration of its complications, as well as new modalities that may provide a cure for the disease. Thanks to the significant improvement in therapy, patients with β-thalassemia may reach an advanced age.

#### **1.1 β-Thalassemia: types and clinical presentation**

In the homozygous state of β-thalassemia which is known as thalassemia major represented with severe, transfusion-dependent anemia within the first 2 years of life. Also associated with skeletal abnormalities and poor growth, in the heterozygous state of β-thalassemia (trait or minor) causes mild to moderate microcytic anemia and not require any specific management. On the other hand, patients in whom clinical severity of the disease lies between that of thalassemia major and thalassemia trait are classified as having β-thalassemia intermedia and require only periodic blood transfusions under special circumstances [1]. Numerous different genotypes are associated with β-thalassemia intermedia, such as HbE which is a common Hb variant found in Southeast Asia, and this variant is included in the beta thalassemia category of diseases. Also, HbS (sickle cell disease) can be presented clinically with severe anemia [2].

Repeated blood transfusions resulting in excessive iron deposition and generation of ROS is a leading cause of morbidity and mortality, in those patients [3].

#### **1.2 Removal of excess iron**

Removal of blood (phlebotomy) is used to remove excess iron in patients with normal Hb levels, such as in patients with hereditary hemochromatosis, where iron overload is caused by mutations in the iron homeostasis system [4].

Most other patients with iron overload are anemic (Hb < 10 g/dL) and, therefore, particularly those who are transfusion dependent, phlebotomy would not be optimum and will require iron chelation therapy to decrease iron overload [5].

#### **1.3 Iron chelators**

Each unit of transfused red blood cells contains approximately 200 mg of elemental iron. In addition to anemia and ineffective erythropoiesis down-regulates the synthesis of hepcidin, so the use of iron chelators is mandatory to remove excess iron from the plasma as well as from tissues through binding the cheatable, labile, iron form and enhancing its excretion through the body excreta.

Three chelation agents are approved for use in the United States. Deferoxamine, the first to be used, is given by continuous intravenous infusion or slow subcutaneous, infusion through a portable pump. Its main disadvantage is poor compliance secondary to its mode of administration [6].

Deferiprone is an oral iron chelator effective in removing excess iron from the organs and mainly from the heart. The main potential complication is neutropenia that may rarely be followed by agranulocytosis. A liquid formulation has been recently introduced [7].

Deferasirox is available as oral dispersible tablets and is considered as an effective oral chelator, and it reduces liver iron concentration and serum ferritin levels. Deferasirox binds iron with high affinity in a 2:1 ratio. Its main side effects are GIT upsets in the form of abdominal pain, nausea, diarrhea, liver, and kidney dysfunction as well as skin rash. A new formulation of film-coated tablets are now available with better compliance, as it can be taken with a light meal [8].

#### *1.3.1 Shuttle mechanism*

The efficacy of chelation may be improved by the use of a combination of chelators. For example, deferiprone may mobilize iron from tissues into the circulation, where deferoxamine binds and facilitates its excretion in the urine this is what is called (the "shuttle mechanism") [9].

#### **1.4 Dyserythropoiesis**

Chronic anemia in addition to associated hypoxia in β-thalassemia stimulates excessive RBC production which is mediated through release of erythropoietin, the main erythropoietic stimulating hormone. This attempt is called "stress erythropoiesis" that passes through four steps: expansion of erythroid progenitors, accelerated erythroid differentiation, maturation arrest, and apoptosis. Many other factors, for example, transforming growth factor-β and activin receptor-II trap ligands contribute to this phenomenon. Binding of EPO to its surface receptor on erythroid precursors activates transduction pathways, including Jak2/Stat5, which inhibit apoptosis and stimulates proliferation as well as differentiation of the new cells. However, this operation is futile termed ineffective erythropoiesis due to oxidative stress-increased apoptosis and abortive differentiation [10].

**5**

**Figure 1.**

*Mechanism of gene editing.*

*Introductory Chapter: β-Thalassemia*

**1.5 Novel therapeutic modalities**

*1.5.1 Gene modification approach*

targets.

*DOI: http://dx.doi.org/10.5772/intechopen.90946*

synthesis, and stimulation of HbF production [11–15].

or interacts with the promoter of the γ-globin gene 80 [17].

stage erythroid cell line can achieve the same results [20].

Recent advances in understanding the molecular mechanism involved in two critical steps of dyserythropoiesis are paving the way to new alternative therapeutic

Several therapeutic modalities aimed at reducing the dyserythropoiesis in thalassemia are currently under research. For example, activin receptor-II trap ligands, JAK2 inhibitors, induction of the Hsp70 chaperone machinery, reducing α-globin

The hematopoietic stem cells of the affected individual are subjected to gene editing techniques ex vivo and then reinjected again to the patient for reconstitution [16]. To increase the production of γ-globin lentiviral vectors that express a zinc finger protein has been used in order to carry microRNAs that silence its repressors

Genome editing of the promoter of BCL11A can be accomplished by several nucleases, such as engineered zinc finger nucleases (ZFNs), transcription activatorlike effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats linked to Cas9 nucleases (CRISPR-Cas9) (**Figure 1**) [18]. Recently, it has been shown that ZFN-driven BCL11A enhancer ablation leads to increased production of HbF in erythroid progenitors derived hematopoietic stem cell from β-thalassemia patient which could be used for autologous transplantation [19]. Similarly, CRISPR-Cas9-mediated BCL11A enhancer inactivation in a human adult-

Recent advances in understanding the molecular mechanism involved in two critical steps of dyserythropoiesis are paving the way to new alternative therapeutic targets.

## **1.5 Novel therapeutic modalities**

*Beta Thalassemia*

**1.3 Iron chelators**

recently introduced [7].

*1.3.1 Shuttle mechanism*

**1.4 Dyserythropoiesis**

differentiation [10].

called (the "shuttle mechanism") [9].

**1.2 Removal of excess iron**

Removal of blood (phlebotomy) is used to remove excess iron in patients with normal Hb levels, such as in patients with hereditary hemochromatosis, where iron

Most other patients with iron overload are anemic (Hb < 10 g/dL) and, therefore, particularly those who are transfusion dependent, phlebotomy would not be optimum and will require iron chelation therapy to decrease iron overload [5].

Each unit of transfused red blood cells contains approximately 200 mg of elemental iron. In addition to anemia and ineffective erythropoiesis down-regulates the synthesis of hepcidin, so the use of iron chelators is mandatory to remove excess iron from the plasma as well as from tissues through binding the cheatable, labile,

Three chelation agents are approved for use in the United States. Deferoxamine, the first to be used, is given by continuous intravenous infusion or slow subcutaneous, infusion through a portable pump. Its main disadvantage is poor compliance

Deferiprone is an oral iron chelator effective in removing excess iron from the organs and mainly from the heart. The main potential complication is neutropenia that may rarely be followed by agranulocytosis. A liquid formulation has been

Deferasirox is available as oral dispersible tablets and is considered as an effective oral chelator, and it reduces liver iron concentration and serum ferritin levels. Deferasirox binds iron with high affinity in a 2:1 ratio. Its main side effects are GIT upsets in the form of abdominal pain, nausea, diarrhea, liver, and kidney dysfunction as well as skin rash. A new formulation of film-coated tablets are now available

The efficacy of chelation may be improved by the use of a combination of chelators. For example, deferiprone may mobilize iron from tissues into the circulation, where deferoxamine binds and facilitates its excretion in the urine this is what is

Chronic anemia in addition to associated hypoxia in β-thalassemia stimulates excessive RBC production which is mediated through release of erythropoietin, the main erythropoietic stimulating hormone. This attempt is called "stress erythropoiesis" that passes through four steps: expansion of erythroid progenitors, accelerated erythroid differentiation, maturation arrest, and apoptosis. Many other factors, for example, transforming growth factor-β and activin receptor-II trap ligands contribute to this phenomenon. Binding of EPO to its surface receptor on erythroid precursors activates transduction pathways, including Jak2/Stat5, which inhibit apoptosis and stimulates proliferation as well as differentiation of the new cells. However, this operation is futile termed ineffective erythropoiesis due to oxidative stress-increased apoptosis and abortive

overload is caused by mutations in the iron homeostasis system [4].

iron form and enhancing its excretion through the body excreta.

with better compliance, as it can be taken with a light meal [8].

secondary to its mode of administration [6].

**4**

Several therapeutic modalities aimed at reducing the dyserythropoiesis in thalassemia are currently under research. For example, activin receptor-II trap ligands, JAK2 inhibitors, induction of the Hsp70 chaperone machinery, reducing α-globin synthesis, and stimulation of HbF production [11–15].
