Gene mutation leads to changes in the quality and quantity of the polypeptide chain, which makes the protein defective. If the occurrence of disease controlled by a pair of alleles, which is called single-gene inheritance disease. According to the influence of defective protein on the body, it is divided into molecular disease and congenital metabolic defect.

Molecular diseases, including hemoglobinopathy, receptor disease, membrane transporter disease, structural protein deficiency disease, immunoglobulin defect, etc, are a type of diseases that can directly cause the body dysfunction. Molecular diseases are mainly caused by inherited genetic mutation or acquired genetic mutation that result in the abnormal molecular structure and abnormal synthetic amount of protein. This chapter mainly introduces the hemoglobinopathy.

Hemoglobinopathy are due to the abnormal molecular structure and synthetic amount of hemoglobin (Hb), and are usually divided into two types of abnormal hemoglobin and thalassemia, the common molecular basis of which are gene mutations or deficiency of globin. Hemoglobinopathy are one of the most common single-gene inheritance diseases. The World Health Organization estimates more than 5% of the world's population carries the genes of hemoglobinopathy. Hemoglobinopathy are mainly distributed in Africa, the Mediterranean region, Southeast Asia, and the southern part of China.

Hemoglobin plays an important physiological function of erythrocytes, which is the carrier of erythrocytes to carry and transport oxygen and carbon dioxide in the blood.

Hemoglobin is a binding protein, the polypeptide chain is a globin, and the auxiliary is heme. Human hemoglobin molecule is a tetramer, consists of two α-like globin peptides chains (α chain or ζ chain) and two β-like globin peptide chains (ε chain, γ chain, δ chain, or β chain). Each globin peptide chain is combined with a heme. Heme is an iron-containing and oxygen-binding pigment that gives hemoglobin molecules the ability to transport oxygen. Various combinations of α-like globin peptide chains and β-like globin peptide chains form several kinds of common human hemoglobin (Table 8-1).The expression of globin gene is precisely regulated, showing typical tissue specificity and time specificity.

Table 8-1 Normal human hemoglobin

Globin gene family includes α-globin gene cluster and β-globin gene cluster, and they exist at chromosome 16 and 11, respectively. Each gene contains three exons and two introns.

I. α-like globin gene cluster

α-like globin gene cluster includes two identical α-genes, named α1 and α2, one ζ gene and two pseudogenes (ψζ、ψα1), locating at 16p13. The total length of the gene is 30 kb.

Ascending order: 5′-ζ-ψζ-ψα1 -α2-α1-3′

Expressed sequences: embryonic period expresses ζ, fetal period and adulthood mainly express α2 and α1.

II. β-like globin gene cluster

β-like globin gene cluster includes ε, Gγ, Aγ, ψβ, δ and β, locating at 11p15. The total length of the gene is 60 kb.

Expressed sequences: early embryonic period expresses ε, fetal period expresses γ, and adulthood mainly expresses δ and β.

There are two α genes on each 16 chromosome, and one β gene on each 11 chromosome. Therefore, there are simultaneous four α genes and two β genes expressions in normal adult diploid cells. However, the production of α chain and β chain are roughly equal in number. Because of the expression efficiency of β gene is two times than that of α gene. The balance of these expressions is the need of normal physiological function. If the balance is destroyed, the person will suffer from thalassemia, and if the structure is abnormal, the person will suffer from sickle cell anemia.

Globin gene mutation leads to the abnormal expression of globin, resulting in thalassemia or sickle cell anemia. Common mutations include a variety of types, such as base substitution, frameshift mutation, fusion gene and so on. Single gene replacement. It is the most common type of mutation in hemoglobinopathy.

Frameshift mutation. The loss or insertion of one or two bases in the globin genes causes the rearrangement of the following base, which leads to the change of the structure or synthesis rate of globin peptide chains.

Deletion or insertion of codon. Some abnormal hemoglobin proteins are found deletion or insertion of some amino acids. Because of the mismatch and unequal exchange of homologous chromosomes happen during meiosis.

Nonsense mutation. The normal codon becomes a stop codon, so the synthesis of the protein chain is terminated prematurely, leading to thalassemia.

Termination codon mutation. Due to the mutation of the DNA sequence of the terminator codon, the synthesis of the globin chain does not terminate at the normal position, resulting in the formation of an extended abnormal globin chain.

Gene deletion. Due to the deletion of genes in different parts leads to abnormal synthesis of the different globin peptide chain and different types of thalassemia.

Fusion gene. Two different gene fragments are spliced together. As Hb Lepore, its α chain structure is normal, but the non α chain is connected by the chain of δ and β. So that, its N end is like δ, and the C end is like the β chain, called the δ-β chain.

Hereditary mode: autosomal recessive inheritance (AR)

Sickle cell anemia is a autosomal recessive disease caused by defects in the β gene. The mechanism of sickle cell anemia is that β globin gene appears base substitution, the sixth codon changes from GAG into GTG, making the corresponding glutamic acid change into valine, and β globin changes from normal hemoglobin adult (HbA) to abnormal sickle cell hemoglobin (HbS). HbS easily aggregates into the gelatinization of rod-like structure, leading to erythrocyte sickling. Sickle cell makes the increase of blood viscosity, easily causing the microvessel embolization and tissue’s hypoxia and even necrosis. The patient will develop symptoms such as musculoskeletal pain and abdominal pain, especially suffer frequent and severe bone pain. At the same time, sickle cell’s plasticity decreases and the cells are apt to break when flowing through capillary vessels, leading to hemolytic anemia. These symptoms can be particularly severe in low-oxygen pressures areas.

Thalassemia is caused by the mutation of the globin gene, resulting in the decrease or deletion of globin chain synthesis, and the imbalance of the number of α chain and β chain leads to hemolytic anemia. The Mediterranean regions,Southeast Asia and southern China are high incidence areas of thalassemia. May 8th is the Mediterranean anemia day.

I. α-thalassemia

α-thalassemia is due to the deletion of the α globin gene on chromosome 16, and the reduction or deletion synthesis of α-chain. Since α gene is expressed in both fetal period and adulthood, α-thalassemia would be harmful to both the fetus and adult.

1. Method of marking α-thalassemia gene

There are two α genes on chromosome 16 of the normal person, which are marked as αA (or αα). Two α genes on one chromosome are missing, then marked as α0 (or –). One α gene is missing marked as α+ (or -α).

2. Types of α-thalassemia

The more α gene is missing, the more severe the disease is. According to the losing number of α gene and the degree of clinical manifestations, α-thalassemia can be divided into four types (Table 8-2).

Table 8-2 The genotypes and clinical features α-thalassemia

II. β-thalassemia

β-thalassemia is a hereditary hematological disease caused by the mutation of the β globin gene, which results in β chain's synthesis decreases (β+) or defects (β0). Because of the β-globulin chain is highly expressed and plays functions mainly in postnatal period. So that β-thalassemia mainly occurs few months after birth.

1. Method of marking β-thalassemia

Normal β chain is recorded as βA. No β chain is recorded as β0, in which two β globin genes have mutations. Only small amounts of β globin are expressed that are recorded as β+.

2. Types of β-thalassemia

The vast majority of β-thalassemia are point mutations that target every part of the β gene. In addition, there is phenomenon of gene fusion that produces the fusion gene of δβ. Gene fusion means that two kinds of different genetic fragments joint with each other, and the forming gene is called fusion gene. It may be the result of unequal crossover of homologous chromosomes during meiosis. According to the severity of the disease, β- thalassaemia is divided into three types: heavy (blood transfusion-dependent), intermediate (moderate anemia) and light (asymptomatic).

Table 8-3 The genotypes and clinical features β-thalassemia

III. β-thalassemia compound with α-thalassemia

If the synthesis of α chain and β chain both decrease. It will trend to a new equilibrium, the ineffectual hematopoiesis is reduced, and so the clinical manifestations will relieve.

The inborn errors of metabolism, also known as hereditary enzymopathy, are usually caused by genetic mutations leading to the abnormal structure or quantity of enzyme proteins, including glycometabolism defect deficiency, amino acid metabolism deficiency, nucleic acid metabolism deficiency, lipid metabolism deficiency and so on.

Due to the genetic defects of enzymes involved in glycometabolism, make the body of the glycometabolism abnormalities and produce glycometabolism diseases, including galactosemia, glucose - 6 - phosphate dehydrogenase deficiency, glycogen storage disease, etc. Here mainly introduces glucose-6-phosphate dehydrogenase deficiency (G6PD deficiency).

Glucose -6- phosphate dehydrogenase deficiency is an X-linked recessive disorder caused by lack of the glucose -6- phosphate dehydrogenase due to mutation of the G6PD gene. The G6PD enzyme is one of the important enzymes of the pentose phosphate pathway in red blood cells. If absence of G6PD, cells will not produce sufficient amounts of co-enzyme nicotinamide adenine dinucleotide phosphate (NADPH). NADPH deficiency makes the two important antioxidative substances of glutathione (GSH) and catalase (CAT) deficient, so that hemoglobin and erythrocyte membrane are prone to oxidative damage.

Erythrocyte glucose-6-phosphate dehydrogenase deficiency is the most common erythrocytic enzyme disease in the world. It is estimated that nearly 400 million people in the world have G6PD deficiency. This disease often occurs in endemic areas of malaria, thalassemia and abnormal blood protein disease, such as, the Mediterranean coast, Southeast Asia, India, Africa and African Americans. The distribution pattern in China shows a trend of “high in the south and low in the north”.

Most of the time, the G6PD mutation carrier have no symptoms. The main clinical symptom brought by G6PD deficiency is hemolytic anemia, including drug hemolysis, neonatal jaundice, favism, etc. The triggers of G6PD deficiency include: (1) foods (e.g. fava beans, the hallmark trigger for G6PD mutation carriers, so G6PD deficiency is also commonly called favism); (2) certain oxidizing drugs (e.g. primaquine diphosphate and quinine); (3) severe stress from bacterial or viral infection. Therefore, in order to avoid hemolytic anemia, G6PD gene mutation carriers should avoid taking a large number of related drugs or foods.

Due to the genetic defects of enzymes involved in the metabolism of amino acids, make the body of the amino acids metabolism abnormalities and produce amino acids metabolism diseases, including phenylketonuria, albinism, alkaptonuria, etc. Here mainly introduces phenylketonuria (PKU).

Phenylketonuria is an autosomal recessive metabolic genetic disorder that causes an unusually high concentration of phenylalanine (Phe) in the blood. The mutation of phenylalanine hydroxylase (PAH) gene leads to a decrease in the production of PAH, which can’t convert phenylalanine into tyrosine to further synthesize thyroxine, epinephrine and melanin, resulting in abnormal accumulation of phenylalanine and phenylpyruvic acid in body.

According to the levels of phenylalanine, the signs and symptoms of PKU range from mild to severe. The most severe form of this disorder, known as classic PKU, can lead to permanent intellectual disability, seizures, delayed development, behavioral problems, mental disorders, and can also result in a musty odor of smell and lighter skin. Once the diagnosis is clear, the active treatment should be given as soon as possible, mainly diet therapy. The younger the beginning of the treatment, the better is the effect. The main treatment of PKU is to let patients eat special foods with low Phe (Lofenalac). They should avoid eating foods with high protein or containing aspartame sweeteners, such as egg whites, fish, chicken, nuts, coke, gum, etc.