Medical Genetics - V. M. Zaporozhan 2005

Monogenic Diseases
Principles of Treatment for Monogenic Disorders

Currently, thanks to advances across all branches of genetics and progress in theoretical and clinical medicine, it can be stated with confidence that many Monogenic Disorders are already successfully treatable.

Etiological, pathogenetic, and symptomatic treatments are employed for Hereditary diseases.

Etiological Treatment is based on correcting the primary genetic defect by altering the genotype (Gene Therapy). Essentially, gene therapy involves curing a specific hereditary disease by introducing a functional genetic construct into the target Cell. Positive gene therapy aims to introduce a normal gene to replace an inactive mutant gene. Negative gene therapy aims to suppress the function of an overexpressed gene. Gene therapy can be performed both ex vivo and in vivo.

Ex Vivo Gene therapy involves correcting a genetic defect in somatic Cells isolated from the body. It consists of the following stages: 1) obtaining cells from the patient; 2) correcting the genetic defect by transferring the required gene. Retroviral and adenoviral vectors, Liposomes, and physical Methods (electroporation, ultrasound, laser microinjections, gene guns—gold microparticles coated with DNA shot into Tissues) have been proposed for gene delivery; 3) selecting and expanding the population of genetically corrected cells; 4) reintroducing these cells into the patient.

The first successful attempt at ex vivo gene therapy was performed in the USA on September 14, 1990, in two young girls with a rare inherited disorder—severe combined immunodeficiency caused by a mutation in the adenosine deaminase A (ADA) gene. This day is considered the birthdate of gene therapy. T-lymphocytes were isolated from the patients, and the ADA gene was introduced in vitro using a retroviral vector. The lymphocytes modified in this way were cultured and periodically reintroduced to the patients over a two-year period. Both patients exhibited ADA Gene Expression and clinical improvement. The treatment proved effective, and the patients are still alive today, though lymphocyte infusions must be repeated every 3–6 months. Today, hereditary combined immunodeficiency is successfully treated by modifying hematopoietic stem cells, even prenatally (in the fetus).

Long-term gene therapy was first successfully applied to familial hypercholesterolemia in a 29-year-old woman. The patient underwent a partial (approximately 15%) hepatectomy. Using a retroviral vector, the low-density lipoprotein (LDL) receptor gene was introduced into the obtained Liver cell culture. The transgenic hepatocytes were returned to the patient via a catheter into the portal vein and successfully homed to the liver. As a result, the patient's Blood LDL levels decreased by 15–30%, which significantly improved her condition.

Unfortunately, isolating a patient's cells for genetic modification and subsequent reinfusion is not feasible in every case.

In Vivo Gene therapy is the direct genetic modification of cells within the patient's body. The main challenge of in vivo gene therapy is delivering the gene to the appropriate tissue and ensuring its expression. For instance, gene therapy for cystic fibrosis is only possible in vivo. However, more than 20 clinical trials of cystic fibrosis gene therapy have failed due to poor gene integration into non-dividing cells and short-lived expression of the introduced gene. Furthermore, potential risks include immune responses against the viral vector, tumor induction, and unpredictable effects on The Cell genome if the vector integrates into an inappropriate DNA site.

Due to the insufficiently understood consequences of interfering with The Human Genome, germline gene therapy is banned in most countries.

At the same time, somatic cell gene therapy is one of the most cutting-edge approaches for treating both hereditary and non-hereditary diseases. Its development is closely linked to decoding gene structures through the Human Genome Project. By 2003, more than 600 gene therapy protocols had been proposed for various disorders, with monogenic diseases accounting for only 12% of them (Table 6.14). The majority of research focuses on gene therapy for oncological, therapeutic, and infectious diseases, as well as genetic Vaccines.

Table 6.14. Monogenic disorders undergoing clinical trials and experimental gene therapy developments

Pathogenetic treatment is based on correcting specific links in the pathogenic chain and is used to manage many Metabolic Disorders, such as enzymopathies and endocrinopathies. While highly effective, this therapy requires a thorough understanding of the disease's Pathogenesis. Depending on the level and Nature of the biochemical defect, Pathogenetic Therapy may aim to achieve the following:

1. Restricting the intake of substances whose METABOLISM is impaired. For example, in phenylketonuria, a phenylalanine-restricted diet helps lower the levels of this Amino Acid and its metabolites in the blood, allowing an affected child to develop normally.

In galactosemia, a lactose-free diet based on specialized formulas prevents the accumulation of galactose-1-phosphate and the onset of the disease.

2. Modulating enzyme function:

— Stimulating The production of the missing enzyme. Administering large doses of Coenzymes (typically Vitamins) can overcome a metabolic block if the enzyme deficiency is related to impaired apoenzyme-coenzyme complex formation or reduced coenzyme synthesis. For instance, high doses of vitamins B6 and B12 are effective in treating homocystinuria, Vitamin B1 is used for maple syrup urine disease (leucinosis), and tetrahydrobiopterin administration treats pterin-deficiency-related phenylketonuria;

— Inhibiting enzyme synthesis. An example is The Use of allopurinol to reduce The activity of xanthine oxidase (The enzyme catalyzing uric acid synthesis) in Gout and uric acid diathesis.

3. Replacing missing substances. Replacement therapy is widely used for hereditary endocrinopathies (Pituitary dwarfism, hypothyroidism, adrenogenital syndrome). Administration of antihemophilic globulin prevents bleeding in hemophilia. Gaucher disease (a type of sphingolipidosis) is treated by supplying the missing enzyme glucocerebrosidase (the drug Ceredase).

Unfortunately, ensuring the delivery of missing Enzymes to the appropriate cells is challenging, which is why effective enzyme replacement therapy is not yet widespread. It has been established, for instance, that intramuscular or subcutaneous administration of enzymes allows them to be detected in the liver, but they do not cross the blood-Brain barrier. Methods utilizing liposomes and enzyme-loaded erythrocytes are currently under development.

4. Binding and excreting accumulating compounds or detoxifying toxic metabolic products. For example, in hepatolenticular degeneration (Wilson's disease), D-penicillamine and unithiol are used to clear copper from tissues. Plasmapheresis is performed in severe forms of hypercholesterolemia. Phlebotomy (bloodletting) is effective in treating hemochromatosis to remove excess iron.

An example of detoxifying harmful metabolic byproducts is the treatment of isovaleric acidemia. This disorder features a block in isovaleryl-CoA oxidation, leading to the accumulation of isovaleric acid. Affected patients develop ketoacidosis and mental retardation. Administering Glycine forces isovaleryl-CoA to bind, forming non-toxic compounds that are excreted in the urine. Carnitine operates via a similar mechanism and is used to treat propionic, glutaric, and methylmalonic acidemias.

5. Modulating gene expression. Treating patients with sickle cell anemia with hydroxyurea reactivates fetal Hemoglobin synthesis, which prevents The formation of sickle-shaped red Blood Cells.

To silence an overexpressed gene, antisense oligonucleotides (complementary to Messenger RNA) have been proposed. By binding to mRNA through complementary base pairing, they form a double-stranded Structure and block the Synthesis of the corresponding protein. Antisense RNAs are expected to be effective in treating DISEASES ASSOCIATED WITH gene overabundances or toxic gene products. For example, experimental antisense microRNAs selectively inactivated the AMYOTROPHIC LATERAL SCLEROSIS allele without turning off the normal gene allele. Antisense RNA therapy is one of the most promising therapeutic avenues not only for monogenic disorders, but also for cancers and viral infections.

6. Surgical treatment. For instance, splenectomy is performed to treat Minkowski–Chauffard syndrome (hereditary spherocytosis). This halts red blood cell destruction and alleviates the symptoms of hemolytic anemia.

7. Organ and tissue transplantation. Bone marrow transplantation is performed for severe combined immunodeficiency, Heart transplantation for primary cardiopathy, and

liver transplantation for Wilson's disease, Adrenal gland transplantation for adrenocortical insufficiency, and lung and liver transplantation for cystic fibrosis, among others.

A promising therapeutic approach is the administration of stem cells. Strictly speaking, transplantation is a form of gene therapy, since a normal donor genome is introduced along with the graft.

Symptomatic treatment is used in the management of all types of hereditary disorders. Such therapy does not require knowledge of the primary genetic defect or the pathogenesis of the condition. For example, in mucopolysaccharidosis, the accumulation of glycosaminoglycans within cells leads to joint stiffness and flexion contractures. Mud therapy, balneotherapy, various types of electrotherapy, and heat therapy significantly improve the General condition of such patients and increase their range of joint motion. Reconstructive surgery plays an important role in the symptomatic Treatment of Hereditary diseases, being used for cleft lip, musculoskeletal malformations, and other conditions.

Unfortunately, even successful treatment of monogenic diseases does not solve all problems. This is due to the persistence of the pathological genotype. For instance, in clinically compensated galactosemia, adult women frequently experience ovarian dysfunction. Patients successfully treated for retinoblastoma have a high probability of subsequently developing osteosarcoma. In patients with Pearson's mitochondrial syndrome (pancytopenia), the hematopoietic defect can be managed through repeated blood transfusions. Over time, the proportion of mutant Mitochondria in bone marrow cells decreases, compensating for the hematopoietic defect. However, this is accompanied by an increase in the proportion of pathological mtDNA in Skeletal Muscle cells. This inevitably leads to The Development of the fatal Kearns-Sayre mitochondrial syndrome. Therefore, a general rule applies to all hereditary diseases, particularly monogenic ones: Prevention is better than cure. The prevention of monogenic disorders includes Mass Newborn Screening, Genetic Counseling, and prenatal Diagnosis.