BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL UNIVERSITIES - E. S. Severin - 2004

SECTION 4. BIOSYNTHESIS OF NUCLEIC ACIDS AND PROTEINS (TEMPLATE BIOSYNTHESIS). BASICS OF MOLECULAR GENETICS

IX. Application of DNA Technologies in Medicine

Today, it is evident that breakthroughs in molecular biology have the potential to profoundly transform clinical medicine. They have not only deepened our understanding of Gene Expression and the Etiology of numerous diseases, but have also driven The Development of novel approaches to their Diagnosis and Treatment.

It has been established that gene polymorphism is widespread in human populations, and a correlation between DNA structural alterations and various diseases has been demonstrated. The identification of genes whose dysfunction leads to hereditary disorders has laid the groundwork for a detailed Analysis of the genetic and biochemical bases of Pathogenesis, paving the way for the most effective therapeutic strategies.

Molecular medicine techniques have enabled The production of Vaccines for hepatitis Prevention, human Insulin for Diabetes Mellitus management, factor VIII for restoring normal Blood clotting in hemophilia, and many other biopharmaceuticals.

Gene Therapy has made it possible to introduce fully functional genes into a patient's body, thereby correcting Metabolic Disorders caused by mutant genes. This approach is currently used to treat children with adenosine deaminase deficiency-induced immunodeficiency. Furthermore, gene correction Methods for hereditary disorders such as familial hypercholesterolemia, hemophilia B, cystic fibrosis, and others are currently in clinical trials.

To detect DNA structural defects, the genetic material must be isolated from an appropriate source (biological fluid, biopsy specimen, Cell culture, etc.) and amplified in quantities sufficient for analysis. Gene therapy Applications require the isolation of normal genes and their delivery into defective Cells in a way that ensures their expression, thereby restoring the patient's health.

This section outlines the fundamental concepts and methods used to address the challenges of DNA-based Diagnostics for Hereditary diseases and gene therapy.

A. Methods of DNA Isolation

DNA can be extracted from any type of nucleated tissue or cell. The DNA isolation Procedure involves rapid cell lysis, the removal of cell organelle fragments and membranes via centrifugation, the enzymatic Digestion of Proteins using proteinases, and the extraction of DNA from solution with phenol and chloroform. Subsequently, the DNA is typically precipitated with ethanol, and after the supernatant is removed, it is dissolved in a buffer solution.

✵ The quality of the extracted DNA is assessed by measuring the optical density of the DNA solution within the PROTEIN AND NUCLEIC acid absorption spectra, specifically at 280 and 260 nm, respectively. For pure DNA samples, the absorbance ratio at 260/280 nm should exceed 1.8.

✵ An average-sized chromosomal DNA molecule contains 150 x 10⁶ nucleotide pairs and is approximately 4 cm long. Molecules of this size are sensitive to the mechanical stress generated in solution during isolation and are frequently fragmented. Consequently, the isolated DNA molecules are significantly smaller than the original ones, yet still very large—comprising thousands or tens of thousands of nucleotide pairs. Such molecules are unwieldy for research purposes and require further fragmentation.

DNA Cleavage Using Restriction Enzymes

Fragmentation is achieved using restriction enzymes (DNA-cleaving enzymes) or Restriction Endonucleases derived from bacterial cells. In vivo, these enzymes participate in recognizing and destroying foreign bacterial DNA by cleaving internal Regions of the molecule into relatively small fragments. Restriction enzymes recognize specific sequences of 4 to 6 (or less frequently 8 to 12) NUCLEOTIDES within a double-stranded DNA molecule (restriction sites) and cut the DNA precisely at these loci. The number of restriction fragments generated by a single restriction enzyme depends on the number of restriction sites, while fragment size is determined by the spacing of these sites along the original DNA molecule.

More than 500 Different types of bacterial restriction enzymes are known, each recognizing its own specific sequence (Fig. 4-62). Using a panel of restriction enzymes, a DNA molecule can be cleaved into fragments of the desired length. For example, for Primary Structure analysis (sequencing), fragments of about 300 nucleotide pairs are ideal. Consequently, a single chromosomal DNA molecule of 150 x 10⁶ nucleotide pairs must be cleaved into 500,000 fragments, with each fragment analyzed individually.

Class="center">Fig. 4-62. DNA nucleotide sequences recognized by the three most commonly used restriction enzymes. Restriction endonucleases are derived from various Bacteria: Hpa I from Haemophilus parainfluenzae; EcoRI from Escherichia coli; HindIII from Haemophilus influenzae. Type 1 restriction enzymes cleave DNA to produce blunt ends, whereas others (Type 2) generate single-stranded sticky ends at the cleavage site.

The DNA fragments resulting from restriction can be analyzed by agarose or Polyacrylamide gel Electrophoresis. DNA detection within the gel is achieved using ethidium bromide, which binds to the molecular fragments and imparts a specific pink fluorescence under ultraviolet light.

B. Identification of Specific Sequences

Digestion of genomic eukaryotic DNA—specifically human DNA—with restriction enzymes yields such a vast number of fragments of varying lengths that they cannot be satisfactorily resolved by polyacrylamide or agarose gel electrophoresis. Following the electrophoresis of restricted genomic DNA and ethidium bromide staining, a uniform smear spanning the entire length of the gel is observed. Identifying specific DNA fragments in such a gel requires Hybridization with labeled DNA probes. DNA probes are synthesized using automated instruments capable of producing single-stranded DNA fragments exceeding 100 nucleotide residues in length with a strictly defined primary structure. Such molecules can be

utilized for the specific binding to target gene regions.

Southern Blot Hybridization

Southern blot hybridization, introduced in 1975, has become a classical method for identifying specific DNA regions. The technique is based on denaturing the continuous "ladder" of DNA fragments—separated by molecular weight via gel electrophoresis—and transferring them from the gel onto a solid support (a nitrocellulose filter or nylon membrane). This transfer, or blotting, is driven by capillary action, an electric field, or a vacuum (Fig. 4-63). The DNA fixed on the filter is then hybridized with a labeled DNA or RNA probe. Autoradiography is subsequently used to determine the Location OF THE target genomic DNA fragment on the electrophoregram. Blot hybridization is a highly sensitive method for identifying specific sequences.

Fig. 4-63. Southern blot hybridization. DNA fragments are separated by electrophoresis, denatured, transferred onto a nitrocellulose filter, and hybridized with a DNA probe.

Many modifications of this method have been developed to date. For instance, DNA probes are not always labeled with radioactive isotopes; compounds that bind covalently to DNA are frequently used instead and can be detected through The formation of a colored product or fluorescence. The length of oligonucleotides in DNA probes can also vary significantly, sometimes being very short—about 15 to 20 nucleotide pairs. Dot-blot and slot-blot hybridization methods have also been described, in which DNA or RNA preparations are applied directly to a solid support without prior restriction or electrophoresis and hybridized with labeled DNA probes.

B. Determination of the Introduction/19.html">Primary structure of DNA-Enzymes (DNA Sequencing)

Dideoxy sequencing is the method most commonly used to determine the primary structure of DNA. Reaction mixtures containing denatured single-stranded DNA, DNA polymerase, deoxynucleoside triphosphates (dNTPs)—dATP, dCTP, dGTP, and dTTP (one of which is radioactive)—and a primer are used to initiate DNA Synthesis in the presence of specific dideoxynucleoside triphosphates (ddNTPs), or terminators, namely ddATP, ddCTP, ddGTP, or ddTTP. Synthesis is carried out simultaneously in four parallel reactions, with one of the four ddNTPs added to each tube along with the standard reaction components. The ddNTPs compete with normal dNTPs for incorporation into the growing polynucleotide chain. When a ddNTP is incorporated instead of the corresponding nucleotide, DNA synthesis is terminated. As a result, each tube yields a set of labeled DNA fragments of varying lengths, each ending with a specific dideoxynucleotide. Following simultaneous Separation of these fragments in an electric field across four adjacent lanes and subsequent autoradiography, the sizes of the synthesized molecules can be determined, which in turn allows the localization of the terminating dideoxynucleotides to be established. Based on

these data, The nucleotide sequence in the newly synthesized fragments complementary to the DNA template is determined (Fig. 4-64).

Fig. 4-64. Dideoxy DNA sequencing. Four reactions are used, each containing the DNA template, a primer, DNA polymerase, and the 4 dNTPs (dATP, dGTP, dCTP, dTTP). Either the primer or one of the nucleotides carries a radioactive label, enabling the bands to be visualized in the gel by autoradiography. One of the four dideoxyribonucleotides (ddNTPs) is added to each tube. Synthesis is halted whenever a ddNTP is incorporated into the growing oligonucleotide chain.

Instruments are currently being developed for the automated, simultaneous sequencing of large numbers of samples using dideoxynucleotides labeled with different fluorochromes.

At the same time, new, more efficient, and economical sequencing methods are being developed. THE PRINCIPLE OF one such method is as follows: a set of oligonucleotides (e.g., octanucleotides) comprising all possible sequence variants is generated from the 4 nucleoside triphosphates (dATP, dGTP, dTTP, dCTP). These octanucleotides are immobilized (attached) in microplate wells, thereby creating a so-called oligonucleotide array. The DNA fragment to be sequenced is labeled at the phosphate group and added to the wells of the array. The DNA fragment hybridizes exclusively with those octanucleotides whose sequences are complementary to its regions. In this way, the set of all possible octanucleotides present in the studied DNA fragment is determined. Special computer software is then used to reconstruct the correct order of the octamers within the analyzed DNA fragment.

C. Generation and Amplification of Recombinant DNAs

Working with nucleotide sequences in genes and other DNA regions requires an adequate amount of material for study. This is a challenging task, particularly when human tissue serves as the source of DNA. Therefore, the DNA fragments under investigation are typically pre-amplified (quantitatively increased millions of times) to ensure they are available in unlimited quantities whenever needed. The Use of recombinant DNAs (i.e., DNA molecules constructed from segments of different origins) has proven to be an exceptionally valuable tool in solving this problem.

1. Generation of Recombinant DNAs

To generate such molecules, DNA is first isolated from two different sources (Fig. 4-65).

Fig. 4-65. Generation of recombinant DNAs. Two DNA samples, DNA_X and DNA_Y, are digested with the same restriction endonuclease to yield fragments with "sticky" ends. Upon Denaturation and subsequent annealing, recombinant DNA molecules DNA_A and DNA_B are formed, initially held together by their sticky ends and subsequently covalently joined by DNA ligase.

Each DNA sample is fragmented separately using the same restriction enzyme, which cleaves the DNA to produce sticky ends. Following a procedure of heating and slow cooling (annealing), alongside the original DNA_X and DNA_Y molecules, recombinant molecules consisting of DNA_X and DNA_Y fragments joined via sticky ends may form. Covalent joining of the fragments is achieved using DNA ligase in the presence of ATP as an energy source.

In Recombinant DNA technology, In addition to DNA fragments isolated from nucleated cells, DNA synthesized using Reverse Transcriptase is widely employed. When four different deoxyribonucleoside triphosphates are added to the reaction medium, the enzyme synthesizes a DNA copy, or cDNA, using mRNA as a template According to the principle of complementary base pairing. Because the information source for cDNA generation is mature cytoplasmic mRNA, such DNA lacks introns, unlike fragments obtained by cleaving eukaryotic genomic DNA.

2. DNA Cloning

To obtain significant quantities of the material of interest, DNA cloning is performed, which involves inserting the desired DNA fragment into a vector DNA molecule (or vector). The vector ensures The entry of this recombinant, or chimeric, DNA into bacterial cells. Plasmids, phages, Retroviruses, and Adenoviruses are commonly used as vectors, with plasmid DNA being the most frequent choice.

Plasmids — small, circular, double-stranded DNA molecules present in bacterial cells in varying copy numbers. They possess an autonomous Replication control system that maintains their intracellular copy number at a specific level, ranging from a few to several hundred genome copies per cell.

The plasmid DNA used for cloning and the DNA of interest are cleaved at specific sites with a restriction enzyme to yield recombinant DNA. The hybrid plasmid is then recircularized and introduced into bacterial cells—a process known as Bacterial Transformation. As the transformed bacteria multiply, the copy number of the DNA fragment inserted into the plasmid increases, thereby allowing foreign genetic material to be produced in substantial quantities (Fig. 4-66).

Fig. 4-66. Schematic representation of DNA cloning in bacterial cells.

Phages are frequently utilized as cloning vectors. When exogenous DNA is introduced into Eukaryotic cells, the procedure is referred to as Transduction.

3. Polymerase Chain Reaction

The polymerase chain reaction (PCR) method, proposed in 1983 by Kary Mullis (Nobel Prize, 1993), was a landmark Discovery of the 20th century in molecular biology. It makes it possible to specifically amplify DNA regions ranging from several tens to several hundred Base Pairs in length under in vitro (test-tube) conditions, using any DNA samples as a template. A prerequisite for PCR is knowing the nucleotide sequence of the region to be amplified. The region of the test DNA is hybridized with two artificially synthesized primers—oligodeoxyribonucleotide sequences from 15 to 30 base pairs long that are complementary to the 3' ends of the amplified region on the co-

ding and non-coding strands of DNA. The distance between the primers determines the length of the synthesized molecules. Any type of DNA can be used as a template for synthesizing PCR products: human genomic DNA, that of various pro- and eukaryotic species, DNA isolated from cell cultures, gene "libraries", and other sources. The method does not require large quantities of test DNA; in principle, even a single molecule contained in a single HEAD Hair, a single drop of blood, or semen is sufficient.

The successful Development of the method was largely due to the use of a thermophilic DNA polymerase as the enzyme, isolated from bacteria living in hot springs and therefore resistant to high temperatures.

The reaction mixture for obtaining the DNA of interest contains the test DNA, the reaction substrates—4 dNTPs, 2 primers, a thermostable or Taq polymerase, and a buffer containing Mg²⁺ ions.

One polymerization cycle includes 3 stages (Fig. 4-67): melting: at this stage, the reaction mixture is heated to 90 — 97 °C. The test double-stranded DNA denatures and converts into a single-stranded form; hybridization or annealing of DNA with primers. As the Temperature drops to 50-60 °C, complementary binding of the primers to the template DNA strands occurs, resulting in the formation of a double-stranded region on each of the DNA strands; elongation, the extension of DNA strands complementary to the template DNA, is catalyzed by Taq polymerase in the 5' to 3' direction.

Fig. 4-67. Polymerase chain reaction.

Then the melting stage occurs again, when, due to the temperature increase, DNA synthesis stops, and the double-stranded region between the template and newly synthesized DNA molecules denatures. In the second and subsequent cycles, the primers hybridize with the original template DNA and with the newly synthesized DNA molecules, the number of which increases exponentially. In the latter case, DNA synthesis ends not because of A change in temperature conditions, but when the DNA polymerase reaches the boundary of the amplified region, which determines a strictly defined product size with an accuracy of a single nucleotide.

Each cycle stage lasts from tens of seconds to 1 — 3 min, resulting in a complete cycle duration of one to several minutes.

The described DNA amplification procedure is carried out automatically in a device called a cycler, or thermocycler, a DNA amplifier. Such a device allows you to set the required number of cycles and select optimal time and temperature parameters. Over 25 — 30 cycles, the number of synthesized DNA copies reaches several million.

Using PCR, one can obtain a sufficient number of copies of DNA regions suspected of containing Mutations or site polymorphisms, and perform DNA diagnostics of patient infection with viral, bacterial, and fungal pathogens.

D. DNA Diagnostics of Diseases

Using recombinant DNA technology, researchers can study variant genes responsible for the development of many diseases. This method has identified point mutations caused by the replacement of a single nitrogenous base, deletions, or insertions leading to the appearance of alleles that encode functionally inactive proteins. Defective "polymorphs" arise both from Changes in the coding regions of a gene and as a result of mutations in non-coding areas that closely flank genes and disrupt their normal function.

The developed technologies allow for targeted mapping of human genes within the framework of the international Human Genome Project. This scientific program, involving leading molecular genetic laboratories in the USA, Western European countries, Russia, and Japan, was officially established in 1990. During the project, 923 genes causing Monogenic Diseases were mapped, and over 100 of them were completely sequenced. By the end of 2001, laboratories in the USA, Great Britain, Japan, and several European countries had completed genome decoding with 90% accuracy. It is expected that within the next 2 — 3 years, all genes responsible for pathological processes in humans will be studied. This will take the diagnostics and treatment of many diseases to a new level.

Let us dwell on some methods widely used to identify monogenic diseases.

1. Restriction Fragment Length Polymorphism (RFLP)

Mutations occurring in the recognition sites of certain restriction enzymes render these DNA regions insensitive to the action of the enzymes. This can be easily detected by a change in the length of DNA Restriction fragments. RFLP analysis includes the following steps: isolation of genomic DNA, its restriction with a specific endonuclease, electrophoretic separation of the resulting DNA fragments, and identification of these fragments by Southern blot hybridization. On electrophoretograms where no restriction has occurred in the test DNA, a single large fragment is revealed, corresponding in length to the DNA sequence between two adjacent restriction sites for the same endonuclease. If restriction occurs at the polymorphic site, the electrophoretogram will show a smaller fragment equal to the distance between the polymorphic restriction site and one of the nearest constant restriction sites (Fig. 4-68).

Fig. 4-68. Restriction analysis of DNA from a human with Sickle-Cell Anemia (HbS). The missense mutation responsible for sickle-cell anemia involves the replacement of the GAG triplet with GTG in the globin β-chain gene. This abolishes the restriction site for the enzyme MstII, which recognizes the sequence CCTNAGG, where N can be any base. In the presence of the mutation, the gene probe hybridizes with a larger 1.3-kilobase DNA fragment that exhibits lower electrophoretic mobility than the normal gene restriction product, which is 1.1 kilobases long.

RFLP analysis can be significantly simplified if specific amplification of the DNA region containing the polymorphic restriction site is feasible. Testing the state of this locus is possible by performing PCR and restriction of the amplified fragment. If the recognition site is absent in the studied DNA region, the size of the amplified fragment will not change after treatment with the restriction enzyme. If the recognition site is intact, treatment with the enzyme will yield 2 small fragments with the same total length as the original fragment.

When screening patients and their family members for carrier status of pathological genes, this method is widely used to:

✵ identify deletions in the dystrophin gene, which account for about 60% of all mutations causing Duchenne muscular dystrophy;

✵ diagnose hemophilia A, certain thalassemias, retinoblastoma, and granulomatosis;

✵ monitor the health of children in families where parents are heterozygous for the sickle-cell anemia gene and other defective genes.

2. Detection of Mutations Using Allele-Specific Probes

Many mutations responsible for genetic diseases do not fall within the sequences recognized by restriction enzymes. In such cases, if The base sequence around the mutation is known, it can be detected using allele-specific oligonucleotides. To this end, short oligonucleotide probes, typically about 19 nucleotides long and complementary to the normal and mutant allele regions in the DNA, are synthesized. The region of The Genome containing the target gene is amplified by PCR, and the resulting DNA samples are transferred onto nitrocellulose filters (dot or slot blotting). The samples are incubated with ³²P-labeled probes to identify the normal or mutant sequence. In individuals homozygous for the investigated mutation, the DNA will hybridize exclusively with the probe complementary to the mutant sequence. The DNA of a normal homozygous individual will bind to the probe corresponding to the unaltered nucleotide sequence, whereas both probes will hybridize with the DNA of heterozygotes. Figure 4-69 shows the results of gene probing in 7 patients for carriership of the most common 3-nucleotide deletion (ΔF508) in the gene responsible for cystic fibrosis.

Fig. 4-69. Gene probing for cystic fibrosis carriership. Genomic DNA is amplified by PCR, and the product is transferred to two nylon filters: one hybridized with a ³²P probe specific for the normal allele, and the second with a ³²P probe specific for the mutant ΔF508 allele. Hybrid formation is detected autoradiographically. Samples 1–3 served as controls: the first contained PCR products of DNA from normal gene homozygotes; the second, from a heterozygous carrier of the ΔF508 mutation; and the third, from a cystic fibrosis patient homozygous for the ΔF508 mutation. Samples 4–10 were obtained from the screening of 7 patients for ΔF508 carriership: samples 4, 5, 6, and 9 turned out to be normal gene homozygotes, while samples 7, 8, and 10 were heterozygous carriers of the mutant gene.

Oligonucleotides allele-specific for particular mutations can be used as primers in PCR for clinical population screening for pathogenic genes. If the DNA obtained from a patient undergoes amplification with the mutant oligonucleotide, it indicates that the patient is a mutation carrier. If the nucleotide sequence in the studied gene is unaltered, the mutation-containing oligonucleotide will not bind to the DNA template, and PCR will not proceed.

E. Application of DNA Technologies for Drug Production and the Treatment of Various Diseases

Vaccines — purified proteins representing the antigenic determinants of various viral and bacterial pathogens. Recently, they have been produced using recombinant DNA technology. The first vaccine synthesized by this method was the hepatitis B vaccine.

Therapeutically significant proteins are produced using this technology in many countries worldwide. For instance, human insulin was among the first to be synthesized (Fig. 4-70). E. coli cells transformed with plasmids containing DNA that encodes the A and B chains of insulin synthesize the respective protein products of these chains. Following purification, they undergo folding and oxidation to form the appropriate disulfide bridges.

Fig. 4-70. Production of Human insulin in E. coli cells. 1 — transformation of E. coli cells with plasmids containing genes encoding the structures of the A and B chains of insulin; 2 — synthesis of insulin A and B chains during the cultivation of transformed E. coli cells; 3 — Isolation and Purification of insulin A and B chains; 4 — spatial folding of the insulin A and B chains and oxidation of Cysteine residues.

Growth Hormone, used to treat children with growth hormone deficiency, has been obtained in a similar manner. More complex proteins are produced in mammalian cell cultures. For example, defects in the factor VIII gene, which encodes one of the proteins involved in Blood Coagulation, are responsible for hemophilia. Before factor VIII was produced via Genetic Engineering, A large number of patients died from AIDS or hepatitis contracted as a result of administering blood-derived factor VIII or receiving transfusions from Donors carrying these diseases.

Tissue plasminogen activator (tPA) — a protease involved in Fibrinolysis that prevents blood clot formation within the Circulatory system; it is produced using recombinant DNA technology. tPA is administered to patients with coronary artery disease to accelerate the dissolution of blood clots that can block CORONARY Arteries AND disrupt oxygen supply to the myocardium.

Recombinant growth factors ensuring the restoration of hemostasis—such as Erythropoietin, interleukins, and colony-stimulating factors—have also been successfully produced. These agents are used in the treatment of anemic patients, following Bone Marrow transplantation or Chemotherapy, to stimulate blood cell production and reduce the risk of immunodeficiency. METHODS FOR PRODUCING human proteins using Transgenic Animals have been developed; these proteins are obtained by artificially introducing a foreign gene into a fertilized egg or early mammalian embryos (Fig. 4-71). Genetic engineering techniques can be designed such that the human protein of interest is secreted into milk proteins.

Fig. 4-71. Use of transgenic animals for the production of human proteins. The human gene is inserted into a vector under the control of the β-lactoglobulin promoter, which is active exclusively in mammary gland cells. The presence of the human gene in transgenic offspring was monitored using PCR with primers specific to the human gene. Fractionation of milk proteins yields the protein product of human gene expression.

Gene therapy — the Treatment of Hereditary, multifactorial, and infectious diseases by introducing genes into patient somatic cells to correct genetic defects or impart new Functions to the cells.

The first clinical trial of gene therapy was conducted in 1990 in Bethesda (USA) on a four-year-old girl suffering from severe combined immunodeficiency caused by a mutation in the adenosine deaminase (ADA) gene. The child was administered her own lymphocytes, which had been previously transformed ex vitro with a gene construct comprising the ADA gene + the neo gene + a retroviral vector. A therapeutic effect was observed for several months, after which the gene administration procedure was repeated multiple times without apparent adverse effects.

For successful gene therapy, it is necessary to:

✵ ensure efficient delivery of the foreign gene into target cells;

✵ establish conditions for long-term gene expression within these cells.

To date, chemical, physical, and biological methods for delivering foreign genes into target cells have been developed. However, currently only viral vectors or genetic constructs containing viral sequences are capable of efficiently delivering the required gene and ensuring its subsequent long-term expression. Consequently, of the more than 175 already approved clinical gene therapy protocols, over 120 are based on the use of retroviral vectors.

Foreign DNA can be introduced into a patient's genome either ex vivo (in cell culture) or directly in vivo (into the patient's body). In the first approach, a specific cell type is isolated from the patient and cultured, a foreign gene is introduced into it, the transformed cells are selected, and they are reinfused into the same patient (Fig. 4-72).

Fig. 4-72. Ex vivo (A) and in vivo (B) delivery of a foreign gene. A. Introduction of a foreign "therapeutic" gene into the patient's body as part of cells containing this gene. B. Introduction of the "therapeutic" gene as part of a construct comprising: DNA including this gene; a protein (e.g., asialoglycoprotein) that interacts with a corresponding receptor on The Cell membrane; and a viral vector (adenovirus) ensuring long-term expression of the "therapeutic" gene.

In Vivo Gene therapy is based on the direct administration into specialized patient Tissues of cloned and specifically packaged DNA sequences that enter specific cell types via receptor-mediated uptake. In this approach, genes are typically administered as aerosols or injectables. Aerosol gene therapy is most frequently used in the treatment of pulmonary diseases (e.g., Lung Cancer) and cystic fibrosis.

Alongside the advancement of research concerning the treatment of hereditary disorders, gene therapy is increasingly being utilized to treat non-hereditary conditions, primarily infectious and oncological diseases (see Section 16).

The sole and mandatory restriction on such work is that all gene therapy Procedures must be directed exclusively at a specific patient and affect only their somatic cells.

The Current state of knowledge does not permit the correction of gene defects at the level of human Germ Cells and early pre-implantation embryos due to the real danger of contaminating the gene pool with unwanted gene constructs and introducing mutations with unpredictable outcomes.

To prevent the spread of defective genes in the human population and the birth of children with hereditary disorders, Genetic Counseling services operate in many countries worldwide, alongside prenatal diagnostics that enable the assessment of fetal health using DNA analysis at the earliest Selection/3.html">Stages of development.