Gene Therapy

Key Concepts:

• Genes
• Genetic engineering
• Genetic disorders
• Types of genetic disorders
• Gene therapy
• Types of gene therapy
• Process of gene therapy
• In vivo and ex vivo delivery
• Advantages of gene therapy
• Disadvantages of gene therapy
  

Introduction:

Genes are the basic unit of heredity that controls all the traits in an organism’s body. A gene is a piece of DNA found on a chromosome. They are arranged along the length of a chromosome in the nucleus. Chromosomes are the thread-like structures present in the nucleus that contain directions for cell activities. 

Genes are the specific sequences of nitrogenous bases that code for information about how to make proteins. The proteins are the molecules responsible for performing most of the life functions and also make up the majority of the cellular structures. 

Explanation: 

Genetic engineering: 

Genetic engineering, often also known as genetic modification, is the process by which alterations are made in the genome of an organism. In genetic engineering, the genetic sequence in DNA is changed intentionally so that a particular trait is produced. 

Genetic engineering can be done by replacing one base pair (A-T or C-G), deleting an entire section of DNA, or introducing an extra copy of a gene. It also entails extracting DNA from another organism’s genome and combining it with that individual’s DNA. 

Genetic engineering comprises various methods or types which help alter an organism’s DNA to produce desired traits. 

Recombinant DNA is one such method of genetic engineering in which recombinant DNA is made by taking a useful segment of DNA from one organism and inserting it into another organism. 

Gene therapy is also a kind of genetic engineering used to treat or prevent disease by correcting the underlying genetic disease. 

Genetic disorders: 

A genetic disorder is an illness caused by a change or a mutation in the DNA sequence of an individual. As a result of such alterations, the encoded proteins are unable to perform their regular functions. 

Mutations are caused due to errors in DNA replication or environmental factors such as cigarette smoke or exposure to radiation. 

Every individual carries about half a dozen of defective genes. About one in ten people have or will develop an inherited genetic disorder at some later stage. 

Approximately about 2800 specific conditions are known to be caused by genetic defects in just one of the patient’s genes. All mutations do not alter the biology of cells in our body and result in a genetic disorder. Mutations can sometimes have little or no effect. 

Most of us are unaffected by the harmful effects of the defective genes that we carry because we possess two copies of nearly all genes, one that we inherit from our mother and one from our father. 

The only exception to this is the male sex chromosomes, as they have one X and one Y chromosome. The X chromosome is passed down/inherited from the mother, and the Y chromosome is passed down from the father. 

Types of genetic disorders:  

1. Single-gene disorder:

The disorders that are caused by defects in one particular gene, often with a simple and predictable inheritance pattern, is called single–gene disorder. These are further divided as follows: 

• Dominant disorders: A single gene disorder that occurs when an individual possesses one altered copy of a particular gene and one healthy copy. For example, Huntington’s disease. 

• Recessive disorders: A single gene disorder that only occurs when an individual possesses two altered copies of the relevant gene. For example, cystic fibrosis. The probability of an individual having a recessive disorder increases when two closely related people have a child together.  
• X-linked disorders: A single-gene disorder that is caused due to an altered gene on the X chromosome. X-linked disorders are mostly observed in males because they only have one X chromosome. As a result, males only need one copy of the altered gene to develop symptoms. For example, muscular dystrophy. 

2. Chromosomal disorder:

The disorders that are caused due to a change in the number (increase or decrease) or structure of chromosomes. For example, Down’s syndrome, in which there are three copies of chromosome 21. 

3. Multifactorial disorders (complex diseases):

The disorders that are caused by alterations in multiple genes, often in a complex association with environmental and lifestyle influences such as diet or cigarette smoke. For example, cancer. 

Gene therapy: 

Gene therapy is when DNA is introduced into a patient’s cells and tissues in order to treat a genetic disorder. The new DNA usually contains a functional gene to replace the effects of a disease-causing mutated gene. 

This technique was first developed in 1972 and was successfully accomplished in the year 1989, but so far has had limited success in treating human diseases. Gene therapy enables doctors to treat a disorder by altering a person’s genetic makeup instead of using drugs or performing surgery. 

What can gene therapy achieve? 

It can replace a mutated gene causing the disease with a healthy and functional gene. The new gene produces a functional product in sufficient quantities to replace the missing protein. 

It can “inactivate” or “knock out” a disease-causing mutated gene that is functioning improperly. It eliminates the activity of a gene that encourages the growth of disease-related cells. 

It can introduce a new gene into a body to help fight diseases. 

It can cause the mutated cell to die by inserting DNA into a diseased cell. The inserted DNA promotes the expression of a protein that marks the cells allowing the body’s natural immune system to fight the diseased cells. 

Types of Gene Therapy: 

Depending upon the types of cells that are treated, there are two different types of gene therapy: 

Somatic gene therapy:

Occurs in the somatic cells of the human body. A section of DNA is transferred to any cell of the body that doesn’t produce sperm or eggs.  

It is related to a single person, and the effects of gene therapy will not be passed on to the patient’s offspring. This technique is considered the best and safest method of gene therapy.  

Germline gene therapy:

Takes place in the germline cells of the human body. This method is utilized to treat genetic disease-causing variations of genes that are passed from parents to their children.  

A section of DNA is delivered to the cells producing eggs or sperm. Effects of gene therapy will be passed down to the patient’s children and future generations. Germline gene therapy is not legal in many places as the risks outweigh the rewards. 

How does gene therapy work? 

In gene therapy, a normal allele is placed in a virus that acts as a vector. A vector is genetically engineered to deliver genes and acts as a vehicle to insert new genes directly into cells.  

The vector then delivers the normal allele into the patient’s target cell. The target cell becomes infected with the vector. 

The normal allele replaces the genetic material of the defective allele. Functional proteins are thereby created from the therapeutic gene, which causes the mutated cell to return to its normal state and treat the patient’s disease. 

Types of delivery systems: 

 There are two processes by which gene therapy is executed which are as follows: 

1. In-vivo delivery:

It refers to the direct delivery of functional genes either intravenously or locally to a specific organ. The target cells or tissues must be easily accessible for injecting the new genetic material. 

• In-vivo uses a vector to directly insert functional copies of a gene into targeted cells to treat a mutated or missing gene.  
• This method may be preferred for genetic disorders affecting a specific gene in the body and is also thought to be preferred when the target is an internal organ, like the heart. 
• In the in-vivo method, it’s easier to target internal organs. It’s also less complicated since cells don’t need to be removed from a person at all. 

2. Ex-vivo delivery:

It refers to the process of removing specific cells from a patient, genetically altering them in a laboratory, and then transplanting them again into the patient. 

• Ex-vivo gene therapy uses a patient’s stem cells and modifies them genetically to replace the target cells having a mutated or missing gene.  
• This method may be preferred in cases of blood disorders. This includes certain cancers, such as leukemia and lymphoma, as well as genetic disorders. 
• The ability to control the entire process of creating new genetic material is an advantage of ex-vivo gene therapy. Ex-vivo methods can be used to test the health of new genetic material before it is given to a person. However, preparing cells outside the body is a time-consuming and complicated process. 

Advantages of gene therapy: 

1. Gene therapy has the prospective to eliminate and prevent hereditary diseases such as cystic fibrosis and ADA-SCID. 
2. It could possibly cure heart diseases, AIDS, and cancer. 
3. It may avoid the need for constant medication and extend life expectancy. 
4. Carriers of a genetic disorder may be able to produce offspring without the disorder. 
5. It enables scientists to have a better understanding of how genes work. 

Challenges of gene therapy: 

1. Delivering the gene to the exact location and switching it on: 

• It is important that the new gene reaches the right cell. If a gene is delivered into the wrong cell, it would be inefficient and also cause health problems for the patient. 
• Even if the right cell has been targeted, the gene has to be turned on as cells sometimes block this process by shutting down genes that are showing unusual activity. 

2. Avoiding the immune response: 

• Sometimes genes that are newly introduced by gene therapy are regarded as potentially-harmful intruders and can generate an immune response in the patient that could be harmful to them. 
• As a result, scientists have to develop a way to deliver genes without the immune system ‘noticing.’ This is usually achieved by using vectors that are less likely to trigger an immune response. 

3. Ensure that the new gene doesn’t disrupt the function of other genes: 

• A new gene introduced through gene therapy should ideally integrate into the patient’s genome and function for the rest of their lives. 
• There is a threat that the new gene will get in the path of another gene, disrupting its function. This could have adverse effects; for example, if it interferes with a significant gene involved in the regulation of cell division, it could lead to cancer. 

4. The cost of gene therapy: 

• Many genetic disorders that can be treated with gene therapy are extremely uncommon. 
• Due to this, gene therapy often demands an individual, case-by-case approach. This could be beneficial but may also be very expensive. 

Summary:

• Genetic engineering is the process by which alterations are made in the genome of an
  organism intentionally so that a particular trait is produced. Gene therapy is a type of
  genetic engineering used to treat or prevent disease by correcting the underlying genetic
  disease.
• A genetic disorder is a disease caused by a change or a mutation in the DNA sequence of
  an individual. As a result of such alterations, the encoded proteins are unable to perform
  their regular functions.
• Genetic disorders can be of three types, namely, single-gene disorders, chromosomal
  disorders, and multifactorial disorders.
• Gene therapy is when DNA is introduced into a patient’s cells and tissues in order to treat
  a genetic disorder. The new DNA usually contains a functional gene to replace the effects
  of a disease-causing mutated gene.
• Depending upon the types of cells that are treated, there are two different types of gene
  therapy, i.e., somatic gene therapy, which occurs in the somatic cells of the human body,
  and germline gene therapy which takes place in the germline cells of the human body.
• In gene therapy, a genetically engineered vector carrying the normal allele delivers the
  gene directly into cells, and the target cell becomes infected with the vector. The normal
  allele replaces the genetic material of the defective allele and produces functional
  proteins.
• There are two methods for introducing new genes into target cells. The first method is in-
  vivo delivery, in which functional genes are delivered directly to a specific organ either
  intravenously or locally. The other method is ex-vivo delivery, which involves extracting
  specific cells from a patient, genetically modifying them in a laboratory, and then
  reinserting them into the patient.
• Gene therapy can prevent hereditary diseases such as cystic fibrosis and ADA-SCI
  could possibly cure heart diseases, AIDS, and cancer.
• Limitations of gene therapy include proper delivery of therapeutic genes, avoidance of
  immune response against therapeutic genes, and cost of gene therapy.