Abstract
Many patients have been able to be treated from complicated diseases due to the improvement that has
been achieved in the field of modern medicine.[1] However, some diseases have become quite a
challenge to the medical world as they do not have a cure. Most of these incurable diseases are as a
result of genetic disorders or deformities. Genetic disorders are not only inherited but can also be
acquired through a number of mutations that occur in the gene resulting in a changes on a number of
gene factors. Cystic fibrosis is such a genetic disorder and it occurs when there is a singular gene
nutation on a chromosome. Another disorder from gene mutation is multiple sclerosis that is linked
to multiple mutations on a chromosome [2]. The two disorders do not have a cure and the treatment
used on such patients are only to manage the available symptoms and not to treat the disease.
However, there also a number of diseases that are not caused by chromosome abnormalities alone.
Introduction
The problem of genetic disorders can be treated using Gene Therapy. The concept has been used since
1989 and currently, technology is being used to help in progressing the process. This has led to a
number of treatment procedures that have been used to treat patients that they have been approved in
the United States. [1] It is evident from researchers that gene therapy can be used to treat genetic
disorders and at the same time prevent the symptoms that arise from these genetic disorders. Hence,
the field of gene therapy has become an important advancement in modern medicine.
Gene therapy can be defined as a field of science that genetically modify cells in order to achieve therapeutic effects. The concept used in the process is to introduce a gene into an organism. This introduced gene has the ability to cure the target disease or prevents its progression. This new gene is a normal gene though it performs the same function as the defective gene in the cell.[3] The main purpose of gene therapy is to correct genetic defects, remove cancerous cells and infectious pathogens and prevent cardiovascular diseases. The main types of gene therapy are somatic cell gene therapy and reproductive gene therapy. In germline therapy, functional genes are introduced in the cell to modify the germ cells thus it ensures that the gene therapy can be inherited by the descendants. However, in somatic gene therapy, the functional genes are introduced to a specific organ in the body. Thus, any changes will be restrained to the patient only and cannot be inherited. In this type of gene therapy is to modify the cell’s genetic material in a way that benefits the patient. However, in order to understand the process of gene therapy, knowledge of genes is important.
Background
There are masses of cells in the human that form the building blocks of any living thing. Each cell
has a nucleus that is made of chromosomes. The chromosomes are made up of deoxyribonucleic acid
(DNA) that is responsible for hereditary features. The DNA makes up the gene. A gene is a series of
nucleotides that are found on the DNA. It is a segment of the DNA. Gene encode for proteins in that
if the production of the functional protein is insufficient then it results to a defective gene that
would lead to a number of disorders such as cancer and hemophilia B. Thus, the function of a gene is
to store information that is used to create a unique protein that an organism will need. Genes have
long unique names and thus are assigned symbols.
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An individual will inherit some features from both parents as each person is made of 2 copies of each gene from each parent. The DNA is passed to the child during reproduction. However, each person has their own genes that are different from that of the parent and this results to a child having some unique features that are different from the parents’ features.
The instructions in the DNA are transformed into a purposeful product through a process called central dogma that was invented in 1958. This is the flow of information stored in the DNA to the RNA and then converted to a protein which is the functional product. The nucleotides in the DNA are in two chains that wind on each other. Each nucleotide is made of sugars and phosphates. RNA is similar to DNA. However, RNA is used to create and break molecules and silence genes. The DNA holds all the information needed in the creation of the protein while the RNA acts as the messenger to this information. The process of converting the information from the DNA to a protein is called gene expression and it involves transcription and translation. Transcription is the process by which the information is converted into a form that can be transported by RNA. On the other hand, translation is the process in which the messages in the RNA are decoded to create a unique protein. Thus, the processes involved in central dogma are DNA replication, transcription and translation.
Disorders of the human genome are called genetic diseases or disorders. The human genome is what is used in human hereditary. A number of genetic disorders arise from gene mutations. However, inheritance complex patterns in some gene often cause certain diseases such as cancer and mental illness. This is because such diseases might require more than one gene mutation for it to manifest. These mutations can increase a person’s vulnerability to the said disease.
Gene mutation can be trough substitution, duplication, and elimination. Regardless of the type of mutation, the process removes a gene form the cell or causes a protein to be inefficient in performing its functions. Mutation can also be inherited from a parent to the child. Although some mutations are harmless, there are some that cause genetic disorders. There are four commonly known types of genetic disorders.
Single gene genetic inheritance is the first genetic abnormality. It is commonly known as monogenetic disorder and these are mutations that occur on a single gene’s DNA sequence. Cystic fibrosis and sickle cell anemia are the most common disorders that are known in this category. However, there other diseases such as hemochromatosis and Huntington’s disease.
The second type of disorder is Multifactorial genetic inheritance that results from mutations in a
number of genes and a mixture of different environmental factors. It uses certain human properties
such as eye and skin color and fingerprints. An example is breast cancer, diabetes, obesity, heart
disease, and Alzheimer’s disease.
The third type of genetic disorders is chromosome abnormalities that result from problems during
cell division. These orders include Down syndrome, Klinefelter syndrome and Turner syndrome.
The final type of disorder is the mitochondrial genetic inheritance that results from mutations from the genes in the mitochondria. The mitochondria are responsible for respiration and each one is made up of about 5-10 molecules of DNA. Examples of mitochondrial diseases are an eye disorder called Leber’s hereditary optic atrophy, a type of dementia called mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes and myoclonic epilepsy with ragged red fibers.
Gene Therapy history and initial challenges
Gene therapy is not a new concept as it has been for more than 50 years. It is just evolving as it
has a lot of potentials to find cures to the numerous genetic disorders. Over the years, human has
evolved in order to adapt to various conditions and this has resulted in a number of genetic changes
in the human body. However, since the beginning, there has always been characteristics that children
inherit from their parents and early ancestors. Gregor Mendel was the initiator of genetic studies
through his experiments on green peas. He explained the inheritance patterns through the
observations in his experiments. The DNA’s double strand was later on invented by two scientists,
Francis Crick and James Watson. In 1970, researchers were able to discover ways of separating genes
and this is when genetic engineering started. This led to the manufacturing of new medicine and
antibodies.
Joshua Lederberg introduced the concept of gene therapy in 1963 but research in the field started in 1980. In 1990, Anderson et al were able to have a successful clinical gene therapy in which they treated a patient with adenosine deaminase deficiency by adding the gene to the white blood cells. In the same year, Rosenberg treated 5 patients affected with metastatic melanoma by introducing a neomycin resistance marker gene through a retroviral vector to the lymphocytes that had the tumor. This led to the conclusion the delivery of retroviral gene was safe and sensible and it led to a number of research on gene therapy later on.
One of the initial strategies in gene therapy was Ex vivo gene therapy that targeted a specific organ and was mainly used to treat blindness. However, due to its inability to treat inner disorders, in vivo therapy was developed in which genetic material was used to alter a target of cells in order to correct a disorder. The therapy is still under research and is being developed to treat both hereditary and acquired diseases.
Gene therapy was initially incorporated by scientists in 1980. However, most the gene therapy techniques are still experimental with trials being carried out in Australia, the U.S., and Europe. The trial is being carried with the sole purpose of finding the cure to recessive gene disorders and a number of viral infections. The most common technique is the recombinant DNA technology. In this technique, a gene that is healthy is introduced to a vector. Gene therapy is quite complex and requires continuous improvement for any more cures to be invented. This includes ensuring that all the need cells are available and an understanding of the gene copies and their unique genetic bonds.
There are a number of challenges that are faced in gene therapy in which a normal gene is inserted in a cell to replace an abnormal cell. The most significant challenge is letting the gene into the stem cell. A vector is often used for the process. A vector is a molecular carrier. The vector is an altered virus that does not have any viral genes. Thus, it able to safely transport the intended gene to its location in the cell without any effects. Once it gets inside the cell, the functional protein is restored. Vectors are inserted in the body through intravenous such as an injection to a specific tissue or dispensed into a vein. However, it can also be administered through surgery. The vector has to be specific and efficient in letting go of the carrier gene. Likewise, the vector must not be recognizable by the immune system and can only be cleansed in big quantities. These properties are important in preventing any allergic reactions and ensure it achieves its purpose of correcting abnormalities. These vectors are efficient but also have their own limitations. The vector can provoke serious immune response due to its aggravating factor.
Gene transfers are mostly carried out in the hematopoietic stem cells as this cell a high longevity probability and can renovate itself. An example is the induced pluripotent stem cells (iPS) that help treat patients with the hepatitis virus and liver diseases. Treating these viruses involve transplant from iPS that converts the cells to hepatocytes. Another different typing of therapy is a technique used in immune therapy in which the immune cells are reprogrammed. This therapy was developed to help in identifying the tumor-specific gene, it has improved from 1st generation and not it’s on the third improvement of the technique. The third generation has better ability to activate the cell that before increasing its action and thus regulated the cell cycle.
Another technique in gene therapy is the Clustered Regularly Interspaced Short Palindromic Repeats and associated (CRISPR-Cas9). It is has a technique that has been used since 2012 and it is a well-known technique for gene edition. In CRISPR-Cas9, the mechanism recognizes the abnormal genetic material, breaks it down to smaller particles and then integrate it to its own DNA. If there is a second infection through the same abnormal DNA then the CRISPR-Cas9 follows a sequence. The first step in transcription that occurs in the CRISPR locus and the RNSm is processed. Finally, small particles of RNA are created and these integrate with Cas proteins to form a complex that eliminates the foreign gene.
CRISPR works by modifying the specific DNA sequence through three fragments. The first molecule is the nuclease (Cas9) that creates the DNS’ double-strand cleavage. The second part is the RNA guide which directs the nuclease to the targeted gene and the final molecule is the target DNA. The CRISPR system is quite a simple and precise technique of gene therapy compared to the other techniques. It engages gene editing by inactivation, elimination, and substitution.
The rapid advancement in technology has resulted in a number of trials using CRISPR to edit genes. A number of researchers in the United States have tried and succeeded in correcting the disorder in the hemoglobin gene that results in sickle cell anemia. CRISPR-Cas9 modified the CD34+ cells of sickle-cell anemia victims. In a period of 16eeks, the mutated gene reduced while wild type increased.
The main challenge experienced in gene therapy is the ethical concerns around genetically modified
organisms. Most researchers do not have a problem with the techniques used in gene therapy in
treating a number of life-threatening disorders. The CRISPR-Cas9 was announced in 2015 by the
Chinese researchers amid ethical discussions as a technique for altering embryonic cells
genetically. This was followed by another group of researchers from China who also used CRISPR-Cas9
to try a find an HIV resistance through by adding CCR5 gene mutation. They used 26 embryos and were
able to modify four of them. This indicates that is more trials are done then the technique could be
improved and there would a cure for HIV.
Gene therapy can be used to treat cancer. T6his was first done in China in 2004. Gene therapy has
been successfully used to treat leukemia. Likewise, it is used in treating prostate cancer.
Additionally, there are clinical trials that are targeting skin cancer and breast cancer. Similarly,
apart from cancer, gene therapy is targeting other disorders such as cystic fibrosis, AIDS and
hemophilia. The process is still in its development stage. There was a clinical study in which CCR5
receptor genes were introduced on an HIV patient and the levels of the disease in the patient
reduced. However, it was not a complete success as the introduced cells had a short life-span and
could not act as a long-term cure.
The main challenge in gene delivery is the determination of the most suitable virus to be used, any virus to be used in gene therapy has to be modified to ensure it does not cause any side effects to the patient. Thus, the process of determining a safe virus is taking up a lot of resources in terms of time and money. Another challenge is immune rejection. The human body is structured to attack or discard any alien DNA in the body. Thus, getting past this immune response has been quite a challenge.
However, in gene specialty, the main challenge is developing a long-term cure. Inserting a missing gene in the body does not guarantee that the gene will stay there. There are a number of processes that take place in the body and thus as the cells divide, there is the possibility that the inserted gen would not be reproduced. Hence this may result in the patient needing more than one treatment. Another challenge is the ethical issues around gene therapy. Most germ line gene therapy are not ethical as they modify even the future generations’ genes without their consent. The final challenge is the expensive cost of gene therapy. It costs $1.6 million to do the first gene therapy in 1970 and the year’s cost is increasing as technology is evolving.
Gene therapy research has given hope to a number of genetic disorders patients and with its future potential, it could be used to eliminate a number of disorders across the globe. As much as it was initially invented a mode of treatment to inherited monogenic diseases, it has evolved and is being used to treat various chronic disorders.
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