Questions such as “What is a point mutation and how is it relevant when it comes to gene editing?” are at the core of genetic research. Knowing this is extremely important for a clear understanding of mutations. Point mutations are typically known to be caused during DNA replication, and are thus a product of other genetic processes.
Although most point mutations are considered to be more or less benign, there is usually a risk that they can lead to loss of protein function and ultimately, to various diseases. They can be random and even lethal in some cases. Depending on what caused the mutation, what type of mutation it is and how the organism responds to it, there are many potential repercussions that could affect the organism in question, some more negative than others.
A single point mutation occurs when a single base pair is substituted with another within the DNA, leading to numerous possibilities with regards to mutations at the protein level. Depending on the nature of the change, the resulting codon expression can vary from the original, leading to a significant change in the final resulting protein. Such missense amino acid changes can lead to an energetic and chemical balance throughout the entire organism. As a result, the protein losing its main function can cause disease. In many cases, the development of a similar or the same amino acid will not lead to anything serious. However, when it comes to missense mutations, this isn’t the case, and cancer is one of the main disorders that can result from this type of change.
Single point mutations can and have been used for a variety of different applications. In most cases, they are used in transgenic mice to help develop simple, easy to obtain but also highly targeted changes in the DNA encoding that accounts for certain diseases or their absence. As a result, the use of a single point mutation can lead to anything from the development of prostate cancer to an increase in immune system deficiencies, which can then be used to develop new potential treatments. For example, sickle cell anemia mice have single mutations consisting of mutations of Antilles b23-I and D-Punjab b121-N. These are known to enhance the polymerization process in sickle cell patients being bS-Antilles heterozygotes or compound S/D- Punjab heterozygotes.
Point mutations can occur not only in the DNA, but in the RNA as well. In humans, the double-stranded DNA and RNA are complementary in their function of generating the vital amino acids required for cells to thrive. However, when a multiple or single point mutation occurs, the result can lead to an alteration (or more) that can be difficult to trace or to fully understand. The difference between multiple and single point mutations is that the point mutation only alters one base of nucleotides, while multiple mutations can also affect entire sections of chromosomes. Multiple point mutations can also occur in a single strand of DNA or RNA.
So what is a point mutation, and why is it important when using animal models in research? A point mutation mouse is a knockin mouse line in which one or more nucleotides in the mouse genome are substituted by variant nucleotides.
This mutation can result in either an in-frame amino acid change of protein sequence or a frameshift mutation. The base pairs can either be deleted or added to the organism’s genetic code through a variety of means. Some might involve DNA replication, while others have to do with the effects of radiation, such as UV or X-rays. The end result and its complications aren’t always fully foreseeable, although new methods for predicting point mutations have improved in recent years. They can range between synonymous mutations (which are typically benign) to more complex frameshift mutations.
The most serious impairment of gene function as a result of a point mutation will typically depend on the location of the mutation itself. If the mutation occurs in an area responsible for protein encoding then the damage can be severe, potentially leading to the loss of stability and/or function within the protein complex.
There are three types of DNA mutations: base substitutions, deletions and insertions.
Base Substitutions: When considering what a point mutation is, it’s also important to note that there are two other types of point mutations depending on whether there is a purine or pyrimidine base. The latter is known as a transition mutation, and the former is commonly named a transversion mutation. An example of a transition mutation is a GC base replacing a naturally occurring AT base pair. When it comes to transversion mutations, a pyrimidine base is substituted by a purine base. In most cases, that translates as a TA or CG pair replacing a wild type AT pair.
Deletions: A deletion, resulting in a frameshift, occurs when one or more base pairs are lost from the DNA. If one or two bases are deleted, the translational frame is altered resulting in a garbled message and nonfunctional product. A deletion of three or more bases leaves the reading frame intact. A deletion of one or more codons results in a protein missing one or more amino acids. This may be deleterious or not. An example in which a deletion of three nucleotides takes place in a recessive inherited disorder is cystic fibrosis. This deletion occurs in the cystic fibrosis transmembrane conductance regular gene, resulting in the loss of the amino acid phenylalanine and causing an incorrectly folded protein.
Insertions: The insertion of additional base pairs may lead to frameshifts depending on whether or not multiples of three base pairs are inserted. Combinations of insertions and deletions leading to a variety of outcomes are also possible. For example, if a sequence of codons in DNA is normally CCT ATG TTT and an extra A is added between the two cytosine bases, the sequence will instead read CAC TAT GTT T. This completely changes the amino acids that would be produced, which in turn alters the structure and function of the resulting protein and can render it useless.
A point mutation can develop when a double stranded DNA molecule creates two separate single strands. In addition, radiation and chemical reactions can result in a point mutation when the reactions are severe enough. Environmental properties such as extreme heat and other temperature changes may also be a factor. High frequency light can affect DNA due to its influence on ionizing electrons. This can lead to reactions between oxygen molecules and free radicals, which can be extremely detrimental to the DNA molecule.
Cystic Fibrosis
Cystic Fibrosis (CF) is a recessive inherited disorder most common among people of European descent. In the United States, 1 in 3500 newborns are born with cystic fibrosis, and 1 in 30 Caucasian Americans is a carrier. There are many different mutations that can cause CF, but the most common one is a deletion of three nucleotides in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that results in the loss of the amino acid phenylalanine and causes an incorrectly folded protein. (Note that this deletion is not a frameshift mutation because three bases next to each other are deleted, and all the other amino acids in the chain remain the same.) CF is associated with thick, sticky mucus in the lungs and trouble breathing, salty sweat, infertility in certain individuals, and a shortened life expectancy (about 42-50 years in developed countries).
Sickle-Cell Anemia
Sickle-cell anemia is a recessive disorder caused by a single substitution in the gene that creates hemoglobin, which carries oxygen in the blood. Normally, glutamic acid is produced in the chain, but the substitution causes valine to be produced at that spot instead. When people have two copies of this mutation, it results in thin sickle-shaped blood cells that sometimes cannot carry oxygen properly. About 80% of people with sickle-cell disease are in sub-Saharan Africa, where being a carrier for sickle-cell anemia (having only one copy of the gene, not two) actually helps protect against malaria. It is also found in other parts of the world such as India and the Middle East, and affects about 1 in 500 African Americans. Symptoms include anemia, obstruction of blood vessels, and chest pain, and it is treated with folic acid, blood transfusions, bone marrow transplants, and certain prescription drugs.
A good example of what a single point mutation can lead to happens in the case of converting the GAG codon into GUG. This would lead to the encoding of amino acid valine instead of glutamic acid. In some cases, activation of the RAF protein is also possible due to the protein exhibiting increased function. When that happens, unlimited proliferative signaling in cancer cells can lead to the development of a severe form of cancer.
Another example could be chemical damage to DNA. There are many chemical mutagens that are exogenous, man-made, or environmental that are capable of damaging DNA. Many chemotherapeutic drugs and intercalating agent drugs function by damaging DNA.
Not all mutations are neutral or negative. In some cases, they can lead to positive changes, such as germline mutations that may protect the organism and will then be passed down through multiple generations. In this case, point mutations can actually be a viable explanation for the theory of evolution, to explain how certain changes occurred in the genetic makeup of evolving organisms.
With modern sequencing and genomic engineering technologies, the precise mutation(s) underlying human disease can be introduced into mice, yielding more accurate and useful disease research data. Through the years, the mouse model has proven to be the best way to reproduce human disease when due to mutations. There has been high physiological relevance of the scientific data obtained from the mouse model and it is a cleaner way than the classical knockout mouse model where the whole gene is deleted. In addition, the phenotype is due only to the mutation: alteration of a single function without disturbing other domains of a protein.
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