Studying genetic diseases: Finding out about the genes
Certain diseases are commonly found in members of the same family. This indicates that a particular gene variation, or gene combination, might be involved.
A person's genetic makeup is called their genotype. The genotype affects their physical characteristics, or phenotype.
Single nucleotide polymorphisms - a single change in the DNA code
Some diseases seem to run in families, suggesting that there might be a genetic link. Sometimes the DNA change only involves a single base. This is called a single nucleotide polymorphism (SNP).
Identifying SNPs, involves screening the DNA of thousands of different people, including those who have the disease and those who don't. Computers can then be used to check for particular DNA sequences that are found only in people who have the disease.
Currently there are about eight different SNPs known to increase the risk that a person will develop Crohn's disease.
Single nucleotide polymorphisms - a single change in the DNA code
Some diseases seem to run in families, suggesting that there might be a genetic link. Sometimes the DNA change only involves a single base. This is called a single nucleotide polymorphism (SNP).
Identifying SNPs, involves screening the DNA of thousands of different people, including those who have the disease and those who don't. Computers can then be used to check for particular DNA sequences that are found only in people who have the disease.
Currently there are about eight different SNPs known to increase the risk that a person will develop Crohn's disease.
Researchers are now able to identify particular gene variations that might increase a person's susceptibility to getting a particular disease. The aim of this is to provide information to help with disease management.
First the researchers need to study the DNA from a huge number of people who have a disease, and those who don’t. The DNA patterns between these two groups can then be compared. The researchers will be looking for particular DNA patterns that only appear in people who have the disease. Often the change only involves a single base in the DNA code.
The DNA is collected from people who have agreed to participate in the study. Usually a blood sample is collected. The DNA is separated from the white blood cells and stored. Often it will be separated into at least two containers, which are stored in different places (one can act as a back-up if necessary). The samples are also coded, so that anyone working with the DNA sample has no idea which person the DNA comes from.
Once the DNA has been purified, millions of copies are produced using the polymerase chain reaction (PCR). This is important because a lot of DNA is needed for results of DNA tests to be seen – more than what is collected from the blood.
Researchers can then use two different techniques to study the DNA and look for gene variants: gel electrophoresis and DNA sequencing.
Using gel electrophoresis to find gene variants associated with a disease
To use gel electrophoresis, the DNA is first cut into pieces, called DNA fragments. Specific enzymes are used to do this. A selected enzyme will always cut the DNA from a particular person in the same places. It will cut the DNA from two different people in different places. This means that the sizes of the DNA fragments produced are unique for each person. The DNA fragments are then separated according to size using gel electrophoresis.
Using DNA sequencing to find gene variants associated with a disease
DNA is composed of bases, called A, T, C and G. These bases are joined together in specific sequences. Each person has a unique sequence of bases.
Genetic testing: Getting the DNA
Researchers can use DNA samples to find genetic combinations that might increase the risk of getting a particular disease. The DNA is often collected from the white blood cells in a blood sample. About 10 ml of blood is collected from patients involved in the study. They patients know that DNA studies are going to be done, and have given their permission to the researchers. It is important that the blood samples are 'anonymised', or coded. The laboratory number and person's name is kept in a database - this is so that the researchers don't lose track of the original identity of the samples. The blood is also divided into at least two samples so that some of it can be stored in a separate place to provide original material in case test results show up something interesting that needs to be checked.
DNA sequencing is used to determine the base sequence of parts of the DNA samples. The sequences from people who have the disease are compared with those who don’t have the disease. Researchers look for specific sequences that are only found in people who have the disease.
Find out more in the article, DNA sequencing can be used to find genes associated with a disease.
Susceptibility to a genetic disease
Most diseases that have a genetic component are caused by variations in more than just one gene region. Environmental effects such as exercise, diet, exposure to radiation, etc. also may have an important influence. Having a certain sequence in a region of DNA therefore does not necessarily mean you will get the disease, but it may mean you are more susceptible to getting the disease.
Activity idea
In this activity, students participate in a simulation that demonstrates that both genetic make-up and environmental factors influence an individual’s likelihood of becoming obese.
Individual DNA testing
Many online companies offer online DNA testing that can give individuals insights into different areas such as ancestory or disease risk. Curious to test your own DNA? Read this article from The Conversation Five things to consider before ordering an online DNA test.
Find out more in this RadioNZ Our Changing World programme Planning for Aotearoa's genomic medicine future from December 2022.
