Understanding Genetics Part 3: Not All Genetic Testing is the Same – Understanding TSP, WES, and WGS
All “genetic testing” is not created equal. Genetic tests, just like the multitude of medical tests your physician may choose to order when you are sick, have their advantages and disadvantages as well as their indications and contraindications. If someone says. “I did genetic testing and it did not identify the cause of the death”, it doesn’t really convey valuable information or guidance to the receiver of that information.
Genetic testing is a type of test that identifies changes in chromosomes, genes or proteins. The results of a genetic test can confirm a suspected genetic condition or help determine a person’s chance of developing or passing on a genetic disorder. They can also establish biological relationships between individuals. Determining the order of DNA building blocks (nucleotides) in an individual’s genetic code called DNA sequencing has advanced the study of genetics and is one technique used to test for genetic disorders.
- Targeted Sequencing Panels (TSP) test for a select number of specific genes, or coding regions within genes that are known to harbor mutations that contribute to disease and may include information that will guide your physician with very specific clinical care for you.
- Whole Exome Sequencing (WES) allows variations in the protein-coding region of any gene to be identified, rather than in only a select few genes. Because most known mutations that cause disease occur in exons, whole-exome sequencing is thought to be an efficient method to identify possible disease-causing mutations. However, exomes make up only about 2% of the whole genome.
- Whole Genome Sequencing (WGS) determines the order of all the nucleotides in the entire genome that makes up an individual’s DNA and can determine variations in any part of the genome that makes up an organism.
While many more genetic changes can be identified with whole exome and whole genome sequencing than with targeted gene sequencing, the significance of much of the information generated is not yet well understood. Because not all genetic changes affect health, it is difficult to know whether identified variants are involved in a condition of interest.
Cost and time to complete testing are just two factors your clinician will consider when evaluating testing options. If a potential disorder is suspected due to other health and diagnostic factors, a targeted sequencing panel may be prioritized. These tests are generally lower cost than WES and WGS and can be performed quickly (about 2 weeks). They also generally include clinically actionable genes of interest. “Clinically actionable” means that some action can be taken to prevent a genetic related disease from occurring or to alter in some way its natural progression. When it is not as clear as to what potential disorder/disease is responsible for specific health concerns, whole exome sequencing (WES) may be considered. It does cost more than targeted testing and can require more time to generate results but does have the ability to look at a very large array of number of genes. Whole genome sequencing (WGS) is the most expensive and time consuming (2-3 times longer) compared to panel testing, and WES. It also generates very large amounts of data which requires analysis and can identify many more variants in which clinical significance is not known yet. Currently, WES and WGS are often utilized in research. They both have the potential to determine whether new genetic variations are associated with health conditions and assist in disease/disorder diagnosis and discovery.