What are basic methods of DNA sequencing?

There are a wide variety of medical applications for DNA sequencing. These techniques can be used to test one gene or several genes to help diagnose medical conditions. Some examples include:

• Targeted sequencing—sequencing of select variants or areas within a gene's exons or full exons (the segments of DNA that code for proteins). When there is a known effect of certain types of changes to one or more genes, it may help guide medical care to only test for these known changes. One example is testing a tissue biopsy sample from a melanoma to determine whether or not the cells have a mutation (disease-causing variant) in the BRAF gene. A mutation in BRAF is found in more than 50% of melanomas, and people with advanced melanoma that have BRAF mutations may respond to drugs that target these mutations, a treatment referred to as targeted cancer therapy. Targeted drugs work differently than standard chemotherapy and may have fewer side effects.
  
  
• Single gene sequencing—sequencing all exons of a gene, often including parts of the non-coding areas (e.g., the sequence before a gene [promoter] or between exons [introns]). An example of this is sequencing the FBN1 gene. Mutations (disease-causing variants) in this gene cause Marfan syndrome, a disorder that affects the connective tissue that makes up many parts of the body, including bones, muscles, ligament, blood vessels, and heart valves. Over 1,000 different mutations have been found in FBN1, so it is important to evaluate the entire sequence of the FBN1 gene.
  
  
• Multi-gene panel sequencing—sequencing parts or all of several genes to detect mutations (disease-causing variants) that can cause a genetic disorder. An example of this is a panel to test for mutations in the MLH1, MSH2, MSH6, PMS2 and EPCAM genes. Mutations in these genes can cause Lynch syndrome, an inherited disorder that increases the risk of many types of cancer, especially colon cancer and endometrial cancer.
  
  
• Whole genome sequencing or whole exome sequencing (described above)—examples include sequencing the genome or exome of infants with rare metabolic disorders or children with developmental delays and/or intellectual disabilities. This method of testing may be used after other testing has failed to reveal a diagnosis.
  
  
• Whole genome sequencing of microbes—in addition to sequencing the genome of humans, whole genome sequencing can be used to sequence the genomes of other organisms. An example is sequencing the genomes of bacteria in suspected outbreaks. By comparing sequences of bacteria and identifying differences, public health scientists can determine how closely related the bacteria are and how likely it is that they are part of the same outbreak. (For more information on this topic, read Infectious Disease Genetic Testing)