You may have heard terms like whole genome sequencing and DNA sequencing and thought that they sound like they’ve come right out of the script for a futuristic movie. But in reality, sequencing technologies are readily available in many parts of the world, and they can be instrumental in understanding more about human genetics and our health.
The human genome contains a wealth of information about each of us, from our ancestry to our risk of developing certain genetic diseases or passing them down to our children. As a result, it’s not surprising DNA sequencing is becoming a widely used tool in both research and healthcare.
What Is DNA?
Deoxyribonucleic acid, more commonly known as DNA, is a molecule that contains all genetic information for the development and functioning of a living organism. . All the living beings have DNA, from complex mammals such as humans to simple organisms like bacteria.
- A sugar molecule
- A nitrogenous base
- A phosphate group
The nitrogenous base portion of each nucleotide can be adenine (A), cytosine (C), guanine (G), and thymine (T). These bases are “strung together” during DNA replication. and form base pairs by binding to a base from a parallel strand — A always binds to T, while C always binds to G — forming the familiar double helix DNA image that we all know. One of the key features of DNA is that it can replicate itself constantly to create new cells that are identical to their parental cell.
Understanding the Science of DNA
The “patterns” created by these nitrogenous bases in our DNA decide our genetic makeup and these specific DNA sequences form genes. Each gene has coding regions that function almost like a blueprint for cells since they contain “instructions” that tell cells in our body how to synthesise amino acids to form different proteins.
There are only 20 different amino acids types, but they can be arranged in different combinations to create thousands of different proteins, each with its structure and function. Proteins carry out a wide range of processes inside our bodies, depending on their type. If abnormal variations are present in our DNA sequence, these instructions could be faulty and lead to various health issues.
However, not all of our DNA is conformed by protein-coding genes. In fact, nearly 99 percent of all our DNA are non-coding DNA, meaning that it doesn’t contain instructions for protein-coding. For a long time, scientists thought that non-coding DNA didn’t have a useful function. Thanks to the advancement of sequencing, we’re learning more about the purpose of non-coding DNA every day.
Some parts of our non-coding DNA act as regulatory agents that activate or repress DNA transcription, whereas others contain instructions for RNA synthesis.
Chromosomes are long strands of DNA that contain our genes — humans normally have 23 pairs of chromosomes. Each of us receive two versions of the same gene — one from each of our biological parents. The different versions of a gene are called alleles, and they determine the biological traits that we will express, such as eye or hair colour.
Some alleles are dominant (meaning that you only need one copy of the same allele to express its associated characteristic). In contrast, others are recessive (meaning that you need to inherit two copies of the same allele to express that characteristic). This explains why DNA testing results will differ for everyone, even for some identical twins.
What Is a DNA Test?
A DNA test is a genetic test used to determine the order of the nucleotides that form our entire genome — both coding and non-coding —allowing scientists and physicians to ascertain whether an individual’s DNA sequence contains any abnormalities.
The first generationfor whole genome sequencing was called Sanger sequencing or chain termination method, and it was invented in 1977. Although this method was revolutionary for its time, it was also very slow and expensive.
The data analysis required to sequence just one person’s DNA could take years to complete.
Purpose of DNA Test
These methods, also called high-throughput or next-generation sequencing methods, have made large-scale DNA testing accessible to a wider audience since they only require a few days to analyse a human genome while still producing high-quality results.
Due to technology advancement, genetic testing offers a wide array of benefits for scientific research, genetic counselling, individualised medical care, and even public health initiatives.
Types of DNA Tests
There are several types of DNA tests, and each processes genetic information differently and for different purposes. Some types of DNA testing include:
- Whole Genome Sequencing (WGS)
As stated above, WGS sequences the entire genome, including both coding and non-coding DNA. As its name suggests, this type of genetic testing can identify variations in any part of your genome.
- Whole Genome Sequencing (WGS)
- Whole Exome Sequencing (WES)
Rather than sequencing an individual’s entire genome, this test only sequences the parts of their DNA and RNA containing coding instructions, also known as exons. Many of the genetic mutations that lead to genetic disorders happen in the exome (which is the combination of all our exons), which is why this test can still be very useful even though it doesn’t analyse your entire genome.
- Whole Exome Sequencing (WES)
- Targeted Re-sequencing
This method isolates specific regions of DNA to analyse them for mutations. This technique can identify variations in specific genes, which can be very useful if you only need to screen for a particular genetic disease or rule out distinct variations.
- Targeted Re-sequencing
- SNP genotyping (DNA microarrays)
This test measures the variations of single nucleotide polymorphisms (SNPs) against reference genome fragments from members of the same species. SNPs are variations that occur in a single base pair of your DNA. SNPs are the most common type of genetic variation among people. Single SNP annotations can also be used in conjunction with WGS to assess the frequency of rare genetic variants and their consequences.
Bear in mind that whole genome sequencing and other sequencing methods shouldn’t be confused with DNA profiling, which is simply used to determine the source of a DNA sample. DNA profiling is usually used to help solve criminal investigations, but it doesn’t provide any further genomic data.
What Is Whole Genome Sequencing?
Put simply, whole genome sequencing (WGS) is a genetic testing technology that is able to read the whole 3 billion base pairs of DNA in our body at one time. In order to understand this testing procedure better, let’s take a look at what DNA is, how it’s formed, and what it does.
Genome Sequencing and Personalised Medicine
Whole genome sequencing plays a huge role in the future of personalised medicine. For example, whole genome sequencing provides adequate data to personalise therapeutic approaches and treatments to diseases, so that patients receive treatment with the most benefit with the least risk of side effects.
Instead of using the same treatment on all patients who suffer from the same condition, physicians could use sequencing data to personalise each patient’s care management plan.
These advancements wouldn’t just potentially improve patients’ outcomes — they could also help create a more comfortable patient experience in which patients do not have to try different treatments in order to find the right fit for them. Additionally, it could lower healthcare costs and improve the workflow at medical facilities by providing a straightforward way to decide between different treatment options.
DNA testing is often used in prenatal genetic counselling, especially for future parents who have a family history of genetic diseases or pregnancy loss. In some cases, genetic testing may be necessary to understand the cause of a patient’s fertility issues. WGS could also be used to detect illnesses and genetic abnormalities in unborn fetuses.
Whole genome sequencing hasn’t been exclusively applied to humans. Sequencing the DNA of bacterial organisms and other pathogens can help scientists understand the disease better, determine the origin of new mutations, synthesise new medications and vaccines, combat drug-resistant pathogens, and even contain outbreaks of contagious diseases worldwide.
What Can Whole Genome Sequencing Reveal?
This information can also inform other family members of possible health risks, even if they’re not the ones who took the DNA test.
Some patients may be advised by their physician to take a DNA sequencing test to provide an accurate diagnosis for their symptoms after failing to discover their cause through other methods. WGS has been successfully used to diagnose genetic disorders in gravely-ill children and to modify the therapeutic management they received.
Others may simply take a DNA test to learn more about their ancestry. Using WGS, it compares the SNPs in your DNA against reference sequences from specific ethnic populations. SNPs have been found to be inherited through generations, which is why they’re reliable indicators that can be used to compare your DNA against different populations around the world, thus determining your most likely DNA ancestry. For this reason, SNP sequencing is also valuable in the field of evolutionary biology.
Since WGS analyses your entire genome, it may reveal unexpected variations that aren’t currently related to any physical symptoms. These variations are often called “secondary findings”. Sometimes, these genetic variants are benign and don’t affect our health at all. And in other cases, the exact implications of a genetic variant are still unknown — these variants are also known as “variants of uncertain significance”.
Keep in mind that not all genetic variations will automatically generate a disease. Certain genetic changes increase a person’s risk of developing a health condition like breast cancer. Depending on the disease, this predisposition can be mitigated through medical treatment or lifestyle changes. That’s why it’s always important to discuss the results of your sequencing test with a specialist, such as a geneticist.
How Is Whole Genome Sequencing Done
In the past, genotyping human DNA was a long and expensive process. In fact, the Human Genome Project lasted 13 years —but now, you can use a DNA kit from the comfort of your own home and get results in a few weeks.
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- Illumina dye sequencing
- Pyrosequencing
- Single-molecule real-time (SMRT) sequencing
- Nanopore sequencing
Once you purchase a commercial direct-to-consumer sequencing test, you’ll receive a sample collection kit that you’ll be able to do it at home. Depending on the provider, you’ll be instructed to either swab the inside of your cheek and place the swab inside a labelled container or spit the salive to a labelled container before shipping it back to the provider.
Collecting samples using these DNA testing kits is safe, quick, and painless. In other cases, the sample may be collected at a doctor’s office or lab.
Since the human genome contains so much information, DNA tests can’t analyse it all at once. Instead, these methods use different techniques to break DNA into smaller pieces, and a sequencer is used to determine the order of the nucleotides in each of these DNA fragments. Then, bioinformatic technology is used to put all the pieces back together correctly so that they can be analysed.
Whole Genome Sequencing Results
Depending on the type of DNA test you’ve purchased, you’ll receive your results in a few days or up to weeks. Whole genome sequencing provides the most comprehensive type of genomic characterisation that is currently available. Each whole genome sequencing test generates a colossal amount of data — after all, the human genome contains approximately 3 billion base pairs.
But of course you won’t be receiving the sequencing results of 3 billion base pairs after mailing your DNA test kit back to the provider. This amount of information would be practically impossible to process, even for a healthcare provider or scientist.
Instead, the genetic testing company will analyse your results and convert it into a detailed report consisting of pertinent information regarding your health and genetic makeup. WGS can identify many types of variations in your DNA, such as single nucleotide variants, insertion/deletion (indel) polymorphisms in a specific nucleotide sequence, structural variants, copy number variants, among others.
The DNA testing results can provide a wide range of information, including but not limited to:
- Single-gene or Mendelian disorders
As their name suggests, these disorders affect a single gene. Your WGS will determine whether you suffer from one of these disorders or are at risk of developing one later on or passing one down to your children. Single-gene disorders include sickle cell anaemia, Huntington’s disease, cystic fibrosis, or muscular dystrophy.
- Single-gene or Mendelian disorders
- Susceptibility to multifactorial disorders
These disorders are caused by mutations in different genes, but they also depend on environmental and/or lifestyle factors. As we mentioned earlier, this also means that your risk of developing these diseases can often be mitigated through medical intervention or a healthier lifestyle. Some of these conditions include obesity, hypertension, and diabetes.
- Susceptibility to multifactorial disorders
- Pharmacogenomic profile
This section of your WGS will provide information on how you can be expected to respond to different drugs according to your genetic profile. This data can be used to provide individualised medical care, decrease drug toxicity, and adjust medication dosages.