Genetic testing for dummies



The term “genetic testing” doesn’t have the same meaning for everyone. What does it mean for you? A DNA test to establish the paternity of an alleged father is not the same as a genetic test to determine someone’s predisposition to certain diseases. In both cases, we’re looking into a person’s genome. But what’s the difference? We hope this text will give you a clearer idea. But first, a 101 course on cellular and molecular biology.
 

 

 

What is the genome?

The human body has about 50,000 billion cells, and they all play special roles: brain cells and nerve cells (neurons) transmit nerve impulses; certain cells in the pancreas produce insulin; the ones in the muscles contract so that we can move. Despite such differences, cells have something in common: they all have the same genetic background, also known as a genome. Whether we’re short or tall or have brown or blue eyes depends on the baggage we inherit from our parents. Each of us has a unique genome that distinguishes us from other humans.

A cell is a kind of kitchen where all kinds of proteins are concocted. All the kitchen utensils are there with an abundance of basic ingredients (amino acids). In this kitchen, amino acids are combined to make proteins according to different recipes. The genome, the recipe book for our cells, is locked securely in a safe: the cell nucleus. Each gene is a recipe that tells the cell how to make a particular protein.

There are about 20,000 recipes in this book: 20,000 genes that contain the recipes for some 20,000 different proteins. But not every cell produces all the proteins in the book. Depending on their role (neurons, pancreatic cells, muscle cells), they specialize in some recipe, but will never make others. 

When a cell is instructed to produce a particular protein (e.g. pancreatic cells, which can tell when the blood contains too much sugar and produce insulin), things get going in the kitchen. The cell follows the recipe and assembles the amino acids in exactly the right order to form the protein.

 

But the recipe book is not foolproof;

sometimes accidents can occur in the genome...

For example, when the cell replicates, one letter in a recipe can be accidentally replaced by another. This is called a mutation. In some cases, the cell still manages to read the recipe; in others, it is more problematic: the protein formed either malfunctions or does not work, or the cell stops producing the protein outright.

Such a disruption can lead to a disease. Cancer, for example, starts when a cell disrupted by a mutation starts to duplicate out of control, eventually forming a mass of invasive, non-functional tissue: a tumor.

While mutations can be “natural” accidents, they may also be due to external causes. Ultraviolet rays, such as those from the sun, can damage the genome and cause mutations that result in skin cancer. The same is true of carcinogens such as tobacco smoke, which causes cancer by triggering a mutation in a lung cell.  

Interrogating the genome

In recent decades, biological research has gradually led to an understanding of the structure and function of the genome. Once long and costly, decoding the genome has since become quick and affordable thanks to recent scientific breakthroughs. It is now possible to explore an individual’s genome and read his “recipes.”

Doing a genetic test on someone involves looking at what is written in their genome based on a small sample of living cells. But seeing what is written in the genome does not necessarily mean understanding what it says. Sequencing someone’s genome is like downloading a text in a foreign language. Although it’s possible to see every letter of every gene, the genome essentially remains an unknown language. With a few exceptions.

 

What we are unable to do

At the present time, while it is possible to sequence a person’s entire genome, it is not possible to arrive at a comprehensive portrait of this person’s potential and all the risks he or she is prone to. Of course, several genes have already been identified and it is possible to deduce some information from a complete sequencing, but as yet the system as a whole remains indecipherable.

For as little as a few hundred dollars, some private companies offer a genome genotyping service. How the results are interpreted may vary from one company to another depending on the algorithms they use. Clients can discover their predisposition to certain diseases or to certain mundane conditions such as baldness or the ability to digest lactose. 

 

Knowing how to read the numbers

Such genetic testing requires a certain measure of skill in analyzing the statistics...

Consider the hypothetical example of a genetic test for measuring the risk of developing Alzheimer’s that announces a 50% higher than average risk of developing the condition. This does not mean that the subject has a fifty-fifty chance of having the disease. If the prevalence of the disease in the general population is 2%, this means that the subject’s risk stands at 3% (50% more than 2). Such tests therefore require a complete, rigorous consulting service if the results are to be fully understood – something that every private company does not necessarily offer.  

 

Paternity and maternity tests

It is possible to compare the genome of one person to that of another to note the similarities and determine how they are related. This is how a paternity or maternity test is performed: it’s not necessary to sequence the entire genome, just compare the degree of resemblance between certain sections of the child's genome and the same sections of the adult’s. To do this, there’s no need to know what the text says, it’s just a matter of comparing the sequences of letters.

 

Comparing DNA

A DNA sample collected at the scene of a crime (drops of blood or semen, a hair root, saliva, a skin fragment, etc.) can be analyzed and compared to DNA from suspects. Just like fingerprints, DNA is evidence that someone was in the place where the traces were found.

Searching for known mutations on specific genes

While the genome is still largely unchartered territory, a number of sequences have been identified as being responsible for certain traits or diseases. We have determined the usefulness of certain genes and have identified mutations that can render them ineffective and cause disease. It is therefore possible to perform genetic testing to obtain information on specific diseases by sequencing small segments of the genome, the ones that are associated with the diseases in question.

BRCA1 and BRCA2 genes, for example, when defective, can cause breast or ovarian cancer. In people with a family history of breast cancer, genetic analysis can detect a mutation in one of these genes and see if the risks of having it are higher too (this was the case with American actress Angelina Jolie who lost her mother to cancer and had her genes analyzed. The test proved positive and prompted her to have her breasts and ovaries removed as a preventive measure, to avoid having the disease).

Prenatal tests

Before the birth of a child, it is possible to analyze its genome and assess its susceptibility to certain diseases. Trisomies, for example, can be detected at a very early stage in a baby’s development. The decision to continue the pregnancy will then be up to the parents.

Pharmacogenomics

This is one of the most promising applications of genetics: some people respond better to certain medications than others and the origin of this favorable response is in their genome. By analyzing patients’ genes, it is possible to prescribe the drugs that are best suited to them. This is already the case with some anti-anxiolytic drugs; psychiatrists are now able to avoid prescribing remedies on a hit-or-miss basis until they find one that is suitable. Currently, 40% of drugs are being developed in parallel with genetic testing, which will allow them to be prescribed to the patients who will derive maximum benefit from them.

 

So what does Génome Québec do?

Génome Québec cannot perform or prescribe genetic testing for individuals. Its role is to fund genomic research in Québec and provide researchers with the tools they need to continue their work. The organization currently supports a dozen large-scale projects for the development, validation and implementation of genetic testing, including:

 

Genomic tests for prenatal screening
Dr François Rousseau
Université Laval

Learn more about this project
Breast cancer

Pr Jacques Simard
Université Laval

Learn more about this project
Cardiovascular diseases and diagnostic tests
Dr Jean-Claude Tardif
Montreal Heart Institute 

Learn more about this project