How long do genetic tests take

What happens in the laboratory of an institute for human genetics?

The following information is intended to give you an insight into what happens to a tissue / blood sample that has been sent to a laboratory at a human genetic institute. The most important facts that are presented here are:

  • the various methods used in genetic testing in the laboratory;
  • the reasons why some of the genetic testing methods take so long while others can be carried out comparatively quickly, and
  • why in some cases no result can be determined in the laboratory.

You can find more detailed information about why you should use genetic testing procedures in the information sheet: " What is predictive genetic testing?

What are genetic tests?

Most genetic testing methods examine that DNA, the chemical substance in our cells that commands our body to grow, develop and function. DNA is a chain of encoded instructions that are grouped into special instructions known as Genes are designated. A person has around 30,000 different genes, which are based on a number of thread-like structures Chromosomes calls, are lined up. We get our chromosomes from our parents, 23 from our mother and 23 from our father. This means that we have two sets of chromosomes, or 23 “chromosome pairs”. If you think of genetics as the book of life, DNA is the letters, genes are words, and chromosomes are the chapters of that book.

Fig .: 1 DNA, genes and chromosomes

Changes in genes or chromosomes are called Mutations designated. A mutation can be thought of as a spelling mistake or a change in the order of a number of words in a sentence. Mutations are very common. We all have a variety of them. A mutation can have beneficial effects, adverse effects, or no effects at all. That depends on environmental conditions, chance, or changes in other genes. Mutations can always cause problems if they prevent a gene or chromosome from giving the body the correct instructions it needs to function properly. Genetic testing is therefore geared towards finding mutations in a specific gene or chromosome. As a rule, these tests are carried out on blood or other tissue. (In some cases, a sample of saliva is sufficient to obtain DNA. However, the researchers need a certain amount of high quality DNA for the tests, so it is better to work with blood). A blood sample is taken from the patient and sent to a laboratory, where the genes or chromosomes are examined.

Usually, human genetic institutes have their own laboratories, but since there are a large number of genetic test methods for many different disorders, not all laboratories do all tests. This is especially true for the rare genetic disorders. As a result, a sample is sometimes sent to a particular laboratory because it specializes in the type of test the doctor wants it to be done.

One should always bear in mind: a genetic test procedure can only provide a result about the disorder for which it was developed. There is no general test for all genetic disorders. The genetic test, which is carried out at an institute for human genetics, is intended to provide information about the state of health of a person or family. In general, no examinations for other purposes, such as paternity examinations, are carried out there, although this information may arise in the course of the examination process.

Human genetic laboratories

There are two main forms of human genetic laboratories. Investigate the one Genes , the other Chromosomes .

1) cytogenetics

When a doctor suspects a genetic disorder is due to a change in one Chromosome has been evoked, it becomes a cytogenetic Ask the laboratory to examine the patient's chromosomes. Blood or skin samples or material obtained from amniocentesis or CVS can be used for this purpose. First, cell cultures are created. These cells are then transferred to slides and the chromosomes are stained to make them more visible.

Fig .: 2 This is what chromosomes look like under the microscope

The first thing the cytogeneticist checks is that Chromosome number . Some disorders are caused by excess chromosomes. One of the most famous examples of this is Down syndrome. People with this disorder have one too many chromosomes in their cells. In addition, the cytogeneticist examines the Chromosome structure . Changes in the chromosome structure can occur when the material of a chromosome has broken off and somehow re-deposited. This can lead to the addition or loss of chromosomal material. The changes that can be made can be so small that it is sometimes difficult to spot. In such cases, a different examination method is sometimes used, which is called one Fluorescence In-Situ Hybridization (FISH) is called. It is used to detect changes that are too small to be seen under the microscope or to be certain of a small change that was seen under the microscope.

Fig .: 3 chromosomes arranged according to size, a so-called karyotype

Cytogenetic studies can be time consuming. First, the laboratory has to create a cell culture and let the cells grow, which takes at least a week. Another week is required to prepare the slides and examine the chromosomes under the microscope because one chromosome at a time has to be examined.

2) molecular genetics

When a doctor suspects a genetic disorder is due to a change (mutation) in one gene evoked, he becomes a molecular genetic laboratory ask to examine the DNA of a particular gene. The commands stored in DNA are written in a code made up of 4 letters: A, C, G and T. The molecular genetic laboratory can examine the exact sequence (sequence) of the code in a particular gene to determine whether incorrect sequences, so to speak "spelling mistakes", have arisen. However, a single gene consists of 10,000 or more letters of the DNA code. That is, the skill of a molecular geneticist is to read this code and find changes. If these changes prevent the gene from instructing the body correctly, a genetic disorder can result.

Unlike chromosomes, DNA cannot be viewed under a microscope. Therefore, the molecular geneticist extracts the DNA from the cells and uses it to run specific chemical reactions in order to read the code of the corresponding gene. There are a number of different techniques for detecting mutations. Usually a certain sequence of the DNA is examined.

Fig .: 4 DNA sequencing: Spot the difference!

Normal sequence Sequence of the patient

Here is a short piece of a gene's code. If you can see the picture in color, you will see that each letter of the DNA code is shown in a different color. The left picture shows the normal sequence, the right picture is the patient's sequence. In the left picture, the curve that belongs to each letter has only one peak. In the picture on the right you can see that the curves of this patient have two peaks in the same place, a G (black curve) and a C (blue curve). This shows that there is a mutation on one of the chromosome pairs at this point.

How can a laboratory tell whether a mutation has harmful consequences?

That is a very important question. In human genetic laboratories a statement is circulating: "Finding a mutation is not difficult, everyone can do it. But not everyone can interpret it correctly." Mutations can have different degrees of severity and to recognize the effects a mutation will have, expert knowledge of the disease and the gene or chromosome associated with the disease is required, as well as careful attention to the slightest changes. So how can a laboratory find out if a mutation is benign, harmful, or completely meaningless?

The prerequisite for this is that an expert, i.e. a clinical geneticist, has examined the respective patient and knows their relatives and their medical histories, as well as the results of other clinical examinations. This information provides the human geneticist with clues as to which gene or chromosome is to be examined. For example, if the human geneticist suspects that the patient has cystic fibrosis (cystic fibrosis) because he has symptoms of this disorder, and other relatives have these symptoms as well, then he will draw blood from the patient and test a human genetic Send laboratory for examination. The laboratory is given all relevant information about the patient and the medical histories of his relatives with the request to look for the changes that cause cystic fibrosis. If two mutations are found on chromosome pair 7 in the laboratory, one on each of the two chromosomes that cause cystic fibrosis, then it is certain that the patient has cystic fibrosis.

Sometimes it happens that a child has a disorder but none of their parents have the mutation. In these cases it is likely that the mutation first appeared at conception. This is called a “de novo” (Latin) “new mutation”.

It also happens that a laboratory cannot make a statement as to whether a mutation found is the cause of a disorder, because it is only a minor change in the DNA. Such mutations are called "unclassified variants". When it occurs, it is extremely unsatisfactory for all concerned. However, it is of the utmost importance that a mutation is not labeled as harmful by a laboratory unless it is established that it is, as this could result in someone being misdiagnosed.

Can the mutations always be found in the laboratory?

Sometimes genetic testing is done to identify the cause of a disorder, but no mutation is found.

There are a number of reasons for this which explain this:

  • A genetic test usually only looks for the most common mutation that causes the disorder in question. It is possible that a patient has an unusual mutation that the laboratory cannot find.
  • Or, scientists may not have identified all of the genes that cause the genetic disorder.
  • It is also possible that the patient does not have the disorder he is supposed to have and that the scientists are therefore not investigating the correct gene

It should therefore always be made clear that genetic testing methods and our knowledge of genetics are developing and expanding extremely quickly. This means that the scientists can identify a mutation that cannot be found now, possibly in the future, with the help of new test methods.

Why do some genetic tests take so long, while others can be carried out comparatively quickly?

If the laboratory knows exactly which mutation to look for because a relative has the same disorder, or if the laboratory knows where to look for the mutation in the gene, the task is much easier. In such cases, the genetic testing procedure takes a week or two.

However, if no mutation has occurred in the family in the past, or if a number of genes are associated with the disorder, it will take longer before a result is available. Instead of focusing on one location in a gene, the laboratory must analyze the entire gene or even more than one gene. This is an extremely lengthy process and can take several months, depending on the size of the gene and the resources available to the laboratory.

For example, in the case of Duchenne muscular dystrophy, the disorder is caused by mutations in a gene called dystrophin. It is one of the longest known genes. This means that thousands of different mutations can occur, which can make finding the specific mutation in that family a very labor-intensive and time-consuming process. On the other hand, in the case of Huntington's disease, the mutations in the huntingtin gene always occur in the same narrow region. The scientists therefore know exactly where to look in which area of ​​the gene. Hence, this genetic testing procedure is quite easy and much faster to perform.

The quality of the available DNA is also an important influencing factor. Some laboratories must first examine the DNA of deceased people to identify the respective mutation. If the deceased's DNA is of poor quality, the time it takes to identify the mutation can double or triple. In some cases it is even impossible to complete the analysis because there is not enough DNA available.

Can test results be wrong?

Because genetic test results can have very important consequences for a person and their relatives, they are prepared very carefully. In order to ensure that a correct result is transmitted, a variety of verification steps must be completed. If a mutation is found, the result is always checked for correctness; (Although many steps in the investigation process are carried out by machines, a scientist always checks the respective results). In addition, the result found first is often checked again with the help of a further test procedure. Defined process steps ensure that the individual samples are not mixed up. In addition, many laboratories participate in quality assurance procedures (QA) that have been set up to ensure reliable human genetic test results and good process quality.

What happens to my sample when the test is complete?

Laboratories usually store DNA and chromosome samples unless a patient insists that their sample be destroyed after the examination is complete. Any laboratory will be happy to provide you with information about your sample at any time, and you can of course request that your sample be destroyed or returned to you at any time. No examinations for other disorders will be carried out without the consent of the respective patient.

Since improved test procedures are constantly being developed, these new test procedures can be carried out on stored samples if the patient has given his / her consent (for example, if the original examinations were inconclusive). In this way, both patients and clinicians can be sure that the latest examination techniques are being used. Laboratories also use anonymized DNA samples to develop new test methods or exchange them with one another as part of quality assurance measures, unless a patient has excluded the use of his sample for such measures. The DNA, like any other stored clinical sample, is part of the patient's medical record and is therefore subject to medical confidentiality. This means that only properly qualified health care personnel have access to it.

Some people fear that the police could also gain access to their DNA. This is an extremely rare occurrence. If the police want to gain access to a DNA sample from a human genetic laboratory, this is only possible if there is a court order (as is the case for all other components of the medical record).

Credits

This information was prepared with the assistance of Dr. Ian M Frayling, Institute of Medical Genetics, University Hospital of Wales, Cardiff, UK; Dr. Domenico Coviello, Laboratory of Medical genetics, Fondazione IRCCS Milan, Italy and The Genetic Interest Group.

Translation: Prof. Dr. R. Peter Nippert

June 2009

This text was prepared with the support of EuroGentest, a Network of Excellence funded by the EU-FP6, contract no: E 512148

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