Most human diseases, perhaps with the sole exception of infectious diseases, recognize a genetic basis or at least a genetic predisposition, although this is often difficult to identify because the clinical phenotype (i.e., disease manifestations) is the result of theinteraction of the individual’s genetic makeup with environmental factors ( multifactorialdiseases ).
Monogenic diseases are those conditions in which thealteration of a single gene is capable of causing disease. To understand the basic rules of transmission of genetic diseases, it is necessary to always keep in mind that our genetic makeup is dual.
In other words, we have two copies of each gene, one inherited from the mother and one inherited from the father.
How is the genetic analysis performed?
The first step in genetic analysis isDNA extraction.
DNA is contained in all cells of our body and therefore, in theory, can be extracted from any tissue.
In everyday clinical practice, it is preferred to use venous blood, which is an easily obtainable source of DNA by simple sampling. DNA is extracted from white blood cells, the only nucleated cells in the blood. Through various reactions, DNA is separated from the proteins that make up our cells.
The DNA extraction process requires 24-48 hours before the DNA is ready to be submitted for genetic analysis. Obviously, the more genes that need to be analyzed, the more DNA we will need to have on hand.
The Polymerase Chain Reaction ( PCR ) technique is the basic methodology of most genetic and molecular biology studies.
By means of PCR it is possible to obtain thousands of copies of the DNA fragment we wish to study. In fact, DNA obtained by extraction from blood is generally in too small an amount to be analyzed.
To achieve this type of selective amplification, it is necessary to “cut” from the total DNA the fragment we are interested in and then multiply it (creating identical copies) until we have the amount needed to perform the analysis.
This “biological photocopier” uses an enzyme (the DNA Taq polymerase) and an apparatus that can very quickly and precisely change the temperature in the tubes containing the DNA to be analyzed (the so-called thermal cycler): in this way, sufficient DNA can be obtained to perform mutation search studies.
It should be noted that PCR allows amplification of only relatively short fragments of DNA (fragments of 150-300 base pairs are used for mutation detection), but a gene is usually composed of several thousand base pairs.
It is therefore clear that to analyze an entire gene, many PCR reactions will be required. This fact often makes genetic investigation very long and laborious, especially for diseases in which many other genes have been identified.
PCR-amplified DNA can be analyzed in various ways for genetic defects.
The most widely used techniques are: SSCP (Single Strand Conformational Polymorphism), DHPLC (Denaturating High Performance Liquid Chromatography) and Sequencing.
Regardless of the technical details of these different methods, the ultimate goal is to identify mutations, that is, errors in the DNA that underlie the disease.
DNA (deoxyribonucleic acid) is composed of a very long chain of molecules, the so-called bases (Adenine, Thymine, Cytosine and Guanine).
As mentioned above, a gene can consist of a chain of many thousands of DNA bases. In some cases, even a single error (for example, an adenine instead of a guanine) can cause a genetic disease.
The 3 main types of mutations are:
– point mutations in which a single DNA base is “wrong”
– deletions in which there is the loss of a piece of DNA of varying size
– listings that are due to the abnormal addition of a DNA fragment in a gene
Polymorphisms may also be present in DNA: these are alterations in the gene sequence that are present in the general population with a frequency of more than 1%.
Some polymorphisms, while not causing disease, may be responsible for modulating clinical manifestations.
The detection of a mutation in a subject’s DNA. with arrhythmogenic disease helps to gain a deeper understanding of the pathogenesis of the disorder and provides an important element for the physician to identify the most appropriate care, making genotype-based risk stratification and a gene-specific treatment approach possible.
In addition, identification of the genetic defect is helpful to the patient’s family members because it makes it possible toidentify silent carriers of the disease and perform prenatal diagnostics.
Cardio-genetic counseling is a procedure directed at helping families in whom a genetically based heart disease has been identified to address the problems associated with the disease.
Counseling is, therefore, an integral part of genetic testing: it should precede it-to give the patient the choice of whether to avail himself or herself of genetic screening knowing in depth its limitations and possible consequences at the level of the person himself or herself and family members, both positive and negative-and then conclude it, in order to ensure that the patient is given as much information as possible about the clinical implications of the diagnosis itself.
In addition, in the course of counseling, the patient may freely express his or her desire to be or not to be made aware of the results of the analysis, to inform or not to inform family members, and to perform or not to perform the analysis in children if they are minors.
Genetic counseling should be carried out by professionals specifically trained to use procedures, norms, and behaviors different from those used in normal clinical practice.
The purpose of genetic counseling is informative and non-directive, with the aim of providing those who use it with the necessary elements to make appropriate decisions.
When and why genetic analysis?
If the patient’s clinical picture is already diagnostic, the genetic analysis provides two key pieces of information: first related to the patient, and second related to the patient’s family members.
As for the patient himself,identifying the genetic variant of a disease in which several genes may be involved can help in better risk stratification and targeting therapy in a gene-specific manner.
Once the genetic defect of the proband (the first affected individual in a family who comes to the physician’s observation) has been identified, the extension of the genetic analysis in family members who are apparently healthy or have a nuanced clinical picture leads to the ‘identification Of the so-called “silent bearers“, individuals who, even if they do not have significant clinical problems at the time of diagnosis, may develop the disease later or under certain conditions.
When the diagnosis of disease is only suspected, identification of a mutated gene makes it certain.
In-depth study at the clinical level
It is crucial to remember that for so many monogenic diseases only a few of the disease genes have been identified: this limitation means that failure to identify a mutation does not allow the diagnosis of disease to be ruled out.
For this reason, subjects undergoing genetic analysis must be previously studied thoroughly at the clinical level. Hence, it is important for the center performing thegenetic analysis to have access to all clinical information about the patient: this is to avoid investigations that are inappropriate or performed on genes that do not correlate with the patient’s clinical picture.
Prenatal diagnosis can be performed in families where the genetic defect responsible for the disease has been identified. Prenatal diagnosis, because of its sensitive role, should always be preceded by accurate genetic counseling with informative purpose toward the users of the investigation itself.