Karyotype - Blood Test

Generality

The karyotype is an exam that allows to study the number and / or structure of an individual's chromosomes. The purpose of this analysis is to identify any anomalies responsible for various hereditary and non-hereditary diseases.

The examination of the karyotype is performed on a sample of venous blood taken from the arm or on the cells collected by a needle aspiration of the haematopoietic cord. In pregnant women, on the other hand, the evaluation of the fetal chromosomal structure can be performed on a sample of amniotic fluid or chorionic villi.

What's this

The karyotype consists of the analysis of chromosomes . These elements are present in the cell nucleus.

Chromosomes contain the genetic heritage of an individual and their genes are, in linear succession, genes .

Genes represent the fundamental unit of genetic information: genes are, in fact, made up of traits of hereditary material, DNA, which is the molecule that contains all the information necessary for the "construction" of the individual (with hereditary physical characteristics of the person to the directives for the growth, development and functioning of the whole organism).

The chromosomes are found in all the cells of the body in a fixed number: as a rule, in the human species there are 46, of which 44 autosomes and 2 sex chromosomes, identical in the female (XX) and different in the male (XY).

The karyotype analysis allows us to highlight, by examining the blood cells, the number anomalies (such as trisomies and monosomies) and structure (translocations, deletions, inversions, etc.) on the chromosomes.

Possible chromosomal alterations can cause malformations and birth defects, repeated abortions or other types of conditions (such as blood and lymphatic system disorders).

What are chromosomes?

• One person's chromosomal kit derives 50% from the mother (female gamete: egg cell) and 50% comes from the father (male gamete: spermatozoon).
• During conception, a spermatozoon with 23 chromosomes joins an egg, which contains another 23, to give life to a human being with 46 chromosomes. If the male gamete carries the X chromosome, a female will be born, while if he wears the Y chromosome a male will be born.
• The karyotype of a normal female will therefore be 46, XX, while that of a male 46, XY.

Synonyms

The examination of the karyotype is also called cytogenetic analysis or chromosome map.

Chromosomal abnormalities: possible consequences

Chromosomal abnormalities can be numerical or structural.

• Numerical anomalies : any change in the number of chromosomes (46, 23 pairs) can lead to health and development problems.
• Structural anomalies : the severity of the consequences depends on the type of chromosome involved. In any case, the same chromosomal anomaly can cause problems of different severity in different people.

Why do you measure

The examination of the karyotype allows the search for possible chromosomal diseases, through the observation of the number and structure of chromosomes.

This information supports the diagnosis of some abnormalities affecting the fetus, which can compromise growth and psychomotor development (eg Down syndrome) or determine infertility / sterility (eg Turner and Klinefelter syndromes).

The study of the karyotype may also be required to detect chromosomal abnormalities acquired in patients with hematologic malignancies (leukemia, lymphoma, myeloma and myelodysplasia), refractory anemia or other types of cancer characterized by "specific tumor" rearrangements.

When is the exam indicated?

The analysis of the karyotype is proposed by the doctor mainly in the case of:

• Carriers of a genetic pathology;
• Parents and family members (brothers and sisters) of subjects with chromosomal abnormalities;
• Poliabortività or sterility in couples or single subjects;
• Suspected chromosomal abnormalities acquired in patients with refractory anemia, leukemia, lymphoma, myeloma, or other types of cancer.

Normal values

The expected result of the analysis coincides with the normal karyotype, ie:

• 46, XX for a female subject;
• 46, XY for a male subject.

Any result other than the above is not necessarily a cause of illness, but must be explained; for this reason, the indication for genetic counseling will be reported on the report.

Altered Cariotype - Causes

Among the chromosomal pathologies identifiable with the karyotype analysis are included:

• Down syndrome (trisomy 21): it is characterized by the presence of a supernumerary chromosome 21 (47, XX / XY, + 21); this anomaly can be found in all or most of the body's cells.
• Edwards syndrome (trisomy 18) : is a disease associated with severe mental retardation, in which the presence of a supernumerary chromosome 18 is observed.
• Klinefelter syndrome : it is the most common anomaly associated with sex chromosomes in males, due to the presence of a supernumerary X chromosome (47, XXY, instead of 46, XY).
• Turner syndrome : is caused by the loss of one X chromosome in females (45, X).
• Patau syndrome (trisomy 13) : chromosome 13 is supernumerary.
• Chronic myeloid leukemia : this hematological pathology is characterized by the presence of the Philadelphia chromosome, which originates from the exchange (translocation) of genetic material between chromosome number 9 and chromosome number 22.

How to measure it

• The examination of the karyotype on peripheral blood is carried out by collecting a blood sample from the arm. Chromosomes are extracted from white blood cells, one of the cellular components of the blood; once placed on a microscope slide, the material is observed to verify the correct number and structure.

To study chromosomes it is necessary to use appropriate culture techniques, as it is possible to visualize them only during the metaphase of cell division; under an optical microscope, these are easily recognized under the microscope as rod-like structures.

• In the case of prenatal diagnosis, the chromosomes can be prepared from fetal cells taken by villocentesis or amniocentesis.
• The search for variations that may be indicative of the presence of haematological diseases or of the lymphatic system (such as leukemia, myeloma or lymphoma) can also be performed on tissue or bone marrow samples.

Research of chromosomal abnormalities

During cellular growth, genetic material (DNA) appears as a non-analysable disorganized cluster. At the time of division, this condenses into ordered structures, the chromosomes.

For karyotype analysis, cells are blocked at a particular time in the division: the metaphase. In this phase, the chromosomes appear as well-defined structures, easily identifiable and recognizable under a microscope.

The examination of the karyotype therefore envisages the following phases:

• Sample collection and growth in culture in an enriched soil, such as to promote cell division in vitro.
• Isolation of chromosomes from the cell nucleus and positioning them on a slide.
• Coloring with specific substances that are selectively fixed to certain chromosomal zones, giving rise to a characteristic banded appearance (Q, G or R, according to the technique used).
• Microscopic observation: the chromosomes are counted, analyzed and photographed.
• Reconstruction, in a similar way to what is done for a puzzle, of the image of chromosomes. The pairs are paired two by two, depending on the size (from the largest to the smallest), the position of the centromere (choke of the chromosome) and banding.
• In the reconstructed image the chromosomes must be placed vertically, from 1 to 22, placing the pair of sex chromosomes (number 23) at the bottom. Each chromosome has the appearance of a striped rod. For each of these, it is possible to identify a short arm (indicated with the letter "p") and a long one (indicated with the letter "q"), a more colored area located in the middle of the chromosome, called centromere, and a series of light and dark horizontal bands. The length of the arms and the position of the bands allows to discriminate the top of the chromosome from the base.
• This leads to the determination of the karyotype: after the reconstruction of the image, the specialized laboratory assesses the chromosomal structure and identifies any anomalies that may be present.

Preparation

The examination of the karyotype is prescribed by the specialist doctor (gynecologist, endocrinologist, pediatrician, urologist or medical geneticist).

Fasting is not necessary prior to sampling.

Interpretation of Results

• The karyotype analysis examines a person's chromosomal structure to verify the presence of the exact number and the absence of structural anomalies.
• The interpretation of the results requires the presence of experienced personnel in cytogenetic analysis: some alterations are quite evident, others may be complex to evaluate.
• Usually, congenital chromosomal aberrations are associated with characteristic signs, but the effects and severity of the related pathology may vary from person to person.
• Cytogenetic analysis can also be performed to study tumors, both hematological (eg leukemia), and solid (eg lung, breast, liver, bladder etc.). Currently, over 100 chromosomal rearrangements characteristic of different types of tumors are known. In this case, the examination of the karyotype can be a useful tool for the diagnostic and / or prognostic evaluation of the disease.
• The examination of the karyotype does not allow to identify the mutations responsible for genetic diseases (such as cystic fibrosis, dystrophy, hemophilia and thalassemia), which must be studied using specific molecular biology techniques.