Laboratory cytogenetic examination
During the cell division the gene carrier – deoxyribonucleic acid (DNA) - compacts in the nucleus and forms stainable corpuses (chromosomes) with characteristic stripes. The chromosomes can be observed by means of a light microscope and arranged according to their size and shape into a set - the karyotype. In a normal human nucleus, there are 23 pairs of chromosomes (each member of the pair comes from one parent), marked according to the size and distribution of the stripes by numbers 1 to 22. The last, 23rd pair, forms the gonadal chromosomes. Thus the location of every gene can be described by the chromosome number and a certain stripe.
If it is possible to see a defect in the shape or number of chromosomes under the light microscope, a whole block of genes (in the order of hundreds) must be affected. Either the genes are missing (deletion, monosomy) or they are, on the contrary, present in multiple copies (duplication, trisomy). Parts of the chromosomes can be dislocated (inverted or translocated). Chromosomes can also have a distorted shape. Defects (aberrations) of chromosomes can be heritable (hereditary) or originating suddenly at around the time of conception (germinal). They can also be acquired in life in certain cells (somatic). The somatic mutations are the cause of tumours. The prevalence of chromosomal aberrations at birth is 6/1000 (0.6%). If the aberration is already present in a fertilised ovum (germinal mutation), then this large genetic mutation is subsequently present in all cells of the organism. This therefore explains the multiple character of defects in the individual affected by the chromosomal aberrations. Some chromosomal aberrations are related to a typical complex of attributes - a syndrome. Syndromes are mostly named in honour of the researchers, who were the first to describe them (e.g., Down, Edwards, Patau, Turner, Klinefelter).
It is assumed that almost half of all embryos have a chromosomal aberration, which leads to their early abortion. The early abortions can be explained by a "hormonal defect". In case of a recognized pregnancy, the frequency of chromosomal aberrations at the beginning of the pregnancy is 7.5% while at childbirth it is only 0.6%. This means that most of the chromosomal aberrations are aborted in the course of the pregnancy. A spectrum of chromosomal defects identified in a newborn is connected to the fact if the defect is compatible with the intrauterine development. For example, in case of abortions, a very frequent trisomy of the chromosome 16 almost never occurs in newborn infants. It is obviously connected to the fact that the chromosome 16 contains genes coding for placental functions. If there are three copies of these genes in the placenta, their orders are chaotic and lead to a placental defect and an almost certain abortion. Foetuses having a trisomy of the chromosome 18 (Edwards syndrome) in case of the recognized pregnancies (i.e. between 12th – 40th week of pregnancy) abort in 85% of cases. Foetuses with a trisomy of the chromosome 21 (Down syndrome) are spontaneously aborted in ca. 50% of recognized pregnancies.
A special group are the chromosomal aberrations affecting the gonadal chromosomes. A man and a woman are different genetically only by the gonadal chromosomes. A woman has a standard human genetic make-up, containing a complete set of paired chromosomes, including two gonadal chromosomes, which are denoted, also according to their shape, as “ X ”. Every mother transmits to her son exactly half of the human gene equipment, containing one of her "female" gonadal chromosomes X. From his father, however, the son receives slightly less than a half of the genetic equipment, because the area of the genes located on the chromosome X is in males represented by a smaller chromosome, called Y. Thus the 23rd pair of chromosomes in every man consists of two different chromosomes (XY). The name “sex (or gonadal) chromosomes” is slightly misleading, because (particularly on the chromosome X) there is a number of genes important for completely different functions: the gene for the coagulation factor VIII, muscular albumin dystrophine, genes for height, intelligence, etc. Although the aberrations of the gonadal chromosomes are often aborted (for example, monozomy of chromosome X - Turner syndrome, even up to 99 %), if the foetus does survive the prenatal period, the aberration may not cause any life-threatening defects to its carrier. A dominant affection is a fertility defect.
The FISH (fluorescent hybridization) method is a more sensitive cytogenetic method. In case of this method the chromosomes are marked with short sections of DNA - probes - labelled with fluorescent agents. The DNA sequence of the probe is complementary to the sequence of the DNA in an accurately determined location of the chromosome. If the sequence of the probe and the DNA of the patient correspond to each other, the chromosome is permanently labelled by the colour agent, which shines in the UV-light. Thus it is possible to demonstrate the presence or the alteration of a certain gene. The green signal on the picture denotes the centre (centromere) of the chromosome and the two red dots represent the ends of the short arms of the chromosome X.
The FISH method is often utilized in preimplantation diagnosis (PGD) as part of the assisted reproduction (IVF), before the embryo is inserted into the uterus. Most frequently, 1 to 2 cells are taken from the embryo, the centres (centromers) of selected chromosomes are marked with fluorescently-labelled probes and thus their number is determined. Thus it is, for example, possible to find out, whether the embryonic cells contain three chromosomes No. 21, which would cause the Down syndrome. As well as that, it is possible to determine the sex of the embryo.
Chromosomal PGD may increase the hope of success of the assisted reproduction, mainly in case of older mothers and the pairs with a repeated non-success of the IVF.