Chromosome disorders are of conditions, caused by constitutional numerical or structural abnormalities of chromosomes.
Normally every cell of the human body has 46 chromosomes, organized in 23 pairs (22 pairs of autosomes, identical in males and females) and one pair of sex chromosomes – XX in females and XY in males. The only exceptions are egg–cells and sperm–cells, which have only haploid set of chromosomes. All normal egg–cells have karyotype 23,X; the sperm–cells may be 23,X and 23,Y. Fertilization of the egg–cell by 23,X–sperm will lead to development of female, fertilization by 23, Y–sperm will produce male organism 46,XY.
Diagnosis of chromosomal disorders requires analysis of chromosomes. Experienced clinicians (geneticists, dysmorphologists) may diagnose many chromosomal disorders by clinical examination. But even if clinical diagnosis is obvious, it has to be confirmed by cytogenetic examination, because almost all chromosomal disorders may exist in different cytogenetic variants with very different prognosis for the family. Therefore, cytogenetic testing is necessary even in patients with a clear clinical diagnosis.
Standard cytogenetic examination requires analysis of chromosomes on the stage of metaphase (metaphase analysis). At this stage of cell division all chromosomes became clearly visible structures. All chromosomes may be recognized by their size, position of a centromere and characteristic pattern of dark and light bands, which can be seen after special staining. A cytogeneticist counts number of chromosomes in each of studied cells and compares its size and banding pattern with a standard. If the studied cells have 46 chromosomes with normal structure karyotype of the person considered as normal. If there are some abnormalities it may be evidence of a chromosomal disorder.
Basically (in normal conditions) all cells of the organism have the same karyotype. Therefore, theoretically all cells may be used for cytogenetic examination. However, the preferential types of cells for chromosomal examination are cells of chorionic villi or amniocytes (in prenatal diagnosis of karyotype) and lymphocytes (for postnatal examination).
Prenatal examination of karyotype is usually performed for several groups of pregnant women. It was shown that pregnancy by a fetus with some chromosomal syndromes (trisomy 21 and trisomy 18) is frequently accompanied by an increase or decrease of several biochemical components of serum. Almost all trisomies (trisomies 13, 18 and 21) occur more often in fetuses of “older” woman (especially after 35 years of age). Age and biochemical parameters (taken together) allow calculation of the risk for Down’s syndrome. If this risk is higher that arbitrarily chosen level (for example, higher than 1%) prenatal examination of karyotype is recommended. Some abnormalities of the fetus, which are noted upon ultrasound examination may be another indication for prenatal cytogenetic diagnosis. The examination may be necessary also for the families where one of the parents is a carrier of a balanced structural chromosomal rearrangement – translocation, inversion, insertion or any complex rearrangement.
There are several ways to obtain cells, identical to fetal cells. The most known test to obtain cells at early stage (~10–11 weeks) is chorionic villus sampling. Under the control of ultrasound the special instrument is inserted via uterine cervix or thorough the abdominal wall. A small piece of placenta with growing chorionic villi is taken for analysis. Short term cultivation is usually needed.
Amniocentesis is a predominant way to obtain cells for prenatal diagnosis. Small amount (5–10 ml) of amniotic fluid is taken from the amniotic cavity via transabdominal amniocentesis. This procedure is usually performed at 14–17 weeks of pregnancy. Amniotic fluid has plenty of amniotic cells. After centrifugation almost all amniotic cells are concentrated at the bottom of the tube. ~1 ml of suspension from the bottom of the tube is placed on the cover slides in the small Petri dishes. A special medium is added to facilitate growth of amniotic cells. After a short-term cultivation (usually 6–7 days) the cells are ready for analysis. A cytogeneticist counts ~20 cells at least from 2 flasks and karyotypes several cells. In some centers the cytogeneticist looks on the cells through the microscope, other centers prefer automatic analysis, when the cytogeneticist looks on the screen of the special computer designed for the selection and analysis of metaphases. There is photographic documentation for every studied person. The results are provided to the patient and (if the results show a chromosomal disorder) the family may decide to continue pregnancy or to terminate it.
Technically amniocentesis may be performed also in a more advanced pregnancy. However amniotic cells obtained after 22 weeks had worse growth potentials (than amniotic cells at 14–17 weeks). If karyotype at late pregnancy became really necessary samples of fetal blood may be obtained by puncture of fetal umbilical cord (under guidance of ultrasound).
Practically, prenatal cytogenetic diagnosis is a very good method to reduce numerical abnormalities, mostly trisomies. Its role in detection of chromosomal disorders, caused by structural abnormalities is far less, because most women pregnant with fetuses having structural chromosomal defects are young and do not have biochemical indications for amniocentesis. The only (but very important) exceptions are families with structural chromosomal abnormalities in one of the parents. In these families prenatal diagnosis of the karyotype may be crucial for decision about fate of the pregnancy. Actually, the last group of families may benefit from preconceptional diagnosis. This method (or better these methods) may allow selection of normal egg–cells for further fertilization in vitro and implantation of the embryo with already known karyotype. If a balanced rearrangement (usually translocation) is found in a father, his sperm cells are used for simultaneous fertilization of several egg–cells with karyotyping of the very early pre–implantational embryo and implantation of the embryo having normal karyotype. In that case the family does not have to decide fate of unborn fetus. However, there are many technical limitations regarding usage of these methods.
Post–natal cytogenetic diagnosis is based in vast majority of situations on examination of the lymphocytes of the peripheral blood. Cells of the peripheral blood are mature cells, they grow and divide in the bone marrow, spleen and lymphatic nodes. Adding of specific stimulator phytohemagglutinin (PHA) is necessary to obtain division of lymphocytes, obtained from peripheral blood. Small amount of blood (less than 1 ml) mixed with PHA and special medium is cultivated in thermostat at 37°C during 72 hours. After it the obtained suspension of dividing cells is treated by Colchicine, which blocks cellular division. Hypotonic solution is added to provide better spreading of chromosomes on the slides. Special staining allows visualization of the chromosomes as structures having an individual pattern of distribution of dark and light bands. Further steps (analysis itself) are basically the same as in analysis of amniotic cells for prenatal diagnosis.
However, the standard (visual) cytogenetic analysis does not allow recognition of small deletions or duplications. Even in ideal technical conditions level of recognition is about 5-6 millions of base pairs (Mb). Practically, however, deletions or duplications less than 10 Mb hardly may be recognizable. Fluorescence in situ hybridization (FISH) is a method, which may improve quality of cytogenetic diagnosis in patients, where some structural abnormalities may be suspected. There are probes to some specific segments of DNA. These probes are tagged by fluorescent stains. In normal condition the person will have two areas of hybridization (2 hybridization spots) on the homologous chromosomes. When the patient has a hybridization spot only on one of the homologous chromosomes it means that this segment of DNA on the other homologous chromosome is lost. Vice versa, three spots of hybridization may indicate evidence of a duplication of this segment of DNA. This method may be used also for the study of undivided (interphase) cells, obtained, for example, from a buccal smear (or uncultivated amniotic fluid). Practically, FISH may be used for exclusion (or confirmation) of trisomies or relatively frequent deletions, for example del 22q11.2, which causes diGeorge syndrome or del 7q11.23, which causes Williams syndrome. Limitations of FISH examination are obvious: a) if you have normal results with probes “a”, “b” and “c” it means that a patient does not have deletions or duplications for these regions, but does not exclude abnormalities for regions “d” and “e”, which have not been tested; b) FISH does not give precise coordinates of the deleted segment.
Sometimes, the patient may have mosaicism: the condition, when he/she has several clones of cells with different chromosomal complement. Mosaicism is very common for numerical anomalies of sex chromosomes, but not so common for autosomal trisomies and for structural chromosomal abnormalities. The methods of cytogenetic examination for diagnosis of mosaicism are the same but number of studied cells should be increased. Usually the number of cells with different karyotypes is shown in brackets after the standard formula. For example, the formula 47,XX,+21 /46,XX  means that the patient have mosaic trisomy 21 with trisomy in 80% of cells.
There are some rare conditions, where an abnormal karyotype may be found predominantly (or even exclusively) in fibroblasts, whereas the lymphocytes show a normal karyotype. This situation is typical for mosaic tetrasomy 12p (Pallister–Killian syndrome) and frequent in some “rare” trisomies. Skin biopsy and cultivation of skin fibroblasts may be necessary for cytogenetic examinations to confirm (or exclude) these syndromes. FISH examination of interphase cells using probes for 12p may facilitate diagnosis of Pallister–Killian syndrome.
The ultimate goal of all these methods is diagnosis of constitutional (inherited) chromosomal abnormalities. Structure of chromosomes may be changed in various tumors. The methods for examination of these acquired chromosome abnormalities are out of our scope.
Non-invasive prenatal diagnosis (NIPD) of chromosomal disorders is a new method introduced in recent years. Almost all human DNA is organized into chromosomes and located in cells. However, a small part of DNA exists outside the cells. It is a so-called cell-free DNA (cfDNA). When a woman is pregnant a small part of the fetal cfDNA enters the maternal blood through the placenta. Analysis of the maternal blood allows 1) to distinguish maternal and fetal cfDNA, and 2) to analyze presence of some specific components in fetal cfDNA. If a fetus has additional chromosomes 13, 18, 21 or X as well as monosomy X these abnormalities may be discovered analyzing fetal DNA obtained from a maternal blood. It has been shown that NIPD detects virtually all cases of trisomy 18 or trisomy 21 as well as the vast majority of other trisomies or monosomy X. Normal NIPD test results also offer the opportunity to avoid CVS or amniocentesis, which are more traumatic and (in rare cases) may lead to a miscarriage. NIPD may be performed after 10 weeks of pregnancy. Although there are some reports of discovery of structural abnormalities (deletions or partial trisomies) via NIPD it is too early to say for certain that this method is reliable for diagnosis of such conditions.
Currently medical insurances cover the cost of NIPD for pregnant women over 35 years old and for the families with chromosomal abnormalities in a previous child or fetus.