Aneuploidy is ... Aneuploidy: a description, causes, symptoms, forms and characteristics of treatment

Aneuploidy is a genetic disorder, in which an abnormal amount of chromosomes is contained in the body cell. To learn more about how aneuploidy manifests and what its causes are, briefly examine the structure of a living cell, cell nucleus and the chromosomes themselves.

A bit of theory

As you know, in every somatic cell of the human body there are in norm 46 chromosomes, that is, a diploid set. Only in the sex cells this set is single. Chromosomes are located in the cell nucleus and represent a long, compacted DNA helix. DNA, in turn, consists of monomers - polypeptides. A certain sequence of polypeptides sets the gene - the structural unit of heredity. DNA contains the entire genetic program for the development of this organism.

In non-sex cells, each chromosome has a homologous, sister chromosome, very similar, but not identical to the first. In the process of meiosis, these chromosomes exchange patches. This phenomenon is called "crossing-over". In the chromosome, a long and short shoulders are distinguished.

Chromosomal set

The totality of the chromosomes of the body is called a karyotype. Normally, as already mentioned, the karyotype is represented by 46 chromosomes, 23 of each parent, but an anomaly occurs in which the karyotype is represented differently. This disorder is called aneuploidy.

Aneuploidy is a kind of karyotype, in which the number of chromosomes is not equal to normal. Aneuploidy of chromosomes, in which one chromosome is absent, is called monosomy. If there is no para - nucleisomy. If instead of a normal pair of homologous chromosomes in the karyotype there are three homologous, then this is trisomy. Any change in the number of chromosomes causes serious developmental disorders. Some of them can be lethal.

Aneuploidy of non-complete chromosomes (autosomes)

There are many genetic diseases that cause exactly aneuploidy. Examples of such diseases are Down's Syndrome, Patau Syndrome, Edwards Syndrome. All these diseases are caused by the presence of an additional chromosome in different homologous pairs. Down Syndrome is the most common of these diseases. It manifests itself as mental retardation, disruption in communication, learning difficulties.

However, trisomy on the 21st chromosome, which causes the disease, does not affect life expectancy at all. Studies on certain techniques can help patients achieve certain successes in training and socialization.

Another disease, the Patau syndrome, also causes aneuploidy. This is a severe disorder - the result of trisomy on the 13th chromosome. Patients rarely survive to 10 years, more than 80% die in the first year of life, others suffer a severe form of oligophrenia. Children with Patau syndrome have microcephaly, often have heart problems, corneal opacity, cleft palate, ears are somewhat deformed.

The main cause of Edwards syndrome is also aneuploidy - trisomy on the 18th chromosome. More than 90% of children with this disease die from cardiac arrest in the first year of life, the rest suffer a severe form of oligophrenia and are practically uneducable.

Trisomy on the 16th chromosome is more common than any other aneuploidy. This genetic disorder is lethal, the fetus dies in the womb within the first three months of pregnancy.

Aneuploidy in humans can be manifested by a rare genetic disease - the Varkani syndrome, trisomy on the 8th chromosome. The main symptoms are mental retardation, heart defects, skeletal anomalies.

Aneuploidy of sex chromosomes

A variety of aneuploidy, which does not always lead to severe consequences, is trisomy on the X chromosome. Such a violation is only in women. Trisomy on the X chromosome is relatively common - in 0.1% of women. The disease can not be manifested in any way throughout life, which occurs in about 30% of cases. For the remaining 70% percent the following symptoms are characteristic: dyslexia, agraphia, speech and motor disorders. The symptomatic treatment used in such cases is very effective and helps to completely or partially eliminate these symptoms.

Other anomalies associated with the number of sex chromosomes are much more serious. These include the Shereshevskoro-Turner syndrome, a particular case of aneuploidy, in which in a karyotype of a woman there is only one X chromosome in place of the normal two. Symptoms - a delay in mental development, low growth, a violation of the development of the genital organs, an easy form of mental retardation is possible.

There is another serious disease caused by aneuploidy. This is Klinefelter's syndrome-the presence of one or more additional X- or Y-chromosomes in men. Symptoms do not appear until puberty. Almost half of the patients have no symptoms throughout their lives, others have gynecomastia, obesity, infertility. In rare cases, oligophrenia is noted.

Causes of aneuploidy

Aneuploidy arises from the fact that during meiosis, homologous chromosomes are not divided and instead of one, a pair of chromosomes or not one has got into the gamete. If in one gamete the number of chromosomes is normal, and in another - anomalous, then the zygotic karyotype will be anomalous.

Non-separation of homologous chromosomes can occur due to unfavorable environmental conditions, exposure to toxins of different nature. But most often the cause is in heredity: in families where there have already been cases of aneuploidy, the risk of having a child with an abnormal karyotype is higher than that of healthy parents.

Frequency

We can say that the basic risk of chromosomal diseases-aneuploidy is on average small, since all genetic diseases are classified as rare. The total probability of a child with a karyotype anomaly is 5% if both parents are healthy. If one of the parents has any pathology of the karyotype, the probability of the birth of a sick child increases.

The frequency of aneuploidy depends on the chromosome. The most common karyotype pathology is Klinefelter's syndrome, sex chromosome aneuploidy, frequency 1 per 500 newborn boys, the most rare is Varkani syndrome, aneuploidy of the 8th chromosome, frequency 1:50 000.

Treatment

Treatment of aneuploidy is always symptomatic. With Shereshevsky-Turner syndrome, low growth and a lack of female sex hormones are noted, therefore anabolic steroids are used for treatment.

With Down's syndrome, treatment is reduced to training with a patient under special programs aimed at developing cognitive abilities.

Patients with Varkani syndrome rarely live up to 20 years. Over time, new anatomical changes appear, mental retardation progresses. If necessary, resort to surgical intervention to correct the condition of the spine and treat contractures.

If girls with trisomy on the X chromosome experience a delay in speech development, a speech therapist may need to consult. In case of problems with writing and reading, you should contact a dyslexia specialist.

Klinefelter's syndrome does not always affect the quality of life of patients. In rare cases, mental retardation is noted. Depending on its degree (light, medium, heavy), an individual training program is developed. If there is gynecomastia, decreased sexual function, infertility, then resort to hormone therapy.

Diagnostics

Identify genetic diseases of the fetus can be using a number of methods (ultrasound examination, the method of biochemical markers).

With the help of ultrasound can identify the fetus Down's syndrome at an early stage of development.

Non-invasive prenatal diagnosis of aneuploidy is a safe and accurate method for determining possible karyotype anomalies. The essence of the method is simple - a specialist examines the fragments of the child's DNA that are in the mother's blood. Invasive diagnosis of aneuploidy is a more accurate method, but since it carries the risk of spontaneous abortion, it is resorted to only in extreme cases.

Factors that increase the probability of aneuploidy

Recent studies have shown that there is a definite correlation between the age of the mother and the probability of a child with Down syndrome, Patau syndrome or Edwards syndrome. The older the woman, the higher the probability of having a baby with an abnormal karyotype.

What factors play a decisive role in the development of aneuploidy sex chromosomes, is unknown. It is assumed that a key role in such cases belongs to heredity.