Chromosome trisomy syndromes

Genetic control of meiosis and causes of aneuploidies

Normal formation of germ cells (gametogenesis) is a complex process under strict genetic control. A vast group of genes, activated in a strictly defined order, participates in the regulation of meiosis. Among them are:

• Genes responsible for the direct activation and deactivation of the meiotic process.
• Genes controlling the proper conjugation of homologous chromosomes and subsequent crossing over.
• Genes regulating the formation of the mitotic spindle.

If mutations occur in these control genes, the normal course of cell division is disrupted. The main pathological consequence of such failures is disruption of chromosome segregation. As a result, aneuploidies are formed—conditions in which the number of chromosomes in a cell is not a multiple of the haploid set. This exact mechanism underlies the development of trisomy syndromes, where the zygote receives an extra chromosome from one of the unbalanced gametes.

Place of trisomies in the classification of hereditary diseases

In medical genetics, all human hereditary pathologies are divided into several large groups. Trisomies belong to chromosomal diseases, which are associated with changes in the number or structure of chromosomes. To understand the general picture, it is important to distinguish them from other groups:

• Gene diseases: monogenic (Mendelian) pathologies where a mutation occurs in a specific gene, leading to the synthesis of a defective protein or enzyme (enzymopathies). Examples: autosomal dominant (Marfan syndrome, polydactyly, Huntington's chorea), autosomal recessive (phenylketonuria, cystic fibrosis, albinism), and X-linked (hemophilia, color blindness).
• Multifactorial diseases: develop due to a combination of hereditary predisposition and the negative impact of environmental factors.
• Mitochondrial diseases: caused by mutations in mitochondrial DNA and inherited exclusively through the maternal line.
• Incompatibility diseases: arise from a conflict between mother and fetus (e.g., Rhesus incompatibility).

Autosomal trisomies are singled out into a separate, highly important category of chromosomal abnormalities due to their high clinical significance.

Main autosomal trisomy syndromes

In clinical practice, three types of autosomal aneuploidies are most common, each with its own eponym:

1. Down syndrome: develops when three chromosomes are present in the 21st pair (trisomy 21).
2. Patau syndrome: caused by non-disjunction of the 13th pair of chromosomes (trisomy 13).
3. Edwards syndrome: occurs with the appearance of an extra 18th chromosome (trisomy 18).

In addition to autosomal abnormalities, segregation disorders can also affect sex chromosomes, causing Turner syndrome (karyotype 45, X0) or Klinefelter syndrome (karyotype 47, XXY).

The presence of an extra chromosome critically affects intrauterine development. The detection rate of chromosomal abnormalities in fetal growth restriction significantly exceeds general population values, reaching 7–19%. Regardless of the specific syndrome, the general pathophysiological mechanism of impaired fetal development boils down to reduced uteroplacental perfusion and critical impairment of embryonic nutrition.

Principles of diagnostics, treatment, and prevention

Medical genetics offers a comprehensive approach to managing patients with hereditary pathologies.

Treatment of chromosomal and gene diseases is divided into three areas:

• Symptomatic: aimed solely at eliminating or alleviating current symptoms of the disease.
• Pathogenetic: includes diet therapy (e.g., in phenylketonuria), replacement therapy (administration of insulin or blood clotting factors), as well as procedures to remove toxins from the body.
• Etiological: targeting the very cause of the disease. Currently, gene therapy is considered the most promising, though still developing, approach.

Since etiological treatment for trisomies is currently unavailable, prevention comes to the forefront. Key measures to prevent the birth of children with chromosomal abnormalities include competent medical genetic counseling for couples, elimination of exposure to known mutagens on the parents' bodies, and mandatory prenatal diagnostics in early pregnancy.

Class and exam questions

What else to study on this topic

• Classification of human hereditary diseases (gene, multifactorial, mitochondrial)
• Sex chromosome abnormalities (Turner and Klinefelter syndromes)
• Pathogenetic treatment of hereditary diseases (diet therapy, replacement therapy)
• Factors of fetal growth restriction (maternal, placental, infectious)
• Maternal effect genes and segregation genes

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.