Down syndrome

Place of the syndrome in the classification of genetic pathologies

In medical genetics, all hereditary human pathologies are subdivided into several broad groups. Down syndrome belongs to the category of chromosomal diseases, which are associated with quantitative changes in autosomes.

To understand the context, in addition to chromosomal anomalies, the following are distinguished:

• Gene disorders (monogenic or Mendelian). Their mechanism is a gene mutation leading to a protein or enzyme defect (development of "enzymopathies"). Examples: phenylketonuria, cystic fibrosis, albinism (autosomal recessive), as well as hemophilia and color blindness (X-linked).
• Multifactorial diseases (a combination of hereditary predisposition and environmental influence).
• Mitochondrial diseases (caused by mtDNA mutations and characterized exclusively by maternal inheritance).
• Maternal-fetal incompatibility diseases (e.g., classic Rhesus incompatibility).

Along with Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18), Down syndrome is a prime example of a chromosomal anomaly in which the root cause is impaired chromosome disjunction during gametogenesis.

Pathophysiology and impact on fetal development

Regardless of the specific etiology that triggered the pathological cascade, the common final mechanism in the development of genetic anomalies is decreased uteroplacental perfusion. This directly leads to inadequate nutrition of the developing fetus and intrauterine growth restriction (IUGR).

According to statistics, the detection rate of chromosomal anomalies in intrauterine growth restriction significantly exceeds the general population norm and can reach 7–19%. This percentage includes the main life-threatening trisomies: chromosomes 13, 18, and 21.

Risk factors and multifactorial nature

Although the syndrome arises due to a genetic error, a colossal complex of factors influences the likelihood of developing severe malformations. About 90% of all congenital malformations (including heart defects) have a polygenic-multifactorial nature — a combination of genetic predisposition and exogenous influences.

Main factor groups:

1. Endogenous parental factors: The age factor is of particular importance (maternal age over 35, paternal age over 45). Chronic somatic, endocrine, or autoimmune diseases in parents create an additional risk.
2. Teratogenic exposure: Maternal intake of drugs with proven damaging effects (lithium preparations, warfarin, thalidomide, antimetabolites, amphetamines, anticonvulsants, trimethadione, progestogens).
3. Infectious agents: Intrauterine infections such as cytomegalovirus infection (CMV), toxoplasmosis, rubella, herpes simplex virus (HSV), syphilis, and adenovirus.
4. Toxic and environmental factors: Ionizing radiation, adverse ecology, as well as bad habits (alcoholism, tobacco smoking, drug addiction).

Association with congenital heart defects (CHD)

Down syndrome is critically often accompanied by impaired formation of the cardiovascular system. Overall, chromosomal anomalies cause syndromic pathology in 6–36% of patients with diagnosed CHD.

Trisomies 21, 18, and 13 play the leading role in these statistics, accounting for the vast majority (75–85%) of the genetically determined heart defects group. At the same time, establishing a single exact cause for the development of a specific defect in a fetus is most often impossible, as the process depends on a complex of embryogenesis factors.

Management and prevention principles

Treatment of chromosomal and gene pathologies is divided into three vectors:

• Symptomatic: Elimination of disease manifestations (e.g., surgical correction of defects).
• Pathogenetic: Metabolic correction, diet therapy, replacement therapy (insulin administration, clotting factors) or removal of accumulated toxins.
• Etiological: Gene therapy, which is currently the most promising, yet complex direction.

The main tool in combating chromosomal diseases remains prevention. It is based on timely medical and genetic counseling of couples, prenatal fetal diagnosis, and strict exclusion of contact with known mutagens at the stage of planning and carrying out pregnancy.

Class and exam questions

What else to study on this topic

• Gene diseases and enzymopathies: classification and mechanisms
• Polygenic-multifactorial nature of congenital heart defects
• Intrauterine infections as a teratogenic factor
• Medical and genetic counseling and prenatal diagnosis
• Inborn errors of metabolism and lysosomal storage diseases

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