Mechanisms of origin (Etiology)
The main cause of Down syndrome is the appearance of a third copy of genes on the 21st chromosome (trisomy). There are two main mechanisms for the formation of this anomaly:
- Chromosomal non-disjunction
Occurs during gametogenesis (during meiosis) or at the very early stages of zygote cleavage. This results in the karyotype 47, 21+. This is the most frequent classical variant of the pathology.
- Translocation variant
In this case, not the entire chromosome is added to the genome, but only a part of the 21st chromosome, which physically fuses with another chromosome. The following translocation variants are possible:
- To a homologous 21st chromosome.
- To a non-homologous 15th chromosome.
Phenotype and clinical picture
Patients with Morbus Down have short stature and a characteristic appearance combined with neurological deficit.
- Head and face: Small, round head with a flat occiput. The face appears flat, and the palpebral fissures are slanted. The nose is usually short, and the nasal bridge is flattened.
- Oral cavity: The mouth is often half-open, the tongue is thickened and exhibits poor mobility (clumsy).
- Limbs: Fingers and toes are shortened, with characteristic clinodactyly of the fifth fingers.
- Neurology and psyche: Marked hypotonia (sharply decreased muscle tone) is noted, making patients sluggish and clumsy. Mental retardation is very pronounced, and speech development is impaired.
Genetic map of chromosome 21
The extra copy of the 21st chromosome leads to an increased dosage of specific genes. According to the biochemical hypothesis, this increased dosage of the superoxide dismutase enzyme gene (which protects cells from free radicals) may underlie mental retardation.
Key genes responsible for the symptomatology are localized on chromosome 21:
| Genes | Name / Function | Involvement in the syndrome |
|---|---|---|
| APP | Amyloid beta precursor protein gene | Pronounced learning difficulties |
| SOD | Superoxide dismutase gene | Pronounced learning difficulties, protection against radicals |
| GART | Purine synthesis protein gene | Phenotypic features (face, heart) |
| ETS | Oncogene | Phenotypic features (face, heart) |
| CRYA1 | Alpha-crystallin gene | Phenotypic features (face, heart) |
Experimental in vitro gene therapy
Modern research (e.g., D. Lawrence's group in the USA) aims to block the "extra" chromosome at the cellular level.
The XIST gene is used for this purpose. Normally, this gene is localized on the X chromosome and is responsible for the natural compaction (inactivation) of one of the sex chromosomes in females. Experimental introduction of the XIST gene into one of the 21st chromosomes successfully achieves inactivation of this extra chromosome in vitro.
Briefly: answers to topical questions
What cardiovascular malformations are most commonly found in patients with Down syndrome?
In patients with Down syndrome, cardiovascular sources indicate congenital heart defects, specifically septal defects. Atrioventricular septal defect is mentioned separately: a ventricular septal defect combined with deformation of the anterior leaflet of the mitral valve. Additionally, sources link the GART, ETS, and CRYA1 genes on chromosome 21 to phenotypic traits affecting the face and heart.
What prenatal screening methods are used to diagnose Down syndrome?
Non-invasive methods are used for prenatal screening of chromosomal pathology: ultrasound examination and maternal serum biochemical screening. Ultrasound is performed at 11–14 weeks and 18–21 weeks; the first trimester evaluates echomarkers of chromosomal pathology, including increased nuchal translucency and nasal bone agenesis or hypoplasia. Biochemical screening is performed at 11–14 weeks; an elevated level of β-hCG is a significant serum marker of Down syndrome.
What gastrointestinal malformations are associated with Down syndrome?
Congenital gastrointestinal malformations are associated with Down syndrome. Sources indicate duodenal atresia or stenosis, esophageal atresia, and rectal and anal atresia/stenosis. Duodenal atresia is described as the most frequent congenital obstructive lesion of the small intestine and is associated with trisomy 21 in 30–40% of cases. Anorectal malformations include anal and rectal atresia or stenosis; Down syndrome occurs in 2–8% of patients with such malformations, with the vast majority having an imperforate anus without a fistula.
What neurodegenerative disease in adults is associated with trisomy 21?
Trisomy 21 is associated with Alzheimer's-type brain changes. For Down syndrome, sources indicate early involution by age 40, accompanied by progressive intellectual disability and Alzheimer's-type brain changes.
To memorize chromosome 21 genes: "Learning difficulties" are associated with brain enzymes and proteins (APP, SOD). "Appearance and heart" depend on purines, oncogenes, and crystallins (GART, ETS, CRYA1).
Class and exam questions
Does maternal age affect the probability of having a child with Down syndrome?
Yes, the risk increases sharply with age. After 35–37 years, the risk reaches 1%, and in mothers over 40, it is 20 times higher than in women under 30.
Why did patients with this pathology often die at an early age in the past?
In the past, high mortality was observed due to increased susceptibility to infections. With the advent of the antibiotic era, the prognosis has improved significantly.
What is the essence of the translocation form of the syndrome?
In translocation, not the entire 21st chromosome is extra, but only a part of it attached to another chromosome (e.g., to the 15th or to a homologous 21st chromosome).
What else to study on this topic
- Mechanisms of chromosomal non-disjunction in meiosis and zygote cleavage stages
- The role of the XIST gene in chromosome compaction and the Barr body
- Biochemistry of the superoxide dismutase (SOD) enzyme and oxidative stress
- Mechanisms of involvement of GART, ETS, and CRYA1 genes in the formation of heart defects
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