Down syndrome trisomy 21 critical risk

We all know that a full body check-up is a great help in understanding our physical condition. Examinations at the hospital can give us a basic understanding of our own physical condition. Compared with ordinary people, the physical condition of pregnant women is more concerned, so pregnant women need to go to the hospital for regular check-ups at the prescribed time. This is the prenatal check-up we often hear about. One of the prenatal checkups is to check whether the fetus has malformations, which is the Down syndrome screening. So what are the critical risks of Down syndrome trisomy 21?

Down syndrome, also known as trisomy 21, is a disease caused by a chromosomal abnormality (an extra chromosome 21). 60% of the affected children are aborted in the early fetus, and the survivors have obvious intellectual disability, special facial features, growth and development disorders and multiple deformities.

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1. Analysis of chromosome karyotype of peripheral blood cells

Cytogenetic studies have found that when the 21q22 region on the long arm of chromosome 21 is trisomic, the individual has clinical manifestations that are completely similar to Down syndrome. On the contrary, individuals whose region is non-trisomic do not have this typical symptom. It can be inferred from this that the 21q22 region may be the key genetic region for Down syndrome, also known as the Down syndrome region. Children with Down syndrome can be divided into three types according to chromosome karyotype analysis:

(1) The standard type accounts for 95% of all cases. The child has 47 somatic chromosomes, with an extra chromosome 21, and the karyotype is 47, XX (or XY), +21.

(2) Translocation type accounts for 2.5% to 5%. The total number of chromosomes in the affected children is 46. Most of them are Robertsonian translocations, which refers to a reciprocal translocation occurring at the acrocentric chromosomes. Most of them are D/G translocations, with chromosome 14 being the main one in the D group, that is, the karyotype is 46, XX (or XY), -14, +t (14q21q); a few are translocations of chromosome 15. About half of the children with this translocation type are hereditary, that is, one of the parents is a carrier of a balanced translocation chromosome. The other is G/G translocation, which is less common and is caused by the centromere fusion of two chromosomes 21 in group G to form an isochromosome t (21q21q), or the translocation of one chromosome 21 to one chromosome 22.

(3) The mosaic type accounts for 2% to 4% of this disease. The child has two or more cell strains in his body (two is more common), one of which is normal and the other is trisomy 21. The severity of the clinical manifestations is related to the percentage of normal cells, which can range from near normal to typical phenotypes. The higher the proportion of trisomy 21 cell strains, the more severe the intellectual disability and deformity.

2. Amniotic fluid cell chromosome examination

Amniotic fluid cell chromosome examination is an effective method for prenatal diagnosis of Down syndrome. Pregnant women whose Down syndrome screening results are "high risk" need to confirm whether the fetus has Down syndrome. Currently, the most commonly used technology for prenatal diagnosis is amniocentesis, which involves inserting a needle through the abdomen of the pregnant woman into the amniotic fluid under the guidance of B-ultrasound, extracting the amniotic fluid, and performing chromosome analysis on the fetal cells. Suitable for pregnant women between 16 and 20 weeks of pregnancy. In addition to amniocentesis, other prenatal diagnostic techniques include chorionic villus sampling, fetal umbilical vein puncture, and fetoscopy. The common karyotype is the same as the chromosome karyotype of peripheral blood cells.

3. Fluorescence in situ hybridization

Using the corresponding site sequence of chromosome 21 as a probe and hybridizing with lymphocytes or amniotic fluid cells in the peripheral blood, three fluorescent signals of chromosome 21 can be seen in the cells of Down syndrome patients. If the specific sequence of the core region of Down syndrome is selected as a probe for FISH hybridization analysis, the abnormal site of chromosome 21 can be accurately located, thereby improving the accuracy of detecting abnormalities in the number and structure of chromosome 21.

4. Prenatal screening serum markers

The exploration of Down syndrome screening by measuring maternal serum human chorionic gonadotropin (HCG), alpha-fetoprotein (AFP), and free estriol (FE3) has been going on for many years. This is a mid-pregnancy indicator (after 13 weeks).

The risk of having a child with Down syndrome can be calculated based on these three serological indicators (HCG and AFP can also be measured) as well as the age and weight of the pregnant woman, and further confirmatory tests can be performed based on the risk rate.

5. Routine examinations such as X-ray, ultrasound, electrocardiogram, and electroencephalogram

Some children may be found to have congenital heart disease, delayed bone age, abnormal EEG and other changes.