What Causes Trisomy 21?

Down syndrome is trisomy 21, also known as congenital stupidity or Down syndrome. It is a disease caused by a chromosomal abnormality (an additional chromosome 21). 60% of children have abortions early in the fetus. Survivors have significant mental retardation, special facial features, growth and development disorders, and multiple malformations.

Basic Information

Causes of Down syndrome

In 1866, Dr. John Langdon Down first described and published the typical signs of Down syndrome including similar facial features in these children. Therefore, this syndrome was named Down syndrome ( Down syndrome). It was confirmed in 1959 that Down's syndrome was caused by a chromosomal abnormality. Modern medicine has confirmed that the incidence of Down's syndrome is related to the mother's age of pregnancy. It is an abnormality of chromosome 21 and has three types of trisomy, translocation and chimerism. Older pregnant women and aging eggs are important reasons for non-segregation.

Clinical manifestations of Down syndrome

1. The child has obvious special facial signs, such as wide eye distance, low nasal roots, small eye fissures, lateral oblique oblique skin, internal epiphyseal skin, small external ears, fat tongue, often sticking out of the mouth, and drooling more. Short stature, smaller than normal head circumference, short front and rear diameter, flat flat head. Short neck and loose skin. Bone age often lags behind, teething is delayed and often misplaced. Hair is soft and less. The anterior condyle is closed late, and there may be a third condylar midline. Short limbs, due to loose ligaments, excessive flexion of the joints, short fingers, osteoplasty of the middle finger of the little finger, bending of the little finger, short phalanges, and displacement of the trigeminal point of the palm to the distal end. About half of the children in the toe bulbous area were arched.

2. It often presents drowsiness and feeding difficulties, and its low intelligence gradually becomes apparent with age, with an IQ of 25 to 50, and both motor and sexual development are delayed.

3. Male Down's babies grow up to puberty and do not have fertility. And female Down's babies have menstruation when they grow up and are likely to have children.

4. Children are often accompanied by other deformities such as congenital heart disease. Due to low immune function, they are susceptible to various infections, and the incidence of leukemia is 10 to 30 times higher than the general. If surviving to adulthood, symptoms of senile dementia often occur after the age of 30.

Down syndrome test

1. Analysis of karyotype of peripheral blood cells

Cytogenetic studies have found that when the 21q22 region of the long arm of chromosome 21 is trisomy, the individual has clinical manifestations that are completely similar to Down's syndrome. On the contrary, individuals with non-trisomy in this region have no typical symptoms. It is inferred from this that 21q22 region may be the key gene region of Down syndrome, also known as Down syndrome region. According to karyotype analysis, children with Down syndrome can be divided into three types:

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

(2) The translocation type accounts for 2.5% to 5%. The total number of chromosomes in children is 46, most of which are Robertsonian translocations, which refers to a type of mutual translocation that occurs on the proximal centromere chromosome, mostly D / G. In group D, chromosome 14 is the main type, that is, the karyotype is 46, XX (or XY), -14, + t (14q21q); a few are chromosome 15 translocations. Half are hereditary, that is, there are carriers of balanced translocation chromosomes in the parents. The other is the G / G translocation, which is less common, due to centromeric fusion of two chromosomes 21 in group G, forming an isobromous chromosome t (21q21q), or one translocation from 21 to 1 22 On chromosome.

(3) The chimera type accounts for 2% to 4% of this disease. There are two or more cell lines in the patient (two are more common). One is normal and the other is 21-trisomy cells. The severity of clinical manifestations is related to the percentage of normal cells, which can be from close to normal to the typical phenotype. The higher the proportion of 21-trisomy cell lines, the more severe the mental retardation and deformity.

2. Amniotic fluid cell chromosome examination

Amniotic fluid cell chromosome examination is an effective method for prenatal diagnosis of Down's syndrome. Pregnant women with Down's screening results of "high risk" need to confirm whether the fetus is a child with Down's syndrome. At present, the most commonly used technique for prenatal diagnosis is amniocentesis, that is, under the guidance of B ultrasound, a needle is inserted into the amniotic fluid through the abdomen of a pregnant woman, the amniotic fluid is extracted, and the fetal cells are analyzed for chromosomes. Suitable for pregnant women from 16 to 20 weeks. In addition to amniocentesis, techniques for prenatal diagnosis include villus biopsy, fetal umbilical vein puncture, and fetal microscopy. The common karyotype is the same as the chromosome karyotype of peripheral blood cells.

3. Fluorescence in situ hybridization

Using the corresponding sequence of chromosome 21 as a probe to hybridize with lymphocytes or amniotic fluid cells in peripheral blood, the cells of patients with Down syndrome can show three fluorescent signals of chromosome 21. If the specific sequence of Down's syndrome core region is selected as a probe and FISH hybridization analysis is performed, the abnormal part of chromosome 21 can be accurately located, and the accuracy of detecting the number and structural abnormality of chromosome 21 can be improved.

4. Prenatal screening for serum markers

The use of pregnant women to measure serum chorionic gonadotropin (HCG), alpha-fetoprotein (AFP), and free estriol (FE3) has been explored for Down syndrome screening for many years, which is an indicator of mid-pregnancy (13 weeks after).

The risk rate of children with Down syndrome was estimated based on these three (also HCG and AFP) serological indicators, as well as the age and weight of the pregnant woman, and further confirmed diagnosis based on the level of risk.

5. Routine X-ray, ultrasound, ECG, EEG, etc.

Some children can find changes in congenital heart disease, backward bone age, and abnormal EEG.

Down syndrome diagnosis

Karyotype analysis and FISH technology: Although the special face, hand characteristics and low intelligence of the disease can provide important clues for clinical diagnosis, the establishment of a diagnosis must rely on karyotype analysis. Therefore, karyotype analysis and FISH technology are The main laboratory test technique for Down syndrome. These two tests also have a positive significance for the prognosis of the chimeric type of Down syndrome. Due to the disparity in phenotype of children with chimeric deformity, they can be from normal or near normal to typical clinical manifestations, and their prognosis depends mainly Percentage of normal cell lines in children's somatic cells. Therefore, understanding the ratio of normal karyotype cells to 21-trisomy karyotype cells in somatic cells of chimeric children can guide children's families and society to educate them according to their specific conditions.

Down syndrome treatment

Because the child's immunity is low, attention should be paid to preventing infection. If there is congenital heart disease, gastrointestinal tract or other deformities, surgical correction can be considered.

Down syndrome prognosis

Recurrent respiratory infections often occur during infancy, and those with congenital heart disease often die early. Muscle tension gradually improves with age, and the gap between growth and development progress and normal children gradually increases. At 15 years of age, she has stopped growing tall, has a short stature, and has a low IQ. Infant performance is "good boy". Childhood emotions are mostly pleasant and friendly to people, but the emotional regulation ability is poor, the fluctuation is large, and sometimes quite stubborn and naughty. Comprehensive measures should be adopted, including medical and social services, long-term patient education and training for patients, preparatory education for mentally handicapped children to enable them to transition to ordinary schools, and training of mentally handicapped children with certain work skills. With patient education and training, under supervision, life can be more self-care, and even simpler social work can be self-reliant. Parents and schools should help children overcome behavioral problems, and society should provide moral support to parents of children with disabilities.

Down syndrome prevention

There is no effective treatment. The best way is to terminate the pregnancy before giving birth to the pregnant mother. The content of prenatal prevention for pregnant women is as follows:

Genetic counseling

The older the pregnant woman, the higher the risk rate. The risk of recurrence of standard Down syndrome is 1%. Parents of children with translocation should perform karyotype analysis to find carriers of balanced translocation: if the mother is D / G translocation, each child has a 10% risk rate; if the father is D / G translocation , The risk rate is 4%. The vast majority of G / G translocation cases are sporadic, and the parental karyotype is mostly normal, but 21/21 translocation carriers have also been found, and the next generation suffers from this disease.

2. Prenatal diagnosis

It is an effective measure to prevent the birth of children with Down syndrome. Couples who have had a childbirth history of the disease should make an antenatal diagnosis, ie, karyotype analysis, when sampling includes analysis of amniocentesis in the second trimester for amniotic fluid cells, fetal villous cells in the second trimester, and cord blood lymphocytes in the second trimester. Prenatal screening of serum markers HCG and AFP has certain clinical significance, because it can reduce the blindness of amniocentesis for prenatal diagnosis, suggesting the existence of high-risk pregnant women, allowing these pregnant women to conduct further prenatal examinations and consultations. Minimize the birth of children with Down syndrome.