Contents
What is trisomy 21?
Coming into the category of “chromosomal abnormalities”, trisomy is a story of chromosomes, in other words genetic material. Indeed, in an individual without Down’s syndrome, the chromosomes go in pairs. There are 23 pairs of chromosomes in humans, or 46 chromosomes in total. We speak of trisomy when at least one of the pairs does not have two, but three chromosomes. This chromosomal anomaly can occur during the distribution of the genetic heritage of the two partners, during the creation of gametes (oocyte and sperm), then during fertilization.
While trisomy can affect any pair of chromosomes, the best known is trisomy 21, which concerns the 21st pair of chromosomes.
The most common and viable trisomy, trisomy 21, also called Down syndrome, is observed on average in 27 pregnancies out of 10. Its frequency increases with the age of the mother. It is estimated that there are around 000 people with Down’s syndrome in France (around 50 births per year in France).
Note that there are three subcategories of Down’s syndrome:
- free, complete and homogeneous trisomy 21, which represents approximately 95% of cases of trisomy 21:
the three chromosomes 21 are separated from each other, the anomaly concerns the whole of chromosome 21 and was observed in all the cells examined (at least those which were analyzed in the laboratory);
- mosaic trisomy 21:
cells with 47 chromosomes (including 3 chromosomes 21) coexist with cells with 46 chromosomes, including 2 chromosomes 21. The proportion of the two categories of cells varies from one subject to another, but also from one organ or another. one tissue to another in the same individual;
- trisomy 21 by translocation:
the karyotype (that is, the arrangement of all the chromosomes) does show three chromosomes 21, but not all grouped together. One of the three chromosomes 21 can for example be with two chromosomes 14, or 12 …
Causes and risk factors of Down’s syndrome
Trisomy 21 results from a poor distribution of chromosomes during cell division that takes place at the time of fertilization. At this time, it is still unclear exactly why this is happening. On the other hand, we know the major risk factor: the age of the future mother.
The older a woman, the more likely she is to give birth to a child with trisomy, and a fortiori with trisomy 21. From 1/1 to 500 years, this probability rises from 20/1 to 1 years, then 000/30 to 1 years, and even from 100/40 to 1 years.
Note that in addition to maternal age, there are two other recognized risk factors of having a child with Down’s syndrome, namely:
- the presence of a chromosomal abnormality linked to chromosome 21 in one of the parents (which then results in a trisomy by translocation);
- having already given birth to a child with Down’s syndrome.
What are the signs of Down’s syndrome?
In addition to the characteristic appearance of the head and face (small and round head, slightly marked root of the nose, eyes wide apart, neck small and wide, etc.), there are other characteristic physical signs: strabismus, stocky hands and short fingers, short stature (rarely more than six feet in an adult), poor muscle tone (hypotonia), and sometimes more or less serious malformations, cardiac, ocular, digestive, orthopedic.
An intellectual disability of varying intensity, affecting abstraction capacities, is also observed. Note, however, that the greater or lesser importance of the intellectual deficit does not exclude a certain autonomy in affected individuals. Everything also depends on the support, education and monitoring of each child with Down’s syndrome.
What are the consequences and complications of Down’s syndrome?
The consequences of Down’s syndrome are none other than the signs it causes, starting with intellectual disability.
But beyond the classic symptoms, trisomy 21 can cause diseases of the ENT sphere, a greater sensitivity to infections, an increased risk of deafness due to frequent ear infections, visual problems (myopia, strabismus, early cataracts ), epilepsy, joint deformities (scoliosis, kyphosis, hyperlordosis, luxable kneecaps, etc.) linked to ligament hyperlaxity, digestive problems, more frequent autoimmune diseases, or even more frequent cancers (childhood leukemia , lymphoma in adulthood in particular) …
Screening: how is the Down’s syndrome test going?
Currently in France, the screening test for trisomy 21 is systematically offered to pregnant women. It consists in the first place of a blood test coupled with an ultrasound, which takes place between 11 and 13 weeks of amenorrhea, but remains possible up to 18 weeks of amenorrhea.
The assay of serum markers (proteins) in maternal blood, coupled with ultrasound measurements (nuchal translucency in particular) and at the age of the future mother, allows the calculation of a risk of giving birth to a child with Down’s syndrome.
Note that this is a probability and not a certainty. If the risk is above 1/250, it is considered “high”.
If the calculated risk is above 1/250, additional examinations are offered: non-invasive prenatal screening, or amniocentesis.
Trisomy 21: what treatment?
At present, there is no treatment to “cure” a trisomy 21. You are a carrier of this chromosomal abnormality for life.
However, avenues of research are being explored, in particular with the aim, not of removing the excess chromosome, but of canceling its influence. But although this chromosome is the smallest of these, it contains over 250 genes. Deleting it altogether, or even silencing it, could have negative repercussions.
Another approach under study consists in targeting certain specific genes of chromosome 21 in excess, in order to act on them. These are genes that have a close link with learning, memory or cognition difficulties in people with Down’s syndrome.
Trisomy 21: its management
So that a child with Down’s syndrome reaches a certain level of autonomy in adulthood, and does not suffer too many complications in terms of his health, a consequent medical follow-up is more than advised.
Because in addition to potential congenital malformations, trisomy 21 increases the risk of other pathologies such as hypothyroidism, epilepsy or sleep apnea syndrome. A complete medical check-up is therefore required at birth. to take stock, but also regularly over the course of life.
In terms of motor skills, language, communication, support from several specialists is necessary in order to help children with Down’s syndrome to develop their capacities to the maximum. Thereby, psychomotor therapists, physiotherapists or speech therapists are specialists that a child with Down’s syndrome will have to see regularly in order to progress.
Spread over the whole of mainland France and overseas, the CAMSPs, or early medico-social action centers, are theoretically able to take care of children with Down’s syndrome. All of the interventions carried out by the CAMSPs are covered by the Health Insurance. The CAMSPs are 80% funded by the Primary Health Insurance Fund, and 20% by the General Council on which they depend.
You should also not hesitate to call on the various associations that exist around Down’s syndrome, because they can refer parents to professionals and structures able to take care of their child.
World Down Syndrome Day
Every year, March 21 is World Down’s Syndrome Day. This date was not chosen by chance, since it is 21/21, or 03/3 in English, which echoes the presence of three chromosomes 21.
Organized for the first time in 2005, this Day aims to make the general public better aware of this pathology, which is currently the main cause of mental retardation in people with intellectual disabilities. The idea is both to raise awareness of this condition for a better integration of people with the disease into society, and to continue research to better take care of them both at the medical and educational level.
Mismatched socks to raise awareness about Down’s syndrome
Initiated in 2015 by the association “Down Syndrome International” (DSI) and relayed by Trisomy 21 France, the # socksbatlle4Ds (Socks battle for Down Syndrom) operation consists of wearing mismatched socks for World Down Syndrome Day, the March 21st. The goal: to promote diversity and difference, but also to make known the life of people with Down’s syndrome and the associations that work to support them.