When we think of Down’s syndrome, the first condition we think of is trisomy 21, or Down syndrome. So much so that when we talk about a person with Down’s syndrome, we actually consider that they have Down’s syndrome.
However, trisomy defines above all a genetic anomaly, and can therefore take different forms. So there are actually several types of Down’s syndrome.
What is Down’s syndrome?
Down’s syndrome is above all a story of chromosomes. We talk about aneuploidy, or more simply chromosomal anomaly. In a normal individual, who does not suffer from Down’s syndrome, the chromosomes go in pairs. There are 23 pairs of chromosomes in humans, or 46 chromosomes in all. 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 the gametes (oocyte and spermatozoon), then during fertilization.
Down’s syndrome can affect any pair of chromosomes, the best known being that which concerns the 21st pair of chromosomes. In reality, there can be as many trisomies as there are pairs of chromosomes. Only, in humans, most trisomies end in miscarriage, because the embryo is not viable. This is particularly the case for trisomy 16 and trisomy 8.
What are the most common types of Down’s syndrome?
- Trisomy 21, or Down syndrome
Trisomy 21 is currently the first diagnosed cause of genetic mental deficit. It is also the most common and viable trisomy. It is observed on average during 27 pregnancies out of 10 and its frequency increases with maternal age. There are around 000 births of children with Down’s syndrome per year in France, according to the Lejeune Institute, which offers medical and psychological support to those affected.
In trisomy 21, chromosome 21 is in triplicate instead of two. But there are “subcategories” to designate trisomy 21:
- free, complete and homogeneous trisomy 21, which represents approximately 95% of cases of trisomy 21: the three chromosomes 21 are separated from each other, and the anomaly was observed in all the cells examined (at least those 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 and, in the same individual, from one person to another. organ or tissue to tissue;
- trisomy 21 by translocation : the genome karyotype 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 …
The main symptoms of Down’s syndrome
« Each person with Down’s syndrome is first of all himself, unique, with a complete genetic heritage and his original way of supporting this excess of genes. », Details the Lejeune Institute. While there is great variability in symptoms from one person with Down’s syndrome to another, there are some common physical and mental characteristics.
Intellectual disability is a constant, even if it is more or less marked from one individual to another. There are also characteristic physical symptoms: small and round head, small and wide neck, wide open eyes, strabismus, indistinct root of the nose, stocky hands and short fingers… congenital complications can sometimes be added to these symptoms, and require extensive medical monitoring: heart, eye, digestive, orthopedic malformations …
To better cope with these complications and limit physical and mental disabilities, multidisciplinary care for people with Down’s syndrome : geneticists, psychomotor therapists, physiotherapists, speech therapists …
- Trisomy 13, or Patau’s syndrome
Trisomy 13 is due to the presence of a third chromosome 13. The American geneticist Klaus Patau was the first to describe it, in 1960. Its incidence is estimated between 1 / 8 and 000 / 1 births. This genetic anomaly unfortunately has serious consequences for the affected fetus: severe cerebral and cardiac malformations, eye anomalies, malformations of the skeleton and the digestive system…. The vast majority (approximately 15-000%) of affected fetuses die in utero. And even if he survives, a baby with Down’s syndrome has a very low life expectancy, a few months to a few years depending on the malformations, and in particular in the event of mosaicism (different genotypes present).
- Trisomy 18, or Edwards syndrome
Trisomy 18 indicates, as its name suggests, the presence of an extra 18 chromosome. This genetic anomaly was first described in 1960 by the English geneticist John H. Edwards. The incidence of this trisomy is estimated at 1/6 to 000/1 births. In 18% of cases, trisomy 95 results in a death in utero, assures the Orphanet site, portal for rare diseases. Due to serious cardiac, neurological, digestive or even renal malformations, newborns with trisomy 18 generally die during their first year of life. With mosaic or translocation trisomy 18, life expectancy is greater, but does not exceed adulthood.
- Down’s syndrome affecting the sex chromosomes
Since trisomy is defined by the presence of an extra chromosome in the karyotype, all chromosomes can be involved, including sex chromosomes. Also there are trisomies affecting the pair of X or XY chromosomes. The main consequence of these trisomies is to affect the functions linked to the sex chromosomes, in particular the levels of sex hormones and the reproductive organs.
There are three types of sex chromosome trisomy:
- trisomy X, or triple X syndrome, when the individual has three X chromosomes. The child with this trisomy is female, and does not present any major health problems. This genetic anomaly is often discovered in adulthood, during in-depth examinations.
- Klinefelter syndrome, or XXY trisomy : the individual has two X chromosomes and one Y chromosome. The individual is generally male, and infertile. This trisomy causes health problems, but no major deformities.
- Jacob’s syndrome, or trisomy 47-XYY : presence of two Y chromosomes and one X chromosome. The individual is male. This genetic anomaly does not cause no characteristic symptoms major, it is often observed in adulthood, during a karyotype carried out for another purpose.
Little symptomatic, these genetic abnormalities concerning the X and Y chromosomes are rarely diagnosed. in utero.
On the other hand, all the other trisomies (trisomy 8, 13, 16, 18, 21, 22 …), if they do not naturally result in spontaneous miscarriage, are usually suspected on ultrasound, given growth retardation, using nuchal translucency measurement, trophoblast biopsy, or ‘an amniocentesis, to perform a karyotype in case of suspected trisomy. If a disabling trisomy is proven, a medical termination of pregnancy is proposed by the medical team.