Up the stairs leading down
What prospects does the medicine of the present and future offer to people with Down syndrome
Daria Spasskaya, N+1
In 2006, at the international scientific symposium dedicated to Down syndrome, a special date was established – the World Day of People with Down Syndrome. Since its cause is a trisomy on chromosome 21, the event was timed to the twenty-first day of the third month. The editorial board of N+1 decided to find out how far medicine has advanced in understanding the mechanism of development of symptoms of this syndrome, and whether it will be possible to achieve its complete cure in the future.
What is this disease?
Scientists are not in vain showing an increased interest in the study of Down syndrome – this disease, caused by the appearance of an extra copy of chromosome 21 in the embryo (usually by chance), is quite common. In 2015, there were more than five million people with this diagnosis in the world. The probability of having a child with such a chromosomal abnormality increases from 0.1 percent in 20-year-old women to three percent in women aged 45. But most sick children are born to mothers younger than 35 years old – because the peak of childbearing falls at this age. In addition, the probability of the appearance of extra copies of chromosomes or their fragments increases with age in men, so that the father can also contribute to the birth of a sick child.
An extra chromosome creates many health hazards. According to According to the U.S. Department of Health, half of children with trisomy on chromosome 21 suffer from a congenital heart defect that requires surgery immediately after birth. In addition, people with the syndrome experience vision problems, including the frequent development of cataracts, in 70 percent of cases they develop hearing disorders. Weakened immunity and susceptibility to infections are combined with an increased risk of leukemia by an order of magnitude.
Preparation of human chromosomes with trisomy on chromosome 21
National Down Syndrome Society
Muscle weakness, problems with the spine, sleep disorders, diseases of the digestive tract, decreased thyroid function – this is an incomplete list of possible (but optional) circumstances that carriers of an extra copy of chromosome 21 may face. One of the important problems of children with Down syndrome is mental retardation. With age, such people have a significantly increased risk of developing Alzheimer's disease. Nevertheless, progress in medicine has led to the fact that in developed countries, the life expectancy of people with this syndrome is currently 50-60 years, and many of them are socially adapted and are full-fledged members of society. Since most of the accompanying medical symptoms can now be eliminated with supportive therapy, the main problem of such people remains social stigma. Society exerts enormous pressure on the mothers of such children, and opportunities for rehabilitation and socialization are not available to everyone.
History of the syndrome
The phenotypic features characteristic of Down syndrome were found on the faces of figurines made 2.5 thousand years ago, and this means that trisomy is not a sign of our time. Trisomy on chromosome 21 was named Down syndrome in honor of the doctor John Down, who first compiled its clinical description in 1866 (Down himself, however, called the syndrome "Mongolism" – because of the characteristic slant of the eyes for patients). However, the genetic cause of the syndrome was discovered by French scientists only in 1959.
This discovery led doctors to formulate the hypothesis of "increasing the dose of genes", which was partially confirmed by biochemical analyses. It implies that the disorders arise from an excess of certain proteins that are produced from normal genes on an extra chromosome. Enzymes of redox metabolism, carbohydrate metabolism and some cofactors were classified as "redundant". Based on this, some researchers have suggested treating the syndrome with vitamin and mineral complexes. In the 1960s, Dr. Henry Turkel claimed to have developed a complex of 48 substances. He has been giving it to his patients for many years, and he allegedly restored their intellectual abilities. Subsequently, doctors declared the benefits of therapy based on vitamins, minerals and thyroid hormone, antioxidants and folic acid. However, no clinical trials were conducted at that time, and there is no documentary evidence of the effectiveness of these complexes.
Where do all these symptoms come from?
Human chromosome 21 (HSA21) is the smallest autosome (non–sex chromosome), however, it contains about 400 genes, 250 of which encode proteins. A study of gene expression in cell lines with an extra copy of HSA21 showed that only about a third of the genes contained on it are expressed at an excessive level. At the same time, an increase in the expression of most of them reflects an increase in the copyicity of the chromosome (that is, it increases by one and a half times), and the expression of seven percent is increased more significantly. It was among this portion of genes that scientists searched for the causes of the development of painful symptoms.
wikimedia commons
Some genes in this group may be responsible for the disruption of DNA synthesis and repair. The COL6A1 gene, which is necessary for the synthesis of a certain type of collagen, may be associated with the development of myopathy and heart disorders. Overexpression of the CRYA1 gene encoding one of the crystalline proteins forming the lens can provoke cataracts. An increase in the amount of tyrosine kinase encoded by the DYRK1A gene and expressed in the brain probably hinders the development of the nervous system. It turned out that a large number of "suspects" are concentrated in one large locus, which was called the "main site responsible for the development of Down syndrome" (DSCR – Down Syndrome critical region). However, for most of these genes, participation in the development of symptoms of the disease has not been experimentally proven.
With the development of genetic engineering, mouse models come to the aid of scientists in the study of a large number of genetically determined diseases. Despite the fact that mouse analogues of the genes of human chromosome 21 are scattered across three different chromosomes, the researchers managed to duplicate large areas containing dozens of the necessary genes and obtain animal lines showing "human" symptoms of Down syndrome. Mice of the Ts65Dn line currently represent one of the most popular models for the study of this syndrome. In particular, this line proved that the increased expression of the APP gene encoding the precursor of the amyloid protein is to blame for the early development of Alzheimer's disease in Down syndrome.
Is it possible to treat it?
To date, there is no generally accepted therapy for Down syndrome, but a number of approaches are being worked out. In addition to the symptomatic treatment of concomitant disorders, such as heart disease and thyroid disorders, the efforts of scientists are aimed at finding drugs that help restore the intellectual functions of people with Down syndrome. When looking for a promising therapy, the above-mentioned mice become the first test subjects.
A moderate positive effect on learning and memory was found when model mice were implanted with neural progenitor stem cells in the brain. However, it is premature to talk about the use of this method in humans. This also includes gene therapy, for example, suppression of the expression of the said CYRK1A gene using RNA interference. This approach also produced significant results in mice. However, it is possible to suppress the activity of the DYRK1A product in a simpler way – its natural inhibitor is epigallocatechin gallate (EGCG), which is contained in large quantities, for example, in tea.
A lot of research has been devoted to restoring brain functions by influencing the amount of neurotransmitters, for example, gamma-aminobutyric acid and serotonin. Lithium salts, which were previously used to treat bipolar disorder, performed well in animal trials. One of the pronounced biochemical disorders that accompany almost all tissues of patients with Down syndrome is oxidative stress. Experiments on mice have shown that taking antioxidants, in particular vitamin E, normalizes the number of neurons and improves the working and spatial memory of animals.
The simplest therapy that portends a minimum of side effects is physical activity and an environment enriched with stimuli. In mice, it has been proven that these conditions increase neurogenesis and the formation of synapses, which are indicators of brain development.
To date, a number of molecules have been tested on humans (mainly children and adolescents) in clinical trials conducted according to modern standards. Scientists evaluated the effectiveness of acetylcholinesterase inhibitors used in the treatment of Alzheimer's and Parkinson's diseases (rivastigmine and donepezil), the popular nootrope piracetam, folic acid, vitamin E, growth hormone, glutamate receptor antagonist memantine and EGCG tea component. Unfortunately, almost all of these drugs did not improve the intellectual performance of patients evaluated by special tests. The only exception was epigallocatechin – a small preliminary study showed an improvement in memory in children as a result of taking the substance for some time. Probably, this molecule is not hopeless, and the optimization of treatment protocols may eventually bear fruit.
Despite the failed trials of most candidates, scientists suggest that treatment can be effective if it is started in the womb, at a time when the brain is still developing. This hypothesis proved itself again in mice, but the researchers have not yet reached the tests on pregnant women.
Is it possible to simply turn off the extra chromosome?
The most radical ways to treat Down syndrome and other chromosomal disorders are offered by genetic engineering. Modern biochemical tools allow both to "turn off" extra genes and to insert new ones almost anywhere in the genome.
For example, researchers from the University of Washington were able to get rid of an extra chromosome in cells using the method of positive-negative selection, which is popular among geneticists. To do this, a cassette containing an antibiotic resistance gene and a nucleotide synthesis enzyme gene was first inserted into the chromosome using a viral vector. At the first stage, the antibiotic resistance gene helped to select the necessary cells with a cassette integrated into the genome (positive selection). At the second stage, a nucleotide precursor substance was added to the growth medium to the cells. The enzyme embedded in the cells turned it into a toxic substance, as a result of which the cells containing the cassette died – or threw out an extra chromosome (negative selection).
Another group of American scientists proposed a more elegant solution for inactivating an extra chromosome. The researchers drew an analogy with the natural mechanism of switching off the X chromosome. Women have two X chromosomes in their cells, but only one of them works, and the other is inactivated to form a compact "glomerulus" of DNA and proteins, called the Barr body. This happens with the participation of the XIST gene, from which a long non-coding RNA molecule is read, which suppresses the activity of all genes on the X chromosome. The researchers inserted the XIST gene using a genome editing tool – recognizing a certain sequence of DNA nuclease – into the locus of the DYRK1A gene already known to us. The activity of the XIST gene led to the fact that the extra 21 chromosome "curled up into a ball" and formed an inactive Barr body.
Inactivation of an extra chromosome in induced stem cells (trisomy 21 iPS cell) made from connective tissue cells (fibroblast). Inactivation is achieved by integrating the XIST gene into the chromosome using ZNF nuclease (Jun Jiang et al / Nature 2013).
Chinese scientists have proposed using CRISPR-Cas9 technology to destroy extra chromosomes. To do this, a cocktail of guide RNA "seeds" for Cas9 nuclease was injected into the cells, indicating where to cut it, and Cas9 itself. As a result of recognizing many sites, the protein "crumbled" the chromosome into pieces. Thus, the researchers managed to get rid of the Y-chromosome with high efficiency, and with an efficiency of about 15 percent – of extra autosomes, including the seventh, 14th and 21st chromosomes.
Alas, genetic engineers have not even reached mice yet – all the mentioned experiments were performed on stem cells. In addition, methods of correcting the genome of "sick" cells leave questions: how to "direct" the editing tool to one copy of the chromosome, while there are three of them in the cell? What if the integration turns out to be too effective, and two chromosomes turn off instead of one? Even if we still want to apply DNA editing methods to humans, how can we ensure that the tools are delivered to all cells?
While enthusiasts of human genetic engineering suggest correcting mutations at the stage of the embryo, however, chromosomal disorders, as a rule, are not hereditary diseases. If parents want to conceive a child "in vitro", which in any case is required for genome editing, it will be easier to select a healthy embryo at the stage of fertilization.
Thus, genetic engineering also does not promise the prospects of a complete cure for people with Down syndrome. However, she is able to offer one completely realizable thing. As mentioned at the beginning of the article, patients with Down syndrome are at high risk of developing leukemia. At the same time, researchers who are engaged in cancer immunotherapy have already learned how to replace "sick" immune cells with genetically modified ones in patients. Thus, if you take immune cells from a person with Down syndrome, turn off the extra chromosome in them and return the cells back, there will be one less potential health problem for a person.
Treat, prevent, or accept?
While it is not possible to cure Down syndrome, medicine provides expectant mothers with another opportunity – to check whether the fetus is a carrier of an extra chromosome, even in the womb. For this purpose, prenatal diagnostics is used, which is done at different stages of pregnancy. "Traditional" methods include ultrasound diagnostics, determination of the concentration of a set of protein markers in the mother's blood serum, examination of amniotic fluid or placental tissues. Their accuracy is about 80-90 percent. The best diagnosis to date is sequencing of circulating fetal DNA in the mother's blood. The accuracy of this method exceeds 99 percent, besides, the method is non-invasive, which means it is the safest for both mother and child.
Prenatal diagnosis of chromosomal diseases, for all its reliability and simplicity, has an ethical component, because the diagnosis is followed by termination of pregnancy in more than 90 percent of cases. While most doctors are convinced that this is the right choice, some experts call this state of affairs "modern eugenics".
Despite the fact that parents spend much more resources on raising a child with Down syndrome than on raising an "ordinary" child, their efforts often pay off. Considering that most of the diseases associated with the syndrome can be eliminated by auxiliary therapy, the main problem in the socialization of such people remains intellectual development. Nevertheless, in most cases, children with Down syndrome are teachable, with the use of special developmental techniques, children turn into full-fledged members of society who are not a burden to him. Some people with this syndrome have graduated from university, are successful actors or musicians. Among them, for example, Karen Gaffney, the head of a non-profit organization helping children with disabilities, or musicians of the Finnish punk rock band Pertti Kurikan Nimipäivät (PKN).
A small survey among one hundred families conducted by the Canadian Down Syndrome Society in British Columbia showed that only 40 percent of parents would like to cure their child of the syndrome if a cure existed. 27 percent of parents said they would not treat a child – they love him the way he is. This once again reminds us that "all kinds of people are needed, all kinds of people are important," and with the support of others, everyone can find their place under the sun.
Literature on the topic:
Mégarbané, André, et al. 2009.
The 50th anniversary of the discovery of trisomy 21: the past, present, and future of research and treatment of Down syndrome // Genetics in Medicine 11.9: p. 611.
Guedj, Fayçal, Diana W. Bianchi, and Jean-Maurice Delabar. 2014. Prenatal treatment of Down syndrome: a reality? // Current Opinion in Obstetrics and Gynecology 26.2: p. 92-103
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