On the pathogenesis of mental deficiency in trisomy 21

J. Lejeune

International symposium on trisomy 21. Rome, 21-24 Mai 1989.

• The symphony of intelligence


• A biochemical scheme


• Genes on chromosome 21
• Superoxide dismutase :
• Cystathionine beta synthase :
• Genes on purine synthesis :
• ß amyloid precursor protein (A4) :
• Thyroid dysfunction :
• Biopterin metabolism :


• Experiments in vitro


• Theoretical considerations and clinical observations
• Clinical data :


• Conclusion


• Summary


• Annexes
• Annexes

With their upward slanting eyelids, their little nose in a round face, their incompletly chiseled features, Down syndrome patients look more children than usual. Every child has short hands with short fingers, but theirs are shorter. All their anatomy is rounded of with no asperity nor stiffness. Their ligaments and their muscles have a suppleness producing a tender langor in their posture. This general softness extend even to their character : cheerful and affectionate, they have special charm, easier to cherish than to describe.

That is not to say that Down syndrome is a desirable condition. It is an implacable disorder depriving the children from the most precious quality afforded by our genetic patrimony, the full power of rational thinking.

This combination of a tragic chromosomal error with a really attracting nature reveals in a glimpse what medecine is all about : to fight against the disease and to love the disabled.

While pondering over this evening talk, I suddenly realized that with all the progress accumulated during the last 30 years, the destiny of the trisomy 21 affected persons has not yet been substantially ameliorated.

Indeed remarkable achievements in cardiac surgery and in management of infectious or malignant diseases have greatly extended their life expectancy. But at the same time, early detection and selective abortion have drastically reduced their rate of survival.

Looking at some statistics, it seems that for a few-month-old trisomic 21 baby in utero, the rate of survival up to 10 years of age was possibly greater 30 years ago than it is today. Such an estimate includes the post natal dangers : deliberate neglect, denial of life-saving interventions or of simple nutrition, and even direct infanticide ; health by death is a desperate mockery of medicine.

Let us look at another terrible and incurable ailment : Alzheimer disease. An enormous effort, worldwide, is very aptly made in its study. Life of many millions depends of its success.

But the gene of the familial form (115) is on the chromosome 21, the gene of the precursor of the amyloid substance (11)(131), is also on 21 and trisomy 21 affected persons are especially prone to presenile dementia (59), (36), (52), although there is no excess of Alzheimer disease in their families (9). Sure enough, a microscope must not be construed into a crystal ball, but I would venture to sav that a victory over the neural disturbances resulting from the genic overdose of trisomy 21 should very likely also lead to a cure or to a prevention of Alzheimer dementia. The reciprocal prediction looks much less likely.

Could it be that in the want of a cure for the late one, it looks futile to try to cope with the inborn form of mental deficiency? Is a genic overdose for ever unamenable to treatment? So grave a matter deserves careful discussion.

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The symphony of intelligence

The message of life can be compared to a symphony : each musician (the genes) reads its score and follows the tempo of the conductor.

During a solo, a too-quick musician (in case of trisomy) could transform an "andante" in a "prestissimo" : the ears will be too small and the fingers too short. Conversly, a slow musician (in case of monosomy) could change an "allegretto" in a "largo" : the ear will be chiseled and the fingers too slender. In both cases, because the musician played a solo, he modified a trait but did not spoiled the whole symphony. Hence the type-countertype opposition between trisomy and monosomy (69).

On the contrary, when the full orchestra is concerting, all the musician playing in a "tutti", it does not matter wether the faulty musician accelerates or slows down ; the result will be cacophonic, even if he reads correctly his music! Hence the mental deficiency in trisomy as well as in monosomic states : human intelligence is the top performance of all our genetic endowment.

Detecting the discording musicians is not an easy task especially when a whole chromosome is involved like in Down syndrome. Surely, most of the genes do not produce harm when in triplicate, because trisomic children would not survive at all. Few of the accelerated reactions are dangerous ; but how will we detect the culprits among so many innocents!

This detective story could be avoided if we knew how to silence a that peculiar chromosome without disturbing the others. Let us suppose (a competent car repair-man has received from the factory a four-cylinder engine equipped by mistake with five spark-plugs. He would certainly notice that the engine does not run smoothly. An ignorant would discard this motor, but an expert would cleverly disconnect the extra plug and thus bring the rhythm to normal. Nature is that shrewd ; She knows how to silence one of the X chromosomes in female cells, so that the woman with her two X chromosomes is not so much superior to the man who has only one X and a tiny Y! We still ignore how this turning off is achieved.

Pending such a "tour de force" applied to chromosome 21 we have to analyse its genic contents and consider how it could affect neural efficiency.

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A biochemical scheme

As already discussed (72) the functioning of the brain necessitates :

- An enormous number of components : Some eleven thousand millions of neurones.

- A logical wiring of a considerable length : Some 5.000 kms if counted in dendrites and axones and from here to the moon and hopefully back if measured in the neurotubules network inside neurones.

- A specific response of the gating system, through chemical mediators acting on appropriate post-synaptic membranes.

To meet these three requirements, the brain has to synthesize a considerable amount of :

- Monocarbons for synthesis of chemical mediators and for their subsequent inactivation, and all the methylation pathways.

- Purine and pyrimidine for RNA and DNA maintenance,

- Tubuline for the wiring and biopterine for the aromatic hydroxylations of the mediators.

It has already been remarked that a block of one step of any of these pathways does produce mental retardation (72).

The painful task of unravelling one by one, the genes of chromosomes 21 is achieved by two methodes :

- The molecular biologists split the DNA and letter by letter decipher the message encoded in each piece.

- The biochemists carefully analyse chemical reactions in order to pick out these running too fast in trisomy 21.

The results of these two convergent approachs can be summarized in a general chemical scheme (see fig. 1)

- The first columms deals with purine synthesis.

- The second deals with pyrimidine (top) and purine (below) interconversions,

- The third deals with folate and monocarbons metabolism (top), biopterin and hydroxylases (middle) and methylation (bottom).

At the far right end products appear, required for an informative network (tubulin), a gating process (chemical mediators) and an insulating system (myelin).

These three categories are absolute prerequisites for the function of the brain (see (72) for general discussion).

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Genes on chromosome 21

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Superoxide dismutase :

First enzyme assigned to a gene on chromosome 21, superoxyde dismutase activity is increased by a 1,5 factor (120). Too much O2- is transformed into hydrogene H2O2. Glutathion peroxydase, which disposes of H2O2 into H2O, is also increased (121), although its gene is in chromosome 3. Remarkably a glutathion peroxydase like gene is located on chromosome 21 (85).

Superoxyde ion is required by indolamine oxydases ( tryptophane and hydroxytryptophane) and biopterines are involved also in these reactions (90).

Experimentally SODI protects the activity of the 5'deiodinase, normally inactivated by superoxide ion (55). Thus excess of SODI could increase the transformation rate of rT3, into inactive T2 thus contributing to the low rT3 level found in trisomy 21 (76).

In transgenic mice (5) excess of the Cu/Zn SOD gene produces abnormal neuromuscular junctions reminiscent of these seen in trisomy 21.

It is also to be remarked that the production of superoxide ion by human neutrophiles is inhibited by adenosine acting upon a membrane receptor (27).

The excess activity of SODI could thus be related to :

- Diminished input of oxydized monocarbons (indoleamine oxydases) and impaired biopterine metabolism

- Decrease of rT3 (5'deiodinase)

- Correlative change in neuro muscular junction

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Cystathionine beta synthase :

Inactivity of the cystathionine beta synthase (CBS), leads to accumulation of homocysteine not transformed in cystathionine - Homocystinuric children are tall, slender with long tapering fingers with extra flexion creases, contrasting to the small stature and short fingers lacking some creases of trisomic 21 children. This type and counter type effect led to prediction of an anomaly of CBS (70) confirmed ten years later by the localisation of the gene on chromosome 21 (123) and the dosage effect demonstration (19).

In homocystinuria, excess of S-adenosyl-homocysteine (SAH) compete with S-adenosyl-methionine (SAM) and inhibits the transmethylases. In trisomy 21, insufficient level of homocysteine (20) could impair the remethylation pathway via 5 methyl-THF and B12, and slow down the recovery of SAM. An insufficient speed of methylation was demonstrated 30 years ago (45) for nicotinamide.

Homocysteic acid promotes the growth of the rat (24), but it remains an open question how homocysteine availability, which regulates the thymidine synthetase, could also modify the thyroid regulation or the growth hormone production.

S100ß. Recently localized on chromosome 21 (2), the S100ß protein subunit plays an important role in neural function. Appearing late in the forebrain (23), (142), its level is high in hippocampal region, it increases during the learning process (56) and resembles the neurite extension factor (66). Modulated by calcium (95),it has also great affinity for phenothiazine (83) and, remarkably,for zinc(7).

Zinc has neurotrophic properties (128), it modulates the thyrotropin excretion (60) and modifies the sensitivity of the N-methyl-D-aspartate receptor in the hippocampus (136). Zinc is reported to be beneficial in Down syndrome : it enhances neutrophil chemotaxis and the immune function (10), it reactivates the serum thymic factor (39) which is low in Down syndrome as well as in hypothyroidism (37).The increase of (CuZn) SOD and of S100ß could very well increase the requirements of Zn ++. In addition 5'nucleotidase, the main producer of adenosine, is also a Zn++ requiring enzyme.

Beside these interactions, an excess of S100ß could be deleterious because of its ability to disassemble brain microtubules (30).

Phosphofructo kinase (PFK). Excess of activity of PFK could increase the fructose-1, 6-diphosphate which is known to accelerate the biotin acetyl-CoA carboxylase, first step of lipid synthesis. Whether this could be related to adiposity is not known.

Curiously, S100ß has a special affinity for fructose-1, 6-diphosphate aldolase (141), the step following the PFK. Could this peculiarity be another indication of a subjacent biochemical logic to gene localization?

The same reflexion could apply to the enhancement of PFK activity by NH4 produced by the AMP deaminase in the "purine cycle" (80) especially important in the brain, and probably abnormal in trisomy 21.

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Genes on purine synthesis :

Slight overproduction and overexcretion of uric acid in Down syndrome patients has been recognized long ago (42),(93),(4). Among the various steps of purine synthesis, three are known to be controlled by genes on chromosome 21 : the third, leading to synthesis of 5-P-rybosylamine, the fourth, formyl-transferase leading to N-formylglycineamidine riboside (FGAM) and the sixth, amino-imidazole synthetase, leading to 5-aminoimidazole riboside (AIR).

Overproduction of purine necessitates more phosphoribosyl pyrophosphate ( PRPP)and the first step in the production of its precursor ribulose-5-P, by the hexose monophosphate shunt is accelerated (121),(88),(105).

Although the demand upon PRPP and monocarbon metabolisms remains moderate, there are reasons to believe that purine methabolism unbalance is worth further investigations.

In trisomic 21 erythocytes (111) and lymphocytes (110), there is an excess of adenosine deaminase ( ADA) and of purine nucleoside phosphorylase (PNP), in accordance with increased urate excretion. An excess of AMP (63) and ADP (129), (81) exists in erythrocytes with however a normal level of ATP.

This would draw attention to adenosine metabolism, especially because a block of adenylosuccinate lyase producing AMP from adenylosuccinate as well as 5-amino-4-imidazole-carboxamide riboside (AICAR) from 4-N-succinocarboxamide-5-aminoimidazole riboside (SCAIR), which is located on chromosome 22 (65), induces a very severe syndrome described by JAEKEN et al. (57)(58). The psychotic behaviour is quite the countertype of the happy character of "easy going" Down syndrome children.

But, in rare case, severely affected trisomic children exhibit autoagressive behaviour, biting their fingers, banging their heads, quite reminiscent of the Lesch-Nyhan syndrome. In this devastating disease the lack of hypoxanthine guanine phosphoribosyl transferase (HPRT) prevents the salvage of guanine, hypoxanthine and xanthine, and necessitates a excessive synthesis of purine to cope with this permanent leakage. Some Lesch-Nyhan patients excrete AICAR, possibly by insufficient availability of 10-formyl tetra-hydrofolate, too severely sollicitafced by the purine production (107). Remarkably AICAR is the step just following SCAIR, the product insufficiently metabolized in adenylosuccinate lyase deficiency.

The block of adenylosuccinate synthase in mouse cells produces an excess of GTP (133), demonstrating the intrication of these regulations.

Adenosine modulates the release of chemical mediator (106), (34) at central and peripheric synapses. Its action essentially presynaptic, has numerous pharmacological consequences (38), (41), (28), (82), (138). From the experimental data one can forsee many effects which could result from excessive adenosine formation and compare them to frequent trisomy 21 symptomes :

- A mild deficit of immune reaction, suggested by the dramatic effect of ADA deficiency (68), vs. the classical sensitivity to various infections.

- A deficiency of the neuromuscular transmission (118), (122), the severe hypotonia.

- A deficit of growth hormone secretion (31), vs. short stature with normal sensitivity to growth hormone (3).

- Inhibition of lipolysis after adrenergic stimulation of the adipocytes (117), (135) especially in hypothyroidism (92), vs. frequent obesity and thyroid deficiency.

- Instability of glycemia (17), (78), (139), vs. the frequent prediabetic state.

- Abnormal pupillar reaction (48), Vs. the hypersensitivity of the iris to atropine (71).

- Relaxation of the vascular muscle (28), vs. the livedo reticularis, observed also in homocytinuria, and relaxation of intestinal muscle (28), vs. the frequent constipation.

- The hypersensitivity of fibroblastes to ß-adrenergic stimulation (87) could possibly be related to adenosine effect.

The importance of adenosine modulation in the cerebellum, in the hippocampus (33) and on the innervation of trigeminal mescencephalic primary afferent neurons from hypothalamus (89) could be compared to the waddling gait, the frequent grinding of teeth, the nystagmus and, possibly, the slight instability of the thermoregulation in the newborn.

Similarly, adenosine deficiency could produce instability, irritability, anxiety or even convulsions, as suggested by the effects of adenosine antagonists like theophylline (106), (82), (112) and caffeine (32), (29), (26), (67) or even psychotic behaviour as suggested by the antipsychotic like properties of adenosine receptors agonists (51).

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ß amyloid precursor protein (A4) :

Different from the locus of the familial predisposition to Alzheimer disease (115), (15) the B amyploid precursor gene (46) is closer to the centromere (21q21.1) than the "Down syndrome region" (21q22.1), (108), (130), (131).

Accumulation of amyloid substance in the plaques is one of the symptoms of Alzheimer disease (86). It occurs also In Down syndrome and other diseases (16), (47), in angiopathy (25), in thyroxin transport defect (transthyretine) (54), (84) or even in pugilistic dementia (53), (114).

The first 28 aminoacids of A4, as a free polypeptide, increase the survival of pyramidal neurons of the hippocampus cultured in vitro (137). Accumulation of A4, could result from an inappropiate utilisation and be a symptom more than a cause of the cerebral impairment.

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Thyroid dysfunction :

BOURNEVILLE was the first, in 1906, to treat by opotherapy, hypothyroid in a trisomic patient (14). The two diseases have always been closely linked and BENDA described "Thyroid exhaustion" after a series of autopsies (8).

Since 1981 (116) an excess of thyreostimuline hormone (TSH) has been widely observed (for a general review, see ret. (76)). Aside from the increased frequency of true hypothyroidism, a moderate excess of TSH is found in nearly half of apparently euthyroid patients. Tetraiodothyronine (T4) and 3, 5, 3' -triiodothyronine (T3) remain within normal values. Moreover, a characteristic deficit of 3, 3', 5' - triiodothyronine, or reverse T3 (rT3) is observed (76).

The ratio rT3/TSH (an index of the yield of rT3 per unit of TSH) is highly significantly diminished. This rT3/TSH ratio, highly correlated with T3 level normally, is not correlated in trisomy 21 (76).

These new facts demonstrate a thyroid dysfunction, especially for rT3 which regulates T4 and T3 (22).

Low rT3, possibly related to superoxide dismutase excess (see above) could impair growth hormone stimulation (94) possibly jeopardized also by an adenosine effect (see below). Intrication of these regulations is illustrated by the fact that the level of 5' nucleosidase, one of the producers of adenosine, is controlled by thyroxine (124).

Remarkably, thyroid hormone induces gene expression through a responsive element common to retinoic acid (134) ; disorder of carotene and vit. A is frequent in Down syndrome (127).

Monocarbon metabolism, which could play a keyrole in mental deficiency (72), is strongly connected to thyroid function. Thyroxin increases the input of oxydized monocarbons (-CHO) by activating the 10-formyl-synthase and the output of reduced monocarbons (-CH3) by activating the 5-10-methylene -THF-reductase (119). Simultaneously thyroxine preserves the monocarbon pool by inhibiting the 10-formyl -THE- dehydrogenase (which eliminates -CHO into CO2) (62), and by inhibiting the cystathionase (21) (which disposes of homocysteine into cysteine and homoserine).

This interference with the methyl carrier is so much more interesting, since methionine has antinomic properties (119). It increases activity of 10-formyl-THF-dehydrogenase (deplenishing the monocarbon pool) and blocks the 10-formyl-THF-synthase and the 5-10-methylene-THF-reductase (diminishing the input and the output).

Hence, thyroxine and methionine metabolisms, both abnormal in trisomy 21, play antagonistic roles upon the monocarbon metabolism, abnormal also in this disease (see purine metabolism).

Tubulin organisation, using GTP (140) is largely controled by thyroid function (91). As already discussed (74), the spindle of the mitotic apparatus and the neurotubules are made of the same building blocks : tubuline. Hence a cell has to choose, either to keep assembling and disassembling tubuline for mifcotic machinery or to mount tubuline into an inner informative circuitry. And this ordinated network is disrupted by neurofibril tangles in three diseases : Down syndrome, Alzheimer and hypothyroidism.

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Biopterin metabolism :

Controlling the production of adrenergic and serotoninergic mediators via hydroxylases, biopterin seems imperfectly regulated in trisomy 21 (49) ; low level of BH. in the brain suggest a deficit of dihydrobiopterin reductase (QDPR).

A recent investigation (18) showed an elevated urinary ratio of neopterin/biopterin , a shift already observed in Alzheimer disease (49). A slight overproduction of GTP, together with a lower quinonoid-dihydropterine-reductase (QDPR) efficiency could be present.

Enzymes controlling folate (a vitamin ) and biopterin (a home made cofactor) can partially replace each other, suggesting a kind of failfree system (74). Both can be eliminated by oxydation into isoxanthopterin.

A trisomic 21 child, overeliminating isoxanthopterin (18) was secondarily found by anamnesis to have been at that time suffering of a typical regression diagnosed two months later as a severe hypothyroidism.

Whether thyroid hormones could prevent folate and biopterin elimination via isoxanthopterine is an open question.

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Experiments in vitro

In 1985, clinical serendipity paved the way to experimental investigation, PEETERS et al (96), (97) discovered that leukemic children could not tolerate normal dosage of methotrexate if they were also affected by Down syndrome. Toxicity of this inhibitor of dehydrofolate reductase appeared at half the normal dose (99). This phenomenon has since be amply confirmed. (40), (98), (45).

In 1986, we demonstrated (73) that trisomic 21 lymphocytes are twice as sensitive than normal ones. Sensitivity to chromosome breaks is also increased (15), (104). These facts are in accordance with the symptomes of folate deficiency in the leucocytes (44).

Generally speaking, it must be remembered that methotrexate is dangerous for the human brain as observed after treatment for leukemias, with or without cranial irradiation (61), (109), (35), (1).

Although time consuming, the mitotic index method allows a first approach of the specific sensitivity of trisomy 21 lymphocyte as exemplified by the results of PEETERS (101). Methotrexate hypersensitivity is fully confirmed (103), (99) and systematic investigation showed that thymidylate synthase and thymidine kinase pathways are normal (103).

Contrasting to this methotrexate toxicity, 6-mercaptopurine (inhibitor of IMP dehydrogenase and also of adenylosuccinate synthase) has at low doses a beneficial effect upon the mitotic index (102). Aracytine has also a beneficial effect (101). All these facts points toward a disregulation of the folate metabolism with a desequilibrium between adenosine and guanosine derivatives as well as between purine and pyrimidine pathways.

IMP dehydrogenase can also be modified by a specific inhibitor : mycophenolic acid (MPA) (79),a natural modulator of 2-3 diphosphoglycerate (2-3 DPG) (77) and reverse triiodotyrosine (rT3). In trisomy 21 erythrocytes, 2-3 DPG is low (63), (64), (81), its synthesis is under thyroxin command (123), (126) and rT3 is also low in trisomy 21 (76). Preliminary data show a very tight correlation between the effects of these three products when tested on the lymphocytes of the same patient.

A possible explanation of the disequilibrium observed in purine metabolism could be that a small excess of GTP (see biopterin ) would accelerate adenylosuccinate synthase and in the same time diminish the IMP deaminase, thus more AMP would be available. In the same time disposal of homo-cysteine by C B S would liberate S-adenosyl-homocysteine hydrolase. These two effects could increase the adenosine production (see purine synthesis).

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Theoretical considerations and clinical observations

From these considerations an heuristic investigation could be directed toward :

- Controlling the dysthyroidism.

- Compensating the abnormal purine derivatives.

- Equilibrating the homocysteine/methionine pathway.

- Increasing folate or biopterin availability.

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Clinical data :

No systematic attempt has been made because each child has been treated with the medication considered the best for his personal state.

Nevertheless some data on thyroxine, methionine and folic acid, however scanty or possibly biased, can be extracted from the observations accumulated with the collaboration of associate Professor M.O. Rethore and Doctors M.C de Blois, C. Panqalos, M. Peeters, M. Prieur, P.M. Sinet and O. Raoul at the Hospital des Enfants Malades.

Thanks to the extraordinary cooperation of the patients and of their sibs, and with the fully informed consent of their dedicated parents, laboratory and/or clinical examinations are gathered at a rate of some 2.000 per year, with a total of some 5.000 recorded files.

Every six months, a patient is submitted to a psychometric test. His personal progression is compared step by step to the general chart established long ago on hundred Down syndrome children, (see fig. 2)

After each period of six months under a given medication, an eventual "inflexion" of the personal curve is calculated according to the expected evolution (see legend of fig. 2). This parameter express the eventual betterment or deterioration of the personal curve of a given child. "Inflexion" has by definition a mathematical expectation of zero and an experimental standard deviation of about 0.5.

All the assays have been followed in this manner. If the mean "inflexion" is statistically different from zero, this is taken as an indication that the "treatment" has modified the developpment which would have probably taken place if no "treatment" had been given.

a) Thyroxin therapy. (see table 1)

Thyroxin (T4) was prescribed only if TSH was elevated with low T3 or T4. Varying from 3 mcg./kg/d. at 6 months to 1 mcg./kg/d. after 12 years of age, the dose was adjusted according the ensuing shift in TSH, T3, T4 values.

For children less than 5 years of age, 95 "inflexions" could be calculated. The mean I.Q. of this sample being 74,3 ± 15,9 the patients could be classified as "gifted" above 70 and "less gifted" below 70. The 36 "less gifted" had a mean inflexion of + 0, 454 ± 0,709, this value being significantly higher than the + 0,140 ± 0,754 observed in 82 equally "less gifted" of the same age receiving no T4 (and no methionine). The 59 "gifted" ones had a mean inflexion of - 0,201 ± 0,724 which did not differ from the - 0,062 ± 0,720 observed in equally "gifted" not receiving T4 nor methionine.

The highly significant difference between "gifted" doing poorly with thyroxine and "less gifted" getting a great benefice of it (t = 4.3; P<<0,001) is remarkable.

b) Methionine medication : (see table 2)

For children less than five years of age 275 "inflexions" are available for patients receiving methionine(61,7 mg/k/j. ± 21,6) with no association to T4 or folinate.

Splitting the sample at the mean I.Q. (69,7 ± 12,7) we observed for the 133 "less gifted" a mean inflexion of ± 0,381 ± 0,777 and for 142 "gifted", a mean of - 0,183 ± 0,655.

The difference is highly significant (t = 6,53 ; P<< 0,001), the effect being significantly favourable for the "less gifted" (0,05> P> 0,025) and being nonsignificantly unfavourable for the "gifted" ones.

For children more than 5 years of age, the same tendency is observed, although not significant ; for 136 "less gifted" (below the mean I.Q. of 55) inflexion = + 0,062 ± 0,509 and for the 150 "gifted" (I.Q. above 55) infl = - 0,038 ± 0,471.

c) Folic acid medication : (see table 3)

143 inflexions are available for less than 5 years old children who received neither methionine nor T4.

69 received moderate folic acid doses ranging from 5 mg to 35 mg per week and 74 received no folate at all. The mean "inflexion" was + 0,1514 ± 0,828 for the folate group against - 0,037 ± 0,649 for the non folate, the difference being not significant.

For children more than 5 years of age, the same tendancy is observed and all the data could be pooled (children from 1 year to 12 years of age).The 109 folate receivers showed a mean inflexion of + 0.129 ± 0.689 and the 172 non folate a mean of - 0.054 ± 0.508. The difference is significant t = 2.56 ; 0,05>P>0.01).

A multivariate analysis on a greater sample (now in progress) will be required in order to confirm this seemingly benefical effect of moderate doses of folic acid and to investigate an eventual dose/effect relation.

d) Folinic acid medication : (see ref. 75)

A trial of medication with folinic acid (5-formyl-fcetrahydrofolate) was performed on 39 severely affected trisomic 21 patients (75). Thirty of them had an infantile psychosis and the other 9 suffered from Alzheimer-type regression.

the 69 assays, 57 were favourable and 52 were not, with no untoward effects. Considering the quasi-inexorable course of these two complications, this modest result is nevertheless very suggestive of a really beneficial effect. Unexpectedly, a dose/effect relationship was noted, suggesting the efficient dose being rather high at some 0.8 to 1 mg per kilo per day.

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Conclusion

No simple management, short of chromosome turning off, can be predicted for trisomy 21.

The chemical basis of the mental deficiency must be a disruption of a fantastically co-ordinated system. The metaphore of a "concerting" orchestra was not purely rethoric. Trisomy 21 is dis-concerting. For each chemically defined disease known to produce mental retardation, one finds a more or less evident anomaly of the sensitive step in Down syndrome.

This evidence is, in a sense, encouraging. For example it could very well be that very regressive infantile psychosis sometimes observed, were sequelae from an hypothyroidism that went unnoticed and reequilibrated itself later.

The same may be true for purine, pyrimidine, folic acid, biopterine or methionine metabolisms. Surely we are not yet able of restoring the destiny, but we are possibly just in the situation of being able to prevent its worsening.

It must be very precisely stressed that this general model is for the moment strictly speculative. Even if the reasoning was sound, it would remain to be seen whether the correction of such troubles will, in the long run, alleviate the mental deficiency.

Mr. President and dear friends, please forgive me for this too technical discussion. Its only purpose was to show that research on the pathogenesis of inborn mental deficiency is really begun.

Sure enough nobody knows the length of the road to be covered before reaching the achievement we are all longing for, dedicated parents, skillfull teachers and research workers. But one thing is certain, thanks to cooperation from all fields of Science, we will indefatigably try to render to the children injured in their intelligence, this marvelous glaring which is the mark of the spirit.

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Summary

In trisomy 21, pathogenesis of mental retardation is still poorly understood although the knowledge of the genic content of chromosome 21 is steadily increasing. Short of discovering how to silence selectively one of the three exemplars of chromosome 21, no rational medication can be envisaged before pathogenis has been unraveled, at least partially.

A biochemical scheme of impairment of mental efficiency is presented. Secondarily, the possible deleterious effects of a given gene overdose, are discussed. Cu / Zn SOD, cystathionine beta synthase, S 100B protein, phosphofructokinase, purine synthesis and adenosine pharmacology, thyroid disturbance and elevated TSH with low rT3, as well as biopterine metabolism interferences are reviewed.

It is observed that the metabolisms controlled by these genes, although unrelated at first glance, are in fact tightly related by their effects. Just as if synteny was in some way related to biochemical cooperation or mutually controlled regulation.

Experiments in vitro, have demonstrated a peculiar sensitivity of trisomic 21 lymphocytes to methotrexate. From this starting point, systematic research of special sensitivities is underway.

Clinical observations and relevant statistical methods allow the study of the speed of mental developpment under various medications. The interest of regulating thyroid metabolism, when needed, is exemplified. Reequilibration of monocarbon's metabolism is discussed and the seemingly favourable effect of folinic acid medication in pseudo-Alzheimer complication is presented.

fig. 1.

fig. 2. (cf annexe)

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Legend of fig. 2.

A cohort of hundred Down's syndrome affected children was followed from one year up to 14 years of age. A psychometric test was administered twice a year (Brunet-Lezine first, Borel Maisonny later and finally Binet-Simon). Mean mental age was reported in ordinates with chronological age in abscisses. The local value of standard deviation (S.D.) is given in months.

Analysis of more than 700 successives tests (independant of those reported here)"demonstrated that each child follows his "personnal trajectory", roughly parallele to the curve of the general mean. For each child fluctuations around his own trajectory happened to be contained inside a corridor having a width sensibly equal to one S.D. of the general population (data not shown).

On the fig. 2 for example the child had a mental age of 3y. at 4y. of age. His next performance six months later, Xe, was thus expected to lie on a line coparallele to the curves of the mean and of the S.D. ; Xe = 3y.3m. at 4y.6m. of chronological age.

The observed performance, Xo, being 3y.10m., the "inflexion" of his curve can be expressed as 3y.10m. - 3y.3m. / 6.5m = + 7/ 6.5 = 1.08

This "inflexion" Xo - Xe / local S.D.has a mathematical expectation equal to zero

The standard deviation of this parameter was found to be of the order of 0.5, ranging from 0,4 for children between 5-12 years of age to 0.7 for children less than 5 years old.

For a cohort of children submitted to a given treatment, the "mean inflexion" ± s.d, can be calculated (see tables 1, 2 and 3). If this "mean inflexion" is more than twice its s.d., this is taken as an indication that the medication has modified the speed of development of the mental age.

Haut

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