Vingt Ans Après
Jérôme Lejeune
Trisomy 21. An International Symposium. Edited by G. R. Burgio, M. Fraccaro, L. Tiepolo, U. Wolf. Springer-Verlag Berlin Heidelberg 1981
Résumé :
Summary. A systematic review of the biochemical troubles associated with Trisomy 21 lead to three constatations : 1) There must be an important perturbation of the oxygen metabolism (Excess of S.O.D.I. and of G.P.X.) 2) There is probably a shift in the regulation of non essential amino acids (Comparison with other forms of mental deficiencies) 3) In conformity with a general hypothesis, an abnormality of the monocarbon metabolism could be the consequence of 1) and 2) and could represent the actual cause of mental deficiency.Sommaire
Au Professeur Marco Fraccaro
En réunissant ce symposium, vous avez fait preuve de la même imagination qu'Alexandre Dumas donnant une suite aux "Trois Mousquetaires". Et cous avez, comme lui, fait preuve dune souveraine indifférence pour le quantitatif puisque comme chacun sait, les trois mousquetaires étaient quatre, et que noun sommes ici, grace à vous, plus de deux cents !
Que savons-nous de la trisomie 21, après vingt ans de recherche ? Quel sujet de meditation et d'inquiètude aussi ! Certes, nous avons appris bien des chosen, et même à reconnaître la maladie chez des enfants très jeunes, encore au ventre de leur mère. Mais si ce pouvoir nouveau a suscité chez certains la tentation d'éliminer les malades extrêmement jeunes, cette connaissance n'a fait en aucun cas régresser la maladie.
Et c'est pourtant la maladie qu'il fact vaincre, et les patients qu il faut guérir !
Introduction
Twenty years after the description of the chromosomal basis of the disease (Lejeune et al. 1959), the main question remains unanswered: "what is the actual cause of mental deficiency in trisomy 21 ?"
Sure enough, the first hypothesis of a genic dosage effect (Lejeune 1963) has been followed by a considerable amount of information. For instance, the gene of superoxide dismutase A (Tan et al. 1973; Sinet et al. 1976 a) has been localized in the region of the bands q2.21 of chromosome 21. Hence, the excess of superoxide dismutase A in trisomic 21 patients (Sichitiu et al. 1974; Sinet et al. 1974) shows the convergence of biochemical, cytogenetic, and clinical investigations.
The shift of the oxygen metabolism is complicated by the fact that glutathione peroxidase (nonlocated in the 21) is also elevated (Sinet et al. 1976 b). More recently, it was demonstrated that the oxygen metabolism disturbance must have some relationship with the mental retardation (Sinet et al. 1979): a positive correlation is observed between glutathione peroxidase activity and the IQ.
Obviously SODA excess could transform O2- to rapidly into H202 and partially deprive the organism of O2 and increase the amount of toxic H202. On the other side, glutathione peroxidase elevation could dispose of the excess of H202 by transforming it in H20. This protective effect seems plausible, but it is entirely unknown how scavenging too much of the superoxide ion 02 could be deleterious.
Before trying to discuss this difficult question, it seems appropriate to ask ourselves whether the anatomoclinical method could reveal some hint, some heuristic models. The goal is to figure out where a link between mental deficiency and trisomy 21 should be sought.
No doubt the tiny 21 contains many genes and some, if not all of them, could exhibit a dosage effect. Truly, such a search could seem entirely hopeless at first glance, but I shall try to show that methodological reflection could tell us that some specific observations are to be performed.
A General Outlook of Metabolic Disturbancies Leading to Mental Retardation: Possible Central Role of the one Carbon Cycle
In a few preceding papers (Lejeune 1977, 1978), a general representation of the chemical reactions going on inside the neurons was presented in the form of a pseudo-pascalian machine. If the genetic diseases actually known are reported on such a scheme, it becomes possible to state the dynamic effect of each disturbance.
Curiously, the blocks are not located randomly on the machine but cluster clearly in two regions: one is related to manufacture and disposal of insulating substances, protecting the nervous network from short circuits. The other is related to the production of the constitutive elements of the membranes (glycoproteins and mucopolysaccharides).
As already discussed (Lejeune 1978), such a clustering is expected if we consider that membrane efficiency on the one hand and reliable circuitry on the other hand are two prerequisites for an engine able to simulate some of the functions of the mind. Here, the analogy with computer is too obvious to be considered again.
However, a third group of points remains, rather scattered on the machine and apparently entirely unrelated. This heteroclite sample includes, for example: phenylketonuria, tyrosinosis, hypothyroidism, maple sirup disease, homocystinuria, histidinemia, carnosinuria, hyperprolinemia (I and II), hyperhydroxyprolinemia, iminodipeptiduria, nonketotic hyperglycinemia, sarcasinemia, hypoxanthine-guanine phosphoribosyltransferase deficiency, Hartnup disease.
Interestingly enough, inspection of the machine shows that all these various conditions could have a common consequence, a disturbance of the one carbon cycle. A simplified, so to speak, diagram of the machine will demonstrate this overall impression (Fig. l ).
Sarcosinemia, homocystinuria, and nonketotic glycinemia would disequilibrate the 5-10 methylene-THF entry to the one carbon cycle as well as the S-adenosylmethionine-methyltransferase system.
Histidinemia, and carnosinuria would deplete the formimino-THF entries. The excessive synthesis of purines in hypaxanthine-guanine phosphoribosyltransferase deficiency (Lesch-Nyhan disease), and Hartnup disease, produce a relative depletion of the 10 formyl-THF.
The general result of prolinemia I and II, hyperhydroxyprolinemia, and iminodipeptiduria is that all of then disequillbrate the glyoxylate metabolism either by the loss of proline and hydroxyproline (iminodipeptiduria) or by a deficiency of recuperation of glyoxylate from the catabolism of hydroxyproline.
Phenylketonuria, tyrosinosis, and maple sirup disease remain, apparently quite distant from this system, but the fact that metabolites of phenylalanine (Ulevitch and Kallen 1977) and of isoleucine (Hillman and Otto 1974) are strong competitive inhibitors of the serine transhydroxymethylase put back the effect of these three diseases in the same part of the one carbon cycle, the 5-10-methylene-THF entry. The sane effect is found in diseases affecting the interconversions of the various forms of folate (Rows 1978).
As a very broad and very tentative hypothesis, it could be postulated that in case of mental retardation in which there is no gross anatomic defect of the brain, no obvious disturbance of the insulating substances, no demonstrated abnormality of the membranes building blocks, a deficiency of the one carbon cycle could be the most likely trouble to be looked for.
Coming back to the diagram, it can be outlined that a main stream seems very sensitive: the recuperation of methyl from acetylcholine, the methyltransferases, and the transformations, serine ? glycine ? glyoxal ? ?1-pyrrolin ? carboxylate (via ?-glutamic semialdehyde) to both glutamate and proline metabolisms. A systematic analysis of trisomy 21 could be tried, following these lines.
Fig. 1. - Simplified scheme of
the mechanisms discussed (after Lejeune 1979)
Special Pathology of Trisomic 21 Children
The Cholinergic System
To start with the acetylcholine manufacture and recuperation, a first constatation is that trisomic 21 children are hypocholinergic. Their hypersensitivity to atropine (Berg and Stern 1962; Harris and Goodman 1968) and the pharmacologic sensitivity of their iris to various chalinomimetric and cholinolytic drugs is amply established (Priest 1960). A constitutional hypocholinergy is the simplest explanation of these findings (Bourdais 1973; Lejeune et al.1976).
Remarkably, trisomic 21 adults are very prone to Alzheimer-like presenile dementia (Jervis 1970) - even the ultrastructural anomaly of the entangled neurofilaments is entirely comparable in both disease (Ellis et al. 1974) - and, as it is well-known, Alzheimer disease is also related to a cholinergic abnormality (deficiency of acetylcholinesterase and of choline acetyltransferase, Davies 1979).
Serine and Glycine Pathway
No obvious trouble is actually known in trisomy 21, but a few hints are available. Compared to their healthy brothers and sisters, trisomic 21 patients have a slight but significant diminution of serine level in the blood (Sinet 1972). They also show a slight increase of ethanolamine. It is worth mentioning that an ethanolamine increase is found in sarcosinemia, which, diminishes the reuptake of methyls from choline.
Glyoxylate Pathway
Glyoxylate is among the most versatile substances. Surely its regulation is not normal in trisomy 21. For example, a tryptophan load (McCoy 1969; E. McCoy, personal communication) increases the urinary excretion of oxalate (the end product of glyoxylate) in normal children much more than in trisomic 21 children. Furthermore, if an artificial deficiency of vitamin B6 has been produced previously by deoxypyridoxine administration, the oxaluria after tryptophan load becomes much more marked among trisomic 21 children than normals.
It could very well be that glyoxylate metabolism in man could be of a quantitative importance in the biosynthesis of nonessential aminoacids, such as glutamate and proline as we will see later.
Glutamate and Proline Pathway
From experimental data already quoted (Sinet 1972), it was shown that glutamate is significantly elevated in the blood of trisomic 21 patients compared to their healthy sibs. This increase could be taken care of by introduction of glutamate to the Krebs cycle, a possibility in accordance with the fact that those with trisomy 21 excrete more urea than their normal sibs (Jérôme et al. 1960; Sinet 1972) and possibily derive more energy from amino acids compared to sugar than ordinarily expected.
For proline, not elevated in the blood of trisomic 21 patients, very few experimental data are available. Nonessential amino acids are rarely extensively studied, but three remarks are worthwhile.
Among the diseases producing disturbance of growth and morphology comparable to trisomy 21, congenital hypothyroidism is preeminent with the short stature, the short fingers, the broad rose and, of course, the mental retardation. Besides trouble of combustion through FAD systems (a possible analogy to the disturbed metabolism of O2- due to SODA in trisomy 21), hypothyroidism implies a diminution of the synthesis of collagen from proline and a diminution of elimination of hydroxyproline after pro-collagen or old collagen has been disassembled during growth and the modeling process (Uitto et al. 1968).
Iminodipeptiduria is also very relevant here. Affected persons lack prolidase activity and thus eliminate X-CO-NPRO and X-CO-NHYPRQ dipeptides that they cannot cleave. Thus, they cannot recuperate the proline and OH proline from used collagen or pro-collagen and daily loose some 40 mg of proline and some 10 mg of hydroxyproline per kilo (Goodman et al. 1968; Jackson et al. 1975). Iminodipeptidurics have many morphological features in common with trisomic 21 patients: the shortness of body and fingers, syndactyly between the second and third toes, unique palmar crease, waddling gait, rough skin, and mental retardation.
Like those with trisomy 21, they are also very susceptible to naso-pharynx infection. It has been postulated (Jackson et al. 1975) that this sensitivity could be related to the fact that the C1q fraction of the complement has a collagen-like polypeptide chain. An effect on the fixation of other immunologic factors and a possible relation with the abnormal uptake to serotonin (Jérôme and Kamoun 1960 and epinephrine (Jérôme and Kamoun 1970) by the platelets of trisomic 21 patients has been discussed (Lejeune 1979).
Tryptophan Pathway
As already mentioned, the platelets of trisomic 21 patients do not take up serotonin as well as normals. A tentative treatment was tried to palliate this defect. 5-Hydroxytryptophan was administered to trisomic 21 children and reported to increase their muscular tone (Coleman 1971). Unfortunately, this medication turned out to be very toxic, producing devastating hypsarythmia (West syndrome). These dramatic clinical results are in accordance, possibly, with a maladjustment of the glyoxylate metabolism already quoted.
Homocystine Pathway
Having thus followed the steps dictated by our general hypothesis, we have effectively noted that each of then was likely to be perturbated slightly in trisomy 21. No blockage has been noted, which was expected in a case of excess of genetic material, but a dynamic disequilibrium is to be suspected at each of these steps.
Homocystinuria remains, which is entirely dissimilar to trisomy 21 as already pointed out (Lejeune 1977) and, phenotypically speaking, quite the countertype of it. If it is remembered that homocystinuria disturbs the 5-methyltransferase system that is the output of the one carbon cycle to the manufacture of methylated neurotransmitters (acetylcholine among others), it could very well be that the countertype aspect could be related to a regulation, a feedback effect, on the ?1-pyrrolin carboxylate pathway and the proline synthesis and incorporation; anyway, homocysteic acid itself has been demonstrated stimulating the growth of the long bones (Lejeune 1977).
Heuristic Prospect
This general analysis of metabolic blocks known to produce mental deficiency has focused our attention on the choline-serine-glycine-glyoxylate-?1-pyrrolin carboxylate pathways and their ultimate action on the one carbon cycle metabolism.
Concomitantly, a brief review of the special pathology of trisomy 21 has shown that each of these steps was possibly slightly abnormal in the patients. A systematic investigation is clearly indicated.
"Le Superflu, Chose si Nécessaire"
It should be first remarked that the amino acids involved are all nonessential, and it could be argued that nonessential products are not of an obvious interest and that essential amino acids are much more important, as demonstrated by phenylketonuria, (phenylalanine), Hartnup disease (tryptophan), and maple sirup disease (leucine, isoleucine, valine), for example.
In case of genic dosage effect, I would be inclined to consider the nonessential amino acids as the essential ones, for a very simple genetic reason. The loss of a synthetic function (which makes a substance essential in the food) is indeed an interesting economy made in the course of evolution. Instead of spending energy to make a particular compound, the organism can thus use its forces to a more interesting task, but such an economy is feasible only if the diet can provide at the precise moment the rewired materials.
Let us consider a metabolite so important that its constant use must be regulated with extreme precision, which cannot be stockpiled in an efficient way, such as glucose in glycogen. Then the organism cannot indulge itself to loose its synthesis function, under the risk of death.
It could thus very well be that the so-called nonessential building blocks are in fact the very essential ones, and if we have retained the faculty of their synthesis during the whole history of living systems, it is dust because we could not afford to do otherwise.
A General Outlook
At this moment, it would be very tempting to try to relate the disequilibrium noted in the choline-serine-glycine-glyoxylate-?1-pyrrolin carboxylate pathway to the shift of the oxygen metabolism due to the SODA excess and to the GSHPX regulation. Although such a discussion has already been attempted (Lejeune 1979), it does not seem necessary to rely on such a unifying hypothesis.
Relations to the oxygen metabolism are indeed suggested by the occurrence of many steps involving FAD-dependent reactions and the concomitant 02 ? H202 transformation. Also, the curious facial anomaly of children suffering from compound hemoglobinopathies (Hashem. 1978), rendering newborns rather comparable to trisomic 21 babies, points out the extreme importance of oxygen metabolism.
However, so many discoveries remain to be made in this field before acceptable models could be presented that another way of research should be pursued simultaneously.
A Step-by-step Approach
Our attention being directed to metabolic pathways involving nonessential amino acids, a very simple experiment would be to consider the possible effect of increasing their availability by adding some of them to the ordinary diet; an eventual effect would pinpoint a shift in their regulation.
Investigations are now in progress and it would be very premature to derive final conclusions from the scanty data now available. Nevertheless, it can be mentioned that the administration of 50 mg/kg/day of serine to trisomic 21 children does not produce, as expected, any unfavorable effect nor a fully demonstrable clinical improvement. As demonstrated in a double-blind test, even this extremely moderate alteration of the diet has a metabolic effect in trisomic 21 children.
Compared to trisomic 21 children receiving only a placebo, the trisomic 21 children heated with serine exhibited a significant rise (although quite moderated) of their glutamate blood level. Such a result, in accordance with the model discussed here, was not predicted by the conventional biochemical dynamics.
Other experiments with special modification of the diet are easy to imagine and some of them are actually in progress. It must be stressed clearly that these laborious assays, very demanding to dedicated parents, are not to be taken as therapeutic trials. These investigations are purely heuristics, at least at present.
Conclusion
This very brief summary of the possible biochemical mechanisms underlying the mental deficiency in trisomy 21 leads to few general propositions for future research.
First, the one carbon cycle could be a central crossroad in most of the severe disturbances of mental efficiency. This general statement suggested by comparative pathogeny of various genetic diseases is possibly applicable also to trisomy 21.
Secondly, a shift in the regulation of common, nonessential amino acids should be looked for in trisomy 21. Again, comparative pathogeny clearly points in that direction.
Thirdly, the relationship between these two general disequilibriums and the perturbation of the oxygen metabolism remains to be carefully investigated.
As a conclusion, it could be ventured that future achievements in these three areas could open new views on the general mechanisms of mental retardation and lead some day to means of controlling directly or indirectly these biochemical problems.
Indeed, the length of road to be covered before reaching such a goal cannot be predicted, but to summarise the situation, I would like to quote a famous phrase of Lyautey:
Arrivant au Maroc, il decida de fournir à la population un supplément de ressources en étendant les palmeraies. "Mais, avant la premiere récolte de ces palmiers-dattiers, il faudra bien vingt ans !" lui dit un spécialiste.
"Vingt ans ?" réplique Lyautey, "Alors, commençons tout de suite."
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