Gene Dosage, Effects In Man [1]

Jérôme Lejeune, M.D., Ph.D.2.

Ala. J. Med. Sci., vol. 3, n° 4, 425-431, 1966.


Sommaire

Human cytogenetics is still in its infancy and its constant progress prevents making in 40 minutes a review of what we know and of what we would like to know. So let's discuss what we can consider as being interesting in chromosomal disorders as far as autosomes are concerned.

I think we better not discuss the sex chromosomes in which so many different diseases are known. The particular interest of chromosomal disorders is that they are introducing a new concept in the understanding of genetic diseases. In most of the well-known genetic afflictions, a genic change is involved. There is an error in the DNA molecule which produces, by a complex mechanism that you know, a kind of misprint in the enzyme controlled by the gene. This misprint changes the behavior of the enzyme and produces a biochemical or morphological variation recognized as a disease. Most of the genetic diseases are thought of in terms of an error in the genetic information. When we are dealing with chromosomal imbalance, the situation is entirely different. Not only in anima viti but also in human beings we know by experimental demonstration, that the genic content of the chromosomes involved is entirely normal. It is only a quantitative change which produces diseases. When a chromosome is in excess, as in trisomy, or is lacking, as in monosomy, we know that the genetic information is unchanged; but the repetition or the omission of this perfectly all right message is the sole cause of the disease. Redundancies and shortcomings have always been the pitfalls of talks and conferences and it is interesting to see that this general law also applies to chromosomal changes. The question is now to know why, and how this genic dosage effect can be achieved.

Unfortunately, it is only too easy to show that excess of an autosome is deleterious. Trisomy 21, which is related to the fact that one of the tiny 21 chromosomes is in excess (Lejeune et al., 1959) does produce the classical picture that all of you know. If I recall this trisomic condition, it is only to consider what kind of mask the disease is able to throw on the personal features of the child, so as to make them apparently like each other. As you know, the bridge of the nose is very flat; there is epicanthus, microcephaly and so on. This particular mask is the same for all children and is superimposed over the particular genetic makeup of the individual.

The same features of trisomy 21 are found in whites, Negroes, and in Asiatic babies. Although vaguely reminiscent at birth of Asiatic traits, these features are not at all racial. The characters change with the growth and in the adult, only the dysmorphy persists, the same for all races. I will not go very far into the description of this syndrome of trisomy 21, for it is very well known to everybody. I would just remind you that together with many deformities of internal organs, the disease has a signature on the hand.

Instead of having normal ridge patterns as everybody has, 21 trisomic people have the ridges abnormally set so that the triradius in which there is a convergence of the ridges, which is normally located in the root of the hand, is in the middle of the palm. The two flexion creases, (let's say the "line of heart" and the "line of head") are fused together in a transversal crease. I will not discuss further the dermataglyphics because Dr. Uchida will speak about this, but I wanted to include it to, give you the idea that the whole individual is shifted from normal by the presence of the extra chromosome and that we can predict just by clinical inspection what chromosome produces the diseases.

In other trisomies, like trisomy 18, in which a member of the pair number 18 is in excess (Edwards et al., 1960) the picture is typical but very different from trisomy 21. The children are rather small and many traits allow the clinical diagnosis. They are microcephalic; they have a very small chin, a short sternum and a narrow pelvis. The hands are held in the surrender position and the first and the fifth fingers cover respectively the third and the fourth. In the external ears the scaphoid dimple is rather flat and the helix is poorly rolled. Those things are very small symptoms but we will see later that they have possibly a very great genetic interest. The digits are very short, have only one flexion crease instead of two; and most of them exhibit arches on the finger prints.

In trisomy 13 a big acrocentric of the D group is in excess (Patau et al., 1960). Here also the clinical picture allows diagnosis, and one can briefly remember the deformities of these children. All of them are microcephalic and have a very peculiar disturbance of the cephalic development: the eyes are poorly developed ranging from pure anophthalmia to simple microphthalmia. Harelip and cleft palate are common, and the ears are abnormal. Among many other congenital deformities, the most typical is an aplasia of the olfactory lobes, which classify the disease as arrhinencephaly. This disease, described in this country rather recently, was in fact entirely and perfectly described 300 years ago in Denmark by Bartholin who found in one stillborn all the characters of the disease. This historical reference shows us that these "new diseases" have always existed and are not an invention of cytogeneticists. There is also a signature of the disease on the palm prints together with extra digits in hands and toes. As Dr. Uchida will discuss later, cytogeneticists are really reading the destiny of people an the hands and these minor stigmata are a great help in the diagnosis.

From just the remembering of those three well-known syndromes, we can draw the first conclusion: If the excess of a given segment of the genome is able to produce a specific clinical entity that we can recognize, it follows logically that genes located in this chromosome control more or less directly the manifestation of those traits. If we find something wrong with an enyzme, this does not prove that the structural gene of that enzyme is surely an the chromosome involved, but at least that something controlling this thing is located on that chromosome. All of the children carrying an extra piece of an autosome have in common a very dramatic symptom-all of them are feebleminded. For the moment this rule has no exception. We, must then ask ourselves why it is so. The straightforward conclusion that there are "genes of intelligence" located in chromosome 21, chromosome 18, chromosome 13 and every other autosome is entirely unwarranted. The best representation of the situation is that in an extremely complex and highly integrated system, any change is likely to produce, besides its specific aspects, some degradation of the best performance of the system. Thus, human intelligence is always suffering if some of its substratum is abnormal. Indeed, it seems possibly a little too anthropomorphic to state bluntly that human intelligence is the top performance of living systems but cytogenetics does not rule out this idea.

Now let's see what not excess but lack of chromosomal material, like in the "cri du chat" disease could do (Lejeune et. al., 1965). All the children affected by this disease have a rather peculiar face. They have wide-spaced eyes, epicanthus, rather small chin, and are very microcephalic. Otherwise, their morphology is not greatly abnormal. In common, all the children have a very particular cry which is the "symptome d'appel." In early infancy, they have a highpitched, prolonged cry, which is by its plaintive tonality, very reminiscent of the cry of distress of a young, suffering cat, hence the name of the disease "cri du chat" disease. This is related to abnormality of maturation of the larynx. Recent spectral analysis of the sound has confirmed the similarities between the cry of the affected children and the cry of kittens. The disease is related to the loss of part of the short arm of chromosome 5. To my knowledge there are more than 70 cases now entirely investigated and all of them have a deletion of a portion of chromosome 5. The signature of the disease in the palm prints is essentially a moderate elevation of the axial triradius and a straight, short, distal flexion crease. The problem was to know whether those children were really suffering from genetic loss because it was generally believed that monosomics having only one exemplar of a given segment of chromosome should be lethal in man. This opinion was very widely accepted and hence it was necessary to know that the fragment was really lost in that disease and not only translocated somewhere else. A very peculiar family gave a quite experimental answer to that question (Lejeune et al., 1963). In this family, one case was a typical "cri du chat" syndrome. This girl had two sisters who were feebleminded and did not have any symptoms of the cri du chat syndrome but were really severely affected. By interrogation, it turned out that another child, dead many years before the examination, had the cri du chat condition. The grandfather remembered and told us spontaneously that the boy had an abnormal cry like a little cat. The mother is phenotypically and mentally normal but she has abnormal chromosomes. Her father has the same abnormality of his chromosome and he transmitted it also to her sister.

The karyotype of these persons exhibited a deletion of the short arm of chromosome 5 as in the typical cri du chat disease. The missing segment is translocated to the end of one of the chromosome 13's. Measurements show that the sum of the lengths of this deleted 5 and the augmented 13 is the same as that of the normal 5 and the normal 13. Thus, the father of this woman carried a balanced 5 to 13 translocation and transmitted it to his daughters, when the mother transmitted a normal 13 and the deleted 5, then the child was a typical cri du chat syndrome. In another child, she transmitted the normal 5 but the big 13 and then the child became trisomic for this little segment, because she had two normal exemplars and an extra one, stuck on the 13. This is then the reciprocal of the deletion, that is trisomy for the very same fragment, which is monosomic in cri du chat disease. Another girl was entirely normal because she received a normal 5 and a normal 13. The two sisters who had received the extra piece and who were trisomies were very severely mentally retarded. The oldest girl has a particularly big and long nose, rather contrasting with the sharp little nose in cri du chat.

The youngest died at seven months of age and unfortunately no examination was possible. She was not obviously morphologically abnormal. She was severely retarded and she had a very abnormal cry. You would ask what should be the contrary of the cri du chat cry. She had a very raucous and discordant voice and the parents said that the oldest one had exactly the same voice when she was young. Unfortunately, we could not examine their larynx.

But with this first example of a disease in which we could know two instances which are mirror images of each other, the idea arises that if we can define a clinical type due, for example, to a trisomy, the contrary could possibly exist and produce the mirror image of deformities and then be the countertype of the disease. In that case the trisomy for the small fragment of short arm of chromosome 5 is the countertype of the cri du chat disease. To make clear, I should say that this instance of monosomy and trisomy for the portion of short arm of chromosome 5 is now known in two other families, and the picture of the reciprocal syndrome of the cri du chat will be, I suppose, correctly described when those three different families can be matched together. If types and countertypes do exist, we have to find out how they should be looking alike. As you know, in case of translocation involving the chromosome 21, a carrier mother should sometimes produce ova without any 21 chromosome and then have pure monosomic children, that is, haplo 21, having only one 21 chromosome given by the father. Although more than 200 children born from such mothers have been recorded, no such case is yet known (Lejeune, 1964). It can be considered that pure monosomy for chromosome 21 is not viable. But it is possible that situations very close to pure monosomy do exist.

A child was observed to be a carrier of ring-21 chromosome (Lejeune et al., 1964). One member of the 21st pair, instead of being shaped like a rod, was a little ring. Some material was probably lost during the formation of the ring. More interestingly the child had lost the ring in many cells which were then entirely monosomic for chromosome 21. In the blood 95 percent of the cells had forty five chromosomes without the ring; thus, the blood was quite purely monosomic 21. In other tissue, 1/3 of the cells were monosomic, 2/3 having kept the ring. All the abnormalities exhibited by the child were exactly the contrary of the stigmata of trisomy 21. The child was very hypertonic instead of hypotonic like the 21 trisomies. He had very big ears and they are small in trisomy 21. The ear channel was very broad but it is very narrow in trisomy 21. The nose is extremely salient (and that was calculated also in radiography) and as you remember it is very flat in trisomy 21. Instead of having the epicanthus in the inner part of eyelids, he had a kind of "hypocanthus" an the lower part of the eye. A second case was recently described in this country (Reismann et al., 1966). The child also had one little ring of the 21 and lost it in many cells; and he had also the big ears, the prominent nose and an enormous rigidity.

As I told you, for biochemical traits, the child examined had also the contrary of trisomy 21, that instead of having elevated alkaline phosphatase of polymorphs, he had a low value. Instead of having hypoeosinophilia, he had an excess of eosinophils in his blood. And also for the tryptophan metabolism, which is abnormal in trisomy 21, the ratio of 5-H.I.A.A. excretion in the urine compared to kynurenin excretion, which is very low compared to normal in trisomy, was very high in this child compared to normal. We have then the notion that when one chromosome is absent, or partly absent, the deformity is contrary to that produced by the excess of this particular choromosome.

I would just discuss another example of this type and countertype pair which is a deletion of part of the long arm of chromosome 18, a disease which has been recently individualized (Lejeune, et al., 1966, de Grouchy et al., 1964). Affected children have an underdevelopment of the middle part of the face with slanting eyes and epicanthus. Also they have abnormality of the superior lip and of the insertion of the nose. The features are the same no matter what the race of the child; the retraction of the middle part of the face, and also the little fugal nodule and the special feature of the lip and of the nose are found.

The syndrome is also producing a kind of "hyper normality" of the ears. The helix is overrolled, the anthelix is prominent and the scaphoid dimple is extremely deep. These are minor points but they are just the contrary of what we have seen in trisomy 18 in which the helix was poorly rolled and the scaphoid dimple was quite flat.

Those children have also frequently a little dimple on the shoulder and on the dorsal part of the phalango-metacarpal articulations. The morphology of their fingers is exactly the contrary of what was seen in trisomy 18. The digits are long "fusi-form," and fingerprints show a great excess of whorls. All of the children have lost about one-half of the long arm of chromosome 18. This disease was described in three cases but there are now other cases known, not yet published. Two cases were observed in England by Dr. Insley who kindly sent me the karyotypes and the picture. Other cases have been detected, in Boston (Dr. Park Gerald) and another instance in Paris, which are mosaics. All together it can be said safely that this new syndrome is an entity clinically recognizable.

We could now try to discuss the interest of this concept of "types and countertypes." First it makes an immediate improvement o£ semeiology: observing a symptom which is shifted one way by excess and the other way by loss and allowing to determine what is the precise point of action of a particular chromosome. The second interest is possibly more general. To understand how chromosome dosage can operate we have to build a logical model. The basic rule in genetics is that one gene controls one enzyme or one specific protein. If we just quantify this rule, we can figure out a first hypothesis to explain these types and countertypes. In monosomy one gene produces in the cells one unit of enzyme; in trisomy, three genes being present, three units of enzyme will be produced. Then compared to normal with two genes the shift will be in different directions in monosomy and in trisomy. Thinking in these terms of quantity of enzyme is quite different from what is the biochemical results of point mutations.

Let's take for example, PKU: one enzyme does not function, the biochemical step is stopped and then the disease occurs. In the case of excess of enzyme, the chemical reactions are entirely normal but just going too fast. And if there is not enough enzyme, it is going too slowly but in both cases the quality of the reaction is normal. We can then understand why a reaction going fast would produce the contrary of a reaction going slow. Still it remains a paradox: if types and countertypes are really mirror images for minor symptoms, they are alike for major disasters. All the children are feebleminded. To understand that, we can take an example and suppose that genes are like the musicians in an orchestra, each of them playing his personal partition. Now if some musician decides to play his partition slower than the rest of the orchestra or if he decides to play faster than the others, in both cases the result is bound to be cacophonic. But if the musician is changing his tempo, not during a tutti but during a solo, then the fact of changing moderato to pretissimo will produce one symptom and the change to a largo will make the contrary. It is probably because the enzymatic processes are so much intricated for building high functions like human intelligence that any shift is deleterious. For morphological traits there is some degree of liberty in their expression, which is obvious by the differences occurring in normal beings. It is this possibility of variations which make possible the recognition of types and countertypes.

One gene-one enzyme, three genes-three enzymes is extremely naive indeed. Nevertheless, even if not exactly true, this model does have a great heuristic interest. If it represents a part of the truth, it would mean that in chromosomal imbalance, children have the whole biochemical machinery right at hand with just some steps running too fast or too slow. To control the speed of a biochemical reaction is surely much more feasible by metabolites or antimetabalites than to replace an entirely lacking enzyme. Indeed, these crucial biochemical steps have yet to be discovered, and this construction is far the mordent purely speculative. But if you remember that at birth one child in every hundred is severely affected by such a chromosomal imbalance and will suffer in his mind and his flesh for all his life, you will find that those speculations are not academic but just an immediate challenge. This way of research must be investigated for it is our only hope that someday we will be able to do something for those children who, having not received an equitable patrimony, are in the true sense of the term, the most disinherited of the children of man.

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Questions

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Dr. Harold Goodman

I wonder if you would be willing to speculate, Dr. Lejeune, about one of the large problems that relates to this area. In particular what about the occurrence in the normal population of deviants who have one or more of these mongoloid traits. Also an occasional individual is seen who has almost all arches who, of course, has apparently normal chromosomes.

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Dr. Lejeune

This is an obvious question. I cannot give a final answer but when we are speaking about trisomies, we have to consider that the children arc all the same for the chromosome involved, but the genes inside the chromosome are not identical, because alleles are different from one individual to the other. Some alleles will produce much more enzyme than other alleles and then produce a trait transmitted in a Mendelian way. We know that those genes are an these chromosomes because they take part in the morphological syndrome, but they can he present in normal people. Let's say that if a crooked little finger was never present in trisomy 21, we would never be surprised that some people have crooked little fingers and transmit it to their progeny. Just because it appears that it is governed by genes in the chromosome 21, we are interested in it. Then these normal variations are in fact just the material with which syndromes arc expressed in trisomies and monosomies.

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Dr. David Smith

Professor Lejeune, cytogenetics being in its infancy and thereby open to changes of interpretation with advancing methods and knowledge, there is one point that I would like to bring up and have you comment on. I would appreciate it if it would be possible also for Dr. Yunis to make a comment in this regard. The question pertains to the nomenclature of the chromosomes themselves and to the decision as to whether it was 17 or 18 in triplicate in the 18 trisomy syndrome. This decision was helped by the advancing methodology in terms of the tritiated thymidine replicating pattern of the chromosome which is ire triplicate. Similar type studies which were carried out by Dr. Yunis an the Down's syndrome patients seem to indicate that it is the smaller of the two chromosomes that is in triplicate which by the original designation would then be chromosome 22 and with your indulgence, I would appreciate very much your comments.

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Dr. Lejeune

My comments will be very brief. When we are giving a number, a numeral to a chromosome, it is purely a conveniece. Then the best is to say by convention that we call 21 that chromosome which produces the syndrome when in triplicate. If we can say later that this chromosome has such and such property, that it is a little smaller than the other, that its thymidine incorporation is not exactly the same and so on, that is for the better; but there is no reason at all to change the etiquette we have put in it. That is the same for trisomy 18 or trisomy 13, We have to make the blunt definition-one day we give this number to that chromosome and no matter what will be the definition of this chromosome we will get later, there is no reason to change the number because, anyway, the numbers are entirely arbitrary. I must say that I have measured as many chromosomes as anybody, and I am still unable to prove that one is bigger than the other, then I just will stick to this very simple decision.

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Dr. Walter Nance

Dr. Eric Engel and I have recently studied a case which illustrates some of the findings which Dr. Lejeune has just described in the 18 (E) long arm deletion syndrome. This patient was a severely retarded, 12-year-old girl. She had the long slender fingers, along with the somewhat abnormally placed thumb that Dr. Lejeune has emphasized as being a component of the long arm deletion syndrome. She also had the midfacial hypoplasia along with the deeply convoluted ears that lie has emphasized. Of some interest, however, is the fact that this patient also demonstrates a broad chest, pterygium colli, and ptosis and these findings are features that have recently been emphasized by Dr. de Grouchy as being common findings in the 18 (E) short arm deletion patients. Chromosome studies in this individual showed an abnormal E group member which was identified on the basis of morphologic and autoradiographic criteria as being the 18 (E) chromosome. Late prophase preparations indicated that this is a ring chromosome from which we infer loss of chromosomal material from both the long and short arms of this E group member.

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Dr. Lemons Yielding

It seems to me that the problem of gene dose might be more understandable if we would include in our model the concept of regulator genes because in that instance we could explain in the trisomy state, for example, the occurrence of decreased enzyme levels as well as increased enzyme levels. I wonder if you could comment on that.

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Dr. Lejeune

I did not understand entirely your question but I think that you put forward the regulating mechanism. This is a complex business but as far as we know, the genie regulation as described in the term of operon does not apply to diploid organisms, at least we don't have any clue to know how it should apply. Obviously regulation plays a tremendous role in the explanation; it is the main reason why I said that the simple arithmetic I was giving was just extremely naive. For the moment there is no logical model which we can safely produce and try to test by experiments.

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Dr. Lemone Yielding

Ithink it is especially interesting when we consider that a more complex organism has a problem of differentiation. If repressors of various sorts play a large role in differentiation, then if we have a gene which produces a large concentration of repressors, then it is easy to see how we could disturb the whole process of differentiation.

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Dr. Lejeune

That is obvious. It doesn't matter in fact to the theory whether the gene is a structural gene or a repressor gene. The genie dosage effect will be different on the end result but the mechanism will be the same.

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Dr. Frederick Hecht

In Down's syndrome, as well as in the other autosomal trisomies, the mean maternal age is increased. This is usually taken to be evidence that the majority of cases are due to maternal nondisjunction; therefore, the individual has an extra addition of the mother's autosome. Is there any evidence that the children resemble the mother more than the father?

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Dr. Lejeune

That is an old question that has been especially investigated by British workers, by Dr. Penrose in particular. He found that there were, as far as blood groups are concerned, large similarity with the mother, but this is a very tricky problem. Susceptibility to a difference in blood groups between fetuses and the mother is great; then it could be just a selection effect. If the trisomies which are weaker than normal fetuses do not resemble their mother, they are possibly discarded. There is no clue as far as I know to tell that trisomy 18 or 13 are more alike the mother than they should be. The notion that the accident is occurring during the meiosis in the woman is for the moment a simple hypothesis which must not be taken for granted. It is just an hypothesis.


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References

Edwards, J. H., D. G. Harnden, A. H. Cameron, M. V. Crosse and O. H. Wolff. 1960. A New Trisomic Syndrome. Lancet 1:787.

Grouchy, J. de, P. Royer, Ch. Salmon and M. Lamy. 1964. Deletion partielle des bras longs du chromosome 18. Path. et Biol. 12:579-582.

Lejeune, J., M. Gauthier and R. Turpin. Les chromosome humains en culture de tissus. 1959. C. R. Acad. Sci. Paris 248:602.

Lejeune, J. 1964. The 21-Trisomy. Current Stage of Chromosomal Research, ed. A. G. Steinberg and A. G. Bearn. Progress in Medical Genetics, Vol. III, New York: Grune and Stratton, Inc. pp. 144-177.

Lejeune, J., J. Lafourcade, R. Berger, J. Vialatte, M. Boeswillwald, Ph. Seringe, and R. Turpin. 1963. Trois cas de délétion du bras court du chromosome 5. C. R. Acad. Sci. Paris 257:3098-3102.

Lejeune, J., J. Lafourcade, R. Berger and M. O. Rethore. 1965. Maladie du cri du chat et sa reciproque. Ann. Genet. 8:11.

Lejeune, J., R. Berger, M. O. Rethore, L. Archambault, H. Jerome, S. Thieffry, J. Aicardi, M. Broyer, J. Lafourcade, J. Cruveiller, and R. Turpin. 1964. Monosomie partielle pour un petit acrocentrique. C. R. Acad Sci. Paris 259:4187.

Lejeune, J., R. Berger J. Lafourcade, and M. O. Rethore. 1966. La délétion partielle du bras long du chomosome 18. Individualisation d'un nouvel état morbide. Am. Genet. 9:32.

Patau, K., D. W. Smith, E. Therman, S. L. Inborn and H. P. Wagner. 1960. Multiple Congenital Anomaly Caused by an Extra Chromosome. Lancet 1:790.

Reismann, L. E., S. Kasahara, C. Y. Chung, A. Darnell and B. Hall. 1966. Antimongolism. Studies in an infant with a Partial Monosomy of the 21 Chromosome. Lancet 1:394.

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Note

1. Presented at the International Seminar on Medical Genetics, University of Alabama Medical Center, Birmingham, Ala. September 2, 1966.