Concluding remarks

J. LEJEUNE

Acad Pont Sc (Rome), 1984, 51, 255-260


Sommaire

After the summary so aptly given by Mrs. Singer, it seems difficult to continue the analysis of our discussions. With your permission I will try, Mr. Chairman, to express the feelings left in us by this week of working together.

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Basic information:

As already said by Mrs. Singer, we have witnessed the progressive unraveling of the coded message carried by macromolecules, DNA and RNA - copy, repeat, include, delete, transfer, invert, replace by, insert, and so on ... - all these instructions look very much like the tricks used by an author editing a manuscript and then correcting the galley-proofs. We now have librairies of DNA segments, and dictionaries to check where some chapters do belong and what is their meaning. Really the book of life is of a fascinating richness.

But other instructions like "go to", "split", "pause", "come back",. "if flag" and so on are more akin to the language of computers. Indeed, the tables of the laws of life are very complex multiplication tables.

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Cooperation:

At the other end of the size scale we find the whole cell, in which so much informations is imbedded. Its structural com-plexity would need more descriptive parameters than astrophysicists would use to figure out a galaxy.

Thanks to the experiments of Mrs. Mintz, we know now that this miniaturization of molecular subtleties confers on the primordial cells a kind of wisdom. The few active cells of the early blastocyst are able to educate and so to speak domesticate teratoma cells, gone wild in their unlimited growth. Apparently the blastocyst's cells know something more than these selfishly dividing tumor cells. By means yet unknown, they teach them how to cooperate harmoniously in the make-up of a new individual. A beautiful lesson!

Among the wealth of new knowledge brought to us, I would thank especially Mr. Illmensee for his rehabilitation of the fecundation process. We geneticists have been lazily thinking that spermatozoa were mere vehicles of the chromosomes, their only payload being the DNA tapes. But nature is more refined. Sperm does carry some information to the cytoplasm as well, a domain previously restricted only to the female contribution. Maybe the rest of the sperm is not only a dispensable re-entry vehicle after all.

With Dr. Edwards, we witnessed the extraordinary achievement of the early blastocyst. Even in the conditions realized in extracorporeal fecundation, very inhabitual for our species, it tends to its own goal, the blossoming of a new member of our kind. It can withstand many aggressions. It can eventually be killed by them. But if it survives, it regulates, compensates with a stupendous stubbornness, and expresses the individual it is bound to by its genetic make-up.

All these data point to the fact that one cell alone cannot read the book of life of complex living beings. In order to exhaust the whole significance, many different cells have to cooperate, helping each other, in this deciphering and its complete expression. As algebrists would do, expanding a formula, reduced to its single expressions, the genetic information of the fecundated egg has to be progressively unfolded. Apparently various specialists are required in this interdisciplinary adventure.

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Signalling:

Obviously these early cells, these would-be specialists have to get committed to their particular way of reading, through some messages they receive from each other.

Dr. Schell showed us a part of their strategy, thanks to the ring of DNA responsible for the crown gall modification. This circular jewel reminds me of the famous ring of Gyges which if twisted in a given way, rendered its bearer invisible. Depending upon which segments of the ring are operating, it renders invisible the roots, the leaves or other parts of the plant. But proper procedures make them reappear at will.

Besides this quite mythological property, Dr. Schell's ring will possibly provide a vast field for the genetic manipulation of plants. Besides the "beer factories" predicted by the breeders of bacteria and molds, we will some day have crops furnishing us very precious drugs and medicaments. A fate to which plants are eminently prone, if we remember that apart from the antibiotics, most of the molecules used in medicine are plant produced.

The external signalling between cells has been exposed in its very different aspects by Drs. Köhler, Hammerling, Scharfstein and Koprowski. Here we deal with recognition, enhancement, competition, advertising or disparagement, exactly as we observe in the "Republique des lettres", the interactions between authors, readers and critics. Another example of a cooperative race.

Maybe the work of Dr. Saxén and of Prof. Ranzi affords us a first glimpse of this system of emission and reception of signals and of their significance. In the embryological development, the commitment to differentiate is transmitted by non-informative molecules, I mean by end products of the long chain of transfer of information from DNA to proteins and from them (through purely enzymatic processes) to glycopeptides. Of course we already know that cellular membranes are sensitive to this kind of molecules. But how is the reception of this information transmitted to the nucleus and how does it modify the way of reading the genetic message? That remains to be discovered, a fascinating endeavor.

It is absolutely necessary that the flow of information from DNA to cytoplasm must be in some way fed back to the blue point lecturing machine. As we have already seen, the message in higher organisms is too important to be read by one reader only (as in microorganisms); specialized readers must interchange their discoveries so that differentiation still leads to a common goal!

Thus we have observed two levels, the molecular one of the coded molecules, and the cellular one, of the supercoded organization of coded molecules. But what about the intermediate size, I mean the DNA tape .and its cartridge, the chromosome?

Three of us addressed ourselves to this intermediate size. Dr. Carbon deciphered the DNA sequence of the anchoring point, the centromere. Dr. Davis manipulated chromosomes of Saccharomyces like an editor of a movie giving orders to a cutter for a new presentation of already filmed sequences.

I addressed myself to another gap of our knowledge, a tiny gap of the x chromosome, responsible for a terrible gap in mental development of affected children.

But these examples do not exhaust the future of chromosome manipulations. Compared to our ability of manipulating the DNA molecule, we know practically nothing about directed modifications of the chromosomes. But Nature does.

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Species Constitution:

With the use of precise techniques the banding pattern of the chromosomes defines precisely the structure of the caryotype. The simplest observation being that each species, whether a whale or a mouse, can be defined by its own caryotype. As was noted in the study group "on the evolution of primates" which preceded this one, the genie differences are seemingly less striking than the chromosomal rearrangements in telling a party related species. With minute variations the words seem common to all of them but the structure of the phrases and the length of the chapters make most of the difference. As Buff on said: "Ie style c'est l'homme". A quite prophetic thought!

We are still confronted with the task of understanding the significance of the distribution of the message in the various volumes of the encyclopedia of life.

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Preferential translocations:

Nobody knows nowadays why a given gene is carried by a particular chromosome in one species and is located in another place in another species. But already pathology teaches us that this allocation must have some functional meaning. For example, some malignancies are related to a single translocation (like the rearrangement of chromosomes 22 and chromosome 9 in chronic granulocytic leukemia). Even in some lymphomas, by looking at the rearranged chromosomes it can be safely predicted what kind of immunoglobulin will be expressed by the tumor cells.

Also, specificity of small deletions (retinoblastoma) or of reciprocal translocations (ataxia telangectasia) is well established. A kind of regulation of position effect is no doubt of extreme importance.

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A tridimensional structure:

All these facts point to the notion that uncoiled active chromosomes cannot be distributed at random inside the nucleus, like spaghetti floating in a bowl of soup. On the contrary they must be anchored by special points to the pores of the nucleus membrane. In this context, may I suggest that the dispersed repetitive sequences of DNA could be candidates for this function, and could also provide some specificity for accidental translocations. Be that as it may, the resting, active nucleus is surely a tridimensional network of precisely interrelated chromatin fibers, a structure very reminiscent of the informative network of a fantastically miniaturized computer.

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Future applications:

If we could understand, even partially, how these phenomena are achieved we would possibly be much more efficient in fighting chromosomal diseases. For example, we know that during meiosis, especially for the maturation of the ovule, the chromosomal mechanism is directly geared to the hormonal regulation. The older the mother, the greater the risk that some "give" will happen between the two systems, and the higher the risk of malsegregation of a chromosome.

If this "give" could be corrected, a true prevention would ensue.

Even, in case of translocations, we know that the risk of malsegregation is much more severe if the translocation is carried by the mother than by the father. A kind of sieve, selecting against the unbalanced gametes, seems to protect partly the male production line. Here also, a means of prevention is awaiting discovery.

In the regulation of a whole chromosome, we know quite well, that in females, one of the X's is turned off, producing the Bar body. Hence the relative innocuity of the excess of the X chromosome, like in XXX conditions. The inactivation of two of the X's leaves only one fully active, a situation very close to the normal equilibrium.

If we could understand how to turn off and turn on whole segments of chromosomes, we would possess a powerful tool for compensating trisomies or monosomies.

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Classical biochemistry:

If the DNA, the genes and the chromosomes are like commanders, generals and staff headquarters of the struggle for life, what about the infantry men, the non-commissioned, the non-informative molecules?

It could very well be that the blossoming of molecular biology is masking the interest, still enormous, of classical biochemistry (like vitamins). In many instances we can either add small molecules or exclude them from the regimen (like phenylalanine in P.K.W.) and effectively protect the soma from inborn errors of the genome. The possibility remains open, that curative medicine can do much to prevent the disastrous effects of a blurred genetic message, even without repairing the abnormal genes. This necessary approach is not as fashionable as genie manipulation but is possibly unjustly neglected.

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Two conclusions:

At the end of these reflections many questions have been raised, but "concluding remarks" were asked for by the program. I feel that conclusions are straightforward.

During this week, we scientists developed our work in the manner of a fully integrated organism. We were offered basic information, shared this in true cooperation, thanks to dependable signaling (turning off and on of the information was even materialized by the microphonic devices). We attained some wisdom thanks to the participants. More than a crude analogy, this week was a demonstration that the deciphering of life processes requires the cooperation and, so to speak, the coming together of various specialists. And we fully appreciated it.

The second one is that each of us, no matter whether he uses a pipette, a microscope, or an electrophoretic system, is a lover of life. We observe it as exactly as we can, but we also admire it, and admiration is really the highest human activity. Never a dog, however cheerful, smelled the perfume of a rose. Never a chimpanzee, however artful, gazed at the sunset or at a starry sky.

And it was very proper for our admiration of life that this meeting took place in this marvelous room of the Casina Pio IV. During our meetings I often stared in wonder at this ceiling's frescoes, which protected our discussions like a shell of beauty, And if you look at the four corners, you will understand why. In the rear at right, Veritas is depicted and indeed truth is our only goal. At the left is Concordia and it was our manner of exchanging ideas. Near the entrance, at the left, is Tranquillitas and we can pursue our endeavor in full tranquillity as long as admiration is our guide and guards us from pride and its dangers. Finally at the right is Letitia, and our joint efforts in unraveling some of the wonders of life, gave us indeed a profound joy.