So much work has been devoted to the chromosomal study of trisomy 21
that it seems unnecessary to add to it. Nevertheless, the organizers of this
conference have rightly believed that research was probably not complete, since
we do not yet know why, how and when the chromosomal aberrations appear. Before
focusing our attention on this most important point for the understanding of
the causes of the disease, we can briefly summarize the general data now
established in regard to many thousands of trisomics.
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General statements
To date, all cases of clearcut clinical 21 trisomy syndrome have been
carriers of extra chromosomal material. In all the cases in which
identification was possible, either by cytological or genealogical analysis,
the extra fragment was chromosome 21. It is then entirely correct to assume
that the morphological and biochemical syndrome results from a pure gene dosage
effect.
The second general statement we can accept is that the risk of the
disease increases exponentially with the aging of the mother, as first
recognized by Shuttelworth [1] and carefully analyzed by Penrose [2]. This
aging factor is related exclusively to the mother; parity and father's age play
no detectable role.
If trisomies 21 are classified by their karyotypes, it appears that
at least 95 % are due to de novo free trisomies; the rest are related to
translocation trisomies. Among these, around half are newly arisen mutations
and half due to abnormal segregation of a preexisting familial balanced
translocation. Interestingly enough, the risk for the progeny of a
translocation carrier is much higher if the carrier is the mother; however, no
maternal age effect has been detected in this category.
Our purpose in this paper is to investigate whether these apparently
disparate data can be fitted together into a coherent picture so that a
tentative answer can be given to our three basic questions, why, how and when
?
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Origin of the extra 21 in free trisomiesHaut
General Morphology
If we could easily recognize each chromosome 21 individually, we
would immediately detect the origin of the extra one and determine more exactly
whether the malsegregation occurs in the male or the female gamete.
Unfortunately, the situation is not so easy. First, chromosomes 21 and 22 are
very similar. In exceptionally good preparations, the five acrocentrics can be
split into two obvious groups; such a classification, however, is often not
possible. Still more troublesome is the fact that if the three 21 chromosomes
are clearly detected, it is due to their intraclass similarity; thus there is
no possibility of differentiating the one coming from one parent from the two
others. Many speculations have been made in regard to the satellites, for it
sometimes happens that one of the parents has large satellites on only one of
these 21 chromosomes. Unfortunately, no case has yet been reported in which the
trisomic received two largesatellited 21 chromosomes.
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The Meiotic Analysis
First Division Error
If for the sake of simplicity the occurrence of crossingover is
neglected, malsegregation occurs if rupture of the synapses does not take place
early enough. The two duplicated chromosomes migrate at the same pole and, as a
consequence, the two chromatids included unduly in the egg are homologous;
i.e., one is received from the father and the other from the mother of the
individual in which meiosis takes place.
Fig. 1. Normal
meiosis: 1 first division; synapses rupture, but centromeres do not split; 2
second division; cleavage of centromeres.
Fig. 2. First division error.
Synapses do not split; both 21s are homologous.
Second Division Error
Again, neglecting crossingover, the accident occurs when the
centromere does not split before anaphase. The two chromatids migrating
together to the same pole are thus sister chromatids.
In the first accident, the homologous chromatids are independent
insofar as their allelic content is concerned. In the second type, on the other
hand, the allelic content of sister chromatides is by necessity identical. It
follows, as has been shown independently by Penrose [3] and Gremy [4], that
quantitative characters would be differently affected by the two types of
accidents and that parentchild or sibsib correlations would be different.
Another conclusion can be drawn. To avoid a full mathematical
demonstration which is quite simple but rather cumbersome, we can focus on the
problem of the mean and the variance. Let us suppose that a gene has two
alleles in the population, a and b, the effect of these alleles being exactly
additive, so that as people have a (2a) value; ab people have a (a+ b) value;
and bb people have a (2b) value.
The three phenotypes have the frequencies p2, 2pq, and
q2, if p is the frequency of allele a and q is the frequency of
allele b; i.e., p2 (aa) + 2pq (ab) + q2 (bb). It follows
that the expected mean is M=2(pa+qb), with a variance V=2pq(a
b)2.
In a trisomic if the extra chromatid is a homologue of the other
(first division accident), the three chromosomes 21 are independent and the
distribution of the aaa aab abb and bbb types would be given by the expansion
of (p+q)3; i.e., p3 (aaa) + 3p2q (aab) +
3pq2 (abb) + q3 (bbb). The expected mean is thus : M=3(pa
+ qb) and the variance V=3pq(ab)2.
If, however, the extra chromatid is a sister of the other, the
trisomic contains only two independent sets of alleles, one of the chromosomes
being a copy of another. Hence the four genotypes and phenotypes will be given
by the expansion of (p + q)2 and have the frequencies p2
(aaa) + pq (aab) + pq(abb) + q2 (bbb) . The mean is obviously 3(pa +
qb), as shown by elementary calculus, however, the variance is now V =
5pq(ab)2; thus it is much greater than in the preceding case.
From these formulas, we can deduce that the ratio V/M should be
the same in normals and in trisomics by first meiotic error: Vn/Mn(normals) =
2pq(a  b)2 / 2(pa + qb) = V1/M1(trisomics) = 3pq(a  b)2
/ 3(pa + qb).
On the other hand, the ratio for the second meiotic error, V2/M2 =
5pq(a  b)2 / 3 (pa +qb), is expected to be 5/3 higher than the V/M
ratio for either normal or first meiotic error.
By analysis of available data on alkaline phosphatase, galactose
uridyl transferase and kynurenine excretion, it can be shown that the V/M ratio
is higher in trisomics than in normals, thus pointing toward a second division
error.
It can be seen intuitively and demonstrated algebraically, that if
crossingover occurs the V2/M2 parameter of the second division will tend to
resemble the V1/M1 parameter of the first division error. The phenotypic
distribution would still be different but the discrepancy would be less
striking; it follows that the increased V/M ratio in trisomics does not prove
that all cases are second division errors, only, that a sizable number of them
must be. This is in agreement with the occurrence of mosaics, which are mitotic
events, with the same phenotypic result as second division error (if
crossingover is overlooked).
From this statistical aspect we can conclude that possibly a very
high number of trisomics are carriers of sister chromatids and, since free
trisomics alone seem to be sensitive to the aging of the mother, we can infer
that aging must act somehow on the segregation mechanism, impairing either the
first or the second divisionpredominantly the second.
Fig. 3
. Second division error. Centromeres do not split; both 21s are sister
chromatids.
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The Marker Chromosome
If these deductions were true, we should find cases in free 21
trisomics in which the extra chromosome is demonstrably a copy of the
other.
Recently, Dr. Jean de Grouchy came across such an instance, and he
was kind enough to allow me to quote these unpublished data. A typical trisomic
21 has two Gpo chromosomes in his karyotype. His mother is carrier, in the
heterozygous stage, of the Gpo marker. This Gpo must be a 21, for the child who
has received two of them is a typical 21 trisomic. Thus, we have here the proof
that noncleavage of the centromere at the second division is the cause of the
disease, at least in this instance.
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Origin of the free 21 in the translocation
trisomiesHaut
The G ~ D Translocation
Without reviewing all kinds of translocations involving chromosome
21 we can focus on the most common type, the translocation between
acrocentrics. If we exclude the very rare 21 ~ 21 type, which can only produce
21 trisomics or monosomic 21 offspring, the effects are comparable no matter
whether the translocation is a 21 ~ D, a 21 ~ 22 or a 21 ~ Z [5].
In observing the most frequent type, the 21 ~ D, it is remarkable
that among the newly arisen mutants for 21 ~ D translocations, most of them are
also carriers of 21 trisomy as demonstrated by Dutrillaux [6]. This phenomenon
could be expected if the exchange occurs at the four chromatids stage.
Moreover, if the translocation process is akin to a chromatid exchange and has
a tendency to stabilize the segregation, most of the newly arisen
translocations should produce a disomic 21 gamete, thus explaining why de novo
balanced translocations are comparatively rare.
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The Origin of the Extra 21
During meiosis in a translocation carrier, the error could occur at
either the first or the second meiosis. If it occurs at the first meiosis, the
resulting trisomic would be a carrier of the translocation and of the free 21
homologue. If it occurs at the second meiosis, the trisomic would either be a
free trisomy or a trisomy for both 21 and D, due to the nondisjunction of the
centromere of the translocated chromosome.
Among more than 300 children born from a D ~ G translocated parent,
all the trisomics but one were carriers of the translocation. Hence, all of
them resulted from a first meiotic error, and the extra 21 was by necessity a
homologue of the translocated one (if crossingover is not taken into
consideration). According to our preceding considerations, the ratio of the
variance to the mean should thus be greater for free trisomics than for
translocated trisomics if a quantitative character is analyzed. From the recent
data of Jerome (this monograph), a simple calculation shows a deviation in the
expected sense, albeit not a significant one.
Fig. 4. Normal
meiosis in a G/D translocation carrier.
Fig. 5. First division error in a G/D carrier. In the trisomic,
carrier of the G/D, both 21s are homologous.
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The Risk of Malsegregation
To estimate correctly the risk of the occurrence of a 21 trisomic in
the progeny of a translocation carrier, a careful statistical analysis of the
published data must be worked out. Not only must the bias of ascertainment by a
patient be corrected, but also the likelihood of detection of the whole
genealogical tree must be accounted for. As a general conclusion, confirming
earlier findings, it can be estimated [2] that the risk of trisomy in the
progeny of a woman carrier of a D/G is approximately 1/5 or 1/6. This is
definitely lower than the 1/3 expected in the case of random segregation with
lethality of the monosomic 21 zygotes. In carrier fathers, the risk is much
lowerbetween 1/20 and 1/100.
A simple way to examine this difference is to count all the D ~ G
trisomics known to be born from D ~ G carrier mothers and all those born from a
D ~ G carrier father. Whey amount, respectively, to 104 vs 11. Without any
other calculation, we can thus conclude that the risk is tenfold higher in
women than in men.
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Discussion of causes of the aberrations
Excluding the eventual effects of viral diseases, xrays, and other
risk exposures, we are finally confronted with apparently disparate findings
which can be summarized as follows: (1) Free trisomies are very often second
meiotic errors, and their global frequency increases with the aging of the
mother. (2) Inherited translocation trisomies are apparently independent of
maternal age, but are first meiotic errors and are much more frequent if the
carrier is the mother.
How can these two observations be combined into a general hypothesis ?
To investigate this possibility it is interesting to consider, as recently
proposed by Gillois,[7] that every meiotic (or mitotic) step is enzymatically
controlled and automatically regulated by the preceding one. For our purposes
we can consider a simplified scheme and suppose that if the whole machinery is
prepared before the starting signal is given, all the steps will succeed each
other in appropriate sequence.
First division :
 fusorial apparatus
 rupture of synapses (without cleavage of centromeres)
 anaphase
Second division :
 fusorial apparatus
 cleavage of centromeres
 anaphase
Let us consider what would be the most appropriate way of using a
system with builtin selfregulation. Obviously, best results would be achieved
if the system was constantly running at its own speed, but a disadvantage would
be to increase the number of point mutations if their frequency was related to
the number of copies of a gene.
To circumvent this difficulty, an immediate answer would be to stop
meiosis and to have it functioning only at the very moment that a new gamete is
necessary. To achieve this trick we would need another regulatory mechanism
controlling the meiotic process, turning it on or off on demand. This solution
would result in a drastic reduction of cell division between the egg and the
gamete, thus, in our hypothesis, reducing the rate of point mutations. The
disadvantage would be to superimpose another regulatory mechanism on the
already builtin regulation of meiosis. Any "give" in the coupling would
increase the risk of error.
Both solutions have advantages that are greater than their
inconveniences; they are, however, mutually exclusive. Thus, an optimization of
the process could be to use both of there with the aid of two separate systems:
one using continuous meiotic flow and the other using meiosis on request. The
fact that this solution was chosen by nature is not proof positive that man and
woman have been invented for that sole purpose. Nevertheless, this broad view
has some practical implications which could be investigated.
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The Translocation Trisomies
As noted earlier, any "give" between ovarian regulation superimposed
upon meiotic regulation would add to abnormal migration. The way this coupling
is realized is poorly understood. Nevertheless, as demonstrated by the
experiments of Edwards, [8] the mechanical rupture of the follicle seems to
trigger the whole meiotic process.
Let us suppose that the D/G translocation slows down the synaptic
process between the free and the translocated 21. No effect is expected in male
meiosis progressing at its own rate; in the ovary, however, if ovulation
triggers the system before synapses are established, the; free 21 will migrate
at random. Every random migration will produce either a trisomic or unviable
monosomic zygote after fertilization.
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The Free Trisomies and Aging
In female meiosis, a second trigger is the entry of the sperm that
occurs before the expulsion of the second polar body. It is even possible that
sperm entry is normally the triggering mechanism for the second anaphase. If it
occurs earlier than the preparation of the necessary enzymatic machinery, the
cleavage of centromeres could fail, leading to a second meiotic accident. This
new type of accident occurs in addition to the first type due to early
triggering by premature ovulation.
On the other hand, if fecundation is delayed, the meiotic mechanism,
not retriggered in due time, could fail and become dormant. Thus, no fusorial
apparatus would be produced at all, and triploidy by digyny could occur. Even
aging of the membrane could let more than one sperm in, Hence triploidy by
dispermy. At the most, a retarded second anaphase could result in the lagging
of one element, hence a free trisomy. The difficulty here again stems from
inappropriate timing between meiotic regulation and the circumstantial
triggering mechanisms.
Various conjectures can be made about the possible correlation
between the follicle chronology and the risk of aberration. For translocation
carriers, early rupture of the follicle could be the plain danger, and that
seemed to be the case in at least one family that was observed. For normal
mothers precocious or late triggering could be dangerous, therefore no
straightforward prediction can be ventured. It can only be surmised that aside
from the possible affect of the aging of the egg, [9] premature rupture of the
follicle could play a very dangerous role. If this were the case, fertilization
would occur too early in the cycle rather than too late.
These reflections do not pretend to provide a solution to the whole
problem, but may possibly lead to some heuristic implication. Geneticists
probably unduly overlook physiological conditions. If we could understand them
and possibly correct or prevent their distortion, many children could be
protected from a severe disease which is considered a chance accident because
its actual causes have not yet been sufficiently studied. In contrast to the
case of amniocentesis, much discussed these days, a physiological understanding
of trisomy could save many lives without destroying any.
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References
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661.
2. PENROSE, L. S. 1934. A method of separating the relative
etiological effects of birth order and maternal age. Ann. Eugen. 6: 108.
3. PENROSE, L. S. 1963. Measurements of likeness in relative of
trisomics. Ann. Hum. Genet. 27: 182187.
4. GREMY, F., D. SALMON & J. LEJEUNE. 1967. Les modèles de
genèse prézygotiques des trisomies. Ann. Génét. 10: 167178.
5. DUTRILLAUX, B. 1968, Etude des translocations du chromosome 21.
Thèse de Médecine. Paris.
6. DUTRILLAUX, B. & J. LEJEUNE. 1969. Etude de la descendance des
porteurs d'une translocation t (21q Dq) Ann. Génét. 12: 7782.
7. GILLOIS, M. 1969. La pseudo sexualité des cellules somatiques en
culture: modèle de l'induction des pseudomeioses. Ann. Génét. 21: 514.
8. EDWARD, R. G. 1965. Maturation in vitro of human ovarian ovocytes.
Lancet ii: 926929.
9. GERMAN, J. 1968. Mongolism, delayed fertilization and human sexual
behaviour. Nature 217: 516518.
10. DE GROUCHY, J. 1969. Personal communication.
