The analysis of the fine structure of chromatids by new techniques
allow a better determination of the type of translocations. A review of 30
cases examined in our laboratory show that centromeric and telomeric regions
are more frequently involved by breakage and rejoining than other parts of the
Material and methods
Our technique of controlled heat denaturation has been applied
successfully to more than 1.000 patients referred to the laboratory.
The patients have been examined for various reasons, mostly because of
an abnormality of the patient himself or of some of his relatives, or of an
abnormality of reproduction such as sterility or repeated spontaneous
a) Controlled heat denaturation (1-3-6) ( =
The slides are dipped for 10 to 20 min in a solution of Earle's
medium at pH 6.5, heated at 87° C.
b) Enzymatic digestion (2-4) ( =
The slides are dipped at room temperature in a solution of 5 mg of
enzyme for 100 ml of water, buffered at the optimal pH, for the enzyme used
(pronase, trypsin, a-chymotrypsine, protease, etc.).
c) Staining o f constitutive heterochromatin (=
This technique established in the laboratory uses a 20% solution of
urea in water, heated at 87° C, in which the slides are dipped for 1 to 6
Giemsa staining was used for all three techniques.
In this test with more than 1.000 patients, 30 were found to be
I. Translocations undetectable by classical
The hypothesis has been verified (7) that some rearrangements which
do not greatly modify the length of the elements should have escaped detection
by the classical procedures.
These translocations seem to occur comparatively seldom, for only
three cases have been detected during the course of our experiments.
1. In one family a child having a supernumerary G-like acrocentric,
was born from a carrier father (Fig. 1).
2. In another family two brothers having also a supernumerary G-like
acrocentric were born from a carrier mother (9).
Although these two families are not related, so far as we know, the
carrier parents exhibit rather the same translocation: The long arm of
chromosome 22 is exchanged with the terminal portion of the long arm of
chromosome 8, the exchanged segments being of similar length. The three
children are thus essentially trisomics for the distal part of the long arm of
chromosome 8 and for the short arms. and centromeric region of chromosome
3. The third case concerns a sterile man exhibiting apparently a Gq-
chromosome. From meiotic studies there was no evidence of a reciprocal
translocation. Denaturation technique demonstrated a translocation of the
distal part of the long arm of chromosome 22 and on the end portion of
chromosome 11. [t(11; 22)] (Fig. 8f).
Fig. 1. - Partial
karyotype of two cells of a patient carrying a t(8 ; 22). The translocation is
detectable only after denaturation (below).
II. Complex translocations
Of the 30 cases, two cases have been demonstrated as complex
rearrangements, which are due to more than two break points.
In one family an apparently reciprocal translocation had been
detected by the classical procedure, the carrier mother having a t(3p-; Cq+)
Denaturation showed (11) that a part of the short arm of the 3 is in
fact inserted in the long arm of chromosome 7 (Fig. 2).
A rearrangement of this kind needs three break points, two in the
donor and one in the recipient chromosome.
Fig. 2. Three brews
accident: A segment of the short arm of chromosome 3 is inserted in the long
arm of chromosome 7.
2. Complex duplication:
A child with typical trisomy 21 had previously been proven, by
classical procedures (8), to be a carrier of an abnormally long G
This element was interpreted as a tandem translocation between two
chromosomes 21, the whole arm of one of them being translocated on the
telomeric portion of the other. Because of a further case of free trisomy 21 in
a relative of the child, the eventuality of an " aneusomie de recombinaison "
The analysis of the band pattern after controlled heat denaturation
as well as after enzymatic digestion, shows that both chromosomes 21 are fused
together by their telomeric region.
At the free end of the element the chromaticism tend to keep close
to each other and show a fuzzy region which could be the remanants of
satellites stalks (Fig. 3b and 3d). Nevertheless this rearranged chromosome is
definitely not a true dicentric as confirmed by the centromeric staining (Fig.
From this aspect the element can be designed, using the Paris
nomenclature (10) as
t (21; 21) (21 pter ? 21 qter : : ? 21 q11 : : 21 p 11 ? 21
pter) (Fig. 4).
A new analysis of the karyotypes of the parents did not demonstrate
a rearrangement of the chromosomes 21 which look normal. This negative result
does not permit to propose a simple model of aneusomy of recombination to
explain this strange chromosomal aberration.
If the accident really occurred de novo in the affected child, 4
break points are needed to explain it. It cannot be excluded that a
particularity of the centromeric region of one of the chromosomes 21,
undetectable with the present techniques, preexisted in one of the parents.
Fig. 3. Partial caryotype of 8 cells with a complex translocation
t(21:21) : a) after controlled denaturation, b) after enzymatic digestion, c)
after urea treatment, d) after conventional staining.
Fig. 4. Interpretation of the
rearrangement t(21; 21) with a four breaks hypothesis.
III. Translocations X-autosome
Among the 30 cases of translocations here discussed, 4 of them were
ascertained for sterility. From the 4, two cases showed a reciprocal autosomal
translocation and two showed an X-autosomal translocation.
1. Translocation: t (6; X)
An amenorrheic woman carries a t (6 q+ ; Xq-) translocation.
Analysis of band patterns after denaturation shows that break points occurred
at the end of the long arm of 6 (6q27) and in band q 21 of the X. One third of
this clear band remained on the Xq- and 2/3 were transferred to the 6q+ (Fig.
2. Translocation: t (1 ; X )
The translocation is observed in an azoospermic man (Fig. 6 c) and
is also found in his mother (Fig. 6a) and his aunt (Fig. 6b). This person is
also sterile. The sister of the proband does not carry the translocation but
one of her chromosome I has an elongated secondary constriction (Fig. 6d). The
same feature is found in the normal chromosome I of the mother and in the 1q-
of the proband. It follows that a crossing over took place between the
secondary constriction and the break point in the maternal meiosis (5).
The study of the meiosis of the proband (Fig. 7) reveals in many
cells a very long element consisting of the rearranged chromosome 1 associated
by the short arms to the normal one, itself associated by long arms to the
segment translocated to the X ; this latter being itself associated by its
short arms to the Y chromosome (Figs. 5 and 6).
Translocation t(6;X) in an amenorrheic woman.
Fig. 6. Translocation t(1;X): a) The
proband's mother. b) The proband's aunt (sterile). c) Proband (sterile). d)
Proband's sister (non-sterile).
Fig. 7. Primary metaphase and interpretation of the tetravalent
observed in the proband's meiosis.
IV. Location of the break points
If we limit the analysis to the 28 instances of reciprocal
translocations, we can divide the 56 break points according to their location
in the centromeric region,
in the telomeric region,
or somewhere else in the chromatid (see Table 1 and Fig. 8, a, b, c,
d, e, f).
The analysis shows that out of these 56 break points,
18 occur in a centromeric region,
20 occur in a telomeric region,
and only 18 occur in another part of a chromatid.
No case of translocation has been recorded in which both breaks are in
the intermediate region.
Although any comparison of lengths of these regions is very difficult,
it appears that the break points tend to lie in either end of a chromatid and
occur rather unfrequently in the middle part of the chromatids.
The particular mechanism of " centric fusion " has often been used to
explain the high frequency of translocation between acrocentrics.
It is possible that a comparable tendency exists also in metacentric
chromosomes, their centromeric and telomeric regions being more prone to
chromosome exchanges than the rest of the chromatid.
Table 1. Classification of 28 cases of reciprocal
translocation according to the location of the break points.
|Location of the break point||number of
Fig. 8. Various types of translocations: a) t(5 ;
12) between two centromeric regions; b) t(5 ; 7) between a telomeric region and
a centromeric region; c) t(10 ; 11) between a telomeric region and a chromatid;
d) t(9 ; 22) between two centromeric regions; e) t(8; 22) between a chromatid
and a centromeric region; f) t(11 ; 22) probably between a telomeric region and
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