Human somatic chromosome chains and rings. A preliminary note on end-to-end fusions

B. DUTRILLAUX1, M.F. CROQUETTE1, E. VIEGAS-PEQIGNOT1, A. AURIAS1, J. COGET2, J. COUTURIER1, and J. LEJEUNE1

Cytogenet. Cell Genet. 20: 70-77 (1978)

Résumé :

• Materials and methods


• Results


• Discussion

In a patient affected by Thiberge-Weissenbach syndrome (scleroderma, calcinosis, and telangiectasia), cytogenetic examination of peripheral blood lymphocytes, performed in 1972 by one of us, had shown in one cell a very long chain element, presumably formed through end-to-end fusion of many chromosomes (MERLEN et al., 1975). This abnormality was not seen during a second examination in 1974.

In 1977, this peculiarity was again found in some cells after a systematic study of several thousand mitoses. The preliminary results of the analysis of the abnormal cells are reported here.

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Materials and methods

Lymphocytes were cultured according to standard technique. Air-dried prepara-tions were obtained as usual and treated sequentially in the following order classic Giemsa staining, quinacrine mustard staining, and acridine orange staining after 10-30 min thermal denaturation of freshly prepared slides (DUTRILLAUX, 1975). Some of the cultures were treated with BrdU, and the slides were stained with Giemsa and acridine orange.

Photographs of the abnormal cells were analyzed by two groups of investigators, and the interpretations were compared afterward.

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Results

Whereas the previous cultures appeared rather poor, the two consecutive ones carried out in 1977 were among the richest obtained in this laboratory; between 20,000 and 30,000 mitoses could be analyzed.

Cells having chain or ring chromosomes ranged from 1 per 1000 to 2 per 500, the frequency varying with the preparations. The rate of abnormal mitoses, very low in diploid cells, was rather high among the tetraploid cells. No increase in the frequency of chromatid breaks was detected.

Various types of anomalies were observed. In diploid cells, the end-to-end fusion of two or more chromosomes lead to chain configurations. The number and the length of the chains vary from cell to cell (figs. 1 and 2). Some of these chains are sometimes present as giant rings produced by the fusion of all chromosome ends. Isolated chromosomes may coexist with the chains, some of then forming rings. In some rare instances, the chains seem to cross or associate at places corresponding to satellites of acrocentrics (fig. 3). In tetraploid cells, the same configurations can be observed. As far as these cells can be correctly analyzed, all the abnormal figures are present in duplicate and the equivalent chains tend to be paired (fig. 4). This pairing is complete in a few cells interpreted as endoreduplications (fig. 5).

Among the abnormal cells scored, only a dozen were clear enough to be analyzed. Table I shows the distribution of the chromosomal associations observed. In a given diploid cell, distribution of the chromosomes in the chains seems to be random. In particular, no two identical chains correspond to the arrangement of the homologs; two homologs can be included in the same chain. On the other hand, there is no similarity between the chains of one cell and those of another.

Fig. 1. - Four cells stained with Giemsa, showing different chains and giant rings.

Fig. 2. - The same cell with (a) ordinary Giemsa, (6) Q-banding, and (c) R-banding. Among others, chromosomes 3, 5, 9, and 16 are free, and give the scale.

Fig. 3. - Prometaphase cell in R-banding (RHA). The arrow indicates a satellite association.

Fig. 4. a. - Tetraploid cell in R-banding (RHG). b. Identification of the chains, showing loose association of the homologs.

Fig. 5. - Endoreduplication.

Table 1. Distribution of 230 end-to-end associations of chromosomes (except for chromosome 19, associations of the short arms [p] and of the long arms [q] have been considered).

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Discussion

The two main types of information arising from these observations are that chromosomes are able to fuse together by the ends of their arms and that, connected in a such way, they can reach metaphase.

As far as can be judged from the few analyzable cells, there is no simple law ruling the associations: (1) No particular sequence of chromosomes was observed in different cells. (2) No obvious relationship could be found between the associations and special terminal structures, such as the T-bands or terminal Q-bands, previously described in some Pongidae (DUTRILLAUX, 1975). (3) These associations also seem to be independent of the replication pattern of the connected regions (DUTRILLAUX et al., 1976) and of the length of the arms. (4) Satellite associations of the acrocentrics, though observed, do not have a particularly high frequency. (5) Distribution of the chromosomes at the ends of the chains seems to be random. (6) End-to-end associations between homologs do not have an increased frequency; in the four instances observed, they formed a mirror figure (p to p and q to q).

Telomeric fusions of the chromosomes might result from the association of a single-stranded palindromic DNA located at the telomeres, according to the model proposed by CAVALIER-SMITH (1974). The base sequence of the palindromic DNA should be common to all telomeres.

The main question is whether these chain configurations do or do not reflect a functional organization at interphase. Three hypothesis may be proposed concerning the interphase arrangement of the chromosomes in normal cells:

1. The chromosomes are all independent. In this case, the abnormal cells we observed have undergone fusion of their chromosomes, leading to chain formation. The accident presumably occurred during the G1 phase because of the systematic involvement of the two chromatids. Its origin and mechanism remain totally unclear.

2. The chains exist in normal cells and consist of a random sequence of chromosomes. The interphase association of the telomeres might be suppressed by the clevage of the palindromes by a unique restriction enzyme. The somatic mutation of a single gene would be sufficient to impair the enzyme activity, and thus to induce the anomaly observed here.

3. Interphase chains exist in normal cells, and their chromosome sequence is well defined. This hypothesis requires the existence of a specific mechanism for the recognition of telomeres. The random association observed here might result from a defect in this recognition system. Afterward, the fission of the abnormal linkages would be impossible, leading to the persistance of the chains.

This last hypothesis is the most satisfactory to explain a rational organization of interphase chromosomes, particularly if somatic pairing of homologs exists. However, it is not in direct agreement with the present observations. On the contrary, the second hypothesis, though less attractive a priori agrees rather well with the observations. It does not exclude the possibility of somatic pairing but implies a more complex disposition of the chromosomes. They should be ordered in a rosette around several centers, to which they would be related by their telomeres (see COMINGS, 1968, for references). Considering the proximity, at least in lawman cells, of certain telomeres and nucleolar organizers, these centers might include the nucleoli.

In conclusion, the particular chromosome configurations observed here might be of great significance in understanding how the interphase chromosome arrangement comes about. For the moment, it is not possible to determine if these peculiarities are related to the disease affecting the patient. Scleroderma has been reported to be associated with chromatid breakage (EMERIT, 1971); however, this was not detected in our patient.

These preliminary results do not support the concept of a simple linear organization of chromosomes during interphase. Additional material of this type is necessary for the eventual clarification of a more complex arrangement.