[Cytogenetic analysis of the Bloom syndrome. A study of the exchange-break relationship].

Cytogenetic study of a case of Bloom's syndrome (number 46 of the international registry) confirms the excess of exchanges in all cellular types with the exception of a minority of lymphocytes and of two lymphoblastoid cell lines. These exchanges are produced in an X or U fashion between sister chromatids or between homologous chromatids and produce both simple and complex figures for which symmetry is the common feature. Some of these structures are rearranged secondarily, producing centric or acentric fragments and marker chromosomes. Triradial configurations are considered to be the result of exchanges rather than of partial endoreduplication. Chromatid and chromosome breaks are interpreted to result from incomplete exchanges. It is suggested that the general propensity for exchanges is the primary event responsible directly or indirectly for the cytogenetic observations rather than a defect in one of the DNA-repair mechanisms. No increase in mitomycin C sensitivity appears in vitro. The excess of SCEs is partially correlated by contact in vitro with normal cells and to a lesser degree by the culture medium in which the cells were grown.