Chromosome 15 anomalies and the Prader-Willi syndrome: cytogenetic analysis.

The behaviour of chromosome 15 is very different from that of the other acrocentric chromosomes. The cytogenetic characteristics of rearrangements associated with Prader-Willi syndrome (PWS) are analyzed as similar rearrangements irrespective of the associated phenotype (reciprocal translocations of chromosome 15, small bisatellited additional chromosomes, Robertsonian translocations, interstitial deletions, pericentric inversions). This study suggests that: (1) The proximal ( 15q ) region and PWS seem to be indissociable ; (2) chromosome 15 has an indisputable cytogenetic originality which could be related to its histochemical properties. Chromosome 15 constitutive heterochromatin usually contains much 5-methylcytosine-rich DNA and a large amount of each of the four satellite DNAs. Furthermore the existence in the proximal ( 15q ) region of one or several palindromic sequences could be postulated to explain the great lability of this region of chromosome 15.

[Individual variability of associations between acrocentrics (author's transl)]. / Variabilite interindividuelle des associations entre chromosomes acrocentriques.

We studied the quantitative and qualitative variation of associations between acrocentric chromosomes in four subjects on a period of ten months; results show a definite tendency of association in a same subject, tendency which is reproducible and clear. Even though each individual has a determined rate of associations, the number of associated chromosomes and the number of associations themselves do not vary with months. Furthermore, associations do not appear at random; indeed, they have a wide spectrum of variations from one subject to the other but if we take the subjects individually, their associative rate concerning each chromosome do not change with months. Therefore, the phenomenon of association could be viewed as a biological characteristic of each individual.