Satellite association and translocation mongolism.

Cytogeneticists before I960 recognized only two pairs of satellited chromosomes in man: a large acrocentric pair of the D group and a small acrocentric pair of the G group. In I960 another pair of the D chromosomes was found to be satellited, and in I96I Ferguson-Smith and Handmaker presented conclusive evidence of all I0 acrocentric autosomes having satellites, though usual staining methods rarely show all of them in a single metaphase plate. Better visibility of satellites is obtained in our laboratory by staining the chromosomes with Giemsa. By careful inspection of many karyograms we noticed that the satellited ends of two or more chromosomes lay closely together more often than expected if random distribution of the chromosomes was assumed. This phenomenon is called satellite association (S.A.). Its occurrence in human karyograms was first described in I96I (Ferguson-Smith and Handmaker, I96I; Harnden, I96I; Ohno, Trujillo, Kaplan, and Kinosita, I96I). A schematic drawing of S.A. as it may appear in mitotic metaphase is shown in the Figure. S.A. is known to occur in meiosis as well as in mitosis (Ferguson-Smith, I964). It has been related to the presence of persistent nucleolar RNA located near the nucleolar organizers which are proximal to the satellited ends of the acrocentric chromosomes. This material has lately been detected by special cytochemical methods, though it is not visible in ordinary karyogram preparations (Hsu, Arrighi, Klevecz, and Brinkley, I965). It is somewhat sticky and has, particularly if present in abundance, a tendency to keep the associated chromosomes together through later phases of meiosis or mitosis. The respective chromosomes (meiosis) or chromatids (mitosis) may fail to disjoin and aneuploid karyotypes result. Thus S.A. may represent a cause of aneuploidy. Furthermore, the heterochromatic site of the nucleolar organizers is prone to chromosomal breakage. Thus fragments of acrocentric chromosomes arise which tend to fuse in various combinations, whereby translocations may be formed (Ohno et al., I96I). This explains why translocations between acrocentric chromosomes are far more frequent than translocations between other non-acrocentric chromosomes of the groups A, B, C, E, and F. As a working hypothesis it can be assumed that non-disjunction and translocations may occur more often if there is (a) increased stickiness of some of the satellited chromosomes, or (b) if the number of S.A. is increased. The first part of this hypothesis cannot be tested in our laboratory, but there are methods available to check its second part: the number of S.A. In a previous study evidence was presented that increased S.A. may be transmitted as an inherited trait (Abbo, Zellweger, and Cuany, I966). In this paper studies concering S.A. in our cases of translocation mongolism and their parents will be presented.