[Clinical and genetic consequences of ring chromosome, a structural genome mutation]

Ring chromosome is a peculiar genetic anomaly which may cause phenotypic abnormalities even without a loss of genetic material. The ring formation of chromosome ends induces difficulties in the sister chromatid separation at cell division, resulting in increased cell death rate through the generation of secondary aneuploid cells. Based on the in vitro analyses of own cases and a review of more than 200 case reports published in the literature, the author presents some conclusions on "ring syndrome", a non-specific clinical manifestation of ring formation of any chromosome, consisting of the lack of (or poorly expressed) organic manifestations, mild or borderline mental retardation, and a severe growth failure which is not explained by organic, biochemical, or endocrine findings. It is also suggested that the phenotypic manifestation of a ring chromosome is even less expressed when the ring is transmitted to the next generation. Based on observations of ring behaviour and the clinical phenotypes in a 3-generational family with a mosaic supernumerary ring, it is not unrealistic to assume that there is a transgenerational increase in the severity of the clinical manifestation resulting from a less efficient elimination process against the ring (cytogenetic anticipation).