MENTAL-RETARDATION, MARKER X-CHROMOSOMES, AND SILVER-STAINING (NORS)

A form of X-linked mental retardation has recently been shown to be associated with a fragile site on the long arm of the X chromosome (band Xq27 or 28). Physically, this fragile site gives the marker X chromosome appearance of a satellited chromosome, indicating the possibility that a nucleolar organizer region (NOR) from a D- or G-group chromosome was translocated to the Xq. This possibility has been discounted based on the fact that the Xq is not found in satellite associations with D- and G-group chromosomes. Such a marker X chromosome has been identified in two newly ascertained families (four males in family E0772-9 and four males in family E0642) studied by growing lymphocyte cultures in medium 199. The marker X in each family was confirmed by G-banding studies. Slides from each family were then stained by a silver-stain technique which preferentially stains the active organizer regions of animal chromosomes. Optimum NOR staining was seen on the D- and G group chromosomes but no silver-stained areas were seen on the marker X in either family. Thus, if the marker X does contain an NOR, it does not silver stain, which means that it is inactive and does not participate in nucleolar organization. This probably would account for the fact that such a marker X would not be found in association with the D- and G-group chromosomes. However, the presence of a NOR at the fragile site cannot be entirely excluded until it is known what chromosomal material is stained by the silver technique. Like most others, they doubt that such marker X chromosomes can be explained as satellites. Their study does offer direct evidence that such sites are not active NORs, at least in these two families.