[Chromosomal characteristics of X-linked recessive mental retardation. I. The X chromosome]

The X-chromosome was studied in blood lymphocytes of 68 males with aspecific mental retardation (MR), their 57 relatives and 15 intellectually normal males. The incidence of a fragile X-chromosome (fra(X)) was found to be 4.7% in an unselected group of 42 patients, 50% among 10 probands in which pedigree data were suggestive of X-linked MR diagnosis, and 75% in the group of 15 patients selected for phenotype characteristic of the fragile X syndrome. The fra(X) was present in 1-43% of metaphases in different individuals, no such marker being observed in cells of 15 normal individuals. No significant difference was found when the incidence of the fra(X) was compared in cells cultured in the medium 199 with low folic acid content and the Eagle's medium supplemented with 5-fluorodeoxyuridine (10.62 +/- 2.94 SEM and 13.53 +/- 2.85 SEM, respectively). The possibility of false-positive diagnosis of the fragile X syndrome was quantitatively appreciated. A half of the patients showing a fra(C) in conventionally stained chromosomes were found to have fragile 6 autosome as the only marker in these cells, and in patients with the evident fragile (X) syndrome the fra(6) constituted about one-third of the fra(X) frequency. Both culture media employed were similar in the fra(6) induction.