The behaviour of fragile X and other aberrations during recovery from low folate conditions.

Whole blood from two mentally retarded fra-X brothers was grown in low folate medium where fra-X expression was enhanced. Bromodeoxyuridine was added to mitigate the low folate conditions and metaphases were sampled sequentially, and stained for replication banding, through one cell cycle of recovery. The replication bands allowed detailed analysis of the cell cycle and the allocation of individual cells to precise sub-phases. Various classes of fra-X and all other types of chromosomal aberrations were scored in these classified cells. The fra-X does not conform in morphology to any of the known simple chromatid intrachange types, which were often present within the same cells, but the subsequent fall in frequency once bromodeoxyuridine was added closely paralleled that of the conventional aberrations. Normal folate level frequencies of fra-X are restored by the time early S-phase cells (sub-phase SkI) reach metaphase. When sub-phased cells are rearranged in true chronological sequence, there is a suggestion of a sudden fall in frequency between SkII-III (about 70% of the transit of S). This suggests that the critical point for low folate enhancement occurs in this region of the S-phase. This is somewhat earlier than the band-appearance distribution curve for Xq27 which lies within sub-phase SkIV.