Expression of fragile X chromosome in human-rodent somatic cell hybrids.

The fragile X chromosome, associated with a common form of X-linked mental retardation, is cytologically observed most often as a gap or fragile site near the distal end of the long arm in band Xq28. Expression of this site is variable and dependent upon lowered thymidylate pools. In order to examine the behavior of this fragile site in a foreign genetic background, interspecific somatic cell hybrids were isolated from crosses of hamster cells and lymphoblastoid cells derived from male patients with fragile X-linked mental retardation. Three hybrid cell lines containing the human X chromosome were analyzed. Following induction with 5-fluorodeoxyuridine, all three hybrids expressed the fragile site in approximately 10% of the metaphases examined. Our data indicate that expression of the fragile site in band Xq27 is dependent neither on the integrity of the human genome nor on the expression of human autosomal genes.