A Cytogenetic Study of Human Fragile Sites

Abstract

Three rare fragile sites (2q13, 12q13 and 17p12) were found to be segregating in a large kindred. Fragile sites 2q13 and 12q13 were readily detectable in culture medium 199+5%FCS, except for 2q13 in family member II-2. The treatment of 199 cultures with fluorodeoxyuridine (FUdR) or methotrexate (MTX) or aphidicolin did not increase the levels of expression of fragile sites 2q13, 12q13 or 17p12, nor did it induce fragile site 2q13 in family member II-2. For family members II-2 and III-7, the levels of expression for fragile site 12q13 was greater in 96hour cultures than in 72hour cultures. The addition of bromodeoxyuridine (BrdU) to 199 cultures reduced expression of fragile sites 2q13 and 12q13. There was a statistically significant difference in the chance of expression for fragile sites 2q13 and 12q13 in F10+20%AB+FUdR compared to F10+20%AB. Treatment of 199+5%FCS cultures with aphidicolin induced the common fragile sites 3p14, 6q26, 16q23 and Xp22, and did so frequently in both homologues. In these aphidicolin treated cultures, the subjects studied showed fragile site expression at different rates with time. At 72hours there was a significant difference between subjects but by 96hours there was no significant difference between the subjects. C-banding studies of the aphidicolin induced fragile site 16q23 indicated that fragile sites induced by aphidicolin may be individually site specific sensitive for each homologue. The addition of uridine or excess thymidine or FUdR to the lymphoblastoid cell line (LCL) established from family member II-2 did not induce fragile site 2ql3, nor did it enhance expression of fragile sites 12ql3 or 17pl2. There was a lower level of fragile site expression in the LCL (B-lymphocytes) compared to the PHA stimulated T- lymphocyte cultures. Co-cultivation experiments between family members (II-2, III-6 and III-7) who were either expressors or non-expressors of the fragile sites (2ql3, 12ql3, 17pl2) showed no evidence of the existence of fragility reducing or fragility enhancing factors. There were no significant differences in the rate of sister chromatid exchange (SCE) between family members (II-2, III-7) and the five controls (CT 1-5). No sister chromatid exchange (SCE) was observed at the fragile sites studied. The clinical history of the family showed no evidence of cancer, mental retardation or phenotypic abnormalities.