High-resolution R-banding and localization of fragile sites in Oryctolagus cuniculus

Genetics Selection Evolution, Jan 1991

BS Poulsen, M. Rønne

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High-resolution R-banding and localization of fragile sites in Oryctolagus cuniculus

High-resolution R-banding and localization of fragile sites in Oryctolagus cuniculus BS Poulsen M Rønne 0 0 Odense University, Institute of Medical Biology, Department of Anatomy and Cytology , Campusvej 55, DK-5230 Odense M , Denmark rabbit/ R-banding/ fragile sites/ X chromosome Fragile sites on chromosomes are points where chromosomes are liable to break. They are divided into two major groups, common and rare, according to their population frequency and their mode of unmasking (Sutherland and Hecht, 1985; Hecht et al, 1990). In humans, fragile sites have been correlated to mental retardation (Lubs, 1968), to cancer breakpoints (De Braekeleer, 1987), to targets for mutagens and carcinogens (Yunis et al, 1987) and to breakpoints involved in the chromosomal evolution of primates (Mir6 et al, 1987). Yunis and Soreng (1984) found several fragile sites shared by man, chimpanzee and gorilla demonstrating that fragile sites are conserved among closely related species. Data on fragile sites in chromosomes of domesticated animals are limited, but they have been demonstrated in the pig (Riggs and Chrisman, 1989) and the horse (Ronne and Poulsen, unpublished). This paper reports on 8 putative fragile sites located in the rabbit karyotype. MATERIALS AND METHODS - INTRODUCTION Four animals, 2 males and 2 females, were randomly selected from the stock of laboratory rabbits at the Institute of Biomedicine, Odense University. R-band induction and fragile site-unmasking were performed simultaneously as follows: peripheral blood was allowed to sediment for 1 h at room temperature and 0.5 ml from the buffy/plasma layer was added to 9.5 ml of RPMI-1640 (Gibco) supplemented with 10% fetal calf serum (Gibco), 5 IU/ml heparin (Sigma), 50 pg/ml gentamycin (Sigma) and phytohemagglutinin (PHA-M) (Gibco). Cultures were synchronized with fluorouracil (FU; Sigma; 5 x 170- M) and processed as previously described (Ronne, 1984) . For each animal, 4 cultures (1 control and 3 test cultures) were exposed to different chemicals as listed in table I. Subsequent harvesting, staining and band induction were performed as described by Ronne (1983 , 1984). RESULTS AND DISCUSSION For each animal, 4 cultures (1 control and 3 test cultures) were examined for the presence of fragile sites. From each culture, 50 metaphases were selected at random, photographed and analyzed on a screen at a magnification of x 5 000. If chromosomes were difficult to identify directly, photographic copies were made and karyotyping performed. For all 4 animals, the putative fragile sites were not expressed in the control cultures (control), while the test cultures (table I: x, y, z) yielded metaphases with non-randomly distributed chromosomal breaks. The localization of chromosomal breaks is shown in figure 1. The mode of action of the 3 agents employed is similar, but APC seems to be the most potent one. Only two sites, lp32 and Xpl4, were sensitive to all 3 agents. Fragility of these sites was expressed in all animals at high frequencies. In the females, Xpl4 was expressed both on the active and the inactive X-chromosome at approximately the same frequencies. Chromosomal breaks were also found in lq26, 4p14, 4ql2, 4ql4, 15ql2, Xq24, but only at low frequencies (range: 2-8%). Breakpoints at 4q12 and 4q14 were only observed in one animal (A) after APC exposure. Both females displayed a breakpoint at Xq24. This specific break was only unmasked at a relatively low frequency after APC exposure. It was only detected on the active X chromosome, but this may be due to the staining technique used or to a sampling error. In rare cases, the X chromosome displayed both breakpoints (fig 1). It is interesting that the laboratory rabbit has a breakpoint at Xq24 similar to that observed for horses and humans. The Y chromosome showed no breaks either in the controls or in the test cultures. The most important findings are summarized in table II. According to Hecht et al (1990 ), the status of a fragile site may be either tentative, provisional or confirmed. Judged from the frequencies and the distribution of the breakpoints observed in the rabbit karyotype, these fall into two separate groups. The breakpoints lp32 and Xpl4 may represent APC-sensitive common fragile sites with a provisional status, while the other breakpoints may be either rare or common fragile sites with a tentative status. De Braeckeleer M (1987) Fragile sites and chromosomal structural rearrangements in human leukemia and cancer. Anticancer Res 7, 417-422 Hecht F , Ramesh KH , Lockwood DH ( 1990 ) A guide to fragile sites on human chromosomes . Cancer Genet Cytogenet 44 , 37 - 45 Lubs HA ( 1969 ) A marker X chromosome . Am J Hum Genet 21 , 231 -244 Miro S , Clemente JC , Fuster C , Egozcue J ( 1987 ) Fragile sites, chromosome evolution and human neoplasia . Hum Genet 75 , 345 - 349 Riggs PK , Chrisman CL ( 1989 ) Preliminary analysis of aphidicolin induced fragile sites in pig chromosomes . 6th North American Colloquium on Cytogenetics of Domestic Animals West Lafayette Indiana abstr 4 Ronne M ( 1983 ) Simultaneous R-banding and localization of dA-dT clusters in human chromosomes . Hereditas 98 , 241 - 248 Ronne M ( 1984 ) Fluorouracil synchronization of human bone marrow cultures. In vitro induction of high resolution R-banding by simultaneous exposure to 5'-bromodeoxyuridine/Hoechst 33258 . Anticancer Res 4 , 279 - 282 Sutherland GR , Hecht F ( 1985 ) Fragile Sites on Human Chromosomes . Oxford University Press, New York Yunis JJ , Soreng AL ( 1984 ) Constitutive fragile sites and cancer . Science 226 , 1199 - 1204 Yunis JJ , Soreng AL , Bowe AE ( 1987 ) Fragile sites are the target of diverse mutagens and carcinogens . Oncogene 1 , 59 - 69

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BS Poulsen, M. Rønne. High-resolution R-banding and localization of fragile sites in Oryctolagus cuniculus, Genetics Selection Evolution, 1991, 183s-186s, DOI: doi:10.1051/gse:19910734