A number of applications of the detection of deoxyribonucleic acid synthesis by fluorescence microscopy are illustrated. These include (a) the analysis of sister chromatid exchanges and sister chromatid segregation at mitosis, (b) the location of chromosome regions containing deoxyribonucleic acid with an asymmetric distribution of thymine residues between polynucleotide chains and (c) the detection of late replicating regions in metaphase chromosomes. The suppression of 33258 Hoechst fluorescence by 5-bromodeoxyuridine incorporated biosynthetically into interphase nuclei is demonstrated both in fixed cytologic preparations and in unfixed cultured cells. Many of the cytologic observations described might form a basis for future biochemical studies.

Sister chromatid exchanges, which may reflect chromosome repair in response to certain types of DNA damage, provide a means of investigating the increased chromosome fragility characteristic of Fanconi's anemia. By a recently developed technique using 33258 Hoechst and 5-bromodeoxyuridine, it was observed that the baseline frequency of sister chromatid exchanges in phytohemagglutinin-stimulated lymphocytes from four males with Fanconi's anemia differed little from that of normal lymphocytes. However, addition of the bifunctional alkylating agent mitomycin C (0.01 or 0.03 ;Ig/ml) to the Fanconi's anemia cells during culture induces less than half of the increase in exchanges found in identically treated normal lymphocytes. This reduced increment in exchanges is accompanied by a partial suppression of mitosis and a marked increase in chromatid breaks and rearrangements. Many of these events occur at sites of incomplete chromatid interchange. The increase in sister chromatid exchanges induced in Fanconi's anemia lymphocytes by the monofunctional alkylating agent ethylmethane sulfonate (0.25 mg/ml) was slightly less than that in normal cells. Lymphocytes from two sets of parents of the patients with Fanconi's anemia exhibited a normal response to alkylating agents, while dermal fibroblasts from two different patients with Fanconi's anemia reacted to mitomycin C with an increase in chromatid breaks, but a nearly normal increment of sister chromatid exchanges. The results suggest that chromosomal breaks and rearrangements in Fanconi's anemia lymphocytes may result from a defect in a form of repair of DNA damage. Fanconi's anemia is a hereditary disease characterized by pancytopenia, congenital malformations, and patches of increased skin pigmentation (1). Chromosomes from affected individuals exhibit structural lability (2, 3), and there is a strong predisposition for the development of neoplasia (4, 5). Because of the high frequency of chromosomal abnormalities, predominantly breaks, it has been suggested that an error in DNA repair exists in Fanconi's anemia (5).Bifunctional alkylating agents such as mitomycin C (6), as well as y-rays (7), are unusually effective in inducing breaks in chromosomes of peripheral lymphocytes from individuals with Fanconi's anemia. Similarly, mitomycin C has been reported to be highly lethal to fibroblasts cultured from patients with Fanconi's anemia (8). Monofunctional alkylating agents exhibit less selective toxicity to Fanconi's anemia cells than do their bifunctional counterparts (6,8). One recent study has suggested that Fanconi's anemia fibroblasts may also have a reduced ability to excise thymine dimers (9).The observation that alkylating agents are able to induce large numbers of sister chromatid exchanges in chromosomes from normal cells (10-12) provides a new approach for investigating Fanconi's anemia. Since MATERIALS AND METHODSSister chromatid exchanges in human lymphocyte chromosomes were detected by fluorescence microscopy after staining with the dye 33258 Hoec...

DNA flow histogram analysis, using 33342 Hoechst as a stain, has been used to detect the effect of the potentially bifunctional alkylating agent, mitomycin C (MMC) on dermal fibroblasts from patients with Fanconi's anemia (FA), a hereditary human disease characterized by pancytopenia, hypersensitivity to DNA-crosslinking agents, congenital abnormalities, and a predisposition for neoplasia. At 24 or 48 hr after a 2-hr exposure to 0.05 or 0.10 pg/ml MMC, 3HdT incorporation was reduced to a greater extent in FA cells than in normal cells. Cells sorted from the last half of S phase showed a slightly greater inhibition of 3HdT incorporation than did those sorted from the first half of S.Fanconi's anemia cells exhibited a marked accumulation in the Gz + M peak of flow histograms following exposure to MMC. Twenty-four hr after treatment with 0.05 pg/ ml MMC, the G2 + M fraction of FA cells (eight lines) increased to more than 0.5 from a control value of approximately 0.2. Both normals (six lines) and heterozygotes (eight lines) showed, on the average, much less of a Gz + M increment than did FA cells, even after exposure to 0.1 pg/ml MMC. Examination of cells sorted from the Gz + M peak revealed that MMC-treated FA cells were blocked prior to mitosis.To determine whether the response of FA cells was specific for a bifunctional alkylating agent, cells were also treated with ethylmethanesulfonate, a monofunctional agent. Twenty-four hours after exposure to 0.25 or 0.5 mg/ml ethylmethanesulfonate, FA and normal cells showed similar, small increases in the G2 + M peak. The results suggest the utility of flow cytometry in the diagnostic evaluation of fibroblasts from patients suspected of having Fanconi's anemia.Key terms: Fanconi's anemia, DNA crosslinking agent, mitomycin C, flow cytometry, 33342 Hoechst, Gz block, Cell cycle analysis, DNA repair Fanconi's anemia (FA), a syndrome of pancytopenia and a variable constellation of congenital anomalies (12,34,44), has been shown to be associated with chromosomal instability (8, 42, 47) and a predisposition for cancer (5,16, 43). Cells from patients with FA show increased sensitivity to agents capable of producing DNA crosslinks. This sensitivity is evident as decreased survival (51), increased chromosome breakage (3, 29,40) and a reduction in both the rate of progression through I This research was supported by grants CD-36F from the American Cancer Society and GM21121 from the National Institute of General Medical Sciences.Presented at the VIII Conference on Analytical Cytology and Cytometry, Wentworth-by-the-Sea, New Hampshire, May 19-25, 1981. the cell cycle (41) and in the proportion of mitotic cells (17, 51), following exposure to such agents.A reduction in the ability of cells from patients with Fanconi's anemia to remove DNA crosslinks, caused e.g. by mitomycin C (MMC), was observed in some studies (15) but not in others (13, 25). There are also isolated reports that cells from patients with FA exhibit other defects, such as a reduced ability to remove damage induc...

A number of DNA-binding dyes, with spectral properties making them suitable as components of energy donor-acceptor pairs, are described. If such pairs are used to stain metaphase chromosomes, and if the energy acceptor (e.g., actinomycin D or methyl green) has a binding specificity opposite to the binding or fluorescence specificity of the donor (e.g,, 33258 Hoechst, quinacrine or chromomycin A3), contrast in donor fluorescence can be enhanced, leading to patterns selectively highlighting standard or reverse chromosome bands or particular polymorphic regions. Such results presumably reflect chromosomal regions enriched in 10-20 base pair clusters to which the donor binds and fluoresces but to which the acceptor cannot bind. For other pairs, involving counterstains such as netropsin or echinomycin, which are not suitable as energy acceptors, specific changes observed in polymorphic region fluorescence are most likely due to binding competition between dyes. Dye pairs producing contrast by either method can be used to differentiate between homologous chromosomes or to facilitate detection of specific chromosomal rearrangements. Preliminary data indicate that contrast enhancement generated in fixed metaphase chromosomes spread on microscopic slides can also be observed in suspensions of unfixed metaphase chromosomes, reinforcing the expectation that the methodology described will be of use in flow cytometry.Key terms: Energy transfer, fluorescence, binding competition, chromosome banding patterns, polymorphisms, rearrangements, flow cytometry.Staining metaphase chromosomes simultaneously with pairs of different dyes can lead t o new fluorescence patterns. In addition to the signals produced when the dyes are used independently, information can be generated by interactions between these dyes. For example, two dyes can compete for binding to certain chromosomal sites, which can thereby be stained differentially (15,41) Alternatively, indirect effects of one dye on another can be mediated via conformational changes induced in DNA (46). In addition, because of a coincidence between the range of separations possible between ' Supported by Grant GM21121 from the National Institutes of Health. S.A.L. is the recipient of a Research Career Development Award GM00122 from the National Institute of General Medical Sciences and E.S. is supported in part by a grant from the Whitaker Foundation.* Presented in part at Automated Cytology VII, Asilomar, California, November 25-30, 1979. dye molecules bound to DNA (20) and the effective distance over which dye electronic transition dipoles can interact (typically 5 50 A) (9, 42), the transfer of electronic excitation energy from one dye to another in doubly stained chromatin can be appreciable (14,16,17,36).We have recently shown that the enhanced contrast in metaphase chromosome staining produced by certain dye pairs is due largely to energy transfer, while that produced by other pairs, which fail to satisfy spectral overlap criteria, is due primarily to binding competition (37)....

Cells from patients with Fanconi’s anemia are unusually sensitive to agents which are capable of crosslinking DNA. This increased sensitivity can be detected both by cytogenetic and flow cytometric methods. An elevated frequency of chromosome aberrations, which is further exaggerated by exposure of cells to DNA crosslinking agents, is a general feature of Fanconi’s anemia. Information about the formation of sister chromatid exchanges in this disease is less consistent. Cytogenetic analysis of cells from patients with Fanconi’s anemia can be compromised by a low mitotic index. This is reflected in an accumulation of cells in the G2 phase of the cycle, after exposure to the bifunctional alkylating agent, mitomycin C. New methods for differentiating individuals with Fanconi’s anemia from unaffected individuals should be of empirical use and might also facilitate mechanistic studies of this disease.