Mapping of sister-chromatid exchanges in human chromosomes using G-banding and autoradiography

Hepatitis B virus infection and replication in a new cell culture system established by fusing HepG2 cells with primary human hepatocytes

BACKGROUND

Hepatitis B virus (HBV) infection is strictly species and tissue specific, therefore none of the cell models established previously can reproduce the natural infection process of HBV in vitro. The aim of this study was to establish a new cell line that is susceptible to HBV and can support the replication of HBV.

METHODS

A hybrid cell line was established by fusing primary human hepatocytes with HepG2 cells. The hybrid cells were incubated with HBV-positive serum for 12 hours. HBV DNA was detected by quantitative fluorescence polymerase chain reaction (QF-PCR). HBsAg (surface antigen) and HBeAg (extracellular form of core antigen) were observed by electrochemiluminescence (ECL). HBcAg (core antigen) was detected by the indirect immunofluorescence technique. HBV covalently closed circular DNA (cccDNA) was analyzed by Southern blot hybridization and quantified using real-time PCR.

RESULTS

A new cell line was established and named HepCHLine-7. The extracellular HBV DNA was observed from Day 2 and the levels ranged from 9.80 (± 0.32) × 10(2) copies/mL to 3.12 (± 0.03) × 10(4) copies/mL. Intracellular HBV DNA was detected at Day 2 after infection and the levels ranged from 7.92 (± 1.08) × 10(3) copies/mL to 5.63 (± 0.11) × 10(5) copies/mL. HBsAg in the culture medium was detected from Day 4 to Day 20. HBeAg secretion was positive from Day 5 to Day 20. HBcAg constantly showed positive signals in approximately 20% (± 0.82%) of hybrid cells. Intracellular HBV cccDNA could be detected as early as 2 days postinfection and the highest level was 15.76 (± 0.26) copies/cell.

CONCLUSION

HepCHLine-7 cells were susceptible to HBV and supported the replication of HBV. They are therefore suitable for studying the complete life cycle of HBV.

A new cell culture system for infection with hepatitis B virus that fuses HepG2 cells with primary human hepatocytes

Hepatitis B virus (HBV) infection exhibits a very narrow host range and shows a strong tropism for liver parenchymal cells, however none of the previously established experimental models can reproduce the natural process of HBV infection. In the present study, primary human hepatocytes were fused with HepG2 cells to establish the hybrid HepCHLine-4 cell line with high susceptibility to HBV. The HepCHLine-4 cells expressed HBV-specific antigen when co-incubated with HBV-positive serum from a hepatitis B patient. Post-infection, HBV relaxed circular DNA and covalently closed circular DNA were detected in HepCHLine-4 cells using a nested polymerase chain reaction, and HBV-specific particles were visualized by electron microscopy of the culture media of HepCHLine-4 cells. HepG2 cells were not susceptible to HBV infection under the same conditions. The HepCHLine-4 cells can be sub-cultured for > 12 months while maintaining susceptibility to HBV and may, therefore, be useful for studying HBV infection and the viral life cycle in human hepatocytes.

Sister chromatid exchange frequency in breast cancer cases

Sister chromatid exchange (SCE) were studied in 22 patients with breast cancer (i.e., four stage II; 12 stage III, six stage IV) and 10 normal healthy females as age-matched controls. The data obtained in these cases followed a Poisson distribution. An apparent increase in the average rate of SCE/cell was observed with the advancing stage of breast cancer.

Sister-chromatid exchange (SCE) rate in normal and abnormal sexual development in males and females

Thirty-three patients with abnormal sexual development (9 male hypogonads, 20 females with primary amenorrhea and 4 cases of ambiguous genitalia), 10 normal males and 8 normal females (below the age of 30 years) were evaluated for SCE/cell and for SCE distribution according to chromosome groups (A to G). Smokers and alcoholics and subjects under medication were excluded from the study. The average rates of SCE/cell in male hypogonads, primary amenorrhea and ambiguous genitalia were 4.23 +/- 1.51, 4.02 +/- 0.90 and 4.33 +/- 1.34, respectively, whereas in normal males and females the average rates were 4.27 +/- 0.69 and 4.49 +/- 0.87, respectively. The SCE data followed a Poisson distribution. Chi-square testing showed a statistically significant difference only in B-group chromosomes when male hypogonads were compared with normal males (p less than 0.02) and females with primary amenorrhea were compared with normal females (p less than 0.02), suggesting the importance of the study of SCE frequency distribution at chromosome group level to bring out the differences otherwise concealed in average rates.

Characterization of the distribution of sister chromatid exchange frequencies: implications for research design

Mitoses from 21 individuals were examined to characterize the within and between subjects distributions of sister chromatid exchange (SCE) frequencies. The importance of accounting for both the within subjects and between subjects variability in the design and interpretation of research is demonstrated. A computer simulation study was performed to evaluate the estimate of an individual's mean SCE rate. A power analysis was performed using our transformed descriptive data to determine the percentage difference in SCE which could be detected between two groups (with 95% probability) as a function of both the number of mitoses per individual and the number of individuals in each group. Tables are provided as guidelines. Conclusions from our work include: (1) The empirical distribution of SCEs within an individual is best described by a negative binomial distribution; (2) The predominant source of variability in baseline SCE rates is between cells within a given individual rather than between individuals and can be accounted for in experiments by counting adequate numbers of mitoses per individual; and (3) There is a systematic relationship between the mean and SD for SCEs which should be removed prior to hypothesis testing by taking either a logarithmic transformation or square root transformation of the data.

Eukaryotic genome: model considerations

The paper presents a new model of chromosome structure based on the assumption that multiple circular subunits of DNA exist. The essential difference with previously described models is the circular DNA unit forms a central chromosome axis. Chromosome configurations during various phases of the cell cycle depend on the various conformations of this central integrating unit. The described model can be generalized for all haploid set of eukaryotic nucleus. Some aspects of the chromosome structure and their functions have been discussed.

Sister chromatid exchange in chromosomes of cattle (Bos taurus)

Lymhocytes from clinically healthy Holstein animals (8 males and 23 females) were cultured in media containing 5-bromodeoxyuridine (10 microgram/ml) for two cycles of deoxyribonucleic acid replication (48 h). The exchange of sister chromatids per cell varied from 1 to 16 with a mean of 5.4 and a standard deviation of 2.1 in 603 differentially stained metaphase chromosome spreads. The major fraction of exchanges in the X chromosome were located in the region of the subcentromeric G negative bands q1 and q2. When the cells were pulse labeled with tritium labeled thymidine (1 muCi/ml) for the last 6 h of the cell cycle, the rate of exchange was higher in the late replicating X chromosome (facultative heterochromatin) in comparison to its homologue (active X). This study characterizes the yield of sister chromatid exchange in lymphocytes exposed to a given amount of 5-bromodeoxyuridine in a population of normal individuals so that it may be used as a standard for 1) a diagnostic tool in pathological conditions and 2) as an assay of chromosome stability in relation to environmental hazards.

Replication bypass model of sister chromatid exchanges and implications for Bloom's syndrome and Fanconi's anemia

A model of the sister chromatid exchange (SCE) process is outlined as a replication mechanism to bypass DNA crosslinks. The model suggests that when normal bidirectional replication advances from both sides towards a crosslink along the two opposite parental strands, the complementary parental strand segments can be temporarily displaced at each contralateral 5' side from the crosslink. The free ends produced in this first step will be terminally aligned but will have opposite polarity. The second step of the bypass can, however, be completed by either of two rejoining processes--terminal ligation of the free ends via nascent Okazaki pieces or aberrant complementation by overlapping the free ends. This bypass mechanism (1) allows replication to continue past a crosslink leaving it intact but (2) results in the switching of parental strands and their attached incomplete nascent strands above and below the crosslink site producing an exchange between sister chromatids. This model is compatible with the findings of current SCE studies using the new BUDR/stain techniques as well as with previous autoradiographic studies. It also suggests that the chromatid breaks and deletions in Fanconi's Anemia represent a defect in step two of the replication bypass mechanism and that the high frequency of SCE's and quadriradials in Bloom's Syndrome represent the SCE overload effects of a defect in crosslink repair.