Why are transformed cells immortal? Is the process reversible?

Normal cells have finite proliferative potential in culture. In contrast, cells derived from tumors immortalized by chemical carcinogens or viruses are able to divide indefinitely. A question of major importance is the mechanism that limits the proliferative potential of normal cells, and conversely, the process by which immortal cells have escaped irreversible growth cessation. To address this question we fused a number of different normal human fibroblast cell lines with various immortal human cell lines and determined the proliferative behavior of the resulting hybrids. In all cases the hybrids had a limited ability to proliferate in culture. These results suggested that the finite proliferative capacity of normal human cells was dominant and that immortal cells had acquired recessive changes in their genetic program, which allowed them to escape senescence. We were also able to assign approximately 30 immortal human cell lines to four complementation groups for indefinite division.

Heterozygote detection through bleomycin-induced G2 chromatid breakage in dyskeratosis congenita families

We determined the mean number of chromatid breaks per cell (b/c) in the bleomycin-treated lymphocytes of 10 patients with dyskeratosis congenita (DC) and 26 of their relatives to ascertain whether bleomycin sensitivity would distinguish DC heterozygotes from normal individuals. We observed a significantly higher mean number of chromatid b/c in DC patients and obligate heterozygotes (patients versus controls, p less than 0.0001; heterozygotes versus controls, p = 0.0076, Mann-Whitney rank-sum test). Unequivocal heterozygote detection was not possible owing to overlap of the b/c values of patients, heterozygotes, and controls, but our findings provided strong evidence of a link between autosomal recessive as well as X-linked recessive DC mutations and bleomycin sensitivity in homozygous, hemizygous, and heterozygous individuals.

Isolation of monochromosomal hybrids following fusion of human diploid fibroblast-derived microcells with mouse A9 cells

Rodent x human hybrids containing single human chromosomes are valuable tools in the study of human genetics. Using a retrovirus carrying the neomycin gene, we generated a neomycin-resistant (neor) pool of human diploid fibroblasts. Following fusion of microcells prepared from this pool to mouse A9 cells, we obtained four neor hybrids containing a single human chromosome 2, 3, 5, or 15.

Prenatal diagnosis of mucopolysaccharidosis by continuous, monodimensional electrophoresis of amniotic fluid glycosaminoglycans

A continuous dimensional electrophoretic method was used in 15 urine samples of clinically diagnosed mucopolysaccharidoses I or IV. Twelve of the 15 were glycosaminoglycans band positive. Of the 12 one was KS band positive but methyl toluidine blue test was negative. The other 11 were positive with both methods. Prenatal diagnosis was done in 10 cases during their next pregnancy with the same method on the amniotic fluid. Two affected fetuses were diagnosed and proved, one after induced abortion and the other after full-term delivery.

Genetic analysis of indefinite division in human cells: evidence for a cell senescence-related gene(s) on human chromosome 4

Earlier studies had demonstrated that fusion of normal with immortal human cells yielded hybrids having limited division potential. This indicated that the phenotype of limited proliferation (cellular senescence) is dominant and that immortal cells result from recessive changes in normal growth-regulatory genes. In additional studies, we exploited the fact that the immortal phenotype is recessive and, by fusing various immortal human cell lines with each other, identified four complementation groups for indefinite division. Assignment of cell lines to specific groups allowed us to take a focused approach to identify the chromosomes and genes involved in growth regulation that have been modified in immortal cells. We report here that introduction of a normal human chromosome 4 into three immortal cell lines (HeLa, J82, T98G) assigned to complementation group B resulted in loss of proliferation and reversal of the immortal phenotype. No effect on the proliferation potential of cell lines representative of the other complementation groups was observed. This result suggests that a gene(s) involved in cellular senescence and normal growth regulation resides on chromosome 4.

Molecular genetic approaches to the study of cellular senescence

Normal cells in culture exhibit limited division potential, which is used as a model for cellular aging. In contrast, tumor-derived, carcinogen- or virus-transformed cells are capable of dividing indefinitely (immortal). Fusion of normal with immortal human cells yielded hybrids having limited life span, indicating that cellular senescence is a dominant phenotype and that immortality is recessive. Fusions of various immortal human cell lines with each other led to the identification of four complementation groups for indefinite division. In order to identify the chromosomes and genes involved in growth regulation, that had been modified in immortal cells, we used the technique of microcell fusion to introduce either a normal human chromosome 11 or 4 into cell lines representative of the different complementation groups. Chromosome 11 had no effect on the in vitro life span of the different immortal human tumor lines. However, when a normal human chromosome 4 was introduced into cell lines assigned to complementation group B, the cells lost the immortal phenotype. No effect on the proliferation potential of cell lines representative of the other complementation groups was observed. These results suggest that a gene(s) on human chromosome 4 has been modified in immortal cell lines assigned to complementation group B, to allow escape from senescence. They also provide evidence for a genetic basis for cellular aging.

Tumor suppression by chromosome 11 is not due to cellular senescence

Previous hybrid studies involving fusion of normal with immortal human cells indicated that the phenotype of cellular senescence is dominant and that immortality results from recessive changes in normal growth regulatory genes. We have further assigned 28 different immortal human cell lines to at least four complementation groups for indefinite division. In order to identify the chromosomes involved in regulating cell proliferation, we have introduced single human chromosomes by microcell fusion into immortal human cells representative of the different complementation groups. Our results demonstrate that the introduction of chromosome 11, implicated in tumor suppression, does not cause cellular senescence in three different immortal human cell lines tested.

Hybrids from fusion of normal human T lymphocytes with immortal human cells exhibit limited life span

A number of normal human cell types have been shown to exhibit cellular senescence in vitro. We and others had found that fusion of normal human fibroblasts with immortal human cells yielded hybrids having limited lifespan. This indicated that the phenotype of cellular senescence is dominant and that immortality results from recessive changes in genes involved in growth control. They also supported the hypothesis that senescence results from genetic mechanisms rather than random damage. Since T lymphocytes are a highly differentiated cell type, in contrast to fibroblasts, it was of interest to determine whether similar mechanisms caused senescence in the T cells. We therefore fused normal human T lymphocytes with an immortal human cell line to determine whether they could restore the senescent, nondividing phenotype in hybrids, as do normal human fibroblasts. Eleven of fifteen hybrid clones studied exhibited limited proliferative potential after achieving a range of population doubling similar to that observed in the cell fusion studies involving normal fibroblasts. These results provide evidence that cellular senescence in T lymphocytes occurs via genetic mechanisms.

Study on BcII/St14 RFLPs in Chinese for DNA diagnosis for hemophilia A

The BcII polymorphism within DXS52 (St14) was reported. It was composed of 4 allelic fragments, 4.0 kb, 3.3 kb, 3.0 kb and 2.3 kb. The frequency of these fragment were 0.09, 0.12, 0.44 and 0.35 respectively in the Chinese. The polymorphism provided the PIC of 0.66. DNA analysis of families with hemophilia A showed that the confidence of the RFLPs was the same as the TaqI/St14 RFLPs and for carrier detection the former is much better than that of the TaqI/St14 RFLPs.

Characteristics of laser diodes for interferometric use

Low-coherence-length light from laser diode sources has applications in extending the useful range of interferometric fiber optic sensors. The characteristics of two commercial low-coherence laser diodes were investigated and compared with theoretical models to determine the operational characteristics of the devices. Reasonable trends in the comparison were seen.