Expression and loss of alleles in cultured mouse embryonic fibroblasts and stem cells carrying allelic fluorescent protein genes.

BACKGROUND: Loss of heterozygosity (LOH) contributes to many cancers, but the rate at which these events occur in normal cells of the body is not clear. LOH would be detectable in diverse cell types in the body if this event were to confer an obvious cellular phenotype. Mice that carry two different fluorescent protein genes as alleles of a locus would seem to be a useful tool for addressing this issue because LOH would change a cell's phenotype from dichromatic to monochromatic. In addition, LOH caused by mitotic crossing over might be discernable in tissues because this event produces a pair of neighboring monochromatic cells that are different colors. RESULTS: As a step in assessing the utility of this approach, we derived primary embryonic fibroblast populations and embryonic stem cell lines from mice that carried two different fluorescent protein genes as alleles at the chromosome 6 locus, ROSA26. Fluorescence activated cell sorting (FACS) showed that the vast majority of cells in each line expressed the two marker proteins at similar levels, and that populations exhibited expression noise similar to that seen in bacteria and yeast. Cells with a monochromatic phenotype were present at frequencies on the order of 10(-4) and appeared to be produced at a rate of approximately 10(-5) variant cells per mitosis. 45 of 45 stably monochromatic ES cell clones exhibited loss of the expected allele at the ROSA26 locus. More than half of these clones retained heterozygosity at a locus between ROSA26 and the centromere. Other clones exhibited LOH near the centromere, but were disomic for chromosome 6. CONCLUSION: Allelic fluorescent markers allowed LOH at the ROSA26 locus to be detected by FACS. LOH at this locus was usually not accompanied by LOH near the centromere, suggesting that mitotic recombination was the major cause of ROSA26 LOH. Dichromatic mouse embryonic cells provide a novel system for studying genetic/karyotypic stability and factors influencing expression from allelic genes. Similar approaches will allow these phenomena to be studied in tissues.

Exposure of mice to arsenic and/or benzo[a]pyrene does not increase the frequency of Aprt-deficient cells recovered from explanted skin of Aprt heterozygous mice.

Exposure to inorganic arsenic in drinking water is linked to cancer in humans, but the mechanism of arsenic-induced cancer is not clear. Arsenic is not a powerful point mutagen, but can cause chromosome malsegregation and mitotic recombination, two events that can cause loss of tumor suppressor alleles and thereby contribute to the evolution of cancerous cells. To determine whether arsenic increases the frequency of allele loss due to either malsegregation or mitotic recombination in vivo, Aprt(+/-) hybrid mice were exposed to sodium arsenite (10 mg/L) in their drinking water for 10 weeks. To determine whether arsenic enhances the action of a known mutagen, half of the arsenic-treated mice were exposed to benzo[a]pyrene (BaP) for 8 weeks by skin painting (500 nmoles/week). Cells were taken from painted dorsal skin and cultured in the presence of 2,6-diaminopurine (DAP), to select colonies lacking adenosine phosphoribosyl transferase (Aprt) activity. The frequency of DAP-resistant (DAP(r)) colonies varied substantially within the treatment groups, but there was no significant difference between the groups. Analysis of DNA from DAP(r) colonies suggested that mitotic recombination contributed to the loss of wild-type Aprt allele. Whether arsenic or BaP enhanced or diminished the frequency of this process could not be deduced from these data.

Co-mutagenic activity of arsenic and benzo[a]pyrene in mouse skin.

Exposure to inorganic arsenic in drinking water is linked to skin, lung and bladder cancer in humans. The mechanism of arsenic-induced cancer is not clear, but exposure to arsenic and polycyclic arylhydrocarbons (PAH) is more carcinogenic than exposure to either type of carcinogen alone. Arsenic can also generate reactive oxygen species, suggesting that oxidation of DNA may play a role in carcinogenesis. Oxidization of guanosines in polyG tracts is known to cause frameshift mutations, and such events can be detected in situ using the G11 placental alkaline phosphatase (PLAP) transgenic mouse model, which reports frameshift mutations in a run of 11 G:C basepairs by generating cells containing heat-resistant alkaline phosphatase activity. PAH can also induce frameshift mutations. In the study described here, FVB/N mice carrying the G11 PLAP transgene were crossed to C57Bl/6 mice. Half of the hybrid mice were given drinking water with sodium arsenite (10 mg/L) for 10 weeks. Half of the arsenic treated mice were also exposed to benzo[a]pyrene (BaP) by skin painting (500 nmol/week) for 8 weeks. Another group of mice was exposed to BaP but not arsenic. The effect on frameshift mutation was assessed by staining sections of skin tissue to detect cells with PLAP activity. Arsenic alone had no significant effect. On average, mice given BaP alone had approximately three times more PLAP-positive (PLAP+) cells. By contrast, mice exposed to both arsenic and BaP exhibited 10-fold more PLAP+ cells in the skin, and these cells were often arranged in large clusters, suggesting derivation from stem cells. Whereas combined treatment produced more PLAP+ cells, stable BaP adduct levels and arsenic burdens were not higher in mice exposed to both agents compared to mice exposed to either one agent or the other.

Impact of mismatch repair deficiency on genomic stability in the maternal germline and during early embryonic development.

The effects of lack of the mismatch repair protein PMS2 on germline and maternal-effect mutations were studied in transgenic mice that allow mutant cells to be visualized in situ. Tg(betaA-G11PLAP) mice are transgenic for the G11 allele of a human placental alkaline phosphatase (PLAP) gene driven by a human beta-actin promoter. The G11 allele of the PLAP gene does not produce enzyme due to a frameshift induced by a mononucleotide repeat containing 11 G:C basepairs. Loss of one G:C basepair restores enzyme production. When the G11 PLAP allele was passed through the germline of female mice lacking PMS2, approximately 25% of the offspring that inherited the transgene exhibited the phenotype expected for germline mutation. The mice transmitted the germline-mutation phenotype normally and their offspring exhibited PLAP enzyme activity in at least 30% of the cells in each tissue examined. By contrast, only 1 of 32 mice that inherited the G11 PLAP transgene from a wild-type male crossed to a Pms2-/- female exhibited a high number of PLAP+ cells. Compared to germline revertants, approximately one half to one quarter as many cells were PLAP+, suggesting that a mutation occurred in one cell of an embryo containing two to four cells. These data suggest that the paternally derived Pms2 gene provided normal levels of PMS2 protein to embryos by the time they reached the eight-cell stage, but that smaller embryos formed from PMS2-deficient eggs lacked PMS2 function.

Increased mutation in mice genetically predisposed to oxidative damage in the brain.

Harlequin (Hq) mice develop ataxia due to an X-linked recessive mutation in the gene encoding apoptosis-inducing factor (Aif). Brain cells in Hq mice contain the modified base 8-hydroxydeoxyguanosine (8-OHdG), suggesting that the defect in Aif causes increased DNA oxidation in these cells. Because oxidative damage is mutagenic, Hq mice might suffer increased mutation in the brain. To examine this possibility, mutation in the brain was assessed using the Tg(betaA-G11PLAP) mouse model, which allows mutant cells to be visualized in tissue sections in situ. Hq mice exhibited more and larger patches of PLAP positive tissue in the brain. PLAP+ cells were observed in all areas of the brain. No increase in the number of PLAP+ cells was seen in three other tissues, suggesting that the effect of Aif deficiency on mutation was specific to brain.

Modeling variation in tumors in vivo.

Transgenic mice that allow mutant cells to be visualized in situ were used to study variation in tumors. These mice carry the G11 placental alkaline phosphatase (PLAP) transgene, a mutant allele rendered incapable of producing its enzyme product by a frameshift caused by insertion of a tract of G:C base pairs in a coding region. Spontaneous deletion of one G:C base pair from this tract restores gene function, and cells with PLAP activity can be detected histochemically. To study tumors, the G11 PLAP transgene was introduced into the polyoma virus middle T antigen mammary tumor model. Tumors in these mice exhibited up to 300 times more PLAP+ cells than normal tissues. PLAP+ cells were located throughout each tumor. Many of the PLAP+ cells were singlets, but clusters also were common, with one cluster containing >30,000 cells. Comparison of these data to simulations produced by computer models suggested that multiple factors were involved in generating mutant cells in tumors. Although genetic instability appeared to have occurred in most tumors, large clusters were much more common than expected based on instability alone.