Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.4.2.7 (adenine phosphoribosyltransferase)
 692 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We describe an in vivo mutagenesis model that utilizes reverse mutation and forward mutation at the endogenous Aprt locus. Reverse mutation provides an in situ method for detecting environments or agents that cause point mutations. Forward mutation detects large chromosomal events, including mitotic recombination, chromosome loss, and large multilocus deletion, all of which can lead to loss of heterozygosity. Detection of reverse mutation in vivo is based on the differential capacity of Aprt and Aprt cells to sequester radiolabeled adenine by catalyzing its conversion to adenosine monophosphate with subsequent incorporation into nucleic acids. Cells lacking APRT activity cannot accumulate exogenously administered, tagged adenine, whereas Aprt+ cells can and will thereby become marked. Thus, genetically modified mice with mutant but revertible Aprt alleles should be a useful vehicle for in situ detection of mutagenic activity in the whole animal. the feasibility of this model has been illustrated, first, by showing that APRT-deficient mice are viable and, second, by demonstrating that the minority of Aprt+ cells within a chimeric tumor growing in an Aprt+ mouse can be selectively labeled following IP injection of [14C]-adenine and can be identified by autoradiography. Forward mutation, detected by growth in selective medium of primary cells derived from Aprt+/- heterozygous mice, provides on independent estimate of in vivo mutation frequency. The frequency with which Aprt colonies arise provides a measure of the frequency of Aprt(-)-negative cells in the tissue at that point in time. Culture of skin fibroblasts in 2,6-diaminopurine (DAP) produced Aprt+ colonies with a frequency of about 10(-4). This frequency is similar to that found for human T lymphocytes from individuals heterozygous at the Aprt locus. In both cases, the majority of mutagenic events involved allele loss. Polymerase chain reaction with linked polymorphic microsatellites on mouse chromosome 8 demonstrated that allele loss was mediated mostly by mitotic recombination, as was the case for human T lymphocytes. The high frequency of mitotic recombination and allele loss at a neutral locus has significant implications for the process of tumorigenesis and argues that spontaneous or induced mitotic recombination may play a causal role in the progression to cancer.
 ...
PMID:APRT: a versatile in vivo resident reporter of local mutation and loss of heterozygosity. 899 Oct 80

Current models suggest that genomic instability is crucial in the accumulation of the multiple alterations required for tumorigenesis. However, the nature of the initial damage responsible for the origin of genomic instability remains poorly understood. In this investigation we demonstrate that the nucleotide analog 2,6-diaminopurine (DAP) can be used to induce highly focused damage to the large blocks of paracentromeric heterochromatin on chromosomes 1, 9 and 16. A large fraction of cells exposed to DAP exhibit undercondensation of alpha and classical heterochromatin which persists into metaphase. Subsequent chromosome breakage was observed for one of the target chromosomes by preferential exclusion of chromosome 16 fragments into micronuclei (P < 0.0001). The specificity of DAP-induced chromosomal breakage enabled us to utilize it as a reagent to demonstrate that paracentromeric heterochromatin is a sensitive target for the induction of persistent genomic instability. We observed a 100-fold increase in mutagenesis affecting a chromosome 16 marker (APRT) compared with marker loci on chromosomes 17 (TK) or X (HPRT). We previously reported that APRT- mutants were recovered at a high rate upon selection in DAP in a process involving recombinationally mediated loss of heterozygosity that extends from the telomere to the boundary region of the paracentromeric heterochromatin. Karyotypic analysis of DAP-resistant APRT- mutant clones demonstrated extensive genomic instability, particularly evidence of multiple and sequential events affecting chromosome 16. These data suggest that the heterochromatic breakage observed cytogenetically immediately following DAP exposure is also responsible for the initiation of persistent genomic instability.
 ...
PMID:Targeted breakage of paracentromeric heterochromatin induces chromosomal instability. 980 Jan 88

Genetic events leading to the loss of heterozygosity (LOH) have been shown to play a crucial role in the development of cancer. However, LOH events do not occur only in genetically unstable cancer cells but also have been detected in normal somatic cells of mouse and man. Mice, in which one of the alleles for adenine phosphoribosyltransferase (Aprt) has been disrupted by gene targeting, were used to investigate the potency of carcinogens to induce LOH in vivo. After 7,12-dimethyl-1,2-benz[a]anthracene (DMBA) exposure, a 3-fold stronger mutagenic response was detected at the autosomal Aprt gene than at the X chromosomal hypoxantine-guanine phosphoribosyltransferase (Hprt) gene in splenic T-lymphocytes. Allele-specific PCR analysis showed that the normal, nontargeted Aprt allele was lost in 70% of the DMBA-induced Aprt mutants. Fluorescence in situ hybridization analysis demonstrated that the targeted allele had become duplicated in almost all DMBA-induced mutants that displayed LOH at Aprt. These results indicate that the main mechanisms by which DMBA caused LOH were mitotic recombination or chromosome loss and duplication but not deletion. However, after treatment with the alkylating agent N-ethyl-N-nitrosourea, Aprt had a similar mutagenic response to Hprt while the majority (90%) of N-ethyl-N-nitrosourea-induced Aprt mutants had retained both alleles. Unexpectedly, irradiation with x-rays, which induce primarily large deletions, resulted in a significant increase of the mutant frequency at Hprt but not at Aprt. This in vivo study clearly indicates that, in normal somatic cells, carcinogen exposure can result in the induction of LOH events that are compatible with cell survival and may represent an initiating event in tumorigenesis.
 ...
PMID:Carcinogen-induced loss of heterozygosity at the Aprt locus in somatic cells of the mouse. 981 74

In response to ionizing radiation and other agents that damage DNA, the p53 tumor suppressor protein activates multiple cellular processes including cell cycle checkpoints and programmed cell death. Although loss of p53 function is associated with radiation-induced genetic instability in cell lines, it is not clear if this relationship exists in vivo. To study the role of p53 in maintenance of genetic stability in normal tissues following irradiation, we have measured mutant frequencies at the adenine phosphoribosyltransferase (Aprt) and hypothanine-guanine phosphoribosyltransferase (Hprt) loci and examined mechanisms of loss of heterozygosity (LOH) in normal T cells of p53-deficient, Aprt heterozygous mice that were subjected to whole-body irradiation with a single dose of 4Gy X-rays. The radiation-induced mutant frequency at both the Aprt and Hprt loci was elevated in cells from mice with different p53 genotypes. The radiation-induced elevation of p53-/- mice was significantly greater than that of p53+/- or p53+/+ mice and was caused by several different kinds of mutational events at the both chromosomal and intragenic levels. Most significantly, interstitial deletion, which occurs rarely in unirradiated mice, became the most common mechanism leading to LOH in irradiated p53 null mice. These observations support the idea that absence or reduction of p53 expression enhances radiation-induced tumorigenesis by increasing genetic instability at various loci, such as those for tumor suppressor genes.
 ...
PMID:Radiation-induced genetic instability in vivo depends on p53 status. 1199 74