Carcinogens induce genome-wide loss of heterozygosity in normal stem cells without persistent chromosomal instability

Abstract

Widespread losses of heterozygosity (LOH) in human cancer have been thought to result from chromosomal instability caused by mutations affecting DNA repair͞genome maintenance. However, the origin of LOH in most tumors is unknown. The present study examined the ability of carcinogenic agents to induce LOH at 53 sites throughout the genome of normal diploid mouse ES cells. Brief exposures to nontoxic levels of methylnitrosourea, diepoxybutane, mitomycin C, hydroxyurea, doxorubicin, and UV light stimulated LOH at all loci at frequencies ranging from 1-8 ؋ 10 ؊3 per cell (10 -123 times higher than in untreated cells). This greatly exceeds the frequencies at which these agents have been reported to induce point mutations and is comparable to the rates of LOH observed in ES cells lacking the gene responsible for Bloom syndrome, an inherited DNA repair defect that results in greatly increased risk of cancer. These results suggest that LOH contributes significantly to the carcinogenicity of a variety of mutagens and raises the possibility that genome-wide LOH observed in some human cancers may reflect prior exposure to genotoxic agents rather than a state of chromosomal instability during the carcinogenic process. Finally, as a practical matter, chemically induced LOH is expected to enhance the recovery of homozygous recessive mutants from phenotype-based genetic screens in mammalian cells.cancer etiology ͉ cancer genetics ͉ carcinogenesis ͉ genome stability

Genetic and epigenetic X-chromosome variations in a parthenogenetic human embryonic stem cell line

“…One possibility is that in the XO cells, the remaining X initiated a duplication process. A similar phenomenon has been observed in SCNT-hES-1 cells and some other cell lines [27,34]. Then, those duplicated cells ultimately adapted to selective pressures and acquired a growth advantage.…”

Diepoxybutane activates the mitochondrial apoptotic pathway and mediates apoptosis in human lymphoblasts through oxidative stress

“…A variety of apoptotic stimuli, including DNA damaging agents and oxidative stress, have been reported to stimulate the intrinsic mitochondrial apoptotic pathway by activating, and upregulating, pro-apoptotic Bcl-2 family proteins (Bax and Bak), thereby provoking dissipation of the mitochondrial trans-membrane potential (Mandic et al, 2001;Jungas et al, 2002;Panaretakis et al, 2002;Chada et al, 2006;Donahue et al, 2006;Kuhar et al, 2006;Zhang et al, 2006;Baysan et al, 2007;Jantova et al, 2007). Our observations of Bax and Bak activation and up-regulation, together with attenuation of mitochondrial trans-membrane potential in lymphoblasts undergoing DEB-induced apoptosis, are in line with these reports.…”

Genome Maintenance and Mutagenesis in Embryonic Stem Cells

“…Indeed, the per-gene frequencies of carcinogen-induced LOH can exceed the reported frequencies of point mutations, particularly when analyzed on a genome-wide level. 16 �herefore, LOH is perhaps best viewed as a somatic mutation when considering the mutagenic potential of many carcinogens.…”

“…First, the spontaneous rates of point mutations, which are in fact quite low in both normal cells and cancer cells, 27,28 are not relevant to spontaneous LOH, which can occur at higher rates. 16,[22][23][24] Second, spontaneous rates for any type of mutation significantly underestimate the levels of genetic alterations expected in most cancers where prior, and possibly extensive, carcinogen exposure must be assumed.…”

“…In a recent study, 16 we examined the ability of carcinogens to induce LOH throughout the genome of diploid mouse embryo-derived stem (ES) cells, a potential model of selfrenewing stem cells that serve as the precursors to cancer. 17 �he study exploited a novel gene trap retrovirus 18 to quantify the rates of carcinogen-induced LOH at 53 sites throughout the genome.…”