Phenotype-Based Identification of Mouse Chromosome Instability Mutants

Corresponding author: John C. Schimenti, 600 Main St., Bar Harbor, ME 04609., jcs{at}jax.org (E-mail)

Communicating editor: D. KINGSLEY

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

There is increasing evidence that defects in DNA double-strand-break (DSB) repair can cause chromosome instability, which may result in cancer. To identify novel DSB repair genes in mice, we performed a phenotype-driven mutagenesis screen for chromosome instability mutants using a flow cytometric peripheral blood micronucleus assay. Micronucleus levels were used as a quantitative indicator of chromosome damage in vivo. Among offspring derived from males mutagenized with the germline mutagen N-ethyl-N-nitrosourea (ENU), we identified a recessive mutation conferring elevated levels of spontaneous and radiation- or mitomycin C-induced micronuclei. This mutation, named chaos1 (chromosome aberration occurring spontaneously 1), was genetically mapped to a 1.3-Mb interval on chromosome 16 containing Polq, encoding DNA polymerase . We identified a nonconservative mutation in the ENU-derived allele, making it a strong candidate for chaos1. POLQ is homologous to Drosophila MUS308, which is essential for normal DNA interstrand crosslink repair and is unique in that it contains both a helicase and a DNA polymerase domain. While cancer susceptibility of chaos1 mutant mice is still under investigation, these data provide a practical paradigm for using a forward genetic approach to discover new potential cancer susceptibility genes using the surrogate biomarker of chromosome instability as a screen.

CHROMOSOME instability is a hallmark of cancer cells. It may arise from defects in chromosome metabolism, including DNA double-strand-break (DSB) repair. DSBs can lead to chromosome aberrations and to mitotic recombination, either of which can result in loss of heterozygosity. As seen in individuals with certain cancer syndromes, DSB repair defects may cause chromosome instability, increasing cancer risk. For example, ataxia telangiectasia and the Nijmegen breakage syndrome (DIGWEED et al. 1999 ; MEYN 1999 ) are attributed to germline mutations in genes regulating DSB repair signaling. Werner, Bloom, and Rothmund-Thomson syndromes (VAN BRABANT et al. 2000 ) are all caused by mutations in RecQ-like genes, which are thought to be involved in repair of DSBs by homologous recombination (HR). Furthermore, the breast cancer susceptibility genes BRCA1 and BRCA2 also function in DSB repair, particularly in the regulation of HR (VENKITARAMAN 2002 ). Inactivation of these genes also causes genomic instability (SHEN et al. 1998 ; TUTT et al. 1999 ; KRAAKMAN-VAN DER ZWET et al. 2002 ). In sum, DSB repair appears to have an important role in genome maintenance and tumor suppression. To achieve a full understanding of DSB repair and its association with cancer, it is necessary to identify all the genes involved.

Many mammalian genes involved in DSB repair, particularly by HR, have been identified on the basis of homology to those of the yeast Saccharomyces cerevisiae, indicating their conserved role in genome maintenance (THACKER 1999 ). However, it is likely that additional mammalian DSB repair genes do not exist in yeast. For example, there are no yeast homologs of BRCA1, BRCA2, or PRKDC (protein kinase, DNA-activated, catalytic polypeptide). Furthermore, mammals have a larger RAD51 family consisting of seven members with nonredundant function (THOMPSON and SCHILD 2001 ; VAN GENT et al. 2001 ). Most notably, yeast and mammalian cells have clear differences in the way they repair DSBs. There are two major pathways in DSB repair in mammalian cells, HR and nonhomologous end joining (NHEJ; KARRAN 2000 ; KHANNA and JACKSON 2001 ; VAN GENT et al. 2001 ). NHEJ is used heavily in mammalian cells, whereas in yeast, DSBs are repaired almost exclusively by HR (THOMPSON and SCHILD 1999 ). The NHEJ pathway, which is also involved in V(D)J recombination, joins broken chromosomal ends with little homology and thus is error prone. In the HR repair pathway, a sister chromatid or homologous chromosome is used as a repair template, resulting in higher fidelity. This pathway may be more important during development, since inactivation of most of HR genes results in early embryonic lethality (THOMPSON and SCHILD 2001 ). It has been suggested that activities of the two DSB repair pathways may be regulated during the cell cycle in mammals (HENDRICKSON 1997 ; THOMPSON and SCHILD 1999 , THOMPSON and SCHILD 2001 ).

DSB repair mutants have been traditionally isolated on the basis of radiation hypersensitivity in yeast and rodent cell lines (THOMPSON et al. 1982 ; JONES et al. 1987 , JONES et al. 1988 ; FRIEDBERG et al. 1995 ). At least 11 complementation groups of X-ray-sensitive rodent cell line mutation have been identified (ZDZIENICKA 1999 ). Eventually, the in vivo consequences of mutations in some of these genes were investigated by the generation of gene-targeted mice. Since most of these mutations are recessive, it has been suggested that the mutant identification depended on aneuploidy and/or presence of hemizygous loci in these cell lines (JONES et al. 1988 ). Thus it is possible that many genes were undetectable by such screens.

Forward genetic mutation screens in mice offer several advantages for the identification of new DSB repair genes. Random N-ethyl-N-nitrosourea (ENU) mutagenesis of the mouse genome is now a well-established method to isolate both dominant and recessive mutations with high efficiency (JUSTICE et al. 1999 ; HRABE DE ANGELIS et al. 2000 ), and positional cloning of these mutations has been vastly simplified by the availability of mouse genomic sequence and various other genetic resources. If an efficient assay were available to detect chromosome instability, it could be exploited in forward genetic mutation screens to identify novel genes required for DSB repair and/or chromosome stability in mice. Moreover, potential cancer susceptibility can be addressed directly in the mutant mice.

To detect mouse DSB repair mutants, we explored the efficacy of a high-throughput micronucleus assay, which provides a quantitative measure of in vivo chromosome damage (HEDDLE 1973 ). Micronuclei (MN) can arise from acentric chromosome fragments or whole chromosomes that have not been incorporated in the main nuclei at cell division (NUSSE et al. 1996 ). The formation of micronuclei can be stimulated by DNA-damaging agents that induce chromosome breaks or abnormal chromosome segregation, and thus the micronucleus assay has been used as a genetic toxicology tool for quantitative analysis of in vivo chromosome damage induced by potential mutagens (MORITA et al. 1997 ). A peripheral blood micronucleus assay has been semiautomated by flow cytometry (DERTINGER et al. 1996 ), making it practical for screening large numbers of mice with high statistical power. We show that this assay has the ability to detect spontaneous and radiation-induced chromosome instability in DSB repair-deficient homozygous Atm (ataxia telangiectasia-mutated) and Prkdc^scid (severe combined immune deficiency) mutant mice.

Here we report a small-scale ENU mutagenesis screen for chromosome instability mutants that yielded three mutations and one potential mutation causing higher spontaneous micronucleus levels. One of the recovered mutations also confers higher radiation-induced micronucleus levels. This newly identified mutation was named chaos1 (chromosome aberration occurring spontaneously 1) and mapped to a 1.3-Mb region of chromosome 16 that does not contain any genes known to cause chromosome instability in humans or mice. However, we identified a mutation in Polq, an ortholog of Drosophila mus308, that resides in this region. Flies mutant at this locus exhibit genome instability and hypermutability in response to certain chemical agents (LEONHARDT et al. 1993 ). These experiments demonstrate the robustness of the flow cytometric micronucleus assay as a high-throughput screening tool to detect mutations causing genome instability and potential cancer susceptibility.

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

ENU mutagenesis:
ENU preparation and the injection protocol was based on described protocols (JUSTICE 1999 ). Male C57BL6/J (B6) mice were intraperitoneally injected with 80 mg ENU/kg body weight (Sigma, St. Louis) once a week for 3 weeks at 8–10 weeks of age. They were mated to C3HeB/FeJ (C3H) females to obtain first generation (G₁) sons. The G₁ males were mated to C3H females to obtain second generation (G₂) daughters. In the whole-genome screens, up to four G₂ daughters were backcrossed to their G₁ fathers to generate third-generation (G₃) offspring who were potentially homozygous for mutations transmitted by the G₁. Micronucleus assays were performed on male G₃ progeny.

Some of the animals screened were derived from a region-specific mutagenesis program focused on the 30-cM region on proximal chromosome 5 spanned by the rump white (Rw) inversion. Mutagenized B6 mice were crossed to C3H-Rw/+ females, and resulting G₁ males inheriting Rw (Rw/+*, where the asterisk represents the mutagenized chromosome 5) were crossed to Rw/Hm females (Hm refers to hammertoe, a semidominant mutation causing webbing of the digits) to yield Rw/+* G₂ offspring. Unlike the previous cross where G₂'s were crossed to the G₁ father, in this case the G₂'s were intercrossed to produce the G₃ generation. Since Rw contains a recessive lethal, only Rw/+* and +*/+* G₃ offspring were produced. Only 1 +*/+* G₃ male per family was screened by the micronucleus assay. Note that, as a result of the intercross of the G₂'s, the non-chromosome 5 mutations could be rendered homozygous, but the proportion of these compared to the former screen is half (G₂ animals carry half the mutational load of a G₁).

Irradiation of mice and flow cytometric micronucleus assay:
Six-week-old G₃ males were exposed to 0.7 Gy of -rays from a ¹³⁷Cs source. Forty-eight hours later, 50 µl of peripheral blood was collected from the retro-orbital sinus into a tube containing 250 µl of anticoagulant solution (500 USP heparin/ml saline, Sigma). A total of 180 µl was transferred to a polypropylene centrifuge tube containing 2 ml methanol at -80°. The tubes were struck sharply several times to break up aggregates and then stored at least overnight before further processing. Flow cytometric analysis was performed on a FACScan cytometer (Becton-Dickinson, San Jose, CA) as described (DERTINGER et al. 1996 ). At least 10,000 reticulocytes and 500,000 normochromatic erythrocytes were analyzed per blood sample.

Microscopic scoring of micronuclei:
A method using acridine orange (Sigma)-coated slides (HAYASHI et al. 1990 ) was used to score micronuclei under a fluorescent microscope. Five thousand reticulocytes per sample were analyzed for the presence of micronuclei.

SCID phenotyping:
A total of 100 µl peripheral blood in anticoagulant solution was added to 1 ml fluorescence-activated cell sorter (FACS) buffer (Ca/Mg free PBS, 5 mM EDTA, 0.02% NaN₃), mixed, and set on ice. Four milliliters of Gey's buffer (HBSS, 650 mM NH₄Cl, 27 mM glucose) was added to the mixture and placed for 5 min on ice. Cells were pelleted at 500 x g for 5 min at 10°. The pellet was washed twice with 4 ml Gey's buffer and once with 4 ml FACS buffer and then resuspended. Fc receptors were blocked for 30 min on ice with a cocktail of anti-CD16/32 (FcII/II Rc, produced in house) and Rat IgG (Sigma) using 10 µg of each per blood sample. They were then stained with 145-2C11 (hamster anti-mouse CD3) phycoerythrin (PE) to label any T-lymphocytes and with anti-mouse Ig , light chain FITC (PharMingen, San Diego) for B-lymphocytes. All antibodies were pretitrated for optimal concentration. Staining occurred on ice for 30 min, after which cells were washed with 2 ml FACS buffer, pelleted, and resuspended in 250 µl FACS buffer. A total of 10 µl propidium iodide solution (20 µg/ml in FACS buffer) was added prior to running samples on the FACScan for live/dead cell discrimination.

Polymerase chain reaction (PCR) analysis of microsatellite markers:
Genomic DNA was prepared from the tails as described elsewhere (TRUETT et al. 2000 ). Three microliters of genomic DNA was amplified in a total reaction volume of 30 µl under standard PCR conditions. PCR products were analyzed on 3.75% MetaPhor gels (BMA, Rockland, ME).

Reverse transcription-PCR analysis of Polq cDNA:
Total RNA was extracted from testes using the RNeasy midi kit (QIAGEN, Valencia, CA). Five micrograms of total RNA were used for RT reactions with Super-ScriptII (GIBCO BRL, Rockville, MD) followed by PCR using Polq primer pairs. The primer sequences are available upon request. Rapid amplification of cDNA ends (RACE) was conducted with the 5' RACE system kit and 3' RACE adapter primer (GIBCO BRL). cDNA was sequenced on an ABI 3700 DNA analyzer (Applied Biosystems, Foster City, CA).

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

High-throughput assay for detecting chromosome instability in mice:
Phenotype-driven mutagenesis is a powerful way to identify new genes and their biological roles in the context of a whole organism. In seeking a high-throughput assay suitable for identifying mutations causing elevated levels of chromosome damage in vivo, a highly sensitive and reproducible flow cytometric peripheral blood micronucleus assay (DERTINGER et al. 1996 ) was evaluated.

In peripheral blood, micronuclei can be enumerated clearly in erythrocytes, which expel their nuclei, but not micronuclei, after their last mitotic division. To facilitate the formation of micronuclei, mice were exposed to 0.7 Gy -rays from a ¹³⁷Cs source. In a control experiment, blood was analyzed from a wild-type mouse before and 48 hr after irradiation. In erythrocytes, normochromatic erythrocytes (NCEs) and reticulocytes (RETs) can be distinguished with an anti-CD71 antibody (SERKE and HUHN 1992 ), and micronuclei are stainable with the nucleic acid binding agent propidium iodide. The frequency of reticulocytes with micronuclei (MN-RETs) increased from 0.29 to 2.6% at 48 hr after -irradiation (Fig 1). Induced micronuclei would be observed in RETs, because micronucleus formation requires a mitosis and RETs in the peripheral blood are products of the most recent mitotic cycle. On the other hand, because NCEs lacked nuclei at the time of irradiation, the frequency of MN-NCEs remained relatively constant before and after irradiation; these micronuclei are of spontaneous origin. Therefore, both spontaneous and radiation-induced micronucleus levels could be measured simultaneously in the same sample.

View larger version (30K): In this window In a new window Download PPT slide   Figure 1. Representative data from the flow cytometric micronucleus assay. Peripheral blood from a wild-type animal before (A) and 48 hr after (B) -irradiation was analyzed. CD71-positive RETs (populations 3 and 4) are separated from CD71-negative NCEs (populations 1 and 2) to enumerate radiation-induced micronuclei. MN-NCEs and MN-RETs are stained with propidium iodide and shown as populations 2 and 4, respectively.

The data obtained by flow cytometry were compared to those obtained by microscopic manual scoring of the same samples. A high correlation (r² = 0.96) was achieved, demonstrating that the flow cytometric scoring accurately reflects classical micronucleus scoring.

Elevated incidence of micronuclei in DSB-repair-deficient mice:
To test the sensitivity and efficacy of the assay in detecting genomic instability/DSB repair mutants, we used two types of radiation-sensitive mice, 129S6/SvEvTac-Atm^tm1Awb (Atm^-/-) and NOD.CB17-prkdc^scid/J (NOD scid). As shown in Fig 2, these mutants had significantly higher micronucleus frequencies at 48 hr after irradiation than did controls (P < 0.0005 for both Atm^-/- and NOD scid using the two-tailed t-test). In contrast to the SCID mice, the percentage of MN-RETs in Atm^-/- mice prior to irradiation was significantly higher (P < 0.0001) than that in controls, indicating that these mice have intrinsically elevated chromosome instability. These results demonstrate the potential usefulness of this assay as a screening tool for mutations causing both spontaneous and radiation-induced chromosome instability.

View larger version (15K): In this window In a new window Download PPT slide   Figure 2. Higher micronucleus levels in radiation-sensitive mutants. Homozygous mutants at Atm or Prkdc and their control strains (wild-type littermates for Atm-/- mutants and NOD/LtJ for NOD scid) were exposed to 0.7 Gy -ray. Peripheral blood was collected from the animals every 24 hr up to 72 hr from the time of irradiation and was analyzed by the flow cytometric micronucleus assay. Each point is shown with standard deviation. At least five animals were used per group.

Mutagenesis screen to isolate chromosome instability mutations:
To identify new mutations, we mutagenized male C57BL6/J (B6) mice with ENU and used them to initiate a three-generation breeding scheme to obtain third-generation (G₃) offspring that were potentially homozygous for induced mutations (see MATERIALS AND METHODS). ENU is a potent germline point mutagen that produces functional mutations at a rate of 1/750/locus/gamete (HITOTSUMACHI et al. 1985 ). G₃ males were screened by the micronucleus assay to detect recessive mutations affecting radiation-induced and/or spontaneous micronucleus frequencies.

In Fig 3, representative distributions of spontaneous and -ray-induced micronucleus frequencies in 127 G₃ males are plotted. The means (with standard deviation) were 0.21 ± 0.08% and 2.35 ± 0.70% for spontaneous and -ray-induced micronucleus frequencies, respectively (Fig 3A and Fig B).

View larger version (17K): In this window In a new window Download PPT slide   Figure 3. Distribution of micronucleus frequency in NCEs and RETs in G3 animals. chaos1 appeared as an outlier with significantly higher spontaneous (A) and radiation-induced (B) micronucleus frequencies. Both exceed three standard deviations (3 SD). The flow plot of chaos1 is shown in C. Two million erythrocytes were analyzed to collect 5000 RETs because of an extremely low number of RETs in this mutant after irradiation.

Three different screens were conducted. In the first, 422 G₃ males descended from 39 G₁ males were tested for elevated spontaneous and radiation-induced micronucleus levels. One variant appeared as an outlier, which was defined as an individual with micronucleus levels higher than three standard deviations of the mean. This variant exhibited significantly elevated levels of both spontaneous and radiation-induced micronucleated erythrocytes (Fig 3A and Fig B). Moreover, the number of RETs was decreased markedly after irradiation (to 0.26% of total erythrocytes) as seen in Fig 3C, indicating its hypersensitivity to -rays. This trait was determined to be recessive and exhibited Mendelian segregation. This mutation, chaos1, is described below.

Since a positive correlation between spontaneous and induced micronucleus levels has been reported in a number of mouse strains (SALAMONE and MAVOURNIN 1994 ), mice generated in the subsequent screens were tested only for spontaneous micronucleus levels. In the second screen, one potential mutation was recovered among 212 G₃ males derived from 20 G₁ males. This potential mutation is being tested to characterize radiation sensitivity and genetic heritability. The third screen involved mice produced in an ongoing region-specific mutagenesis project in our laboratory designed to detect various mutations on proximal chromosome 5 (SCHIMENTI and BUCAN 1998 ). Out of 336 G₃ males screened from 336 families (see MATERIALS AND METHODS), we have identified two mutations conferring higher spontaneous micronucleus levels. However, these two mutations were not linked to chromosome 5. Overall, three mutations and one potential mutation have been recovered among 970 G₃ males derived from 395 G₁ males. The results of all the screens are summarized in Table 1.

chaos1 mutation:
Since the DNA content of micronuclei in chaos1/chaos1 mice spans a wide range, it is likely that the micronuclei contain fragments of chromosomes, indicative of a failure to properly repair DSBs. DSBs are repaired in mammalian cells by one of two pathways: NHEJ and HR. Mice deficient in all known components of the NHEJ pathway show a SCID phenotype, due to defects in V(D)J recombination that lead to serious impairment of immune function (MULLER et al. 1999 ). Although there were no overt indications that chaos1 mutants were immunodeficient, experiments were performed to test this possibility. Peripheral blood of chaos1/chaos1 mice was stained with anti-CD3-PE and anti-Ig light chain-FITC antibodies to mark T- and B-cells, respectively. chaos1/chaos1 mice had normal numbers of B- and T-cells, unlike a classic SCID profile (Fig 4A). To evaluate chaos1 mutants for potential HR repair defects, we challenged them with mitomycin C (MMC), which causes DNA interstrand crosslinks. Since mutations in the RAD51-related genes XRCC2 and XRCC3 confer MMC hypersensitivity, it has been suggested that crosslinks are repaired by the HR pathway (THACKER 1999 ). chaos1/chaos1 mice had higher levels of micronuclei in response to MMC than did their wild-type littermates (P = 0.0039; see Fig 4B), suggesting that these mice are defective in HR repair or crosslink repair.

View larger version (29K): In this window In a new window Download PPT slide   Figure 4. Phenotypes of chaos1/chaos1 mice. (A) chaos1 mutants have normal B- and T-cell production. Peripheral blood from chaos1/chaos1 mice and their wild-type littermate was stained with anti-Ig light chain and anti-CD3-PE to label B- and T-cells, respectively. NOD/LtJ and NOD scid mice were used as positive and negative controls, respectively. (B) Mitomycin C sensitivity of chaos1 mutants revealed by the micronucleus assay. chaos1 mutants and their wild-type littermates were given 1 mg/kg of MMC. Peripheral blood was sampled every 24 hr and analyzed for MMC-induced micronuclei in RETs. At least five animals were used per group and data are shown with standard deviations.

Aside from the phenotypes of elevated micronuclei and radiosensitivity of reticulocytes, chaos1/chaos1 mutants are fertile and appear normal in all other respects up to 18 months of age. Radiation-induced tumorigenesis is being investigated in chaos1/chaos1 mice rendered congenic in particular strain backgrounds. Tail fibroblasts isolated from chaos1/chaos1 mice did not appear to be significantly sensitive to radiation compared to those from wild type (data not shown); thus this phenotype might be restricted to hematopoietic cells.

chaos1 mapping:
We genetically mapped chaos1 to an 3-cM interval between D16Mit4 and D16Mit125 on chromosome 16 by performing genome scans of affected animals (97 meioses) produced in matings of homozygous G₃ animals to their heterozygous G₁ or G₂ parents (using microsatellite markers polymorphic between B6 and C3H). We then conducted a larger intersubspecific backcross by crossing chaos1/chaos1 mice to Mus castaneus (CAST/Ei) and then backcrossing the F₁'s to chaos1 homozygotes. The resulting 1710 progeny were typed with existing and newly developed polymorphic microsatellite markers in the critical region of MMU16, and recombinants were phenotyped by the micronucleus assay. Exploiting the mouse genomic sequences in the Celera Discovery System (CDS), we localized chaos1 to a 1.3-Mb interval between D16Mit11 and a new marker, D16Jcs23 (Fig 5). Information on chaos1 mapping has been deposited in the Mouse Genome Database (accession no. J:73427).

View larger version (31K): In this window In a new window Download PPT slide   Figure 5. The chaos1 critical region. Physical location of all known genes and microsatellite markers are shown, using data from the Celera Discovery System. D16Jcs markers, which are polymorphic between B6 and CAST/Ei, were designed for this study. Data from the intersubspecific mapping backcross between chaos1 and CAST/Ei, which were used to determine the chaos1 critical region, are shown. Twenty-two genes predicted by the CDS are in this region.

Polq as a candidate gene for chaos1:
The chaos1 critical region is homologous to human chromosome 3q13.31, which contains the POLQ gene, encoding DNA polymerase (SHARIEF et al. 1999 ). Among 22 genes predicted by the CDS in the chaos1 critical region (see Fig 5), Polq is an attractive candidate for chaos1, because its protein sequence is homologous to that encoded by Drosophila melanogaster mus308 (mutagen sensitivity 308), a gene believed to be involved in interstrand crosslink repair (BOYD et al. 1990 ). mus308 encodes a 229-kD polypeptide containing seven conserved motifs characteristic of DNA and RNA helicases in an amino-terminal domain. The carboxy-terminal domain shares similarity with the polymerase domain of prokaryotic DNA polymerase I-like enzymes (HARRIS et al. 1996 ). The presence of two such domains in a protein is unique.

The CDS predicted the existence of a gene sharing homology with human POLQ in the chaos1 critical region. RT-PCR was performed with primers designed to the predicted mouse gene, yielding partial cDNAs from testis of B6 mice. The overlapping partial cDNAs were used to identify an open reading frame of 7635 nucleotides (Fig 6A), which encodes a polypeptide of 2544 amino acids (GenBank accession no. AY074936). As predicted, this polypeptide contains helicase and DNA polymerase motifs and it has 68% amino acid identity to human POLQ containing 2724 amino acids (AY032677; Clustal W 1.4 alignment).

View larger version (22K): In this window In a new window Download PPT slide   Figure 6. Structure of the Polq gene and transcripts. (A) An open reading frame flanked by 5'- and 3'-untranslated regions (UTR) in Polq cDNA is shown to scale (above) with the start codon and stop codon. The Celera mouse genome sequence was used to reveal that a total of 30 exons encode this Polq cDNA (AY074936). Exons (small rectangles) and introns in the Polq gene are shown to scale (below). Exons 1 and 31 (small unfilled rectangles) contain the 5'- and 3'-UTRs, respectively. The T C transition found in chaos1 mutant is indicated. The regions of helicase and DNA polymerase homology and the corresponding exons are also indicated as arrows. (B) Schematic illustration of exons comprising the different Polq transcripts. A shorter transcript that skips exons 6–10 was also identified (AY147862). (*) Longer isoforms containing one extra exon (exon 4) were also identified; however, the presence of this exon creates a stop codon (AY147863, AY147864). () Full-length exons 2 and 5 were found in a Riken mouse cDNA from neonatal thymus (AK020790), whereas Polq cDNAs from testis contain only part of exons 2 and 5, because of alternative splicing of these exons. Collectively, 31 Polq exons were found.

The Celera mouse genome sequence was used to reveal that a total of 30 exons comprise this Polq cDNA (Fig 6A). Similarly, exploiting the Celera human genome sequence, 31 exons were found for POLQ. As shown in Fig 6B, a shorter transcript, which skips exons 6–10, was also found in mouse testis, giving rise to a predicted polypeptide of 2265 amino acids (AY147862). Moreover, each of the Polq transcripts has a longer isoform containing one extra exon (exon 4); however, the presence of this exon creates a stop codon (AY147863, AY147864). A Riken mouse cDNA from neonatal thymus has full-length exons 2 and 5, also containing a stop codon (AK020790). Collectively, 31 Polq exons were found; however, the roles of these various transcripts remain to be elucidated.

There was no indication of differences in transcript size or expression levels in mutant RNA compared to that from wild-type B6 mice (data not shown). Nevertheless, a single T C base substitution was identified at residue 5794 in the coding region (exon 19) of the chaos1 mutant allele (Fig 6A), which is not present in B6 cDNA. This mutation gives rise to a serine-to-proline change at residue 1932.

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

To our knowledge, this is the first successful phenotype-driven screen for chromosome instability mutants in mice. The micronucleus assay adapted from the method developed by DERTINGER et al. 1996 was highly sensitive and readily implemented as a high-throughput screen for mutagenized mice. The dose of -rays used in this screen had little effect on the reproductive ability of the G₃ males, which were subsequently used for mapping studies or maintenance of the mutation. Only males were screened in this study due to logistical factors associated with the mutagenesis program from which the mice were derived. However, the spontaneous micronucleus frequencies tended to be lower in females than in males.

Importantly, the high reproducibility of the assay (DERTINGER et al. 2000 ; TOROUS et al. 2001 ) allowed accurate phenotyping of mice in the mapping crosses, which is critical for positional cloning. In the case of chaos1 phenotyping, there was no need to irradiate mice, because the mutants were easily identified by high spontaneous micronucleus levels. Actually, we find it more practical and feasible to perform screens only for spontaneously elevated micronucleus levels. Potential mutants isolated can be characterized later as to sensitivity to radiation or other agents. Moreover, micronuclei can be isolated by flow sorting or microdissection for further analysis (NUSSE et al. 1996 ; PEACE et al. 1999 ) to characterize phenotypes of identified mutants.

The presence of a mutated Polq allele in chaos1 mice makes this gene a strong candidate for chaos1. The identified T C transition is one of the two most frequent classes of ENU-induced mutations in the mouse germline (MARKER et al. 1997 ; JUSTICE et al. 1999 ). This mutation causes a drastic amino acid change in POLQ from serine to proline, which may alter the secondary structure of the molecule. However, since the mutation is not located in either the helicase or the polymerase domains of the predicted protein (the two regions of the protein about which we can make informed speculation as to key enzymatic activity), it is difficult to draw a conclusion about the functional importance of this particular amino acid residue or to determine if the mutation actually compromises protein function. To gain some insight into the functional importance of this amino acid residue, we compared the mouse POLQ sequence to those of other organisms. Homologs of mus308 have been reported in Caenorhabditis elegans, Arabidopsis thaliana (HARRIS et al. 1996 ; MARINI and WOOD 2002 ), and humans (SHARIEF et al. 1999 ), but not yeast. By BLAST searching databases, we also found a presumed ortholog in Anopheles gambiae (mosquitos; accession no. EAA04696). The serine is conserved in human and mosquito. It exists as alanine in Drosophila (L76559) and asparagine in Arabidopsis (CAA18591), both of which are semiconservative differences. The region containing this residue in C. elegans (AAB93325) is too divergent to align with other mus308 homologs.

Additional support for Polq as a candidate for chaos1 is the similarity in phenotypes of chaos1 mutants to flies containing mutations in mus308, a Polq homolog. Drosophila mus308 encodes a unique protein with helicase and prokaryotic DNA polymerase I-like motifs in a single polypeptide (HARRIS et al. 1996 ). mus308 mutants were identified as strains hypersensitive to nitrogen mustard, a crosslinking agent (BOYD et al. 1981 ), but not to a monofunctional alkylating agent, methyl methanesulfonate (BOYD et al. 1990 ), suggesting that mus308 is specifically involved in crosslink repair. However, it has been reported that mus308 might be also involved in postreplicational repair (AGUIRREZABALAGA et al. 1995 ; TOSAL et al. 2000 ). Homozygous mus308 flies showed elevated embryonic mortality associated with chromosome instability and a mutator phenotype in response to certain mutagens (LEONHARDT et al. 1993 ).

Recently, human POLQ was purified as a high-fidelity DNA polymerase with the ability to bypass DNA lesions (MAGA et al. 2002 ). This is quite unique among recently discovered DNA polymerases, most of which are error prone (GOODMAN and TIPPIN 2000 ). However, the purified POLQ did not show detectable helicase activity (MAGA et al. 2002 ). New helicase genes, HEL308 and Hel308, which are homologous to the helicase motif of mus308, have been identified in humans and mice (MARINI and WOOD 2002 ). Purified human HEL308 exhibited a DNA helicase activity (MARINI and WOOD 2002 ). The existence of these paralogs may reflect redundancy in mammalian crosslink repair in which at least two pathways have been found: recombination-dependent and recombination-independent error-prone pathways (MCHUGH et al. 2001 ; WANG et al. 2001 ). It remains to be elucidated how exactly these proteins function to repair crosslinks. If chaos1 is truly a Polq mutation, the chaos1 mutant may fill a unique niche that would allow in vivo investigation of crosslink repair in mammals.

Despite the higher micronucleus levels, chaos1/chaos1 mutants showed no apparent abnormalities up to 18 months of age. This may not be surprising, since mouse models for Fanconi anemia, which have defects in crosslink repair, do not show a predisposition to cancer (CHEN et al. 1996 ; CHENG et al. 2000 ; YANG et al. 2001 ). It has also been reported that Xpa or Xpc knockouts, deficient in nucleotide excision repair, rarely developed tumors without carcinogen treatment (WIJNHOVEN et al. 2000 ; VAN KREIJL et al. 2001 ), although Xpc^-/- mice had a higher spontaneous mutation rate at the Hprt locus. In general, genome instability itself may not be sufficient to cause cancer. Other events, such as loss of cell-cycle checkpoints, could be more critical. Nevertheless, chromosome instability may facilitate the occurrence of these critical events. Indeed, introduction of a Trp53 null allele significantly enhanced mammary tumor formation in the Brca1 conditional mutant mice; otherwise tumorigenesis occurred after long latency and at a low frequency (XU et al. 1999 ; DENG and SCOTT 2000 ). We have also observed a synergistic increase in genome instability and growth retardation in mice doubly mutant for Atm and chaos1 (N. SHIMA and J. SCHIMENTI, unpublished data).

The data presented here demonstrate the efficacy of the micronucleus screen for detecting new chromosome instability mutants and subsequently mapping them in a robust way. With this screen, it is also possible that hypomorphic alleles of important DSB repair genes such as Rad51 paralogs, whose complete inactivation causes embryonic lethality, may be detected (THOMPSON and SCHILD 1999 ). The genes responsible for elevated micronuclei might be involved not only in DNA repair, but also in processes such as mitotic-spindle checkpoints, defects of which could lead to aneuploidy, the most frequent genetic abnormality observed in cancer cells (LENGAUER et al. 1998 ). While it is still controversial that genome instability always leads to carcinogenesis (MARX 2002 ), elevated micronucleus levels could be used as a surrogate phenotype that predicts cancer predisposition at an early age. In some cases, higher micronucleus levels in peripheral blood of humans have been linked with increased cancer risk (DONEDA et al. 1995 ; SCOTT et al. 1999 ). Development of such a surrogate phenotype would enable the screening and mapping of recessive mutations causing cancers without aging mice to the point where late-onset cancers develop.

In conclusion, the flow cytometric screen for elevated micronuclei has proven to be a useful approach to identifying mutations in novel genes that cause genome instability as a consequence of DSB repair defects. The incorporation of these screens into major mutagenesis efforts may yield new and hitherto unknown genes that contribute to cancer and concomitantly may yield the cognate mutant mouse models.

	FOOTNOTES

Sequence data from this article have been deposited with the EMBL/GenBank Data Libraries under accession nos. AY074936, AY147862, AY147863, and AY147864.

	ACKNOWLEDGMENTS

We thank Leonard Shultz, Susan Ackerman, and Jeremy Ward for critical reading of the manuscript and helpful comments. This work is supported by National Institutes of Health grants GM-45415 and HD-35984 to J.S. N.S. is supported by a postdoctoral fellowship from the Breast Cancer Research Program, Department of Defense (DAMD 17-01-1-0277).

Manuscript received September 13, 2002; Accepted for publication November 20, 2002.

	LITERATURE CITED

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