Microsatellite analysis of recurrent chromosome 2 deletions in acute myeloid leukaemia induced by radiation in F1 hybrid mice

Genomic mapping in outbred mice reveals overlap in genetic susceptibility for HZE ion- and γ-ray-induced tumors

Cancer risk from galactic cosmic radiation exposure is considered a potential "showstopper" for a manned mission to Mars. Calculating the actual risks confronted by spaceflight crews is complicated by our limited understanding of the carcinogenic effects of high-charge, high-energy (HZE) ions, a radiation type for which no human exposure data exist. Using a mouse model of genetic diversity, we find that the histotype spectrum of HZE ion-induced tumors is similar to the spectra of spontaneous and γ-ray-induced tumors and that the genomic loci controlling susceptibilities overlap between groups for some tumor types. Where it occurs, this overlap indicates shared tumorigenesis mechanisms regardless of the type of radiation exposure and supports the use of human epidemiological data from γ-ray exposures to predict cancer risks from galactic cosmic rays.

Tracking preleukemic cells in vivo to reveal the sequence of molecular events in radiation leukemogenesis

Epidemiological studies have demonstrated an increased leukemia incidence following ionizing radiation exposure, but to date, the target cells and underlying mechanisms of radiation leukemogenesis remain largely unidentified. We engineered a mouse model carrying a different fluorescent marker on each chromosome 2, located inside the minimum deleted region occurring after radiation exposure and recognized as the first leukemogenic event. Using this tailored model, we report that following radiation exposure, more than half of asymptomatic CBA Sfpi1^GFP/mCh mice presented with expanding clones of preleukemic hematopoietic cells harboring a hemizygous interstitial deletion of chromosome 2. Moreover, following isolation of preleukemic hematopoietic stem and progenitor cells irradiated in their native microenvironment, we identified the presence of Sfpi1 point mutations within a subpopulation of these preleukemic cells expanding rapidly (increasing from 6% to 55% in 21 days in peripheral blood in one case), hence identifying for the first time the presence of such cells within a living animal. Importantly, we also report a previously undescribed gender difference in the phenotype of the preleukemic cells and leukemia, suggesting a gender imbalance in the radiation-induced leukemic target cell. In conclusion, we provide novel insights into the sequence of molecular events occurring during the (radiation-induced) leukemic clonal evolution.

Leukemogenesis in heterozygous PU.1 knockout mice

Most murine radiation-induced acute myeloid leukemias involve biallelic inactivation of the PU.1 gene, with one allele being lost through a radiation-induced chromosomal deletion and the other allele affected by a recurrent point mutation in codon 235 that is likely to be spontaneous. The short latencies of acute myeloid leukemias occurring in nonirradiated mice engineered with PU.1 conditional knockout or knockdown alleles suggest that once both copies of PU.1 have been lost any other steps involved in leukemogenesis occur rapidly. Yet, spontaneous acute myeloid leukemias have not been reported in mice heterozygous for a PU.1 knockout allele, an observation that conflicts with the understanding that the PU.1 codon 235 mutation is spontaneous. Here we describe experiments that show that the lack of spontaneous leukemia in PU.1 heterozygous knockout mice is not due to insufficient monitoring times or mouse numbers or the genetic background of the knockout mice. The results reveal that spontaneous leukemias that develop in mice of the mixed 129S2/SvPas and C57BL/6 background of knockout mice arise by a pathway that does not involve biallelic PU.1 mutation. In addition, the latency of radiation-induced leukemia in PU.1 heterozygous mice on a genetic background susceptible to radiation-induced leukemia indicates that the codon 235 mutation is not a rate-limiting step in radiation leukemogenesis driven by PU.1 loss.

Live cell detection of chromosome 2 deletion and Sfpi1/PU1 loss in radiation-induced mouse acute myeloid leukaemia

The CBA/H mouse model of radiation-induced acute myeloid leukaemia (rAML) has been studied for decades to bring to light the molecular mechanisms associated with multistage carcinogenesis. A specific interstitial deletion of chromosome 2 found in a high proportion of rAML is recognised as the initiating event. The deletion leads to the loss of Sfpi, a gene essential for haematopoietic development. Its product, the transcription factor PU.1 acts as a tumour suppressor in this model. Although the deletion can be detected early following ionising radiation exposure by cytogenetic techniques, precise characterisation of the haematopoietic cells carrying the deletion and the study of their fate in vivo cannot be achieved. Here, using a genetically engineered C57BL/6 mouse model expressing the GFP fluorescent molecule under the control of the Sfpi1 promoter, which we have bred onto the rAML-susceptible CBA/H strain, we demonstrate that GFP expression did not interfere with X-ray induced leukaemia incidence and that GFP fluorescence in live leukaemic cells is a surrogate marker of radiation-induced chromosome 2 deletions with or without point mutations on the remaining allele of the Sfpi1 gene. This study presents the first experimental evidence for the detection of this leukaemia initiating event in live leukemic cells.

Intestinal tumours induced in Apc(Min/+) mice by X-rays and neutrons

PURPOSE

To compare the development of intestinal adenomas following neutron and X-ray exposure of Apc(Min/+) mice (Apc - adenomatous polyposis coli; Min - multiple intestinal neoplasia).

MATERIALS AND METHODS

Adult mice were exposed to acute doses of X-rays or fission neutrons. Tumour counting was undertaken 200 days later and samples were taken for Loss of Heterozygosity (LOH) analysis.

RESULTS

Tumour numbers (adenomas and microadenomas) increased by 1.4-fold, 1.7-fold, 2.7-fold and 9-fold, after 0.5, 1, 2 and 5 Gy X-rays, respectively, and by 2.4-fold and 5.7-fold, after 0.5 and 1 Gy fission neutrons, respectively. LOH analysis of tumours from neutron-exposed mice showed that 63% had lost Apc and 90% (cf. 53% in controls) had lost D18mit84, a marker for Epb4.1l4a/NBL4 (erythrocyte protein band 4.1-like 4a/novel band 4.1-like 4), known to be involved in the Wnt (wingless-related mouse mammary tumour virus integration site) pathway. Some tumours from neutron-exposed mice appeared to have homozygous loss of some chromosomal markers.

CONCLUSIONS

X-ray or fission neutron irradiation results in strongly enhanced tumour multiplicities. Comparison of tumour yields indicated a low Relative Biological Effectiveness of around 2-8 for fission neutrons compared with X-rays. LOH in intestinal tumours from neutron-exposed mice appeared to be more complex than previously reported for tumours from X-irradiated mice.

Microsatellite instability in radiation-induced murine tumours; influence of tumour type and radiation quality

PURPOSE

To investigate microsatellite instability (MSI) in radiation-induced murine tumours, its dependence on tissue (haemopoietic, intestinal, mammary, brain and skin) and radiation type.

MATERIALS AND METHODS

DNA from spontaneous, X-ray or neutron-induced mouse tumours were used in Polymerase Chain Reactions (PCR) with mono- or di-nucleotide repeat markers. Deviations from expected allele size caused by insertion/deletion events were assessed by capillary electrophoresis.

RESULTS

Tumours showing MSI increased from 16% in spontaneously arising tumours to 23% (P = 0.014) in X-ray-induced tumours and rising again to 83% (P << 0.001) in neutron-induced tumours. X-ray-induced Acute Myeloid Leukaemias (AML) had a higher level of mono-nucleotide instability (45%) than di-nucleotide instability (37%). Fifty percent of neutron-induced tumours were classified as MSI-high for mono-nucleotide markers and 10% for di-nucleotide markers. Distribution of MSI varied in the different tumour types and did not appear random.

CONCLUSIONS

Exposure to ionising radiation, especially neutrons, promotes the development of MSI in mouse tumours. MSI may therefore play a role in mouse radiation tumourigenesis, particularly following high Linear Energy Transfer (LET) exposures. MSI events, for a comparable panel of genome-wide markers in different tissue types, were not randomly distributed throughout the genome.

Implication of replicative stress-related stem cell ageing in radiation-induced murine leukaemia

BACKGROUND

The essential aetiology of radiation-induced acute myeloid leukaemia (AML) in mice is the downregulation of the transcription factor PU.1. The causative mutation of the PU.1-endocing Sfpi1 gene consists mostly of C:G to T:A transitions at a CpG site and is likely to be of spontaneous origin. To work out a mechanism underlying the association between radiation exposure and the AML induction, we have hypothesised that replicative stress after irradiation accelerates the ageing of haematopoietic stem cells (HSCs), and the ageing-related decline in DNA repair could affect the spontaneous mutation rates.

METHODS

Mathematical model analysis was conducted to examine whether and to what extent the cell kinetics of HSCs can be modified after irradiation. The haematopoietic differentiation process is expressed as a mathematical model and the cell-kinetics parameters were estimated by fitting the simulation result to the assay data.

RESULTS

The analysis revealed that HSCs cycle vigourously for more than a few months after irradiation. The estimated number of cell divisions per surviving HSC in 3 Gy-exposed mice reached as high as ten times that of the unexposed.

INTERPRETATION

The mitotic load after 3 Gy irradiation seems to be heavy enough to accelerate the ageing of HSCs and the hypothesis reasonably explains the leukaemogenic process.

Upregulation of c-myc gene accompanied by PU.1 deficiency in radiation-induced acute myeloid leukemia in mice

OBJECTIVE

High-dose radiation exposure induces acute myeloid leukemia (AML) in C3H mice, most of which have a frequent hemizygous deletion around the D2Mit15 marker on chromosome 2. This region includes PU.1, a critical candidate gene for initiation of leukemogenesis. To identify novel cooperative genes with PU.1, relevant to radiation-induced leukemogenesis, we analyzed the copy number alterations of tumor-related gene loci by array CGH, and their expressions in primary and transplanted AMLs.

MATERIALS AND METHODS

For the induction of AMLs, C3H/He Nrs mice were exposed to 3 Gy of x-rays or gamma-rays. The genomic alterations of 35 primary AMLs and 34 transplanted AMLs obtained from the recipient mice transplanted the primary AMLs were analyzed by array CGH. According to the genomic alterations and mutations of the 235th arginine of PU.1 allele, we classified the radiogenic AMLs into three types such as Chr2(del) PU.1(del/R235-) AML, Chr2(del) PU.1(del/R235+) AML and Chr2(intact) PU.1(R235+/R235+) AML, to compare the expression levels of 8 tumor-related genes quantitatively by real-time polymerase chain reaction and cell-surface antigen expression. Results. In addition to well-known loss of PU.1 with hemizygous deletion of chromosome 2, novel genomic alterations such as partial gain of chromosome 6 were recurrently detected in AMLs. In this study, we found similarity between cell-surface antigen expressions of bone marrows and those of spleens in AML mice and significantly higher expressions of c-myc and PU.1 expression, especially in the PU.1-deficient (Chr2(del) PU.1(del/R235-)) AML and Chr2(del) PU.1(del/R235+) compared to Chr2(intact) PU.1(R235+/R235+) AMLs.

CONCLUSION

The new finding on upregulation of c-myc and PU.1 in both and hemizygous PU.1-deficient AMLs and different genomic alterations detected by array CGH suggests that the molecular mechanism for development of radiation-induced AML should be different among three types of AML.

Radiation leukemogenesis: a proteomic approach

OBJECTIVE

The objectives of this study were to identify protein biomarkers of radiation-induced acute myeloid leukemia (rAML) in CBA/CaJ mice, and to examine the similarities or differences in the patterns of protein-expression profiles among AMLs induced by low linear energy transfer (LET) radiation (e.g., gamma- or x-rays), and high LET radiation (i.e., neutrons).

MATERIALS AND METHODS

We used two-dimensional electrophoresis gel in combination with mass spectrometry (MS), i.e., matrix-assisted laser desorption ionization/time-of-flight MS and electrospray ionization-liquid chromatography/tandem mass spectrometry, to identify protein signatures in blood-plasma samples collected from control and rAML mice. There were nine cases of rAML (three cases induced by high LET radiation; six induced by low LET radiation) and eight control mice at similar ages.

RESULTS

The results showed differences in the patterns of protein profiles from blood-plasma samples collected from rAML vs control mice. Moreover, our data demonstrated, both qualitatively and quantitatively, differences between the plasma protein profiles obtained from mice with AML induced by low vs high LET radiation. Most of the proteins that were present at greater levels in normal samples than in rAML samples were associated with normal metabolism and growth. Several acute-phase proteins were upregulated in rAML samples.

CONCLUSION

The data present, for the first time, evidence for increased expression of clusterin and a loss of gelsolin expression in blood plasma as potential biomarkers of rAML in the CBA/CaJ mouse. Results also indicate that two-dimensional electrophoresis, in combination with MS, is a highly sensitive technique for identification of blood-based biomarkers of rAML.

Deregulation of homologous recombination DNA repair in alkylating agent-treated stem cell clones: a possible role in the aetiology of chemotherapy-induced leukaemia

Chemotherapeutic regimes involving alkylating agents, such as methylators and crosslinking nitrogen mustards, represent a major risk factor for acute myeloid leukaemia. A high frequency of microsatellite instability and evidence of MSH2 loss in alkylating chemotherapy-related acute myeloid leukaemia (t-AML) suggests that DNA mismatch repair (MMR) dysfunction may be an initiating event in disease evolution. Subsequent accumulation of secondary genetic changes as a result of DNA MMR loss may ultimately lead to the gross chromosomal abnormalities seen in t-AML. Homologous recombination repair (HRR) maintains chromosomal stability by the repair of DNA double-strand breaks, and is therefore a possible target for deregulation in MMR dysfunctional t-AML. In order to test this hypothesis Msh2- proficient and -deficient murine embryonic stem (ES) cells were used to examine the effects of MMR status and methylating agent treatment on cellular expression of DNA double-strand break repair genes. HRR gene expression was significantly deregulated in Msh2 null ES cell clones compared to wild-type clones. Furthermore, some Msh2 null clones expressed high levels of Rad51 specifically, a critical component of HRR. Such Rad51 superexpressing clones were also observed when expression was determined in monocytic myeloid cells differentiated from ES cells. A deregulated HRR phenotype could be partially recapitulated in MMR-competent wild-type cells by treatment with the methylating agent, N-methyl-N-nitrosourea. Furthermore, treatment with melphalan, a leukaemogenic DNA crosslinking chemotherapy nitrogen mustard predicted to elicit HRR, selected against cells with deregulated HRR. These data suggest a t-AML mechanism whereby DNA MMR loss promotes the emergence of HRR gene superexpressing clones, with concomitant chromosomal instability. However, melphalan selection against clones with deregulated HRR suggests that persistence and expansion of unstable clones may require additional genetic alterations that promote cell survival.

Mutations of the PU.1 Ets domain are specifically associated with murine radiation-induced, but not human therapy-related, acute myeloid leukaemia

Murine radiation-induced acute myeloid leukaemia (AML) is characterized by loss of one copy of chromosome 2. Previously, we positioned the critical haematopoietic-specific transcription factor PU.1 within a minimally deleted region. We now report a high frequency (>65%) of missense mutation at codon 235 in the DNA-binding Ets domain of PU.1 in murine AML. Earlier studies, outside the context of malignancy, determined that conversion of arginine 235 (R235) to any other amino-acid residue leads to ablation of DNA-binding function and loss of expression of downstream targets. We show that mutation of R235 does not lead to protein loss, and occurs specifically in those AMLs showing loss of one copy of PU.1 (P=0.001, Fisher's exact test). PU.1 mutations were not found in the coding region, UTRs or promoter of human therapy-related AMLs. Potentially regulatory elements upstream of PU.1 were located but no mutations found. In conclusion, we have identified the cause of murine radiation-induced AML and have shown that loss of one copy of PU.1, as a consequence of flanking radiation-sensitive fragile domains on chromosome 2, and subsequent R235 conversion are highly specific to this mouse model. Such a mechanism does not operate, or is extremely rare, in human AML.

Functional evidence from microcell-mediated chromosome transfer of myeloid leukemia suppressor genes on human chromosomes 7 and 11

The long arm of human chromosome 7 between 7q22 and 7q36 has been identified as a region harboring one or more tumor-suppressor genes (TSGs) inactivated in acute myeloid leukemia (AML). Additional TSGs mapping to other chromosomes may well be involved in the etiology of this disease. For example, experiments using a mouse model system have indicated the possible presence of an AML TSG at 11p11-12. Microcell-mediated chromosome transfer (MMCT) has been used to introduce human chromosomes 7 and 11 into a murine myeloid leukemia cell line. A proportion of MMCT hybrid clones containing either whole chromosome 7 or fragments of chromosome 11 showed a significant delay in leukemogenic onset when injected into syngeneic mice. Screening of hybrid clones did not associate any human microsatellite markers with decreased leukemogenic potential in vivo. However, preliminary evidence was obtained of allelic loss at chromosomal regions homologous with human 7q22 in murine F1 hybrid AMLs. Our data provide functional evidence of AML-associated TSGs localized to human chromosomes 7 and 11 in support of previously published studies on cytogenetic and allelic losses associated with AML development.

Evidence for two commonly deleted regions on mouse chromosome 2 in gamma ray-induced acute myeloid leukemic cells

OBJECTIVE

The objective of this study was to delineate a precise molecular map of the commonly deleted region (CDR) on mouse chr2 in radiation-induced mouse acute myeloid leukemic (AML) cells.

MATERIALS AND METHODS

We used a PCR-based loss of heterozygosity (LOH) assay to map the chr2-CDR in AML cells isolated from F1 hybrid mice (BALB/cJ x CBA/CaJ) which developed AML following exposure to a single dose of 3 Gy of 137Cs gamma rays. A total of 30 polymorphic microsatellite markers, mapping within or close to chr2(D-E), were used under optimized PCR conditions that generate a single major band for each marker on a nondenaturing polyacrylamide gel.

RESULTS

Detailed LOH mapping identified two distinct AML-CDRs: one localized to a 4.6 centiMorgan (cM) interval between markers D2Mit272 and D2Mit394; the other mapped to a 0.8 cM interval between markers D2Mit276 and D2Mit444. Both CDRs span the mouse chr2E region.

CONCLUSION

The data present, for the first time, evidence for two distinctly noncontiguous CDRs on mouse chr2 harboring gene(s) involved in AML development. These CDRs are orthologous to human chromosomes 11p11-13 and 15q11-15 that have been implicated in subsets of AML. This finding indicates the region of mouse chr2 that must be searched for candidate genes involved in radiation-induced AML.

A major breakpoint cluster domain in murine radiation-induced acute myeloid leukemia

Cytogenetic and molecular studies have provided evidence of the clustering of chromosome 2 deletion breakpoints in radiation-induced murine acute myeloid leukemia (AML). Moreover, clustering occurs in at least two fragile domains rich in telomere-like arrays. Here we describe a physical map of the distal breakpoint cluster and confirm the presence of inverted head-to-head telomeric sequence arrays. These potentially recombinogenic sequences were not, however, the direct focus for post-irradiation chromosome breakage in AML. Instead, the two arrays bordered a 2.5-kb sequence with properties expected of a nuclear matrix attachment region (MAR). The putative MAR co-localized in the fragile domain with genes important to the hemopoietic system (leukocyte tyrosine kinase, zinc finger protein 106, erythrocyte protein band 4.2, and beta(2)-microglobulin (beta2m)); the beta2m subdomain was a particular focus of breakage. On the basis of these and other data, we suggest that AML-associated chromosome 2 fragility in the mouse is a consequence of domain-specific fragility in genomic domains containing numerous genes critical to the hemopoietic system. Recorded with the permission of the controller of Her Majesty's Stationery Office. Published by Wiley-Liss, Inc.

A PCR-based clonal analysis of radiation-induced loss of heterozygosity in haemopoietic stem cells

CBA mouse strains have been used for many years as a model of radiation-induced acute myeloid leukaemia and the leukaemias in CBA and their F1 hybrids are characterised by a specific loss of heterozygosity involving one homologue of chromosome 2. Previous cytogenetic studies of transplanted irradiated bone marrow, or of bone marrow obtained from irradiated mice significantly before the appearance of leukaemia, have been interpreted as the chromosome 2 deletion being a high frequency, possibly initiating event. However, these studies had not specifically addressed the question of whether the characteristic deletion was induced at a high frequency in stem cells. Using a PCR-based technique, we have studied the induction of chromosome 2 LOH in the progeny of (CBA/H x C57BL/6)F1 stem cells after a potentially leukaemogenic radiation exposure. Whilst chromosome 2 LOH can be induced directly by irradiation and there is a preferential loss of the CBA allele, the frequency is no greater than LOH induced in other chromosomal regions studied. The data do not support radiation-induced deletion involving one homologue of chromosome 2 in long-term repopulating stem cells (<1 in 200) being as high a frequency event as might be inferred by previous cytogenetic studies of total bone marrow.

Analysis of loss of heterozygosity in lymphoma and leukaemia arising in F1 hybrid mice locates a common region of chromosome 4 loss

Previous studies have identified five lymphoma-related tumour suppressor gene regions on murine chromosome 4. Using detailed allelotype analysis on a range of lympho-haematopoietic tumour types arising in F1 hybrid mice, we now show a consistent pattern of loss of heterozygosity (LOH) which identifies a common region of loss delineated by microsatellites D4Mit21 and D4Mit53 on proximal chromosome 4. This critical segment corresponds to the thymic lymphoma tumour suppressor region 5 (TLSR5) identified in an earlier study. Tumours of this type have also been reported as showing allelic loss from the Trp53 and Ikaros regions on chromosome 11. In the present study, only a small fraction of tumours showed LOH in the Ikaros region, while a minority of lymphomas, but not acute myeloid leukaemias, showed allelic loss of the chromosome 11 segment encoding Trp53. These and other data indicate strongly that the genomic regions identified as showing recurrent LOH depend on the genetic background of the mice. Overall, the results indicate a key role for a tumour suppressor gene(s) encoded in an approximately 3 cM segment on proximal chromosome 4 and provide an experimental basis for the further investigation of the functional role of candidate genes which include Pax5 and Tgfbr1.

Loss of heterozygosity in spontaneous and X-ray-induced intestinal tumors arising in F1 hybrid min mice: evidence for sequential loss of APC(+) and Dpc4 in tumor development

Min (multiple intestinal neoplasia) mice carry a mutant allele of the murine Apc (adenomatous polyposis coli) locus and are predisposed to intestinal adenoma formation in the intestinal tract. Early studies have shown complete loss of function of Apc by whole chromosome loss on the tumor-sensitive C57BL/6J genetic background and in AKR x B6 F1 hybrids. Gamma-radiation-induced chromosomal losses focus the critical region on wt Apc, but because of the limited number of polymorphic markers used, no other critical regions of loss on chromosome 18 were identified. Using intestinal tumors arising spontaneously and induced by X-rays in CBA/H x C57BL/6J F1 hybrid mice and high-resolution microsatellite loss of heterozygosity (LOH) techniques, we provide mapping data for wt Apc loss, which confirms and extends earlier observations. In addition, high-frequency loss events at the Dpc4 locus were found in both spontaneous and radiation-induced tumors. These data identified LOH of Dpc4 as a critical secondary event following complete functional loss of Apc. LOH across the Trp53 genomic region of chromosome 11 was not observed. No LOH was recorded for the Mom1 candidate gene Pla2g2a or for 9 out of 10 polymorphic markers from the Mom1 genomic region on murine chromosome 4. One marker mapping distal to Pla2g2a showed LOH in a small minority of spontaneous tumors. These data support the contention that Mom1 does not act as a classical tumor suppressor. Overall, our data indicates a significant role for Dpc4 mutation in intestinal tumor progression in the mouse and provides further evidence for the importance of interstitial chromosome losses in radiation tumorigenesis.

Gain of chromosomes 15 and 19 is frequent in both mouse hepatocellular carcinoma cell lines and primary tumors, but loss of chromosomes 4 and 12 is detected only in the cell lines

Chromosomal alterations were investigated in hepatocellular carcinoma cell lines, primary tumors and liver epithelial cell lines derived from normal livers of C57BL/6JxC3H/HeJ F(1) and C3H/HeJxC57BL/6J F(1) mice. In the primary tumors, non-random gain of chromosomes 15 and 19 was found in seven and five of 14 hepatocellular carcinomas, respectively. On the other hand, in the cases of both liver epithelial and hepatocellular carcinoma cell lines, frequent changes were loss of chromosomes 4 (4/9 cell lines) and 12 (3/9) as well as gain of chromosomes 15 (5/9) and 19 (4/9). These results indicate that the chromosomal gain is associated with both in vivo carcinogenesis and establishment of cell lines, while the loss is specific for the latter. PCR analysis using polymorphic microsatellite DNA markers revealed that the loss of chromosome 12 as well as chromosome 4 was much more frequent for the C57BL/6J hepatocarcinogenesis-resistant rather than the susceptible C3H/HeJ strain.