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

Spi1 R235C point mutation confers hypersensitivity to radiation-induced acute myeloid leukemia in mice

Ionizing radiation (IR) is a risk factor for acute myeloid leukemia (rAML). Murine rAMLs feature both hemizygous chromosome 2 deletions (Del2) and point mutations (R235) within the hematopoietic regulatory gene Spi1. We generated a heterozygous CBA Spi1 R235 mouse (CBASpm/+) which develops de novo AML with 100% incidence by ∼12 months old and shows a dose-dependent reduction in latency following X-irradiation. These effects are reduced on an AML-resistant C57Bl6 genetic background. CBASpm/Gfp reporter mice show increased Gfp expression, indicating compensation for Spm-induced Spi1 haploinsufficiency. Del2 is always detected in both de novo and rAMLs, indicating that biallelic Spi1 mutation is required for AML. CBASpm/+ mice show that a single Spm modification is sufficient for initiating AML development with complete penetrance, via the "two-hit" mechanism and this is accelerated by IR exposure. Similar SPI1/PU.1 polymorphisms in humans could potentially lead to enhanced susceptibility to IR following medical or environmental exposure.

TGFβ Inhibitor A83-01 Enhances Murine HSPC Expansion for Gene Therapy

Murine hematopoietic stem and progenitor cells (HSPCs) are commonly used as model systems during gene therapeutic retroviral vector development and preclinical biosafety assessment. Here, we developed cell culture conditions to maintain stemness and prevent differentiation during HSPC culture. We used the small compounds A83-01, pomalidomide, and UM171 (APU). Highly purified LSK SLAM cells expanded in medium containing SCF, IL-3, FLT3-L, and IL-11 but rapidly differentiated to myeloid progenitors and mast cells. The supplementation of APU attenuated the differentiation and preserved the stemness of HSPCs. The TGFβ inhibitor A83-01 was identified as the major effector. It significantly inhibited the mast-cell-associated expression of FcεR1α and the transcription of genes regulating the formation of granules and promoted a 3800-fold expansion of LSK cells. As a functional readout, we used expanded HSPCs in state-of-the-art genotoxicity assays. Like fresh cells, APU-expanded HSPCs transduced with a mutagenic retroviral vector developed a myeloid differentiation block with clonal restriction and dysregulated oncogenic transcriptomic signatures due to vector integration near the high-risk locus Mecom. Thus, expanded HSPCs might serve as a novel cell source for retroviral vector testing and genotoxicity studies.

Cellular kinetics of hematopoietic cells with Sfpi1 deletion are present at different frequencies in bone-marrow and spleen in X-irradiated mice

PURPOSE

Several past studies using a mouse model of radiation-induced AML (rAML) have shown that hemizygous deletion of the Sfpi1 gene (HDSG) is an initiating event for the development of rAML. In this study, we examined the difference in frequency of HDSG in hematopoietic stem cells (HSCs) Rich hematopoietic Cell population (HRCs) from bone marrow (BM) and spleen of C3H mice irradiated with 3 Gy X-rays.

MATERIALS AND METHODS

8-weeks old male C3H mice were irradiated 3Gy of whole body X-ray (1 Gy/min) and mice were sacrificed at 1, 4, 8, and 26 weeks. Then, HSPCs were isolated from BM of femur and spleen, the frequency of HRCs with Sfpi1 gene deletion was analyzed by fluorescence in situ hybridization (FISH).

RESULTS AND CONCLUSIONS

The frequency of HRCs with HDSG in both BM and spleen was increased 1 week after X-irradiation. Then, the frequency of HRCs with HDSG in BM showed a gradual decrease from 4 to 26 weeks, whereas HRCs with HDSG in spleen remained high, even at 26 weeks after X-irradiation. HDSG is less likely to be eliminated, particularly in the spleen, after X-irradiation. The spleen as well as BM of the femur may be major sites of rAML development.

Thymic precursor cells generate acute myeloid leukemia in NUP98-PHF23/NUP98-HOXD13 double transgenic mice

Transgenic mice that express either a NUP98–PHF23 (NP23) or NUP98-HOXD13 (NHD13) fusion in the hematopoietic compartment develop a wide spectrum of leukemias, including myeloid, erythroid, megakaryocytic and lymphoid, at age 9–14 months. NP23-NHD13 double transgenic mice were generated by interbreeding NP23 and NHD13 mice. Remarkably, 100% of the NP23-NHD13 double transgenic mice developed acute myeloid leukemia (AML) within three months, characterized by replacement of the thymus with leukemic myeloblasts. The marked infiltration of thymus led to the intriguing hypothesis that AML generated in NP23-NHD13 mice arose in the thymus, as opposed to the bone marrow (BM). Transplantation of CD4-CD8- double negative (DN) thymocytes (which were also negative for Mac1 and Gr1) from leukemic NHD13/NP23 mice demonstrated that DN thymocytes could transmit AML, and limiting dilution studies showed that leukemia initiating cells were increased 14-fold in the thymus compared to BM. Further thymocyte fractionation demonstrated that DN1 and DN2, but not DN3 or DN4 fractions transmitted AML, and a marked expansion (100-fold) of Lineage-Sca1 + Kit + (LSK) cells in the thymus of the NP23-NHD13 mice. Taken together, these results show that the thymus of NP23-NHD13 mice acts as a reservoir for AML initiating cells and that thymic progenitors can transmit AML.

Dose-Rate-Dependent PU.1 Inactivation to Develop Acute Myeloid Leukemia in Mice Through Persistent Stem Cell Proliferation After Acute or Chronic Gamma Irradiation

Radiation-induced acute myeloid leukemia (rAML) in C3H mice is commonly developed through inactivation of PU.1 transcription factor encoded in Sfpi1 on chromosome 2. PU.1 inactivation involves two steps: hemizygous deletion of the Sfpi1 gene (DSG) and point mutation of the allele Sfpi1 gene (PMASG). In this study, we investigated the dose-rate dependence of the frequency of both DSG and PMASG in hematopoietic stem cells (HSCs) of C3H mice that received a total of 3 Gy gamma-ray exposure at dose rates of 20 mGy/day, 200 mGy/day or 1,000 mGy/min. All mice were followed for 250 days from start of irradiation. Fluorescent in situ hybridization of the Sfpi1 gene site indicated that frequency of HSCs with DSG was proportional to dose rate. In cell surface profiles, PU.1-inactivated HSCs by both DSG and PMASG were still positive for PU.1, but negative for GM-CSF receptor-α (GMCSFRα), which is transcriptionally regulated by PU.1. Immunofluorescent staining analysis of both PU.1 and GM-CSFRα also showed dose-rate-dependent levels of PU.1-inactivated HSCs. This study provides evidence that both DSG and PMASG are dose-rate dependent; these experimental data offer new insights into the dose-rate effects in HSCs that can lead to radiation-induced leukemogenesis.

Hematopoietic stem and progenitor cell responses to low radiation doses - implications for leukemia risk

Studies of the responses of hematopoietic stem and progenitor cells (HSPCs) to low doses of ionizing radiation formed an important aspect of the RISK-IR project ( www.risk-ir.eu ). A brief overview of these studies is presented here. The findings confirm the sensitivity of HSPCs to radiation even at low doses, and illustrate the substantial impact that differentiation state has upon cell sensitivity. The work provides mechanistic support for epidemiological findings of leukemia risk at dose levels used in diagnostic CT imaging, and further suggests that low-dose irradiation may facilitate bone marrow transplantation, a finding that could lead to refinements in clinical practice.

Transcriptomic and proteomic analysis of mouse radiation-induced acute myeloid leukaemia (AML)

A combined transcriptome and proteome analysis of mouse radiation-induced AMLs using two primary AMLs, cell lines from these primaries, another cell line and its in vivo passage is reported. Compared to haematopoietic progenitor and stem cells (HPSC), over 5000 transcriptome alterations were identified, 2600 present in all materials. 55 and 3 alterations were detected in the proteomes of the cell lines and primary/in vivo passage material respectively, with one common to all materials. In cell lines, approximately 50% of the transcriptome changes are related to adaptation to cell culture, and in the proteome this proportion was higher. An AML 'signature' of 17 genes/proteins commonly deregulated in primary AMLs and cell lines compared to HPSCs was identified and validated using human AML transcriptome data. This also distinguishes primary AMLs from cell lines and includes proteins such as Coronin 1, pontin/RUVBL1 and Myeloperoxidase commonly implicated in human AML. C-Myc was identified as having a key role in radiation leukaemogenesis. These data identify novel candidates relevant to mouse radiation AML pathogenesis, and confirm that pathways of leukaemogenesis in the mouse and human share substantial commonality.

Mouse models for radiation-induced cancers

Potential ionising radiation exposure scenarios are varied, but all bring risks beyond the simple issues of short-term survival. Whether accidentally exposed to a single, whole-body dose in an act of terrorism or purposefully exposed to fractionated doses as part of a therapeutic regimen, radiation exposure carries the consequence of elevated cancer risk. The long-term impact of both intentional and unintentional exposure could potentially be mitigated by treatments specifically developed to limit the mutations and precancerous replication that ensue in the wake of irradiation The development of such agents would undoubtedly require a substantial degree of in vitro testing, but in order to accurately recapitulate the complex process of radiation-induced carcinogenesis, well-understood animal models are necessary. Inbred strains of the laboratory mouse, Mus musculus, present the most logical choice due to the high number of molecular and physiological similarities they share with humans. Their small size, high rate of breeding and fully sequenced genome further increase its value for use in cancer research. This chapter will review relevant m. musculus inbred and F1 hybrid animals of radiation-induced myeloid leukemia, thymic lymphoma, breast and lung cancers. Method of cancer induction and associated molecular pathologies will also be described for each model.

PU.1 downregulation in murine radiation-induced acute myeloid leukaemia (AML): from molecular mechanism to human AML

The transcription factor PU.1, encoded by the murine Sfpi1 gene (SPI1 in humans), is a member of the Ets transcription factor family and plays a vital role in commitment and maturation of the myeloid and lymphoid lineages. Murine studies directly link primary acute myeloid leukaemia (AML) and decreased PU.1 expression in specifically modified strains. Similarly, a radiation-induced chromosome 2 deletion and subsequent Sfpi1 point mutation in the remaining allele lead to murine radiation-induced AML. Consistent with murine data, heterozygous deletion of the SPI1 locus and mutation of the −14kb SPI1 upstream regulatory element were described previously in human primary AML, although they are rare events. Other mechanisms linked to PU.1 downregulation in human AML include TP53 deletion, FLT3-ITD mutation and the recurrent AML1-ETO [t(8;21)] and PML-RARA [t(15;17)] translocations. This review provides an up-to-date overview on our current understanding of the involvement of PU.1 in the initiation and development of radiation-induced AML, together with recommendations for future murine and human studies.

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.

Radiation-induced myeloid leukemia in murine models

The use of radiation therapy is a cornerstone of modern cancer treatment. The number of patients that undergo radiation as a part of their therapy regimen is only increasing every year, but this does not come without cost. As this number increases, so too does the incidence of secondary, radiation-induced neoplasias, creating a need for therapeutic agents targeted specifically towards incidence reduction and treatment of these cancers. Development and efficacy testing of these agents requires not only extensive in vitro testing but also a set of reliable animal models to accurately recreate the complex situations of radiation-induced carcinogenesis. As radiation-induced leukemic progression often involves genomic changes such as rearrangements, deletions, and changes in methylation, the laboratory mouse Mus musculus, with its fully sequenced genome, is a powerful tool in cancer research. This fact, combined with the molecular and physiological similarities it shares with man and its small size and high rate of breeding in captivity, makes it the most relevant model to use in radiation-induced leukemia research. In this work, we review relevant M. musculus inbred and F₁ hybrid animal models, as well as methods of induction of radiation-induced myeloid leukemia. Associated molecular pathologies are also included.

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.

Frequency of acute myeloid leukaemia-associated mouse chromosome 2 deletions in X-ray exposed immature haematopoietic progenitors and stem cells

Exposure to ionising radiation can lead to an increased risk of cancer, particularly leukaemia. In radiation-induced acute myeloid leukaemia (rAML), a partial hemizygous deletion of mouse chromosome 2 is a common feature in several susceptible strains. The deletion is an early event detectable 24h after exposure in bone marrow cells using cytogenetic techniques. Expanding clones of bone marrow cells with chromosome 2 deletions can be detected less than a year after exposure to ionising radiation in around half of the irradiated mice. Ultimately, 15-25% of exposed animals develop AML. It is generally assumed that leukaemia originates in an early progenitor cell or haematopoietic stem cell, but it is unknown whether the original chromosome damage occurs at a similar frequency in committed progenitors and stem cells. In this study, we monitored the frequency of chromosome 2 deletions in immature bone marrow cells (Lin(-)) and haematopoietic stem cells/multipotent progenitor cells (LSK) by several techniques, fluorescent in situ hybridisation (FISH) and through use of a reporter gene model, flow cytometry and colony forming units in spleen (CFU-S) following ex vivo or in vivo exposure. We showed that partial chromosome 2 deletions are present in the LSK subpopulation, but cannot be detected in Lin(-) cells and CFU-S12 cells. Furthermore, we transplanted irradiated Lin(-) or LSK cells into host animals to determine whether specific irradiated cell populations acquire an increased proliferative advantage compared to unirradiated cells. Interestingly, the irradiated LSK subpopulation containing cells carrying chromosome 2 deletions does not appear to repopulate as well as the unirradiated population, suggesting that the chromosomal deletion does not provide an advantage for growth and in vivo repopulation, at least at early stages following occurrence.

Mouse models of radiation-induced cancers

Radiation-induced (RI) secondary cancers were not a major clinical concern even as little as 15 years ago. However, advances in cancer diagnostics, therapy, and supportive care have saved numerous lives and many former cancer patients are now living for 5, 10, 20, and more years beyond their initial diagnosis. The majority of these patients have received radiotherapy as a part of their treatment regimen and are now beginning to develop secondary cancers arising from normal tissue exposure to damaging effects of ionizing radiation. Because historically patients rarely survived past the extended latency periods inherent to these RI cancers, very little effort was channeled towards the research leading to the development of therapeutic agents intended to prevent or ameliorate oncogenic effects of normal tissue exposure to radiation. The number of RI cancers is expected to increase very rapidly in the near future, but the field of cancer biology might not be prepared to address important issues related to this phenomena. One such issue is the ability to accurately differentiate between primary tumors and de novo arising secondary tumors in the same patient. Another issue is the lack of therapeutic agents intended to reduce such cancers in the future. To address these issues, large-scale epidemiological studies must be supplemented with appropriate animal modeling studies. This work reviews relevant mouse (Mus musculus) models of inbred and F1 animals and methodologies of induction of most relevant radiation-associated cancers: leukemia, lymphoma, and lung and breast cancers. Where available, underlying molecular pathologies are included.

Mouse models for efficacy testing of agents against radiation carcinogenesis—a literature review

As the number of cancer survivors treated with radiation as a part of their therapy regimen is constantly increasing, so is concern about radiation-induced cancers. This increases the need for therapeutic and mitigating agents against secondary neoplasias. Development and efficacy testing of these agents requires not only extensive in vitro assessment, but also a set of reliable animal models of radiation-induced carcinogenesis. The laboratory mouse (Mus musculus) remains one of the best animal model systems for cancer research due to its molecular and physiological similarities to man, small size, ease of breeding in captivity and a fully sequenced genome. This work reviews relevant M. musculus inbred and F₁ hybrid animal models and methodologies of induction of radiation-induced leukemia, thymic lymphoma, breast, and lung cancer in these models. Where available, the associated molecular pathologies are also included.

Molecular characterisation of murine acute myeloid leukaemia induced by 56Fe ion and 137Cs gamma ray irradiation

Exposure to sparsely ionising gamma- or X-ray irradiation is known to increase the risk of leukaemia in humans. However, heavy ion radiotherapy and extended space exploration will expose humans to densely ionising high linear energy transfer (LET) radiation for which there is currently no understanding of leukaemia risk. Murine models have implicated chromosomal deletion that includes the hematopoietic transcription factor gene, PU.1 (Sfpi1), and point mutation of the second PU.1 allele as the primary cause of low-LET radiation-induced murine acute myeloid leukaemia (rAML). Using array comparative genomic hybridisation, fluorescence in situ hybridisation and high resolution melt analysis, we have confirmed that biallelic PU.1 mutations are common in low-LET rAML, occurring in 88% of samples. Biallelic PU.1 mutations were also detected in the majority of high-LET rAML samples. Microsatellite instability was identified in 42% of all rAML samples, and 89% of samples carried increased microsatellite mutant frequencies at the single-cell level, indicative of ongoing instability. Instability was also observed cytogenetically as a 2-fold increase in chromatid-type aberrations. These data highlight the similarities in molecular characteristics of high-LET and low-LET rAML and confirm the presence of ongoing chromosomal and microsatellite instability in murine rAML.

MYC oncogene in myeloid neoplasias

MYC is a transcription factor that regulates many critical genes for cell proliferation, differentiation, and biomass accumulation. MYC is one of the most prevalent oncogenes found to be altered in human cancer, being deregulated in about 50 % of tumors. Although MYC deregulation has been more frequently associated to lymphoma and lymphoblastic leukemia than to myeloid malignancies, a body of evidence has been gathered showing that MYC plays a relevant role in malignancies derived from the myeloid compartment. The myeloid leukemogenic activity of MYC has been demonstrated in different murine models. Not surprisingly, MYC has been found to be amplified or/and deregulated in the three major types of myeloid neoplasms: acute myeloid leukemia, myelodysplastic syndromes, and myeloproliferative neoplasms, including chronic myeloid leukemia. Here, we review the recent literature describing the involvement of MYC in myeloid tumors.

Nuclear EGFR suppresses ribonuclease activity of polynucleotide phosphorylase through DNAPK-mediated phosphorylation at serine 776

Nuclear existence of epidermal growth factor receptor (EGFR) has been documented for more than two decades. Resistance of cancer to radiotherapy is frequently correlated with elevated EGFR expression, activity, and nuclear translocation. However, the role of nuclear EGFR (nEGFR) in radioresistance of cancers remains elusive. In the current study, we identified a novel nEGFR-associated protein, polynucleotide phosphorylase (PNPase), which possesses 3' to 5' exoribonuclease activity toward c-MYC mRNA. Knockdown of PNPase increased radioresistance. Inactivation or knock-down of EGFR enhanced PNPase-mediated c-MYC mRNA degradation in breast cancer cells, and also increased its radiosensitivity. Interestingly, the association of nEGFR with PNPase and DNA-dependent protein kinase (DNAPK) increased significantly in breast cancer cells after exposure to ionizing radiation (IR). We also demonstrated that DNAPK phosphorylates PNPase at Ser-776, which is critical for its ribonuclease activity. The phospho-mimetic S776D mutant of PNPase impaired its ribonuclease activity whereas the nonphosphorylatable S776A mutant effectively degraded c-MYC mRNA. Here, we uncovered a novel role of nEGFR in radioresistance, and that is, upon ionizing radiation, nEGFR inactivates the ribonuclease activity of PNPase toward c-MYC mRNA through DNAPK-mediated Ser-776 phosphorylation, leading to increase of c-MYC mRNA, which contributes to radioresistance of cancer cells.

Myc roles in hematopoiesis and leukemia

Hematopoiesis is a process capable of generating millions of cells every second, as distributed in many cell types. The process is regulated by a number of transcription factors that regulate the differentiation along the distinct lineages and dictate the genetic program that defines each mature phenotype. Myc was first discovered as the oncogene of avian leukemogenic retroviruses; it was later found translocated in human lymphoma. From then on, evidence accumulated showing that c-Myc is one of the transcription factors playing a major role in hematopoiesis. The study of genetically modified mice with overexpression or deletion of Myc has shown that c-Myc is required for the correct balance between self-renewal and differentiation of hematopoietic stem cells (HSCs). Enforced Myc expression in mice leads to reduced HSC pools owing to loss of self-renewal activity at the expense of increased proliferation of progenitor cells and differentiation. c-Myc deficiency consistently results in the accumulation of HSCs. Other models with conditional Myc deletion have demonstrated that different lineages of hematopoietic cells differ in their requirement for c-Myc to regulate their proliferation and differentiation. When transgenic mice overexpress c-Myc or N-Myc in mature cells from the lymphoid or myeloid lineages, the result is lymphoma or leukemia. In agreement, enforced expression of c-Myc blocks the differentiation in several leukemia-derived cell lines capable of differentiating in culture. Not surprising, MYC deregulation is recurrently found in many types of human lymphoma and leukemia. Whereas MYC is deregulated by translocation in Burkitt lymphoma and, less frequently, other types of lymphoma, MYC is frequently overexpressed in acute lymphoblastic and myeloid leukemia, through mechanisms unrelated to chromosomal translocation, and is often associated with disease progression.

Cell surface marker phenotypes and gene expression profiles of murine radiation-induced acute myeloid leukemia stem cells are similar to those of common myeloid progenitors

Radiation exposure induces acute myeloid leukemia (AML) in humans and mice. Recent studies postulated that AML stem cells of spontaneous human AML arise from hematopoietic stem cells. However, other studies support the possibility that short-lived committed progenitors transform into AML stem cells, accompanied by a particular gene mutation. It remains unclear whether AML stem cells are present in radiation-induced AML, and information regarding AML-initiating cells is lacking. In this study, we identified and analyzed AML stem cells of mice with radiation-induced AML. The AML stem cells were identified by transplanting 100 bone marrow cells from mice with radiation-induced AML. We injected 100 cells of each of seven cell populations corresponding to different stages of hematopoietic cell differentiation and compared the latencies of AMLs induced in recipient mice. The identified radiation-induced AML stem cells frequently displayed similarities in both CD antigen and gene expression profiles with normal common myeloid progenitors. The number of common myeloid progenitor-like AML stem cells was significantly increased in mice with radiation-induced AML, but the progeny of common myeloid progenitors was decreased. In addition, analysis of radiation effects on the hematopoietic system showed that common myeloid progenitor cells were extremely radiosensitive and that their numbers remained at low levels for more than 2 months after radiation exposure. Our results suggest that murine radiation-induced AML stem cells arise from radiosensitive cells at a common myeloid progenitor stage.

Kinetics of blood glucose in mice carrying hemizygous Pax6

The genotype-phenotype relationship was examined experimentally for the Pax6(Sey-4H) mutant, which carries deletion of its chromosome 2 middle region hemizygously. The genotyping has indicated that this deleted segment is between 102.6 and 109.2 Mb from the centromere. The glucose-6-phosphatase gene followed by the glucagon and carboxyl ester lipase genes were mapped adjacent to the deleted region. Phenotyping indicates that the Pax6(Sey-4H) mutant is more susceptible to diabetes. The glucose tolerance test showed that the mutants were less capable of reducing their level of blood glucose to the standard level than the normal sibs. The insulin-loading test revealed their inability to elevate their blood glucose levels up to normal levels. The time it took for the onset of diabetes induced by streptozotocin was shorter in the mutants than in normal sibs. Both the haploinsufficiency of the genes in the hemizygous segment of chromosome 2 and the quantitative imbalance of the whole genome could contribute the development of this phenotype in the mutant.

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.