California Breast Cancer Prevention Initiatives: Setting a research agenda for prevention.

The environment is an underutilized pathway to breast cancer prevention. Current research approaches and funding streams related to breast cancer and the environment are unequal to the task at hand. We undertook the California Breast Cancer Prevention Initiatives, a four-year comprehensive effort to set a research agenda related to breast cancer, the environment, disparities and prevention. We identified 20 topics for Concept Proposals reflecting a life-course approach and the complex etiology of breast cancer; considering the environment as chemical, physical and socially constructed exposures that are experienced concurrently: at home, in the community and at work; and addressing how we should be modifying the world around us to promote a less carcinogenic environment. Redirecting breast cancer research toward prevention-oriented discovery could significantly reduce the incidence and associated disparities of the disease among future generations.

Target cell frequency is a genetically determined risk factor in radiation leukaemogenesis.

Whole body exposure to ionizing radiation increases the risk of radiation-induced acute myeloid leukaemia (r-AML). r-AML is the result of the accumulation of mutations in a single haemopoietic stem cell, so risk is therefore a function of the number of mutations required to transform the stem cell and the mutation rate. There is a genetic component to the risk of AML within the general population, and low penetrance variant alleles encoding DNA repair enzymes have been genetically implicated in therapy-related AML susceptibility. However, what is largely ignored is that target cell number, which defines the number of genomes at risk from DNA damaging agents, is also part of the equation that defines risk. We will review the evidence from genetic studies of inbred mouse models that target cell frequency is a risk factor in radiation leukaemogenesis. Inbred mouse strains that differ in their susceptibility to radiation-induced r-AML and thymic lymphoma (r-TL), spontaneous TL and pristane-induced plasmacytoma (PCT) have been exploited to identify susceptibility loci. The target cell in AML is the haemopoietic stem cell, whereas TLs and PCT arise from more mature lymphoid progenitor cells. Inbred mice also differ significantly in all aspects of haemopoiesis, and these differences have been used to identify quantitative trait loci (QTL) that determine the frequency of specific haemopoietic stem, progenitor or mature blood cells. The co-localization of QTL that determine risk and target cell frequency in all three haemopoietic malignancies is strong evidence that target cell frequency is a risk factor in radiation leukaemogenesis.

The late radiotherapy normal tissue injury phenotypes of telangiectasia, fibrosis and atrophy in breast cancer patients have distinct genotype-dependent causes.

The relationship between late normal tissue radiation injury phenotypes in 167 breast cancer patients treated with radiotherapy and: (i) radiotherapy dose (boost); (ii) an early acute radiation reaction and (iii) genetic background was examined. Patients were genotyped at single nucleotide polymorphisms (SNPs) in eight candidate genes. An early acute reaction to radiation and/or the inheritance of the transforming growth factor-beta1 (TGFbeta1 -509T) SNP contributed to the risk of fibrosis. In contrast, an additional 15 Gy electron boost and/or the inheritance of X-ray repair cross-complementing 1 (XRCC1) (R399Q) SNP contributed to the risk of telangiectasia. Although fibrosis, telangiectasia and atrophy, all contribute to late radiation injury, the data suggest that they have distinct underlying genetic and radiobiological causes. Fibrosis risk is associated with an inflammatory response (an acute reaction and/or TGFbeta1), whereas telangiectasia is associated with vascular endothelial cell damage (boost and/or XRCC1). Atrophy is associated with an acute response, but the genetic predisposing factors that determine the risk of an acute response or atrophy have yet to be identified. A combined analysis of two UK breast cancer patient studies shows that 8% of patients are homozygous (TT) for the TGFbeta1 (C-509T) variant allele and have a 15-fold increased risk of fibrosis following radiotherapy (95% confidence interval: 3.76-60.3; P=0.000003) compared with (CC) homozygotes.

Mouse bone marrow and peripheral blood erythroid cell counts are regulated by different autosomal genetic loci.

Erythropoiesis is under fine control and genetic loci that affect it are likely to be important in a range of conditions. To assess the relative contributions of different genetic loci to parameters of erythropoiesis, we have measured RBC counts in the peripheral circulation and committed erythroid cells (RBC and small normoblasts) in the bone marrow in a cohort of (CBA/H x C57BL/6) F2 mice to map quantitative trait loci (QTL). Candidate genes were assessed using bioinformatics and DNA sequencing. Different autosomal loci affect bone marrow (chromosomes 5, 11 and 19) and peripheral blood (chromosome 4) erythroid cell counts but there may be a common chromosome X locus. Spleen weight QTL were found on chromosomes 3, 15 and 17. Surprisingly, erythropoietin (Epo) is the best candidate quantitative trait gene (QTG) in the chromosome 5 locus that affects bone marrow but not peripheral blood erythroid cell counts. Epo gene expression is known to be genetically regulated in mice, but our data suggest a tissue-specific role for epo in mouse erythropoiesis that is also genetically determined. The identity of the other QTG will be important both to further knowledge of the control of erythropoiesis and as potential modifier genes for haematological disorders.

The response of elderly human articular cartilage to mechanical stimuli in vitro.

OBJECTIVE: To investigate the biosynthetic response of elderly human femoral head articular cartilage to mechanical stimulation in vitro and its variation with site. METHOD: Full-depth cartilage biopsies of articular cartilage were removed from defined sites on 10 femoral heads from patients aged 68-95 years. Cartilage explants were subjected to either static or cyclic (2s on/2s off) loading in unconfined compression at a stress of 1MPa for 24h, or no load. Metabolic activity was assessed by adding medium containing (35)S-sulphate and (3)H-leucine during the last 4h of loading and measuring the incorporated radioisotope. Matrix composition was measured in terms of the amounts of collagen, sulphated glycosaminoglycans (GAG) and water content. RESULTS: Loading of elderly human articular cartilage at 1MPa significantly inhibited incorporation of (35)S-sulphate (P=0.023) into cartilage explants. Pairwise comparisons showed that the difference in incorporation was only for static loading (43% decrease compared to unloaded) (P<0.05). (3)H-leucine incorporation appeared to follow the same trends but neither static nor cyclic load was significantly different from control (P=0.31). Significant topographical variation was found for % GAG wet and GAG:collagen but not water content, % GAG dry or collagen. Isotope incorporation rates were in the order anterior>superior>posterior. CONCLUSION: Static loading inhibits matrix biosynthesis in elderly human cartilage, and cyclic loading is not stimulatory. This is in contrast to previous studies on young bovine tissue where cyclic loading is stimulatory.

Evidence of genetic instability in 3 Gy X-ray-induced mouse leukaemias and 3 Gy X-irradiated haemopoietic stem cells.

PURPOSE: If radiation-induced genetic instability is causal in mouse radiation leukaemogenesis, then genetic instability should be detectable in the irradiated target untransformed haemopoietic stem cell, and evidence of genetic instability detected in the clonal radiation-induced leukaemia. We have tested this hypothesis using the CBA/H mouse model of radiation-induced acute myeloid leukaemia (r-AML). MATERIALS AND METHODS: Fluorescence in situ hybridisation (FISH) was employed to screen for chromosomal aberrations in mouse 3 Gy X-ray-induced r-AMLs and in the clonal descendents of control and 3 Gy X-irradiated bone marrow haemopoietic stem cells using the in vitro clonogenic CFU-A colony assay. RESULTS: High levels of clonal non-specific chromosomal aberrations were detected in the r-AML (approximately 4-5 aberrations/r-AML), and ongoing chromosomal instability as defined by subclonal variants detected in 5/10 r-AML. A similar analysis of CFU-A colonies revealed chromosomal aberrations in 25% of colonies derived from irradiated bone marrow (2% in controls). However, 66% of the aberrant colonies (2% in controls) exhibited ongoing genetic instability as defined by non-clonal chromosomal aberrations. Overall, 6% (121/1884) of the CFU-A cells derived from irradiated bone marrow were aberrant (0.05% in controls) of which 12% (15/121) were subclonal. No one CFU-A cell exhibited aberrations on more than one of the three chromosomes painted. CONCLUSIONS: The high levels of non-specific genetic damage observed in the r-AMLs is therefore attributed to the accumulation of genetic lesions in the target haemopoietic stem cell over a longer time-scale after exposure than assessed in the in vitro CFU-A clonogenic assay. This is consistent with the long latency of the multi-stage radiation leukaemogenic process, and a role for radiation-induced genetic instability is inferred.

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.

Allelic loss on chromosome 4 (Lyr2/TLSR5) is associated with myeloid, B-lympho-myeloid, and lymphoid (B and T) mouse radiation-induced leukemias.

The CBA/H mouse model of radiation-induced acute myeloid leukemia (AML) was re-examined using molecular approaches. In addition to the typical promyelocytic AMLs, 34% were reclassified as early pre-B lympho-myeloid leukemias (L-ML) based on leukemic blood cell morphology, immunoglobulin heavy-chain gene re-arrangements (IgH(R)), or expression of both lymphoid (Vpre-B1 and Rag1) and myeloid (myeloperoxidase and lysozyme M) genes. Allelic loss on chromosome 4 was frequently detected in AMLs (53%) and L-MLs (more than 95%), and the preferential loss of the maternally transmitted allele suggests the locus may be imprinted. A minimally deleted region (MDR) maps to a 3.4-cM interval, which is frequently deleted in radiation-induced thymic lymphomas (TLSR5) and contains a recessive, maternally transmitted genetic locus (Lyr2) that confers resistance to spontaneous and radiation-induced pre-B and T cell lymphomas, suggesting they are one and the same. Thus, the Lyr2/TLSR5 locus is frequently implicated in myeloid, lymphoid (B and T), and mixed-lineage mouse leukemias and lymphomas. Epigenetic inactivation of one Lyr2/TLSR5 allele during normal mouse development suggests that only a single hit is required for its inactivation during leukemogenesis, and this may be a significant contributing factor to the efficiency of the leukemogenic process in the mouse.

Undercounts and overstatements: will the IOM report on lesbian health improve research?

In January 1999, the Institute of Medicine (IOM) released a report on lesbian health research that fulfills 3 goals: it provides an extensive review of much of the research that has been done on the health of women who have sex with other women, it addresses the methodological and ethical issues inherent in conducting research on this population, and it suggests avenues for further research. This report will likely help lesbian health researchers gain funding, publish further research in medical journals, and receive support and validation from medical and research institutions. To ensure that such research is useful, benefits the lesbian community, and expands the understanding of lesbian health conditions, particular attention needs to be paid to the methods and definitions used and to the involvement of the lesbian community in designing, implementing, and analyzing the research itself.

Susceptibility to radiation-induced leukaemia/lymphoma is genetically separable from sensitivity to radiation-induced genomic instability.

PURPOSE: To determine whether there is a relationship between the genetics underlying the susceptibility to radiation-induced leukaemia in CBA/H (acute myeloid leukaemia, AML) and C57BL/6 (thymic lymphoma, TL) mice, and the genetics underlying the sensitivity of CBA/H (sensitive) and C57BL/6 (resistant) mice to radiation-induced chromosomal instability. MATERIALS AND METHODS: CBA/H, (CBA/H x C57BL/6)F1, F1 x CBA/H, F1 x C57BL/6 and F1 x F1 mice were exposed to a single acute dose of 3.0 Gy X-rays. AML and TL were diagnosed over the subsequent 30 months. RESULTS: There was no statistically significant difference in the incidence of AML in F1, F1 x F1, F1 x CBA/H and F1 x C57BL/6 mice, which was approximately 50% that in CBA/H mice. AML susceptibility is therefore a dominant polygenic trait, and both susceptibility and resistance (variable penetrance) CBA/H and C57BL/6 loci are involved. The incidence of TL in the FM and F1 x CBA/H mice was negligible, indicating that TL susceptibility is a recessive trait. As the TL incidence in the F1 x C57BL/6 mice was about half that in C57BL/6 mice, one recessive locus is probably involved. CONCLUSIONS: AML susceptibility in CBA/H mice is a dominant trait in contrast to the recessive inheritance of CBA/H sensitivity to radiation-induced chromosomal instability. TL-susceptibility in C57BL/6 is a recessive trait in contrast to the dominant inheritance of C57BL/6 resistance to radiation-induced chromosomal instability.

No correlation between germline mutation at repeat DNA and meiotic crossover in male mice exposed to X-rays or cisplatin.

To test the hypothesis that mouse germline expanded simple tandem repeat (ESTR) mutations are associated with recombination events during spermatogenesis, crossover frequencies were compared with germline mutation rates at ESTR loci in male mice acutely exposed to 1Gy of X-rays or to 10mg/kg of the anticancer drug cisplatin. Ionising radiation resulted in a highly significant 2.7-3.6-fold increase in ESTR mutation rate in males mated 4, 5 and 6 weeks after exposure, but not 3 weeks after exposure. In contrast, irradiation had no effect on meiotic crossover frequencies assayed on six chromosomes using 25 polymorphic microsatellite loci spaced at approximately 20cM intervals and covering 421cM of the mouse genome. Paternal exposure to cisplatin did not affect either ESTR mutation rates or crossover frequencies, despite a report that cisplatin can increase crossover frequency in mice. Correlation analysis did not reveal any associations between the paternal ESTR mutation rate and crossover frequency in unexposed males and in those exposed to X-rays or cisplatin. This study does not, therefore, support the hypothesis that mutation induction at mouse ESTR loci results from a general genome-wide increase in meiotic recombination rate.

Human complement protein C8 gamma.

Human C8 gamma is a 22 kDa subunit of complement component C8, which is one of five components (C5b, C6, C7, C8, C9) that interact to form the cytolytic membrane attack complex (MAC) of complement. C8 contains three nonidentical subunits (alpha, beta, gamma) that are products of different genes. These subunits are arranged asymmetrically to form a disulfide-linked C8 alpha-gamma dimer that is noncovalently associated with C8 beta. C8 alpha and C8 beta are homologous to C6, C7 and C9 and together these proteins comprise what is referred to as the 'MAC protein family'. By comparison, C8 gamma is distinct in that it belongs to the lipocalin family of small, secreted proteins which have the common ability to bind small hydrophobic ligands. While specific roles have been identified for C8 alpha and C8 beta in the formation and function of the MAC, a function for C8 gamma and the identity of its ligand are unknown. This review summarizes the current status of C8 gamma structure and function and the progress made from efforts to determine its role in the complement system.

An indel within the C8 alpha subunit of human complement C8 mediates intracellular binding of C8 gamma and formation of C8 alpha-gamma.

Human C8 is one of five complement components (C5b, C6, C7, C8, and C9) that interact to form the cytolytic membrane attack complex, or MAC. It is an oligomeric protein composed of three subunits (C8alpha, C8beta, C8gamma) that are products of different genes. In C8 from serum, these are arranged as a disulfide-linked C8alpha-gamma dimer that is noncovalently associated with C8beta. In this study, the site on C8alpha that mediates intracellular binding of C8gamma to form C8alpha-gamma was identified. From a comparative analysis of indels (insertions/deletions) in C8alpha and its structural homologues C8beta, C6, C7, and C9, it was determined that C8alpha contains a unique insertion (residues 159-175), which includes Cys(164) that forms the disulfide bond to C8gamma. Incorporation of this sequence into C8beta and coexpression of the resulting construct (iC8beta) with C8gamma produced iC8beta-gamma, an atypical disulfide-linked dimer. In related experiments, C8gamma was shown to bind noncovalently to mutant forms of C8alpha and iC8beta in which Cys(164)-->Gly(164) substitutions were made. In addition, C8gamma bound specifically to an immobilized synthetic peptide containing the mutant indel sequence. Together, these results indicate (a) intracellular binding of C8gamma to C8alpha is mediated principally by residues contained within the C8alpha indel, (b) binding is not strictly dependent on Cys(164), and (c) C8gamma must contain a complementary binding site for the C8alpha indel.

Induction of minisatellite mutations in the mouse germline by low-dose chronic exposure to gamma-radiation and fission neutrons.

Germline mutation induction at mouse minisatellite loci by paternal low-dose (0.125-1 Gy) exposure to chronic (1.66 x 10(-4) Gy min(-1)) low-linear energy transfer (low-LET) gamma-irradiation and high-LET fission neutrons (0.003 Gy min(-1)) was studied at pre-meiotic stages of spermatogenesis. Both types of radiation produced linear dose-response curves for mutation of the paternal allele. In contrast to previous results using higher doses, the pattern of induction of minisatellite mutation after chronic gamma-irradiation was similar to acute (0.5 Gy min(-1)) exposure to X-rays, indicating that the elevated mutation rate was independent of the ability of the cell to repair damage induced immediately or over a period of up to 100 h. Chronic exposure to fission neutrons was more effective than acute or chronic low-LET exposure (relative biological effectiveness, RBE=3.36). The data also provide strong support for the previous conclusion that increases in minisatellite mutation rate are not caused by radiation-induced DNA damage at minisatellite loci themselves, but rather from damage induced by ionising radiation elsewhere in the genome/cell.

Allelic loss and promoter hypermethylation of the p15INK4b gene features in mouse radiation-induced lymphoid - but not myeloid - leukaemias.

Mouse radiation-induced acute myeloid leukaemias (AMLs) which arose in a (CBA/H x C57BL/6) genetic background have a 45% incidence of loss of heterozygosity (LOH) on chromosome 4. Frequent chromosome 4 LOH in mouse radiation-induced (C57BL/6 x RF/J) thymic lymphomas (TLs) is associated with promoter/exon 1 region hypermethylation of the remaining p15INK4b and p16INK4a alleles, so this may be common to mouse radiation myeloid and lymphoid leukaemogenesis. We addressed the question of p15INK4b/p16INK4a/p19ARF gene promoter hypermethylation in radiation-induced AMLs by comparison to TLs which arose in a similar (C57BL/6 x CBA/H) genetic background as a consequence of the same initiating dose of 3 Gy X-rays. Only one homozygous deletion was detected in the approximately 100 leukaemias analysed. p15INK4b gene promoter/exon 1 hypermethylation was readily detected (21%) in the lymphoid but not myeloid (3.1%) leukaemias, and p16INK4a and p19ARF gene promoter/exon 1 methylation was rare (<3%) in both. Thus, allelic loss and promoter hypermethylation of the p15INK4b gene is particular to radiation-induced lymphoid leukaemias and is independent of p16INK4a and p19ARF gene promoter/exon 1 hypermethylation.

Specificity of loss of heterozygosity in radiation-induced mouse myeloid and lymphoid leukaemias.

PURPOSE: To determine whether loss of heterozygosity (LOH) at specific chromosomal loci in radiation-induced leukaemias, arising in a similar genetic background, is leukaemia-type specific (myeloid versus lymphoid) or common to both. MATERIALS AND METHODS: Leukaemias that arose in 3 Gy X-irradiated (CBA/H x C57BL/6)F1 intercross and backcross mice were diagnosed as acute myeloid leukaemia (AML) or thymic lymphoma (TL). LOH was determined using 28 polymorphic microsatellite markers distributed over seven chromosomes using control and leukaemic DNA from individual mice. RESULTS: LOH incidences of 0-20% were observed at most loci in both leukaemia types. Specific LOH incidences of 38-76% were observed for myeloid (chromosome 2) and lymphoid (chromosomes 11 and 14) leukaemias. Chromosome 4 LOH was frequently (38-50%) observed in both types, although the commonly deleted regions differed. LOH was detected at either chromosome 2 or 4 in AML and either chromosome 4 or 11 in TL. CONCLUSIONS: LOH incidences of 38-76% suggest a causal role of particular loci which is mainly, but not exclusively, dependent on leukaemia type. LOH incidences of 0-20% at other loci in both leukaemias suggest that many genetic deletions are non-causal and incidental in radiation-leukaemogenesis.

An E mu-BCL-2 transgene facilitates leukaemogenesis by ionizing radiation.

Clonogenic murine B cell precursors are normally ultrasensitive to apoptosis following genotoxic exposure in vitro but can be protected by expression of an E mu-BCL-2 transgene. Such exposures are likely to be mutagenic. This in turn suggests that a level of in vivo genotoxic exposure that usually has minimal pathological consequences might become leukaemogenic when damaged cells fail to abort by apoptosis. If this were to be the case, then the cell type that becomes leukaemic and the chromosomal/molecular changes that occur would also be of considerable interest. We tested this possibility by exposing E mu-BCL-2 and wild-type mice of differing ages to a single dose of X-irradiation of 1-4 Gy. Young (approximately 4-6 weeks) transgenic mice developed leukaemia at a high rate following exposure to 2 Gy but adult mice (4-6 months) did not. Exposure to 4 Gy produced leukaemia in both young and adult transgenic mice but at a higher frequency in the former. Leukaemic cell populations showed clonal rearrangements of the IGH gene but in most cases analysed had immunophenotypic features of an early B lympho-myeloid progenitor population which has not previously been recorded in radiation leukaemogenesis. Molecular cytogenetic analysis of leukaemic cells by banded karyotype and FISH revealed a consistent double abnormality: trisomy 15 plus an interstitial deletion of chromosome 4 that was confirmed by LOH analysis.

Chimeric and truncated forms of human complement protein C8 alpha reveal binding sites for C8 beta and C8 gamma within the membrane attack complex/perforin region.

Human C8 is one of five components of the membrane attack complex of complement. It is an oligomeric protein composed of three subunits (C8 alpha, C8 beta, and C8 gamma) that are derived from different genes. C8 alpha and C8 beta are homologous and both contain a pair of tandemly arranged N-terminal modules [thrombospondin type 1 (TSP1) + low-density lipoprotein receptor class A (LDLRA)], an extended middle segment referred to as the membrane attack complex/perforin region (MACPF), and a pair of C-terminal modules [epidermal growth factor (EGF) + TSP1]. During biosynthetic processing, C8 alpha and C8 gamma associate to form a disulfide-linked dimer (C8 alpha-gamma) that binds to C8 beta through a site located on C8 alpha. In this study, the location of binding sites for C8 beta and C8 gamma and the importance of the modules in these interactions were investigated by use of chimeric and truncated forms of C8 alpha in which module pairs were either exchanged for those in C8 beta or deleted. Results show that exchange or deletion of one or both pairs of modules does not abrogate the ability of C8 alpha to form a disulfide-linked dimer when coexpressed with C8 gamma in COS cells. Furthermore, each chimeric and truncated form of C8 alpha-gamma retains the ability to bind C8 beta; however, only those containing the TSP1 + LDLRA modules from C8 alpha are hemolytically active. These results indicate that binding sites for C8 beta and C8 gamma reside within the MACPF region of C8 alpha and that interaction with either subunit is not dependent on the modules. They also suggest that the N-terminal modules in C8 alpha are important for C9 binding and/or expression of C8 activity.

DNA-PK activation by ionizing radiation-induced DNA single-strand breaks.

PURPOSE: To assess the ability of 60Co gamma-radiation-induced plasmid DNA single-strand breaks (gamma-ssb) to activate the DNA-dependent protein kinase (DNA-PK) in vitro. MATERIALS AND METHODS: Plasmid DNA was gamma-irradiated under aerobic conditions to yield 0-6 gamma-ssb and <0.1 double-strand breaks (dsb) per plasmid molecule. The irradiated DNA was used to stimulate DNA-PK in crude HF19 fibroblast nuclear extracts and/or purified HeLa cell DNA-PK protein, and the activation compared with that obtained with a single enzymatically generated plasmid DNA ssb (GpII endonuclease) or dsb (EcoRI endonuclease). RESULTS: Gamma-Irradiated plasmid DNA activates DNA-PK in both crude and purified preparations and the kinase activity increases linearly with dose. As significant DNA-PK activation was detectable using irradiated plasmids which contain <0.1 dsb/molecule, it was concluded that this activation is due to gamma-ssb. However, using purified DNA-PK, this activation is relatively weak as approximately 3 approximately-ssb is equivalent to one GpII-generated DNA ssb or one end of an EcoRI-generated dsb in DNA-PK assays. CONCLUSIONS: As gamma-ssb are in a approximately 20-fold excess of approximately-dsb in vivo for low LET radiation, gamma-ssb may contribute significantly to DNA-PK signalling of gamma-radiation-induced DNA damage in vivo.