DNA-dependent kinase (p350) as a candidate gene for the murine SCID defect

DNA-PKcs: a T-cell tumour suppressor encoded at the mouse scid locus

Severe combined immunodeficiency (SCID) mice are defective in their ability to rearrange their variable (V), diversity (D) and joining (J) genetic elements to generate functional immunoglobulin (Ig) and T-cell receptor (TCR) molecules; as a result, they lack mature B and T cells. These mice are highly sensitive to ionizing radiation, suggesting that the product of the scid gene plays a critical role in both V(D)J recombination and DNA double-strand break repair. Recent studies suggest that the SCID defect lies in the gene encoding the catalytic subunit of DNA-dependent protein kinase (DNA-PK; refs 6-8), a nuclear protein made up of the Ku 70 and Ku 86 subunits as well as the large catalytic subunit, DNA-PKcs. Other reports have implied that the SCID phenotype correlates with nonsense mutations at the extreme 3' end of Prkdc, the DNA-PKcs gene. The identity of the gene remains in doubt, however, because the consequences of genetic inactivation of Prkdc have not been determined. This study shows that complete inactivation of Prkdc in a novel insertional mouse mutant recapitulates the SCID phenotype and that Prkdc and scid are alleic. Significantly, DNA-PKcs null mice demonstrate complete penetrance of thymic lymphoblastic lymphomas, strongly suggesting that Prkdc functions in mice as a T-cell tumour suppressor and, by virtue of its association with DNA repair and recombination, belongs to the 'caretaker' class of tumour-suppressor genes that includes ATM, BRCA1 and BRCA2 (ref. 15).

RAG-1 and RAG-2-dependent assembly of functional complexes with V(D)J recombination substrates in solution

V(D)J recombination is initiated by RAG-1 and RAG-2, which introduce double-strand DNA breaks at recombination signal sequences (RSSs) of antigen receptor gene segments to produce signal ends, terminating in blunt, double-strand breaks, and coding ends, terminating in DNA hairpins. While the formation of RAG-RSS complexes has been documented, observations regarding the individual contributions of RAG-1 and RAG-2 to RSS recognition are in conflict. Here we describe an assay for formation and maintenance of functional RAG-RSS complexes in the course of the DNA cleavage reaction. Under conditions of in vitro cleavage, the RAG proteins sequester intact substrate DNA in a stable complex which is formed prior to strand scission. The cleavage reaction subsequently proceeds through nicking and hairpin formation without dissociation of substrate. Notably, the presence of both RAG-1 and RAG-2 is essential for formation of stable, functional complexes with substrate DNA under conditions of the sequestration assay. Two classes of substrate mutation are distinguished by their effects on RAG-mediated DNA cleavage in vitro. A mutation of the first class, residing within the RSS nonamer and associated with coordinate impairment of nicking and hairpin formation, greatly reduces the stability of RAG association with intact substrate DNA. In contrast, a mutation of the second class, lying within the RSS heptamer and associated with selective abolition of hairpin formation, has little or no effect on the half-life of the RAG-substrate complex.

V(D)J recombination: in vitro coding joint formation

Antigen receptor genes are assembled through a mechanism known as V(D)J recombination, which involves two different joining reactions: signal and coding joining. Formation of these joints is essential for antigen receptor assembly as well as maintaining chromosomal integrity. Here we report on a cell-free system for coding joint formation using deletion and inversion recombination substrates. In vitro coding joint formation requires RAG1, RAG2, and heat-labile factors present in the nuclear extract of nonlymphoid cells. Both inversion- and deletion-mediated coding joint reactions produce diverse coding joints, with deletions and P nucleotide addition. We also show that deletion-mediated coding joint formation follows the 12/23 rule and requires the catalytic subunit of DNA-dependent protein kinase.

Increased sensitivity of scid heterozygous mice to ionizing radiation

In the present study, acute effects of ionizing radiation on animal survival, bone marrow cells and fibroblast cell lines of scid homozygous, scid heterozygous and wild-type mice with the same C.B-17 genetic background were examined. The sensitivities to ultraviolet light (UV) and various chemicals, bleomycin, mitomycin C, N-methyl-N'-nitro-N-nitrosoguanidine, methyl methanosulphonate, 5-fluorouracil, 6-mercaptopurine, 4-nitroquinoline 1-oxide and potassium bromate) were also investigated. In addition, micronucleus testing of whole-body irradiated mice was performed. Scid heterozygous mice were found to be less sensitive than the homozygotes but more sensitive to ionizing radiation than wild-type mice, not only in vivo but also for bone marrow cells in vitro, suggesting partial dominance under both conditions. In contrast, there were no differences in sensitivity to UV light and various chemicals, as compared with wild-type and scid heterozygous cell lines, either in vitro or in the micronucleus test.

Nicking is asynchronous and stimulated by synapsis in 12/23 rule-regulated V(D)J cleavage

The first step in DNA cleavage at V(D)J recombination signals by RAG1 and RAG2 is creation of a nick at the heptamer/coding flank border. Under proper conditions in vitro the second step, hairpin formation, requires two signals with spacers of 12 and 23 bp, a restriction referred to as the 12/23 rule. Under these conditions hairpin formation occurs at the two signals at or near the same time. In contrast, we find that under the same conditions nicking occurs at isolated signals and hence is not subject to the 12/23 rule. With two signals the nicking events are not concerted and the signal with a 12 bp spacer is usually nicked first. However, the extent and rate of nicking at a given signal are diminished by mutations of the other signal. The appearance of DNA nicked at both signals is stimulated by more than an order of magnitude by the ability of the signals to synapse, indicating that synapsis accelerates nicking and often precedes it. These observations allow formulation of a more complete model of catalysis of DNA cleavage and how the 12/23 rule is enforced.

Growth retardation and leaky SCID phenotype of Ku70-deficient mice

Ku70, Ku80, and DNA-PKcs are subunits of the DNA-dependent protein kinase (DNA-PK), an enzyme implicated in DNA double-stranded break repair and V(D)J recombination. Our Ku70-deficient mice were about 50% the size of control littermates, and their fibroblasts were ionizing radiation sensitive and displayed premature senescence associated with the accumulation of nondividing cells. Ku70-deficient mice lacked mature B cells or serum immunoglobulin but, unexpectedly, reproducibly developed small populations of thymic and peripheral alpha/beta T lineage cells and had a significant incidence of thymic lymphomas. In association with B and T cell developmental defects, Ku70-deficient cells were severely impaired for joining of V(D)J coding and recombination signal sequences. These unanticipated features of the Ku70-deficient phenotype with respect to lymphocyte development and V(D)J recombination may reflect differential functions of the three DNA-PK components.

DNA damage-induced phosphorylation of p53 alleviates inhibition by MDM2

DNA-damaging agents signal to p53 through as yet unidentified posttranscriptional mechanisms. Here we show that phosphorylation of human p53 at serine 15 occurs after DNA damage and that this leads to reduced interaction of p53 with its negative regulator, the oncoprotein MDM2, in vivo and in vitro. Furthermore, using purified DNA-dependent protein kinase (DNA-PK), we demonstrate that phosphorylation of p53 at serines 15 and 37 impairs the ability of MDM2 to inhibit p53-dependent transactivation. We present evidence that these effects are most likely due to a conformational change induced upon phosphorylation of p53. Our studies provide a plausible mechanism by which the induction of p53 can be modulated by DNA-PK (or other protein kinases with similar specificity) in response to DNA damage.

Ku80 gene expression is Sp1-dependent and sensitive to CpG methylation within a novel cis element

The Ku70/80 complex, known as Ku, constitutes the DNA end binding component of the DNA-dependent protein kinase (DNA-PK). We have characterized the promoter region of the mouse and human Ku80 genes to delineate transcriptional elements necessary for basal gene expression and proliferation-dependent regulation. Consensus Sp1 recognition elements were identified in both promoters, and were determined to be essential for basal expression. We further identified a near-perfect palindrome of 21 base pairs located immediately 5' to one Sp1 element. This sequence was present once within the mouse Ku80 promoter and seven times, in a head-to-tail tandem array, within the human Ku80 promoter. This sequence possessed homology with a methylation-sensitive promoter element, Enh2, present in the LTR of mouse intractisternal A-particles. Promoter deletion studies and expression analysis of in-vitro methylated reporter gene constructs provided strong evidence that, in vivo, this repeat sequence regulates Ku80 gene expression in cis, through a mechanism involving CpG methylation. Evidence is also presented, suggesting that Ku is directly involved in this regulatory process.

p53-dependent apoptosis and transcription of p21waf/cip1/sdi1 in SCID mice following gamma-irradiation

The recruitment and activation of DNA-repair mechanisms at the sites of DNA-damage after exposure of cells to genotoxic stress are poorly understood. The DNA-dependent kinase (DNA-PK) was considered to be a likely candidate for initiating these events because of the conditions required for its activation, its phosphorylation of p53 in vitro and the extreme radiosensitivity induced by its inactivation in vivo. We analyzed irradiation-induced p53-activation in SCID mice, which lack DNA-PK activity, and found that p53-dependent apoptosis and p21waf/cip1/sdi1 transcription in these animals are at least as efficient as in wild-type mice. Thus, our results show that DNA-PK is not the main sensor for genotoxic stress and is not required for p53 activation. In fact, they rather suggest that DNA-PK may play a role in p53 down-regulation.

Minisatellite instability in severe combined immunodeficiency mouse cells

We have recently found that okadaic acid, which shows strong inhibitory activity on protein serine/threonine phosphatases and tumor-promoting activity in vivo and in vitro, induces minisatellite mutation (MSM). Human tumors and chemically induced counterparts in experimental animals are also sometimes associated with MSM. In the present study, we demonstrated minisatellite (MS) instability in severe combined immunodeficiency (SCID) cells in which the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is impaired. Cells from a SCID fibroblast cell line transformed by simian virus 40 large tumor antigen, SC3VA2, and from an embryonal SCID fibroblast cell line, SC1K, were cloned and propagated to 10(7) to 10(8) cells, and then subjected to subcloning. After propagation of each subclone to 10(7) to 10(8) cells, DNA samples were digested with HinfI and analyzed by Southern blotting using the Pc-1 MS sequence as a probe. Under low-stringency conditions, about 40 MS bands were detected, with 45% +/- 6% and 37% +/- 3% of SC3VA2 and SC1K cells, respectively, having MSM. In contrast, cells from the RD13B2 cell line, which was established from SCVA2 by introducing human chromosome 8q fragments, on which DNA-PKcs is known to reside, to complement the SCID phenotype, showed a very low frequency of MSM (3% +/- 3%). The high frequencies of MSM in SC3VA2 and SC1K were significant, with no difference between the two. The present study clearly demonstrates that MS instability exists in SCID fibroblasts, suggesting that DNA-PKcs might be involved in the stable maintenance of MS sequences in the genome.

Ku70 is required for DNA repair but not for T cell antigen receptor gene recombination In vivo

Ku is a complex of two proteins, Ku70 and Ku80, and functions as a heterodimer to bind DNA double-strand breaks (DSB) and activate DNA-dependent protein kinase. The role of the Ku70 subunit in DNA DSB repair, hypersensitivity to ionizing radiation, and V(D)J recombination was examined in mice that lack Ku70 (Ku70(-/-)). Like Ku80(-/-) mice, Ku70(-/-) mice showed a profound deficiency in DNA DSB repair and were proportional dwarfs. Surprisingly, in contrast to Ku80(-/-) mice in which both T and B lymphocyte development were arrested at an early stage, lack of Ku70 was compatible with T cell receptor gene recombination and the development of mature CD4+CD8- and CD4-CD8+ T cells. Our data shows, for the first time, that Ku70 plays an essential role in DNA DSB repair, but is not required for TCR V(D)J recombination. These results suggest that distinct but overlapping repair pathways may mediate DNA DSB repair and V(D)J recombination.

Molecular processes and radiosensitivity

BACKGROUND

DNA double-strand breaks (DSB) are the most genotoxic lesions induced by ionizing radiation. At least 2 different pathways for DSB repair have been identified, homologous and non-homologous recombination.

METHODS

Studies on X-ray-sensitive mutants have led to the identification of several genes involved in processing of DSB in bacteria, yeast and mammalian cells.

RESULTS AND CONCLUSION

In mammalian cells non-homologous recombination is the main pathway for DSB repair, while the role of homologous recombination in DSB repair awaits clarification. It is known that, in addition to DNA repair, other safeguards control the human cellular response to ionizing radiation, such as cell cycle regulation and mechanisms involved in scavenging of free radicals produced by ionizing radiation.

Identification of defective illegitimate recombinational repair of oxidatively-induced DNA double-strand breaks in ataxia-telangiectasia cells

Ataxia-telangiectasia (A-T) is an autosomal-recessive lethal human disease. Homozygotes suffer from a number of neurological disorders, as well as very high cancer incidence. Heterozygotes may also have a higher than normal risk of cancer, particularly for the breast. The gene responsible for the disease (ATM) has been cloned, but its role in mechanisms of the disease remain unknown. Cellular A-T phenotypes, such as radiosensitivity and genomic instability, suggest that a deficiency in the repair of DNA double-strand breaks (DSBs) may be the primary defect; however, overall levels of DSB rejoining appear normal. We used the shuttle vector, pZ189, containing an oxidatively-induced DSB, to compare the integrity of DSB rejoining in one normal and two A-T fibroblast cells lines. Mutation frequencies were two-fold higher in A-T cells, and the mutational spectrum was different. The majority of the mutations found in all three cell lines were deletions (44-63%). The DNA sequence analysis indicated that 17 of the 17 plasmids with deletion mutations in normal cells occurred between short direct-repeat sequences (removing one of the repeats plus the intervening sequences), implicating illegitimate recombination in DSB rejoining. The combined data from both A-T cell lines showed that 21 of 24 deletions did not involve direct-repeats sequences, implicating a defect in the illegitimate recombination pathway. These findings suggest that the A-T gene product may either directly participate in illegitimate recombination or modulate the pathway. Regardless, this defect is likely to be important to a mechanistic understanding of this lethal disease.

SV40 large tumor antigen nuclear import is regulated by the double-stranded DNA-dependent protein kinase site (serine 120) flanking the nuclear localization sequence

Nuclear localization sequence (NLS)-dependent nuclear import of SV40 large tumor antigen (T-Ag) fusion proteins is regulated by phosphorylation sites for casein kinase II (CKII) and the cyclin-dependent kinase Cdc2 amino-terminal to the NLS (amino acids 126-132). Between the T-Ag CKII and Cdc2 sites is a site (Ser120) for the double-stranded DNA-dependent protein kinase (dsDNA-PK), which we show here for the first time to play a role in regulating T-Ag nuclear import. We replaced Ser120 by aspartic acid or alanine using site-directed mutagenesis and assessed the effects on nuclear transport kinetics both in vivo (microinjected cells) and in vitro (mechanically perforated cells) in HTC rat hepatoma cells. Maximal nuclear accumulation of the Asp120 and Ala120 protein derivatives was approximately 40% and 70% reduced in vivo, respectively, compared with that of the wild type protein, and similarly reduced in vitro, although to a lesser extent. This implies that the dsDNA-PK site regulates the maximal level of nuclear accumulation, normally functioning to enhance T-Ag nuclear transport; the higher accumulation of the Asp120 protein compared with the Ala120 protein indicates that negative charge at the dsDNA-PK site is mechanistically important in regulating nuclear import. The Asp120 protein accumulated in the nucleus at a faster rate than the wild type protein, implying that phosphorylation at Ser120 may also regulate the nuclear import rate. CKII phosphorylation of the Asp120 protein in cytosol or by purified CKII was approximately 30% higher than that of the Ser120 and Ala120 proteins, while negative charge at the CKII site increased dsDNA-PK phosphorylation of Ser120 by approximately 80% compared with wild type, implying physical and functional interactions between the two phosphorylation sites. Quantitation of NLS recognition by the importin 58/97 subunits using an enzyme-linked immunosorbent assay indicated that while the Ala120 protein derivative had a binding affinity very similar to that of wild type, the Asp120 derivative showed 40% higher affinity. In vitro CKII phosphorylation increased importin binding by about 30% in all cases. These results imply that negative charge at the dsDNA-PK site may enhance nuclear import through increasing both NLS recognition by importin subunits, and phosphorylation at the CKII site, which itself also facilitates NLS recognition by importin 58/97.

The human XRCC9 gene corrects chromosomal instability and mutagen sensitivities in CHO UV40 cells

The Chinese hamster ovary (CHO) mutant UV40 cell line is hypersensitive to UV and ionizing radiation, simple alkylating agents, and DNA cross-linking agents. The mutant cells also have a high level of spontaneous chromosomal aberrations and 3-fold elevated sister chromatid exchange. We cloned and sequenced a human cDNA, designated XRCC9, that partially corrected the hypersensitivity of UV40 to mitomycin C, cisplatin, ethyl methanesulfonate, UV, and gamma-radiation. The spontaneous chromosomal aberrations in XRCC9 cDNA transformants were almost fully corrected whereas sister chromatid exchanges were unchanged. The XRCC9 genomic sequence was cloned and mapped to chromosome 9p13. The translated XRCC9 sequence of 622 amino acids has no similarity with known proteins. The 2.5-kb XRCC9 mRNA seen in the parental cells was undetectable in UV40 cells. The mRNA levels in testis were up to 10-fold higher compared with other human tissues and up to 100-fold higher compared with other baboon tissues. XRCC9 is a candidate tumor suppressor gene that might operate in a postreplication repair or a cell cycle checkpoint function.

Drosophila P-element transposase is a novel site-specific endonuclease

We developed in vitro assays to study the first step of the P-element transposition reaction: donor DNA cleavage. We found that P-element transposase required both 5' and 3' P-element termini for efficient DNA cleavage to occur, suggesting that a synaptic complex forms prior to cleavage. Transposase made a staggered cleavage at the P-element termini that is novel for all known site-specific endonucleases: the 3' cleavage site is at the end of the P-element, whereas the 5' cleavage site is 17 bp within the P-element 31-bp inverted repeats. The P-element termini were protected from exonucleolytic degradation following the cleavage reaction, suggesting that a stable protein complex remains bound to the element termini after cleavage. These data are consistent with a cut-and-paste mechanism for P-element transposition and may explain why P elements predominantly excise imprecisely in vivo.

Enhancer control of local accessibility to V(D)J recombinase

We have studied the role of transcriptional enhancers in providing recombination signal sequence (RSS) accessibility to V(D)J recombinase by examining mice carrying a transgenic human T-cell receptor (TCR) delta gene minilocus. This transgene is composed of unrearranged variable (Vdelta and Vdelta2), diversity (Ddelta3), joining (Jdelta1 and Jdelta3), and constant (Cdelta) gene segments. Previous data indicated that with the TCR delta enhancer (Edelta) present in the Jdelta3-Cdelta intron, V(D)J recombination proceeds stepwise, first V to D and then VD to J. With the enhancer deleted or mutated, V-to-D rearrangement is intact, but VD-to-J rearrangement is inhibited. We proposed that Edelta is necessary for J segment but not D segment accessibility and that J segment inaccessibility in the enhancerless minilocus resulted in the observed V(D)J recombination phenotype. In this study, we tested this notion by using ligation-mediated PCR to assess the formation of recombination-activating gene (RAG)-dependent double-strand breaks (DSBs) at RSSs 3' of Ddelta3 and 5' of Jdelta1. In five lines of mice carrying multicopy integrants of constructs that either lacked Edelta or carried an inactivated Edelta, the frequency of DSBs 5' of Jdelta1 was dramatically reduced relative to that in the wild type, whereas the frequency of DSBs 3' of Ddelta3 was unaffected. We interpret these results to indicate that Edelta is required for Jdelta1 but not Ddelta3 accessibility within the minilocus, and we conclude that enhancers regulate V(D)J recombination by providing local accessibility to the recombinase. cis-acting elements other than Edelta must maintain Ddelta3 in an accessible state in the absence of Edelta. The analysis of DSB formation in a single-copy minilocus integrant indicates that efficient DSB formation at the accessible RSS 3' of Ddelta3 requires an accessible partner RSS, arguing that RSS synapsis is required for DSB formation in chromosomal substrates in vivo.

Phosphorylation of human general transcription factors TATA-binding protein and transcription factor IIB by DNA-dependent protein kinase--synergistic stimulation of RNA polymerase II basal transcription in vitro

DNA-dependent protein kinase (DNA-PK) has been known to catalyze phosphorylation of a number of regulatory factors involved in DNA replication and transcription such as simian virus 40 T antigen, p53, c-Myc, Sp1, and RNA polymerase II (Pol II). We examined the possibility that DNA-PK phosphorylates the general transcription factors TATA-binding protein (TBP) and transcription factor (TF) IIB, which play key roles in the formation of transcription initiation complex with Pol II. By using a highly purified preparation of DNA-PK from Raji cells, both TBP and TFIIB were shown to be phosphorylated in vitro by DNA-PK. We then investigated the effect of the phosphorylation of these factors on Pol II basal transcription. Stepwise analysis of preinitiation complex formation by electrophoretic mobility shift assay revealed that the phosphorylation of TBP and TFIIB by DNA-PK did not affect the formation of promoter (P)-TBP and P-TBP-TFIIB complexes but synergistically stimulated the formation of P-TBP-TFIIB-TFIIF-Pol II complex. Similarly, combination of the phosphorylated TBP and TFIIB synergistically stimulated Pol II basal transcription from adenovirus major late promoter. These observations suggest that DNA-PK could positively regulate the Pol II basal transcription by phosphorylating TBP and TFIIB.

Mammalian DNA double-strand break repair protein XRCC4 interacts with DNA ligase IV

BACKGROUND

Mammalian cells deficient in the XRCC4 DNA repair protein are impaired in DNA double-strand break repair and are consequently hypersensitive to ionising radiation. These cells are also defective in site-specific V(D)J recombination, a process that generates the diversity of antigen receptor genes in the developing immune system. These features are shared by cells lacking components of the DNA-dependent protein kinase (DNA-PK). Although the XRCC4 gene has been cloned, the function(s) of XRCC4 in DNA end-joining has remained elusive.

RESULTS

We found that XRCC4 is a nuclear phosphoprotein and was an effective substrate in vitro for DNA-PK. Human XRCC4 associated extremely tightly with another protein(s) even in the presence of 1 M NaCl. Co-immunoprecipitation and adenylylation assays demonstrated that this associated factor was the recently identified human DNA ligase IV. Consistent with this, XRCC4 and DNA ligase IV copurified exclusively and virtually quantitatively over a variety of chromatographic steps. Protein mapping studies revealed that XRCC4 interacted with ligase IV via the unique carboxy-terminal ligase IV extension that comprises two tandem BRCT (BRCA1 carboxyl terminus) homology motifs, which are also found in other DNA repair-associated factors and in the breast cancer susceptibility protein BRCA1.

CONCLUSIONS

Our findings provide a function for the carboxy-terminal region of ligase IV and suggest that BRCT domains of other proteins may mediate contacts between DNA repair components. In addition, our data implicate mammalian ligase IV in V(D)J recombination and the repair of radiation-induced DNA damage, and provide a model for the potentiation of these processes by XRCC4.

Sp1 binds to the precise locus of end processing within the terminal repeats of Epstein-Barr virus DNA

Interconversion between the linear genome of Epstein-Barr virus (EBV) present in virions and intracellular circular EBV DNA is a novel DNA recombination process. A previously characterized DNA binding activity called terminal repeat or tandem repeat binding protein (TRBP) was found to recognize several G-rich recombinogenic sequences in the EBV genome and in cellular DNA. TRBP was also found to be an autoantigen recognized by sera from certain patients with undifferentiated connective-tissue disorders. Here the transcription factor Sp1 has been identified as a component of TRBP and has been shown to be an autoantigen. Sp1 bound to recombination junctions of EBV DNA, such as those in the terminal repeats and in the large internal repeats, as well as to recombinogenic regions of cellular DNA, such as variable-number tandem repeats and switch regions of the immunoglobulin genes. We defined the ends of the linear EBV genome present in virions and showed that Sp1 binds to the sequence (GGGGTGGGGCATGGG) within EBV terminal repeats at the precise locus of interconversion of linear and circular viral DNA. Sp1 may be involved in DNA recombination.

Activity of DNA ligase IV stimulated by complex formation with XRCC4 protein in mammalian cells

Mutation of the XRCC4 gene in mammalian cells prevents the formation of the signal and coding joints in the V(D)J recombination reaction, which is necessary for production of a functional immunoglobulin gene, and renders the cells highly sensitive to ionizing radiation. However, XRCC4 shares no sequence homology with other proteins, nor does it have a biochemical activity to indicate what its function might be. Here we show that DNA ligase IV co-immunoprecipitates with XRCC4 and that these two proteins specifically interact with one another in a yeast two-hybrid system. Ligation of DNA double-strand breaks in a cell-free system by DNA ligase IV is increased fivefold by purified XRCC4 and seven- to eightfold when XRCC4 is co-expressed with DNA ligase IV. We conclude that the biological consequences of mutating XRCC4 are primarily due to the loss of its stimulatory effect on DNA ligase IV: the function of the XRCC4-DNA ligase IV complex may be to carry out the final steps of V(D)J recombination and joining of DNA ends.

Ku70-deficient embryonic stem cells have increased ionizing radiosensitivity, defective DNA end-binding activity, and inability to support V(D)J recombination

V(D)J recombination requires both lymphoid-specific and generally expressed enzymatic activities. All three known generally expressed activities involved in V(D)J recombination are also involved in DNA double-strand break repair (DSBR). Two of these are components of the DNA-dependent protein kinase (DNA-PK) and include Ku80 and DNA-PK catalytic subunit (DNA-PKcs); the third, XRCC4, is a protein of unknown function. The Ku70 protein is an additional component of DNA-PK; Ku70 forms a heterodimer with Ku80 to generate the DNA end-binding component of the enzyme. To test putative functions for Ku70, we have used gene-targeted mutation to generate a murine embryonic stem cell line which lacks Ku70 expression. We find that the Ku70(-/-) cells produce no detectable Ku70 and very little Ku80, suggesting a direct interrelationship between their levels. Correspondingly, these cells lack the nonspecific DNA end-binding activity associated with Ku. Significantly, the Ku70(-/-) embryonic stem cells have markedly increased sensitivity to gamma-irradiation relative to Ku70(+/-) or wild-type embryonic stem cells. Furthermore, the Ku70(-/-) cells lack the ability to effectively rejoin signal and coding ends liberated in transiently introduced V(D)J recombination substrates by enforced RAG-1 and RAG-2 expression. We conclude that the Ku70 gene product is involved in DSBR and V(D)J recombination and confirm that the Ku70 gene can be classified as a member of the x-ray cross-complementation group 6 (XRCC6). Potential differences between the Ku70(-/-) and Ku80(-/-) V(D)J recombination defects are discussed.

The composition of coding joints formed in V(D)J recombination is strongly affected by the nucleotide sequence of the coding ends and their relationship to the recombination signal sequences

V(D)J recombination proceeds in two stages. Precise cleavage at the border of the conserved recombination signal sequences (RSSs) and the coding ends results in flush double-stranded signal ends and coding ends terminating in hairpins. In the second stage, the signal and coding ends are processed into signal and coding joints. Coding ends containing certain nucleotide homopolymers affect the efficiency of V(D)J recombination. In this study, we have tested the effect of small changes in coding-end nucleotide composition on the frequency of coding- and signal joint formation. Furthermore, we have determined the sequences of coding joints resulting from recombination of coding ends with different compositions. We found that the presence of two T nucleotides 5' of both RSSs, but not a single T, reduces the frequency of signal joint formation, i.e., interferes with the cleavage stage of V(D)J recombination. However, coding-joint processing is sensitive even to a single T. Both the sequence of the coding ends and the particular RSS (12-mer or 23-mer) with which the coding end is associated affect the final composition of the coding joints. Thus, the presence of P nucleotides, the conservation of one undeleted coding end, the formation of joints without any deletions, and the template-dependent insertion of nucleotides are strongly influenced by the coding-end nucleotide composition and/or RSS association. The implications of these results with respect to the processing of coding ends are discussed.

Molecular aspects of cellular responses to radiotherapy

Advances have been made in unravelling the molecular chains of cause and effect that determine cellular responses to radiotherapy, including cell cycle arrest, DNA repair and apoptosis. To begin with, cells must have mechanisms that enable them to sense DNA damage. Little was known about this until recently, when a DNA-protein kinase (DNA-PK) system for detecting radiation-induced strand breaks was described. The ataxia telangiectasia (ATM) gene has amino acid sequence similarities to DNA-PK, raising the possibility that the ATM protein also functions in some way as a sensor of DNA damage. However, just knowing the DNA damage is present is not enough. Signals must be transmitted via afferent biochemical pathways to proteins, such as p53, that determine which cellular responses are activated. The responses include cell cycle arrest, apoptosis and DNA repair, all of which relate closely to loss of clonogenic capacity and the outcome of treatment in our patients.

KARP-1: a novel leucine zipper protein expressed from the Ku86 autoantigen locus is implicated in the control of DNA-dependent protein kinase activity

The Ku autoantigen plays an integral role in mammalian DNA double-strand break repair as the DNA binding component of the DNA-dependent protein kinase (DNA-PK) complex. Here, we demonstrate that a second gene, KARP-1 (Ku86 Autoantigen Related Protein-1), is expressed from the Ku86 locus. The KARP-1 gene utilizes an upstream promoter and additional exons which results in an extra 9 kDa of protein appended onto the normal Ku86 polypeptide. The KARP-1-specific domain encodes interdigitating hexa- and penta-heptad repeats of leucine residues flanked by a very basic region. Intriguingly, the catalytic subunit of DNA-PK also contains a hexa-heptad repeat of leucines. Consistent with this observation, we observed that human cell lines stably expressing dominant-negative constructs of KARP-1 resulted in diminished DNA-PK activity and X-ray hypersensitivity and that a KARP-1 antibody significantly neutralized DNA-PK activity in vitro. Finally, we present data which suggests that KARP-1 may be primate-specific. These observations have important repercussions for mammalian DNA double-strand break repair.

A possible mechanism for hyperthermic radiosensitization mediated through hyperthermic lability of Ku subunits in DNA-dependent protein kinase

DNA-dependent protein kinase (DNA-PK), composed of p470 catalytic subunit and p85/p70 heterodimer of Ku autoantigen, is considered a critical enzyme in DNA double-strand break repair. We purified DNA-PK from human leukaemic MOLT-4 cells by successive column chromatography and separated into p470 and Ku subunits by ultracentrifugation in glycerol gradient. We studied hyperthermic stability of DNA-PK holoenzyme and its separated subunits to test a possible role of DNA-PK in hyperthermic radiosensitization. DNA-PK was found to lose its activity rapidly at hyperthermic 44 degrees C, and further, Ku subunits instead of p470 catalytic subunits were found to be sensitive to hyperthermia. These results indicate a possibility that hyperthermic radiosensitization is mediated through the heat lability of Ku subunits of DNA-PK, impairing repair of radiation-induced double-strand break of DNA.

DNA looping by Ku and the DNA-dependent protein kinase

The DNA-dependent protein kinase (DNA-PK) is required for DNA double-strand break (DSB) repair and immunoglobulin gene rearrangement and may play a role in the regulation of transcription. The DNA-PK holoenzyme is composed of three polypeptide subunits: the DNA binding Ku70/86 heterodimer and an approximately 460-kDa catalytic subunit (DNA-PKcs). DNA-PK has been hypothesized to assemble at DNA DSBs and play structural as well as signal transduction roles in DSB repair. Recent advances in atomic force microscopy (AFM) have resulted in a technology capable of producing high resolution images of native protein and protein-nucleic acid complexes without staining or metal coating. The AFM provides a rapid and direct means of probing the protein-nucleic acid interactions responsible for DNA repair and genetic regulation. Here we have employed AFM as well as electron microscopy to visualize Ku and DNA-PK in association with DNA. A significant number of DNA molecules formed loops in the presence of Ku. DNA looping appeared to be sequence-independent and unaffected by the presence of DNA-PKcs. Gel filtration of Ku in the absence and the presence of DNA indicates that Ku does not form nonspecific aggregates. We conclude that, when bound to DNA, Ku is capable of self-association. These findings suggest that Ku binding at DNA DSBs will result in Ku self-association and a physical tethering of the broken DNA strands.

Disruption of mouse RAD54 reduces ionizing radiation resistance and homologous recombination

Double-strand DNA break (DSB) repair by homologous recombination occurs through the RAD52 pathway in Saccharomyces cerevisiae. Its biological importance is underscored by the conservation of many RAD52 pathway genes, including RAD54, from fungi to humans. We have analyzed the phenotype of mouse RAD54-/- (mRAD54-/-) cells. Consistent with a DSB repair defect, these cells are sensitive to ionizing radiation, mitomycin C, and methyl methanesulfonate, but not to ultraviolet light. Gene targeting experiments demonstrate that homologous recombination in mRAD54-/- cells is reduced compared to wild-type cells. These results imply that, besides DNA end-joining mediated by DNA-dependent protein kinase, homologous recombination contributes to the repair of DSBs in mammalian cells. Furthermore, we show that mRAD54-/- mice are viable and exhibit apparently normal V(D)J and immunoglobulin class-switch recombination. Thus, mRAD54 is not required for the recombination processes that generate functional immunoglobulin and T cell receptor genes.

Functional interaction between DNA-PK and c-Abl in response to DNA damage

How DNA damage is converted into intracellular signals that can control cell behaviour is unknown. The c-Abl protein tyrosine kinase is activated by ionizing radiation and certain other DNA-damaging agents, whereas the DNA-dependent protein kinase (DNA-PK), consisting of a serine/threonine kinase and Ku DNA-binding subunits, requires DNA double-strand breaks or other DNA lesions for activation. Here we demonstrate that c-Abl interacts constitutively with DNA-PK. Ionizing radiation stimulates binding of c-Abl to DNA-PK and induces an association of c-Abl with Ku antigen. We show that DNA-PK phosphorylates and activates c-Abl in vitro. Cells deficient in DNA-PK are defective in c-Abl activation induced by ionizing radiation. In a potential feedback mechanism, c-Abl phosphorylates DNA-PK, but not Ku, in vitro. Phosphorylation of DNA-PK by c-Abl inhibits the ability of DNA-PK to form a complex with DNA. We also show that treatment of cells with ionizing radiation results in phosphorylation of DNA-PK that is dependent on c-Abl. Our results support the hypothesis that there are functional interactions between c-Abl and DNA-PK in the response to DNA damage.

Characterization of two DNA double-stranded break repair-deficient cell lines that express inactive DNA-dependent protein kinase catalytic subunits

The DNA-dependent protein kinase (DNA-PK) is a trimeric enzyme consisting of a 460-kDa catalytic subunit (DNA-PKcs) and a heterodimeric regulatory complex called Ku, which is comprised of 70 (Ku70) and 86 (Ku80) kDa subunits. Mutations that affect the expression of the catalytic or Ku80 subunits of DNA-PK disrupt both V(D)J recombination and DNA double-stranded break repair pathways. In this report, we show that two previously uncharacterized rodent cell lines that are defective in DNA double-stranded break repair express catalytically inactive DNA-PK. The DNA-PKcs from the DNA double-stranded break repair mutant cell lines IRS-20 and SX-9 assembles on double-stranded DNA but fails to function as a protein kinase. In addition to the kinase defect, the abundance of the DNA-PKcs from both of these cell lines is reduced relative to wild-type controls. These results suggest that the DNA-PKcs gene from each of these cell lines contains mutations that inactivate the enzymatic activity and the expression or stability of the gene product. These data further strengthen the hypothesis that DNA-PK-mediated protein phosphorylation is a necessary component of the DNA double-stranded break repair pathway.

Early alpha beta T cell development in the thymus of normal and genetically altered mice

The vast majority of T lymphocytes, with the exception of gut-associated, intraepithelial lymphocytes, differentiate and mature inside the thymus. Early T cell development is characterized by expansion and differentiation of thymocytes which do not yet express mature TCRs on their cell surface. Important events in early thymocyte development are controlled by a pre-TCR complex consisting of a conventional TCR beta chain and a novel transmembrane protein termed pre-TCR alpha (p T alpha chain) which are noncovalently associated with components of CD3. Recent studies of pre-TCR function have led to a better understanding of the molecular events in early thymocyte development.

Ku86 is not required for protection of signal ends or for formation of nonstandard V(D)J recombination products

Ku, a heterodimer of 70- and 86-kDa subunits, serves as the DNA binding component of the DNA-dependent protein kinase (DNA-PK). Cells deficient for the 86-kDa subunit of Ku (Ku86-deficient cells) lack Ku DNA end-binding activity and are severely defective for formation of the standard V(D)J recombination products, i.e., signal and coding joints. It has been widely hypothesized that Ku is required for protection of broken DNA ends generated during V(D)J recombination. Here we report the first analysis of V(D)J recombination intermediates in a Ku-deficient cell line. We find that full-length, ligatable signal ends are abundant in these cells. These data show that Ku86 is not required for the protection or stabilization of signal ends, suggesting that other proteins may perform this function. The presence of high levels of signal ends in Ku-deficient cells prompted us to investigate whether these ends could participate in joining reactions. We show that nonstandard V(D)J recombination products (hybrid joints), which involve joining a signal end to a coding end, form with similar efficiencies in Ku-deficient and wild-type fibroblasts. These data support the surprising conclusion that Ku is not required for some types of V(D)J joining events. We propose a novel RAG-mediated joining mechanism, analogous to disintegration reactions performed by retroviral integrases, to explain how formation of hybrid joints can bypass the requirement for Ku and DNA-PK.

Nonsense mutation at Tyr-4046 in the DNA-dependent protein kinase catalytic subunit of severe combined immune deficiency mice

The severe combined immune deficiency (SCID) mouse was reported as an animal model for human immune deficiency. Through the course of several studies, the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) gene came to be considered a candidate for the SCID-responsible gene. We isolated an ORF of the murine DNA-PKcs gene from SCID mice and their parent strain C.B-17 mice and determined the DNA sequences. The ORF of the murine DNA-PKcs gene contained 4128-aa residues and had 78.9% homology with the human DNA-PKcs gene. A particularly important finding is that a T to A transversion results in the substitution of termination codon in SCID mice for the Tyr-4046 in C.B-17 mice. No other mutation was detected in the ORF of the gene. The generality of this transversion was confirmed using four individual SCID and wild-type mice. The substitution took place in the phosphatidylinositol 3-kinase domain, and the mutated gene encodes the truncated products missing 83 residues of wild-type DNA-PKcs products. Furthermore, the quantity of DNA-PKcs transcript in wild-type and SCID cells was almost equal. These observations indicate that the DNA-PKcs gene is the SCID-responsible gene itself and that the detected mutation leads to the SCID aberration.

NF-kappa B RelA-deficient lymphocytes: normal development of T cells and B cells, impaired production of IgA and IgG1 and reduced proliferative responses

To investigate the function of NF-kappa B RelA (p65), we generated mice deficient in this NF-kappa B family member by homologous recombination. Mice lacking RelA showed liver degeneration and died around embryonic day 14.5. To elucidate the role of RelA in lymphocyte development and function, we transplanted fetal liver cells of 13.5-day embryos from heterozygote matings into irradiated SCID mice. Within 4 weeks, both T and B cells had developed in the SCID mice receiving relA-/- fetal liver transplants, similar to the relA+/+ and +/- cases. T cells were found to mature to Thy-1+/TCR alpha beta +/CD3+/CD4+ or CD8+, while B cells had the ability to differentiate to IgM+/B220+ and to secrete immunoglobulins. However, the secretion of IgG1 and IgA was reduced in RelA-deficient B cells. Furthermore, both T and B cells lacking RelA showed marked reduction in proliferative responses to stimulation with Con A, anti-CD3, anti-CD3 + anti-CD28, LPS, anti-IgM, and PMA + calcium ionophore. The results indicate that RelA plays a critical role in production of specific Ig isotypes and also in signal transduction pathways for lymphocyte proliferation.

Molecular and biochemical characterization of xrs mutants defective in Ku80

The gene product defective in radiosensitive CHO mutants belonging to ionizing radiation complementation group 5, which includes the extensively studied xrs mutants, has recently been identified as Ku80, a subunit of the Ku protein and a component of DNA-dependent protein kinase (DNA-PK). Several group 5 mutants, including xrs-5 and -6, lack double-stranded DNA end-binding and DNA-PK activities. In this study, we examined additional xrs mutants at the molecular and biochemical levels. All mutants examined have low or undetectable levels of Ku70 and Ku80 protein, end-binding, and DNA-PK activities. Only one mutant, xrs-6, has Ku80 transcript levels detectable by Northern hybridization, but Ku80 mRNA was detectable by reverse transcription-PCR in most other mutants. Two mutants, xrs-4 and -6, have altered Ku80 transcripts resulting from mutational changes in the genomic Ku80 sequence affecting RNA splicing, indicating that the defects in these mutants lie in the Ku80 gene rather than a gene controlling its expression. Neither of these two mutants has detectable wild-type Ku80 transcript. Since the mutation in both xrs-4 and xrs-6 cells results in severely truncated Ku80 protein, both are likely candidates to be null mutants. Azacytidine-induced revertants of xrs-4 and -6 carried both wild-type and mutant transcripts. The results with these revertants strongly support our model proposed earlier, that CHO-K1 cells carry a copy of the Ku80 gene (XRCC5) silenced by hypermethylation. Site-directed mutagenesis studies indicate that previously proposed ATP-binding and phosphorylation sites are not required for Ku80 activity, whereas N-terminal deletions of more than the first seven amino acids result in severe loss of activities.

On the role of DNA double-strand breaks in toxicity and carcinogenesis

DNA double-strand breaks are associated with various endogenous processes, such as transcription, recombination, replication, and with the process of active cell death, which aims to eliminate cells. In addition, DNA double-strand breaks can be induced by irradiation, exposure to chemicals, increased formation of reactive oxygen species, and, indirectly, during repair of other types of DNA damage or as a consequence of extranuclear lesions. In addition to the neutral filter elution of DNA, the recently introduced pulsed-field gel electrophoresis is capable of determining DNA double-strand breaks with higher accuracy and sensitivity and is expected to increase our knowledge on the frequency and the role of DNA breakage. Parallel determination of parameters for cytotoxicity is necessary to elucidate the causal primary lesion. Although the repair of DNA double-strand breaks is a complex task, cells are capable of repairing--with or without errors and up to a certain extent--and surviving this DNA lesion. Gene translocations, rearrangements, amplifications, and deletions arising during repair and misrepair of double-strand breaks may contribute to cell transformation and tumor development.

Human severe combined immunodeficiency: genetic, phenotypic, and functional diversity in one hundred eight infants

OBJECTIVE

To determine the relative frequencies of the different genetic forms of severe combined immunodeficiency (SCID) and whether there are distinctive characteristics of the particular genotypes.

STUDY DESIGN

The demographic, genetic, and immunologic features of 108 infants with SCID who were treated consecutively at Duke University Medical Center were analyzed.

RESULTS

Eighty-nine subjects were boys and 19 were girls; there were 84 white infants, 16 black infants, and 8 Hispanic infants. Forty-nine had X-linked SCID with mutations of common cytokine receptor gamma chain (gamma c), 16 had adenosine deaminase (ADA) deficiency, 8 had Janus kinase 3 (Jak3) deficiency, 21 had unknown autosomal recessive mutations, 1 had reticular dysgenesis, 1 had cartilage hair hypoplasia, and 12 (all boys) had SCID of undetermined type. Deficiency of ADA caused the most profound lymphopenia; gamma c or Jak3 deficiency resulted in the most B cells and fewest natural killer (NK) cells; NK cells and function were highest in autosomal recessive and unknown types of SCID.

CONCLUSIONS

Different SCID genotypes are associated with distinctive lymphocyte characteristics. The presence of NK function in ADA-deficient, autosomal recessive, and unknown type SCIDs, and low NK function in a majority of gamma c and Jak3 SCIDs indicates that some molecular lesions affect T, B, and NK cells (gamma c and Jak3), others primarily T cells (ADA deficiency), and others just T and B cells.

Evidence for DNA-PK-dependent and -independent DNA double-strand break repair pathways in mammalian cells as a function of the cell cycle

Mice homozygous for the scid (severe combined immune deficiency) mutation are defective in the repair of DNA double-strand breaks (DSBs) and are consequently very X-ray sensitive and defective in the lymphoid V(D)J recombination process. Recently, a strong candidate for the scid gene has been identified as the catalytic subunit of the DNA-dependent protein kinase (DNA-PK) complex. Here, we show that the activity of the DNA-PK complex is regulated in a cell cycle-dependent manner, with peaks of activity found at the G1/early S phase and again at the G2 phase in wild-type cells. Interestingly, only the deficit of the G1/early S phase DNA-PK activity correlated with an increased hypersensitivity to X-irradiation and a DNA DSB repair deficit in synchronized scid pre-B cells. Finally, we demonstrate that the DNA-PK activity found at the G2 phase may be required for exit from a DNA damage-induced G2 checkpoint arrest. These observations suggest the presence of two pathways (DNA-PK-dependent and -independent) of illegitimate mammalian DNA DSB repair and two distinct roles (DNA DSB repair and G2 checkpoint traversal) for DNA-PK in the cellular response to ionizing radiation.

Ku selectively transfers between DNA molecules with homologous ends

Double strand break repair and V(D)J recombination in mammalian cells require the function of the Ku protein complex and the DNA-dependent protein kinase. The DNA-dependent protein kinase is targeted to DNA through its interaction with the Ku protein complex, and thus the specificity of template recognition in the repair and recombination reactions depend on Ku. We have studied Ku binding to DNA using competitive gel shift analysis. We find that Ku bound to one DNA molecule can transfer directly to another DNA molecule when the two DNA molecules have homologous ends containing a minimum of four matched bases. This remarkable reaction can give a false impression of sequence specificity of Ku DNA binding under certain assay conditions. A model is proposed for the DNA binding function of Ku on the basis of these results and the discovery of a novel type of DNA-Ku complex formed at high Ku/DNA ratios is discussed.

Sequence-specific DNA binding and transcription factor phosphorylation by Ku Autoantigen/DNA-dependent protein kinase. Phosphorylation of Ser-527 of the rat glucocorticoid receptor

NRE1 is a DNA sequence element through which Ku antigen/DNA-dependent protein kinase (DNA-PK) catalytic subunit represses the induction of mouse mammary tumor virus transcription by glucocorticoids. Although Ku is an avid binder of DNA ends and has the ability to translocate along DNA, we report that direct sequence-specific Ku binding occurs with higher affinity (Kd = 0.84 +/- 0.24 nM) than DNA end binding. Comparison of Ku binding to several sequences over which Ku can accumulate revealed two classes of sequence. Sequences with similarity to NRE1 competed efficiently for NRE1 binding. Conversely, sequences lacking similarity to NRE1 competed poorly for Ku and were not recognized in the absence of DNA ends. Phosphorylation of glucocorticoid receptor (GR) fusion proteins by DNA-PK reflected Ku DNA-binding preferences and demonstrated that co-localization of GR with DNA-PK on DNA in cis was critical for efficient phosphorylation. Phosphorylation of the GR fusion protein by DNA-PK mapped to a single site, Ser-527. This site occurs adjacent the GR nuclear localization sequence between the DNA and ligand binding domains of GR, and thus its phosphorylation, if confirmed, has the potential to affect receptor function in vivo.

Specificity in V(D)J recombination: new lessons from biochemistry and genetics

Recent in vitro work on V(D)J recombination has helped to clarify its mechanism. The first stage of the reaction, which can be reproduced with the purified RAG1 and RAG2 proteins, is a site-specific cleavage that generates the same broken DNA species found in vivo. The cleavage reaction is closely related to known types of transpositional recombination, such as that of HIV integrase. All the site specificity of V(D)J recombination, including the 12/23 rule, is determined by the RAG proteins. The later steps largely overlap with the repair of radiation-induced DNA double-strand breaks, as indicated by the identity of several newly characterized factors involved in repair. These developments open the way for a thorough biochemical study of V(D)J recombination.

Potential molecular targets for manipulating the radiation response

Recent advances in our understanding of the molecular events that occur following ionizing radiation leading to DNA damage and repair, apoptosis, and cell-cycle arrests suggest new ways in which the radiation response might be manipulated. Specific targets which, if inactivated, might increase radiosensitivity include Ras, which has been implicated in the radioresistant phenotype, and components of DNA-dependent protein kinase or other molecules involved in the recognition or repair of DNA damage. In some tumors, apoptosis is an important mode of cell death following radiation, so agents that promote this may prove useful therapeutically. Conversely, side effects may result from radiation-induced apoptosis of normal tissues: for example, pneumonitis following the destruction of endothelial cells in the pulmonary vasculature. Therefore, decreasing apoptosis in these tissues may reduce late effects. It may also be possible to prevent late effects such as fibrosis by blocking the induction of certain genes such as transforming growth factor beta. Cell-cycle regulation is another area that could be manipulated to increase radiosensitivity. There is evidence that the G2 delay following radiation is important in protecting cells from death. Abolition of this delay may increase radiosensitivity, especially in cells with mutant p53 that have lost the G1 checkpoint.

Replication protein A: a heterotrimeric, single-stranded DNA-binding protein required for eukaryotic DNA metabolism

Replication protein A [RPA; also known as replication factor A (RFA) and human single-stranded DNA-binding protein] is a single-stranded DNA-binding protein that is required for multiple processes in eukaryotic DNA metabolism, including DNA replication, DNA repair, and recombination. RPA homologues have been identified in all eukaryotic organisms examined and are all abundant heterotrimeric proteins composed of subunits of approximately 70, 30, and 14 kDa. Members of this family bind nonspecifically to single-stranded DNA and interact with and/or modify the activities of multiple proteins. In cells, RPA is phosphorylated by DNA-dependent protein kinase when RPA is bound to single-stranded DNA (during S phase and after DNA damage). Phosphorylation of RPA may play a role in coordinating DNA metabolism in the cell. RPA may also have a role in modulating gene expression.

A role for molecular radiobiology in radiotherapy?

As we learn more about the cellular response to radiation and its genetic control, new avenues are opened up that have the potential to have a significant impact on radiotherapy practice. The recognition of the importance of the control of DNA damage induction and repair, cell cycle arrest and apoptosis gives us the primary areas to investigate, and the improvements in molecular technology make the application of our new knowledge more feasible. It can only be hoped that specific means can be found to assist in the prediction of normal tissue and tumour radiosensitivity and to manipulate sensitivity when that is desirable.