Prediction of human cell radiosensitivity: comparison of clonogenic assay with chromosome aberrations scored using premature chromosome condensation with fluorescence in situ hybridization

PURPOSE

The purpose of the present investigation was to determine whether chromosome aberrations scored by premature chromosome condensation (PCC) and fluorescence in situ hybridization (FISH) can predict the radiosensitivity of human cell lines, thereby providing a possible means of assessing the in situ radiosensitivity of normal tissues and the radiocurability of individual human cancers.

METHODS AND MATERIALS

We used four cells lines of different radiosensitivity: normal human fibroblasts (AG1522), ataxia-telangiectasia fibroblasts (AT2052), a human fibrosarcoma cell line (HT1080), and a human melanoma cell line (melanoma 903). These were irradiated in plateau phase with a range of doses and assessed both for clonogenic cell survival and for aberrations in a single chromosome (number 4) immediately after, and 24 h after irradiation.

RESULTS

The initial number of breaks in chromosome 4 was proportional to irradiation dose and was identical for all the different human cell lines, irrespective of radiosensitivity. On the other hand, the number of chromosome 4 breaks remaining 24 h after irradiation reflected the radiosensitivity of the cells such that the relationship between residual chromosome aberrations and cell survival was the same for the different cell lines.

CONCLUSIONS

These results suggest that the scoring of chromosome aberrations in interphase using FISH with PCC holds considerable promise for predicting the radiosensitivity of normal and tumor tissues in situ.

Technique to stent the open sternum after cardiac operations

Extreme myocardial edema may preclude sternal closure after a cardiac operation. We describe a technique to stent the sternum open to optimize cardiac function.

Influence of muscle activation dynamics on reaction time in the elderly

The aim of this investigation was to determine whether age-related changes in the dynamics of muscle activation were, in part, responsible for longer reaction times (RT) in the elderly. A group of 12 young (mean age, 20.6 years) and 12 elderly (mean age, 64.3 years) women performed a series of ballistic forearm supination movements in response to an auditory stimulus while using a simple reaction time test. Surface electromyographic waveforms from biceps brachii (agonist) and pronator teres (antagonist) muscles were recorded, together with the angle-time curves representing the motion of the forearm, on to an IBM compatible microcomputer. The results showed that an age-related increase (P < 0.05) in motor reaction time (MRT) contributed to longer RT in the elderly. In addition, the longer (P < 0.05) MRTs in the elderly were associated with a significantly slower rate (P < 0.05) of biceps brachii muscle activation and a significantly increased proportion (P < 0.05) of the initial biceps brachii muscle burst required to initiate the movement. This data suggested that an important part of the slowing of motor behaviour, commonly observed with increasing age, may be due to either decreases in the ability of aged skeletal muscle to rapidly generate tension or to a reduction in motor drive.

Discrepancies between measured changes of radiobiological hypoxic fraction and oxygen tension monitoring using two assay systems

PURPOSE

This study was conducted to assess the ability of computerized pO2 histography to measure changes in tumor oxygenation produced by low oxygen breathing.

METHODS AND MATERIALS

Female syngeneic C3H/Km mice bearing SCC VII/St carcinomas were used in these experiments. Changes in tumor oxygenation produced by the mice breathing 10% oxygen were assessed with computerized pO2 histography, 3H-misonidazole binding, and the paired survival curve assay of radiosensitivity.

RESULTS

The hypoxic cell fraction of the tumors in mice breathing 10% oxygen was 3.1 times higher than that of tumors in mice breathing normal air determined by an in vivo-in vitro clonogenic assay. Binding of radiolabeled misonidazole to the tumors in mice breathing 10% oxygen was also significantly higher than that to tumors in mice breathing normal air (p < 0.05). In addition, oxygen tension histograms for normal tissue showed a dramatic shift to a lower oxygen tension when the mice were breathing 10% oxygen. However, under identical conditions, there was only a minimal shift in the oxygen tension of tumor tissue. Although the number of oxygen tension readings in the relatively oxic class in tumor tissue was lower when the mice were breathing 10% oxygen than when breathing normal air, there was not a significant decrease in the median pO2 value for the tumor. The number of pO2 readings lower than 5 mmHg in the tumor was not affected by the 10% oxygen breathing.

CONCLUSION

These findings indicate that increases in radiobiological hypoxic fraction produced by lower blood oxygen levels may not correlate well with the results of polarographic measurements of tumor pO2 levels.

Calcineurin-mediated protein dephosphorylation in brain nerve terminals regulates the release of glutamate

In response to Ca2+ entry, several prominent brain nerve terminal phosphoproteins undergo dephosphorylation, but the relation between dephosphorylation and neurotransmitter release is unknown. Using the immunosuppressants cyclosporin A (CsA) and L-683,590 (FK-520) to inhibit specifically the Ca2+/calmodulin-dependent protein phosphatase calcineurin, we demonstrate here that Ca(2+)-dependent dephosphorylation in isolated rat brain nerve terminals (synaptosomes) is mediated by calcineurin. Pretreatment with micromolar CsA resulted in a 76-95% inhibition of stimulation-induced decreases in 32P-labeled dynamin (previously referred to as dephosphin), a phosphoprotein of M(r) = 145,000 (145-kDa protein), and a phosphoprotein of M(r) = 170,000 (170-kDa protein). Pretreatment with FK-520 also inhibited Ca(2+)-dependent dephosphorylation. Using hypotonic lysates of 32P-labeled synaptosomes, the addition of Ca2+ plus calmodulin, but not either agent alone, induced dynamin dephosphorylation. CsA and FK-520 had little to no effect on the release of glutamate induced by either K(+)-depolarization or the Ca2+ ionophore ionomycin. In contrast, calcineurin inhibition led to a substantial enhancement of glutamate release evoked by the K(+)-channel blocker 4-aminopyridine, an agent whose action most closely mimics physiological stimulation. Calcineurin inhibition had no effect on stimulation-induced changes in synaptosomal Ca2+ levels. Based on our findings, we hypothesize that Ca(2+)-dependent protein dephosphorylation resulting from calcineurin activation during physiological stimulation limits neurotransmitter release from brain nerve terminals, perhaps being dependent upon cyclic repolarization of the membrane during stimulation.

An electromyographic validation of an abdominal muscle test

A recently developed abdominal muscle test (AMT), using four levels of difficulty to assess abdominal muscle strength, was validated by examination of the functional interrelationship between the individual muscles during the AMT, and by determining the relationship between the AMT level of difficulty and electromyographical (EMG) activity. EMG data were collected from the upper and lower rectus abdominis, and the external and internal oblique in 22 subjects as they participated in the AMT. Typically, throughout the AMT, lower rectus abdominis was most active, followed by upper rectus abdominis, external oblique, and internal oblique. EMG activity increased significantly (p < .05) for all muscles from Levels 1 to 3. At Level 4, although upper and lower rectus abdominis remained more active than the obliques, the relative contribution from the obliques was increased. The results of this study show that the AMT is a valid indicator of ability to develop voluntary abdominal muscle tension in Levels 1 to 3, although, Level 4 is more indicative of the oblique muscles' functional abilities and therefore may be considered a distinctly different muscle test.

Examining the time course of prime effects on Stroop processing

This study was conducted to assess the effect of priming on the Stroop task over time. Color-congruent, color-incongruent, and neutral stimuli were randomly presented. Five prime conditions were also used. The prime conditions included valid color, invalid color, valid word, and invalid word primes and no prime. Primes were presented to 8 subjects at varying stimulus onset asynchronies ranging from -200 msec., i.e., 200 msec, before the color-word stimulus, to 200 msec, i.e., 200 msec, after the color-word stimulus. Analysis suggested the facilitory or inhibitory effects of semantic information on the Stroop task are reduced when the prime follows the color-word stimulus by 200 msec. This implies 200 msec. are needed to make the proper color response. A model is proposed to account for the findings. Methodological considerations for studies using priming and the Stroop task are also discussed.

Brain abscess due to Gordona terrae in an immunocompromised child: case report and review of infections caused by G. terrae

A brain abscess complicated antineoplastic chemotherapy for a primary cerebral rhabdoid tumor in an immunocompromised boy. Culture of purulent exudate obtained by surgical puncture of an intracranial hematoma yielded a gram-positive microorganism initially identified as a Rhodococcus species by conventional biochemical analysis; however, the isolate was subsequently identified as Gordona terrae by ribosomal DNA analysis. To our knowledge, this is the third case of human infection caused by G. terrae and the first case of a brain abscess due to this organism. As this case demonstrates, this species may cause opportunistic invasive infection in severely immunocompromised patients. The identity of clinical isolates believed to be G. terrae should be confirmed by molecular methods until better species-specific phenotypic markers become available.

Mechanisms of coronary vasomotor dysfunction in the transplanted heart

The transplanted heart sustains both cold ischemic and reperfusion injuries. These can produce coronary vascular endothelial or smooth muscle injury or both, which, in turn, can produce coronary vasomotor dysfunction. Using a canine model of autologous heart transplantation, we examined the following coronary vasomotor control mechanisms in isolated coronary artery rings: (1) endothelial-dependent cyclic guanosine monophosphate (cGMP)-mediated vasorelaxation (response to acetylcholine); (2) endothelial-independent cGMP-mediated vasorelaxation (response to sodium nitroprusside); and (3) beta-adrenergic cyclic adenosine monophosphate (cAMP)-mediated vasorelaxation (response to isoproterenol hydrochloride). Further, these mechanisms were related to 3 hours of cold ischemia alone and to 3 hours of cold ischemia plus 1 hour of reperfusion. Autologous heart transplantation was performed in dogs, and isolated distal left anterior descending coronary artery rings were studied in individual organ chambers. Cold ischemia alone produced significant dysfunction of beta-adrenergic cAMP-mediated vasorelaxation, which was exacerbated after reperfusion. Neither endothelial-dependent nor endothelial-independent cGMP-mediated vasorelaxation was dysfunctional after cold ischemia alone, but both were significantly impaired after reperfusion. We conclude that cold ischemia and reperfusion each produce coronary vasomotor dysfunction in the transplanted heart. Cumulatively, such coronary vasomotor dysfunction can acutely impair coronary vasodilatation and potentially jeopardize myocardial blood flow in the transplanted heart.

The medically important aerobic actinomycetes: epidemiology and microbiology

The aerobic actinomycetes are soil-inhabiting microorganisms that occur worldwide. In 1888, Nocard first recognized the pathogenic potential of this group of microorganisms. Since then, several aerobic actinomycetes have been a major source of interest for the commercial drug industry and have proved to be extremely useful microorganisms for producing novel antimicrobial agents. They have also been well known as potential veterinary pathogens affecting many different animal species. The medically important aerobic actinomycetes may cause significant morbidity and mortality, in particular in highly susceptible severely immunocompromised patients, including transplant recipients and patients infected with human immunodeficiency virus. However, the diagnosis of these infections may be difficult, and effective antimicrobial therapy may be complicated by antimicrobial resistance. The taxonomy of these microorganisms has been problematic. In recent revisions of their classification, new pathogenic species have been recognized. The development of additional and more reliable diagnostic tests and of a standardized method for antimicrobial susceptibility testing and the application of molecular techniques for the diagnosis and subtyping of these microorganisms are needed to better diagnose and treat infected patients and to identify effective control measures for these unusual pathogens. We review the epidemiology and microbiology of the major medically important aerobic actinomycetes.

Reoxygenation and rehypoxiation in the SCCVII mouse tumor

PURPOSE

To test the hypothesis that, following preferential killing of tumor hypoxic cells, the fraction of hypoxic cells in the tumor will reestablish itself to pretreatment levels (rehypoxiation) with the same kinetics as for reoxygenation.

METHODS AND MATERIALS

Mouse squamous cell carcinoma VII (SCCVII) tumors were treated with a single dose of 10 Gy or a single dose of the bioreductive hypoxic cell cytotoxin, tirapazamine (SR 4233, 0.2 mmol/kg), which preferentially kills hypoxic cells within the tumor. Hypoxic fractions were determined by the paired survival curve technique using the in vivo-in vitro clonogenic assay 0-24 h after treatment.

RESULTS

Immediately after irradiation with 10 Gy, the hypoxic fraction of the tumors increased to 80% and rapidly returned to pretreatment levels 3-6 h later. Within 1 h of injecting tirapazamine, the hypoxic fraction fell to 0.57% (about 7% of pretreatment levels) and returned to pretreatment levels 3-5 h later.

CONCLUSION

The return to pretreatment levels of hypoxia among tumor cells surviving a single dose of radiation (reoxygenation) and of the hypoxic cell toxin tirapazamine (rehypoxiation) was rapid and occurred with similar kinetics for the two processes. These data support the hypothesis that reoxygenation and rehypoxiation are different manifestations of the same phenomenon and result from fluctuating tumor blood flow which creates acute hypoxia.

Human infections caused by Brevibacterium casei, formerly CDC groups B-1 and B-3

Forty-one clinical strains of CDC coryneform groups B-1 and B-3 were compared biochemically, by analysis of cell wall sugars, amino acids, and cellular fatty acids, and by DNA relatedness to the type strains of Brevibacterium casei, Brevibacterium epidermidis, and Brevibacterium linens. Twenty-two strains were shown to be B. casei, while five other strains formed a phenotypically inseparable genomospecies in the same genus. The remaining isolates were genetically heterogeneous, and most are probably members of the genus Brevibacterium. They were not further identified, but they were biochemically distinguishable from B. casei. Eleven of the clinical strains of B. casei were isolated from blood, and two each were isolated from cerebrospinal fluid and from pleural fluid. At least five isolates were from multiple blood or cerebrospinal fluid cultures. To our knowledge, these strains are the first described clinical isolates identified as B. casei, which was previously considered to be a nonpathogenic species.

Fluorescence in situ hybridization in cancer and radiation biology

This review describes molecular cytogenetic approaches to genetic analysis including fluorescence in situ hybridization, primed in situ labeling and comparative genomic hybridization. It also summarizes the applications of this technology to physical mapping, cancer diagnosis and prognostication and in radiation biology.

Effects of practice on tip-of-the-tongue states

Tip-of-the-tongue (TOT) states were examined in relation to acquisition manipulations, using named imaginary animals (TOTimals) as targets. High levels of TOT states were found in three experiments. In the first experiment an increase in the duration of initial exposure to target material improved recall and recognition, and reduced the number of unrecalled items not in TOT states (NTOTs), but did not affect TOT levels. In Experiment 2 practice at writing target names, as compared with only reading them, improved recall performance and decreased TOT levels, but did not reduce NTOTs. Experiment 3 replicated the finding that writing during practice reduced TOT states, but did not reduce NTOTs, and also found that more frequent practice trials increased recall without affecting TOT levels. The results suggest that practice writing target names prevents TOT states by strengthening otherwise deficient phonological connections in memory, a deficiency that can cause TOT states when visual-to-lexical connections give only partial access to a target in memory. The results also demonstrate the usefulness of the TOTimal technique for testing effects of acquisition variables on TOT experiences.

Increased cytotoxicity of chronic hypoxic cells by molecular inhibition of GRP78 induction

PURPOSE

To develop a molecular strategy of increasing cytotoxicity of chronically hypoxic cells by inhibiting Glucose Regulated Protein 78 kDal (GRP78) induction.

METHODS AND MATERIALS

A mutant nonGRP78 inducing cell line (78WO) was developed from its parent (DG44) by overexpressing antisense GRP78 mRNA. Following exposure to varying durations of hypoxia, Northern and Western blot analysis were used to characterize the amount of GRP78 expression both at the RNA and protein level. Hypoxia was achieved by placing cells in specially designed hypoxic chambers which were subjected to successive rounds of evacuation and flushing with 95% CO2/N2 to reduce the oxygen in the environment to 0.02% oxygen. After treatment with hypoxia, cells were assayed for colony forming ability.

RESULTS

GRP78 mRNA and protein induction following exposure to hypoxia was 3-4 fold lower in the 78WO cell line than in the parental DG44 cell line. Furthermore, it was observed that there was no difference in the cytotoxicity of 78WO and DG44 cells after 10 h of hypoxia. However, after 15 h of hypoxia, the survival of 78WO cells decreased by 1 log and after 20 h of hypoxia, the survival of 78WO decreased by another log.

CONCLUSION

These results show that stress protein induction is important for cellular survival to chronic hypoxia and that inhibition of GRP78 induction may represent a novel therapeutic strategy by selectively sensitizing chronically hypoxic cells within solid tumors.

Radiation-induced damage, repair and exchange formation in different chromosomes of human fibroblasts determined by fluorescence in situ hybridization

We have used fluorescence in situ hybridization with whole-chromosome probes for human chromosomes 1, 4, 8 and 13 to investigate the extent to which the induction of damage and its repair after exposure to ionizing radiation is distributed randomly among these chromosomes. All the studies were performed with AG1522 human fibroblasts irradiated with 6 Gy and maintained in a nondividing state for at least 6 h after irradiation except for the measurements of initial damage. The extent of initial damage was determined by fusion of the cells immediately after irradiation with metaphase HeLa cells to obtain premature chromosome condensation (PCC). Breaks and exchanges were also scored by PCC 24 h after irradiation and in metaphase spreads at the first division after irradiation. The data obtained were consistent with random breakage and repair in these chromosomes. Comparing PCC 24 h after irradiation with first metaphase, there was a deficit in aberrations at metaphase, particularly in unrejoined breaks, implying loss or slowing of cells containing aberrations prior to the first division. An analysis of dicentrics and translocations in chromosome 4 at first and in subsequent divisions showed that there was an equal number of dicentrics and translocations at first metaphase with loss of dicentrics, but no loss of translocations in subsequent divisions. These data are supportive of the hypothesis tht the total number of chromosome aberrations in cells can be estimated from a single chromosome pair.

Neuromuscular control of lifting in the elderly

The aim of this investigation was to determine, utilising an electromyographic (EMG) technique, the influence of the ageing process on the neuromotor control of the stoop-lift. Seven elderly (60-75 years) and seven young (18-25 years) subjects completed a series of ten unresisted (no weight) stoop-lifts. EMG potentials were recorded with 4-mm bipolar surface electrodes from the erector spinae and medial hamstring muscle groups. The temporal features of the stoop-lift itself were recorded with an electrogoniometer. The results of this investigation showed that the elderly subjects had a significantly (p < 0.05) earlier onset of the erector spinae muscle during the up-phase of the stoop-lift compared to the younger subjects. In addition, reactivation of the erector spinae muscle in the up-phase occurred at hand positions significantly (p < 0.05) closer to the floor in the elderly group indicating more stooped postures. Thus the EMG profiles of the erector spinae within the elderly subjects performing unresisted stoop-lifts approximated those previously reported in younger subjects performing resisted (with a weight) lifting. The earlier onset of the erector spinae in the up-phase of the stoop-lift was partially explained by a significant (p < 0.05) reduction in flexibility (sit and reach test) associated with the elderly subjects. The results further stress the potential for injury associated with the utilisation of the stoop-lift, particularly by elderly populations.

Comparison of the enhancement of tumor responses to fractionated irradiation by SR 4233 (tirapazamine) and by nicotinamide with carbogen

PURPOSE

This study was undertaken to compare in a fractionated regimen, with clinically relevant radiation doses, two radiation response modifiers that function by different mechanisms: SR 4233, a bioreductive agent toxic to hypoxic cells, and nicotinamide with carbogen, a combination that has been shown to improve tumor oxygenation.

METHODS AND MATERIALS

Cell survival assays were used to examine the response of three different tumors: KHT, RIF-1 and SCCVII/St in C3H/Km mice. Regrowth delay studies were also performed with the RIF-1 tumor. A fractionated irradiation schedule, consisting of twice daily 2.5 Gy treatments was investigated with and without drug pretreatment. SR 4233 was given IP at 0.12 mmol/kg one half hour before each irradiation. Nicotinamide (250, 500, 1000 mg/kg) was given IP 1 h before each irradiation with carbogen exposure 5 min prior to and during the irradiation.

RESULTS

Both treatment strategies enhanced the response of all three tumors to the fractionated radiation regimen. However, for two of the tumors (KHT and SCCVII), SR 4233 produced a significantly greater enhancement than did the combination of nicotinamide + carbogen. For the RIF-1 tumor (which has the lowest hypoxic fraction of the three), the response was comparable for the two modalities. For nicotinamide + carbogen, there was no significant change in the radiation enhancement at nicotinamide doses between 250 and 1000 mg/kg.

CONCLUSION

Adding the bioreductive cytotoxin SR 4233 or nicotinamide + carbogen to fractionated irradiation enhances the response of the three transplanted tumors used in this study to fractionated irradiation. The radiation enhancement was significantly greater, however, for SR 4233 for two of the tumors with comparable results in the third. The data are consistent with the prediction that killing tumor hypoxic cells can produce a similar or greater enhancement of the efficacy of fractionated radiation in enhancing tumor response than either oxygenating or radiosensitizing these cells.

Phylogeny of Greya (Lepidoptera: Prodoxidae), based on nucleotide sequence variation in mitochondrial cytochrome oxidase I and II: congruence with morphological data

The phylogeny of Greya Busck (Lepidoptera: Prodoxidae) was inferred from nucleotide sequence variation across a 765-bp region in the cytochrome oxidase I and II genes of the mitochondrial genome. Most parsimonious relationships of 25 haplotypes from 16 Greya species and two outgroup genera (Tetragma and Prodoxus) showed substantial congruence with the species relationships indicated by morphological variation. Differences between mitochondrial and morphological trees were found primarily in the positions of two species, G. variabilis and G. pectinifera, and in the branching order of the three major species groups in the genus. Conflicts between the data sets were examined by comparing levels of homoplasy in characters supporting alternative hypotheses. The phylogeny of Greya species suggests that host-plant association at the family level and larval feeding mode are conservative characters. Transition/transversion ratios estimated by reconstruction of nucleotide substitutions on the phylogeny had a range of 2.0-9.3, when different subsets of the phylogeny were used. The decline of this ratio with the increase in maximum sequence divergence among taxa indicates that transitions are masked by transversions along deeper internodes or long branches of the phylogeny. Among transitions, substitutions of A-->G and T-->C outnumbered their reciprocal substitutions by 2-6 times, presumably because of the approximately 4:1 (77%) A+T-bias in nucleotide base composition. Of all transversions, 73%-80% were A<-->T substitutions, 85% of which occurred at third positions of codons; these estimates did not decrease with an increase in maximum sequence divergence of taxa included in the analysis. The high frequency of A<-->T substitutions is either a reflection or an explanation of the 92% A+T bias at third codon positions.

Tumour hypoxia: the picture has changed in the 1990s

Since the 1950s, the presence of hypoxic cells in human tumours has been widely regarded as a problem, and a variety of strategies have been developed and tested, both in experimental and clinical studies, to overcome this perceived problem. One of these strategies was the development of bioreductive cytotoxins--drugs which in themselves were relatively innocuous, but when metabolized under hypoxic conditions, became highly cytotoxic, thereby preferentially killing the hypoxic cells. Modelling studies and experimental data with newly developed hypoxic cytotoxins, such as SR 4233 (tirapazamine) and RSU 1069, have led to the realization not only that it is better to kill hypoxic cells in tumours than to radiosensitize or oxygenate them, but also that with these bioreductive cytotoxins hypoxic cells in tumours can be an advantage in cancer therapy. However, to realize the advantage of adding the drug with each radiation dose, the tumour must undergo a process analogous to reoxygenation, which we have termed 'rehypoxiation', by which hypoxic cells are regenerated after each dose of the hypoxic cytotoxin. In addition, we also discuss the fact that hypoxia is a cellular stress which activates many new genes. The activation of these genes will be a major focus for research in coming years and will undoubtedly lead to new approaches in cancer detection and treatment. In summary, the 1990s are bringing a fundamental change in our perception of tumour hypoxia, from a position of being a problem to that of being a solution in cancer treatment.

Complementation of the radiosensitive phenotype in severe combined immunodeficient mice by human chromosome 8

Severe combined immunodeficient (scid) C.B-17 mice are deficient in variable (diversity) joining region recombination, the process of assembling the immunoglobulin and T-cell receptor genes from gene segments, thereby creating much of the enormous diversity of antigen-binding capacity, scid mice are also sensitive to ionizing radiation, as a result of their deficiency in double-strand break repair. Here we report the complementation of the radiation-sensitive scid phenotype by transferring human chromosome 8 into scid cells. Somatic cell hybrids were generated by fusing scid cells with human HT-1080 cells, resulting in radioresistant hybrids with several human chromosomes. One of the identified human chromosomes in the radioresistant scid cell line 4.61, which retains only two human chromosomes, is a rearranged 8/21 translocation. Proof that chromosome 8 confers the complementation was achieved by transferring only human chromosome 8 into scid cells by microcell-mediated chromosome transfer (scid/hu8 cell line). The presence of chromosome 8 in our scid/hu8 cell line was monitored by fluorescence in situ hybridization and polymerase chain reaction. We demonstrated the radioresistance of this hybrid not only to high dose rate but also to low dose rate radiation. We also showed that transference of human chromosome 8 to scid cells fully complements the DNA double-strand break repair deficiency and the high sensitivity of scid cells to radiation-induced chromosome aberrations. Mapping the scid gene to human chromosome 8 is an important first step in cloning the scid gene, which will enhance our understanding of double-strand break repair pathways in humans.

Oxygen in human tumors: correlations between methods of measurement and response to therapy. Summary of a workshop held November 19-20, 1992, at the National Cancer Institute, Bethesda, Maryland

Knowledge about the oxygenation of human tumors and its importance in the response to radiotherapy is crucial to the effort to develop improved treatment methods for radiotherapy. The measurement of oxygenation of human tumors and correlations with response to radiotherapy were the subjects of a recent workshop sponsored by the National Cancer Institute. The following methods for measuring oxygen or hypoxia, or a parameter related to either, were presented: polarographic oxygen electrodes, the comet and alkaline elution assays for radiation-induced DNA damage, nitroimidazole binding assays, hemoglobin saturation assays, magnetic resonance spectroscopy, electron spin resonance spectroscopy, phosphorescence imaging, and an assay for tumor interstitial pressure. The electron spin resonance, alkaline elution, and phosphorescence imaging methods have not been used in human tumors. The comet assay, nitroimidazole binding assays, magnetic resonance spectroscopy, cryospectroscopy, and near-infrared spectroscopy have been employed in human tumors, but correlations to treatment response have not been made. Polarographic measurements have indicated that the presence of hypoxia correlates with a poor response to radiotherapy in cervical cancers, but additional data are needed on early-stage disease, and with long-term follow-up on local control and survival. If these confirm the correlation between hypoxia and poor response to radiotherapy, additional tumor sites should be studied. Future clinical trials of treatments that sensitize, exploit, or kill hypoxic cells should identify and include the individual patients with hypoxic tumors. Fundamental unanswered questions regarding the assessment of tumor oxygenation concern the need for invasive procedures, the spatial resolution needed for prediction of response to radiotherapy, the importance of reoxygenation, differences between tumors in rates and degrees of reoxygenation, whether measurements made during a course of therapy are of value, and correlations among methods and with other predictive assays such as intrinsic radiosensitivity and potential doubling time.

Genetic analysis of the sevenless signal transduction pathway of Drosophila

The specification of the R7 photoreceptor cell fate in the developing eye of Drosophila depends on the local activation of the sevenless (sev) receptor tyrosine kinase (RTK) by boss, a protein expressed on the membrane of the neighboring R8 cell. Constitutive activation of the scv receptor results in a dosage dependent increase in the number of R7 cells per ommatidium. Genetic screens have been used to identify mutations that alter the efficiency of signal transduction. Subsequent molecular characterization of the corresponding genes has led to the identification of a number of proteins involved in transducing the signal from the receptor to the nucleus. In contrast to the receptor and its ligand, these components are shared between different signal transduction pathways not only in Drosophila but homologous components are also involved in signal transduction in other organisms.