Exploring the utility of the Threshold of Toxicological Concern (TTC) as a screening approach for complex substances

The Threshold of Toxicological Concern (TTC) has been applied to assess chemical safety for use, particularly in the food safety area. Although the TTC was developed for application to an individual chemical structure, more recently this concept has been suggested for the assessment of combined exposures to multiple chemicals. This study evaluated the potential for applying the TTC to a specific type of co-exposure, that of a complex substance of variable composition which contains multiple constituents, following the World Health Organization/International Programme on Chemical Safety framework for risk assessment of combined exposure to multiple chemicals. The results indicated that the TTC threshold was lower (i.e., more conservative) than regulatory thresholds derived for the same substance or even its most toxic constituent, providing assurance that the TTC could meet the requirements for a conservative screening process. This case study indicates that the TTC concept can be a useful tool to screen for potential risks from complex substances, with the consideration of additional aspects such as variability in chemical constituents and their relative proportions within the substance.

Publisher Correction: A protein assembly mediates Xist localization and gene silencing

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

X chromosome reactivation dynamics reveal stages of reprogramming to pluripotency

Reprogramming to iPSCs resets the epigenome of somatic cells, including the reversal of X chromosome inactivation. We sought to gain insight into the steps underlying the reprogramming process by examining the means by which reprogramming leads to X chromosome reactivation (XCR). Analyzing single cells in situ, we found that hallmarks of the inactive X (Xi) change sequentially, providing a direct readout of reprogramming progression. Several epigenetic changes on the Xi occur in the inverse order of developmental X inactivation, whereas others are uncoupled from this sequence. Among the latter, DNA methylation has an extraordinary long persistence on the Xi during reprogramming, and, like Xist expression, is erased only after pluripotency genes are activated. Mechanistically, XCR requires both DNA demethylation and Xist silencing, ensuring that only cells undergoing faithful reprogramming initiate XCR. Our study defines the epigenetic state of multiple sequential reprogramming intermediates and establishes a paradigm for studying cell fate transitions during reprogramming.

Mediator and SAGA have distinct roles in Pol II preinitiation complex assembly and function

A key feature of RNA polymerase II (Pol II) preinitiation complexes (PICs) is their ability to coordinate transcription initiation with chromatin modification and remodeling. To understand how this coordination is achieved, we employed extensive proteomic and mechanistic analyses to study the composition and assembly of PICs in HeLa cell and mouse embryonic stem cell (ESC) nuclear extracts. Strikingly, most of the machinery that is necessary for transcription initiation on chromatin is part of the PIC. The PIC is nearly identical between ESCs and HeLa cells and contains two major coactivator complexes: Mediator and SAGA. Genome-wide analysis of Mediator reveals that it has a close correlation with Pol II, TATA-binding protein, and messenger RNA levels and thus may play a major role in PIC assembly. Moreover, Mediator coordinates assembly of the Pol II initiation factors and chromatin machinery into a PIC in vitro, whereas SAGA acts after PIC assembly to allow transcription on chromatin.

Differential alterations in tachykinin NK2 receptors in isolated colonic circular smooth muscle in inflammatory bowel disease and idiopathic chronic constipation

Inflammatory bowel disease (IBD) and idiopathic chronic constipation (ICC) are intestinal disorders which disrupt normal colonic motility. Enteric tachykinins are well-recognised to play a role in the motor control of the gut, and increased colonic levels of substance P are seen in IBD, whereas decreased levels have been reported in ICC. In this investigation, we have characterised the tachykinin receptor population of normal human colonic circular smooth muscle and examined any changes that occur in IBD and ICC. The selective tachykinin NK2 receptor agonist, [beta-Ala8]neurokinin A(4-10), caused concentration-dependent contractions in healthy tissues; neither NK1 receptor-selective nor NK3 receptor-selective agonists were contractile. In diseased preparations also, only [beta-Ala8]neurokinin A(4-10) caused contractions with EC50 values similar to health. The maximum contractile responses (Emax), however, were significantly decreased in both forms of IBD but significantly increased in ICC. The muscarinic acetylcholine receptor agonist, carbachol, also caused contractions in diseased tissues, but EC50 and Emax values were not significantly different from health. The differential changes in contractility found in IBD and ICC are specific to NK2 receptors, and may reflect the altered levels of substance P or other tachykinins found in these intestinal disorders.

Breeding of retroviruses by DNA shuffling for improved stability and processing yields

Manufacturing of retroviral vectors for gene therapy is complicated by the sensitivity of these viruses to stress forces during purification and concentration. To isolate viruses that are resistant to these manufacturing processes, we performed breeding of six ecotropic murine leukemia virus (MLV) strains by DNA shuffling. The envelope regions were shuffled to generate a recombinant library of 5 x 106 replication-competent retroviruses. This library was subjected to the concentration process three consecutive times, with amplification of the surviving viruses after each cycle. Several viral clones with greatly improved stabilities were isolated, with the best clone exhibiting no loss in titer under conditions that reduced the titers of the parental viruses by 30- to 100-fold. The envelopes of these resistant viruses differed in DNA and protein sequence, and all were complex chimeras derived from multiple parents. These studies demonstrate the utility of DNA shuffling in breeding viral strains with improved characteristics for gene therapy.

Rates of small intestinal mucosal protein synthesis in human jejunum and ileum

We investigated possible differences in the rates of mucosal protein synthesis between the proximal and distal regions of the small intestine. We took advantage of access to the gut mucosa available in otherwise healthy patients with ileostomy in whom the terminal ileum was histologically normal. All subjects received primed, continuous intravenous infusions of L-[1-(13)C]leucine after an overnight fast. After 4 h of tracer infusion, jejunal biopsies were obtained using a Crosby-Kugler capsule introduced orally; ileal biopsies were obtained via endoscopy via the ileostomy. Protein synthesis was calculated from protein labeling relative to intracellular leucine enrichment obtained by appropriate mass spectrometric measurements. Rates of jejunal and ileal mucosal protein synthesis were significantly different (P < 0.001) at 2.14 +/- 0.2 and 1.2 +/- 0.2 %/h (means +/- SD). These are lower than rates in normal healthy duodenum (2.53 +/- 0.25 %/h), suggesting a gradation of rates of synthesis along the bowel. Together with other data, these results suggest that mucosae of the bowel contribute not more than 10% to whole body protein turnover.

Toxicology of n-pentane (CAS no. 109-66-0)

n-Pentane (CAS no. 109-6-0) is a hydrocarbon solvent with an estimated production volume of 50000 metric tons in Europe. The present work was carried out to strengthen the toxicological information available for regulatory decision-making, particularly product classification, occupational exposure limits and risk assessment. The work described in this report was sponsored either by the Pentane Special Interest Group under the auspices of the Hydrocarbon Solvent Producers Association of CEFIC (European Chemical Industry Council) or by Exxon Chemical Europe. The following results were obtained: n-pentane was not acutely toxic by oral or inhalation routes, it was not a skin or eye irritant and did not induce skin sensitization. It did not exhibit cumulative toxicity at levels up to 20000 mg m(-3), which is approximately one-half of the lower explosive limit and the highest level considered safe to test. It did not induce developmental toxicity and was not mutagenic. From this and other existing information, it is concluded that n-pentane does not require classification for potential toxic hazards (as defined by Annex VI of the EU Dangerous Substance Directive, 1993), although the physical properties indicate that it would be appropriate to warn for the potential for aspiration. Further, there is no reason to reduce the current occupational exposure recommendations. Finally, although there are some safety issues relating to the flammability of n-pentane, the absence of any demonstrable toxicity at high treatment levels indicates that the risk of adverse health effects is minimal for all segments of the population.

Chromosomal characterization of MRC-5 cell banks utilizing G-banding technique

We have performed chromosomal monitoring on 18 MRC-5 cell banks using the G-banding technique. A higher frequency of structural abnormalities and hyperdiploidy was detected with respect to the numerical values that were established with conventional staining techniques and reported in the 1979 Ad Hoc Committee guideline on karyology controls of human cell substrates. These numerical criteria have been adopted by international regulatory agencies to release human diploid cell banks (WI-38 and MRC-5) for vaccine manufacture. In the process of characterizing these 18 cell banks, a 7;12 translocation clone was detected in 7 cell banks at a frequency ranging from 0.2 to 5.6%. The presence of the t(7;12) appears to be biphasic. At low population doubling levels (PDLs) (< 30), t(7;12) is rarely observed. However, the incidence of t(7;12) increases and plateaus between PDL 40-50. A decrease in frequency is observed at higher PDLs. Before senescence of the cell bank, t(7;12) is not observed. Investigation of the growth characteristics of MRC-5 cells revealed that cell banks containing the translocation senesced at similar PDLs compared to MRC-5 cells with no detectable 7;12 translocation. In addition, MRC-5 cell banks containing the t(7;12) have successfully completed tumorigenicity testing in a nude mouse model. We conclude that MRC-5 cells obtained from either National Institute for Biological Standards and Control (NIBSC) or American Type Culture Collection (ATCC) contain a 7;12 translocation at a low frequency. This abnormality does not provide MRC-5 cell bank mass cultures with a growth advantage nor is it tumorigenic in nude mice. Furthermore, the presence of this clone and employment of the G-banding technique may be responsible for the increased observation of structural abnormalities in our laboratories. In view of this information, the cytogenetic criteria that were established in 1979 with conventional staining techniques are not appropriate for human diploid cell banks that are examined with more sensitive methodology. Since it has been recognized that MRC-5 and WI-38 cells are safe biological substrates, we recommend that MRC-5 and WI-38 cell banks should only be identified by using an appropriate identity test and should not require any chromosomal analysis before being used as a cell substrate for the manufacture of live virus vaccines.

A prospective trial of endoscopic sclerotherapy v oesophageal transection and gastric devascularisation in the long term management of bleeding oesophageal varices

In a prospective three centre study oesophageal transection and gastric devascularisation have been compared with endoscopic sclerotherapy in the long term management of bleeding oesophageal varices. Cirrhotic patients (Child's A or B grade) with documented bleeding oesophageal varices were treated initially with emergency sclerotherapy, and after five days stability, were allocated to one of the two treatment regimes. The endoscopic sclerotherapy group underwent regular sclerotherapy until variceal obliteration while those undergoing surgery were not endoscoped unless bleeding recurred, when they were treated by sclerotherapy if appropriate. Ninety two patients were eligible for analysis (68% alcoholic cirrhosis; mean age 50.1 years) and follow up was achieved for a mean of 52.5 months (range 17-83). Mortality in the first three months was greater in the oesophageal transection and gastric devascularisation group (20% v 1%) but by two years the survival curves were the same and thereafter there was no difference in mortality. Rebleeding occurred in 13/41 (31%) patients, undergoing oesophageal transection and gastric devascularisation. The costs incurred during the first year of oesophageal transection and gastric devascularisation treatment were significantly greater than with endoscopic sclerotherapy (4369 pounds v 1067 pounds, p < 0.0001) and the high rate of rebleeding in the surgical group meant that no cost savings occurred in subsequent years. It is concluded that oesophageal transection and gastric devascularisation confers no benefit over endoscopic sclerotherapy in terms of long term survival and that it is not cost effective as judged by the current health care costs in the United Kingdom.

Influence of prior events on cognitive judgments in amnesia

Amnesic patients and control Ss read the names of famous and nonfamous persons. Subsequently, both groups were more likely to designate a name as famous if it had been encountered previously. The facilitatory effect of prior presentation was similar for amnesic patients and control Ss and similar for famous and nonfamous names. For amnesic patients, the effect occurred despite severely impaired recognition memory for the names. In a 2nd experiment, recombining the first and last names that had been presented together did not diminish the facilitatory effect of prior presentation, which indicates that the effect does not depend on forming an association between first and last names. The results show that nondeclarative (implicit) memory can support the acquisition of information that is specific (e.g., names of persons) and that has no preexisting representation (e.g., nonfamous names).

Influence of prior events on cognitive judgments in amnesia

Amnesic patients and control Ss read the names of famous and nonfamous persons. Subsequently, both groups were more likely to designate a name as famous if it had been encountered previously. The facilitatory effect of prior presentation was similar for amnesic patients and control Ss and similar for famous and nonfamous names. For amnesic patients, the effect occurred despite severely impaired recognition memory for the names. In a 2nd experiment, recombining the first and last names that had been presented together did not diminish the facilitatory effect of prior presentation, which indicates that the effect does not depend on forming an association between first and last names. The results show that nondeclarative (implicit) memory can support the acquisition of information that is specific (e.g., names of persons) and that has no preexisting representation (e.g., nonfamous names).

Impaired anal sensation and early diabetic faecal incontinence

Faecal incontinence develops in up to 20% of diabetic patients. To try to determine the relative contributions of sensory and motor neuropathy in this troublesome complication, anorectal function was examined in 10 male diabetic patients with early faecal incontinence (mucus leakage or faecal staining without the need to wear a pad), 10 asymptomatic male diabetic patients, and 10 normal control subjects. Motor function was tested using anal manometry to determine the resting and maximum squeeze pressure, and the functional anal canal length. No significant differences were found between the groups. Sensory function was tested by measuring the mucosal sensitivity to electrical stimulation, and the response to inflation of a balloon in the rectum. In the mid-anal canal position the symptomatic patients had a significantly higher sensory threshold at 6.6 +/- 2.8 mA compared with 3.0 +/- 1.2 mA in the normal control subjects (p less than 0.002), and in the high anal zone symptomatic patients had a significantly elevated sensory threshold at 9.1 +/- 2.0 mA compared with 4.6 +/- 1.6 mA in asymptomatic patients and 3.6 +/- 1.3 mA in the normal control subjects (both p less than 0.001). There were no significant differences in the first sensation of fullness, maximum tolerated volume or percentage fall from resting pressure between the groups on inflation of the balloon. Elevation of the sensory threshold in the upper anal canal is an early abnormality in the development of diabetic faecal incontinence.

A study of octreotide in oesophageal varices

A comparison of octreotide infusion (25 micrograms/h) and placebo in 16 stable cirrhotic patients revealed a 30% reduction in transhepatic venous gradient between 0 and 60 min in the octreotide group without an effect on systemic haemodynamics. In a separate trial, 40 patients with active variceal bleeding were randomized to octreotide infusion (25 micrograms/h for 48 h) or oesophageal tamponade. The 2 treatments gave comparable control of variceal bleeding. Tolerance of treatment was significantly better in the octreotide group. In summary, octreotide infusion is simple to administer, has few side effects, and may be of use in the immediate control of oesophageal bleeding.

A simple and efficient non-organic procedure for the isolation of genomic DNA from blood

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Muscle creatine content in rats given repeated large doses of nicotinamide: effects of dietary methionine, choline, carnitine, and other supplements

Rats fed a 12% casein diet without added choline were chronically injected with pharmacologic doses of nicotinamide (NAM) for 18 to 28 d in three experiments. In addition to fatty livers and lower weight gains, the creatine content of heart and skeletal muscle was lower in NAM-treated than in control saline-injected rats. Dietary supplements of methionine prevented these alterations, and choline, depending on the level of supplementation, also had some similar effects. No consistent responses could be demonstrated by supplements of carnitine, histidine or folic acid plus vitamin B-12.

Does antibiotic prophylaxis at the time of catheter insertion reduce the incidence of catheter-related sepsis in intravenous nutrition?

Fifty-five patients referred for intravenous nutrition were randomised to receive either vancomycin (500 mg) or no vancomycin immediately before catheter insertion. Catheter insertion and management were standard in all patients. The mean duration of intravenous feeding was similar in both groups (vancomycin: 17.7 +/- 9.6 days; no vancomycin: 16.5 +/- 8.8 days). Overall, the bacteriologically confirmed catheter-related sepsis was 25% and was not reduced by the prophylactic administration of vancomycin.