Investigating causation in cancer clusters

Questions concerning clusters of cancer cases frequently arise in public health practice. The process of investigating any such cluster requires awareness that such case groupings can easily occur by chance and that any search for biologically meaningful causes will be severely constrained by various methodologic difficulties. These include (1) the long and probably variable latent periods between causative events and cancer diagnosis, (2) the limited numbers of cases available for study in any given cluster situation, and (3) the clinical non-specificity of cancer cases whereby no readily available means are at hand to identify the specific causes for any particular case. Evaluation of any given cluster should involve careful consideration of such limitations, together with a preliminary assessment of the specific cases involved and their community or workplace setting, before more intensive study is undertaken. Received: 22 February 1996 / Accepted in revised form: 13 June 1996     

The effect of age on the recognition thresholds of three sweeteners: sucrose,saccharin and aspartame

It iS believed that people's sensitivity to taste declines with age but the evidence is inconclusive. This study was designed to test the hypothesis that taste recognition thresholds (TRTs) for sweetness are higher in older than in younger individuals, using groups of 16 younger subjects (18–30) and 16 older subjects (60–85). Three test substances were used: sucrose, aspartame and saccharin. A questionnaire recorded variables which might have affected TRTs, but data failed to show any trend that might have biased the principle variate-age. There was a significant alteration with age of recognition thresholds, at least for sucrose and saccharin. The differences between the groups for the three sweeteners were due to the fact that all the very sensitive subjects were young. None of the older subjects had particularly poor discrimination: all but one had TRTs within the range of younger subjects. Although there are age-related taste changes, they are much less dramatic than commonly occurs with other senses, such as sight and hearing. The findings of this study have implications for institutional catering and the dietary management of older people using non-sugar sweeteners.     

Construction of recombination-deficient strains of Streptococcus gordonii by disruption of the recA gene.

Degenerate oligonucleotide primers were used in a polymerase chain reaction (PCR) to amplify a region of the recA sequence of Streptococcus gordonii Challis. The resulting PCR fragment was cloned into the suicide vector pAM6199 and introduced into strain Challis, giving rise to recombination-deficient strains in which the recA gene was specifically inactivated.     

Cleavage of Nuclear DNA into Oligonucleosomal Fragments during Cell Death Induced by Fungal Infection or by Abiotic Treatments

It is often claimed that programmed cell death (pcd) exists in plants and that a form of pcd known as the hypersensitive response is triggered as a defense mechanism by microbial pathogens. However, in contrast to animals, no feature in plants universally identifies or defines pcd. We have looked for a hallmark of pcd in animal cells, namely, DNA cleavage, in plant cells killed by infection with incompatible fungi or by abiotic means. We found that cell death triggered in intact leaves of two resistant cowpea cultivars by the cowpea rust fungus is accompanied by the cleavage of nuclear DNA into oligonucleosomal fragments (DNA laddering). Terminal deoxynucleotidyl transferase-mediated dUTP nick end in situ labeling of leaf sections showed that fungus-induced DNA cleavage occurred only in haustorium-containing cells and was detectable early in the degeneration process. Such cytologically detectable DNA cleavage was also observed in vascular tissue of infected and uninfected plants, but no DNA laddering was detected in the latter. DNA laddering was triggered by [greater than or equal to]100 mM KCN, regardless of cowpea cultivar, but not by physical cell disruption or by concentrations of H2O2, NaN3, CuSO4, or ZnCl2 that killed cowpea cells at a rate similar to that of ladder-inducing KCN concentrations. These and other results suggest that the hypersensitive response to microbial pathogens may involve a pcd with some of the characteristics of animal apoptosis and that DNA cleavage is a potential indicator of pcd in plants.     

Mycobacterium kansasii infection in squirrel monkeys (Saimiri sciureus sciureus)

Abstract: This report documents asymptomatic infections of Mycobacterium kansasii in four of five tuberculin positive squirrel monkeys (Saimiri sciureus sciureus). The mycobacterial DNA amplified by polymerase chain reaction (PCR) from a bronchial lymph node had no affinity for the species specific probes of M. tuberculosis, M. avium, and M. intracellular, thus allowing the presumptive diagnosis of an atypical mycobacterial infection. Infection by Mycobacterium kansasii was confirmed by culture of bronchial lymph nodes from three monkeys. The source of the infection was never identified.     

The mechanism of inhibition of ureogenesis by acidosis

In perfused rat liver a decrease of cytosol pH, determined with pH-sensitive microelectrodes7 from 7.2 to 6.85 is associated with a 50% fall in ureogenesis from ammonium chloride. In isolated rat hepatocytes the fall in ureogenesis due to acidosis is associated with decrease in the mitochondrial and cytosolic concentration of citrulline. Limitation of carbamoyl phosphate synthesis and thus citrulline supply could be responsible for the inhibition of ureogenesis observed.     

Mechanism of inhibition of sickling by dimethyl adipimidate. Effects of intertetramer cross-linking

Dimethyl adipimidate (DMA), an effective antisickling agent in vitro, reacts with free amino groups producing chemically modified and cross-linked molecules. In this report, we have investigated the effect of cross-linked hemoglobin tetramers on sickle hemoglobin polymerization. Since the extent of cross-linking is pH-dependent, we first compared the solubilities of deoxygenated hemolysates prepared from sickle cells previously treated with dimethyl adipimidate at either pH 7.4 or 8.4. The solubility of the hemolysate increased from 18.6 +/- 0.8 g/dl in the untreated sample to 20.9 +/- 1.5 g/dl (pH 7.4) and to 25.4 +/- 3.0 g/dl (pH 8.4) after dimethyl adipimidate treatment. Removal of cross-linked hemoglobin tetramers from hemolysate obtained from dimethyl adipimidate-treated cells abolished part of this effect; at pH 7.4, the solubility decreased from 20.9 +/- 1.5 to 19.4 +/- 0.2 and at pH 8.4 from 25.4 +/- 3.0 to 21.0 +/- 1.5. However, the ratio of [14C]DMA-labelled hemoglobin in the sol phase to that in the gel phase in the unfractionated hemolysate was 1.17 at pH 7.4 and 1.25 at pH 8.4, suggesting that part of the cross-linked hemoglobin tetramers was incorporated into the gel. In order to further investigate the effect of cross-linked hemoglobin tetramers on sickle hemoglobin polymerization, we separated cross-linked hemoglobin tetramers on a gel-filtration column, prepared mixtures of untreated sickle hemoglobin and cross-linked hemoglobin tetramers and studied the polymerization of these mixtures. The Csat of the untreated hemolysate increased progressively from 18.6 +/- 0.8 to 22.5 +/- 0.8 g/dl with 33% cross-linked hemoglobin tetramers. The hemoglobin concentration in the gel decreased from 43 +/- 1.0 to 33.8 +/- 1.0 g/dl with 33% cross-linked hemoglobin tetramers, while the pellet volume fraction, phi p, increased with and almost approached 1 at 50% cross-linked hemoglobin tetramers. In addition, the sol phase contained a higher molecular weight distribution of cross-linked hemoglobin tetramers than the gel phase. These observations suggest that a loose polymer was formed in the gel phase with a hemoglobin concentration much lower than that of the control. Thus, polymerization of sickle hemoglobin is inhibited by: (1) exclusion of higher molecular weight cross-linked hemoglobin tetramers from the gel, and (2) loose incorporation of cross-linked hemoglobin tetramers into the gel, perhaps preventing lateral packing and formation of tightly ordered fibers.     

The taxonomic significance of the mitotic spindle and nucleus-associated organelle ofEntomophaga aulicae

Summary Nuclei in protoplasts ofEntomophaga aulicae contain abundant condensed chromatin and a large central nucleolus. The metaphase spindle occupies a small eccentric area of the nucleus while the remainder of the nucleus is filled with condensed chromatin. Small portions of condensed chromatin are aligned along a broad metaphase plate and connected to the spindle poles by kinetochore microtubules. The nucleus associated organelle (NAO) is a solid barlike structure which lies at the spindle poles and is closely associated with the outer membrane of the nuclear envelope. Comparison of the nuclear characteristics ofE. aulicae with those of other members of theEntomophthorales supports the separation of theEntomophthoraceae from theBasidiobolaceae andAncylistaceae. Further comparison of details of nuclear division in theEntomophthoraceae, specifically NAO morphology, may be useful in helping to delineate evolutionary lines within the family.     

Ozone alteration of transport of cations and the Na+/K+-ATPase in human erythrocytes.

We have purified glutaminase 65-fold from cow brain; the final specific activity is 24 μmol/min/mg. The enzyme is stable between pH 7.5 and 9.0 and has maximal activity at pH 8.8. It requires P_i for activity. The dependence of activity on P_i concentration is sigmoidal; 50 mmP_i gives half-maximal velocity at pH 8.8. At 0.2 mP_i, pH 8.8, the dependence of activity on glutamine concentration is hyperbolic; the observed K_Gln was 30 mm. Increasing P_i concentrations increase the apparent V_m and decrease the apparent K_Gln. NH₄⁺ does not inhibit at concentrations up to 0.1 m. Glutamic acid inhibits competitively with respect to glutamine; at 0.2 mP_i pH 8.8, K_Gln was 30 mm and K_Glu was 19 mm. The results are consistent with a model in which NH₄⁺ is released irreversibly from the enzyme-substrate complex and is the first product released. The activity of glutaminase appears to be independent of the nature of the buffer with which it is equilibrated before being assayed.