The cystathionine beta synthase (CBS) gene is localized on chromosome 21 (21q22.3). This enzyme is one of three enzymes able to produce hydrogen sulfide. CBS is overexpressed in Down syndrome with levels 166% of normal values in fibroblasts [Chadefaux et al., 1985] and 1,200% in myeloblasts [Taub et al., 1999]. The CBS overexpression could induceanoverproduction ofhydrogen sulfide in Down syndrome patients, and this overproduction is potentially able to induce some of the clinical signs of Down syndrome such as hypotonia and mental retardation. As thiosulfate is the main catabolite of hydrogen sulfide [Kangas and Savolainen, 1987], we compared the levels of this molecule in the urine of Down syndrome patients and control subjects. Human erythrocytes containvarious formsofhemoglobin.These include sulfhemoglobin, which is formed by transformation of the ferric derivative of hemoglobin, methemoglobin. Sulfhemoglobin production requires hydrogen sulfide (or another sulfide) andmethemoglobin [Nichols et al., 1968]. The determination of sulfhemoglobin in erythrocytes was therefore also used to assess hydrogen sulfide production in Down syndrome. Informed consent was obtained from Down syndrome patients and their parents and from controls. The subjects were assigned to three groups. Group 1 (diet-matched pairs) consisted of 21 pairs of subjects (17 of them were included in a previously published study [Belardinelli et al., 2001]. In each volunteer family, one Down syndrome subject and one relative (mother or father in most families, brother or sister in rare cases) were given identical diets. This group consisted of 13 male and 8 female Down syndrome subjects and matched controls (10 male and 11 female). Sulfur compounds were excluded from the daily treatments of Downsyndromepatientsandcontrols.Group2consisted of 30 patients with Down syndrome (19 male and 11 female) and 20 controls (volunteers from the laboratory; 10 male and 10 female). In this group, age distribution was similar for Down syndrome patients and controls (Table I). The first urine produced in the morning was collected from the subjects of groups 1 and 2 in vials containing boric acid used as a preservative. Thiosulfate was determined in urine by colorimetry after chromatographic separation [Voroteliak et al., 1993]. Creatinine was determined by the manual Jaffe method. Group 3 consisted of 60 Down syndrome patients (33 male and 27 female) and 60 age-matched controls (35 male and 25 female). Venous blood was withdrawn in fasting subjects of group 3; erythrocytes were separated by centrifugation and hemolysates were frozen until use. Sulfhemoglobin was determined by spectrophotometry and the results are expressed as ratio of absorbance (A) at various wawelengths: (A622 nm A636 nm)/(A535 nmþA560 nm) 0.5 10. This ratio was used because it is not affected by differences between the respective concentrations of oxygenated and unoxygenated hemoglobin. A significant difference was observed in the urinary excretion of thiosulfate between Down syndrome patients and relatives of group 1 (diet-matched pairs) (Table I). In group 2, statistical analysis indicates that the differences in thiosulfate excretion persisted. To confirm that hydrogen sulfide was overproduced in Down syndrome patients, we studied erythrocyte sulfhemoglobin content in subjects of group 3. The wawelength ratios were 2.51 0.04 and 2.00 0.08 (SEM) for patientswithDownsyndrome and controls, respectively (P<0.001). We obtained two different types of evidence for the overproduction of hydrogen sulfide in Down syndrome patients. The main function of CBS is to catalyze the first step of transsulphuration pathway, producing cysteine from homocysteine. In vivo, the high level of CBS activity in Down syndrome results in low concentrations of the substrate of CBS (homocysteine) in plasma [Chadefaux et al., 1988]. CBS also has another enzymatic activity: the production of hydrogen sulfide from cysteine [Stipanuck and Beck, 1982]. The endogenous productionofhydrogensulfidecanbeestimatedbymonitoring thiosulfate excretion in urine (31 mmoles/day in control adults) [Sorbo and Ohman, 1978]. After hydrogen sulfide poisoning, the excretion of thiosulfate in urine increased significantly [Kangas and Savolainen, *Correspondence to: Pierre Kamoun, Biochimie B–Tour Lavoisier, Hôpital Necker Enfants Malades, 149, rue de Sèvres, 75743 Paris Cedex 15, France. E-mail: pierre.kamoun@nck.ap-hop-paris.fr

Endogenous hydrogen sulfide overproduction in Down syndrome.

Recent research shows that occupational exposure to several solvents, metals and other industrial chemicals can impair color vision in exposed workers. Occupation-related color vision impairment usually results in blue–yellow color discrimination loss or, less frequently, a combination of blue–yellow and red–green loss. The eyes may be unequally involved, and the course is variable depending on exposure and other factors. The pathogenesis of occupational color vision loss has not been elucidated; it may be due to, e.g. a direct action of neurotoxins on receptors, possibly on the cone’s membrane metabolism, and/or to an interference with neurotransmitters within the retina. Other possible pathogenetic mechanisms, such as a direct effect to the optic nerve, have also been suggested. Occupational color vision loss is usually sub-clinical, and workers are unaware of any deficit. It can be assessed using sensitive tests, such as the Farnsworth–Munsell 100 Hue (FM-100) or the Lanthony D-15 desaturated panel (D-15 d). The latter is the most widely used for studies in groups of exposed workers, and offers the possibility of a quantitative evaluation of the results by calculation of the Bowman’s Color Confusion Index (CCI), or of the Vingrys’ and King Smith’s Confusion Index (CI). Other advantages of D-15 d are the possibility to perform the test directly at the workplace, and the reproducibility when performed in standardized conditions. In most cases, occupation-related color vision impairment is correlated to exposure levels, and has often been observed in workers exposed to environmental concentrations below the current occupational limit proposed by the ACGIH. Progression with increasing cumulative exposure has been reported, while reversibility is still discussed. Acquired color vision impairment related to occupational exposure to styrene, perchloroethylene (PCE), toluene, carbon disulfide, n-hexane, solvent mixtures, mercury and some other chemicals are discussed. Results show that color vision testing should be included in the evaluation of early neurotoxicity of chemicals in exposed workers. The D-15 d would be useful in the surveillance of workers exposed to solvents and other chemicals toxic to the visual system.

Color vision impairment in workers exposed to neurotoxic chemicals.

Objectives The present study was undertaken to examine the relation between visual functions and occupational exposure to styrene. Methods A total of 128 workers (85% of the total population), from three glass-reinforced plastics plants in Canada, agreed to participate in the study. Environmental and biological measures were made on the day(s) prior to the assessment of near visual acuity (National Optical Visual Chart), chromatic discrimination (Lanthony D-15 desaturated panel) and near contrast sensitivity (Vistech 6000). The analyses were performed on 81 workers with near visual acuity of at least 1 minute of arc at 0.5 m. Results Subjects were relatively young [29 (SD 8) years], with little seniority [5 (SD 4) years]. Styrene exposure for 8 hours ranged from 6 to 937 (first quartile 21 mg m-3, third quartile 303 mg m-3), depending on the job site. The end-shift concentrations of urinary mandelic acid ranged from nondetectable to 1.90 mmol mmol creatinine-1. Significant positive relations were found between internal and external styrene exposure measurements and color vision loss adjusted for age, alcohol consumption and seniority in a multiple regression analysis. The multiple regression analysis also showed that end-shift concentration of urinary mandelic acid was inversely related to contrast sensitivity at 6 and 12 cycles degree-1. Logistic multiple regression models indicated that end-shift concentration of urinary mandelic acid was related to the prevalences of blurred vision, tearing and eye irritation. Conclusions These findings suggest that there is a positive relation between styrene exposure and early color and contrast vision dysfunction.

Visual dysfunction among styrene-exposed workers.

Tetrachloroethylene (also called perchloroethylene, or perc), a volatile organic compound, has been the predominant solvent used by the dry-cleaning industry for many years. The U.S. Environmental Protection Agency (EPA) classified perc as a hazardous air pollutant because of its potential adverse impact on human health. Several occupational studies have indicated that chronic, airborne perc exposure adversely affects neurobehavioral functions in workers, particularly visual color discrimination and tasks dependent on rapid visual-information processing. A 1995 study by Altmann and colleagues extended these findings, indicating that environmental perc exposure at a mean level of 4,980 microg/m(3) (median=1,360 microg/m(3)) alters neurobehavioral functions in residents living near dry-cleaning facilities. Although the U.S. EPA has not yet set a reference concentration guideline level for environmental exposure to airborne perc, the New York State Department of Health set an air quality guideline of 100 microg/m(3). In the current residential study, we investigated the potential for perc exposure and neurologic effects, using a battery of visual-system function tests, among healthy members of six families living in two apartment buildings in New York City that contained dry-cleaning facilities on the ground floors. In addition, a day care investigation assessed the potential for perc exposure and effects among workers at a day care center located in the same one-story building as a dry-cleaning facility. Results from the residential study showed a mean exposure level of 778 microg/m(3) perc in indoor air for a mean of 5.8 years, and that perc levels in breath, blood, and urine were 1-2 orders of magnitude in excess of background values. Group-mean visual contrast sensitivity (VCS), a measure of the ability to detect visual patterns, was significantly reduced in the 17 exposed study participants relative to unexposed matched-control participants. The groups did not differ in visual acuity, suggesting that the VCS deficit was of neurologic origin. Healthy workers in the day care investigation were chronically exposed to airborne perc at a mean of 2,150 microg/m(3) for a mean of 4.0 years. Again, group-mean VCS, measured 6 weeks after exposure cessation, was significantly reduced in the nine exposed workers relative to matched controls, and the groups did not differ significantly in visual acuity. These results suggested that chronic, environmental exposure to airborne perc adversely affects neurobehavioral function in healthy individuals. Further research is needed to assess the susceptibility of the young and elderly to perc-induced effects, to determine whether persistent solvent-induced VCS deficits are a risk factor for the development of neurologic disease, and to identify the no observable adverse effect level for chronic, environmental, perc exposure in humans.

Apartment residents' and day care workers' exposures to tetrachloroethylene and deficits in visual contrast sensitivity.

Apartment Residents' and Day Care Workers' Exposures to Tetrachloroethylene and Deficits in Visual Contrast Sensitivity. (Research Articles)

A possible effect of chronic carbon disulfide exposure on the optic nerve was studied by giving the Farnsworth Munsell 100-Hue Test for color discrimination to 62 exposed and 40 nonexposed men. Carbon disulfide exposure did not relate to specific pattern defects in color discrimination, but impaired color discrimination occurred significantly more often in the exposed group than among the referents. The abnormal findings suggest an impairment in the receptiveness of the ganglion cells or demyelination of the optic nerve fibers.

Helpiö E

Papers

Impaired color discrimination among viscose rayon workers exposed to carbon disulfide.