Genetic variants associated with arsenic susceptibility: study of purine nucleoside phosphorylase, arsenic (+3) methyltransferase, and glutathione S-transferase omega genes.

BACKGROUND: Individual variability in arsenic metabolism may underlie individual susceptibility toward arsenic-induced skin lesions and skin cancer. Metabolism of arsenic proceeds through sequential reduction and oxidative methylation being mediated by the following genes: purine nucleoside phosphorylase (PNP), arsenic (+3) methyltransferase (As3MT), glutathione S-transferase omega 1 (GSTO1), and omega 2 (GSTO2). PNP functions as arsenate reductase; As3MT methylates inorganic arsenic and its metabolites; and both GSTO1 and GSTO2 reduce the metabolites. Alteration in functions of these gene products may lead to arsenic-specific disease manifestations. OBJECTIVES: To find any probable association between arsenicism and the exonic single nucleotide polymorphisms (SNPs) of the above-mentioned arsenic-metabolizing genes, we screened all the exons in those genes in an arsenic-exposed population. METHODS: Using polymerase chain reaction restriction fragment length polymorphism analysis, we screened the exons in 25 cases (individuals with arsenic-induced skin lesions) and 25 controls (individuals without arsenic-induced skin lesions), both groups drinking similar arsenic-contaminated water. The exonic SNPs identified were further genotyped in a total of 428 genetically unrelated individuals (229 cases and 199 controls) for association study. RESULTS: Among four candidate genes, PNP, As3MT, GSTO1, and GSTO2, we found that distribution of three exonic polymorphisms, His20His, Gly51Ser, and Pro57Pro of PNP, was associated with arsenicism. Genotypes having the minor alleles were significantly overrepresented in the case group: odds ratio (OR) = 1.69 [95% confidence interval (CI), 1.08-2.66] for His20His; OR = 1.66 [95% CI, 1.04-2.64] for Gly51Ser; and OR = 1.67 [95% CI, 1.05-2.66] for Pro57Pro. CONCLUSIONS: The results indicate that the three PNP variants render individuals susceptible toward developing arsenic-induced skin lesions.

Arsenic-induced health effects and genetic damage in keratotic individuals: involvement of p53 arginine variant and chromosomal aberrations in arsenic susceptibility.

In West Bengal, India, more than 6 million people are exposed to arsenic through drinking water. Chronic arsenic exposure results in several multisystemic non-cancerous as well as cancerous effects in humans. Among non-cancerous effects, arsenic-specific skin lesions, conjunctivitis, peripheral neuropathy and respiratory diseases are prominent. One of the major consequences of chronic arsenic exposure is keratosis, the precancerous state of skin cancer. The tumor suppressor protein p53 consists of a polymorphism proline72arginine reported to be associated with various types of cancers. Previously we have reported that the p53 codon 72 arginine (Arg) homozygous genotype is associated with the development of arsenic-induced keratosis. In the present study we have investigated the distribution of health effects and chromosomal aberrations (CAs) in the individuals with keratosis. We have compared individuals with keratosis with those without arsenic-induced skin lesions but drinking similar level of arsenic-contaminated water. Attempts have also been made to find out the association of the p53 risk genotype with health effects and chromosomal aberrations. This study comprises of 349 unrelated exposed individuals (162 individuals with keratosis and 187 individuals without arsenic-specific skin lesions) from highly arsenic-affected districts of West Bengal, India. The results showed that health effects (i.e. peripheral neuropathy, conjunctivitis and respiratory illness) and chromosomal aberrations were significantly higher in the keratotic group compared to individuals with no skin lesions. Moreover, individuals with the arginine homozygous genotype showed increased levels of chromosomal aberrations compared to individuals with other genotypes; however, we did not find any significant association of the risk genotype with health effects. This study suggests that individuals with keratosis are more susceptible to arsenic-induced health effects and genetic damage and that the arginine variant of p53 can further influence the repair capacity of arsenic-exposed individuals, leading to increased accumulation of chromosomal aberrations.

Increased chromosome aberration frequencies in the Bowen's patients compared to non-cancerous skin lesions individuals exposed to arsenic.

Cytogenetic biomarkers are essential for assessing environmental exposure that can predict adverse human health effects such as cellular damage. Chromosomal aberrations are the most important cytogenetic end-points successfully used for the cancer risk assessment of populations occupationally or environmentally exposed to different toxic chemicals. Previous reports suggest that, increased frequency of chromosomal aberration (CA), in peripheral blood lymphocytes, is a predictor of cancer. Arsenic is a paradoxical human carcinogen, clastogen and aneugen. Despite of exposure at similar extent, only 15-20% of individuals show arsenic induced skin lesions including Bowen's disease (BD). Previously we have reported the significant increase in CA in the individuals with arsenic induced skin lesions when compared to individuals without any skin lesions, drinking arsenic contaminated water at similar extent. Presently, a matched case-control study was performed to examine whether biomarkers such as chromosomal aberrations can predict the development of arsenic induced Bowen's (in situ carcinoma) diseases. Chromosomal aberrations (both chromosome and chromatid types) and mitotic index were analyzed from the lymphocytes of 25 cases of Bowen's patient which was compared to matched control from the individuals with arsenic induced non-cancerous skin lesions such as raindrop pigmentation, keratosis of palm and sole, hypo and hyper pigmentation. Chromosomal aberrations/cell, chromosome type aberrations and total percentage of aberrant cells were significantly higher in cases compared to control (p<0.01). These results suggest that chromosomal aberrations can be used for cancer risk assessment of the population exposed to arsenic through drinking water.

Comparison of health effects between individuals with and without skin lesions in the population exposed to arsenic through drinking water in West Bengal, India.

A study was conducted to explore the effect of arsenic causing conjunctivitis, neuropathy and respiratory illness in individuals, with or without skin lesions, as a result of exposure through drinking water, contaminated with arsenic to similar extent. Exposed study population belongs to the districts of North 24 Parganas and Nadia, West Bengal, India. A total of 725 exposed (373 with skin lesions and 352 without skin lesions) and 389 unexposed individuals were recruited as study participants. Participants were clinically examined and interviewed. Arsenic content in drinking water, urine, nail and hair was estimated. Individuals with skin lesion showed significant retention of arsenic in nail and hair and lower amount of urinary arsenic compared to the group without any skin lesion. Individuals with skin lesion also showed higher risk for conjunctivitis ((odd's ratio) OR: 7.33, 95% CI: 5.05-10.59), peripheral neuropathy (OR: 3.95, 95% CI: 2.61-5.93) and respiratory illness (OR: 4.86, 95% CI: 3.16-7.48) compared to the group without any skin lesion. The trend test for OR of the three diseases in three groups was found to be statistically significant. Again, individuals without skin lesion in the exposed group showed higher risk for conjunctivitis (OR: 4.66, 95% CI: 2.45-8.85), neuropathy (OR: 3.99, 95% CI: 1.95-8.09), and respiratory illness (OR: 3.21, 95% CI: 1.65-6.26) when compared to arsenic unexposed individuals. Although individuals with skin lesions were more susceptible to arsenic-induced toxicity, individuals without skin lesions were also subclinically affected and are also susceptible to arsenic-induced toxicity and carcinogenicity when compared to individuals not exposed to arsenic.

Association of specific p53 polymorphisms with keratosis in individuals exposed to arsenic through drinking water in West Bengal, India.

Although, more than six million people are endemically exposed to inorganic arsenic in West Bengal, India by drinking heavily contaminated groundwater, only about 300,000 people show arsenic induced skin lesions. This suggests that genetic variability plays an important role in arsenic induced skin lesions and skin cancers. Arsenic induced keratosis is considered as a possible precancerous state of in situ carcinoma. Several reports have suggested the role of p53 polymorphisms as potential marker for risk assessment of different types of cancers. This prompted us to study the association of three p53 polymorphisms with arsenic induced keratosis in a population exposed to arsenic through drinking water. A total of 366 unrelated individuals (177 individuals with arsenic induced keratosis and 189 individuals with no arsenic induced skin lesions) were recruited from North 24 Parganas, Nadia and Murshidabad districts between January 2003 and February 2005 for the study of the genotypic distribution of three p53 polymorphisms (16bp duplication at intron 3, codon 72 Arg/Pro and G>A at intron 6 [nt 13,494]) by PCR-RFLP. The arginine homozygous genotype at codon 72, and homozygous genotype of no duplication polymorphism at intron 3 were over represented in the individuals with keratosis compared with individuals with no skin lesions (OR=2.086; 95% CI=1.318-3.299 and OR=2.086; 95% CI=1.257-3.457, respectively). This study indicates that individuals carrying the arginine homozygous genotype at codon 72, and/or no duplication homozygous genotype at intron 3 are at risk for the development of arsenic induced keratosis.