- Email: [email protected]
Inducible nitric oxide synthase (NOS2) gene polymorphism and parasitic diseases
CORRESPONDENCE
although we presume that they evolved mainly for the latter task. Irrespective of teleology, the most likely mechanism for the association between polymorphisms in xenobiotic metabolising genes and Parkinson’s disease is altered metabolism of neurotoxins. Pesticides are thought to be neurotoxic on the basis of epidemiological studies of Parkinson’s disease, and experimental studies indicate that pesticides are mostly detoxified by glutathione transferases. However, as we had commented, pi class glutathione transferase (GSTP1-1) might activate pesticides. GSTP1-1 variants arise as a result of polymorphisms at two loci (bp105 and 114), and we noted an excess of heterozygotes in patients exposed to pesticides. Because GSTP11 is a dimer, heterozygous individuals will express both homodimers and heterodimers. How can the expression of all GSTP1-1 phenotypes confer increased susceptibility to an environmentally derived neurotoxin? We suggest that at least two sites on the toxin may need to be modified selectively by each of the GSTP1-1 variants to generate the final neurotoxin. This characteristic could identify the Parkinson’s disease neurotoxin. Our study was small and hypothesis generating. Even so, the results emphasise the point that investigations of polymorphic xenobiotic metabolism enzymes and Parkinson’s disease will only show an association in people exposed to neurotoxins unless, of course, the toxins are ubiquitous or the polymorphisms have effects unrelated to xenobiotic metabolism. Other associations have been reported with Parkinson’s and polymorphisms in monoamine oxidase B in cigarette smokers,1 and CYP2D6 in individuals exposed to pesticides.2 Paolini and colleagues state that associations between polymorphic xenobiotic metabolising enzymes, in particular GSTP, and disease are inconsistent but have ignored many positive studies, some of which we had discussed. We believe that such associations may offer critical clues to the pathogenesis of Parkinson’s disease. If polymorphisms in genes for xenobiotic metabolising enzymes alter risk of Parkinson’s disease, then other non-genetic modulators of enzyme activity should similarly affect this risk. Thus, modulation of xenobiotic metabolism by cigarette smoking, diet, and organochlorine pesticides is a plausible mechanism for their effect on the risk of Parkinson’s disease. The greatest risk factor for Parkinson’s disease is old age, yet a mechanism for this association has been
72
elusive.3 Old age is associated with striking changes in xenobiotic and drug metabolism,4 which provides logical explanation of the predisposition of elderly people to Parkinson’s and possibly other diseases of old age. We have postulated that age-related changes in hepatic drug and xenobiotic metabolism are secondary to impaired oxygenation of hepatocytes (the oxygen diffusion barrier theory of ageing).4 It should be possible to improve hepatic oxygenation with locally active and selective hepatic artery vasodilators, which could include low oral doses of prazosin, calcium channel blockers, nitrates, and even sildenafil. *David G Le Couteur, Allan J McLean, Anneke C Blackburn, George D Mellick, Philip G Board *Canberra Clinical School of the University of Sydney, Canberra Hospital, ACT 2605, Australia; John Curtin School of Medical Research, Australian National University, ACT; and University of Queensland, Princess Alexandra Hospital, Woolloongabba (e-mail: [email protected]) 1
2
3
4
Checkoway H, Franklin GM, Costa-Mallen P, et al. A genetic polymorphism of MAO-B modifies the association of cigarette smoking and Parkinson’s disease. Neurology 1998; 50: 1458–61. Hubble JP, Kurth JH, Glatt SL, et al. Gene-toxin interaction as a putative risk factor for Parkinson’s disease with dementia. Neuroepidemiology 1998; 17: 96–104. Langston JW. Epidemiology versus genetics in Parkinson’s disease: progress in resolving an age-old debate. Ann Neurol 1998; 44 (suppl 1): S45–52. Le Couteur DG, McLean AJ. The aging liver: drug clearance and an oxygen diffusion barrier hypothesis. Clin Pharmacokinet 1998; 34: 359–73.
Inducible nitric oxide synthase (NOS2) gene polymorphism and parasitic diseases Sir—David Burgner and colleagues (Oct 10, p 1193)1 report an association between a polymorphism in the promoter region of the inducible nitric oxide synthase (NOS2) gene and the risk of fatal cerebral malaria. This result accords with the findings of Kun and co-workers early this year.2 Altogether these results suggest that susceptibility to severe malaria seems to be determined by complex genetic factors at the NOS2 locus We investigated whether the single nucleotide polymorphism (G-C) in the NOS2 promoter region,2 which is located at position –954 (not at –969 as previously reported2), is associated with susceptibility to or severe forms of Chagas’ disease. We included 85 patients who were serologically positive
for Chagas’ disease and 87 healthy controls from Arequipa, Peru, South America.3 Patients were grouped according to the presence (n=33) or absence (n=52) of cardiomyopathy.3 The NOS2 promoter mutation was analysed by PCR-RFLP with amplification-created restriction site (ACRS) for Xho 1 enzyme. A 60 bp fragment was amplified with 5' primer: 5'-CACTTGAGCTTCAGAGCTC and 3' primer: 5'TGGTAGAGACTGGGTTTCAC. The mutated 5' primer introduces a restriction site for Xho I that recognises the wild type allele. There is a natural restriction site for Bsa I that directly detects the mutation,2 however, Xho I is readily available and cheaper. Analysis of all the samples in the Peruvian population showed that only the wild type occurred. Although studies show association between NOS2 promoter polymorphisms and severe malaria, the functional consequences of these genetic variations in the NOS2 expression is not known. Given the absence of NOS2 promoter mutation in our population where Trypanasoma cruzi infection is endemic and the important role of nitric oxide in the defence against the parasite,4 we believe that NOS2 promoter mutation at –954 may have no functional relevance. On the other hand, Kremsner’s group2 reported that this mutation was not found in 100 controls. Therefore, the NOS2 polymorphisms may be restricted to populations subjected to selective pressure of Plasmodium falciparum. Further studies of distribution of the NOS2 promoter polymorphisms in other areas or ethnic groups where malaria is also endemic will help to elucidate this topic. *Javier Martin, José E Calzada, Antonio Nieto Instituto de Prasitologia y Biomedicina “López Neyra”, CSIC, Calle Ventanilla 11, 18001, Granada, Spain (e-mail:[email protected]) 1
2
3
4
Burgner D, XU Weiming, Rockett K, et al. Inducible nitric oxide synthase polymorphism and fatal cerebral malaria. Lancet 1998; 352: 1193–94. Kun JFJ, Mordmüller B, Lell B, Luckner D, Kremsner PG. Polymorphism in promoter region of inducible nitric oxide synthase gene and protection against malaria. Lancet 1998; 351: 265–66. Beraún Y, Nieto A, Collado MD, González A, Martin J. Polymorphisms at tumor necrosis factor (TNF) loci are not associated with Chagas’ disease. Tissue Antigens 1998; 51: 1–3. Hölscher C, Köhler G, Müller U, Mossmann H, Schaub GA, Brombacher F. Defective nitric oxide effector functions lead to extreme susceptibility of Trypanosoma cruzi-infected mice deficient in gamma interferon receptor or inducible nitric oxide synthase. Infect Immun 1998; 66: 1208–15.
THE LANCET • Vol 353 • January 2, 1999
although we presume that they evolved mainly for the latter task. Irrespective of teleology, the most likely mechanism for the association between polymorphisms in xenobiotic metabolising genes and Parkinson’s disease is altered metabolism of neurotoxins. Pesticides are thought to be neurotoxic on the basis of epidemiological studies of Parkinson’s disease, and experimental studies indicate that pesticides are mostly detoxified by glutathione transferases. However, as we had commented, pi class glutathione transferase (GSTP1-1) might activate pesticides. GSTP1-1 variants arise as a result of polymorphisms at two loci (bp105 and 114), and we noted an excess of heterozygotes in patients exposed to pesticides. Because GSTP11 is a dimer, heterozygous individuals will express both homodimers and heterodimers. How can the expression of all GSTP1-1 phenotypes confer increased susceptibility to an environmentally derived neurotoxin? We suggest that at least two sites on the toxin may need to be modified selectively by each of the GSTP1-1 variants to generate the final neurotoxin. This characteristic could identify the Parkinson’s disease neurotoxin. Our study was small and hypothesis generating. Even so, the results emphasise the point that investigations of polymorphic xenobiotic metabolism enzymes and Parkinson’s disease will only show an association in people exposed to neurotoxins unless, of course, the toxins are ubiquitous or the polymorphisms have effects unrelated to xenobiotic metabolism. Other associations have been reported with Parkinson’s and polymorphisms in monoamine oxidase B in cigarette smokers,1 and CYP2D6 in individuals exposed to pesticides.2 Paolini and colleagues state that associations between polymorphic xenobiotic metabolising enzymes, in particular GSTP, and disease are inconsistent but have ignored many positive studies, some of which we had discussed. We believe that such associations may offer critical clues to the pathogenesis of Parkinson’s disease. If polymorphisms in genes for xenobiotic metabolising enzymes alter risk of Parkinson’s disease, then other non-genetic modulators of enzyme activity should similarly affect this risk. Thus, modulation of xenobiotic metabolism by cigarette smoking, diet, and organochlorine pesticides is a plausible mechanism for their effect on the risk of Parkinson’s disease. The greatest risk factor for Parkinson’s disease is old age, yet a mechanism for this association has been
72
elusive.3 Old age is associated with striking changes in xenobiotic and drug metabolism,4 which provides logical explanation of the predisposition of elderly people to Parkinson’s and possibly other diseases of old age. We have postulated that age-related changes in hepatic drug and xenobiotic metabolism are secondary to impaired oxygenation of hepatocytes (the oxygen diffusion barrier theory of ageing).4 It should be possible to improve hepatic oxygenation with locally active and selective hepatic artery vasodilators, which could include low oral doses of prazosin, calcium channel blockers, nitrates, and even sildenafil. *David G Le Couteur, Allan J McLean, Anneke C Blackburn, George D Mellick, Philip G Board *Canberra Clinical School of the University of Sydney, Canberra Hospital, ACT 2605, Australia; John Curtin School of Medical Research, Australian National University, ACT; and University of Queensland, Princess Alexandra Hospital, Woolloongabba (e-mail: [email protected]) 1
2
3
4
Checkoway H, Franklin GM, Costa-Mallen P, et al. A genetic polymorphism of MAO-B modifies the association of cigarette smoking and Parkinson’s disease. Neurology 1998; 50: 1458–61. Hubble JP, Kurth JH, Glatt SL, et al. Gene-toxin interaction as a putative risk factor for Parkinson’s disease with dementia. Neuroepidemiology 1998; 17: 96–104. Langston JW. Epidemiology versus genetics in Parkinson’s disease: progress in resolving an age-old debate. Ann Neurol 1998; 44 (suppl 1): S45–52. Le Couteur DG, McLean AJ. The aging liver: drug clearance and an oxygen diffusion barrier hypothesis. Clin Pharmacokinet 1998; 34: 359–73.
Inducible nitric oxide synthase (NOS2) gene polymorphism and parasitic diseases Sir—David Burgner and colleagues (Oct 10, p 1193)1 report an association between a polymorphism in the promoter region of the inducible nitric oxide synthase (NOS2) gene and the risk of fatal cerebral malaria. This result accords with the findings of Kun and co-workers early this year.2 Altogether these results suggest that susceptibility to severe malaria seems to be determined by complex genetic factors at the NOS2 locus We investigated whether the single nucleotide polymorphism (G-C) in the NOS2 promoter region,2 which is located at position –954 (not at –969 as previously reported2), is associated with susceptibility to or severe forms of Chagas’ disease. We included 85 patients who were serologically positive
for Chagas’ disease and 87 healthy controls from Arequipa, Peru, South America.3 Patients were grouped according to the presence (n=33) or absence (n=52) of cardiomyopathy.3 The NOS2 promoter mutation was analysed by PCR-RFLP with amplification-created restriction site (ACRS) for Xho 1 enzyme. A 60 bp fragment was amplified with 5' primer: 5'-CACTTGAGCTTCAGAGCTC and 3' primer: 5'TGGTAGAGACTGGGTTTCAC. The mutated 5' primer introduces a restriction site for Xho I that recognises the wild type allele. There is a natural restriction site for Bsa I that directly detects the mutation,2 however, Xho I is readily available and cheaper. Analysis of all the samples in the Peruvian population showed that only the wild type occurred. Although studies show association between NOS2 promoter polymorphisms and severe malaria, the functional consequences of these genetic variations in the NOS2 expression is not known. Given the absence of NOS2 promoter mutation in our population where Trypanasoma cruzi infection is endemic and the important role of nitric oxide in the defence against the parasite,4 we believe that NOS2 promoter mutation at –954 may have no functional relevance. On the other hand, Kremsner’s group2 reported that this mutation was not found in 100 controls. Therefore, the NOS2 polymorphisms may be restricted to populations subjected to selective pressure of Plasmodium falciparum. Further studies of distribution of the NOS2 promoter polymorphisms in other areas or ethnic groups where malaria is also endemic will help to elucidate this topic. *Javier Martin, José E Calzada, Antonio Nieto Instituto de Prasitologia y Biomedicina “López Neyra”, CSIC, Calle Ventanilla 11, 18001, Granada, Spain (e-mail:[email protected]) 1
2
3
4
Burgner D, XU Weiming, Rockett K, et al. Inducible nitric oxide synthase polymorphism and fatal cerebral malaria. Lancet 1998; 352: 1193–94. Kun JFJ, Mordmüller B, Lell B, Luckner D, Kremsner PG. Polymorphism in promoter region of inducible nitric oxide synthase gene and protection against malaria. Lancet 1998; 351: 265–66. Beraún Y, Nieto A, Collado MD, González A, Martin J. Polymorphisms at tumor necrosis factor (TNF) loci are not associated with Chagas’ disease. Tissue Antigens 1998; 51: 1–3. Hölscher C, Köhler G, Müller U, Mossmann H, Schaub GA, Brombacher F. Defective nitric oxide effector functions lead to extreme susceptibility of Trypanosoma cruzi-infected mice deficient in gamma interferon receptor or inducible nitric oxide synthase. Infect Immun 1998; 66: 1208–15.
THE LANCET • Vol 353 • January 2, 1999