Some 100,000 Iranians were exposed to chemical warfare agents during the 8-year Iraq-Iran war, with the approximately 50,000 mustard gas-affected individuals exhibiting a pattern of late respiratory, eye and skin complications 1. Chronic bronchitis, asthma, bronchiectasis and pulmonary fibrosis account for the most frequent long-term respiratory sequelae, with progressive decline occurring over many years 2345.

Mustard gas, bis (2- chloroethyl) sulphide, is a bifunctional alkylating agent. It is a potent vesicant, whose rapid penetration leads to extensive blistering in all epithelial tissues exposed to it. The immediate toxicity of mustard gas is thought to be due to the consequences of both DNA and protein alkylation reactions 6. However, the factors which drive the development and progression of the long term respiratory complications remain unclear.

The circulating or endocrine renin-angiotensin system (RAS) plays a key role in circulatory homeostasis. Angiotensin Converting Enzyme (ACE) converts angiotensin I to the potent vasoconstrictor angiotensin II. However, local RAS exist in diverse human tissues, where they play proinflammatory and profibrotic roles 789. A lung RAS is now known to exist and is implicated in the genesis of lung inflammatory and fibrotic responses 10.

Whether in the circulation or the tissues 1112, the absence (deletion, D allele) rather than the presence (insertion, I allele) of a 287 base pair fragment in the human ACE gene is associated with increased ACE activity. In keeping with the postulated roles for ACE in the lung, the D-allele has been associated with development of the acute respiratory distress syndrome, and poorer markers of respiratory function in acute illness 1314. However, such detrimental impact of the D-allele in acute illness might be counterbalanced by more positive systemic affects in chronic disease, as suggested by the association of the D-allele with preserved skeletal muscle strength amongst patients with chronic pulmonary disease 15.

We thus postulated that ACE genotype might influence the severity of the late respiratory complications of mustard gas exposure, and have tested this hypothesis in a pilot study.

Thus study was approved by the Ethics Committee of the Iranian Janbazan [Veterans'] organization. Written informed consent was obtained from each subject.

This study is the first to suggest an association between the Angiotensin Converting Enzyme Insertion/Deletion polymorphism and the severity of late respiratory complications of mustard gas exposure, as measured by FEV₁. 33.6% of subjects in the better lung function group carry the DD-genotype, whereas only 12% in this group carry the II-genotype.

The finding that the D-allele was over-represented in subjects with less severe late pulmonary impairment following exposure to mustard gas might be considered unexpected given the higher ACE activity associated with it and the recognized pro-inflammatory and pro-fibrotic roles of increased RAS activity in the lung. In contrast to this finding, previous studies have demonstrated associations of the D-allele with increased severity of pulmonary sarcoid 18, acute lung injury responses in adults 19, non-infectious pulmonary complications of bone marrow transplantation 20 and of esophageal surgery 21, and with the development of bronchopulmonary dysplasia after premature birth 22.

Recently a study of transbronchial lung biopsies in patients with severe mustard gas related lung disease has revealed histopathological changes diagnosable as organizing pneumonia 323. Previously fibrosis had been considered to be the predominant pathology in the late respiratory complications of mustard gas exposure. The pathogenesis of organizing pneumonia may interact with the RAS in a different way to the generation of fibrosis and help explain the association of the D-allele with less severe pulmonary impairment.

Most studies examining the role of the RAS in lung inflammation and fibrosis have associated the I-allele with less severe disease in the short term response to a pathological insult. This study examined pulmonary impairment nearly 20 years after exposure to mustard gas and may suggest that the RAS has a different influence on chronic pulmonary disease.

Additionally we found the severity of pulmonary impairment following mustard gas exposure to be associated with increasing age – findings that accord with those of Zarchi et al, in a study of 1337 soldiers exposed to mustard gas 24. The mechanism underlying this increased susceptibility with age is unclear. It could represent a greater susceptibility at a pulmonary level or could be that removal of children from exposure (both physically and the removal of clothing to prevent ongoing exposure) may have been more effective

A further study is warranted – and one which not only includes far greater numbers, but which obtains far greater phenotypic detail – including detailed non-invasive pulmonary function testing, and imaging.

These findings, and the issue of further study, are of importance for a number of reasons. Certainly, gene-environment studies such as this may prove powerful in exploring the fundamental mechanisms driving progressive lung pathology of diverse origin. In addition, however, there are many tens of thousands of patients suffering the pulmonary sequelae of mustard gas exposure, for whom no specific therapeutic modality is yet available. The demonstration that ACE influences such pathology may open the way to new therapeutic options.

ACE genotype influences the severity of the late respiratory complications of mustard gas exposure with the D allele being associated with higher FEV₁ % predicted 18 years after exposure.

ACE: Angiotensin Converting Enzyme; D: Deletion; FEV₁ Forced Expiratory Volume over 1 second; FVC: Forced vital capacity; I: Insertion; PCR: Polymerase chain reaction amplification; RAS: Renin Angiotensin System; SD: Standard deviation; SEM: Standard error mean.

Authors Dr Julian Thompson and Dr Ali Reza Hosseini Khalili; Dr Anthony Kehoe, Dr Nicholas S Hopkinson, Professor A Khoshbaten, Dr Mohammad Reza Soroush, Professor Steve Humphries; Dr Hugh Montgomery and Professor Mostafa Ghanei have no competing interests to disclose.

JT, ARH–K, AKe, NSH, HM drafted the manuscript and performed the statistical analysis. AKh, MRS, SEH, HM and MG conceived of the study, and participated in its design, data acquisition and coordination. All authors read and approved the final manuscript.