PURPOSE - To investigate the relationship between loss of heterozygosity (LOH) at the mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R) gene locus and the development of radiation-induced lung injury.
MATERIAL AND METHODS - Thirty-five lung cancer patients with both stored plasma for Transforming Growth Factor beta1 (TGFbeta1) analysis and sufficient quantities of archival pathology tissue to screen for LOH were studied. All patients had been treated with thoracic radiotherapy for their malignancy and had radiographically detectable tumor present before beginning radiotherapy. Tumor and normal cells were microdissected from archival lung cancer pathology specimens. Two polymorphisms in the 3' untranslated region of the M6P/IGF2R were used to screen for LOH. Plasma TGFbeta1 levels were measured using acid-ethanol extraction and an ELISA. TGFbeta1 and M6P/IGF2R protein expression was estimated by immunofluorescence and immunohistochemical staining. Symptomatic radiation pneumonitis was scored according to National Cancer Institute Common Toxicity Criteria without knowledge of the results of TGFbeta or LOH analyses.
RESULTS - Of the 35 patients, 10 were homozygous for this polymorphism (noninformative) and were excluded. Of the 25 informative patients, 13 had LOH. Twelve of 13 patients with LOH had increased pretreatment plasma TGFbeta1 levels, vs. 3/12 patients without LOH (p < 0.01). A decrease or loss of M6P/IGF2R protein in the malignant cell accompanied by increased latent TGFbeta1 protein in extracellular matrix and tumor stroma was found in tumors with LOH, suggesting that this mutation resulted in loss of function of the receptor. Seven of 13 (54%) LOH patients developed symptomatic radiation-induced lung injury vs. 1/12 (8%) of patients without LOH (p = 0.05).
CONCLUSION - Loss of the M6P/IGF2R gene strongly correlates with the development of radiation pneumonitis after thoracic radiotherapy (RT). Furthermore, patients with LOH (in the setting of measurable tumor) are much more likely to have elevated plasma TGFbeta, suggesting an inability to normally process this cytokine. Thus, loss of the M6P/IGF2R gene may predispose patients to the development of radiation-induced lung injury.