Double Homeobox Protein DUX 4 in the Human Lung: Expression under Normal and Pathological Conditions

Abstract

The double homeobox gene DUX 4 is encoded by a highly conserved open reading frame located in a 3.3 kb macrosatellite repeat sequence on chromosome 4, named D4Z4 locus. Current knowledge of DUX 4 protein expression has been focused on its role in muscle derived pathological disorders. Previous transcriptome analyses of the human lung revealed ubiquitous expression in both normal and non-small cell lung carcinoma tissues. The lack of existing data of functional DUX 4 in the lung prompted a detailed investigation on DUX 4 protein expression in specimens from normal lung and lung tumors as well as NSCLC cell lines by immunohistochemistry, immunofluorescence and western blot analysis. The majority of all tested lung (tumor) tissues and all cell lines consistently exhibited measurable levels of DUX 4 protein. Immunohistochemical DUX 4 expression was evident in the cytoplasm and to a lesser extent in the nucleus, with positively stained tissue ranging between 10 and 50% of the whole tumor in most cases. Likewise, in 20-30% of selected lung tumor cell lines, nucleus-associated DUX 4 was detected by both IHC and IF. Western blot analysis consistently revealed DUX 4 protein bands in the tissue lysates. For the first time, DUX 4 protein has been reported in normal human lung tissues as well as NSCLC. Further evaluation of transcription and functionality of this particular homeobox protein are in progress in order to validate these results and to elucidate the possible role of DUX 4 in the human lung.