Expression analysis of HMGB1 in histological samples of malignant pleural mesothelioma

High mobility group box 1 (HMGB1) is a chromatin structural protein, expressed ubiquitously in the nuclei of mammalian cells. When transported extracellularly, it acts as a tumour suppressor and oncogenic protein. In malignant pleural mesothelioma (MPM), high serum levels of HMGB1 have been related to a poor prognosis. Conversely, the significance of HMGB1 expression in MPM tissues is still unclear.

Biopsy samples from 170 patients with MPM were assessed by immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) to evaluate HMGB1 protein and gene expression. The expression level of HMGB1 protein was scored using a semiquantitative system that sums the intensity (0-3) and the percentage (from 0 to 4) of positively stained cells in nuclei, cytoplasm and in both. The final score was considered as high (>3) or low (<3) expression. Gene expression levels were calculated using the ΔΔC_t method. High expression levels of HMGB1 as total (P = 0.0011) and cytoplasmic score (P = 0.0462) were related to a worse disease-specific survival (DSS) in the entire cohort and in the clinicopathological subgroups. No significant correlation was found between HMGB1 gene expression and DSS.

These findings indicate that HMGB1 may be a useful prognostic biomarker in MPM when detected by immunohistochemistry. Conversely, as it is also expressed in normal and reactive mesothelial cells, HMGB1 cannot be considered a diagnostic biomarker in histological samples of mesothelioma.