Immunohistochemical metallothionein expression in thymoma: correlation with histological types and cellular origin

Metallothioneins (MTs) are low molecular weight and cysteine-rich intracellular proteins involved in metal homeostasis and detoxication. They are found in certain normal tissues, and are overexpressed in various tumours with correlation to more aggressive behaviour in certain tumours. Since the histopathological types of thymoma have unpredictable invasive potential, MT over-expression was investigated as a possible marker of the invasive potential of thymomas. We studied immunohistochemical MT expression in 27 non-invasive thymomas, 20 micro-invasive thymomas, and 23 macro-invasive thymomas with a mouse monoclonal anti-MT antibody E9 on formalin-fixed paraffin-embedded tissues. MT expression was significantly different among the three groups of thymomas ( P  = 0.02) with a stronger expression in invasive thymomas ( P  = 0.003). However, MT expression was not exclusively limited to invasive thymomas. Therefore, it could not be used as a marker of aggressive potential in individual thymomas. Analysis of MT expression according to the histological types of the thymomas revealed that eight of nine spindle cell thymomas, none of 10 small polygonal cell thymomas, four of 14 mixed thymomas, seven of 29 large polygonal cell thymomas, and seven of eight squamoid thymomas significantly expressed MT. There was a statistically significant difference in MT expression among different histological types of thymomas ( P  = 0.000). The strongest and most consistent expression was observed in spindle cell thymoma and squamoid thymoma. Since spindle cell thymoma was usually non-invasive and squamoid thymoma was more aggressive, MT expression does not correlate with the invasive potential of different histological types of thymomas. But because medullary epithelial cells of the thymus were positive for MT, our results suggest that both spindle cell thymoma and squamoid thymoma might derive from the medullary compartment of the thymus.