The importance of FISH signal cut-off values for 9p21 deletion in malignant pleural mesothelioma: Is it underestimated?

Malignant mesothelioma (MM) is an aggressive cancer with a poor prognosis. The most common genetic alteration in MM is the deletion of the INK4a/ARF locus, which encodes the p16 protein and is located on the short arm of chromosome 9 (9p21). Recently, it has been shown that homozygous deletion of 9p21 has both diagnostic and prognostic significance in MM. It is a known fact that, to interpret fluorescence in situ hybridization (FISH) signals, a cut-off value for each probe should be determined for a correct diagnosis. To our knowledge, there is no consensus or confirmed protocol for cut-off values to evaluate FISH signals in MMs. Therefore, the aim of our research was to address 9p21 deletion status and p16 expression profiles of MM by determining our own cut-off values and the effectiveness of using p16 negativity and 9p21 deletion as markers for differentiating MMs from benign mesothelial proliferations in 114 cases. We established a cut-off value for the detection of 9p21 deletion by using 13 benign reactive cases (6 reactive mesothelial hyperplasias and 7 chronic fibrinous pleuritis cases) and found between 0-7%. According to our calculations, homozygous deletion was defined by loss of both p16 gene signals in at least 13.3% of the nuclei that showed at least 1 signal for the CEP 9 probe. Our FISH results showed homozygous 9p21 deletion in 82 of the 114 cases of MM (71.9%), and p16 expression was negative in 75 of the 114 cases (65.8%). The correlation between loss of p16 protein expression and 9p21 deletion was statistically significant. Among the p16-negative cases, 86.7% also had the 9p21 deletion. The combined examination of the 9p21 deletion and loss of p16 expression is helpful for diagnostic purposes, but because the FISH method is an expensive technique and loss of p16 expression is not specific for mesotheliomas, p16 negativity can guide practitioners to eliminate cases that require further investigation by FISH. The variability in the significance of 9p21 homozygous deletion results from inconsistencies among different institutes, suggesting that each institute should establish its own cut-off value using reactive mesothelial proliferations. Alternatively, global studies are needed to assess cut-off values.