Low homozygous/high heterozygous deletion status by p16 FISH correlates with a better prognostic group than high homozygous deletion status in malignant pleural mesothelioma. (September 2016) - Hamasaki, Makoto
Highlights: We set appropriate cutoff values of p16 FISH in malignant pleural mesothelioma (MPM). p16 FISH with appropriate cutoff values was useful to diagnose MPM. Homozygous deletion of p16 in MPM was significantly correlated with shorter survival. Low homo/high heterozygous deletion status of p16 correlated with longer OS in MPM. Abstract: Objectives: Homozygous deletion (homo-d) of the p16 ( CDKN2A ) gene, as determined by fluorescence in situ hybridization (FISH), helps differentiate malignant pleural mesothelioma (MPM) from reactive mesothelial hyperplasia (RMH). Heterozygous deletion (hetero-d) has also been identified variably in p16 FISH. This study aimed to investigate the significance of homo-d and hetero-d of p16 in the diagnosis and prognosis of MPM. Materials and methods: p16 FISH was performed in 93 MPMs and 47 RMHs. Real-time polymerase chain reaction (PCR) and methylation specific PCR (MSP) were also performed for cases in which DNA was available. Overall survival (OS) was assessed via the Kaplan-Meier method and logrank test. Results: Cutoff values for homo-d and hetero-d were set at 10% and 47%, respectively, based on p16 FISH results in RMH. In MPM, 80/93 (86.0%) were homo-d positive, and 15/93 (16.1%) were hetero-d positive. No RMH was homo/hetero-d positive. In nine cases of MPM with the low homo-d (<30%)/high hetero-d (>47%) pattern, FISH with a shorter probe caused a slight increase (from 20.1% to 26.5%) in the mean percentage of homo-d and a decrease in that of hetero-d (from 59.6% to 55.6%). Four cases in which the low homo-d/high hetero-d pattern was maintained with the shorter probe were further analyzed by real-time PCR, which separated them into a two (n = 2) or one allele deletion group (n = 2). MSP revealed no promoter methylation in the two cases with one allele deletion. The OS was significantly shorter in homo-d positive cases (n = 24) than homo-d negative cases (n = 5, p = 0.0002) in the 29 MPM cases with follow-up data. Also, low homo-d/high hetero-d cases (n = 5) had a significantly better prognosis than high homo-d (≥30%) cases (n = 17, p = 0.011). Conclusions: Within p16 homo-d positive MPMs with poorer prognosis, the low homo-d/high hetero-d pattern may belong to a better prognostic subgroup in homo-d positive MPMs.