Low homozygous/high heterozygous deletion status by p16 FISH correlates with a better prognostic group than high homozygous deletion status in malignant pleural mesothelioma.

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.