[1913] Combined Use of p16 FISH and Bap1 Immunohistochemistry Optimizes Diagnosis of Malignant Pleural Mesothelioma

Stephanie M McGregor, Agata Minor, Carrie Fitzpatrick, Aliya N Husain, Elizabeth Hyjek, Wickii Vigneswaran, Thomas Krausz. University of Chicago Medicine, Chicago, IL

Background: Malignant pleural mesothelioma (MPM) is a challenging diagnosis that must frequently be made on small biopsies, making useful molecular markers highly desirable. Homozygous deletion of 9q21 (including the CDKN2A locus, which encodes p16) and loss of Bap1 expression by deletion or mutation are common in MPM. Combined analysis of these genes may improve diagnostic accuracy.
Design: We analyzed 22 primary MPM cases (9 epithelioid, 10 biphasic, 3 sarcomatoid) for p16 and Bap1 using immunohistochemistry (IHC) and fluorescence in-situ hybridization (FISH). Seven reactive mesothelial hyperplasia cases served as controls. Nuclear and cytoplasmic p16 staining was scored as negative (<1%), sporadic (<5%), focal (<25%) or diffuse (>25%). Bap1 nuclear staining was scored as positive or negative. Fibroblasts and lymphocytes served as internal positive controls. FISH was scored according to the raw number of signals per cell using the beta inverse statistic to determine cut-off values.
Results: Bap1 staining was diffusely positive in all controls and was either diffusely positive or negative in mesothelioma cases. Importantly, p16 staining was not diffuse in controls, ranging from negative (1/7) to focal (3/7). FISH detected no deletions in any controls. Results for mesothelioma cases are depicted in Table 1.

Table 1: IHC and FISH for p16 and Bap1 in Primary Mesothelioma Cases
 EpithelioidBiphasicSarcomatoidSensitivitySpecificityPositive Predictive ValueNegative Predictive Value
p16 Negative IHC4/910/103/377%86%94%45%
p16 Homozygous Deletion2/98/103/359%100%100%56%
Bap1 Negative IHC8/96/100/364%100%100%47%
Bap1 Homozygous Deletion3/95/100/336%100%100%67%
Combined Bap1 Negative IHC and/or p16 Homozygous Deletion8/910/103/396%100%100%88%



Conclusions: The sporadic nature of p16 staining is concerning for false positives (negative staining) in the setting of limited material, whereas Bap1 IHC staining is diffuse, making it appropriate for use on small biopsies. In contrast to p16 IHC, the use of FISH to detect p16 deletions is highly specific and has a high PPV. When combined, Bap1 IHC and p16 FISH serve as a highly sensitive and specific panel in the diagnosis of MPM that is also cost effective. These results should be interpreted as an adjunct to morphology and must be validated in a larger case series before being applied for routine clinical use.
Category: Pulmonary

Wednesday, March 6, 2013 1:00 PM

Poster Session VI # 281, Wednesday Afternoon

 

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