Use of PTEN protein dosage to predict for underlying germ-line PTEN mutations among patients presenting with thyroid cancer and Cowden-like phenotypes.

Abstract

1508 Background: Thyroid cancer is a major component of Cowden Syndrome (CS), a heritable syndrome characterized also by breast, uterine and kidney cancers. CS patients with an underlying germline PTEN mutation have a 70-fold increased risk of developing epithelial thyroid cancer. In contrast, <1% of sporadic epithelial thyroid cancer harbor a germline PTEN mutation. Thus, cost-efficient markers capable of shortlisting thyroid cancers for CS genetic testing would be clinically useful. Here, we analyzed the relationship of patient PTEN protein levels in predicting the presence of germline PTEN mutations. Methods: We conducted a 5-year, multi-center prospective study of 2792 CS and CS-like patients, all of whom had comprehensive PTEN analysis done. Analysis of PTEN and downstream proteins by immunoblotting was performed on total protein lysates from patient-derived lymphoblast lines. We compared PTEN protein expression levels between patients with pathogenic PTEN mutations (PTENmut+), and those with variants of unknown significance (VUS) or wildtype PTEN. PTEN immunohistochemistry (IHC) was performed on available thyroid samples. Results: Of 2792 CS/CS-like patients, 722 had thyroid cancer. PTEN germline pathogenic mutations were present in 36/722 (5%) patients. PTENmut+ cases presented at an earlier age (35 vs 42 years; p=0.02). PTEN frameshift and truncation mutations accounted for 55% of mutations. 95% (19/20) of PTENmut+ patients had blood-PTEN protein levels in the lowest quartile as compared to 27% (90/330) of PTEN wildtype/PTEN VUS patients (p<0.001). Low blood-PTEN levels predicted for PTENmut+ cases with a 99.6 % NPV (95%CI 97.7- 99.9) and a positive test likelihood ratio of 3.5 (95%CI 2.8-4.3). Affected thyroid tissues (n=22) from PTENmut+ cases showed loss of PTEN protein by IHC, correlating with patient blood PTEN levels (r2=0.30, p=0.016). Conclusions: Our study shows that PTEN protein dosage could serve as a molecular correlate predicting for germline PTEN mutation in CS and CS-like presentations of thyroid cancer. The clinical utility of PTEN IHC in identifying thyroid cancer patients for germline PTEN testing should be further explored.