Utility of PTEN protein dosage in predicting for underlying germline PTEN mutations among patients presenting with thyroid cancer and Cowden-like phenotypes

Abstract

Context: Thyroid cancer is a major component of Cowden syndrome (CS). CS patients with an underlying PTEN mutation (PTEN(mut+)) have a 70-fold increased risk of developing epithelial thyroid cancer. In contrast, less than 1% of sporadic epithelial thyroid cancer patients carry a germline PTEN mutation. Cost-efficient markers capable of shortlisting thyroid cancers for CS genetic testing would be clinically useful.

Objective: Our objective was to analyze the utility of patient blood phosphate and tensin homolog deleted on chromosome 10 (PTEN) protein levels in predicting germline PTEN mutations.

Design, setting, and patients: We conducted a 5-yr, multicenter prospective study of 2792 CS and CS-like patients, all of whom had comprehensive PTEN analysis. Analysis of PTEN and downstream proteins by immunoblotting was performed on total protein lysates from patient-derived lymphoblast lines. We compared blood PTEN protein levels between PTEN(mut+) patients and those with variants of unknown significance or wild-type PTEN (PTEN(wt/vus)).

Main outcome measures: We assessed the utility of PTEN protein levels in predicting germline PTEN mutations.

Results: Of 2792 CS/CS-like patients, 721 patients had thyroid cancer; 582 of them (81%) had blood PTEN protein analyzed. PTEN germline pathogenic mutations were present in 27 of 582 patients (4.6%). Ninety-six percent (26 of 27) of PTEN(mut+) patients had blood PTEN protein levels in the lowest quartile as compared with 25% (139 of 555) of PTEN(wt/vus) patients (P < 0.001). Low blood PTEN levels predicted for PTEN(mut+) cases with a 99.76% negative predictive value (95% confidence interval = 98.67-99.96) and a positive test likelihood ratio of 3.84 (95% confidence interval = 3.27-4.52).

Conclusions: Our study shows that low blood PTEN protein expression could serve as a screening molecular correlate to predict for germline PTEN mutation in CS and CS-like presentations of thyroid cancer.

Fig. 1.

Mean PTEN fluorescent intensity according to PTEN mutation status and type of mutation. Error bars are representative of 95% CI. WT, Wild type.

Fig. 2.

ROC curve of low blood PTEN levels (PTEN^Q1) in predicting germline PTEN mutation in CS patients who present with thyroid cancer. AUC, Area under the curve.

Fig. 3.

Calibration plot for PTEN^Q1 predicted probabilities of PTEN mutation with actual outcomes.

Fig. 4.

PTEN immunohistochemistry of germline PTEN^mut+-related thyroid cancer showing both nuclear and cytoplasmic loss of PTEN distinct from that seen in PTEN^wt/vus-related thyroid cancer (×40 magnification). A, Papillary thyroid cancer with absent (−) PTEN staining; underlying c.210–1G→A germline PTEN mutation with blood PTEN^Q1 expression; B, follicular variant of papillary thyroid cancer with weak (+) nuclear PTEN staining; underlying c.80-?_164+?del germline PTEN mutation with blood PTEN^Q1 expression; C, papillary thyroid cancer with strong (3+) cytoplasmic PTEN staining; PTEN wild-type with blood-PTEN^Q3 expression; D, papillary thyroid cancer with moderate (2+) nuclear PTEN staining; PTEN wild-type with blood-PTEN^Q1 expression.