PTEN promoter methylation and activation of the PI3K/Akt/mTOR pathway in pediatric gliomas and influence on clinical outcome

Abstract

The signaling pathways that underlie the pathogenesis of pediatric gliomas are poorly understood. We characterized the PI3K/Akt/mTOR pathway in pediatric gliomas of all grades. Using immunohistochemistry, we assessed activation of the PI3K/Akt/mTOR pathway by evaluating the downstream signaling molecules phospho(p)-S6, phospho(p)-4BP1, and phospho(p)-PRAS40; PTEN; and PTEN promoter methylation, as well as the MIB labeling index. We correlated these findings with the clinical outcomes of 48 children with gliomas. Eighty percent of high-grade gliomas (12/15) showed activation of the PI3K/Akt/mTOR pathway based on p-S6 and p-4EBP1 expression. The majority of high-grade gliomas were negative for PTEN expression (10/15), and 50% had PTEN promoter methylation (grade III: 2/4; grade IV: 3/6). Low-grade gliomas demonstrated PI3K/Akt/mTOR pathway activation in 14/32 (43.8%) by p-S6 and 16/32 (50%) by p-4EBP1. Over 50% of grade I (6/11) and almost all grade II tumors (6/7) showed PTEN promoter methylation. Tumor grade correlated negatively with PTEN expression and positively with expression of p-S6 and p-4EBP1 (PTEN: P = .0025; pS6: P = .0075; p-4EBP1: P = .0066). There was a trend toward inverse correlation of methylation of the PTEN promoter with expression of PTEN protein (P= .0990) and direct correlation of expression of p-S6 and p-4EBP1 with poorer clinical outcome, as measured by progression-free survival (p-S6: P= .0874; p-4EBP1: P= .0475). Tumors with no PTEN expression had a higher MIB labeling index (P= .007). The majority of pediatric gliomas show activation of the PI3K/Akt/mTOR pathway, with methylation of the PTEN promoter occurring commonly in these tumors.

Fig. 1.

Progression-free survival according to extent of surgical resection in pediatric (A) low-grade gliomas and (B) high-grade gliomas. GTR: gross total resection; STR: subtotal resection.

Fig. 2.

Progression-free survival according to supratentorial vs infratentorial location of (A) pediatric low-grade gliomas (grades I and II) and (B) high-grade gliomas (HGGs).

Fig. 3.

(A) Representative images of tumors that were positive (top row) and negative (bottom row) stained for PTEN, p-PRAS40 (Thr246), p-4EBP1 (Thr37/46), p-S6 (Ser 235/235) and p-S6 (Ser 240/244). PTEN-positive endothelial cells serve as an internal control for PTEN staining. Original magnification 400×. (B) Representative images of sections immunostained for p-4EPB1 demonstrating tumors scored from left to right as 0,1,2, and 3. Note there is both cytoplasmic and nuclear immune-positivity. Original magnification 400x.

Fig. 4.

MIB-labeling index according to PTEN expression. N, negative PTEN expression; Y, positive PTEN expression (P= .007).

Fig. 5.

Progression-free survival of pediatric gliomas according to (A) phospho-S6 staining (P= .0874) and (B) phospho-4EBP1 staining (P= .0475).