Loss of PTEN in Pediatric T-Cell Acute Lymphoblastic Leukemia Patients: Proteomic and Molecular Characterization

Signaling networks such as the PI3K/Akt/PTEN/mTOR pathway play a role in the modulation of the aggressiveness of T-Cell Acute Lymphoblastic Leukemia (T-ALL), but the significance of an aberrant activation of this pathway is poorly investigated. The master regulator of PI3K-AKT signaling is the lipid phosphatase and tensin homolog (PTEN); decreased or absent PTEN expression or activity could be associated with a constitutive activation of PI3K/Akt/PTEN/mTOR signaling pathway in T-ALLs. We investigated PTEN Exon7 mutations in diagnostic DNA samples from 67 pediatric T-ALL enrolled in AIEOP ALL2000 and R2006 protocols according to methods by Bandapalli et al. (Haematologica, 2012). We also investigated the PI3K/Akt/PTEN/mTOR pathway using Western Blotting (WB) and Flow Cytometry (FC) in parallel. WB analysis was performed using standard RIPA buffer for protein extraction. FC analysis of protein expression was applied as previously described (Gaipa G et al, Leukemia 2008), and protein levels were measured as % of positive cells compared to isotype control. Positivity or negativity by WB was established by presence or absence of a protein band, while for FC a threshold for positivity was set at ≥ 1% of positive cells. PTEN Exon 7 mutations were identified in 11 out of 67 (16.4%) of patients. According to samples availability, PI3K/Akt/PTEN/mTOR pathway was studied in 9 out of 67 patients (3 PTEN Exon 7 mutated and 6 wild type). PTEN protein resulted completely absent in all three PTEN Exon 7 mutated patients, by contrast PTEN was expressed in all 6 PTEN exon 7 wild type patients (mean by FC 46.98% ± 28.58%). This finding was fully confirmed when WB was applied to the same samples. We did not observe any statistically significant differences in p4EBP1 or mTOR levels in Exon 7 mutated patients as compared to wild type. By contrast, PTEN Exon 7 mutated blasts showed lower phospho-S6 levels compared to PTEN Exon7 wild type patients (mean 3.0% of positive cells vs 32.0%, p=ns). In conclusion, our data show a frequency of PTEN Exon 7 mutations of 16.4%, in agreement with the report by Bandapalli et al. (17.3%). Interestingly, we observed a strong association between the PTEN Exon 7 mutation and the total absence of PTEN protein in the pediatric T-ALL patients studied (data not reported so far to our knowledge). Moreover, although the differences are not statistically significant, p-S6 expression resulted consistently lower in mutated patients as compared to wild type patients. If confirmed in a larger cohort of pediatric T-ALLs, our data could open new insights in the significance of PTEN Exon 7 mutation in pediatric T-ALL and its associated functional profile.