PTEN Expression Is Decreased in Pediatric Burkitt Lymphoma Tissues but Does Not Correlate with AKT Activation.

We have recently shown that a subset of pediatric Burkitt lymphoma (pBL) show gain of 13q31, containing the microRNA (miRNA) MIR-17–92 locus, as well as associated increased expression of miRNA from that locus. PTEN, a tumor suppressor phosphatase that negatively regulates the PI3K/AKT/mTOR pro-survival pathway, is negatively regulated by MIR-17–92. Therefore, we hypothesized that PTEN protein and mRNA expression would be decreased in pBL cases with 13q31 locus gain and high expression of MIR-17–92. Because PTEN negatively regulates the PI3K/AKT/mTOR pathway, we also assessed AKT activation in pBL tissues.

We assessed PTEN protein expression by immunohistochemistry (IHC) in formalin-fixed, paraffin-embedded (FFPE) pBL tissues from a cohort of patients (n=25) where MIR-17–92 expression levels and status of the 13q31 locus have been previously determined (Schiffman/Miles, Brit J Haematol 155:477, 2011). From FFPE tissues, we assayed PTEN mRNA levels by quantitative RT-PCR. The results of protein and mRNA expression were then correlated with presence or absence of a 13q31 gain and the expression level of MIR17. To evaluate AKT activation in pBL, we assayed for phospho-(p)-AKT by IHC. In addition, PTEN, AKT, and p-AKT protein levels were evaluated by western blotting in Raji and Ramos BL cell lines.

PTEN protein was weakly expressed in 5/25 cases (20%) of pBL. PTEN protein showed moderate nuclear and cytoplasmic staining in a subset of paracortical and germinal center lymphocytes in reactive lymph node control tissues, and vascular endothelial cells provided internal positive control in tumor tissue. pBL tissues showed decreased relative PTEN mRNA expression compared to reactive lymph node controls (1.56 vs. 2.61, p<3E-07). There was no significant difference in PTEN mRNA levels between PTEN protein positive and negative cases (1.27 vs. 1.61, p=0.22); cases with or without 13q31gain (1.49 vs. 1.57, p=0.60); or cases with high or low expression of MIR17. p-AKT was detected by IHC in 1/25 cases (4%). The BL cell lines Raji and Ramos showed very low levels of PTEN protein but no detectable p-AKT; total AKT was readily detectable.

pBL shows decreased PTEN mRNA, and most cases are negative for PTEN protein expression. Unexpectedly, higher MIR-17–92 expression level or 13q31 gain did not show an inverse relationship with PTEN expression at the protein or mRNA level. Despite decreased PTEN expression, p-AKT was detected in only 1/25 pBL cases and was not detected in BL cell lines. These findings suggest that decreased PTEN expression does not lead to AKT activation in pBL. Ongoing studies will investigate the potential of alternate PTEN targets and/or alternate targets for MIR-17–92 in pBL.

No relevant conflicts of interest to declare.