Loss of the tyrosine phosphatase PTPRD leads to aberrant STAT3 activation and promotes gliomagenesis

PTPRD, which encodes the protein tyrosine phosphatase receptor-δ, is one of the most frequently inactivated genes across human cancers, including glioblastoma multiforme (GBM). PTPRD undergoes both deletion and mutation in cancers, with copy number loss comprising the primary mode of inactivation in GBM. However, it is unknown whether loss of PTPRD promotes tumorigenesis in vivo, and the mechanistic basis of PTPRD function in tumors is unclear. Here, using genomic analysis and a glioma mouse model, we demonstrate that loss of Ptprd accelerates tumor formation and define the oncogenic context in which Ptprd loss acts. Specifically, we show that in human GBMs, heterozygous loss of PTPRD is the predominant type of lesion and that loss of PTPRD and the CDKN2A/p16^INK4A tumor suppressor frequently co-occur. Accordingly, heterozygous loss of Ptprd cooperates with p16 deletion to drive gliomagenesis in mice. Moreover, loss of the Ptprd phosphatase resulted in phospho-Stat3 accumulation and constitutive activation of Stat3-driven genetic programs. Surprisingly, the consequences of Ptprd loss are maximal in the heterozygous state, demonstrating a tight dependence on gene dosage. Ptprd loss did not increase cell proliferation but rather altered pathways governing the macrophage response. In total, we reveal that PTPRD is a bona fide tumor suppressor, pinpoint PTPRD loss as a cause of aberrant STAT3 activation in gliomas, and establish PTPRD loss, in the setting of CDKN2A/p16^INK4A deletion, as a driver of glioma progression.Glioblastoma multiforme (GBM) is a devastating disease. It is the most common and aggressive type of glioma and outcomes remain poor despite current treatments (1). To increase our understanding of the genetic basis of this malignancy, several mutational survey studies examining GBM have been completed and provide a detailed view of the molecular changes underlying this cancer (2–4). Because GBM is a highly heterogeneous tumor, a challenge remains to determine which molecular alterations drive tumorigenesis and to understand the underlying mechanisms of action. Recent work by our group and others have identified inactivation of protein tyrosine phosphatase receptor-δ (PTPRD) as a frequent alteration in GBM and other tumors, and showed that PTPRD copy number loss correlates with poor prognosis (5–10). Despite the high prevalence of PTPRD inactivation in human tumors, it is not known whether loss of PTPRD can promote tumorigenesis. Furthermore, the mechanisms of action and the oncogenic context in which PTPRD acts remain obscure.PTPRD belongs to a family of protein-tyrosine phosphatases that collectively have been implicated in functions, including the regulation of receptor tyrosine kinases, growth, cell migration, and angiogenesis (11). Previously, we demonstrated that phosphorylated STAT3 (p-STAT3) is a substrate of PTPRD and that cancer-specific mutations in PTPRD abrogate the ability of the phosphatase to dephosphorylate STAT3 (5). Interestingly, accumulation of phosphorylated STAT3 and STAT3 hyperactivation are frequent events in solid tumors like GBM, yet the genetic basis of aberrant STAT3 activation is poorly understood. p-STAT3 has been implicated in a number of tumor-promoting processes, including blocking differentiation, maintaining the stem cell pool, promoting growth and angiogenesis, and regulating the immune response and tumor microenvironment (12–14). In this study, we show that allelic loss of Ptprd results in p-Stat3 accumulation and Stat3 hyperactivation, elucidating one genetic root cause for aberrant STAT3 activation in GBM.Chromosome 9p, a region frequently lost in gliomas, contains the genes encoding PTPRD and the cyclin dependent kinase inhibitor 2A (CDKN2A). The CDKN2A locus produces the p16^INK4A and p14/p19^ARF tumor suppressors by alternate splicing (15). We and others have shown that selective pressure exists for inactivation of both PTPRD and CDKN2A, on chromosome 9p, in many types of cancer (5, 6, 10, 16). Both genes are frequently deleted or mutated. In this study, we develop a murine tumor model in which we inactivate both genes to model the genetic events that occur on 9p. We demonstrate that Ptprd loss cooperates with Cdkn2a deletion to promote tumorigenesis.We define the cooperative effects of PTPRD and CDKN2A by using Ptprd knockout and Cdkn2a/p16^Ink4a knockout mice in conjunction with the replication-competent avian sarcoma-leukosis virus long terminal repeat with splice acceptor retrovirus (RCAS) PDGFB/Nestin-tvA glioma mouse model. In this well-established RCAS model, the PDGFB oncogene drives glioma formation. PDGFB is specifically introduced into Nestin-expressing glial progenitor cells via infection of the avian RCAS virus into mice that express the avian tvA receptor under the Nestin promoter (17–19). Intracranial gliomas generated by the RCAS PDGFB/Nestin tvA mouse model reflect the histology of human GBM (20). Furthermore, as opposed to traditional genetically engineered mouse models, genes can be introduced into adult somatic cells of mice with excellent temporal specificity (19). Here, we show that Ptprd is a haploinsufficient tumor suppressor that cooperates with deletion of Cdkn2a/p16^Ink4a to promote glioma progression.