From www.bloodjournal.org by guest on July 4, 2016. For personal use only. Review article PTPN11 is the first identified proto-oncogene that encodes a tyrosine phosphatase Rebecca J. Chan1 and Gen-Sheng Feng2,3 1Department of Pediatrics, the Herman B. Wells Center for Pediatric Research, Indiana University School of Medicine, Indianapolis; 2Programs in Signal Transduction and Stem Cells & Regeneration, Burnham Institute for Medical Research, La Jolla, CA; 3Institute for Biomedical Research, Xiamen University, Xiamen, China Elucidation of the molecular mechanisms 2 Src-homology 2 (SH2) domains (Shp2). vation of the Ras-Erk pathway. This underlying carcinogenesis has benefited This tyrosine phosphatase was previ- progress represents another milestone in tremendously from the identification and ously shown to play an essential role in the leukemia/cancer research field and characterization of oncogenes and tumor normal hematopoiesis. More recently, so- provides a fresh view on the molecular suppressor genes. One new advance in matic missense PTPN11 gain-of-function mechanisms underlying cell transforma- this field is the identification of PTPN11 mutations have been detected in leuke- tion. (Blood. 2007;109:862-867) as the first proto-oncogene that encodes mias and rarely in solid tumors, and have a cytoplasmic tyrosine phosphatase with been found to induce aberrant hyperacti- © 2007 by The American Society of Hematology Introduction Leukemia and other types of cancer continue to be a leading cause tumor suppressor activity when overexpressed in vitro, and Ptprj of death in the United States, and biomedical scientists sorely note maps to the mouse colon cancer susceptibility locus,3 implicating that victories against cancer remain unacceptably rare. Neverthe- Ptprj as a potential tumor suppressor gene. Germline PTEN less, due to the genetic and biochemical analyses of multiple mutations are found in Cowden syndrome, characterized by oncogenes and tumor suppressor genes, significant progress has multiple hamartomas and a high proclivity for cancer development, been made in understanding the molecular basis for transformation and somatic loss-of-function PTEN mutations are detected in of a normal cell to a cancer cell. Gain-of-function mutations of several human cancers; however, substantial data suggest that normal cellular genes, termed proto-oncogenes, generate onco- PTEN mainly acts as a lipid phosphatase, rather than a tyrosine genes that confer a proliferative or survival advantage to the cell. In phosphatase catalytically, and suppresses the phosphoinositide contrast, loss-of-function mutations of tumor suppressor genes lead 3-kinase (PI3K) signaling pathway needed for survival and to dysregulated cellular proliferation and survival. proliferation.4 Reversible phosphorylation of protein tyrosine residues plays a Recent studies identified somatic gain-of-function mutations in critical role in signaling cascades for control of cell proliferation, PTPN11, which encodes the Shp2 tyrosine phosphatase, in child- differentiation, migration, and death. Tyrosyl phosphorylation hood leukemias and, rarely, in adult leukemias and solid tumors.5,6 levels are determined by the opposing activities of protein tyrosine Similar to multiple mutant tyrosine kinases, the missense PTPN11 kinases (PTKs) and phosphatases (PTPs). Many PTKs are known mutations destabilize noncovalent interactions between the N-SH2 to transmit signals resulting in cell proliferation, and thus onco- and PTP domains, resulting in constitutively active phosphatase genic mutations in human cancers commonly target PTKs. Typi- activity (Figure 1B,D). Therefore, PTPN11 was identified as the cally, the mutation disrupts the structural integrity of the kinase, first proto-oncogene encoding a PTP. such that autoinhibitory mechanisms are impaired, causing consti- tutive activation.1 One well-illustrated example is the v-Src onco- gene, first identified as an essential oncogenic component in Rous sarcoma virus. Phosphorylation of tyrosine 527 on the wild-type The tyrosine phosphatase Shp2 (WT) Src induces an intramolecular phosphotyrosine–Src-homol- ogy 2 (SH2) interaction and intramolecular contact between the Shp2 is a widely expressed cytoplasmic PTP containing 2 tandemly SH3 domain and the SH2-kinase linker region, both of which arranged SH2 domains at its amino terminal end, a central contribute to repression of the kinase activity (Figure 1A).1 v-Src phosphatase domain, and a carboxy terminal tail.7,8 Mice homozy- encodes a constitutively active mutant kinase lacking the C- gous for a targeted deletion of exon 3, encoding residues 46 to 110 terminal tail containing Y527 and also possessing missense muta- in the N-SH2 domain of Shp2, are embryonic lethal due to tions in the SH3 and kinase domains (Figure 1C). abnormal gastrulation.9 More recently, Yang et al10 showed that PTPs conventionally are thought to reverse PTK activities, homozygous Shp2 null mutants displayed peri-implantation lethal- thereby attenuating signals for cell proliferation. Although genes ity and that Shp2 had a critical role in trophoblast stem cell encoding PTPs are strong candidates for tumor suppressor genes, survival. Shp2 associates with activated receptor PTKs or cytokine for example in colorectal cancers,2 few phosphatase genes have receptors, which lack intrinsic kinase activity, either directly by been unequivocally identified as tumor suppressors. Density- docking to phosphorylated tyrosine residues on the receptors or enhanced phosphatase 1 (Dep-1) is a receptor PTP that exhibits indirectly via adaptor/scaffolding proteins, such as Grb2-associated Submitted July 11, 2006; accepted September 17, 2006. Prepublished The online version of this article contains a data supplement. online as Blood First Edition Paper, October 19, 2006; DOI 10.1182/blood- 2006-07-028829. © 2007 by The American Society of Hematology 862 BLOOD, 1 FEBRUARY 2007 ⅐ VOLUME 109, NUMBER 3 From www.bloodjournal.org by guest on July 4, 2016. For personal use only. BLOOD, 1 FEBRUARY 2007 ⅐ VOLUME 109, NUMBER 3 PTPN11, A PTP-ENCODING PROTO-ONCOGENE 863 Figure 1. Schematic diagram of inactive and active forms of Src kinase (PTK) and Shp2 phosphatase (PTP). (A) c-Src is maintained in an inactive conformation by intramolecular interactions between phosphorylated tyrosine 527 on the C-terminal tail and the SH2 domain and between the SH3 domain and the kinase linker region. Dephosphorylation of tyrosine 527 and binding of a p-Tyr–containing peptide to the SH2 domain leads to a switch of c-Src to an open conformation with activation of the kinase function. (B) Deletion of sequences encoding for tyrosine 527 and missense mutations within the SH3 or kinase domains (schematically represented by red stars) converts the c-Src proto-oncogene to an oncogene encoding for mutant v-Src with constitutive kinase activity. (C) Shp2 is maintained in an inactive conformation by hydrophobic interactions between amino acid residues within the N-SH2 domain and the PTP domain. Binding of a p-Tyr–containing peptide to the N-SH2 domain causes Shp2 to assume an open conformation with activation of the phosphatase function. (D) Mutations within the N-SH2 or phosphatase domains cause disruption of the hydrophobic interactions resulting in constitu- tive activation of the phosphatase activity. The residues most com- monly mutated in childhood leukemias are shown. binder 1-3 (GAB1-3), fibroblast growth factor receptor substrate-2 found to be essential for normal hematopoietic cell develop- (FRS-2), and insulin receptor substrate 1-4 (IRS1-4), all of which ment.27-30 In an in vitro hematopoietic differentiation assay, homozy- possess 2 conserved tyrosine sites for engagement of the 2 SH2 gous mutant embryonic stem (ES) cells for the Shp2⌬46-110 deletion domains of Shp2.7,8 Genetic and biochemical analyses in Caeno- mutation exhibited severely decreased differentiation capacity to rhabditis elegans, Drosophila, Xenopus, and mammals support the erythroid and myeloid progenitors.27 This in vitro result was notion that Shp2 promotes Ras activation by growth factors and supported by the in vivo chimeric animal analysis, in which neither cytokines.11-18 Although different models have been proposed erythroid nor myeloid progenitor cells of Shp2⌬46-110 origin were involving phosphatase-dependent and -independent mechanisms, detected in the fetal liver or bone marrow of chimeric animals that most functional analyses suggest that the catalytic activity of Shp2 were derived from mutant ES cells and wild-type embryos.28 is required for promotion of signaling through the Ras-Erk Notably, Shp2 mutant ES cells did have significant contributions to pathway. Several studies suggest that Shp2 dephosphorylates, and many other organs or tissues of the chimeras, suggesting a more thus inhibits, RasGAP and sprouty proteins, both negative regula- stringent requirement for Shp2 in hematopoiesis than development tors of Ras activation.8,19 Alternatively, Shp2 has been proposed to of many other cell types in mammals.28 A RAG-2–deficient lead to the dephosphorylation of Src kinase, either directly or blastocyst complementation assay further demonstrated a critical indirectly, leading to Src activation with subsequent Ras activa- role of Shp2 for lymphopoiesis in a cell-autonomous manner, as tion.8,20 However, the unequivocal identification of Shp2 substrates development of lymphoid cell lineages in Shp2Ϫ/Ϫ/Rag-2Ϫ/Ϫ chi- continues to be sought. 29 Shp2 enzymatic studies revealed low basal phosphatase