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172 Part II Cellular Basis of Hematology

response to IL-3, STAT5 phosphorylation, and cell survival following activation of JAKs and STAT proteins. The removal of CD45 also
IL-3 withdrawal. increases erythroid colony formation and antiviral activity, which is
Consistent with the negative role of SHP-1 on the JAK/STAT consistent with the fact that CD45 negatively regulates EPO and IFN
pathway, silencing of SHP-1 by promoter methylation is often associ- signaling. Inactivating mutations occur in patients with T-cell acute
ated with various kinds of leukemia and lymphomas, myeloma, and lymphoblastic leukemia. The Src family kinase members Lck and Lyn
acute myeloid leukemia. On the other hand, in a limited number of are key substrates for CD45 in T and B lymphocytes, respectively.
cases, SHP-1 has been described to have a positive role in promoting CD45 positively regulates T cell receptor-mediated signaling through
JAK/STAT signaling. For example, epidermal growth factor– and the activation of Src-family kinases.
IFN-γ–induced STAT activation was suppressed by expressing a PTP1B (PTP1B or PTPN1) and T-cell PTP (TC-PTP or PTPN2)
catalytically inactive form of SHP-1 in HeLa cells, while this pathway are closely related PTP, sharing 74% homology in their catalytic
was essentially unaffected by the expression of wild-type SHP-1. domain. PTP1B is expressed in many tissues and TC-PTP is ubiqui-
There is no known molecular explanation for this observation. tously expressed with particularly high expression in hematopoietic
tissues. PTP1B is involved in multiple signaling pathways by down-
Src-Homology 2 Containing Phosphatase 2 regulating several tyrosine kinases. For example, PTP1B-deficient
mice display increased insulin sensitivity. Increased phosphorylation
(PTPN11) of JAK2, Tyk2, STAT3, and STAT5 has been observed in PTP1B-
deficient embryonic fibroblasts. TC-PTP targets multiple STAT and
SHP-2 is a protein-tyrosine phosphatase that is widely expressed, JAK proteins in addition to growth factor receptors for dephosphory-
with high levels of expression in hematopoietic cells. It contains two lation. TC-PTP–deficient mice develop anemia, lymphadenopathy
tandem SH2 domains (N-SH2 and C-SH2), a PTP domain and a and splenomegaly and die at early age. These mice display excessive
C-terminal tail. SHP-2 has low basal enzymatic activity caused by inflammation and demonstrate increased numbers of bone marrow,
autoinhibition of the PTP domain by the N-SH2 domain. SHP-2 HSCs and HPCs. Other phosphatases such as PTP-receptor type T
directly or indirectly (via adaptor proteins) associates with activated (PTPRT) and PTP-Basophil like (PTP-BL) and the adapter protein
receptor protein tyrosine kinases or cytokine receptors via its two SH2 LNK have also been implicated in cytokine signaling. LNK muta-
domains. Binding of SH2 domains to phosphotyrosine sites of these tions have been reported in patients with myeloproliferative neoplasms
receptors alters the conformation of the N-SH2 domain, releasing its including essential thrombocythemia and primary myelofibrosis
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binding to PTP domain and causing catalytic activation. Despite (Chapters 69 and 70).
being a phosphatase, SHP-2 promotes activation of the Ras and ERK
pathway by cytokines. Its catalytic activity is required for cytokine
activation of phosphatidylinositol 3-kinase pathway. SHP-2 plays an Protein Inhibitors of Activated STAT
essential role in hematopoietic cell development. Embryonic lethality
is observed at day 8.5 in mice with a truncated version of SHP-2 PIAS3 was the first family member to be identified as a repressor of
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because of severe defects in gastrulation and mesodermal patterning. STAT3 activity. Three additional family members PIAS1, PIASy
Complete loss of SHP-2 causes embryonic death in the periimplanta- (or PIAS4), and PIASx (or PIAS2) were later identified with high
tion period and SHP-2 is required for trophoblast stem cell survival. sequence homology. Other proteins with weak homology (hZIMP7
HSCs from SHP-2 haploinsufficient mice display a competitive and hZIMP10) have been reported. PIAS1 was identified as a
repopulating defect. Embryonic stem (ES) cells lacking SHP-2 STAT1-interacting protein and subsequently found to inhibit STAT1-
exhibit severely decreased differentiation to erythroid and myeloid mediated transcriptional activation. The PIAS family members PIASx
progenitors in vitro and fail to contribute to erythroid and myeloid and PIASy were identified based on sequence similarity to PIAS1 and
−/−
lineages in chimeric mice derived from SHP-2 ES cells and wild- have been shown to inhibit STAT1 and STAT4, respectively. PIAS
type embryos. SHP-2 loss-of-function causes an early block of lym- proteins have been shown to affect the function of many different
phocyte development before Pro-T and Pro-B stages. The exact proteins, with particular effects on gene transcription.
mechanisms by which SHP-2 regulates cytokine signaling and PIAS proteins contain a domain known as SP-RING (Fig. 16.9)
hematopoiesis are uncertain. SHP-2 both enhances and inhibits the with structural similarity to ubiquitin E3 ligase RING fingers. PIAS
JAK/STAT pathway depending on the context. It functions as a binds the protein modifier SUMO (small ubiquitin-like modifier)
negative regulator of the IFN-stimulated JAK1/STAT pathway and and recruits the E2 SUMO ligase UBC9, which transfers SUMO to
it dephosphorylates STAT1 and STAT5. At the same time SHP-2 is target proteins, particularly to transcription factors such as STATs.
required for optimal JAK2 activation. PIAS1, PIAS3, and PIASx sumoylate STAT1 at Lys-703, close to the
Activating germline mutations of PTPN11 (the gene encoding for site at which it is phosphorylated by JAKs (Tyr-701). A mutation of
SHP-2 protein) are seen in patients with Noonan syndrome while Lys-703 results in an increased response to IFN-γ.
loss-of-function mutations are seen in patients with LEOPARD Thus the binding of PIAS to STAT results in the inhibition of
syndrome. Both are congenital disorders associated with abnormal STAT-mediated gene activation. PIAS1 and PIAS3 inhibit STAT
hematopoiesis. Somatic activating mutations are seen in approxi- DNA binding activity, while PIASy and PIASx repress STAT1 and
mately 35% of patients with juvenile myelomonocytic leukemia. STAT4-mediated gene activation without affecting DNA binding.
PTPN11 mutations are also seen in patients with myelodysplastic PIAS proteins also act by recruiting other corepressor molecules. One
syndrome, acute lymphoblastic leukemia, and acute myelogenous example is the inhibition of natural regulatory T-cell differentiation
leukemia. These gain-of-function mutations induce hyperactivation by PIAS1 through chromatin-based epigenetic repression. Sumoylation
of the Ras pathway, which results in growth factor and cytokine may affect nuclear localization of proteins as is the case for the
independent proliferation and survival of hematopoietic progenitor transcriptional factor LEF1, where LEF1 sumoylation by PIASy
cells (HPCs). Increased SHP-2 expression has also been observed in results in its sequestration into nuclear bodies hindering its transcrip-
acute leukemia cells, suggesting a potential role in leukemogenesis. tional activity. Given its impact on STAT1 and IFN signaling, PIAS1
has a tangible role in innate immunity. The antiviral activity of IFNs
is significantly increased in PIAS−/− cells. In addition, PIAS−/− mice
CD45, PTP1B, TC-PTP, PTPRT, and PTP-BL have increased protection against bacterial and viral infection.

CD45 is a receptor-like tyrosine phosphatase highly expressed by
hematopoietic cells. CD45 was identified as a JAK family phosphatase. Suppressor of Cytokine Signaling
CD45 is able to dephosphorylate all JAKs in murine cells, and dephos-
phorylate JAK1 and JAK3 in human cells. Targeted disruption of the The SOCS family consists of eight proteins that antagonize the
CD45 gene leads to enhanced cytokine and IFN-receptor-mediated signaling of STAT proteins. STATs activate the transcription of genes