The SHP2 phosphatase consists of one protein tyrosine phosphatase catalytic domain (PTP domain), two tandem Src homology 2 (SH2) domains (N-SH2 and C-SH2), and a C-terminal tail with two tyrosine phosphorylation sites (Tyr542 and Tyr580) [1] (Figure 1A). SHP2 activity is normally auto-inhibited by the binding of the N-SH2 domain with the PTP domain [2]. Upon stimulation of growth factors or cytokines, the N-SH2 domain binds to specific phospho-tyrosine residues and induces a conformational change that leads to exposure of the PTP domain and an increase in the catalytic activity [3] (Figure 1B). Phosphorylated Tyr542 interacts intramolecularly with the N-SH2 domain to relieve steady-state inhibition of the phosphatase, whereas phosphorylated Tyr580 stimulates the phosphatase activity by interaction with the C-SH2 domain [4].
Germline gain of function (GOF) mutations in PTPN11occur in about 50% of patients with Noonan syndrome [5], which is characterized by abnormal facial features, skeletal malformations, congenital heart disease, short stature and an elevated risk of leukemia and other cancers. In contrast, more than 80% of patients with LEOPARD syndrome (lentigines, EKG abnormalities, ocular hypertelorism, pulmonary stenosis, abnormal genitalia, retardation of growth, and deafness) harbor heterozygous germline inactivating (phosphatase-defective) mutations in PTPN11, despite the overlapping clinical presentations between this syndrome and those with Noonan syndrome [6,7]. Both conditions are associated with an increased risk for malignancies, including leukemia and neuroblastoma