The Association Between SHP-2 Mutations and Platelet Function in Noonan Syndrome.

Noonan syndrome (NS) is an autosomal dominant disorder characterized by variable expression of multiple defects including short stature, facial dysmorphias, congenital heart defects, developmental delay, and hematological abnormalities. A bleeding tendency has been variably described in patients with NS; Factor XI deficiency and platelet function abnormalities are reported most frequently. Recently, germline missense mutations in the PTPN11 gene have been identified in up to 50% of individuals with NS. PTPN11 encodes the ubiquitously expressed cell-signaling mediator SHP-2, a non-transmembrane protein tyrosine phosphatase involved in the normal activation of the Ras-MAPK signaling cascade. SHP-2 has a pivotal role in cell proliferation, differentiation, and survival, but also modulates platelet responses to some stimuli. Some PTPN11 mutations lead to altered SHP-2 phosphatase activity, and have been shown to impact cardiac development in a mouse model of NS.

This study examined the potential association among PTPN11 mutations, platelet function abnormalities, and platelet SHP-2 activity in patients with NS. Eighteen patients with a clinical diagnosis of NS were studied: 14 (78%) were female and the mean age was 20.7 years (range 4–56 years). A validated bleeding questionnaire was administered to each participant, and platelet aggregometry was performed on platelet-rich plasma according to standard clinical practice. SHP-2 was isolated by immunoprecipitation from agoniststimulated platelets and phosphatase activity measured spectrophotometrically. PTPN11 mutations were identified by dHPLC and defined by DNA sequencing in each participant to characterize the presence and the precise nature of a mutation associated with their NS.

Seven (39%) participants were classified as bleeders on the basis of the questionnaire; of these, four had no hemostasis abnormality identified. One participant had mild Factor XI deficiency. Seven participants had abnormalities of platelet aggregation, and four participants had decreased platelet dense granule numbers and ATP release. PTPN11 mutations were identified in 12 (67%) participants: seven were the commonly described mutation in Exon 8; four members of a three-generation family had a previously described Exon 3 mutation; and one individual had a mutation in Exon 12 which is usually associated with LEOPARD syndrome. Mutations in other genes associated with NS were not evaluated in this study. Six patients with PTPN11 mutations were clinically classified as bleeders, but no correlation was found between PTPN11 mutation and agonist-induced platelet aggregation or SHP-2 phosphatase activity. Three patients with platelet function abnormalities had no PTPN11 mutation identified.

Not all patients with NS have a clinical bleeding profile or platelet function abnormalities. Neither clinical bleeding history nor platelet function abnormalities correlated with PTPN11 mutations, suggesting that additional factors may be required for a bleeding phenotype associated with PTPN11 mutations. Mutations in other NS-associated genes may contribute to a bleeding phenotype. Abnormalities in platelet aggregometry could not be explained on the basis of SHP-2 activity, although it is possible that changes in enzyme activity downstream in the Ras-MAPK pathway may impact platelet function in some patients with NS. Ultimately, the identification of a molecular basis for the frequently observed platelet aggregation defects could provide a means to predict the risk of bleeding in patients with NS.