A de novo T73I Mutation in PTPN11 in a Neonate with Severe and Prolonged Congenital Thrombocytopenia and Noonan Syndrome

We observed a neonate with severe congenital thrombocytopenia and features of Noonan syndrome where evaluations were negative for immune-mediated thrombocytopenia, congenital infections, and Fanconi anemia. The marrow findings and a significantly elevated plasma thrombopoietin (Tpo) level were consistent with congenital amegakaryocytic thrombocytopenia; we sought a genetic mutation that could explain this phenotype. No mutations were identified in c-MPL (the Tpo receptor gene). Microarray analysis of peripheral blood did not reveal an abnormality. DNA sequencing of the PTPN11 gene showed a heterozygous C>T nucleotide substitution in exon 3 (c.218C>T) predicted to result in a threonine-to-isoleucine change at residue 73 (T73I). A 6-week trial of eltrombopag (an agonist of the Tpo receptor) failed to increase the platelet count. We propose this specific PTPN11 mutation results in abnormalities of the protein product SHP-2, which, because of its role in signal transduction, results in severe congenital thrombocytopenia refractory to c-MPL agonists.