Clinical and molecular characterization of Rubinstein-Taybi syndrome patients carrying distinct novel mutations of the EP300 gene.

Rubinstein-Taybi syndrome (RSTS) is a rare congenital neurodevelopmental disorder characterized by postnatal growth deficiency, skeletal abnormalities, dysmorphic features and cognitive deficit. Mutations in two genes, CREBBP and EP300, encoding two homologous transcriptional co-activators, have been identified in ˜55% and ˜3-5% of affected individuals, respectively. To date, only eight EP300-mutated RSTS patients have been described and 12 additional mutations are reported in the database LOVD. In this study, EP300 analysis was performed on 33 CREBBP-negative RSTS patients leading to the identification of six unreported germline EP300 alterations comprising one deletion and five point mutations. All six patients showed a convincing, albeit mild, RSTS phenotype with minor skeletal anomalies, slight cognitive impairment and few major malformations. Beyond the expansion of the RSTS-EP300-mutated cohort, this study indicates that EP300-related RSTS cases occur more frequently than previously thought (˜8% vs 3-5%); furthermore, the characterization of novel EP300 mutations in RSTS patients will enhance the clinical practice and genotype-phenotype correlations.

Killing of K562 cells with conjugates between human transferrin and a ribosome-inactivating protein (SO-6).

Cellular iron uptake is mediated by binding of transferrin with specific surface receptors and internalization of the Fe-transferrin-receptor complex. This has been examined as a possible pathway for carrying into leukaemic cells a ribosome-inactivating protein (RIP), SO-6, derived from Saponaria officinalis. Purified human differic transferrin was conjugated with SO-6 and a pool of proteins was obtained, with variable numbers of SO-6 molecules linked to a single transferrin molecule. Human erythroleukaemic K562 cells were grown in the presence of human transferrin, SO-6 and human transferrin conjugated with SO-6. The conjugate was found to be internalized via binding with transferrin receptor. Whereas the presence of unconjugated human transferrin and SO-6 in the medium did not significantly influence K562 cell growth, the conjugated proteins displayed an inhibitory activity on cell proliferation. This was maximal after 72 h at a transferrin concentration of 10(-9) M, with about 50% of cells being killed. Bovine transferrin, present in fetal calf serum, did not appear to compete with human diferric transferrin in binding to K562 cells in suspension culture. In a clonogenic assay, colony formation by leukaemic cells was not influenced by free SO-6 or transferrin, whereas the conjugated proteins were markedly inhibitory (about 100% at 10(-9) M). Our findings indicate that SO-6 can be efficiently carried into mammalian cells via the transferrin-transferrin receptor cycle and exert its ribosome inactivating activity. This is in keeping with the existence of an alternative pathway of transferrin endocytosis in addition to the classic acidic endosome pathway. From a practical viewpoint, conjugates between transferrin and SO-6 can be useful tools for studying the expression of transferrin receptors, and deserve also to be investigated for a possible use in cancer therapy.