guanosine-triphosphate and Neutropenia

The SRP54 GTPase is a key component of co-translational protein targeting by the signal recognition particle (SRP). Point mutations in SRP54 have been recently shown to lead to a form of severe congenital neutropenia displaying symptoms overlapping with those of Shwachman-Diamond syndrome. The phenotype includes severe neutropenia, exocrine pancreatic deficiency, and neurodevelopmental as well as skeletal disorders. Using a combination of X-ray crystallography, hydrogen-deuterium exchange coupled to mass spectrometry and complementary biochemical and biophysical methods, we reveal extensive structural defects in three disease-causing SRP54 variants resulting in critical protein destabilization. GTP binding is mostly abolished as a consequence of an altered GTPase core. The mutations located in conserved sequence fingerprints of SRP54 eliminate targeting complex formation with the SRP receptor as demonstrated in yeast and human cells. These specific defects critically influence the entire SRP pathway, thereby causing this life-threatening disease.

Topics: Binding Sites; Congenital Bone Marrow Failure Syndromes; Guanosine Triphosphate; HEK293 Cells; Humans; Mutation; Neutropenia; Protein Binding; Protein Stability; Protein Transport; Signal Recognition Particle

It appeared possible that abnormal purine or pyrimidine metabolism could cause cyclic hematopoiesis by analogy with the defective lymphopoiesis associated with inherited deficiencies of adenosine deaminase and purine nucleoside phosphorylase. Therefore, we examined erythrocyte purine and pyrimidine nucleotide levels, as well as plasma purine and pyrimidine nucleosides and bases in three patients and in normal controls. These studies showed that during neutropenia there was a significant elevation in the levels of guanosine triphosphate (P = 0.005) and adenosine triphosphate (P less than 0.001) in the patients' red cells not attributable to reticulocyte variation. Serial analysis of a patient's plasma showed a fivefold elevation of hypoxanthine (10.6 mumol/L) during neutropenia, with a return to normal values (1.4 mumol/L) as neutrophil numbers increased. Plasma inosine was also significantly elevated in comparison with normal control values (2.0 mumol/L vs. 0.8 mumol/L), whereas plasma and urinary uric acid were within the normal range. Serial analysis of red cells and plasma from two patients with chronic neutropenia showed no elevations of purine or pyrimidine metabolites. These results provide evidence of a link between abnormal concentrations of purine metabolites and cyclic hematopoiesis, and permit the speculation that aberrant purine metabolism is primarily related to the defective hematopoietic cell proliferation that is characteristic of this disease.

Topics: Adenosine Deaminase; Adult; Agranulocytosis; Child; Chromatography, High Pressure Liquid; Chronic Disease; Female; Guanosine Triphosphate; Hematocrit; Hematopoiesis; Humans; Hypoxanthine; Hypoxanthines; Inosine; Male; Neutropenia; Nucleosides; Nucleotides; Purines; Pyrimidines; Uric Acid