Biomedical subjects

Christine Wittevrongel

Publications and source records attributed to Christine Wittevrongel.

3 recordsLinked to original sources

PubMed2003

Functional polymorphisms in the paternally expressed XLalphas and its cofactor ALEX decrease their mutual interaction and enhance receptor-mediated cAMP formation.

The paternally expressed extra-large stimulatory G protein gene (XLalphas) is a splice variant of the stimulatory G-protein gene (Gsalpha) consisting of XL-exon1 and exons 2-13 of Gsalpha. A second open reading frame (ORF) in XL-exon1, that completely overlaps the XL-domain ORF, encodes ALEX, which is translated from the XLalphas mRNA and binds the XL-domain of XLalphas. We previously demonstrated that a paternally inherited functional polymorphism in XL-exon1, consisting of a 36 bp insertion and two nucleotide substitutions, is associated with Gs hyperfunction in platelets, leading to an increased trauma-related bleeding tendency and is accompanied by neurological problems and brachydactyly in two families. Here, we describe eight additional patients with brachydactyly, who inherited the same XLalphas polymorphism paternally and who show Gs hyperfunction in their platelets and fibroblasts. All carriers also have an elongated ALEX protein, as a consequence of the paternally inherited insertion. The in vitro interaction between the two elongated XLalphas and ALEX proteins is markedly reduced. Moreover, XLalphas or ALEX can be co-immunoprecipitated with an antibody against either ALEX or XLalphas in platelets from a control but hardly from patients with the XLalphas/ALEX insertion. In contrast to the strong interaction between the two wild-type proteins, we suggest that this defective association results in unimpeded receptor-stimulated activation of XLalphas. The paternally inherited double XLalphas/ALEX functional polymorphism is also associated with elevated platelet membrane Gsalpha protein levels. Both phenomena contribute to increased Gs signaling in patients with platelet hypersensitivity towards Gs-agonists and may be accompanied by neurological problems or growth deficiency.

Blood Platelets↗

PubMed2002

Pseudohypoparathyroidism type Ib with disturbed imprinting in the GNAS1 cluster and Gsalpha deficiency in platelets.

Pseudohypoparathyroidism Ib (PHPIb), characterized by parathyroid hormone-resistant hypocalcemia and hyperphosphatemia, is caused by a deregulation in the imprinting status of the GNAS1 cluster, comprising exons XL, NESP55 and 1A and the coding exons of Gsalpha. Differences in methylation of exon 1A and sporadically also of exons XL and NESP55 were found and thought to result in long-range effects on Gsalpha expression, limited to the proximal renal tubules. The exact imprinting defect is not precisely localized, and the expected differences in Gsalpha protein level and function are mainly hypothetical. We describe a PHPIb patient with lack of methylation of the exon XL and 1A promoters, and biallelic methylation of the NESP55 promoter. Platelets of this patient show a functional Gs defect, decreased cAMP formation upon Gs-receptor stimulation, normal Gsalpha sequence but reduced Gsalpha protein levels. Transcriptional deregulation between the now biallelically active promoters of both exon 1A and exon 1 of Gsalpha could explain the decreased Gsalpha expression in platelets and presumably in the proximal renal tubules. We found decreased NESP55 and increased XLalphas protein levels in platelets, in agreement with the methylation status of their corresponding first exons. In a megakaryocytic cell line MEG-01, exon 1A is methylated on both alleles, in contrast to the normally maternally methylated exon 1A in leukocytes. Experimental demethylation of exon 1A in MEG-01 cells led to reduced Gsalpha expression, in agreement with the observations in the patient. Platelet studies may therefore allow easy evaluation of disturbances of the GNAS1 cluster in PHPIb patients.

Adult↗

PubMed2002

Thrombogenicity of beta 2-glycoprotein I-dependent antiphospholipid antibodies in a photochemically induced thrombosis model in the hamster.

We previously showed that beta(2)-glycoprotein I (beta(2)GPI)-dependent lupus anticoagulants (LAs) form bivalent antigen-antibody complexes with high affinity for phospholipids; these complexes are responsible for their in vitro anticoagulant effect. We now studied the role of these bivalent complexes in arterial thrombosis in the hamster. Three monoclonal antibodies (mAbs) raised against human beta(2)GPI were selected on the basis of their cross-reactivity with hamster beta(2)GPI. Two of these, one with LA activity, 5H2, and one with only anticardiolipin properties, 11E8, were infused at 0 to 10 mg/kg prior to photochemically induced vessel damage. 5H2 promoted thrombus formation dose dependently, raising the thrombus size from 6.0 arbitrary units (AU) in controls (n = 9) to 65.0 AU in the high-dose group (10 mg/kg, n = 6, P =.007). The LA(-) mAb 11E8 and mAb 27A8, reactive with human beta(2)GPI exclusively, did not significantly promote thrombus formation. In a second set of experiments, intact mAb 5H2 was compared to its fragments. Intact mAb 5H2 at 3.3 mg/kg and the equimolar dose of F(ab')(2) fragments (2.2 mg/kg) promoted thrombus formation equally well (55.8 AU, n = 8 and 62.5 AU, n = 7, respectively); mAb 5H2-derived Fab' fragments were inactive. Immunohistochemical analysis showed platelet-rich thrombi, with 5H2 or its F(ab')(2) fragments mainly bound to individual platelets. Our results indicate that bivalent immune complex formation plays an important role in the genesis of arterial thrombosis by certain antiphospholipid antibodies. Cellular activation via the Fc portion of these immune complexes, however, is not essential, because F(ab')(2) fragments of 5H2 still promote thrombus formation.

Adenosine Diphosphate↗