GNAS defects identified by stimulatory G protein alpha-subunit signalling studies in platelets.
BACKGROUND
GNAS is an imprinted region that gives rise to several transcripts, antisense transcripts, and noncoding RNAs, including transcription of RNA encoding the alpha-subunit of the stimulatory G protein (Gsalpha). The complexity of the GNAS cluster results in ubiquitous genomic imprints, tissue-specific Gsalpha expression, and multiple genotype-phenotype relationships. Phenotypes resulting from genetic and epigenetic abnormalities of the GNAS region include Albright's hereditary osteodystrophy, pseudohypoparathyroidism types Ia (PHPIa) and Ib (PHPIb), and pseudopseudohypoparathyroidism (PPHP).


OBJECTIVE
The aim was to study the complex GNAS pathology by a functional test as an alternative to the generally used but labor-intensive erythrocyte complementation assay.


METHODS
We report the first platelet-based diagnostic test for Gsalpha hypofunction, supported by clinical, biochemical, and molecular data for six patients with PHPIa or PPHP and nine patients with PHPIb. The platelet test is based on the inhibition of platelet aggregation by cAMP, produced after Gsalpha stimulation.


RESULTS
Platelets are easily accessible, and platelet aggregation responses were found to reflect Gsalpha signaling defects in patients, in concordance with the patient's phenotype and genotype. Gsalpha hypofunction in PHPIa and PPHP patients with GNAS mutations was clearly detected by this method. Mildly decreased or normal Gsalpha function was detected in patients with PHPIb with either an overall or exon 1A-only epigenetic defect, respectively. Platelet Gsalpha expression was reduced in both PHPIb patient groups, whereas XLalphas was up-regulated only in PHPIb patients with the broad epigenetic defect.


CONCLUSIONS
The platelet-based test is a novel tool for establishing the diagnosis of Gsalpha defects, which may otherwise be quite challenging.
Freson
Kathleen
K
Center for Molecular and Vascular Biology, University of Leuven, Herestraat 49, B-3000 Leuven, Belgium. Kathleen.freson@med.kuleuven.be
Izzi
Benedetta
B
Labarque
Veerle
V
Van Helvoirt
Monique
M
Thys
Chantal
C
Wittevrongel
Christine
C
Bex
Marie
M
Bouillon
Roger
R
Godefroid
Nathalie
N
Proesmans
Willem
W
de Zegher
Francis
F
Jaeken
Jaak
J
Van Geet
Chris
C
D000293:Adolescent; D000328:Adult; D001792:Blood Platelets / Q000502:physiology*; D015139:Blotting, Southern; D002648:Child; D002675:Child, Preschool; D002864:Chromogranins; D004247:DNA / Q000235:genetics; D019175:DNA Methylation; D004252:DNA Mutational Analysis; D005260:Female; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000235:genetics* / Q000502:physiology*; D017353:Gene Deletion; D018392:Genomic Imprinting; D006801:Humans; D015151:Immunoblotting; D008297:Male; D005810:Multigene Family; D010974:Platelet Aggregation / Q000187:drug effects / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000235:genetics* / Q000502:physiology*; D013447:Sulfites / Q000737:chemistry; D051438:Syntaxin 16 / Q000235:genetics; D055815:Young Adult
D002864:Chromogranins; C496118:STX16 protein, human; D013447:Sulfites; D051438:Syntaxin 16; D004247:DNA; C406732:GNAS protein, human; D019205:GTP-Binding Protein alpha Subunits, Gs; C042345:hydrogen sulfite