Methylation defect in imprinted genes detected in patients with an Albright's hereditary osteodystrophy like phenotype and platelet Gs hypofunction.
BACKGROUND
Pseudohypoparathyroidism (PHP) indicates a group of heterogeneous disorders whose common feature is represented by impaired signaling of hormones that activate Gsalpha, encoded by the imprinted GNAS gene. PHP-Ib patients have isolated Parathormone (PTH) resistance and GNAS epigenetic defects while PHP-Ia cases present with hormone resistance and characteristic features jointly termed as Albright's Hereditary Osteodystrophy (AHO) due to maternally inherited GNAS mutations or similar epigenetic defects as found for PHP-Ib. Pseudopseudohypoparathyroidism (PPHP) patients with an AHO phenotype and no hormone resistance and progressive osseous heteroplasia (POH) cases have inactivating paternally inherited GNAS mutations.


RESULTS
We here describe 17 subjects with an AHO-like phenotype that could be compatible with having PPHP but none of them carried Gsalpha mutations. Functional platelet studies however showed an obvious Gs hypofunction in the 13 patients that were available for testing. Methylation for the three differentially methylated GNAS regions was quantified via the Sequenom EpiTYPER. Patients showed significant hypermethylation of the XL amplicon compared to controls (36 ± 3 vs. 29 ± 3%; p<0.001); a pattern that is reversed to XL hypomethylation found in PHPIb. Interestingly, XL hypermethylation was associated with reduced XLalphaS protein levels in the patients' platelets. Methylation for NESP and ExonA/B was significantly different for some but not all patients, though most patients have site-specific CpG methylation abnormalities in these amplicons. Since some AHO features are present in other imprinting disorders, the methylation of IGF2, H19, SNURF and GRB10 was quantified. Surprisingly, significant IGF2 hypermethylation (20 ± 10 vs. 14 ± 7%; p<0.05) and SNURF hypomethylation (23 ± 6 vs. 32 6%; p<0.001) was found in patients vs. controls, while H19 and GRB10 methylation was normal.


CONCLUSIONS
In conclusion, this is the first report of methylation defects including GNAS in patients with an AHO-like phenotype without endocrinological abnormalities. Additional studies are still needed to correlate the methylation defect with the clinical phenotype.
Center for Molecular and Vascular Biology, University of Leuven, Leuven, Belgium.
Benedetta
B
Izzi
Inge
I
Francois
Veerle
V
Labarque
Chantal
C
Thys
Christine
C
Wittevrongel
Koen
K
Devriendt
Eric
E
Legius
Annick
A
Van den Bruel
Marc
M
D'Hooghe
Diether
D
Lambrechts
Francis
F
de Zegher
Chris
C
Van Geet
Kathleen
K
Freson
D002864:Chromogranins; D019175:DNA Methylation / Q000235:genetics*; D005359:Fibrous Dysplasia, Polyostotic / Q000235:genetics*; D051383:GRB10 Adaptor Protein / Q000235:genetics; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000235:genetics; D006801:Humans; D007335:Insulin-Like Growth Factor II / Q000235:genetics; D009687:Nuclear Proteins / Q000235:genetics; D010641:Phenotype; D011547:Pseudohypoparathyroidism
D002864:Chromogranins; C096358:GRB10 protein, human; C035170:IGF2 protein, human; D009687:Nuclear Proteins; C445239:SNURF protein, human; D051383:GRB10 Adaptor Protein; D007335:Insulin-Like Growth Factor II; C406732:GNAS protein, human; D019205:GTP-Binding Protein alpha Subunits, Gs