Quantification of the methylation at the GNAS locus identifies subtypes of sporadic pseudohypoparathyroidism type Ib.
BACKGROUND
Pseudohypoparathyroidism type Ib (PHP-Ib) is due to epigenetic changes at the imprinted GNAS locus, including loss of methylation at the A/B differentially methylated region (DMR) and sometimes at the XL and AS DMRs and gain of methylation at the NESP DMR.


OBJECTIVE
To investigate if quantitative measurement of the methylation at the GNAS DMRs identifies subtypes of PHP-Ib.


METHODS
In 19 patients with PHP-Ib and 7 controls, methylation was characterised at the four GNAS DMRs through combined bisulfite restriction analysis and quantified through cytosine specific real-time PCR in blood lymphocyte DNA.


RESULTS
A principal component analysis using the per cent of methylation at seven cytosines of the GNAS locus provided three clusters of subjects (controls n=7, autosomal dominant PHP-Ib with loss of methylation restricted to the A/B DMR n=3, and sporadic PHP-Ib with broad GNAS methylation changes n=16) that matched perfectly the combined bisulfite restriction analysis classification. Furthermore, three sub-clusters of patients with sporadic PHP-Ib, that displayed different patterns of methylation, were identified: incomplete changes at all DMRs compatible with somatic mosaicism (n=5), profound epigenetic changes at all DMRs (n=8), and unmodified methylation at XL in contrast with the other DMRs (n=3). Interestingly, parathyroid hormone concentration at the time of diagnosis correlated with the per cent of methylation at the A/B DMR.


CONCLUSIONS
Quantitative assessment of the methylation in blood lymphocyte DNA is of clinical relevance, allows the diagnosis of PHP-Ib, and identifies subtypes of PHP-Ib. These epigenetic findings suggest mosaicism at least in some patients.
INSERM, Université Paris Descartes, Hôpital Saint Vincent de Paul, France.
Stéphanie
S
Maupetit-Méhouas
Virginie
V
Mariot
Christelle
C
Reynès
Guylène
G
Bertrand
Francois
F
Feillet
Jean-Claude
JC
Carel
Dominique
D
Simon
Hélène
H
Bihan
Vincent
V
Gajdos
Eve
E
Devouge
Savitha
S
Shenoy
Placide
P
Agbo-Kpati
Anne
A
Ronan
Catherine
C
Naud-Saudreau
Anne
A
Lienhardt
Caroline
C
Silve
Agnès
A
Linglart
D000293:Adolescent; D000328:Adult; D002648:Child; D002864:Chromogranins; D019175:DNA Methylation / Q000235:genetics*; D044127:Epigenesis, Genetic; D005260:Female; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000235:genetics*; D005799:Genes, Dominant; D006801:Humans; D008297:Male; D010641:Phenotype; D011547:Pseudohypoparathyroidism / Q000145:classification / Q000235:genetics; D017422:Sequence Analysis, DNA
D002864:Chromogranins; C406732:GNAS protein, human; D019205:GTP-Binding Protein alpha Subunits, Gs