Several large maternal deletions involve not only GNAS exons 1-13, but also one or several GNAS DMRs, thus causing PHP1A combined with apparent GNAS epigenetic changes that are indistinguishable from those observed in PHP1B.
PHP1B is associated with epigenetic changes at one or several differentially-methylated regions (DMRs) within GNAS, which encodes the α-subunit of the stimulatory G protein (Gsα) and splice variants thereof.
We report a case of a sporadic PHP1B patient with broad GNAS methylation changes that were presumably caused by an unknown genetic mutation outside the GNAS locus.
Genomic DNA from PHP1B patients shows epigenetic changes at one or multiple differentially methylated regions (DMRs) within GNAS, the gene encoding Gαs and splice variants thereof.
Although GNAS methylation changes were not detected in additional pediatric and adult patients with subclinical hypothyroidism (23 pediatric and 39 adult cases), hypothyroidism can obviously be the initial finding in PHP-Ib patients.
Microdeletions that alter GNAS methylation and, thereby, diminish Gsα expression in tissues in which the paternal Gsα allele is normally silenced also cause hormone resistance, which occurs typically in the absence of AHO, a disorder termed pseudohypoparathyroidism type-Ib.
We identified two duplications in the GNAS region in two patients with PHP-Ib: one of them, comprising ∼ 320 kb, occurred 'de novo' in the patient, whereas the other one, of ∼ 179 kb in length, was inherited from the maternal allele.
Pseudohypoparathyroidism (PHP) is associated with compromised signal transductions via PTH receptor (PTH-R) and other G-protein-coupled receptors including GHRH-R. To date, while GH deficiency (GHD) has been reported in multiple patients with PHP-Ia caused by mutations on the maternally expressed GNAS coding regions and in two patients with sporadic form of PHP-Ib accompanied by broad methylation defects of maternally derived GNAS differentially methylated regions (DMRs), it has not been identified in a patient with an autosomal dominant form of PHP-Ib (AD-PHP-Ib) accompanied by an STX16 microdeletion and an isolated loss of methylation (LOM) at exon A/B-DMR.
Pseudohypoparathyroidism type Ib (PHP-Ib) is a rare genetic disorder characterized by hypocalcemia and hyperphosphatemia due to imprinting defects in the maternally derived GNAS allele.
In addition, deletions removing the entire NESP55 DMR, located within GNAS, associated with loss of all the maternal GNAS imprints have been identified in some AD-PHP-Ib kindreds.
Familial and sporadic forms of PHP-Ib have distinct GNAS imprinting patterns: familial PHP-Ib patients have an exon A/B-only imprinting defect whereas sporadic PHP-Ib cases have abnormal imprinting of the three differentially methylated regions (DMRs) in GNAS.
Maternally inherited 3-kb STX16 deletions cause autosomal dominant pseudohypoparathyroidism type Ib (PHP-Ib) characterized by PTH resistance with loss of methylation restricted to the GNAS exon A/B.
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.
Based on our data, it appears plausible that some forms of PHP-Ib are caused by homozygous or compound heterozygous mutation(s) in an unknown gene involved in establishing or maintaining GNAS methylation.
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.
Patients with methylation defects at the GNAS locus show a significant decrease in erythrocyte Gsα activity compared to normal controls (PHP-Ib versus controls, p < .001).
Most patients with pseudohypoparathyroidism type Ib (PHP-Ib) exhibit imprinting defects affecting the maternal GNAS allele, which are thought to reduce/abolish Gsalpha expression in renal proximal tubules and thereby cause resistance to PTH.
Familial and sporadic forms of PHPIb have distinct GNAS imprinting patterns: familial PHPIb patients have an exon A/B-only imprinting defect and an intragenic STX16 deletion, whereas sporadic PHPIb cases have abnormal imprinting of the three differentially methylated regions (DMRs) in GNAS without the STX16 deletion.