A Large Inversion Involving GNAS Exon A/B and All Exons Encoding Gsα Is Associated With Autosomal Dominant Pseudohypoparathyroidism Type Ib (PHP1B)

Abstract

Pseudohypoparathyroidism type Ib (PHP1B) is characterized primarily by resistance to parathyroid hormone (PTH) and thus hypocalcemia and hyperphosphatemia, in most case without evidence for Albright Hereditary Osteodystrophy (AHO). PHP1B is associated with epigenetic changes at one or several differentially methylated regions (DMR) within GNAS, which encodes the α-subunit of the stimulatory G protein (Gsα) and splice variants thereof. Heterozygous, maternally inherited STX16 or GNAS deletions leading to isolated loss-of-methylation (LOM) at exon A/B alone or at all maternal DMRs are the cause of autosomal dominant PHP1B (AD-PHP1B). In this study, we analyzed three affected individuals, the female proband and her two sons. All three revealed isolated LOM at GNAS exon A/B, while the proband’s healthy maternal grandmother and uncle showed normal methylation at this locus. Haplotype analysis was consistent with linkage to the STX16/GNAS region, yet no deletion could be identified. Whole genome sequencing of one of the patients revealed a large heterozygous inversion (1,882,433 bp). The centromeric breakpoint of the inversion is located 7,225 bp down-stream of GNAS exon XL, but its DMR showed no methylation abnormality raising the possibility that the inversion disrupts a regulatory element required only for establishing or maintaining exon A/B methylation. Because our three patients presented phenotypes consistent with PHP1B, not PHP1A, the Gsα promoter is probably unaffected by the inversion. Our findings expand the spectrum of genetic mutations that lead to loss-of-methylation at exon A/B alone and thus biallelic expression of the transcript derived from this alternative first GNAS exon.

From cytogenetics to cytogenomics: whole-genome sequencing as a first-line test comprehensively captures the diverse spectrum of disease-causing genetic variation underlying intellectual disability

“…In addition, the possibility of dual diagnosis due to multi-locus variation [22] has been reported for up to 5% of individuals with Mendelian diseases and can explain apparent phenotypic expansion [23]. In research, WGS has been used to detect a wide range of mutations, including copy number variations [24–26] as well as balanced chromosomal rearrangements such as translocations [27, 28], inversions [29], and short tandem repeats (STRs) [30]. A few studies have performed CNV calling from WGS in small cohorts, showing diagnostic rates of 15% (10/79) [24], 33% (20/60) [31], and 14% (7/50) [32].…”

Clinical and Molecular Characteristics of GNAS Inactivation Disorders Observed in 18 Korean Patients

“…However, Patient 11 did not show an exonal deletion in STX16, NESP55, and GNAS AS on the results of MS-MLPA and STX16 qPCR. A large inversion involving GNAS exon A/B and all exons of GNAS has been reported to cause AD PHP1B [20]. The reported patient showed isolated and complete loss of methylation (LOM) at GNAS A/B:TSS-DMR, however copy numbers of the covered exons of GNAS, STX16, NESP55, and GNAS AS were normal on the result of MS-MLPA.…”

Heterotrimeric G proteins in the control of parathyroid hormone actions

“…Genetic changes that lead to these GNAS methylation abnormalities have been discovered in most familial cases with PHP-Ib and include various microdeletions either at the neighboring STX16 locus, located ~200 kb centromeric to GNAS , or at the NESP55 DMR (Bastepe, et al 2003; Bastepe, et al 2005; Chillambhi, et al 2010; Elli, et al 2014; Linglart, et al 2005; Rezwan, et al 2015; Richard, et al 2012; Takatani, et al 2016). A recent study has also revealed a large genomic inversion that disrupts the GNAS locus as a cause of PHP-Ib (Grigelioniene, et al 2017). Sporadic PHP-Ib cases display broad methylation abnormalities within GNAS , including loss of A/B methylation, and in some of these cases, the underlying cause is paternal uniparental disomy involving chromosome 20 (Bastepe, et al 2001a).…”