Parental origin of Gsα inactivation differentially affects bone remodeling in a mouse model of Albright hereditary osteodystrophy

Abstract

Albright hereditary osteodystrophy (AHO) is caused by heterozygous inactivation of GNAS, a complex locus that encodes the alpha-stimulatory subunit of GPCRs (Gsα) in addition to NESP55 and XLαs due to alternative first exons. AHO skeletal manifestations include brachydactyly, brachymetacarpia, compromised adult stature, and subcutaneous ossifications. AHO patients with maternally-inherited GNAS mutations develop pseudohypoparathyroidism type 1A (PHP1A) with resistance to multiple hormones that mediate their actions through GPCRs requiring Gsα (eg., PTH, TSH, GHRH, calcitonin) and severe obesity. Paternally-inherited GNAS mutations cause pseudopseudohypoparathyroidism (PPHP), in which patients have AHO skeletal features but do not develop hormonal resistance or marked obesity. These differences between PHP1A and PPHP are caused by tissue-specific reduction of paternal Gsα expression. Previous reports in mice have shown loss of Gsα causes osteopenia due to impaired osteoblast number and function and suggest AHO patients could display evidence of reduced bone mineral density (BMD). However, we previously demonstrated PHP1A patients display normal-increased BMD measurements without any correlation to body mass index or serum PTH. Due to these observed differences between PHP1A and PPHP, we utilized an AHO mouse model generated in our laboratory to address whether Gsα heterozygous inactivation by the targeted disruption of exon 1 of Gnas differentially affects bone remodeling based on the parental inheritance of the mutation. Mice with paternally-inherited (Gnas E1+/-p) and maternally-inherited (Gnas E1+/-m) mutations displayed reductions in osteoblasts along the bone surface compared to wildtype. Gnas E1+/-p mice displayed reduced cortical and trabecular bone parameters due to impaired bone formation and excessive bone resorption. Gnas E1+/-m mice however displayed enhanced bone parameters due to increased osteoblast activity and normal bone resorption. These distinctions in bone remodeling between Gnas E1+/-p and Gnas E1+/-m mice appear to be secondary to changes in the bone microenvironment driven by calcitonin-resistance within Gnas E1+/-m osteoclasts and therefore warrant further studies into understanding how Gsα influences osteoblast-osteoclast coupling interactions.

Prevalence of Chiari malformation type 1 is increased in pseudohypoparathyroidism type 1A and associated with aberrant bone development

“…C-telopeptide levels were increased in female Gnas E1+/-p mice, but there were no significant changes in Gnas E1+/-mice. [26]. Furthermore, previous reports have described the presence of a thickened cranium on head CT imaging of PHP1A patients [11,58,91] as well as increased bone mineral density in PHP1A [25].…”

“…In order to understand further the etiologies surrounding the craniofacial malformations occurring in PHP1A, the craniofacial phenotype of our AHO mouse model (previously generated through targeted disruption of exon 1 of Gnas [22]) was examined. This model results in the global heterozygous inactivation of Gnas, and we have previously shown that this model phenotypically recapitulates many of the hormonal, metabolic, and skeletal abnormalities seen in patients with PHP1A and PPHP [22,26,68]. In particular, mice with maternally-inherited Gnas mutations (Gnas E1+/-m) phenotypically and hormonally recapitulate PHP1A; when compared to wildtype littermates, these mice have shortened lengths, are obese, develop subcutaneous ossifications [68], display hormonal resistance, and have decreased fertility [22].…”

“…Bone formation activity was assessed in 12-week-old WT and Gnas E1+/-mice by performing alizarin complexone (AC) and calcein double-labeling using methods as previously described [26,84]. Briefly, WT and Gnas E1+/-mice received an intraperitoneal injection of alizarin complexone [dose 30mg/kg (Sigma A-3882)] 7 days prior to sacrifice, in addition to an injection of calcein [dose 10mg/kg (Sigma C-0875)] 2 days prior to sacrifice.…”

“…It has been established in both humans and mouse models [1] that the metabolic and hormonal distinctions between PHP1A and PPHP are secondary to partial paternal imprinting of GNAS in specific tissues, many of which are endocrine, such as the renal proximal convoluted tubules, thyroid, gonads, and pituitary [13][14][15][21][22][23][24]. In addition, we have found in both human and mouse models that regulation of bone remodeling and bone mineral density may be impacted by imprinting as well [25][26][27].…”

“…We also performed secondary analyses to identify whether there were independent factors that positively correlated with CM1/LLCT development, such as GH status, clinical findings, biological sex, GNAS mutation severity, and bone age. On the basis of these clinical observations, we then systematically examined our AHO mouse model generated through the targeted disruption of Gnas exon 1 (Gnas E1+/-) [22,26,68] to evaluate how Gnas heterozygous inactivation influences craniofacial development and bone formation. By utilizing various investigative techniques, we systematically analyzed the various hypothesized structural causes of CM1 formation in our mouse model.…”