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

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.

A Case of Psuedohypoparathyroidism Unmasked by COVID-19 and Rhabdomyolysis

Background: Psuedohypoparathyroidism type 1 b (PHP 1b) is a rare condition characterized by hormone resistance with PTH. It is caused by imprinting defect of the GNAS gene and is acquired as autosomal dominant. Compared with Pseudohypoparathyroidism type 1a (PHP1a), PHP1b does not have the characteristic physical features know as Albright hereditary osteodystrophy or AHO. Results: 12-year-old female with unremarkable past medical history presented with seizures. She has been complaining of leg pain for a week but on the day of presentation noted to have stiffening and shortness of breath. There was no history of cough, fever, leg trauma, headache, dysuria, hematuria or dark urine. She had no significant family history. Work up revealed severe hypocalcemia of 4.6 mg/ dL and elevated phosphorus (7.1mg/dl). There was no hypoglycemia and other electrolytes including renal function were normal. Patient also had normal inflammatory markers, normal fibrinogen, and normal ferritin. Urine was positive for trace protein and was positive for myoglobin. Alkaline phosphatase was normal. Urine toxicology screen was negative. Hand X-rays did not showed the shortening of metacarpal bones and kidney ultrasound was normal. After IV calcium bolus, calcium barely increased at 5.0. Interestingly CK was noted to be elevated at 3,794 U/L. Patient physical exam was normal and there was no signs of Albright hereditary osteodystrophy (AHO). Patient was positive for COVID19. Patient required intensive fluid therapy to correct CK which increased up to 11,223 U/L on 3rd day of admission. It eventually came down back to normal on the 6th day. Creatinine levels remained normal. Patient continued to receive high dose calcium and calcitriol supplement and discharged with calcium of 8.5 mg/dL and phosphorus of 7.8 mg/dL. Additional work up showed PTH of 885 pg/mL consistent with pseudohypoparathyroidism. Vitamin D levels and thyroid function were normal. Genetic testing for pseudohypoparathyroidism is awaited. Conclusion: Psuedohypoparathyroidism type 1 b (PHP 1b) is a rare endocrine disorder presenting with hypocalcemia, hyperphosphatemia and increased PTH values due to a variable resistance to target organs. As in our case it was unmasked by COVID 19 infection and rhabdomyolysis. Best of our knowledge there is no such unusual case reported in children. Severe hypocalcemia likely due to combination of factors including tissue calcium deposition, hyperphosphatemia, and skeletal resistance to PTH. As published in some recent reports, this case also illustrates that rhabdomyolysis could be potential complication of SARS- CoV2 infection in early stage with normal renal function, which warrants further research.

GNAS Heterozygous Inactivation Differentially Affects Osteoclast-Specific Calcitonin Receptor Bioactivity in a Mouse Model of Albright Hereditary Osteodystrophy Based Upon Parental Inheritance

Albright hereditary osteodystrophy (AHO) is caused by the heterozygous inactivation of GNAS, encoding the α-stimulatory subunit (Gαs) of G protein-coupled receptors. Skeletal manifestations of AHO include adult short stature, brachydactyly and subcutaneous ossifications. AHO patients with maternally-derived GNAS mutations develop pseudohypoparathyroidism type 1A (PHP1A) and are obese with resistance to hormones requiring Gαs (eg., PTH, TSH and GHRH) due to tissue-specific GNAS imprinting. Paternally-derived GNAS mutations cause pseudopseudohypoparathyroidism (PPHP) in which patients have AHO skeletal features but do not develop severe obesity or hormonal resistance. Mouse models have shown loss of Gα s signaling in osteoblasts or osteoclasts leads to osteopenia, and suggest AHO patients would display a reduced bone mineral density (BMD). Interestingly, PHP1A patients have been shown to have normal to increased BMD without any correlation to body mass index or serum PTH measurement. Based on the differences observed clinically and hormonally between PHP1A and PPHP, we hypothesize that there may also be distinctions in overall bone remodeling between these two disorders due to GNAS imprinting. This study addressed whether the heterozygous inactivation of Gnas differentially affects Gα s-receptor bioactivity within osteoclasts (OCs) based upon parental inheritance. Bone Marrow Macrophages (BMMs) were harvested from our laboratory’s AHO mouse model with either maternally-inherited (Gnas+/-m) mutations correlating to PHP1A or paternally-inherited (Gnas+/-p) mutations correlating to PPHP. BMMs were exposed to 10-7M salmon calcitonin (sCT), 10-5M forskolin or PBS for 6 hrs. OC receptor activity was measured by fluorescent microscopy to visualize actin ring morphology and RT-PCR analysis of Gα s-PKA signaling transcripts Crem and Ramp3. Forskolin treatment displayed no significant variations in OC ring morphology or Crem and Ramp3 mRNA expression between Gnas+/-m, Gnas+/-p and WT cultures. Both WT and Gnas+/-p OCs displayed appropriate responses to sCT, as indicated by a significant disruption in actin ring morphology and increased Crem and Ramp3 mRNA expression when compared to vehicle-treated controls. SCT-treated Gnas+/-m OCs, however, displayed only mild disruptions in actin ring morphology, and we observed significant reductions in Ramp3 expression compared to WT as well as reductions in Crem compared to WT and Gnas+/-p. These data suggest evidence of partial calcitonin resistance within Gnas+/-m OCs due to impaired Gα s- signaling. These data correlate with previous clinical observations of calcitonin resistance in PHP1A patients. Because these findings were observed only within Gnas+/-m cultures, future work is warranted to determine whether this impaired receptor activity may be attributed to partial Gnas imprinting within OCs or the myeloid lineage.

Acromesomelic and Acromelic Dysplasias/Dysostoses

This chapter discusses acromesomelic and acromelic dysplasias/dysostoses and related disorders and includes discussion on acromesomelic dysplasias (Maroteaux type), Grebe dysplasia, brachydactyly A1, brachydactyly B, brachydactyly C, brachydactyly D, brachydactyly E, brachydactyly (Christian type), tricho-rhino-phalangeal dysplasia (type 1), tricho-rhino-phalangeal dysplasia (type 2), acrocapitofemoral dysplasia, Albright hereditary osteodystrophy, acrodysostosis, geleophysic dysplasia, acromicric dysplasia, Myhre syndrome, and SOFT syndrome. Each discussion includes major radiographic features, major clinical findings, genetics, major differential diagnoses, and a bibliography.