Albright Hereditary Osteodystrophy Patients Research Articles

Hair follicle-resident progenitor cells are a major cellular contributor to heterotopic subcutaneous ossifications in a mouse model of Albright hereditary osteodystrophy.

Heterotopic ossifications (HOs) are the pathologic process by which bone inappropriately forms outside of the skeletal system. Despite HOs being a persistent clinical problem in the general population, there are no definitive strategies for their prevention and treatment due to a limited understanding of the cellular and molecular mechanisms contributing to lesion development. One disease in which the development of heterotopic subcutaneous ossifications (SCOs) leads to morbidity is Albright hereditary osteodystrophy (AHO). AHO is caused by heterozygous inactivation of GNAS, the gene that encodes the α-stimulatory subunit (Gαs) of G proteins. Previously, we had shown using our laboratory's AHO mouse model that SCOs develop around hair follicles (HFs). Here we show that SCO formation occurs due to inappropriate expansion and differentiation of HF-resident stem cells into osteoblasts. We also show in AHO patients and mice that Secreted Frizzled Related Protein 2 (SFRP2) expression is upregulated in regions of SCO formation and that elimination of Sfrp2 in male AHO mice exacerbates SCO development. These studies provide key insights into the cellular and molecular mechanisms contributing to SCO development and have implications for potential therapeutic modalities not only for AHO patients but also for patients suffering from HOs with other etiologies.

Albright hereditary osteodystrophy: Current progress and new frontiers

Albright hereditary osteodystrophy (AHO) is a rare genetic disorder characterized by phenotypic abnormalities including brachydactyly , brachymetacarpia, short stature , subcutaneous ossifications , and dental abnormalities. This disorder is caused by heterozygous, inactivating mutations in GNAS , the gene encoding the alpha chain of the heterotrimeric G protein, Gs, that couples with receptors for many hormones and neurotransmitters for the activation of adenylyl cyclase. Transcripts encoding Gαs are preferentially expressed from the maternally inherited allele in the renal proximal tubule, thyroid, gonad, and pituitary. AHO patients who have GNAS mutations on maternally inherited alleles manifest resistance to multiple hormones (including PTH, TSH, gonadotropins, and GHRH) as well as obesity and cognitive deficits. This condition is termed pseudohypoparathyroidism type Ia (PHP Ia) and is due to tissue-specific paternal imprinting. Conversely, patients with AHO and GNAS mutations on their paternally inherited alleles have no evidence of hormonal resistance, are typically not obese, and cognitive function may be normal. This condition is termed pseudopseudohypoparathyroidism (PPHP). Current knowledge as well as the research in progress in both humans and mouse models regarding the major clinical problems in AHO will be reviewed. Dr. Germain-Lee will focus on her past and current research involving clinical trials with growth hormone in patients with AHO, as well as her research on the heterotopic ossifications that develop in this condition, which she is examining through the use of a mouse model. Her research on the etiology and mechanisms underlying the formation of the heterotopic ossifications has broader implications in terms of the potential to understand more about the mechanisms of osteogenesis overall. In addition, Dr. Germain-Lee will discuss her clinical findings related to the severe obesity in pseudohypoparathyroidism type Ia; this severe obesity is not present in pseudopseudohypoparathyroidism. Dr. Weinstein will discuss his basic science research on metabolism and obesity in AHO, which is based on his studies of his mouse models of this condition. In addition, he will discuss his current ongoing clinical studies of obesity and metabolism in AHO.

Gsα Deficiency in the Paraventricular Nucleus of the Hypothalamus Partially Contributes to Obesity Associated with Gsα Mutations

The G protein α-subunit G(s)α mediates receptor-stimulated cAMP generation. Heterozygous inactivating G(s)α mutations on the maternal allele result in obesity primarily due to reduced energy expenditure in Albright hereditary osteodystrophy patients and in mice. We previously showed that mice with central nervous system (CNS)-specific G(s)α deletion on the maternal allele (mBrGs KO) also develop severe obesity with reduced energy expenditure and that G(s)α is primarily expressed from the maternal allele in the paraventricular nucleus (PVN) of the hypothalamus, an important site of energy balance regulation. We now generated mice with PVN-specific G(s)α deficiency by mating Single-minded 1-cre and G(s)α-floxed mice. Homozygous G(s)α deletion produced early lethality. Heterozygotes with maternal G(s)α deletion (mPVNGsKO) also developed obesity and had small reductions in energy expenditure. However, this effect was much milder than that found in mBrGsKO mice and was more prominent in males. We previously showed mBrGsKO mice to have significant reductions in melanocortin receptor agonist-stimulated energy expenditure and now show that mBrGsKO mice have impaired cold-induced brown adipose tissue stimulation. In contrast, these effects were absent in mPVNGsKO mice. mPVNGsKO mice also had minimal effects on glucose metabolism as compared with mBrGsKO mice. Consistent with the presence of G(s)α imprinting, paternal heterozygotes showed no changes in energy or glucose metabolism. These results indicate that although G(s)α deficiency in PVN partially contributes to the metabolic phenotype resulting from maternal G(s)α mutations, G(s)α imprinting in other CNS regions is also important in mediating the CNS effects of G(s)α mutations on energy and glucose metabolism.

No evidence for GNAS copy number variants in patients with features of Albright's hereditary osteodystrophy and abnormal platelet Gs activity

Albright's hereditary osteodystrophy (AHO) is characterized by short stature, round face, calcifications, obesity, brachydactyly and intellectual disability. AHO without hormone resistance is called pseudopseudohypoparathyroidism (PPHP), a rare clinical condition difficult to diagnose with highly variable features. PPHP is caused by paternally inherited loss-of-function mutations in the GNAS. Patients with 2q37 microdeletions or HDAC4 mutations are also defined as having an AHO-like phenotype with normal stimulatory G (Gs) function. We have studied 256 patients with AHO features but no other diagnosis. Their platelet Gs activity was determined via the aggregation-inhibition test showing Gs hypo- or hyperfuncton in 24% and 15% of the patients, respectively. Before initiating with detailed (epi)genetic GNAS studies, we here wanted to excluded copy number variants (CNVs) in GNAS as cause of AHO with a novel large-scale screening technique. Multiplex amplicon quantification (MAQ) for CNVs screening was developed for the 20q13.3 region including GNAS and potential long-range imprinting control elements such as STX16. This is the first large-scale GNAS CNV study in patients with common AHO features but no CNVs were detected. In conclusion, CNVs in the GNAS region are not likely to cause an AHO-like phenotype with or without abnormal platelet Gs activity. Future studies will be undertaken to find out whether these AHO patients with abnormal Gs function are characterized by GNAS coding or methylation defects.

Heterotopic ossifications in a mouse model of albright hereditary osteodystrophy.

Albright hereditary osteodystrophy (AHO) is characterized by short stature, brachydactyly, and often heterotopic ossifications that are typically subcutaneous. Subcutaneous ossifications (SCO) cause considerable morbidity in AHO with no effective treatment. AHO is caused by heterozygous inactivating mutations in those GNAS exons encoding the α-subunit of the stimulatory G protein (Gαs). When inherited maternally, these mutations are associated with obesity, cognitive impairment, and resistance to certain hormones that mediate their actions through G protein-coupled receptors, a condition termed pseudohypoparathyroidism type 1a (PHP1a). When inherited paternally, GNAS mutations cause only AHO but not hormonal resistance, termed pseudopseudohypoparathyroidism (PPHP). Mice with targeted disruption of exon 1 of Gnas (Gnas E1−/+) replicate human PHP1a or PPHP phenotypically and hormonally. However, SCO have not yet been reported in Gnas E1+/− mice, at least not those that had been analyzed by us up to 3 months of age. Here we now show that Gnas E1−/+ animals develop SCO over time. The ossified lesions increase in number and size and are uniformly detected in adult mice by one year of age. They are located in both the dermis, often in perifollicular areas, and the subcutis. These lesions are particularly prominent in skin prone to injury or pressure. The SCO comprise mature bone with evidence of mineral deposition and bone marrow elements. Superficial localization was confirmed by radiographic and computerized tomographic imaging. In situ hybridization of SCO lesions were positive for both osteonectin and osteopontin. Notably, the ossifications were much more extensive in males than females. Because Gnas E1−/+ mice develop SCO features that are similar to those observed in AHO patients, these animals provide a model system suitable for investigating pathogenic mechanisms involved in SCO formation and for developing novel therapeutics for heterotopic bone formation. Moreover, these mice provide a model with which to investigate the regulatory mechanisms of bone formation.

Osteoma Cutis as the Presenting Feature of Albright Hereditary Osteodystrophy Associated with Pseudopseudohypoparathyroidism

Primary osteoma cutis is characterized by the formation of normal bone tissue in the dermis or subcutis without any underlying tissue abnormality or pre-existing calcification. This illness is associated with Albright hereditary osteodystrophy (AHO), which is characterized by such physical features as a short stature, round face, obesity, brachydactyly and osteoma cutis. Pseudohypoparathyroidism (PHP) is an inherited metabolic disorder that's characterized by resistance to parathyroid hormone, and PHP is present in most AHO patients. An AHO phenotype without hormonal resistance is called pseudopseudohypoparathyroidism (PPHP). Osteoma cutis is less common in patients with PPHP than in patients with PHP. We present here a case of osteoma cutis as the cardinal manifestation of AHO associated with PPHP. Osteoma cutis is an important sign of AHO and its significance should not be overlooked, even if the patient has normal values on the serum biochemical tests.

Patients with Mutations in Gsα Have Reduced Activation of a Downstream Target in Epithelial Tissues due to Haploinsufficiency

Patients with Albright hereditary osteodystrophy (AHO) have defects in stimulatory G protein signaling due to loss of function mutations in GNAS. The mechanism by which these mutations lead to the AHO phenotype has been difficult to establish due to the inaccessibility of the affected tissues. The objective of the study was to gain insight into the downstream consequences of abnormal stimulatory G protein signaling in human epithelial tissues. We assessed transcription of GNAS and Gsalpha-stimulated activation of the cystic fibrosis transmembrane conductance regulator (CFTR) in AHO patients, compared with normal controls and patients with cystic fibrosis. Relative expression of Gsalpha transcripts from each parental GNAS allele and cAMP measurements from nasal epithelial cells were compared among normal controls and AHO patients. In vivo measurements of CFTR function, pulmonary function, and pancreatic function were assessed in AHO patients. GNAS was expressed equally from each allele in normals and two of five AHO patients. cAMP generation was significantly reduced in nasal respiratory epithelial cells from AHO patients, compared with normal controls (0.4 vs. 0.6, P = 0.0008). Activation of CFTR in vivo in nasal (P = 0.0065) and sweat gland epithelia (P = 0.01) of AHO patients was significantly reduced from normal. In three patients, the reduction in activity was comparable with patients with cystic fibrosis due to mutations in CFTR. Yet no AHO patients had pulmonary or pancreatic disease consistent with cystic fibrosis. In humans, haploinsufficiency of GNAS causes a significant reduction in the activation of the downstream target, CFTR, in vivo.

Growth hormone deficiency in pseudohypoparathyroidism type 1a: another manifestation of multihormone resistance.

Albright hereditary osteodystrophy (AHO) is a genetic disorder caused by heterozygous inactivating mutations in GNAS1, the gene encoding the alpha-chain of G(s), and is associated with short stature, obesity, brachydactyly, and sc ossifications. AHO patients with GNAS1 mutations on maternally inherited alleles also manifest resistance to multiple hormones (e.g. PTH, TSH, LH, FSH), a variant termed pseudohypoparathyroidism (PHP) type 1a, due to paternal imprinting of G alpha(s) transcripts in specific tissues. Recent evidence has shown that G alpha(s) transcripts are also imprinted in the pituitary somatotrophs that secrete GH. Because this imprinting could influence GHRH-dependent stimulation of somatotrophs, we hypothesized that maternally inherited GNAS1 mutations would impair GH secretion. We studied GH status in 13 subjects with PHP type 1a. GH responses to arginine/L-dopa and arginine/GHRH were deficient in nine subjects, all of whom were obese and had low serum concentrations of IGF-I. By contrast, none of the four GH-sufficient subjects were obese, and all had normal IGF-I levels. Our data indicate that GH deficiency is common (69%) in PHP type 1a and may contribute to the obesity and short stature typical of AHO. We propose that GH status be evaluated in all patients with PHP type 1a.

Paternal imprinting of Gα s in the human thyroid as the basis of TSH resistance in pseudohypoparathyroidism type 1a

Albright hereditary osteodystrophy (AHO) is characterized by multiple somatic defects secondary to mutations in the GNAS1 gene. AHO patients with mutations on maternally inherited alleles are resistant to multiple hormones (e.g., PTH, TSH), a variant termed pseudohypoparathyroidism (PHP) type 1a, due to presumed tissue-specific paternal imprinting of the α chain of G s as demonstrated in murine renal proximal tubule and fat cells. Studies in human tissues thus far revealed imprinting only in pituitary. Because mild hypothyroidism due to TSH resistance occurs in most PHP type 1a patients, we investigated whether Gα s is imprinted in thyroid. Examination of eight normal thyroids demonstrated significantly greater expression from the maternal GNAS1 allele, with paternal Gα s transcripts accounting for only 25.9–40.4%. Expression of NESP55, XLα s, and 1A was uniallelic. We conclude that Gα s is incompletely imprinted in the thyroid, which provides an explanation for mild TSH resistance in PHP type 1a.

GNAS1 imprinting: the latest twist

In all branches of science, results that appear anomalous or depart from the norm force us to rethink the rules and challenge accepted dogma. The discovery that some genes are imprinted was one such departure. Whereas in classical mendelian genetics, the parental origin of a gene is unimportant, a small proportion of genes are imprinted with a ‘memory’ of whether they are maternally or paternally derived, and their transcription patterns are set accordingly. These imprints are often uncovered by unusual patterns of inheritance of genetic diseases.The GNAS1 locus on human chromosome 20 is already known to be complex. At least three transcripts derive from various combinations of exons: NESP55 is maternally expressed, XLαs is paternally expressed, and Gsα is biallelically expressed in most tissues, but imprinted in a few. Imprinting of this locus was first indicated by the unusual inheritance observed in families with Albright hereditary osteodystrophy (AHO), a condition characterized by short stature, developmental delay, obesity and metaphalangeal shortening. Some AHO patients also show pseudohypoparathyroidism type 1A (PHP1A), a resistance to the effects of several hormones, particularly parathyroid hormone and thyroid-stimulating hormone. PHP1A almost always results from maternally inherited mutations of Gsα; paternal transmission of the same mutation in the same family causes AHO without hormone resistance. This could be explained if Gsα is imprinted in the target sites for these hormones; for instance, the thyroid and renal tubule.A study by Shore and colleagues [1xPaternally inherited inactivating mutations of the GNAS1 gene in progressive osseous heteroplasia. Shore, E.M et al. New Engl. J. Med. 2002; 346: 99–106CrossRef | PubMed | Scopus (144)See all References][1] now shows that even this view is simplistic. First, they demonstrate that mutations of Gsα can give rise to a very different condition, progressive osseous heteroplasia (POH), in which extensive inappropriate bone formation in soft tissue causes progressive disability. In some cases, the POH mutations were identical to those already described in families with AHO. Where the parental origin could be determined, the mutations were all paternally derived. Two transmitting fathers were not clinically affected themselves. Most intriguingly, in one family where three females with POH passed the mutation on, their children were affected by AHO, not POH. The hormone status of these children is not mentioned, so it is unclear whether they have PHP1A.So why do paternally inherited Gsα mutations lead to AHO in some families and POH in others? Existing models of imprinting cannot fully explain these observations, and further analysis of potential genetic and epigenetic modifiers is required. This complex locus and its associated genetic disorders have yet more interesting biological secrets to yield.

Activating and inactivating mutations in the human GNAS1 gene.

GNAS1 on chromosome 20 is a complex locus, encoding multiple proteins, of which G(s)alpha, the alpha-subunit of the heterotrimeric stimulatory G protein G(s), is of particular interest clinically. Amino acid substitutions at two specific codons lead to constitutive activation of G(s)alpha. Such gain-of-function mutations are found in a variety of sporadic endocrine tumors and in McCune-Albright syndrome, a sporadic condition characterized by multiple endocrine abnormalities. Heterozygous loss of G(s)alpha function results in the dominantly inherited condition, Albright hereditary osteodystrophy (AHO). Here we present a review of published GNAS1 mutations and report 19 additional mutations, of which 15 are novel. A diverse range of inactivating mutations has been detected, scattered throughout the gene but showing some evidence of clustering. Only one, a recurring 4 bp deletion in exon 7, could be considered common among AHO patients. The parental origin of the mutation apparently determines whether or not the patient shows end-organ resistance to hormones such as parathyroid hormone. G(s)alpha is biallelically expressed in all tissues studied to date and thus there is no direct evidence that this transcript is imprinted. However, the recent identification of other imprinted transcripts encoded by GNAS1 and overlapping G(s)alpha, together with at least one imprinted antisense transcript, raises intriguing questions about how the primary effect of mutations in GNAS1 might be modulated.

Variable and tissue-specific hormone resistance in heterotrimeric Gs protein alpha-subunit (Gsalpha) knockout mice is due to tissue-specific imprinting of the gsalpha gene.

Albright hereditary osteodystrophy (AHO), an autosomal dominant disorder characterized by short stature, obesity, and skeletal defects, is associated with heterozygous inactivating mutations of GNAS1, the gene encoding the heterotrimeric G protein alpha-subunit (Gsalpha) that couples multiple receptors to the stimulation of adenylyl cyclase. It has remained unclear why only some AHO patients present with multihormone resistance and why AHO patients demonstrate resistance to some hormones [e.g., parathyroid hormone (PTH)] but not to others (e.g., vasopressin), even though all activate adenylyl cyclase. We generated mice with a null allele of the mouse homolog Gnas. Homozygous Gs deficiency is embryonically lethal. Heterozygotes with maternal (m-/+) and paternal (+/p-) inheritance of the Gnas null allele have distinct phenotypes, suggesting that Gnas is an imprinted gene. PTH resistance is present in m-/+, but not +/p-, mice. Gsalpha expression in the renal cortex (the site of PTH action) is markedly reduced in m-/+ but not in +/p- mice, demonstrating that the Gnas paternal allele is imprinted in this tissue. Gnas is also imprinted in brown and white adipose tissue. The maximal physiological response to vasopressin (urinary concentrating ability) is normal in both m-/+ and +/p- mice and Gnas is not imprinted in the renal inner medulla (the site of vasopressin action). Tissue-specific imprinting of Gnas is likely the mechanism for variable and tissue-specific hormone resistance in these mice and a similar mechanism might explain the variable phenotype in AHO.

Immunochemical Analysis of theα-Subunit of the Stimulatory G-Protein of Adenylyl Cyclase in Patients with Albright's Hereditary Osteodystrophy*

Albright's hereditary osteodystrophy (AHO) is an autosomal dominant disorder characterized by an unusual phenotypic appearance and reduced biological activity of the alpha-subunit of the stimulatory G-protein of adenylyl cyclase (Gs alpha). In most AHO patients deficient Gs alpha activity is associated with generalized target organ resistance to hormones that act via stimulation of adenylyl cyclase. This form of the disorder is termed pseudohypoparathyroidism type Ia (PHP Ia). By contrast, other patients with Gs alpha deficiency fail to demonstrate clinical evidence of hormone resistance and are considered to have the related disorder pseudopseudohypoparathyroidism (pseudoPHP). Previous studies demonstrating deficient Gs alpha bioactivity in cell membranes from patients with AHO used functional assays that were unable to distinguish between reduced amounts of normal Gs alpha protein and normal amounts of defective Gs alpha protein. In the present study we used specific Gs alpha antisera to analyze immunoactive Gs alpha protein in erythrocyte and fibroblast membranes from 20 patients with AHO who had either normal or reduced levels of Gs alpha mRNA. Cell membranes were subjected to immunoblot analysis using Gs alpha antisera developed against synthetic peptides corresponding to amino acid sequences in the amino- or carboxyl-terminus of the Gs alpha molecule. Fibroblast membranes from patients with AHO who had reduced or normal levels of Gs alpha mRNA contained both the 45- and 52-kDa forms of the Gs alpha protein in quantities that were significantly less [mean +/- SE, 52 +/- 6%; (n = 8) for reduced mRNA and 35 +/- 19% (n = 2) for normal mRNA, percentage of control values] than those present in membranes from normal subjects. Similar reductions were found in the level of the 45-kDa form of Gs alpha in erythrocyte membranes from all AHO patients studied [40 +/- 4% (mean +/- SE) of control values]. No abnormal forms of Gs alpha protein were detected. Cell membranes from patients with PHP type Ia and from patients with pseudoPHP contained levels of immunoactive Gs alpha that were equivalently reduced (43 +/- 4% vs. 42 +/- 5%, respectively). By contrast, erythrocyte membranes from patients with PHP type Ib, who have normal Gs alpha activity, had normal levels of Gs alpha immunoactivity (101 +/- 7%). These results indicate that most patients with AHO have reduced levels of Gs alpha protein as the basis for deficient Gs alpha bioactivity.

Activity of the stimulatory guanine nucleotide-binding protein is reduced in erythrocytes from patients with pseudohypoparathyroidism and pseudopseudohypoparathyroidism: biochemical, endocrine, and genetic analysis of Albright's hereditary osteodystrophy in six kindreds.

Multiple hormone resistance in many patients with pseudohypoparathyroidism (PHP) type Ia and Albright's hereditary osteodystrophy (AHO) is associated with deficient activity of the stimulatory guanine nucleotide-binding protein (Gs) of adenylate cyclase. To study further the relationship of deficient Gs activity to hormone resistance, we evaluated endocrine function and measured Gs activity of erythrocyte membranes from AHO patients with clinical hormone resistance (PHP type Ia) and from family members with AHO alone (pseudopseudohypoparathyroidism). The results of erythrocyte membrane Gs determinations were compared to those of unaffected relatives and normal subjects. Patients with pseudopseudohypoparathyroidism (pseudoPHP) had reductions in erythrocyte membrane Gs activity comparable to those in patients with PHP type Ia [43.4 +/- 11.9% (+/- SD) for PHP type Ia vs. 47.8 +/- 9.5% for pseudoPHP]. However, in contradistinction to patients with PHP type Ia, individuals with pseudoPHP did not have obvious endocrine dysfunction. Although deficient Gs activity appears to play an important role in the pathogenesis of these disorders, it is possible that Gs deficiency must be combined with other factors that limit cAMP production to cause clinically overt endocrine disease.