The genetic basis of progressive osseous heteroplasia

GNAS locus: bone related diseases and mouse models

GNASis a complex locus characterized by multiple transcripts and an imprinting effect. It orchestrates a variety of physiological processes via numerous signaling pathways. Human diseases associated with the GNAS gene encompass fibrous dysplasia (FD), Albright's Hereditary Osteodystrophy (AHO), parathyroid hormone(PTH) resistance, and Progressive Osseous Heteroplasia (POH), among others. To facilitate the study of the GNAS locus and its associated diseases, researchers have developed a range of mouse models. In this review, we will systematically explore the GNAS locus, its related signaling pathways, the bone diseases associated with it, and the mouse models pertinent to these bone diseases.

Engineering and Characterization of a Biomimetic Microchip for Differentiating Mouse Adipocytes in a 3D Microenvironment

Although two-dimensional (2D) cell cultures are the standard in cell research, one pivotal disadvantage is the lack of cell-cell and cell-extracellular matrix (ECM) signaling in the culture milieu. However, such signals occur in three-dimensional (3D) in vivo environments and are essential for cell differentiation, proliferation, and a range of cellular functions. In this study, we developed a microfluidic device to proliferate and differentiate functional adipose tissue and adipocytes by utilizing 3D cell culture technology. This device was used to generate a tissue-specific 3D microenvironment to differentiate 3T3-L1 preadipocytes into either visceral white adipocytes using visceral adipose tissue (VAT) or subcutaneous white adipose tissue (SAT). The microchip has been tested and validated by functional assessments including cell morphology, inflammatory response to a lipopolysaccharide (LPS) challenge, GLUT4 tracking, and gene expression analyses. The biomimetic microfluidic chip is expected to mimic functional adipose tissues that can replace 2D cell cultures and allow for more accurate analysis of adipose tissue physiology.

Progressive Osseous Heteroplasia is not an Autosomal Dominant Trait but Reflects Superimposed Mosaicism in Different GNAS Inactivation Disorders

Progressive osseous heteroplasia (POH) is a rarely occurring genetic condition characterized by severe segmental ossification involving the skin and deep connective tissues including the muscles. So far, the disorder is generally described as an autosomal dominant trait. By contrast, the following arguments are in favor of the alternative concept that POH should rather be taken as a non-specific segmental manifestation of different GNAS inactivation disorders such as Albright hereditary osteodystrophy (AHO) with hormone resistance, AHO without hormone resistance, and osteomatosis cutis. Presently, POH has got its own OMIM number 166350 but this is obviously wrong because the disorder does not reflect heterozygosity for a GNAS mutation. Conversely, the disorder is most likely due to an early event of postzygotic loss of heterozygosity with loss of the corresponding wild-type allele. This alternative concept, as proposed in 2016, offers a plausible explanation for the following features of POH. Familial occurrence is usually absent. POH is usually observed in families with one of the three GNAS inactivation disorders as mentioned above. Mosaicism is suggested by the pronounced segmental manifestation of POH and by its lateralization. Some patients have, in addition to POH, bilaterally disseminated features of osteomatosis cutis or AHO, and other patients have family members with one of these nonsegmental disorders. Remarkably, POH tends to appear much earlier than the nonsegmental GNAS inactivation disorders. - Molecular support of the concept was documented in a superficial variant of POH called 'plate-like osteoma cutis'. In several other autosomal dominant skin disorders, molecular corroboration of the theory of superimposed mosaicism has been provided. - For all of these reasons, it is unlikely that POH can further be taken as a distinct autosomal dominant trait. Generation of more molecular data in multiple cases of POH occurring in GNAS inactivation disorders will be crucial to corroborate the proposed concept.

Differential Vascularity in Genetic and Nonhereditary Heterotopic Ossification

Introduction. Nonhereditary heterotopic ossification (NHO) is a common complication of trauma. Progressive osseous heteroplasia (POH) and fibrodysplasia ossificans progressiva (FOP) are rare genetic causes of heterotopic bone. In this article, we detail the vascular patterning associated with genetic versus NHO. Methods. Vascular histomorphometric analysis was performed on patient samples from POH, FOP, and NHO. Endpoints for analysis included blood vessel (BV) number, area, density, size, and wall thickness. Results. Results demonstrated conserved temporal dynamic changes in vascularity across all heterotopic ossification lesions. Immature areas had the highest BV number, while the more mature foci had the highest BV area. Most vascular parameters were significantly increased in genetic as compared with NHO. Discussion. In sum, both genetic and NHO show temporospatial variation in vascularity. These findings suggest that angiogenic pathways are potential therapeutic targets in both genetic and nonhereditary forms of heterotopic ossification.

High frequency of paternal iso or heterodisomy at chromosome 20 associated with sporadic pseudohypoparathyroidism 1B

Pseudohypoparathyroidism 1B (PHP1B) is caused by maternal epigenetic defects in the imprinted GNAS cluster. PHP1B can follow an autosomal dominant mode of inheritance or occur sporadically (spor-PHP1B). These latter patients present broad methylation changes of two or more differentially methylated regions (DMR) that, when mimicking the paternal allele, raises the suspicious of the occurrence of paternal uniparental disomy of chromosome 20 (upd(20)pat). A cohort of 33 spor-PHP1B patients was screened for upd(20)pat using comparative genomic hybridization with SNP-chip. Methylation analyses were assessed by methylation specific-multiplex ligation-dependent probe amplification. Upd(20)pat was identified in 6 patients, all exhibiting typical paternal methylation pattern compared to normal controls, namely a complete loss of methylation of GNAS A/B:TSS-DMR, negligible methylation at GNAS-AS1:TSS-DMR and GNAS-XL:Ex1-DMR and complete gain of methylation at GNAS-NESP:TSS-DMR. The overall frequency of upd(20) is 18% in our cohort when searched considering both severe and partial loss of imprinting. However, twenty five patients displayed severe methylation pattern and the upd(20)pat frequency reaches 24% when searching in this group. Consequently, up to day, upd(20)pat is the most common anomaly than other genetic alterations in spor-PHP1B patients. Upd(20)pat occurrence is not linked to the parental age in contrast to upd(20)mat, strongly associated with an advanced maternal childbearing age. This study provides criteria to guide further investigations for upd(20)pat needed for an adequate genetic counseling.

The concept of type 2 segmental mosaicism, expanding from dermatology to general medicine

In autosomal dominant skin disorders, the well-known type 1 segmental mosaicism reflects heterozygosity for a postzygotic new mutation. By contrast, type 2 segmental mosaicism originates in a heterozygous embryo from an early postzygotic mutational event giving rise to loss of the corresponding wild-type allele, which results in a pronounced segmental involvement being superimposed on the ordinary, non-segmental phenotype. Today, this concept has been proven by molecular analysis in many cutaneous traits. The purpose of this review was to seek publications of cases suggesting an extracutaneous manifestation of type 2 segmental mosaicism. Case reports documenting a pronounced extracutaneous segmental involvement were collected from the literature available in PubMed and from personal communications to the author. Pertinent cases are compared to the description of cutaneous segmental mosaicism of type 1 or type 2 as reported in a given trait. In total, reports suggesting extracutaneous type 2 segmental mosaicism were found in 14 different autosomal dominant skin disorders. In this way, clinical evidence is accumulated that extracutaneous type 2 segmental mosaicism does likewise occur in many autosomal dominant skin disorders. So far, however, molecular proof of this particular form of mosaicism is lacking. The present review may stimulate readers to inform colleagues of other specialties on this new concept, in order to initiate further research in this particular field of knowledge that has important implications for diagnosis, treatment and genetic counselling.

Progressive osseous heteroplasia is not a Mendelian trait but a type 2 segmental manifestation of GNAS inactivation disorders: A hypothesis

Progressive osseous heteroplasia (POH) is a segmental disorder characterized by progressive heterotopic ossification that extends from dermal and subcutaneous tissues to deeper structures. So far, it has been taken as a rarely occurring bone disease with autosomal dominant inheritance. Here, arguments are presented in favor of the alternative concept that the disorder is merely a type 2 segmental manifestation of autosomal dominant GNAS inactivation disorders. Type 2 segmental mosaicism arises, in a heterozygous embryo, from a somatic mutational event that occurs at an early developmental stage, resulting in loss of the corresponding wild-type allele and giving rise to a homozygous or hemizygous cell clone. As a characteristic feature, such type 2 segmental involvement is far more pronounced than the type 1 segmental mosaicism as noted in otherwise healthy individuals. The concept of type 2 segmental mosaicism has been proven at the molecular level in six human traits including neurofibromatosis 1, Hailey-Hailey disease, and Gorlin syndrome. In POH, molecular proof of principle is so far lacking. The following lines of reasoning, however, support the hypothesis that POH can be explained by a similar mechanism. Firstly, POH has been found to be associated with different phenotypes caused by inactivating GNAS mutations, which is why it cannot be categorized as one distinct Mendelian trait. Secondly, POH occurs as a rather rare complication of these autosomal dominant traits, which is not compatible with the assumption of a separate Mendelian disorder. Thirdly, in a case of plate-like osteoma that represents a more superficial variant of POH, molecular proof of the concept of type 2 segmental manifestation has already been provided, and the available literature suggests that POH can be best explained by a similar mechanism. Moreover, findings obtained in animal experiments support the assumption that human POH represents such superimposed segmental manifestation of GNAS inactivation disorders.

Screening for GNAS genetic and epigenetic alterations in progressive osseous heteroplasia: first Italian series

Progressive osseous heteroplasia (POH) is a rare autosomal dominant disorder of mesenchymal differentiation characterized by progressive heterotopic ossification (HO) of dermis, deep connective tissues and skeletal muscle. Usually, initial bone formation occurs during infancy as primary osteoma cutis (OC) then progressively extending into deep connective tissues and skeletal muscle over childhood. Most cases of POH are caused by paternally inherited inactivating mutations of GNAS gene. Maternally inherited mutations as well as epigenetic defects of the same gene lead to pseudohypoparathyroidism (PHP) and Albright's hereditary osteodystrophy (AHO). During the last decade, some reports documented the existence of patients with POH showing additional features characteristic of AHO such as short stature and brachydactyly, previously thought to occur only in other GNAS-associated disorders. Thus, POH can now be considered as part of a wide spectrum of ectopic bone formation disorders caused by inactivating GNAS mutations. Here, we report genetic and epigenetic analyses of GNAS locus in 10 patients affected with POH or primary OC, further expanding the spectrum of mutations associated with this rare disease and indicating that, unlike PHP, methylation alterations at the same locus are absent or uncommon in this disorder.

Animal models of typical heterotopic ossification

Heterotopic ossification (HO) is the formation of marrow-containing bone outside of the normal skeleton. Acquired HO following traumatic events is a common and costly clinical complication. In contrast, hereditary HO is rarer, progressive, and life-threatening. Substantial effort has been directed towards understanding the mechanisms underlying HO and finding efficient treatments. However, one crucial limiting factor has been the lack of relevant animal models. This article reviews the major currently available animal models, summarizes some of the insights gained from these studies, and discusses the potential future challenges and directions in HO research.

Progressive osseous heteroplasia: a model for the imprinting effects of GNAS inactivating mutations in humans

CONTEXT

Heterozygous GNAS inactivating mutations are known to induce pseudohypoparathyroidism type 1a when maternally inherited and pseudopseudohypoparathyroidism when paternally inherited. Progressive osseous heteroplasia (POH) is a rare disease of ectopic bone formation, and studies in different families have shown that POH is also caused by paternally inherited GNAS mutations.

OBJECTIVE

Our purpose was to characterize parental origin of the mutated allele in de novo cases of POH and to draw phenotype/genotype correlations according to maternal or paternal transmission of a same GNAS mutation.

DESIGN AND SETTING

We conducted a retrospective study on patients addressed to our referral center for the rare diseases of calcium and phosphorus metabolism.

PATIENTS AND METHODS

We matched 10 cases of POH with cases of pseudohypoparathyroidism type 1a carrying the same GNAS mutations.

MAIN OUTCOME MEASURES

The parental origin of the mutated allele was studied using informative intragenic polymorphisms and subcloning of PCR products.

RESULTS

Paternal origin of GNAS mutations was clearly demonstrated in eight POH cases including one patient with mutation in exon 1. Genotype/phenotype analyses suggest that there is no direct correlation between the ossifying process and the position of the inactivating GNAS mutation. It is, however, more severe in patients in whom origin of the mutation is paternal. Severe intrauterine growth retardation was clearly evidenced in paternally inherited mutations.

CONCLUSIONS

Clinical heterogeneity makes genetic counseling a delicate matter, especially in which paternal inheritance is concerned because it can lead to either a mild expression of pseudopseudohypoparathyroidism or a severe expression of POH.

Chronic ulcers, calcification and calcified fibrous tumours: phenotypic manifestations of a congenital disorder of heterotopic ossification

Calcification is a rarely reported cause for chronic, cutaneous ulceration. Although dystrophic calcification occurs in chronic ulcers, idiopathic calcification and ossification leading to recurrent ulcerations is seldom reported. This report illustrates a challenging case with various non healing wounds, calcification/ossification and calcified fibrous tumours. A 56-year-old woman presented with chronic, painful wounds and calcified deposits in her feet, hands and abdomen, some dating back to childhood. The surgeries she had had for various unrelated conditions healed with hard, calcified deposits, which later ulcerated. Skeletal radiological imaging revealed multiple soft-tissue deposits, with calcification and areas of ossification, along with gross distortion of the bony architecture of her feet. All biochemical investigations were normal apart from raised serum alkaline phosphatase. The management has encompassed a combination of conservative and surgical measures (culminating in a left below-knee amputation) with varying degrees of success. Having ruled out other causes of subcutaneous calcification, three congenital disorders of heterotopic ossification fit this patient's presentation: Albright hereditary osteodystrophy (AHO), fibrodysplasia ossificans progressiva (FOP) and progressive osseous heteroplasia (POH). Although AHO and FOP are possibilities, POH is most likely. In addition to describing the diverse phenotypic manifestations of this disorder, this report discusses the diagnostic dilemmas, difficulties in optimising the management plan and issues relating to health-related quality of life in this patient.

Adipose-derived stem cells for regenerative medicine

The emerging field of regenerative medicine will require a reliable source of stem cells in addition to biomaterial scaffolds and cytokine growth factors. Adipose tissue represents an abundant and accessible source of adult stem cells with the ability to differentiate along multiple lineage pathways. The isolation, characterization, and preclinical and clinical application of adipose-derived stem cells (ASCs) are reviewed in this article.

Imprinting the Gnas locus

Gnas is an enigmatic and rather complex imprinted gene locus. A single transcription unit encodes three, and possibly more, distinct proteins. These are determined by overlapping transcripts from alternative promoters with different patterns of imprinting. The canonical Gnas transcript codes for Gsalpha, a highly conserved signalling protein and an essential intermediate in growth, differentiation and homeostatic pathways. Monoallelic expression of Gnas is highly tissue-restricted. The alternative transcripts encode XLalphas, an unusual variant of Gsalpha, and the chromogranin-like protein Nesp55. These transcripts are expressed specifically from the paternal and maternal chromosomes, respectively. Their existence in the Gnas locus might imply functional connections amongst them or with Gsalpha. In this review, we consider how imprinting of Gnas was discovered, the phenotypic consequences of mutations in each of the gene products, both in the mouse and human, and provide some conjectures to explain why this elaborate imprinted locus has evolved in this manner in mammals.

Reduction in Gsalpha induces osteogenic differentiation in human mesenchymal stem cells

We hypothesized that a decrease in Gsalpha expression occurs with osteogenic differentiation and that when Gsalpha expression was decreased by antisense oligonucleotides or direct inhibition of protein kinase A there was a concomitant increase in Runx2/Cbfa1. We also investigated the mechanism involved in the change in Runx2/Cbfa1 levels and whether the expression of other genes known to be involved in bone formation was altered. There was a decrease in Gsalpha expression with osteogenic differentiation and antisense oligonucleotides, and protein kinase A inhibition led to increased expression and DNA binding of the osteoblast-specific Runx2/Cbfa1. Additionally, with decreased Gsalpha expression or protein kinase A inhibition, Runx2/Cbfa1 protein was serine phosphorylated and ubiquitinated less. Microarray analysis, after the addition of antisense Gsalpha, showed a more than 10-fold increase in collagen Type I Alpha 2 mRNA (a target of Runx2/Cbfa1). These data show that reduced expression of Gsalpha can induce an osteoblast-like phenotype. The results also indicate a potential pathophysiologic role in patients with heterozygous inactivating mutations in GNAS1, the gene for the alpha chain (Gsalpha) of the heterotrimeric G protein, present in three disorders with ectopic intramembranous bone: Albright's hereditary osteodystrophy, progressive osseous heteroplasia, and osteoma cutis.

Genomic Imprinting and Kinship: How Good is the Evidence?

The kinship theory of genomic imprinting proposes that parent-specific gene expression evolves at a locus because a gene's level of expression in one individual has fitness effects on other individuals who have different probabilities of carrying the maternal and paternal alleles of the individual in which the gene is expressed. Therefore, natural selection favors different levels of expression depending on an allele's sex-of-origin in the previous generation. This review considers the strength of evidence in support of this hypothesis for imprinted genes in four "clusters," associated with the imprinted loci Igf2, Igf2r, callipyge, and Gnas. The clusters associated with Igf2 and Igf2r both contain paternally expressed transcripts that act as enhancers of prenatal growth and maternally expressed transcripts that act as inhibitors of prenatal growth. This is consistent with predictions of the kinship theory. However, the clusters also contain imprinted genes whose phenotypes as yet remain unexplained by the theory. The principal effects of imprinted genes in the callipyge and Gnas clusters appear to involve lipid and energy metabolism. The kinship theory predicts that maternally expressed transcripts will favor higher levels of nonshivering thermogenesis (NST) in brown adipose tissue (BAT) of animals that huddle for warmth as offspring. The phenotypes of reciprocal heterozygotes for Gnas knockouts provide provisional support for this hypothesis, as does some evidence from other imprinted genes (albeit more tentatively). The diverse effects of imprinted genes on the development of white adipose tissue (WAT) have so far defied a unifying hypothesis in terms of the kinship theory.

Progressive osseous heteroplasia resulting from a new mutation in the GNAS1 gene

Progressive osseous heteroplasia (OMIM 166350) is a rare autosomal dominant condition that presents in childhood as dermal ossification and may progress deeper to involve subcutaneous fat and connective tissue. Recently, paternally inherited inactivating mutations in the GNAS1 gene on chromosome 20q13 have been implicated in the pathogenesis, although sporadic cases have also been reported. We report a 9-year-old British Chinese girl with progressive osseous heteroplasia resulting from a de novo missense mutation (W281R) in the GNAS1 gene. She is of small stature (0.4th centile) and started to develop skin lesions at the age of 9 months. These have been confirmed histologically as osteoma cutis. She is of normal intelligence and development and has no dysmorphic features. The GNAS1 gene exhibits imprinting and maternally inherited mutations have previously been shown to result in Albright's hereditary osteodystrophy (OMIM 103580) with pseudohypothyroidism type 1a, whereas paternally inherited mutations result in progressive osseous heteroplasia or the Albright's hereditary osteodystrophy phenotype with pseudopseudohypothyroidism (OMIM 300800). With only nine mutations of the GNAS1 gene reported so far in progressive osseous heteroplasia, this new mutation helps to extend further the genotype-phenotype correlation.

Progressive extensive osteoma cutis associated with dysmorphic features: a new syndrome? Case report and review of the literature

Osteoma cutis, also called cutaneous ossification, refers to the rare occurrence of bone in the skin. It may be primary, occurring in normal skin, or secondary, occurring in disrupted skin tissue. A 42-year-old white woman presented with long-standing progressive primary osteoma cutis involving her head and neck, trunk and extremities. She had craniofacial dysmorphism with mid-face hypoplasia, including saddle nose deformity, mild to moderate generalized joint hypermobility, extensive paravertebral ossification, and disc space calcification. The differential diagnosis for this entity is presented. This phenotype may be a previously undescribed syndrome.