Multi-system involvement in a severe variant of fibrodysplasia ossificans progressiva (ACVR1 c.772G>A; R258G): A report of two patients

Timing and Graded BMP Signalling Determines Fate of Neural Crest and Ectodermal Placode Derivatives from Pluripotent Stem Cells

Pluripotent stem cells (PSCs) offer many potential research and clinical benefits due to their ability to differentiate into nearly every cell type in the body. They are often used as model systems to study early stages of ontogenesis to better understand key developmental pathways, as well as for drug screening. However, in order to fully realise the potential of PSCs and their translational applications, a deeper understanding of developmental pathways, especially in humans, is required. Several signalling molecules play important roles during development and are required for proper differentiation of PSCs. The concentration and timing of signal activation are important, with perturbations resulting in improper development and/or pathology. Bone morphogenetic proteins (BMPs) are one such key group of signalling molecules involved in the specification and differentiation of various cell types and tissues in the human body, including those related to tooth and otic development. In this review, we describe the role of BMP signalling and its regulation, the consequences of BMP dysregulation in disease and differentiation, and how PSCs can be used to investigate the effects of BMP modulation during development, mainly focusing on otic development. Finally, we emphasise the unique role of BMP4 in otic specification and how refined understanding of controlling its regulation could lead to the generation of more robust and reproducible human PSC-derived otic organoids for research and translational applications.

A blocking monoclonal antibody reveals dimerization of intracellular domains of ALK2 associated with genetic disorders

Mutations in activin receptor-like kinase 2 (ALK2) can cause the pathological osteogenic signaling seen in some patients with fibrodysplasia ossificans progressiva and other conditions such as diffuse intrinsic pontine glioma. Here, we report that intracellular domain of wild-type ALK2 readily dimerizes in response to BMP7 binding to drive osteogenic signaling. This osteogenic signaling is pathologically triggered by heterotetramers of type II receptor kinases and ALK2 mutant forms, which form intracellular domain dimers in response to activin A binding. We develop a blocking monoclonal antibody, Rm0443, that can suppress ALK2 signaling. We solve the crystal structure of the ALK2 extracellular domain complex with a Fab fragment of Rm0443 and show that Rm0443 induces dimerization of ALK2 extracellular domains in a back-to-back orientation on the cell membrane by binding the residues H64 and F63 on opposite faces of the ligand-binding site. Rm0443 could prevent heterotopic ossification in a mouse model of fibrodysplasia ossificans progressiva that carries the human R206H pathogenic mutant.

Transcriptome Profile in the Mouse Brain of Hepatic Encephalopathy and Alzheimer's Disease

Hepatic encephalopathy (HE) is a chronic metabolic disease accompanied by neuropathological and neuropsychiatric features, including memory deficits, psychomotor dysfunction, depression, and anxiety. Alzheimer's disease (AD), the most common neurodegenerative disease, is characterized by tau hyperphosphorylation, excessive amyloid beta (Aβ) accumulation, the formation of fibrillary tangles, hippocampus atrophy, and neuroinflammation. Recent studies have suggested a positive correlation between HE and AD. Some studies reported that an impaired cholesterol pathway, abnormal bile acid secretion, excessive ammonia level, impaired Aβ clearance, astrocytic dysfunction, and abnormal γ-aminobutyric acid GABAergic neuronal signaling in HE may also be involved in AD pathology. However, the mechanisms and related genes involved in AD-like pathology in the HE brain are unclear. Thus, we compared the cortical transcriptome profile between an HE mouse model, bile duct ligation (BDL), and an AD mouse model, the 5×FAD. Our study showed that the expression of many genes implicated in HE is associated with neuronal dysfunction in AD mice. We found changes in various protein-coding RNAs, implicated in synapses, neurogenesis, neuron projection, neuron differentiation, and neurite outgrowth, and non-coding RNAs possibly associated with neuropathology. Our data provide an important resource for further studies to elucidate AD-like pathophysiology in HE patients.

Multi-omics therapeutic perspective on ACVR1 gene: from genetic alterations to potential targeting

Activin A receptor type I (ACVR1), a transmembrane serine/threonine kinase, belongs to the transforming growth factor-β superfamily, which signals via phosphorylating the downstream effectors and SMAD transcription factors. Its central role in several biological processes and intracellular signaling is well known. Genetic variation in ACVR1 has been associated with a rare disease, fibrodysplasia ossificans progressive, and its somatic alteration is reported in rare cancer diffuse intrinsic pontine glioma. Furthermore, altered expression or variation of ACVR1 is associated with multiple pathologies such as polycystic ovary syndrome, congenital heart defects, diffuse idiopathic skeletal hyperostosis, posterior fossa ependymoma and other malignancies. Recent advancements have witnessed ACVR1 as a potential pharmacological target, and divergent promising approaches for its therapeutic targeting have been explored. This review highlights the structural and functional characteristics of receptor ACVR1, associated signaling pathways, genetic variants in several diseases and cancers, protein-protein interaction, gene expression, regulatory miRNA prediction and potential therapeutic targeting approaches. The comprehensive knowledge will offer new horizons and insights into future strategies harnessing its therapeutic potential.

Rapid Progression of Heterotopic Ossification in Severe Variant of Fibrodysplasia Ossificans Progressiva with p.Arg258Gly in ACVR1: A Case Report and Review of Clinical Phenotypes

Fibrodysplasia ossificans progressiva (FOP) is a rare skeletal disorder characterized by congenital malformation of the great toes and progressive heterotopic ossification. Malformation of the great toes appears at birth, while heterotopic ossification generally occurs during childhood and rarely occurs during infancy. Classical FOP results from the heterozygous p.Arg206His variant of the ACVR1 gene, which encodes Activin A receptor type 1. Recently, some atypical FOP patients with other ACVR1 gene variants and clinical features that are not observed in classical FOP patients have been reported. Herein, we describe a girl with severe FOP and multiple anomalies, including syndactyly of the hands and feet, nail agenesis, mandibular hypoplasia, heterotopic ossification occurring from infancy, and congenital cardiac malformation. In our patient, we identified de novo occurrence of the heterozygous p.Arg258Gly variant of ACVR1, which has previously been reported in only two severe FOP patients. Heterotopic ossification occurred earlier and more frequently compared with classical FOP patients. We present the time-series changes in heterotopic ossification in our patient and compare her clinical features with those of the previously reported patients with p.Arg258Gly. Our report deepens understanding of the clinical features in severe FOP with p.Arg258Gly and of FOP as a systemic disorder.

BMP Signaling Pathway in Dentin Development and Diseases

BMP signaling plays an important role in dentin development. BMPs and antagonists regulate odontoblast differentiation and downstream gene expression via canonical Smad and non-canonical Smad signaling pathways. The interaction of BMPs with their receptors leads to the formation of complexes and the transduction of signals to the canonical Smad signaling pathway (for example, BMP ligands, receptors, and Smads) and the non-canonical Smad signaling pathway (for example, MAPKs, p38, Erk, JNK, and PI3K/Akt) to regulate dental mesenchymal stem cell/progenitor proliferation and differentiation during dentin development and homeostasis. Both the canonical Smad and non-canonical Smad signaling pathways converge at transcription factors, such as Dlx3, Osx, Runx2, and others, to promote the differentiation of dental pulp mesenchymal cells into odontoblasts and downregulated gene expressions, such as those of DSPP and DMP1. Dysregulated BMP signaling causes a number of tooth disorders in humans. Mutation or knockout of BMP signaling-associated genes in mice results in dentin defects which enable a better understanding of the BMP signaling networks underlying odontoblast differentiation and dentin formation. This review summarizes the recent advances in our understanding of BMP signaling in odontoblast differentiation and dentin formation. It includes discussion of the expression of BMPs, their receptors, and the implicated downstream genes during dentinogenesis. In addition, the structures of BMPs, BMP receptors, antagonists, and dysregulation of BMP signaling pathways associated with dentin defects are described.

ACVR1 antibodies exacerbate heterotopic ossification in fibrodysplasia ossificans progressiva (FOP) by activating FOP-mutant ACVR1

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder whose most debilitating pathology is progressive and cumulative heterotopic ossification (HO) of skeletal muscles, ligaments, tendons, and fascia. FOP is caused by mutations in the type I BMP receptor gene ACVR1, which enable ACVR1 to utilize its natural antagonist, activin A, as an agonistic ligand. The physiological relevance of this property is underscored by the fact that HO in FOP is exquisitely dependent on activation of FOP-mutant ACVR1 by activin A, an effect countered by inhibition of anti–activin A via monoclonal antibody treatment. Hence, we surmised that anti-ACVR1 antibodies that block activation of ACVR1 by ligands should also inhibit HO in FOP and provide an additional therapeutic option for this condition. Therefore, we generated anti-ACVR1 monoclonal antibodies that block ACVR1’s activation by its ligands. Surprisingly, in vivo, these anti-ACVR1 antibodies stimulated HO and activated signaling of FOP-mutant ACVR1. This property was restricted to FOP-mutant ACVR1 and resulted from anti-ACVR1 antibody–mediated dimerization of ACVR1. Conversely, wild-type ACVR1 was inhibited by anti-ACVR1 antibodies. These results uncover an additional property of FOP-mutant ACVR1 and indicate that anti-ACVR1 antibodies should not be considered as therapeutics for FOP.

An ACVR1R375P pathogenic variant in two families with mild fibrodysplasia ossificans progressiva

Genetic variants are vital in informing clinical phenotypes, aiding physical diagnosis, guiding genetic counseling, understanding the molecular basis of disease, and potentially stimulating drug development. Here we describe two families with an ultrarare ACVR1 gain-of-function pathogenic variant (codon 375, Arginine > Proline; ACVR1^R375P ) responsible for a mild nonclassic fibrodysplasia ossificans progressiva (FOP) phenotype. Both families include people with the ultrarare ACVR1^R375P variant who exhibit features of FOP while other individuals currently do not express any clinical signs of FOP. Thus, the mild ACVR1^R375P variant greatly expands the scope and understanding of this rare disorder.

Social and Clinical Impact of COVID-19 on Patients with Fibrodysplasia Ossificans Progressiva

Background

COVID-19, caused by the SARS-CoV-2 virus, is a severe inflammatory condition. Patients with pre-existing conditions including diabetes, hypertension, and cardiovascular disease are at particularly high risk of complications. Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare and debilitating genetic disorder that is characterized by a pro-inflammatory state, which leads to progressive heterotopic ossification and complications after trauma, including intramuscular vaccinations. To better understand the impact of COVID-19 on patients with FOP, we first examined the social impact of the pandemic using data from the FOP Registry managed by the International FOP Association. We also identified patients with FOP who were exposed to or contracted the SARS-CoV-2 virus, or who received a COVID-19 vaccine, to investigate if patients with FOP were at increased risks of complications from SARS-CoV2 exposure.

Results

Data from 326 individuals in 69 countries were examined in the International FOP Association FOP Connection Registry using patient-reported outcomes measurement information system (PROMIS) global health scale scores. Twenty-six (28.9%) participants aged ≥ 15 years old rated their satisfaction with their social activities and relationships as poor in 2020, which was an increase from 18 (18.9%) in 2019, prior to the SARS-CoV-2 outbreak. Similar trends were noted for physical and mental health in the pediatric population. Frequency of physician visits was not changed, but a larger portion of patients reported missing dental visits in 2020 compared with 2019 (31.5% vs. 41.7%). A second cohort with 32 subjects was tracked after SARS-CoV-2 exposure or vaccination. Ten subjects were positively diagnosed with COVID-19, 15 received a COVID-19 vaccine, and seven had high-risk SARS-CoV-2 exposure but either did not have a confirmed clinical diagnosis or tested negative. Subjects who tested positive for the virus showed no major complications or increased FOP disease activity, though our sample size is very limited. Among the 15 subjects who received a COVID-19 vaccine, using the International Clinical Council on FOP guidelines for prophylaxis with ibuprofen or acetaminophen, only one person experienced flare activity at the injection site.

Conclusions

Patients with FOP showed a significant decrease in social activities that was reflective of the isolation and mobility changes in this debilitated population. In our limited cohort, the majority of the patients with FOP who tested positive for COVID-19 showed no major complications. Also, although limited in sample size, the majority of patients who received a COVID-19 vaccination and followed guidelines from the FOP International Clinical Council tolerated vaccination well. Only one person experiencing flare activity following their injection. Thus, the risks and benefits of COVID-19 vaccination needs to be discussed carefully so as to support informed decisions.

A case of Fibrodysplasia Ossificans Progressiva associated with a novel variant of the ACVR1 gene

Background

Fibrodysplasia Ossificans Progressiva (FOP) is a rare autosomal dominant disease characterized by congenital malformation of the great toes and progressive heterotopic ossification of soft tissues leading to cumulative disability. The genetic cause of FOP are mutations in the ACVR1 gene that encodes a type I receptor of Bone Morphogenetic Proteins. The most recurrent mutation in FOP patients is R206H affecting the Glycine‐Serine rich domain and causing the hyper‐activation of the receptor and the responsivity to the non‐canonical ligand, Activin A.

In the present study, we described a 3‐years old child with early and highly suggestive clinical features of FOP who was found negative for the recurrent p.R206H substitution.

Methods

Molecular screening of the whole ACVR1 coding sequence and functional characterization in transfection‐based assays.

Results and Conclusions

We identified a novel, de novo variant in the fifth ACVR1 coding exon (NM_001111067.4:c.772A>T; NP_001104537.1:p.(R258W)). This substitution, never reported in association with FOP, affects a conserved arginine residue in the kinase domain of the protein. In silico analysis predicted the pathogenicity of this substitution, demonstrated by in vitro assays showing that the p.R258W ACVR1 mutated receptor acquires the ability to transduce the aberrant Activin A‐mediated signaling, as observed for the gene variants associated with FOP.

Fibrodysplasia Ossificans Progressiva: A Challenging Diagnosis

Fibrodysplasia ossificans progressiva (FOP) is an ultrarare genetic condition characterized by extraskeletal bone formation. Most of the musculoskeletal characteristics of FOP are related to dysregulated chondrogenesis, with heterotopic ossification being the most typical feature. Activating mutations of activin receptor A type I (ACVR1), a bone morphogenetic protein (BMP) type I receptor, are responsible for the skeletal and nonskeletal features. The clinical phenotype is always consistent, with congenital bilateral hallux valgus malformation and early-onset heterotopic ossification occurring spontaneously or, more frequently, precipitated by trauma. Painful, recurrent soft-tissue swellings (flare-ups) precede localized heterotopic ossification that can occur at any location, typically affecting regions near the axial skeleton and later progressing to the appendicular bones. A diagnosis of FOP is suspected in a proband presenting with hallux valgus malformation, heterotopic ossification, and confirmed by the identification of a heterozygous pathogenic variant in the ACVR1/ALK2 gene. Avoiding unnecessary surgical procedures, prescribing prophylactic corticosteroids, preventing falls, and using protective headgear represent essential interventions for care management. Different classes of medications to contain acute inflammation flare-ups have been proposed, with high dose corticosteroids and nonsteroidal anti-inflammatory drugs usually utilized. Here, we report on two FOP patients, with typical clinical features summarizing the principal aspects of FOP, and we aim to provide comprehensive information outlining some unusual findings, possibly contributing to FOP's definition and management.

Accumulated Knowledge of Activin Receptor-Like Kinase 2 (ALK2)/Activin A Receptor, Type 1 (ACVR1) as a Target for Human Disorders

Activin receptor-like kinase 2 (ALK2), also known as Activin A receptor type 1 (ACVR1), is a transmembrane kinase receptor for members of the transforming growth factor-β family. Wild-type ALK2/ACVR1 transduces osteogenic signaling in response to ligand binding. Fifteen years ago, a gain-of-function mutation in the ALK2/ACVR1 gene was detected in patients with the genetic disorder fibro-dysplasia ossificans progressiva, which is characterized by heterotopic ossification in soft tissues. Additional disorders, such as diffuse intrinsic pontin glioma, diffuse idiopathic skeletal hyperostosis, primary focal hyperhidrosis, and congenital heart defects, have also been found to be associated with ALK2/ACVR1. These findings further expand in vitro and in vivo model system research and promote our understanding of the molecular mechanisms of the pathogenesis and development of novel therapeutics and diagnosis for disorders associated with ALK2/ACVR1. Through aggressive efforts, some of the disorders associated with ALK2/ACVR1 will be overcome in the near future.

Embryology, Malformations, and Rare Diseases of the Cochlea

Despite the low overall prevalence of individual rare diseases, cochlear dysfunction leading to hearing loss represents a symptom in a large proportion. The aim of this work was to provide a clear overview of rare cochlear diseases, taking into account the embryonic development of the cochlea and the systematic presentation of the different disorders. Although rapid biotechnological and bioinformatic advances may facilitate the diagnosis of a rare disease, an interdisciplinary exchange is often required to raise the suspicion of a rare disease. It is important to recognize that the phenotype of rare inner ear diseases can vary greatly not only in non-syndromic but also in syndromic hearing disorders. Finally, it becomes clear that the phenotype of the individual rare diseases cannot be determined exclusively by classical genetics even in monogenetic disorders.

A Three-Generation Pedigree of Multifocal Heterotopic Ossification With Bilateral Involvement

Heterotopic ossification (HO) can be classified as nonhereditary HO (NHHO) or hereditary HO (HHO). Hereditary HO includes 2 different forms: fibrodysplasia ossificans progressiva (FOP) and progressive osseous heteroplasia (POH). Each of these disorders is caused by mutations in a single (different) gene. Clinical diagnosis of FOP can be confirmed by sequence analysis of the ACVR1 gene. Fewer than 10 families with autosomal dominant inheritance of FOP have been reported. The authors report clinical manifestations, pedigree analysis, and mutational analysis of the ACVR1 in a 3-generation Chinese family with 4 individuals presenting multifocal and bilateral involvement of HO. Clinical features of affected individuals of the family were not consistent with the classic FOP or atypical FOP. Exon sequencing was performed on every family member and 3 sporadic NHHO patients who did not belong to this family. All family members and 3 sporadic NHHO patients carried 2 hereditary homozygous silent mutations- c.270C>T and c.690G>A-the new and unique mutations in ACVR1. These results may suggest that the affected individuals in this family present a novel FOP-variant syndrome or a new form of HHO. To the authors' knowledge, this is the first well-documented instance of a 3-generation Chinese family with multifocal and bilateral involvement of HO. [Orthopedics. 2021;44(1):e139-e145.].

Clinical Aspects and Current Therapeutic Approaches for FOP

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare heritable disorder of connective tissues characterized by progressive heterotopic ossification in various skeletal sites. It is caused by gain-of-function mutations in the gene encoding activin A receptor type I (ACVR1)/activin-like kinase 2 (ALK2), a bone morphogenetic protein (BMP) type I receptor. Heterotopic ossification is usually progressive leading to severe deformities in the trunk and extremities. Early clinical diagnosis is important to prevent unnecessary iatrogenic harm or trauma. Clinicians should become aware of early detectable skeletal malformations, including great toe deformities, shortened thumb, neck stiffness associated with hypertrophy of the posterior elements of the cervical spine, multiple ossification centers in the calcaneus, and osteochondroma-like lesions of the long bones. Although there is presently no definitive medical treatment to prevent, stop or reverse heterotopic ossification in FOP, exciting advances of novel pharmacological drugs focusing on target inhibition of the activated ACVR1 receptor, including palovarotene, REGN 2477, rapamycin, and saracatinib, have developed and are currently in clinical trials.

Design of primers for direct sequencing of nine coding exons in the human ACVR1 gene

The human ACVR1 gene encodes a transmembrane protein consisting of 509 amino acids called activin A receptor, type I (ACVR1) or activin receptor-like kinase 2 (ALK2) and has nine coding exons. The ALK2 protein functions as a signaling receptor for ligands of the transforming growth factor-β family. In the human ACVR1 gene, approximately 20 types of heterozygotic mutations in the coding exons have been associated with congenital disorders and somatic cancer, such as fibrodysplasia ossificans progressiva (FOP), diffuse intrinsic pontine glioma, diffuse idiopathic skeletal hyperostosis and some congenital heart disorders. In the present study, we designed primers for direct sequencing of the nine coding exons in the human ACVR1 gene. The reliability of the primers was examined by PCR and DNA sequencing using genomic DNA prepared from peripheral blood or swab samples of three patients with FOP who had different mutations in the ACVR1 gene. A single nucleotide heterozygotic mutation was identified in each genomic sample without additional mutations in other regions. Therefore, the primers designed for the nine coding exons of the ACVR1 gene could be useful for the genetic diagnosis of patients who may have disorders associated with mutations in the ACVR1 gene.

Emerging roles of the αC-β4 loop in protein kinase structure, function, evolution, and disease

The faithful propagation of cellular signals in most organisms relies on the coordinated functions of a large family of protein kinases that share a conserved catalytic domain. The catalytic domain is a dynamic scaffold that undergoes large conformational changes upon activation. Most of these conformational changes, such as movement of the regulatory αC-helix from an "out" to "in" conformation, hinge on a conserved, but understudied, loop termed the αC-β4 loop, which mediates conserved interactions to tether flexible structural elements to the kinase core. We previously showed that the αC-β4 loop is a unique feature of eukaryotic protein kinases. Here, we review the emerging roles of this loop in kinase structure, function, regulation, and diseases. Through a kinome-wide analysis, we define the boundaries of the loop for the first time and show that sequence and structural variation in the loop correlate with conformational and regulatory variation. Many recurrent disease mutations map to the αC-β4 loop and contribute to drug resistance and abnormal kinase activation by relieving key auto-inhibitory interactions associated with αC-helix and inter-lobe movement. The αC-β4 loop is a hotspot for post-translational modifications, protein-protein interaction, and Hsp90 mediated folding. Our kinome-wide analysis provides insights for hypothesis-driven characterization of understudied kinases and the development of allosteric protein kinase inhibitors.

ACVR1 Function in Health and Disease

Activin A receptor type I (ACVR1) encodes for a bone morphogenetic protein type I receptor of the TGFβ receptor superfamily. It is involved in a wide variety of biological processes, including bone, heart, cartilage, nervous, and reproductive system development and regulation. Moreover, ACVR1 has been extensively studied for its causal role in fibrodysplasia ossificans progressiva (FOP), a rare genetic disorder characterised by progressive heterotopic ossification. ACVR1 is linked to different pathologies, including cardiac malformations and alterations in the reproductive system. More recently, ACVR1 has been experimentally validated as a cancer driver gene in diffuse intrinsic pontine glioma (DIPG), a malignant childhood brainstem glioma, and its function is being studied in other cancer types. Here, we review ACVR1 receptor function and signalling in physiological and pathological processes and its regulation according to cell type and mutational status. Learning from different functions and alterations linked to ACVR1 is a key step in the development of interdisciplinary research towards the identification of novel treatments for these pathologies.

Molecular diagnostics of disorders of sexual development: an Indian survey and systems biology perspective

We aimed to survey the monogenic causes of disorders of sex development (DSD) and thereby its prevalence in India. This study revealed mutations resulting in androgen insensitivity syndrome, 5α-reductase type 2 deficiency, and gonadal dysgenesis were commonly reported. Intriguingly, AR deficits were the most prevalent (32 mutations) and of 11/26 missense mutations were in exons 4-8 (encoding ligand binding domain). The unique features of SRD5A2 defects were p.R246Q (most prevalent) and p.G196S could be mutational hotspots, dual gene defects (p.A596T in AR and p.G196S in SRD5A2) in a patient with hypospadias and novel 8 nucleotide deletion (exon 1) found in a patient with perineal hypospadias. Deficits in SRY, WT1, DHH, NR5A1, and DMRT1 caused 46,XY gonadal dysgenesis. Notably, mutations in AR, SRD5A2, MAMLD1, WT1, and MAP3K1 have led to hypospadias and only one CYP19A1 mutation caused aromatase deficiency was reported to date. Data mining from various databases has not only reinforced the role of well-established genes (e.g., SRY, WT1, DHH, NR5A1, DMRT1, AR, SRD5A2, MAMLD1) involved in DSD but also provided us 12 more potential candidate genes (ACVR1, AMHR2, CTNNB1, CYP11A1, CYP19A1, FGFR2, FGF9, PRKACA, PRKACG, SMAD9, TERT, ZFPM2), which benefit from a close association with the well-established genes involved in DSD and might be useful to screen owing to their direct gene-phenotype relationship or through direct functional interaction. As more genes have been revealed in relation to DSD, we believe ultimately it holds a better scenario for therapeutic regimen. Despite the advances in translational medicine, hospitals are yet to adopt genetic testing and counseling facilities in India that shall have potential impact on clinical diagnosis. Abbreviations: 5α-RD2: 5α-Reductase type 2; AIS: androgen insensitivity syndrome; AMH: antimullerian hormone; AMHR: antimullerian hormone receptor; AR: androgen receptor gene; CAH: congenital adrenal hyperplasia; CAIS: complete AIS; CAH: congenital adrenal hyperplasia; CHH: congenital hypogonadotropic hypogonadism; CXORF6: chromosome X open reading frame 6 gene; CYP19A1: cytochrome P450 family 19 subfamily A member 1 gene; DHT: dihydrotestosterone; DMRT1: double sex and mab-3 related transcription factor 1 gene; DSD: disorders of sexual development; GD: gonadal dysgenesis; HGMD: human gene mutation database; IH: isolated hypospadias; MAMLD1: mastermind like domain containing 1 gene; MIS: mullerian inhibiting substance; NTD: N-terminal domain; OT DSD: ovotesticular DSD; PAIS: partial AIS; SOX9: SRY-related HMG-box 9 gene; SRY: sex-determining region Y gene; STAR: steroidogenic acute regulatory protein gene; SRD5A2: steroid 5 alpha-reductase 2 gene; T DSD: testicular DSD; T: testosterone; WNT4: Wnt family member 4 gene; WT1: Wilms tumor 1 gene; Δ₄: androstenedione.

Hypoxia-selective allosteric destabilization of activin receptor-like kinases: A potential therapeutic avenue for prophylaxis of heterotopic ossification

Heterotopic ossification (HO), the pathological extraskeletal formation of bone, can arise from blast injuries, severe burns, orthopedic procedures and gain-of-function mutations in a component of the bone morphogenetic protein (BMP) signaling pathway, the ACVR1/ALK2 receptor serine-threonine (protein) kinase, causative of Fibrodysplasia Ossificans Progressiva (FOP). All three ALKs (-2, -3, -6) that play roles in bone morphogenesis contribute to trauma-induced HO, hence are well-validated pharmacological targets. That said, development of inhibitors, typically competitors of ATP binding, is inherently difficult due to the conserved nature of the active site of the 500+ human protein kinases. Since these enzymes are regulated via inherent plasticity, pharmacological chaperone-like drugs binding to another (allosteric) site could hypothetically modulate kinase conformation and activity. To test for such a mechanism, a surface pocket of ALK2 kinase formed largely by a key allosteric substructure was targeted by supercomputer docking of drug-like compounds from a virtual library. Subsequently, the effects of docked hits were further screened in vitro with purified recombinant kinase protein. A family of compounds with terminal hydrogen-bonding acceptor groups was identified that significantly destabilized the protein, inhibiting activity. Destabilization was pH-dependent, putatively mediated by ionization of a histidine within the allosteric substructure with decreasing pH. In vivo, nonnative proteins are degraded by proteolysis in the proteasome complex, or cellular trashcan, allowing for the emergence of therapeutics that inhibit through degradation of over-active proteins implicated in the pathology of diseases and disorders. Because HO is triggered by soft-tissue trauma and ensuing hypoxia, dependency of ALK destabilization on hypoxic pH imparts selective efficacy on the allosteric inhibitors, providing potential for safe prophylactic use.

Effects of FKBP12 and type II BMP receptors on signal transduction by ALK2 activating mutations associated with genetic disorders

Various substitution mutations in ALK2, a transmembrane serine/threonine kinase receptor for bone morphogenetic proteins (BMPs), have been identified in patients with genetic disorders such as fibrodysplasia ossificans progressiva (FOP), diffuse intrinsic pontine glioma (DIPG) and heart defects. In this study, we characterized the ALK2 mutants R258G, G328V and F246Y, which were identified in patients with severe FOP, DIPG and unusual hereditary skeletal dysplasia, respectively. Both R258G and G328V were gain-of-function mutations, but F246Y was equivalent to wild-type ALK2. We also examined the effect of the suppressor FKBP12 on the signal transduction of a further 14 ALK2 mutations associated with FOP and/or DIPG. To varying extents FKBP12 over-expression suppressed the basal signaling induced by thirteen of the ALK2 mutants, whereas PF197-8L was uniquely resistant. In the PF197-8L mutant, the modelled ALK2 residue L197 induced a steric clash with the D36 residue in FKBP12 and dissociated their interaction. The co-expression of BMP type II receptors or stimulation with ligands relieved the suppression by FKBP12 by disrupting the interaction between mutant ALK2 and FKBP12. Taken together, FKBP12 binds to and suppresses mutant ALK2 proteins associated with FOP and DIPG, except for PF197-8L.

Shared ACVR1 mutations in FOP and DIPG: Opportunities and challenges in extending biological and clinical implications across rare diseases

Gain-of-function mutations in the Type I Bone Morphogenic Protein (BMP) receptor ACVR1 have been identified in two diseases: Fibrodysplasia Ossificans Progressiva (FOP), a rare autosomal dominant disorder characterized by genetically driven heterotopic ossification, and in 20-25% of Diffuse Intrinsic Pontine Gliomas (DIPGs), a pediatric brain tumor with no effective therapies and dismal median survival. While the ACVR1 mutation is causal for FOP, its role in DIPG tumor biology remains under active investigation. Here, we discuss cross-fertilization between the FOP and DIPG fields, focusing on the biological mechanisms and principles gleaned from FOP that can be applied to DIPG biology. We highlight our current knowledge of ACVR1 in both diseases, and then describe the growing opportunities and barriers to effectively investigate ACVR1 in DIPG. Importantly, learning from other seemingly unrelated diseases harboring similar mutations may uncover novel mechanisms or processes for future investigation.

The obligatory role of Activin A in the formation of heterotopic bone in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia Ossificans Progressiva (FOP) is a rare genetic disorder that presents at birth with only minor patterning defects, but manifests its debilitating pathology early in life with episodic, yet progressive and cumulative, heterotopic ossification (HO) of ligaments, tendons, and a subset of major skeletal muscles. The resulting HO lesions are endochondral in nature, and appear to be linked to inflammatory stimuli arising in association with known injuries, or from inflammation linked to normal tissue repair. FOP is caused by gain-of-function mutations in ACVR1, which encodes a type I BMP receptor. Initial studies on the pathogenic mechanism of FOP-causing mutations in ACVR1 focused on the enhanced function of this receptor in response to certain BMP ligands, or independently of ligands, but did not directly address the fact that HO in FOP is episodic and inflammation-driven. Recently, we and others demonstrated that Activin A is an obligate factor for the initiation of HO in FOP, signaling aberrantly via mutant ACVR1 to transduce osteogenic signals and trigger heterotopic bone formation (Hatsell et al., 2015; Hino et al., 2015). Subsequently, we identified distinct tissue-resident mesenchymal progenitor cells residing in muscles and tendons that recognize Activin A as a pro-osteogenic signal (solely in the context of FOP-causing mutant ACVR1), and give rise to the cartilaginous anlagen that form heterotopic bone (Dey et al., 2016). During the course of these studies, we also found that the activity of FOP-causing ACVR1 mutations does not by itself explain the triggered or inflammatory nature of HO in FOP, suggesting the importance of other, inflammation-introduced, factors or processes. This review presents a synthesis of these findings with a focus on the role of Activin A and inflammation in HO, and lays out perspectives for future research.

Variant BMP receptor mutations causing fibrodysplasia ossificans progressiva (FOP) in humans show BMP ligand-independent receptor activation in zebrafish

The large majority of cases of the autosomal dominant human disease fibrodysplasia ossificans progressiva (FOP) are caused by gain-of-function Arg206His mutations in the BMP type I receptor ACVR1 (ALK2). The Arg206His mutation is located in the GS domain of the type I receptor. This region is normally phosphorylated by the BMP type II receptor, which activates the type I receptor to phosphorylate its substrate, the signal transducer Smad1/5/8. A small subset of patients with FOP carry variant mutations in ACVR1 altering Gly328 to Trp, Glu or Arg. Since these mutations lie outside the GS domain, the mechanism through which ACVR1 Gly328 mutations cause disease remains unclear. We used a zebrafish embryonic development assay to test the signaling of human ACVR1 Gly328 mutant receptors comparing them to the Arg206His mutant. In this assay increased or decreased BMP pathway activation alters dorsal-ventral axial patterning, providing a sensitive assay for altered BMP signaling levels. We expressed the human ACVR1 Gly328 mutant receptors in zebrafish embryos to investigate their signaling activities. We found that all ACVR1 Gly328 human mutations ventralized wild-type embryos and could partially rescue Bmp7-deficient embryos, indicating that these mutant receptors can activate BMP signaling in a BMP ligand-independent manner. The degree of ventralization or rescue was similar among all three Gly328 mutants. Smad1/5 phosphorylation, a readout of BMP receptor signaling, was mildly increased by ACVR1 Gly328 mutations. Gene expression analyses demonstrate expanded ventral and reciprocal loss of dorsal cell fate markers. This study demonstrates that Gly328 mutants increase receptor activation and BMP ligand-independent signaling through Smad phosphorylation.

Heterotopic bone induction via BMP signaling: Potential therapeutic targets for fibrodysplasia ossificans progressiva

More than 50years ago, Marshal M. Urist detected "heterotopic bone-inducing activity" in demineralized bone matrix. This unique activity was referred to as "bone morphogenetic protein (BMP)" because it was sensitive to trypsin digestion. Purification of the bone-inducing activity from demineralized bone matrix using a bone-inducing assay in vivo indicated that the original "BMP" consisted of a mixture of new members of the transforming growth factor-β (TGF-β) family. The establishment of new in vitro assay systems that reflect the bone-inducing activity of BMPs in vivo have revealed the functional receptors and downstream effectors of BMPs. Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by progressive heterotopic bone formation in soft tissues similar to the event induced by the transplantation of BMPs in skeletal muscle. In patients with FOP, genetic mutations have been identified in the ACVR1 gene, which encodes the BMP receptor ALK2. The mutations in ALK2 associated with FOP are hypersensitive to type II receptor kinases. Recently, activin A, a non-osteogenic member of the TGF-β family, was identified as the ligand of the mutant ALK2 in FOP, and various types of signaling inhibitors for mutant ALK2 are currently under development to establish effective treatments for FOP.

Novel asymptomatic CNS findings in patients with ACVR1/ALK2 mutations causing fibrodysplasia ossificans progressiva

BACKGROUND

Fibrodysplasia ossificans progressiva is an autosomal dominant disorder due to germline mutations of ACVR1/ALK2 causing progressive heterotopic endochondral ossifications. Evidence of central nervous system involvement has emerged only recently.

METHODS

We performed an observational cross-sectional brain MRI study in 13 patients (8 females, mean age 20 years), examining the relationship of clinical and neuroradiological findings.

RESULTS

All patients presented small asymptomatic lesions similar to hamartomas at the level of the dorsal medulla and ventral pons, associated with minor brainstem dysmorphisms and abnormal origin of the vestibulocochlear and facial nerves. The size of the brainstem lesions did not correlate with patient's age (p=0.061), age at first flare-up (p=0.733), severity of disability (p=0.194), history of head trauma (p=0.415) or hearing loss (p=0.237). The radiologic features and the absence of neurological symptoms were consistent with a benign process. Variable signal abnormalities and/or calcifications of the dentate nuclei were noted in all patients, while basal ganglia abnormalities were present in nine subjects. Brain calcifications positively correlated with patient's age (p<0.001) and severity of disability (p=0.002).

CONCLUSIONS

Our data support the hypothesis that the effects of mutation of the ACVR1/ALK2 gene are extended to the central nervous system. Brainstem hamartomatous lesions and dysmorphisms, variably associated with dentate nucleus and basal ganglia signal abnormalities and/or calcifications, may represent useful disease hallmarks.

Common mutations in ALK2/ACVR1, a multi-faceted receptor, have roles in distinct pediatric musculoskeletal and neural orphan disorders

Activin receptor-like kinase-2 (ALK2), the product of ACVR1, is a member of the type I bone morphogenetic protein (BMP) receptor family. ALK2 exerts key and non-redundant roles in numerous developmental processes, including the specification, growth and morphogenesis of endochondral skeletal elements. There is also strong evidence that BMP signaling plays important roles in determination, differentiation and function of neural cells and tissues. Here we focus on the intriguing discovery that common activating mutations in ALK2 occur in Fibrodysplasia Ossificans Progressiva (FOP) and Diffuse Intrinsic Pontine Gliomas (DIPGs), distinct pediatric disorders of significant severity that are associated with premature death. Pathogenesis and treatment remain elusive for both. We consider recent studies on the nature of the ACVR1 mutations, possible modes of action and targets, and plausible therapeutic measures. Comparisons of the diverse - but genetically interrelated - pathologies of FOP and DIPG will continue to be of major mutual benefit with broad biomedical and clinical relevance.