Editorial of Special Issue "Fibrodysplasia Ossificans Progressiva: Studies on Disease Mechanism towards Novel Therapeutic Approaches".

The Special Issue on "Fibrodysplasia Ossificans Progressiva: Studies on Disease Mechanism towards Novel Therapeutic Approaches" has published interesting and useful review articles and original experimental articles on fibrodysplasia ossificans progressiva (FOP), a very rare genetic disorder for which much effort is being devoted to search for a cure. In this editorial, I briefly cite the essential content of all the published articles.

Fibrodysplasia Ossificans Progressiva: What Have We Achieved and Where Are We Now? Follow-up to the 2015 Lorentz Workshop.

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare progressive genetic disease effecting one in a million individuals. During their life, patients with FOP progressively develop bone in the soft tissues resulting in increasing immobility and early death. A mutation in the ACVR1 gene was identified as the causative mutation of FOP in 2006. After this, the pathophysiology of FOP has been further elucidated through the efforts of research groups worldwide. In 2015, a workshop was held to gather these groups and discuss the new challenges in FOP research. Here we present an overview and update on these topics.

Genomic Context and Mechanisms of the ACVR1 Mutation in Fibrodysplasia Ossificans Progressiva.

Basic research in Fibrodysplasia Ossificans Progressiva (FOP) was carried out in the various fields involved in the disease pathophysiology and was important for designing therapeutic approaches, some of which were already developed as ongoing or planned clinical trials. Genetic research was fundamental in identifying the FOP causative mutation, and the astonishing progress in technologies for genomic analysis, coupled to related computational methods, now make possible further research in this field. We present here a review of molecular and cellular factors which could explain why a single mutation, the R206H in the ACVR1 gene, is absolutely prevalent in FOP patients. We also address the mechanisms by which FOP expressivity could be modulated by cis-acting variants in the ACVR1 genomic region in human chromosome 2q. Finally, we also discuss the general issue of genetic modifiers in FOP.

Severe Heterotopic Ossification in the Skeletal Muscle and Endothelial Cells Recruitment to Chondrogenesis Are Enhanced by Monocyte/Macrophage Depletion.

Altered macrophage infiltration upon tissue damage results in inadequate healing due to inappropriate remodeling and stem cell recruitment and differentiation. We investigated in vivo whether cells of endothelial origin phenotypically change upon heterotopic ossification induction and whether infiltration of innate immunity cells influences their commitment and alters the ectopic bone formation. Liposome-encapsulated clodronate was used to assess macrophage impact on endothelial cells in the skeletal muscle upon acute damage in the ECs specific lineage-tracing Cdh5CreERT2:R26REYFP/dtTomato transgenic mice. Macrophage depletion in the injured skeletal muscle partially shifts the fate of ECs toward endochondral differentiation. Upon ectopic stimulation of BMP signaling, monocyte depletion leads to an enhanced contribution of ECs chondrogenesis and to ectopic bone formation, with increased bone volume and density, that is reversed by ACVR1/SMAD pathway inhibitor dipyridamole. This suggests that macrophages contribute to preserve endothelial fate and to limit the bone lesion in a BMP/injury-induced mouse model of heterotopic ossification. Therefore, alterations of the macrophage-endothelial axis may represent a novel target for molecular intervention in heterotopic ossification.

Identification of reference genes for quantitative PCR during C3H10T1/2 chondrogenic differentiation.

C3H10T1/2, a mouse mesenchymal stem cell line, is a well-known in vitro model of chondrogenesis that can be easily employed to recapitulate some of the mechanisms intervening in this process. Moreover, these cells can be used to validate the effect of candidate molecules identified by high throughput screening approaches applied to the development of targeted therapy for human disorders in which chondrogenic differentiation may be involved, as in conditions characterized by heterotopic endochondral bone formation. Chondrogenic differentiation of C3H10T1/2 cells can be monitored by applying quantitative polymerase chain reaction (qPCR), one of the most sensitive methods that allows detection of small dynamic changes in gene expression between samples obtained under different experimental conditions. In this work, we have used qPCR to monitor the expression of specific markers during chondrogenic differentiation of C3H10T1/2 cells in micromass cultures. Then we have applied the geNorm approach to identify the most stable reference genes suitable to get a robust normalization of the obtained expression data. Among 12 candidate reference genes (Ap3d1, Csnk2a2, Cdc40, Fbxw2, Fbxo38, Htatsf1, Mon2, Pak1ip1, Zfp91, 18S, ActB, GAPDH) we identified Mon2 and Ap3d1 as the most stable ones during chondrogenesis. ActB, GAPDH and 18S, the most commonly used in the literature, resulted to have an expression level too high compared to the differentiation markers (Sox9, Collagen type 2a1, Collagen type 10a1 and Collagen type 1a1), therefore are actually less recommended for these experimental conditions. In conclusion, we identified nine reference genes that can be equally used to obtain a robust normalization of the gene expression variation during the C3H10T1/2 chondrogenic differentiation.

Pharmacological Inhibition of the Ubiquitin Ligase RNF5 Rescues F508del-CFTR in Cystic Fibrosis Airway Epithelia.

In cystic fibrosis (CF), deletion of phenylalanine 508 (F508del) in the CFTR channel is associated with misfolding and premature degradation of the mutant protein. Among the known proteins associated with F508del-CFTR processing, the ubiquitin ligase RNF5/RMA1 is particularly interesting. We previously demonstrated that genetic suppression of RNF5 in vivo leads to an attenuation of intestinal pathological phenotypes in CF mice, validating the relevance of RNF5 as a drug target for CF. Here, we used a computational approach, based on ligand docking and virtual screening, to discover inh-02, a drug-like small molecule that inhibits RNF5. In in vitro experiments, treatment with inh-02 modulated ATG4B and paxillin, both known RNF5 targets. In immortalized and primary bronchial epithelial cells derived from CF patients homozygous for the F508del mutation, long-term incubation with inh-02 caused significant F508del-CFTR rescue. This work validates RNF5 as a drug target for CF, providing evidence to support its druggability.

The Horizon of a Therapy for Rare Genetic Diseases: A "Druggable" Future for Fibrodysplasia Ossificans Progressiva.

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic condition characterized by progressive extra-skeletal ossification leading to cumulative and severe disability. FOP has an extremely variable and episodic course and can be induced by trauma, infections, iatrogenic harms, immunization or can occur in an unpredictable way, without any recognizable trigger. The causative gene is ACVR1, encoding the Alk-2 type I receptor for bone morphogenetic proteins (BMPs). The signaling is initiated by BMP binding to a receptor complex consisting of type I and II molecules and can proceed into the cell through two main pathways, a canonical, SMAD-dependent signaling and a p38-mediated cascade. Most FOP patients carry the recurrent R206H substitution in the receptor Glycine-Serine rich (GS) domain, whereas a few other mutations are responsible for a limited number of cases. Mutations cause a dysregulation of the downstream BMP-dependent pathway and make mutated ACVR1 responsive to a non-canonical ligand, Activin A. There is no etiologic treatment for FOP. However, many efforts are currently ongoing to find specific therapies targeting the receptor activity and the downstream aberrant pathway at different levels or targeting cellular components and/or processes that are important in modifying the local environment leading to bone neo-formation.

Thymosin α-1 does not correct F508del-CFTR in cystic fibrosis airway epithelia.

In cystic fibrosis (CF), deletion of phenylalanine 508 (F508del) in the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel causes misfolding and premature degradation. Considering the numerous effects of the F508del mutation on the assembly and processing of CFTR protein, combination therapy with several pharmacological correctors is likely to be required to treat CF patients. Recently, it has been reported that thymosin α-1 (Tα-1) has multiple beneficial effects that could lead to a single-molecule-based therapy for CF patients with F508del. Such effects include suppression of inflammation, improvement in F508del-CFTR maturation and gating, and stimulation of chloride secretion through the calcium-activated chloride channel (CaCC). Given the importance of such a drug, we aimed to characterize the underlying molecular mechanisms of action of Tα-1. In-depth analysis of Tα-1 effects was performed using well-established microfluorimetric, biochemical, and electrophysiological techniques on epithelial cell lines and primary bronchial epithelial cells from CF patients. The studies, which were conducted in 2 independent laboratories with identical outcome, demonstrated that Tα-1 is devoid of activity on mutant CFTR as well as on CaCC. Although Tα-1 may still be useful as an antiinflammatory agent, its ability to target defective anion transport in CF remains to be further investigated.

Genetic inactivation of mGlu5 receptor improves motor coordination in the Grm1crv4 mouse model of SCAR13 ataxia.

Deleterious mutations in the glutamate receptor metabotropic 1 gene (GRM1) cause a recessive form of cerebellar ataxia, SCAR13. GRM1 and GRM5 code for the metabotropic glutamate type 1 (mGlu1) and type 5 (mGlu5) receptors, respectively. Their different expression profiles suggest they could have distinct functional roles. In a previous study, homozygous mice lacking mGlu1 receptors (Grm1crv4/crv4) and exhibiting ataxia presented cerebellar overexpression of mGlu5 receptors, that was proposed to contribute to the mouse phenotype. To test this hypothesis, we here crossed Grm1crv4 and Grm5ko mice to generate double mutants (Grm1crv4/crv4Grm5ko/ko) lacking both mGlu1 and mGlu5 receptors. Double mutants and control mice were analyzed for spontaneous behavior and for motor activity by rotarod and footprint analyses. In the same mice, the release of glutamate from cerebellar nerve endings (synaptosomes) elicited by 12mM KCl or by α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) was also evaluated. Motor coordination resulted improved in double mutants when compared to Grm1crv4/crv4 mice. Furthermore, in in vitro studies, glutamate release elicited by both KCl depolarization and activation of AMPA autoreceptors resulted reduced in Grm1crv4/crv4 mice compared to wild type mice, while it presented normal levels in double mutants. Moreover, we found that Grm1crv4/crv4 mice showed reduced expression of GluA2/3 AMPA receptor subunits in cerebellar synaptosomes, while it resulted restored to wild type level in double mutants. To conclude, blocking of mGlu5 receptor reduced the dysregulation of glutamate transmission and improved motor coordination in the Grm1crv4 mouse model of SCAR13, thus suggesting the possible usefulness of pharmacological therapies based on modulation of mGlu5 receptor activity for the treatment of this type of ataxia.

High-throughput screening identifies FAU protein as a regulator of mutant cystic fibrosis transmembrane conductance regulator channel.

In cystic fibrosis, deletion of phenylalanine 508 (F508del) in the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel causes misfolding and premature degradation. One possible approach to reducing the detrimental health effects of cystic fibrosis could be the identification of proteins whose suppression rescues F508del-CFTR function in bronchial epithelial cells. However, searches for these potential targets have not yet been conducted, particularly in a relevant airway background using a functional readout. To identify proteins associated with F508del-CFTR processing, we used a high-throughput functional assay to screen an siRNA library targeting 6,650 different cellular proteins. We identified 37 proteins whose silencing significantly rescued F508del-CFTR activity, as indicated by enhanced anion transport through the plasma membrane. These proteins included FAU, UBE2I, UBA52, MLLT6, UBA2, CHD4, PLXNA1, and TRIM24, among others. We focused our attention on FAU, a poorly characterized protein with unknown function. FAU knockdown increased the plasma membrane targeting and function of F508del-CFTR, but not of wild-type CFTR. Investigation into the mechanism of action revealed a preferential physical interaction of FAU with mutant CFTR, leading to its degradation. FAU and other proteins identified in our screening may offer a therapeutically relevant panel of drug targets to correct basic defects in F508del-CFTR processing.

Correction to: Rarely occurring mutation of ACVR1 gene in Moroccan patient with fibrodysplasia ossificans progressiva.

One of the author's name on this article was incorrectly spelled as "Renata Borcciadi". The correct spelling is "Renata Bocciardi" and is now presented correctly in this article.

Peripheral Blood Mononuclear Cell Immunophenotyping in Fibrodysplasia Ossificans Progressiva Patients: Evidence for Monocyte DNAM1 Up-regulation.

BACKGROUND: Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder caused by sporadic heterozygous mutations in ACVR1 gene which progressively leads to severe heterotopic ossification. FOP is characterized by episodic flare-ups triggered by different factors such as viral infections, tissue injuries, vaccinations, or occurring without a recognizable cause. The sporadic course of the disease, the documented presence of an important inflammatory reaction in early lesions and the partial response to corticosteroids support the idea that the immune system, and in particular the innate component, may play a role in FOP pathogenesis. However, an extensive expression profile of the peripheral blood mononuclear cells (PBMC) of FOP patients has never been done. METHODS: In this study, we carried out a wide PBMC immunophenotyping on a cohort of FOP patients and matching controls by multiparametric analysis of the expression of a panel of 37 markers associated with migration, adhesion, inhibition, activation, and cell death of circulating immune cells. RESULTS: We observed a statistically significant increase of the expression of DNAM1 receptor in patients' monocytes as compared to controls, and little but significant differences in the expression profile of CXCR1 (CD181), CD62L, CXCR4 (CD184), and HLA-DR molecules. CONCLUSIONS: DNAM1 had been previously shown to play a pivotal role in monocyte migration through the endothelial barrier and the increased expression detected in patients' monocytes might suggest a role of this surface receptor during the early phases of FOP flare-ups in which the activation of the immune response is believed to represent a crucial event. © 2017 International Clinical Cytometry Society.

Hints on transcriptional control of essential players in heterotopic ossification of Fibrodysplasia Ossificans Progressiva.

Signaling of the Bone Morphogenetic Protein (BMP) pathway is influenced by the level of expression of its components, in particular receptors, intracellular molecules and target genes which largely depends on gene transcription. One peculiar aspect of Fibrodysplasia Ossificans Progressiva (FOP) relates to the cell types in which the genetic mutation exerts its effects, then not only those involved in the heterotopic ossification processes but also others that participate in the inflammatory phases preceding and triggering heterotopic ossification. Such effects are in part detectable as variation in gene expression, which is also variably manifesting in term of time of appearance in different phases of the inflammatory or ossification processes.

Targeting of PHOX2B expression allows the identification of drugs effective in counteracting neuroblastoma cell growth.

The pathogenic role of the PHOX2B gene in neuroblastoma is indicated by heterozygous mutations in neuroblastoma patients and by gene overexpression in both neuroblastoma cell lines and tumor samples. PHOX2B encodes a transcription factor which is crucial for the correct development and differentiation of sympathetic neurons. PHOX2B overexpression is considered a prognostic marker for neuroblastoma and it is also used by clinicians to monitor minimal residual disease. Furthermore, it has been observed that neuronal differentiation in neuroblastoma is dependent on down-regulation of PHOX2B expression, which confirms that PHOX2B expression may be considered a target in neuroblastoma. Here, PHOX2B promoter or 3' untranslated region were used as molecular targets in an in vitro high-throughput approach that led to the identification of molecules able to decrease PHOX2B expression at transcriptional and likely even at post-transcriptional levels. Further functional investigations carried out on PHOX2B mRNA levels and biological consequences, such as neuroblastoma cell apoptosis and growth, showed that chloroquine and mycophenolate mofetil are most promising agents for neuroblastoma therapy based on down-regulation of PHOX2B expression. Finally, a strong correlation between the effect of drugs in terms of down-regulation of PHOX2B expression and of biological consequences in neuroblastoma cells confirms the role of PHOX2B as a potential molecular target in neuroblastoma.

Cryopyrin-associated Periodic Syndromes in Italian Patients: Evaluation of the Rate of Somatic NLRP3 Mosaicism and Phenotypic Characterization.

OBJECTIVE: To evaluate the rate of somatic NLRP3 mosaicism in an Italian cohort of mutation-negative patients with cryopyrin-associated periodic syndrome (CAPS). METHODS: The study enrolled 14 patients with a clinical phenotype consistent with CAPS in whom Sanger sequencing of the NLRP3 gene yielded negative results. Patients' DNA were subjected to amplicon-based NLRP3 deep sequencing. RESULTS: Low-level somatic NLRP3 mosaicism has been detected in 4 patients, 3 affected with chronic infantile neurological cutaneous and articular syndrome and 1 with Muckle-Wells syndrome. Identified nucleotide substitutions encode for 4 different amino acid exchanges, with 2 of them being novel (p.Y563C and p.G564S). In vitro functional studies confirmed the deleterious behavior of the 4 somatic NLRP3 mutations. Among the different neurological manifestations detected, 1 patient displayed mild loss of white matter volume on brain magnetic resonance imaging. CONCLUSION: The allele frequency of somatic NLRP3 mutations occurs generally under 15%, considered the threshold of detectability using the Sanger method of DNA sequencing. Consequently, routine genetic diagnostic of CAPS should be currently performed by next-generation techniques ensuring high coverage to identify also low-level mosaicism, whose actual frequency is yet unknown and probably underestimated.

Common PHOX2B poly-alanine contractions impair RET gene transcription, predisposing to Hirschsprung disease.

HSCR is a congenital disorder of the enteric nervous system, characterized by the absence of neurons along a variable length of the gut resulting from loss-of-function RET mutations. Congenital Central Hypoventilation Syndrome (CCHS) is a rare neurocristopathy characterized by impaired response to hypercapnia and hypoxemia caused by heterozygous mutations of the PHOX2B gene, mostly polyalanine (polyA) expansions but also missense, nonsense, and frameshift mutations, while polyA contractions are common in the population and believed neutral. HSCR associated CCHS can present in patients carrying PHOX2B mutations. Indeed, RET expression is orchestrated by different transcriptional factors among which PHOX2B, thus suggesting its possible role in HSCR pathogenesis. Following the observation of HSCR patients carrying in frame trinucleotide deletions within the polyalanine stretch in exon 3 (polyA contractions), we have verified the hypothesis that these PHOX2B variants do reduce its transcriptional activity, likely resulting in a down-regulation of RET expression and, consequently, favouring the development of the HSCR phenotype. Using proper reporter constructs, we show here that the in vitro transactivation of the RET promoter by different HSCR-associated PHOX2B polyA variants has resulted significantly lower compared to the effect of PHOX2B wild type protein. In particular, polyA contractions do induce a reduced transactivation of the RET promoter, milder compared to the severe polyA expansions associated with CCHS+HSCR, and correlated with the length of the deleted trait, with a more pronounced effect when contractions are larger.

Phenotypic characterization of Grm1crv4 mice reveals a functional role for the type 1 metabotropic glutamate receptor in bone mineralization.

Recent increasing evidence supports a role for neuronal type signaling in bone. Specifically glutamate receptors have been found in cells responsible for bone remodeling, namely the osteoblasts and the osteoclasts. While most studies have focused on ionotropic glutamate receptors, the relevance of the metabotropic glutamate signaling in bone is poorly understood. Specifically type 1 metabotropic glutamate (mGlu1) receptors are expressed in bone, but the effect of its ablation on skeletal development has never been investigated. Here we report that Grm1crv4/crv4 mice, homozygous for an inactivating mutation of the mGlu1 receptor, and mainly characterized by ataxia and renal dysfunction, exhibit decreased body weight, bone length and bone mineral density compared to wild type (WT) animals. Blood analyses of the affected mice demonstrate the absence of changes in circulating factors, such as vitamin D and PTH, suggesting renal damage is not the main culprit of the skeletal phenotype. Cultures of osteoblasts lacking functional mGlu1 receptors exhibit less homogeneous collagen deposition than WT cells, and present increased expression of osteocalcin, a marker of osteoblast maturation. These data suggest that the skeletal damage is directly linked to the absence of the receptor, which in turn leads to osteoblasts dysfunction and earlier maturation. Accordingly, skeletal histomorphology suggests that Grm1crv4/crv4 mice exhibit enhanced bone maturation, resulting in premature fusion of the growth plate and shortened long bones, and further slowdown of bone apposition rate compared to the WT animals. In summary, this work reveals novel functions of mGlu1 receptors in the bone and indicates that in osteoblasts mGlu1 receptors are necessary for production of normal bone matrix, longitudinal bone growth, and normal skeletal development.

Assessment of copy number variations in 120 patients with Poland syndrome.

BACKGROUND: Poland Syndrome (PS) is a rare congenital disorder presenting with agenesis/hypoplasia of the pectoralis major muscle variably associated with thoracic and/or upper limb anomalies. Most cases are sporadic, but familial recurrence, with different inheritance patterns, has been observed. The genetic etiology of PS remains unknown. Karyotyping and array-comparative genomic hybridization (CGH) analyses can identify genomic imbalances that can clarify the genetic etiology of congenital and neurodevelopmental disorders. We previously reported a chromosome 11 deletion in twin girls with pectoralis muscle hypoplasia and skeletal anomalies, and a chromosome six deletion in a patient presenting a complex phenotype that included pectoralis muscle hypoplasia. However, the contribution of genomic imbalances to PS remains largely unknown. METHODS: To investigate the prevalence of chromosomal imbalances in PS, standard cytogenetic and array-CGH analyses were performed in 120 PS patients. RESULTS: Following the application of stringent filter criteria, 14 rare copy number variations (CNVs) were identified in 14 PS patients in different regions outside known common copy number variations: seven genomic duplications and seven genomic deletions, enclosing the two previously reported PS associated chromosomal deletions. These CNVs ranged from 0.04 to 4.71 Mb in size. Bioinformatic analysis of array-CGH data indicated gene enrichment in pathways involved in cell-cell adhesion, DNA binding and apoptosis processes. The analysis also provided a number of candidate genes possibly causing the developmental defects observed in PS patients, among others REV3L, a gene coding for an error-prone DNA polymerase previously associated with Möbius Syndrome with variable phenotypes including pectoralis muscle agenesis. CONCLUSIONS: A number of rare CNVs were identified in PS patients, and these involve genes that represent candidates for further evaluation. Rare inherited CNVs may contribute to, or represent risk factors of PS in a multifactorial mode of inheritance.