Disruption of FLNB leads to skeletal malformation by interfering with skeletal segmentation through the HOX gene

Filamin B (FLNB) plays an important role in skeletal development. Mutations in FLNB can lead to skeletal malformation such as an abnormal number of ossification centers, indicating that the skeletal segmentation in the embryonic period may be interfered with. We established a mouse model with the pathogenic point mutation FLNB NM_001081427.1: c.4756G > A (p.Gly1586Arg) using CRISPR-Cas9 technology. Micro-CT, HE staining and whole skeletal preparation were performed to examine the skeletal malformation. In situ hybridization of embryos was performed to examine the transcription of HOX genes during embryonic development. The expression of FLNB was downregulated in FLNBG1586R/G1586R and FLNBWT/G1586R mice, compared to FLNBWT/WT mice. Fusions in tarsal bones were found in FLNBG1586R/G1586R and FLNBWT/G1586R mice, indicating that the skeletal segmentation was interfered with. In the embryo of FLNBG1586R/G1586R mice (E12.5), the transcription levels of HOXD10 and HOXB2 were downregulated in the carpal region and cervical spine region, respectively. This study indicated that the loss-of-function mutation G1586R in FLNB may lead to abnormal skeletal segmentation, and the mechanism was possibly associated with the downregulation of HOX gene transcription during the embryonic period.

Cell-Dependent Pathogenic Roles of Filamin B in Different Skeletal Malformations

Mutations of filamin B (FLNB) gene can lead to a spectrum of autosomal skeletal malformations including spondylocarpotarsal syndrome (SCT), Larsen syndrome (LRS), type I atelosteogenesis (AO1), type III atelosteogenesis (AO3), and boomerang dysplasia (BD). Among them, LRS is milder while BD causes a more severe phenotype. However, the molecular mechanism underlying the differences in clinical phenotypes of different FLNB variants has not been fully determined. Here, we presented two patients suffering from autosomal dominant LRS and autosomal recessive vitamin D-dependent rickets type IA (VDDR-IA). Whole-exome sequencing revealed two novel missense variants in FLNB, c.4846A>G (p.T1616A) and c.7022T>G (p.I2341R), which are located in repeat 15 and 22 of filamin B, respectively. The expression of FLNB^I2341R in the muscle tissue from our LRS patient was remarkably increased. And in vitro studies showed that both variants led to a lack of filopodia and accumulation of the mutants in the perinuclear region in HEK293 cells. We also found that c.4846A>G (p.T1616A) and c.7022T>G (p.I2341R) regulated endochondral osteogenesis in different ways. c.4846A>G (p.T1616A) activated AKT pathways through inhibiting SHIP2, suppressed the Smad3 pathway, and impaired the expression of Runx2 in both Saos-2 and ATDC5 cells. c.7022T>G (p.I2341R) activated both AKT and Smad3 pathways and increased the expression of Runx2 in Saos-2 cells, while in ATDC5 cells it activated AKT pathways through inhibiting SHIP2, suppressed the Smad3 pathway, and reduced the expression of Runx2. Our study demonstrated the pathogenic mechanisms of two novel FLNB variants in two different clinical settings and proved that FLNB variants could not only directly cause skeletal malformations but also worsen skeletal symptoms in the setting of other skeletal diseases. Besides, FLNB variants differentially affect skeletal development which contributes to clinical heterogeneity of FLNB-related disorders.

Whole Exome Sequencing in Individuals with Idiopathic Clubfoot Reveals a Recurrent Filamin B (FLNB) Deletion

BACKGROUND

Clubfoot, a congenital deformity that presents as a rigid, inward turning of the foot, affects approximately 1 in 1000 infants and occurs as an isolated birth defect in 80% of patients. Despite its high level of heritability, few causative genes have been identified, and mutations in known genes are only responsible for a small portion of clubfoot heritability.

QUESTIONS/PURPOSES

(1) Are any rare gene variants enriched (that is, shared) in unrelated patients with isolated clubfoot? (2) Are there other rare variants in the identified gene (Filamin B) in these patients with clubfoot?

METHODS

Whole-exome sequence data were generated from a discovery cohort of 183 unrelated probands with clubfoot and 2492 controls. Variants were filtered with minor allele frequency < 0.02 to identify rare variants as well as small insertions and deletions (indels) resulting in missense variants, nonsense or premature truncation, or in-frame deletions. A candidate deletion was then genotyped in another cohort of 974 unrelated patients with clubfoot (a replication cohort). Other rare variants in the candidate gene were also investigated. A segregation analysis was performed in multigenerational families of individuals with clubfoot to see if the genotypes segregate with phenotypes. Single-variant association analysis was performed using the Fisher two-tailed exact test (exact p values are presented to give an indication of the magnitude of the association).

RESULTS

There were no recurrent variants in the known genes causing clubfoot in this study. A three-base pair in-frame codon deletion of Filamin B (FLNB) (p.E1792del, rs1470699812) was identified in 1.6% (3 of 183) of probands with clubfoot in the discovery cohort compared with 0% of controls (0 of 2492) (odds ratio infinity (inf) [95% CI 5.64 to inf]; p = 3.18 x 10-5) and 0.0016% of gnomAD controls (2 of 125,709) (OR 1.01 x 103 [95% CI 117.42 to 1.64 x 104]; p = 3.13 x 10-8). By screening a replication cohort (n = 974 patients), we found two probands with the identical FLNB deletion. In total, the deletion was identified in 0.43% (5 of 1157) of probands with clubfoot compared with 0% of controls and 0.0016% of gnomAD controls (OR 268.5 [95% CI 43.68 to 2.88 x 103]; p = 1.43 x 10-9). The recurrent FLNB p.E1792del variant segregated with clubfoot, with incomplete penetrance in two families. Affected individuals were more likely to be male and have bilateral clubfoot. Although most patients had isolated clubfoot, features consistent with Larsen syndrome, including upper extremity abnormalities such as elbow and thumb hypermobility and wide, flat thumbs, were noted in affected members of one family. We identified 19 additional rare FLNB missense variants located throughout the gene in patients with clubfoot. One of these missense variants, FLNB p.G2397D, exhibited incomplete penetrance in one family.

CONCLUSION

A recurrent FLNB E1792 deletion was identified in 0.43% of 1157 isolated patients with clubfoot. Given the absence of any recurrent variants in our discovery phase (n = 183) for any of the known genes causing clubfoot, our findings support that novel and rare missense variants in FLNB in patients with clubfoot, although rare, may be among the most commonly known genetic causes of clubfoot. Patients with FLNB variants often have isolated clubfoot, but they and their family members may be at an increased risk of having additional clinical features consistent with Larsen syndrome.

CLINICAL RELEVANCE

Identification of FLNB variants may be useful for determining clubfoot recurrence risk and comorbidities.

Combined exome sequencing and deep phenotyping in highly selected fetuses with skeletal dysplasia during the first and second trimesters improves diagnostic yield

OBJECTIVE

To investigate the genetic etiology of skeletal dysplasia in highly selected fetuses during the first and second trimesters using deep phenotyping and exome sequencing (ES).

METHOD

Fetuses with short femurs were identified using the established prenatal diagnostic approach. A multidisciplinary team reviewed fetal phenotypic information (prenatal ultrasound findings, fetal postmortem, and radiographs) in a cohort of highly selected fetuses with skeletal dysplasia during the first and second trimesters. The affected families underwent multiplatform genetic tests.

RESULTS

Of the 27 affected fetuses, 21 (77.8%) had pathogenic or potential pathogenic variations in the following genes: COL1A1, FGFR3, COL2A1, COL1A2, FLNB, DYNC2LI1, and TRIP11. Two fetuses had compound heterozygous mutations in DYNC2LI1 and TRIP11, respectively, and the other 19 carried de novo autosomal dominant variants. Novel variants were identified in COL1A1, COL2A1, COL1A2, DYNC2LI1, and TRIP11 in 11 fetuses. We also included the first description of the phenotype of odontochondrodysplasia in a prenatal setting.

CONCLUSIONS

ES or panel sequencing offers a high diagnostic yield for fetal skeletal dysplasia during the first and second trimesters. Comprehensive and complete phenotypic information is indispensable for genetic analysis and the expansion of genotype-phenotype correlations in fetal skeletal abnormalities.

Spondylocarpotarsal synostosis syndrome due to a novel loss of function FLNB variant: a case report

Background

Loss of function or gain of function variants of Filamin B (FLNB) cause recessive or dominant skeletal disorders respectively. Spondylocarpotarsal synostosis syndrome (SCT) is a rare autosomal recessive disorder characterized by short stature, fused vertebrae and fusion of carpal and tarsal bones. We present a novel FLNB homozygous pathogenic variant and present a carrier of the variant with short height.

Case presentation

We describe a family with five patients affected with skeletal malformations, short stature and vertebral deformities. Exome sequencing revealed a novel homozygous frameshift variant c.2911dupG p.(Ala971GlyfsTer122) in FLNB, segregating with the phenotype in the family. The variant was absent in public databases and 100 ethnically matched control chromosomes. One of the heterozygous carriers of the variant had short stature.

Conclusion

Our report expands the genetic spectrum of FLNB pathogenic variants. It also indicates a need to assess the heights of other carriers of FLNB recessive variants to explore a possible role in idiopathic short stature.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-020-03890-2.

TK, Ahmad MN, Okashah SS, Kamaraj B, Al-Subaie AM, C. GPD, Zayed H. Deciphering the Role of Filamin B Calponin-Homology Domain in Causing the Larsen Syndrome, Boomerang Dysplasia, and Atelosteogenesis Type I Spectrum Disorders via a Computational Approach

Filamins (FLN) are a family of actin-binding proteins involved in regulating the cytoskeleton and signaling phenomenon by developing a network with F-actin and FLN-binding partners. The FLN family comprises three conserved isoforms in mammals: FLNA, FLNB, and FLNC. FLNB is a multidomain monomer protein with domains containing an actin-binding N-terminal domain (ABD 1-242), encompassing two calponin-homology domains (assigned CH1 and CH2). Primary variants in FLNB mostly occur in the domain (CH2) and surrounding the hinge-1 region. The four autosomal dominant disorders that are associated with FLNB variants are Larsen syndrome, atelosteogenesis type I (AOI), atelosteogenesis type III (AOIII), and boomerang dysplasia (BD). Despite the intense clustering of FLNB variants contributing to the LS-AO-BD disorders, the genotype-phenotype correlation is still enigmatic. In silico prediction tools and molecular dynamics simulation (MDS) approaches have offered the potential for variant classification and pathogenicity predictions. We retrieved 285 FLNB missense variants from the UniProt, ClinVar, and HGMD databases in the current study. Of these, five and 39 variants were located in the CH1 and CH2 domains, respectively. These variants were subjected to various pathogenicity and stability prediction tools, evolutionary and conservation analyses, and biophysical and physicochemical properties analyses. Molecular dynamics simulation (MDS) was performed on the three candidate variants in the CH2 domain (W148R, F161C, and L171R) that were predicted to be the most pathogenic. The MDS analysis results showed that these three variants are highly compact compared to the native protein, suggesting that they could affect the protein on the structural and functional levels. The computational approach demonstrates the differences between the FLNB mutants and the wild type in a structural and functional context. Our findings expand our knowledge on the genotype-phenotype correlation in FLNB-related LS-AO-BD disorders on the molecular level, which may pave the way for optimizing drug therapy by integrating precision medicine.

A Further Case of Larsen's Syndrome: Clinical and Genotypic Challenges in Diagnosis

Larsen's syndrome is characterized by dislocation of multiple large joints, digital anomalies, craniofacial dysmorphism, and short stature. In this paper, we describe a case of a 5-month-old boy with a triad of cardinal features in association with other signs. The diagnosis was confirmed by exome sequencing, which led to the identification of a novel missense variant NM_001457.4:c.4928C > G (p.Ala1643Gly) in the FLNB gene. We describe the role of protein modelling for the establishment of pathogenicity of this variant. We also outline the challenges in genetic diagnosis due to variable expressivity of the variant and discuss the clinicogenetic profile of previously reported patients with Larsen's syndrome in India.

Novel GZF1 pathogenic variants identified in two Chinese patients with Larsen syndrome

GZF1 was recently reported as a genetic factor associated with Larsen syndrome. Two patients presenting hip dislocation, scoliosis and severe myopia, as well as hearing loss and other abnormal features, were found to carry two novel compounds heterozygous variants in GZF1 (c.397400del, p. Leu133fs; and c.1474del, p. Met492fs) through whole-exome sequencing. The mRNA expression level of L133fs-GZF1 did not significantly differ from that of WT-GZF1. However, no HA-conjugated mutant protein was detected by western blotting, which was also confirmed by immunofluorescence staining. In addition, both mRNA transcription and protein expression levels of M492fs-GZF1 were significantly lower than those of wild type, and HA-tagged M492fs-GZF1 was mainly distributed in the cytoplasm of HEK 293 T cells. These results suggested that the two variants could lead to loss of function of GZF1. Our study was the second to report the association between GZF1 variants and Larsen syndrome. We also provided functional evidence for the pathogenicity of GZF1 variants, which expands the mutation spectrum and offers a basis for functional research on the role of GZF1 in the development of Larsen syndrome.

Exome sequencing analysis identifies frequent oligogenic involvement and FLNB variants in adolescent idiopathic scoliosis

Background

Adolescent idiopathic scoliosis (AIS) is a genetically heterogeneous disease characterised by three-dimensional deformity of the spine in the absence of a congenital spinal anomaly or neurological musculoskeletal disorder. The clinical variability and incomplete penetrance of some genes linked with AIS indicate that this disease constitutes an oligogenic trait.

Objective

We aimed to explore the oligogenic nature of this disease and identify novel AIS genes.

Methods

We analysed rare damaging variants within AIS-associated genes by using exome sequencing in 40 AIS trios and 183 sporadic patients.

Results

Multiple variants within AIS-associated genes were identified in eight AIS trios, and five individuals harboured rare damaging variants in the FLNB gene. The patients showed more frequent oligogenicity than the controls. In the gene-based burden test, the top signal resided in FLNB. In functional studies, we found that the AIS-associated FLNB variants altered the protein’s conformation and subcellular localisation and its interaction with other proteins (TTC26 and OFD1) involved in AIS. The most compelling evidence of an oligogenic basis was that the number of rare damaging variants was recognised as an independent prognostic factor for curve progression in Cox regression analysis.

Conclusion

Our data indicate that AIS is an oligogenic disease and identify FLNB as a susceptibility gene for AIS.

Novel in-frame FLNB deletion causes Larsen syndrome in a three-generation pedigree

A 4-yr-old female with congenital knee dislocations and joint laxity was noted to have a strong maternal family history comprising multiple individuals with knee problems and clubfeet. As the knee issues were the predominant clinical features, clinical testing included sequencing of LMX1B, TBX2, and TBX4, which identified no significant variants. Research genome sequencing was performed in the proband, parents, and maternal grandfather. A heterozygous in-frame deletion in FLNB c. 5468_5470delAGG, which predicts p.(Glu1823del), segregated with the disease. The variant is rare in the gnomAD database, removes a residue that is evolutionarily conserved, and is predicted to alter protein length. Larsen syndrome may present with pathology that primarily involves one joint and thus may be difficult to differentiate clinically from other skeletal dysplasias or arthrogryposis syndromes. The p.(Glu1823del) variant maps to a filamin repeat domain where other disease-causing variants are clustered, consistent with a probable gain-of-function mechanism. It has reportedly been observed in two individuals in the gnomAD database, suggesting that mild presentations of Larsen syndrome, like the individual reported here, may be underdiagnosed in the general population.

Critical Structural Defects Explain Filamin A Mutations Causing Mitral Valve Dysplasia

Mitral valve diseases affect ∼3% of the population and are the most common reasons for valvular surgery because no drug-based treatments exist. Inheritable genetic mutations have now been established as the cause of mitral valve insufficiency, and four different missense mutations in the filamin A gene (FLNA) have been found in patients suffering from nonsyndromic mitral valve dysplasia (MVD). The filamin A (FLNA) protein is expressed, in particular, in endocardial endothelia during fetal valve morphogenesis and is key in cardiac development. The FLNA-MVD-causing mutations are clustered in the N-terminal region of FLNA. How the mutations in FLNA modify its structure and function has mostly remained elusive. In this study, using NMR spectroscopy and interaction assays, we investigated FLNA-MVD-causing V711D and H743P mutations. Our results clearly indicated that both mutations almost completely destroyed the folding of the FLNA5 domain, where the mutation is located, and also affect the folding of the neighboring FLNA4 domain. The structure of the neighboring FLNA6 domain was not affected by the mutations. These mutations also completely abolish FLNA's interactions with protein tyrosine phosphatase nonreceptor type 12, which has been suggested to contribute to the pathogenesis of FLNA-MVD. Taken together, our results provide an essential structural and molecular framework for understanding the molecular bases of FLNA-MVD, which is crucial for the development of new therapies to replace surgery.

Genetic Analysis in Fetal Skeletal Dysplasias by Trio Whole-Exome Sequencing

Skeletal dysplasias (SDs) comprise a series of severe congenital disorders that have strong clinical heterogeneity and usually attribute to diverse genetic variations. The pathogenesis of more than half of SDs remains unclear. Additionally, the clinical manifestations of fetal SDs are ambiguous, which poses a big challenge for accurate diagnosis. In this study, eight unrelated families with fetal SD were recruited and subjected to sequential tests including chromosomal karyotyping, chromosomal microarray analysis (CMA), and trio whole-exome sequencing (WES). Sanger sequencing and quantitative fluorescence PCR (QF-PCR) were performed as affirmative experiments. In six families, a total of six pathogenic/likely pathogenic variations were identified in four genes including SLC26A2, FGFR3, FLNB, and TMEM38B. These variations caused disorders following autosomal dominant or autosomal recessive inheritance patterns, respectively. The results provided reliable evidence for the subsequent genetic counseling and reproductive options to these families. With its advantage in variation calling and interpreting, trio WES is a promising strategy for the investigation of fetal SDs in cases with normal karyotyping and CMA results. It has considerable prospects to be utilized in prenatal diagnosis.

A case study of atypical Larsen syndrome with absent hallmark joint dislocations

BACKGROUND

A family with skeletal and craniofacial anomalies is presented. Whole-exome sequencing (WES) analysis indicated a diagnosis of Larsen syndrome, although their clinical presentation does not include the hallmark joint dislocations typically observed in Larsen syndrome.

METHODS

Patient consent for the sharing of de-identified clinical and genetic information, along with use of photographs for publication, was obtained. WES and variant segregation analysis by WES were performed by commercial laboratory, GeneDx (Gaithersburg, MD), on peripheral blood samples from the proband, her brother, and her parents using methods detailed on their website for test XomeDx Whole Exome Sequencing Trio (https://www.genedx.com/test-catalog/available-tests/xomedx-whole-exome-sequencing-trio/). WES uses next-generation sequencing (NGS) technology to assess for variants within the coding regions, or exons, of approximately 23,000 genes. For the FLNB gene (NM_001457.3), 100% of the coding region was covered at a minimum of 10x. GeneDx uses Sanger sequencing to confirm NGS variants.

RESULTS

WES revealed a heterozygous pathogenic variant, p.Glu227Lys (c.679G>A), in the FLNB gene in three out of the four family members tested. This variant is associated with Larsen syndrome, a skeletal dysplasia condition with a wide range of phenotypic variability that usually includes congenital joint dislocations.

CONCLUSION

This is a highly unusual presentation of Larsen syndrome in which the identifying hallmark trait is absent in the patients' phenotypes.

An alternative splicing switch in FLNB promotes the mesenchymal cell state in human breast cancer

Alternative splicing of mRNA precursors represents a key gene expression regulatory step and permits the generation of distinct protein products with diverse functions. In a genome-scale expression screen for inducers of the epithelial-to-mesenchymal transition (EMT), we found a striking enrichment of RNA-binding proteins. We validated that QKI and RBFOX1 were necessary and sufficient to induce an intermediate mesenchymal cell state and increased tumorigenicity. Using RNA-seq and eCLIP analysis, we found that QKI and RBFOX1 coordinately regulated the splicing and function of the actin-binding protein FLNB, which plays a causal role in the regulation of EMT. Specifically, the skipping of FLNB exon 30 induced EMT by releasing the FOXC1 transcription factor. Moreover, skipping of FLNB exon 30 is strongly associated with EMT gene signatures in basal-like breast cancer patient samples. These observations identify a specific dysregulation of splicing, which regulates tumor cell plasticity and is frequently observed in human cancer.

As the human body develops, countless cells change from one state into another. Two important cell states are known as epithelial and mesenchymal. Cells in the epithelial state tend to be tightly connected and form barriers, like skin cells. Mesenchymal state cells are loosely organized, move around more and make up connective tissues. Some cells alternate between these states via an epithelial-to-mesenchymal transition (EMT for short) and back again. Without this transition, certain organs would not develop and wounds would not heal. Yet, cancer cells also use this transition to spread to distant sites of the body. Such cancers are often the most aggressive, and therefore the most deadly.

The epithelial-to-mesenchymal transition is dynamically regulated in a reversible manner. For example, the genes for some proteins might only be active in the epithelial state and further reinforce this state by turning on other ‘epithelial genes’. Alternatively, there might be differences in the processing of mRNA molecules – the intermediate molecules between DNA and protein – that result in the production of different proteins in epithelial and mesenchymal cells.

Li, Choi et al. wanted to know which of the thousands of human genes can endow epithelial state cells with mesenchymal characteristics. A better understanding of the switch could help to prevent cancers undergoing an epithelial-to-mesenchymal transition.

From a large-scale experiment in human breast cancer cells, Li, Choi et al. found that a group of proteins that bind and modify mRNA molecules are important for the epithelial-to-mesenchymal transition. Two proteins in particular promoted the transition, most likely by binding to the mRNA of a third protein called FLNB and removing a small piece of it. FLNB normally works to prevent the epithelial-to-mesenchymal transition, but the smaller protein encoded by the shorter mRNA promoted the transition by turning on ‘mesenchymal genes’.

This switching between different FLNB proteins happens in some of the more aggressive breast cancers, which also contain mesenchymal cells. Finding out which FLNB protein is made in a given cancer may provide an indication of its aggressiveness. Also, looking for drugs that can target the mRNA-binding proteins or FLNB may one day lead to new treatments for some of the most aggressive breast cancers.

Development of a comprehensive noninvasive prenatal test

Our aim was to develop and apply a comprehensive noninvasive prenatal test (NIPT) by using high-coverage targeted next-generation sequencing to estimate fetal fraction, determine fetal sex, and detect trisomy and monogenic disease without parental genotype information. We analyzed 45 pregnancies, 40 mock samples, and eight mother-child pairs to generate 35 simulated datasets. Fetal fraction (FF) was estimated based on analysis of the single nucleotide polymorphism (SNP) allele fraction distribution. A Z-score was calculated for trisomy of chromosome 21 (T21), and fetal sex detection. Monogenic disease detection was performed through variant analysis. Model validation was performed using the simulated datasets. The novel model to estimate FF was robust and accurate (r2= 0.994, p-value < 2.2e-16). For samples with FF > 0.04, T21 detection had 100% sensitivity (95% CI: 63.06 to 100%) and 98.53% specificity (95% CI: 92.08 to 99.96%). Fetal sex was determined with 100% accuracy. We later performed a proof of concept for monogenic disease diagnosis of 5/7 skeletal dysplasia cases. In conclusion, it is feasible to perform a comprehensive NIPT by using only data from high coverage targeted sequencing, which, in addition to detecting trisomies, also make it possible to identify pathogenic variants of the candidate genes for monogenic diseases.

Piepkorn type of osteochondrodysplasia: Defining the severe end of FLNB-related skeletal disorders in three fetuses and a 106-year-old exhibit

The Piepkorn type of lethal osteochondrodysplasia (POCD) is a rare and lethal dwarfing condition. Four cases have been reported to date. The characteristic features are distinctly shortened "flipper-like" limbs, polysyndactyly, excessive underossification, especially of the limb bones and vertebrae, and large (giant) chondrocytes in the cartilaginous bone primordia. These characteristics allowed the diagnosis of Piepkorn type of osteochondrodysplasia in four new cases, three fetuses of 15 to 22 weeks and one 106-year-old museum exhibit. Piepkorn type of osteochondrodysplasia has been assigned to the giant cell chondrodysplasias such as atelosteogenesis type 1 (AO1) and boomerang dysplasia (BD). Analysis of the Filamin B gene in 3p14.3, which is associated with these disorders, allowed the identification of the first FLNB mutations in Piepkorn type of osteochondrodysplasia. The heterozygous missense mutations, found in the three fetuses, were located in exons 28 and 29, encoding the immunoglobulin-like repeat region R15, one of three mutational hot spots in dominant FLNB-related skeletal disorders. Direct preparations and alcian blue staining revealed single upper and lower arm and leg bone primordia, preaxial oligodactyly, and polysyndactyly with complete fusion and doubling of the middle and end phalanges II-V to produce eight distal finger rays. Considering the unique clinical features and the extent of underossification, Piepkorn type of osteochondrodysplasia can be regarded as a distinct entity within the AO1-BD-POCD continuum.

The filamin-B–refilin axis – spatiotemporal regulators of the actin-cytoskeleton in development and disease

During development, cycles of spatiotemporal remodeling of higher-order networks of actin filaments contribute to control cell fate specification and differentiation. Programs for controlling these dynamics are hard-wired into actin-regulatory proteins. The filamin family of actin-binding proteins exert crucial mechanotransduction and signaling functions in tissue morphogenesis. Filamin-B (FLNB) is a key player in chondrocyte progenitor differentiation for endochondral ossification. Biallelic loss-of-function mutations or gain-of-function mutations in FLNB cause two groups of skeletal disorders that can be attributed to either the loss of repressive function on TGF-β signaling or a disruption in mechanosensory properties, respectively. In this Review, we highlight a unique family of vertebrate-specific short-lived filamin-binding proteins, the refilins (refilin-A and refilin-B), that modulate filamin-dependent actin crosslinking properties. Refilins are downstream TGF-β effectors in epithelial cells. Double knockout of both refilin-A and refilin-B in mice results in precocious ossification of some axial skeletal elements, leading to malformations that are similar to those seen in FLNB-deficient mice. Based on these findings, we present a model summarizing the role of refilins in regulating the mechanosensory functions of FLNB during skeletal development. We also discuss the possible contribution of refilins to FLNB-related skeletal pathologies that are associated with gain-of-function mutations.

Biallelic mutations in FLNB cause a skeletal dysplasia with 46,XY gonadal dysgenesis by activating β-catenin

Filamin B (FLNB) functions as a switch that can affect chrondrocyte development and endochondral bone formation through a series of signaling molecules and transcription factors that also affect Sertoli cell development. Here, we report a subject with a novel skeletal dysplasia and co-existing 46,XY gonadal dysgenesis and biallelic mutations in FLNB. Whole exome sequencing was performed to identify mutations. Quantitative polymerase chain reaction (qPCR) and flow variant assays were performed to quantify RNA, proteins and phosphorylated proteins. The TOPFLASH reporter was performed to quantify β-catenin activity. Mutations were identified in the FLNB gene (FLNB:p.F964L, FLNB:p.A1577V). These mutations increased binding of FLNB protein to the MAP3K1 and RAC1 signal transduction complex and activated β-catenin and had different effects on phosphorylation of MAP kinase pathway intermediates and SOX9 expression. Direct activation of β-catenin through the FLNB-MAP3K1-RAC1 complex by FLNB mutations is a novel mechanism for causing 46,XY gonadal dysgenesis. The mechanism of action varies from those reported previously for loss of function mutations in SOX9 and gain-of-function mutations in MAP3K1.

Quantification of transmission risk in a male patient with a FLNB mosaic mutation causing Larsen syndrome: Implications for genetic counseling in postzygotic mosaicism cases

We report the case of a male patient with Larsen syndrome found to be mosaic for a novel point mutation in FLNB in whom it was possible to provide evidence-based personalized counseling on transmission risk to future offspring. Using dideoxy sequencing, a low-level FLNB c.698A>G, encoding p.(Tyr233Cys) mutation was detected in buccal mucosa and fibroblast DNA. Mutation quantification was performed by deep next-generation sequencing (NGS) of DNA extracted from three somatic tissues (blood, fibroblasts, saliva) and a sperm sample. The mutation was detectable in all tissues tested, at levels ranging from 7% to 10% (mutation present in ∼20% of diploid somatic cells and 7% of haploid sperm), demonstrating the involvement of both somatic and gonadal lineages in this patient. This report illustrates the clinical utility of performing targeted NGS analysis on sperm from males with a mosaic condition in order to provide personalized transmission risk and offer evidence-based counseling on reproductive safety.

Filamin B: The next hotspot in skeletal research?

Filamin B (FLNB) is a large dimeric actin-binding protein which crosslinks actin cytoskeleton filaments into a dynamic structure. Up to present, pathogenic mutations in FLNB are solely found to cause skeletal deformities, indicating the important role of FLNB in skeletal development. FLNB-related disorders are classified as spondylocarpotarsal synostosis (SCT), Larsen syndrome (LS), atelosteogenesis (AO), boomerang dysplasia (BD), and isolated congenital talipes equinovarus, presenting with scoliosis, short-limbed dwarfism, clubfoot, joint dislocation and other unique skeletal abnormalities. Several mechanisms of FLNB mutations causing skeletal malformations have been proposed, including delay of ossification in long bone growth plate, reduction of bone mineral density (BMD), dysregulation of muscle differentiation, ossification of intervertebral disc (IVD), disturbance of proliferation, differentiation and apoptosis in chondrocytes, impairment of angiogenesis, and hypomotility of osteoblast, chondrocyte and fibroblast. Interventions on FLNB-related diseases require prenatal surveillance by sonography, gene testing in high-risk carriers, and proper orthosis or orthopedic surgeries to correct malformations including scoliosis, cervical spine instability, large joint dislocation, and clubfoot. Gene and cell therapies for FLNB-related diseases are also promising but require further studies.

Skeletal Dysplasia Mutations Effect on Human Filamins' Structure and Mechanosensing

Cells' ability to sense mechanical cues in their environment is crucial for fundamental cellular processes, leading defects in mechanosensing to be linked to many diseases. The actin cross-linking protein Filamin has an important role in the conversion of mechanical forces into biochemical signals. Here, we reveal how mutations in Filamin genes known to cause Larsen syndrome and Frontometaphyseal dysplasia can affect the structure and therefore function of Filamin domains 16 and 17. Employing X-ray crystallography, the structure of these domains was first solved for the human Filamin B. The interaction seen between domains 16 and 17 is broken by shear force as revealed by steered molecular dynamics simulations. The effects of skeletal dysplasia associated mutations of the structure and mechanosensing properties of Filamin were studied by combining various experimental and theoretical techniques. The results showed that Larsen syndrome associated mutations destabilize or even unfold domain 17. Interestingly, those Filamin functions that are mediated via domain 17 interactions with other proteins are not necessarily affected as strongly interacting peptide binding to mutated domain 17 induces at least partial domain folding. Mutation associated to Frontometaphyseal dysplasia, in turn, transforms 16-17 fragment from compact to an elongated form destroying the force-regulated domain pair.

Structural Analysis of G1691S Variant in the Human Filamin B Gene Responsible for Larsen Syndrome: A Comparative Computational Approach

Larsen syndrome (LRS) is a rare genetic disease associated with variable manifestations including skeletal malformations, dislocations of the large joints, and notable changes in facial and limb features. Genetic variants in the Filamin B (FLNB) gene are associated with the development of LRS. We searched two literature databases (OMIM and PubMed) and three gene variant databases (HGMD, UniProt, & dbSNP) to capture all the possible variants associated with LRS phenotype, which may have an impact on the FLNB function. Our search yielded 77 variants that might impact the FLNB protein function in patients with LRS. We performed rigorous computational analysis such as conservational, biochemical, pathogenicity, and structural computational analyses to understand the deleterious effect of the G1691S variant. Further, the structural changes of the G1691S variant was compared with a null variant (G1691A) and the native protein through a molecular dynamic simulation study of 50 ns. We found that the variant G1691S was highly deleterious and destabilize the protein when compared to the native and variant G1691A. This might be due to the physicochemical changes in the variant G1691S when compared to the native and variant G1691A. The destabilization was further supported by transformation of bend to coil in variant G1691S whereas bend was retained in native and variant G1691A through molecular dynamics analysis. Our study shed light on the importance of computational methods to understand the molecular nature of genetic variants and structural insights on the function of the FLNB protein. J. Cell. Biochem. 118: 1900-1910, 2017. © 2017 Wiley Periodicals, Inc.

Filamin B Loss-of-Function Mutation in Dimerization Domain Causes Autosomal-Recessive Spondylocarpotarsal Synostosis Syndrome with Rib Anomalies

Spondylocarpotarsal synostosis syndrome (SCT) is a distinct group of disorders characterized by short stature, disrupted vertebral segmentation with vertebral fusion, scoliosis, lordosis, carpal/tarsal synostosis, and lack of rib anomalies. Mutations in filamin B (FLNB) and MYH3 have been reported for autosomal-recessive and autosomal-dominant SCT, respectively. We present a family with two patients suffering from autosomal-recessive SCT with rib anomalies, including malalignment, crowding, and uneven size and shape of ribs. Whole-exome sequencing revealed a novel p.S2542Lfs^* 82 (c.7621dup) frameshift mutation in FLNB. This frameshift mutation lies in the C-terminal-most domain involved in FLNB dimerization and resulted in a 20-residue elongation, with complete familial segregation and absence in 376 normal controls. The mutant p.S2542Lfs^* 82 FLNB demonstrated a complete loss of ability to form a functional dimer in transiently transfected HEK293T cells. The p.S2542Lfs^* 82 mutation also led to significantly reduced protein levels and accumulation of the mutant protein in the Golgi apparatus. This is the first identified mutation in the dimerization domain of FLNB. This loss-of-function frameshift mutation in FLNB causes autosomal-recessive SCT with rarely reported rib anomalies. This report demonstrates the involvement of rib anomaly in SCT and its causative mutation in the dimerization domain of FLNB.

Filamin C is a highly dynamic protein associated with fast repair of myofibrillar microdamage

Filamin c (FLNc) is a large dimeric actin-binding protein located at premyofibrils, myofibrillar Z-discs and myofibrillar attachment sites of striated muscle cells, where it is involved in mechanical stabilization, mechanosensation and intracellular signaling. Mutations in the gene encoding FLNc give rise to skeletal muscle diseases and cardiomyopathies. Here, we demonstrate by fluorescence recovery after photobleaching that a large fraction of FLNc is highly mobile in cultured neonatal mouse cardiomyocytes and in cardiac and skeletal muscles of live transgenic zebrafish embryos. Analysis of cardiomyocytes from Xirp1 and Xirp2 deficient animals indicates that both Xin actin-binding repeat-containing proteins stabilize FLNc selectively in premyofibrils. Using a novel assay to analyze myofibrillar microdamage and subsequent repair in cultured contracting cardiomyocytes by live cell imaging, we demonstrate that repair of damaged myofibrils is achieved within only 4 h, even in the absence of de novo protein synthesis. FLNc is immediately recruited to these sarcomeric lesions together with its binding partner aciculin and precedes detectable assembly of filamentous actin and recruitment of other myofibrillar proteins. These data disclose an unprecedented degree of flexibility of the almost crystalline contractile machinery and imply FLNc as a dynamic signaling hub, rather than a primarily structural protein. Our myofibrillar damage/repair model illustrates how (cardio)myocytes are kept functional in their mechanically and metabolically strained environment. Our results help to better understand the pathomechanisms and pathophysiology of early stages of FLNc-related myofibrillar myopathy and skeletal and cardiac diseases preceding pathological protein aggregation.

Phenotype and genotype in patients with Larsen syndrome: clinical homogeneity and allelic heterogeneity in seven patients

Background

Larsen syndrome is an autosomal dominant skeletal dysplasia characterized by large joint dislocations and craniofacial dysmorphism. It is caused by missense or small in-frame deletions in the FLNB gene. To further characterize the phenotype and the mutation spectrum of this condition, we investigated seven probands, five sporadic individuals and a mother-son-duo with Larsen syndrome.

Methods

The seven patients from six unrelated families were clinically and radiologically evaluated. All patients were screened for mutations in selected exons and exon-intron boundaries of the FLNB gene by Sanger sequencing. FLNB transcript analysis was carried out in one patient to analyse the effect of the sequence variant on pre-mRNA splicing.

Results

All patients exhibited typical facial features and joint dislocations. Contrary to the widely described advanced carpal ossification, we noted delay in two patients. We identified the five novel mutations c.4927G > A/p.(Gly1643Ser), c.4876G > T / p.(Gly1626Trp), c.4664G > A / p.(Gly1555Asp), c.2055G > C / p.Gln685delins10 and c.5021C > T / p.(Ala1674Val) as well as a frequently observed mutation in Larsen syndrome [c.5164G > A/p.(Gly1722Ser)] in the hotspot regions. FLNB transcript analysis of the c.2055G > C variant revealed insertion of 27 bp intronic sequence between exon 13 and 14 which gives rise to in-frame deletion of glutamine 685 and insertion of ten novel amino acid residues (p.Gln685delins10).

Conclusions

All seven individuals with Larsen syndrome had a uniform clinical phenotype except for delayed carpal ossification in two of them. Our study reveals five novel FLNB mutations and confirms immunoglobulin-like (Ig) repeats 14 and 15 as major hotspot regions. The p.Gln685delins10 mutation is the first Larsen syndrome-associated alteration located in Ig repeat 5. All mutations reported so far leave the filamin B protein intact in accordance with a gain-of-function effect. Our findings underscore the characteristic clinical picture of FLNB-associated Larsen syndrome and add Ig repeat 5 to the filamin B domains affected by the clustered mutations.

F-actin clustering and cell dysmotility induced by the pathological W148R missense mutation of filamin B at the actin-binding domain

Filamin B (FLNB) is a dimeric actin-binding protein that orchestrates the reorganization of the actin cytoskeleton. Congenital mutations of FLNB at the actin-binding domain (ABD) are known to cause abnormalities of skeletal development, such as atelosteogenesis types I and III and Larsen's syndrome, although the underlying mechanisms are poorly understood. Here, using fluorescence microscopy, we characterized the reorganization of the actin cytoskeleton in cells expressing each of six pathological FLNB mutants that have been linked to skeletal abnormalities. The subfractionation assay showed a greater accumulation of the FLNB ABD mutants W148R and E227K than the wild-type protein to the cytoskeleton. Ectopic expression of FLNB-W148R and, to a lesser extent, FLNB-E227K induced prominent F-actin accumulations and the consequent rearrangement of focal adhesions, myosin II, and septin filaments and results in a delayed directional migration of the cells. The W148R protein-induced cytoskeletal rearrangement was partially attenuated by the inhibition of myosin II, p21-activated protein kinase, or Rho-associated protein kinase. The expression of a single-head ABD fragment with the mutations partially mimicked the rearrangement induced by the dimer. The F-actin clustering through the interaction with the mutant FLNB ABD may limit the cytoskeletal reorganization, preventing normal skeletal development.

The E3 ubiquitin ligase Asb2β is downregulated in a mouse model of hypertrophic cardiomyopathy and targets desmin for proteasomal degradation

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is an autosomal-dominant disease with mutations in genes encoding sarcomeric proteins. Previous findings suggest deregulation of the ubiquitin proteasome system (UPS) in HCM in humans and in a mouse model of HCM (Mybpc3-targeted knock-in (KI) mice). In this study we investigated transcript levels of several muscle-specific E3 ubiquitin ligases in KI mice and aimed at identifying novel protein targets.

METHODS AND RESULTS

Out of 9 muscle-specific E3 ligases, Asb2β was found with the lowest mRNA level in KI compared to wild-type (WT) mice. After adenoviral-mediated Asb2β transduction of WT neonatal mouse cardiomyocytes with either a WT or inactive Asb2β mutant, desmin was identified as a new target of Asb2β by mass spectrometry, co-immunoprecipitation and immunoblotting. Immunofluorescence analysis revealed a co-localization of desmin with Asb2β at the Z-disk of the sarcomere. Knock-down of Asb2β in cardiomyocytes resulted in higher desmin protein levels. Furthermore, desmin levels were higher in ventricular samples of HCM mice and patients than controls.

CONCLUSIONS

This study identifies desmin as a new Asb2β target for proteasomal degradation in cardiomyocytes and suggests that accumulation of desmin could contribute to UPS impairment in HCM mice and patients.

[Larsen-syndrome: final diagnosis following multiple surgical interventions]

Larsen-syndrome is a rare genetic skeletal dysplasia belonging to the group of actin-binding filamin B associated diseases. The features include congenital dislocations of the large joints, scoliosis and cervical kyphosis, short, broad, spatulate distal phalanges, and distinctive craniofacies. Diagnosis is based on clinical and radiographic findings and confirmed by molecular genetic testing. The authors have performed filamin B molecular genetic analysis since 2005 and have found several cases with unusual phenotypes since. This case report presents the diagnostic difficulties of a 30-year-old woman, who was operated several times with congenital hip dislocations and foot deformities. The craniofacial features, short, broad, spatulate fingers, scoliosis and cervical kyphosis directed diagnosis towards Larsen-syndrome and molecular genetic analysis confirmed a previously-described heterozygous missense mutation (c.G679A). They conclude that genetic analysis performed in time would prevent additional superfluous long diagnostic procedures in patients with rare diseases and would ensure adequate supportive therapy and management of the symptoms.