AMACO is a component of the basement membrane-associated Fraser complex

The Fraser Complex Proteins (Frem1, Frem2, and Fras1) Can Form Anchoring Cords in the Absence of AMACO at the Dermal–Epidermal Junction of Mouse Skin

AMACO (VWA2 protein), secreted by epithelial cells, is strongly expressed at basement membranes when budding or invagination occurs in embryos. In skin, AMACO associates with proteins of the Fraser complex, which form anchoring cords. These, during development, temporally stabilize the dermal–epidermal junction, pending the formation of collagen VII-containing anchoring fibrils. Fraser syndrome in humans results if any of the core members of the Fraser complex (Fras1, Frem1, Frem2) are mutated. Fraser syndrome is characterized by subepidermal blistering, cryptophthalmos, and syndactyly. In an attempt to determine AMACO function, we generated and characterized AMACO-deficient mice. In contrast to Fraser complex mutant mice, AMACO-deficient animals lack an obvious phenotype. The mutually interdependent basement membrane deposition of the Fraser complex proteins, and the formation of anchoring cords, are not affected. Furthermore, hair follicle development in newborn AMACO-deficient mice showed no gross aberration. Surprisingly, it appears that, while AMACO is a component of the anchoring cords, it is not essential for their formation or function.

Cryptophthalmos, dental anomalies, oral vestibule defect, and a novel FREM2 mutation

FREM2 is a member of the FREM2-FRAS1-FREM1 protein complex which contributes to epithelial-mesenchymal coupling. We report a Thai woman with cryptophthalmos, dental anomalies, and oral vestibule defect. A compound heterozygous mutation (c.6499C>T; p.Arg2167Trp and c.641_642del; p.Glu214GlyfsTer135) in the FREM2 gene was identified. The frameshift variant p.Glu214GlyfsTer135 is de novo and novel. It is predicted to result in the loss of most of the functional domains. The p.Arg2167Trp mutation was predicted to disrupt both Ca²⁺ binding and conformational change. The Arg2167Trp mutant protein has been shown to cause partial loss of function, decrease its interaction with FREM1 and result in impaired function of the FRAS1-FREM2-FREM1 complex. Frem2 was shown to be expressed in the developing tooth and vestibular lamina. It is hypothesized that these mutations resulted in aberration of the FRAS1-FREM2-FREM1 protein complex, resulting in loss of nephronectin, basement membrane disruption, and abnormal epithelial-mesenchymal interactions leading to dental and oral vestibule malformations.

Contribution of homozygous and compound heterozygous missense mutations in VWA2 to Alzheimer's disease

Alzheimer's disease is the most frequent diagnosis of neurodegenerative dementia with early (≤65 years) and late (>65 years) onset ages in familial and sporadic patients. Causal mutations in 3 autosomal dominant Alzheimer genes, i.e. amyloid precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2), explain only 5%-10% of early-onset patients leaving the majority of patients genetically unresolved. To discover potential missing genetics, we used whole genome sequencing data of 17 early-onset patients with well-documented clinical diagnosis of Alzheimer's disease. In the discovery group, the mean onset age was 55.71 ± 6.83 years (range 37-65). Six patients had a brain autopsy and neuropathology confirmed Alzheimer's disease. Analysis of the genetic data identified in one patient a homozygous p.V366M missense mutation in the Von Willebrand factor A domain containing 2 gene (VWA2). Resequencing of the VWA2 coding region in an Alzheimer's disease patient cohort from Flanders-Belgium (n = 1148), including 152 early and 996 late onset patients, identified additional homozygous and compound heterozygous missense mutations in 1 early and 3 late-onset patients. Allele-sharing analysis identified common haplotypes among the compound heterozygous VWA2 mutation carriers, suggesting shared ancestors. Overall, we identified 5 patient carriers of homozygous or compound heterozygous missense mutations (5/1165; 0.43 %), 2 in early (2/169; 1.18 %) and 3 in late-onset (3/996; 0.30 %) patients. The frequencies of the homozygous and compound heterozygous missense mutations in patients are higher than expected from the frequencies calculated based on their combined single alleles. None of the homozygous/compound heterozygous missense mutation carriers had a family history of autosomal dominant Alzheimer's disease. Our findings suggest that homozygous and compound heterozygous missense mutations in VWA2 might contribute to the risk of Alzheimer's disease in sporadic patients.

Fraser syndrome: review of the literature illustrated by a historical adult case

Fraser syndrome (cryptophthalmos-syndactyly syndrome) is a rare autosomal recessive malformation disorder. The first description of the syndrome was reported by George Fraser in 1962. Diagnosis is based on the major and minor criteria established by van Haelst et al. in 2007. Unilateral or bilateral cryptophthalmos, syndactyly, unilateral renal agenesis, and genital anomalies are the most frequent anomalies. Several maxillofacial, oro-dental, ear-nose-throat, hormonal, and anorectal disorders are reported. Cardiac malformations and musculoskeletal anomalies are uncommon. The syndrome is related to mutations in three different genes (FRAS1, FREM2, and GRIP1) resulting in failure of the apoptosis program and disruption of the epithelial-mesenchymal interactions during embryonic development. Prenatal diagnosis is based on the detection of renal agenesis and laryngeal atresia, together with a family history. Most foetuses with severe anomalies are terminated or are stillborn. All patients or pregnancies with a diagnosis of Fraser syndrome should be referred to expert centres. A collaborative approach including anaesthetists, ENT specialists, maxillofacial surgeons, and geneticists is necessary for the management of this syndrome. In vivo and in vitro research models are available to better understand the underlying aetiology.

Epigenetic and transcriptional dysregulation of VWA2 associated with a MYC-driven oncogenic program in colorectal cancer

VWA2 encodes AMACO, a secreted protein up-regulated in most colorectal carcinomas (CRC), constituting a promising biomarker. The mechanism responsible for its aberrant up-regulation has not been previously described. In this work, we analyzed VWA2 DNA methylation in over 400 primary CRCs. No epigenetic alterations were found in its promoter-associated CpG island. However, the region located downstream of the transcriptional start site was hypomethylated in most CRCs. ChIP-Seq revealed increased levels of the active mark H3K4me3 and reduction of the repressive mark H3K27me3. In contrast, several CRC cell lines exhibited hypermethylation of VWA2. 5-AZA-2-deoxycitidine treatment led to transcriptional activation of VWA2, supporting a functional link between DNA methylation and transcription. VWA2 expression in primary CRCs correlated with that of Myc and Myc-target genes. Transcriptional up-regulation of VWA2 is extremely frequent (78%) and strong (average fold change >15) in CRC, but not in other types of cancer. VWA2 undergoes hypomethylation in the majority of CRCs. This alteration could partly underlie the previously reported over-expression of AMACO. Co-expression profiling suggests that VWA2 might be a constituent of a larger oncogenic transcriptional program regulated by c-Myc. Up-regulation of VWA2 is virtually exclusive of CRC, reinforcing its potential as a specific biomarker.

Gene profile of zebrafish fin regeneration offers clues to kinetics, organization and biomechanics of basement membrane

How some animals regenerate missing body parts is not well understood. Taking advantage of the zebrafish caudal fin model, we performed a global unbiased time-course transcriptomic analysis of fin regeneration. Biostatistics analyses identified extracellular matrix (ECM) as the most enriched gene sets. Basement membranes (BMs) are specialized ECM structures that provide tissues with structural cohesion and serve as a major extracellular signaling platform. While the embryonic formation of BM has been extensively investigated, its regeneration in adults remains poorly studied. We therefore focused on BM gene expression kinetics and showed that it recapitulates many aspects of development. As such, the re-expression of the embryonic col14a1a gene indicated that col14a1a is part of the regeneration-specific program. We showed that laminins and col14a1a genes display similar kinetics and that the corresponding proteins are spatially and temporally controlled during regeneration. Analysis of our CRISPR/Cas9-mediated col14a1a knockout fish showed that collagen XIV-A contributes to timely deposition of laminins. As changes in ECM organization can affect tissue mechanical properties, we analyzed the biomechanics of col14a1a^-/- regenerative BM using atomic force microscopy (AFM). Our data revealed a thinner BM accompanied by a substantial increase of the stiffness when compared to controls. Further AFM 3D-reconstructions showed that BM is organized as a checkerboard made of alternation of soft and rigid regions that is compromised in mutants leading to a more compact structure. We conclude that collagen XIV-A transiently acts as a molecular spacer responsible for BM structure and biomechanics possibly by helping laminins integration within regenerative BM.

A homozygous missense variant in VWA2, encoding an interactor of the Fraser-complex, in a patient with vesicoureteral reflux

Congenital anomalies of the kidney and urinary tract (CAKUT) are the most common cause (40-50%) of chronic kidney disease (CKD) in children. About 40 monogenic causes of CAKUT have so far been discovered. To date less than 20% of CAKUT cases can be explained by mutations in these 40 genes. To identify additional monogenic causes of CAKUT, we performed whole exome sequencing (WES) and homozygosity mapping (HM) in a patient with CAKUT from Indian origin and consanguineous descent. We identified a homozygous missense mutation (c.1336C>T, p.Arg446Cys) in the gene Von Willebrand factor A domain containing 2 (VWA2). With immunohistochemistry studies on kidneys of newborn (P1) mice, we show that Vwa2 and Fraser extracellular matrix complex subunit 1 (Fras1) co-localize in the nephrogenic zone of the renal cortex. We identified a pronounced expression of Vwa2 in the basement membrane of the ureteric bud (UB) and derivatives of the metanephric mesenchyme (MM). By applying in vitro assays, we demonstrate that the Arg446Cys mutation decreases translocation of monomeric VWA2 protein and increases translocation of aggregated VWA2 protein into the extracellular space. This is potentially due to the additional, unpaired cysteine residue in the mutated protein that is used for intermolecular disulfide bond formation. VWA2 is a known, direct interactor of FRAS1 of the Fraser-Complex (FC). FC-encoding genes and interacting proteins have previously been implicated in the pathogenesis of syndromic and/or isolated CAKUT phenotypes in humans. VWA2 therefore constitutes a very strong candidate in the search for novel CAKUT-causing genes. Our results from in vitro experiments indicate a dose-dependent neomorphic effect of the Arg446Cys homozygous mutation in VWA2.

Pharyngeal morphogenesis requires fras1-itga8-dependent epithelial-mesenchymal interaction

Both Fras1 and Itga8 connect mesenchymal cells to epithelia by way of an extracellular 'Fraser protein complex' that functions in signaling and adhesion; these proteins are vital to the development of several vertebrate organs. We previously found that zebrafish fras1 mutants have craniofacial defects, specifically, shortened symplectic cartilages and cartilage fusions that spare joint elements. During a forward mutagenesis screen, we identified a new zebrafish mutation, b1161, that we show here disrupts itga8, as confirmed using CRISPR-generated itga8 alleles. fras1 and itga8 single mutants and double mutants have similar craniofacial phenotypes, a result expected if loss of either gene disrupts function of the Fraser protein complex. Unlike fras1 mutants or other Fraser-related mutants, itga8 mutants do not show blistered tail fins. Thus, the function of the Fraser complex differs in the craniofacial skeleton and the tail fin. Focusing on the face, we find that itga8 mutants consistently show defective outpocketing of a late-forming portion of the first pharyngeal pouch, and variably express skeletal defects, matching previously characterized fras1 mutant phenotypes. In itga8 and fras1 mutants, skeletal severity varies markedly between sides, indicating that both mutants have increased developmental instability. Whereas fras1 is expressed in epithelia, we show that itga8 is expressed complementarily in facial mesenchyme. Paired with the observed phenotypic similarity, this expression indicates that the genes function in epithelial-mesenchymal interactions. Similar interactions between Fras1 and Itga8 have previously been found in mouse kidney, where these genes both regulate Nephronectin (Npnt) protein abundance. We find that zebrafish facial tissues express both npnt and the Fraser gene fibrillin2b (fbn2b), but their transcript levels do not depend on fras1 or itga8 function. Using a revertible fras1 allele, we find that the critical window for fras1 function in the craniofacial skeleton is between 1.5 and 3 days post fertilization, which coincides with the onset of fras1-dependent and itga8-dependent morphogenesis. We propose a model wherein Fras1 and Itga8 interact during late pharyngeal pouch morphogenesis to sculpt pharyngeal arches through epithelial-mesenchymal interactions, thereby stabilizing the developing craniofacial skeleton.

Epidermal Basement Membrane in Health and Disease

Skin, as the organ protecting the individual from environmental aggressions, constantly meets external insults and is dependent on mechanical toughness for its preserved function. Accordingly, the epidermal basement membrane (BM) zone has adapted to enforce tissue integrity. It harbors anchoring structures created through unique organization of common BM components and expression of proteins exclusive to the epidermal BM zone. Evidence for the importance of its correct assembly and the nonredundancy of its components for skin integrity is apparent from the multiple skin blistering disorders caused by mutations in genes coding for proteins associated with the epidermal BM and from autoimmune disorders in which autoantibodies target these molecules. However, it has become clear that these proteins not only provide mechanical support but are also critically involved in tissue homeostasis, repair, and regeneration. In this chapter, we provide an overview of the unique organization and components of the epidermal BM. A special focus will be given to its function during regeneration, and in inherited and acquired diseases.

A new component of the Fraser complex

In embryos, the Fraser Complex (FC) mediates epithelial-connective tissue interactions. Loss of expression of FC components leads to Fraser Syndrome (FS) in which cohesion of epithelial tissues and stroma is perturbed. Using zebrafish, Richardson et al (in this issue) identified the protein AMACO in the FC. We discuss the utility of zebrafish in determining FC functions and identifying FS targets.