Recessive mutations in the gene encoding frizzled 6 cause twenty nail dystrophy--expanding the differential diagnosis for pachyonychia congenita

International Union of Basic and Clinical Pharmacology CXV: The Class F of G Protein-Coupled Receptors

The class F of G protein-coupled receptors (GPCRs) consists of 10 Frizzleds (FZD1-10) and Smoothened (SMO). FZDs bind and are activated by secreted lipoglycoproteins of the Wingless/Int-1 (WNT) family, and SMO is indirectly activated by the Hedgehog (Hh) family of morphogens acting on the transmembrane protein Patched. The advance of our understanding of FZDs and SMO as dynamic transmembrane receptors and molecular machines, which emerged during the past 14 years since the first-class F GPCR IUPHAR nomenclature report, justifies an update. This article focuses on the advances in molecular pharmacology and structural biology providing new mechanistic insight into ligand recognition, receptor activation mechanisms, signal initiation, and signal specification. Furthermore, class F GPCRs continue to develop as drug targets, and novel technologies and tools such as genetically encoded biosensors and CRISP/Cas9 edited cell systems have contributed to refined functional analysis of these receptors. Also, advances in crystal structure analysis and cryogenic electron microscopy contribute to the rapid development of our knowledge about structure-function relationships, providing a great starting point for drug development. Despite the progress, questions and challenges remain to fully understand the complexity of the WNT/FZD and Hh/SMO signaling systems. SIGNIFICANCE STATEMENT: The recent years of research have brought about substantial functional and structural insight into mechanisms of activation of Frizzleds and Smoothened. While the advance furthers our mechanistic understanding of ligand recognition, receptor activation, signal specification, and initiation, broader opportunities emerge that allow targeting class F GPCRs for therapy and regenerative medicine employing both biologics and small molecule compounds.

A novel pathogenic variant in the FZD6 gene causes recessive nail dysplasia in a Moroccan family

This report describes a patient with a novel homozygous nonsense variant, nail dysplasia and diabetes. Only a few patients have been described with FZD6 mutations and nail dysplasia, and the spectrum of associated manifestations has not been identified. Indeed, it is questionable whether there is a link between pancreatic involvement and the genetic abnormality identified.

The role of GPCRs in bone diseases and dysfunctions

The superfamily of G protein-coupled receptors (GPCRs) contains immense structural and functional diversity and mediates a myriad of biological processes upon activation by various extracellular signals. Critical roles of GPCRs have been established in bone development, remodeling, and disease. Multiple human GPCR mutations impair bone development or metabolism, resulting in osteopathologies. Here we summarize the disease phenotypes and dysfunctions caused by GPCR gene mutations in humans as well as by deletion in animals. To date, 92 receptors (5 glutamate family, 67 rhodopsin family, 5 adhesion, 4 frizzled/taste2 family, 5 secretin family, and 6 other 7TM receptors) have been associated with bone diseases and dysfunctions (36 in humans and 72 in animals). By analyzing data from these 92 GPCRs, we found that mutation or deletion of different individual GPCRs could induce similar bone diseases or dysfunctions, and the same individual GPCR mutation or deletion could induce different bone diseases or dysfunctions in different populations or animal models. Data from human diseases or dysfunctions identified 19 genes whose mutation was associated with human BMD: 9 genes each for human height and osteoporosis; 4 genes each for human osteoarthritis (OA) and fracture risk; and 2 genes each for adolescent idiopathic scoliosis (AIS), periodontitis, osteosarcoma growth, and tooth development. Reports from gene knockout animals found 40 GPCRs whose deficiency reduced bone mass, while deficiency of 22 GPCRs increased bone mass and BMD; deficiency of 8 GPCRs reduced body length, while 5 mice had reduced femur size upon GPCR deletion. Furthermore, deficiency in 6 GPCRs induced osteoporosis; 4 induced osteoarthritis; 3 delayed fracture healing; 3 reduced arthritis severity; and reduced bone strength, increased bone strength, and increased cortical thickness were each observed in 2 GPCR-deficiency models. The ever-expanding number of GPCR mutation-associated diseases warrants accelerated molecular analysis, population studies, and investigation of phenotype correlation with SNPs to elucidate GPCR function in human diseases.

A possible founder mutation in FZD6 gene in a Turkish family with autosomal recessive nail dysplasia

BACKGROUND

Autosomal recessive nail dysplasia is characterized by thick and hard nails with a very slow growth on the hands and feet. Mutations in FZD6 gene were found to be associated with autosomal recessive nail dysplasia in 2011. Presently, only seven mutations have been reported in FZD6 gene; five mutations are clustered in the C-terminus, one is at the seventh transmembrane domain, and another is at the very beginning of third extracellular loop.

METHODS

Whole exome sequencing (WES) was applied to the index case, her one affected sister and her healthy consanguineous parents. The mutation was verified via Sanger sequencing. Molecular dynamics simulations of the predicted structures of native and mutant proteins were compared to gain insight into the pathogenicity mechanism of the mutation.

RESULTS

Here, we report a homozygous 8 bp deletion mutation, p.Gly559Aspfs*16; c.1676_1683delGAACCAGC, in FZD6 gene which causes a frameshift and creates a premature stop codon at position 16 of the new reading frame. Our molecular dynamics calculations predict that the pathogenicity of this frameshift mutation may be caused by the change in entropy of the protein with negative manner, disturbing the C-terminal domain structure, and hence interaction partners of FZD6.

CONCLUSION

We identified a homozygous deletion mutation in FZD6 in a consanguineous Turkish family with nail dysplasia. We also provide a molecular mechanism about the effects of the deletion on the protein structure and its possible motions. This study provides a pathogenicity mechanism for this mutation in nail dysplasia for the first time.

A novel pathogenic variant in the FZD6 gene causes recessive nail dysplasia in a large Iranian kindred

BACKGROUND

Nail disorder nonsyndromic congenital (NDNC) is a very rare clinically and genetically heterogeneous disease inherited both in recessive or dominant modes. FZD6 is a component of Wnt-FZD signaling pathway in which recessive loss-of-function variants in the corresponding genes could lead to nail anomalies.

OBJECTIVE

A large multiplex family with NDNC was referred for genetic counselling. Thorough genetic evaluation was performed.

METHODS

PCR-Sanger sequencing was carried out for the coding exons and exon-intron boundaries of the FZD6 gene. Co-segregation analysis, in silico evaluation and computational protein modeling was accomplished.

RESULTS

A homozygous 1bp deletion variant, c.1859delC (p.Ser620Cysfs*75), leading to a truncating protein was found in the patient. Parents were heterozygous for the variant. The variant was found to be co-segreagting with the phenotype in the family. Computational analysis and protein modeling revealed its pathogenic consequence by disturbing the cytoplasmic domain structure and signaling through loss of phosphorylation residues. The variant met the criteria of being pathogenic according to the ACMG guideline.

CONCLUSIONS

This is the first report of the genetic diagnosis of NDNC in Iran. We also report a novel pathogenic variant. The study of the FZD6 gene is recommended as the first step in the diagnostic routing of the autosomal recessive NDNC patients with enlarged nails.

Whole-exome sequencing on deceased fetuses with ultrasound anomalies: expanding our knowledge of genetic disease during fetal development

PurposeThe aim of this study was to determine the diagnostic yield of whole-exome sequencing (WES) in fetuses with ultrasound anomalies that resulted in fetal demise or pregnancy termination. The results were also utilized to aid in the identification of candidate genes for fetal development and to expand the clinical phenotype of known genetic conditions.MethodsWES was performed on specimens from 84 deceased fetuses. Data were analyzed and final results were classified into one of four categories: positive, possible, negative, and candidate gene only. WES analysis was predominantly performed in fetus-parent trios or quads (61%, n=52).ResultsOverall, 20% (n = 17) of cases were positive, 45% (n=38) were possible, 9% (n=7) had only candidate gene variants and 26% (n = 22) tested negative. The diagnostic yield for definitive findings for trio analysis was 24% (n = 11) compared to 14% (n = 4) for singletons. The most frequently reported ultrasound anomalies were central nervous system (37%, n = 31), hydrops/edema (36%, n = 30), and cardiovascular anomalies (31%, n = 26).ConclusionOur experience supports the use of WES to identify the molecular etiology of fetal ultrasound anomalies, to identify candidate genes involved in fetal development, and to expand our knowledge of the clinical phenotype of known genetic conditions.

Planar cell polarity genes frizzled4 and frizzled6 exert patterning influence on arterial vessel morphogenesis

Quantitative analysis of the vascular network anatomy is critical for the understanding of the vasculature structure and function. In this study, we have combined microcomputed tomography (microCT) and computational analysis to provide quantitative three-dimensional geometrical and topological characterization of the normal kidney vasculature, and to investigate how 2 core genes of the Wnt/planar cell polarity, Frizzled4 and Frizzled6, affect vascular network morphogenesis. Experiments were performed on frizzled4 (Fzd4^-/-) and frizzled6 (Fzd6^-/-) deleted mice and littermate controls (WT) perfused with a contrast medium after euthanasia and exsanguination. The kidneys were scanned with a high-resolution (16 μm) microCT imaging system, followed by 3D reconstruction of the arterial vasculature. Computational treatment includes decomposition of 3D networks based on Diameter-Defined Strahler Order (DDSO). We have calculated quantitative (i) Global scale parameters, such as the volume of the vasculature and its fractal dimension (ii) Structural parameters depending on the DDSO hierarchical levels such as hierarchical ordering, diameter, length and branching angles of the vessel segments, and (iii) Functional parameters such as estimated resistance to blood flow alongside the vascular tree and average density of terminal arterioles. In normal kidneys, fractal dimension was 2.07±0.11 (n = 7), and was significantly lower in Fzd4^-/- (1.71±0.04; n = 4), and Fzd6^-/- (1.54±0.09; n = 3) kidneys. The DDSO number was 5 in WT and Fzd4^-/-, and only 4 in Fzd6^-/-. Scaling characteristics such as diameter and length of vessel segments were altered in mutants, whereas bifurcation angles were not different from WT. Fzd4 and Fzd6 deletion increased vessel resistance, calculated using the Hagen-Poiseuille equation, for each DDSO, and decreased the density and the homogeneity of the distal vessel segments. Our results show that our methodology is suitable for 3D quantitative characterization of vascular networks, and that Fzd4 and Fzd6 genes have a deep patterning effect on arterial vessel morphogenesis that may determine its functional efficiency.

Isolated recessive nail dysplasia caused by FZD6 mutations: report of three families and review of the literature

Congenital abnormalities of the nail are rare conditions that are most frequently associated with congenital ectodermal syndromes involving several of the epidermal appendages including the skin, teeth, hair and nails. Isolated recessive nail dysplasia (IRND) is much rarer but has recently been recognized as a condition resulting in 20‐nail dystrophy in the absence of other cutaneous or extracutaneous findings. A few case reports have identified mutations in the Frizzled 6 (FZD6) gene in families presenting with abnormal nails consistent with IRND. These reports have highlighted the role of Wnt–FZD signalling in the process of nail formation. We report three families presenting with features of IRND, in whom we identified mutations in FZD6, including one previously unreported mutation.

Frizzled Receptors in Development and Disease

Frizzled proteins are the principal receptors for the Wnt family of ligands. They mediate canonical Wnt signaling together with Lrp5 and Lrp6 coreceptors. In conjunction with Celsr, Vangl, and a small number of additional membrane and membrane-associated proteins, they also play a central role in tissue polarity/planar cell polarity (PCP) signaling. Targeted mutations in 9 of the 10 mammalian Frizzled genes have revealed their roles in an extraordinarily diverse set of developmental and homeostatic processes, including morphogenetic movements responsible for palate, ventricular septum, ocular furrow, and neural tube closure; survival of thalamic neurons; bone formation; central nervous system (CNS) angiogenesis and blood-brain barrier formation and maintenance; and a wide variety of processes that orient subcellular, cellular, and multicellular structures relative to the body axes. The last group likely reflects the mammalian equivalent of tissue polarity/PCP signaling, as defined in Drosophila, and it includes CNS axon guidance, hair follicle and tongue papilla orientation, and inner ear sensory hair bundle orientation. Frizzled receptors are ubiquitous among multicellular animals and, with other signaling molecules, they very likely evolved to permit the development of the complex tissue architectures that provide multicellular animals with their enormous selective advantage.

Genetics of human isolated hereditary nail disorders

Human hereditary nail disorders constitute a rare and heterogeneous group of ectodermal dysplasias. They occur as isolated and/or syndromic ectodermal conditions where other ectodermal appendages are also involved, and can occur associated with skeletal dysplasia. 'Nail disorder, nonsyndromic congenital' (OMIM; Online Mendelian Inheritance in Man) is subclassified into 10 different types. The underlying genes identified thus far are expressed in the nail bed and play important roles in nail development and morphogenesis. Here, we review the current literature on nail disorders and present a coherent review on the genetics of nail disorders. This review will pave the way to identifying putative genes and pathways involved in nail development and morphogenesis.

Exploring the biology of the nail: An intriguing but less-investigated skin appendage

The nail is a highly keratinized structure covering the tip of the digit, and considered to have several important functions in our daily life. In recent years, as biological aspects of the nail organ have been characterized, we realize that the nail unit and the hair follicle share various biological and immunological features. In particular, development and homeostasis of the nail unit also requires intimate epithelial-mesenchymal interactions that involve signaling pathways such as Wnt. There is also a striking immunological resemblance between both appendages, since the nail matrix, like the anagen hair bulb and the bulge, was shown to present unique characteristics of an immune privileged site. On the other hand, considerable progress in identifying nail stem cells has succeeded in locating putative stem cell niches in the nail unit. In this context, it is intriguing that nail stem cells residing in the nail matrix were recently shown to possess the ability to organize the process leading to digit regeneration. Further elucidation of signaling pathways governing epithelial-mesenchymal interactions in the nail unit seems to be a key to develop a novel therapeutic tool to treat amputees using nail epithelium. However, it is at least certain that the nail unit has a promising potential for the future of regenerative medicine. This review explores the biology of the nail organ by focusing on intriguing knowledge gained from recent studies.

The molecular genetic analysis of the expanding pachyonychia congenita case collection

BACKGROUND

Pachyonychia congenita (PC) is a rare autosomal dominant keratinizing disorder characterized by severe, painful, palmoplantar keratoderma and nail dystrophy, often accompanied by oral leucokeratosis, cysts and follicular keratosis. It is caused by mutations in one of five keratin genes: KRT6A, KRT6B, KRT6C, KRT16 or KRT17.

OBJECTIVES

To identify mutations in 84 new families with a clinical diagnosis of PC, recruited by the International Pachyonychia Congenita Research Registry during the last few years.

METHODS

Genomic DNA isolated from saliva or peripheral blood leucocytes was amplified using primers specific for the PC-associated keratin genes and polymerase chain reaction products were directly sequenced.

RESULTS

Mutations were identified in 84 families in the PC-associated keratin genes, comprising 46 distinct keratin mutations. Fourteen were previously unreported mutations, bringing the total number of different keratin mutations associated with PC to 105.

CONCLUSIONS

By identifying mutations in KRT6A, KRT6B, KRT6C, KRT16 or KRT17, this study has confirmed, at the molecular level, the clinical diagnosis of PC in these families.

Report of the 10th Annual International Pachyonychia Congenita Consortium Meeting

The International Pachyonychia Congenita Consortium (IPCC) was founded in 2004 in Park City, Utah, USA. Its goal is to find a cure for pachyonychia congenita, a rare keratinizing disorder. From February 14th–17th, 2013, the group convened in Park City for their tenth annual meeting. The 2013 meeting focused on how to best move forward with clinical trials and on learning from work in other scientific areas, with an emphasis on understanding mechanisms of pain and hyperkeratosis. Considerable time was spent on discussing the best way to move forward with development of new treatments and how to obtain or develop tools that can measure treatment outcomes in PC.