The Connective Tissue Disorder Associated with Recessive Variants in the SLC39A13 Zinc Transporter Gene (Spondylo-Dysplastic Ehlers-Danlos Syndrome Type 3): Insights from Four Novel Patients and Follow-Up on Two Original Cases

Possible involvement of zinc transporter ZIP13 in myogenic differentiation

Ehlers-Danlos syndrome spondylodysplastic type 3 (EDSSPD3, OMIM 612350) is an inherited recessive connective tissue disorder that is caused by loss of function of SLC39A13/ZIP13, a zinc transporter belonging to the Slc39a/ZIP family. We previously reported that patients with EDSSPD3 harboring a homozygous loss of function mutation (c.221G > A, p.G64D) in ZIP13 exon 2 (ZIP13G64D) suffer from impaired development of bone and connective tissues, and muscular hypotonia. However, whether ZIP13 participates in the early differentiation of these cell types remains unclear. In the present study, we investigated the role of ZIP13 in myogenic differentiation using a murine myoblast cell line (C2C12) as well as patient-derived induced pluripotent stem cells (iPSCs). We found that ZIP13 gene expression was upregulated by myogenic stimulation in C2C12 cells, and its knockdown disrupted myotubular differentiation. Myocytes differentiated from iPSCs derived from patients with EDSSPD3 (EDSSPD3-iPSCs) also exhibited incomplete myogenic differentiation. Such phenotypic abnormalities of EDSSPD3-iPSC-derived myocytes were corrected by genomic editing of the pathogenic ZIP13G64D mutation. Collectively, our findings suggest the possible involvement of ZIP13 in myogenic differentiation, and that EDSSPD3-iPSCs established herein may be a promising tool to study the molecular basis underlying the clinical features caused by loss of ZIP13 function.

Systemic Associations with Keratoconus

Keratoconus is a disease of the cornea that results in progressive steepening and thinning of the cornea and subsequent vision loss. It nearly always presents as a bilateral disease, suggesting that there is an underlying abnormality of the corneas that becomes manifest with time. However, the mechanisms underlying the development of keratoconus are largely unknown. Associations reported between keratoconus and systemic diseases are abundant in the literature, and the list of possible associations is very long. We found that atopy, Down syndrome, and various connective tissue diseases were the most frequently cited associations in our broad literature search. Additionally, Diabetes Mellitus has been increasingly studied as a possible protective factor against keratoconus. In this review, we have summarized the evidence for and against these particular systemic conditions and keratoconus and have discussed some of the implications of keratoconus patients having these conditions.

Osteogenesis Imperfecta/Ehlers-Danlos Overlap Syndrome and Neuroblastoma-Case Report and Review of Literature

Osteogenesis imperfecta/Ehlers−Danlos (OI/EDS) overlap syndrome is a recently described disorder of connective tissue, characterized by mutation of COL1A1 (17q21.33) or COL1A2 (7q21.3) genes, that are involved in α-1 and α-2 chains of type 1 collagen synthesis. The clinical spectrum of this new clinical entity is broad: patients could present a mixed phenotype that includes features of both osteogenesis imperfecta (bone fragility, long bone fractures, blue sclerae, short stature) and Ehlers−Danlos syndrome (joint hyperextensibility, soft and hyperextensible skin, abnormal wound healing, easy bruising, vascular fragility). We reported the case of a young Caucasian girl with severe short stature and a previous history of neuroblastoma, who displayed the compound phenotype of OI/EDS. Next generation sequencing was applied to the proband and her parent genome. Our patient presented a de novo heterozygous COL1A1 variant (c.3235G>A, p.Gly1079Ser), whose presence might be indicative of diagnosis of OI/EDS overlap syndrome. We also hypothesize that the association with the previous history of neuroblastoma could be influenced by the presence of COL1A1 mutation, whose role has been already described in the behavior and progression of some cancers.

Animal Models of Ehlers-Danlos Syndromes: Phenotype, Pathogenesis, and Translational Potential

The Ehlers–Danlos syndromes (EDS) are a group of heritable connective tissues disorders mainly characterized by skin hyperextensibility, joint hypermobility and generalized tissue fragility. Currently, 14 EDS subtypes each with particular phenotypic features are recognized and are caused by genetic defects in 20 different genes. All of these genes are involved in the biosynthesis and/or fibrillogenesis of collagens at some level. Although great progress has been made in elucidating the molecular basis of different EDS subtypes, the pathogenic mechanisms underlying the observed phenotypes remain poorly understood, and consequentially, adequate treatment and management options for these conditions remain scarce. To date, several animal models, mainly mice and zebrafish, have been described with defects in 14 of the 20 hitherto known EDS-associated genes. These models have been instrumental in discerning the functions and roles of the corresponding proteins during development, maturation and repair and in portraying their roles during collagen biosynthesis and/or fibrillogenesis, for some even before their contribution to an EDS phenotype was elucidated. Additionally, extensive phenotypical characterization of these models has shown that they largely phenocopy their human counterparts, with recapitulation of several clinical hallmarks of the corresponding EDS subtype, including dermatological, cardiovascular, musculoskeletal and ocular features, as well as biomechanical and ultrastructural similarities in tissues. In this narrative review, we provide a comprehensive overview of animal models manifesting phenotypes that mimic EDS with a focus on engineered mouse and zebrafish models, and their relevance in past and future EDS research. Additionally, we briefly discuss domestic animals with naturally occurring EDS phenotypes. Collectively, these animal models have only started to reveal glimpses into the pathophysiological aspects associated with EDS and will undoubtably continue to play critical roles in EDS research due to their tremendous potential for pinpointing (common) signaling pathways, unveiling possible therapeutic targets and providing opportunities for preclinical therapeutic interventions.

Identification of the novel SDR42E1 gene that affects steroid biosynthesis associated with the oculocutaneous genital syndrome

Hereditary connective tissue diseases form a heterogeneous group of disorders that affect collagen and extracellular matrix components. The cornea and the skin are among the major forms of connective tissues, and syndromes affecting both organs are often due to mutations in single genes. Brittle cornea syndrome is one of the pathologies that illustrates this association well. Furthermore, sex hormones are known to play a role in the maintenance of the structure and the integrity of the connective tissue including the skin and cornea, and may be involved in pathogenesis of oculocutaneous diseases. Herein, a double consanguineous family of Moroccan origin with two affected siblings, with suspected brittle cornea syndrome, was recruited. Ophthalmic examinations and genetic testing were performed in all the nuclear family individuals. Clinical examinations showed that the two affected boys presented with thinning of the cornea, blue sclera, keratoconus, hyperelasticity of the skin, joint hypermobility, muscle weakness, hearing loss and dental abnormalities that are compatible with the diagnosis of BCS disease. They showed however additional clinical signs including micropenis, hypospadias and cryptorchidism, suggesting abnormalities in endocrine pathways. Using a duo exome sequencing analysis performed in the mother and the propositus, we identified the novel homozygous missense mutation c.461G > A (p.Arg154Gln) in the short-chain dehydrogenase/reductase family 42E member 1 (SDR42E1) gene. This novel mutation, which co-segregated with the disease in the family, was predicted to be pathogenic by bioinformatics tools. SDR42E1 stability analysis using DynaMut web-server showed that the p.Arg154Gln mutations has a destabilizing effect with a ΔΔG value of -1.039 kcal/mol. As this novel gene belongs to the large family of short-chain dehydrogenases/reductases (SDR) thought to be involved in steroid biosynthesis, endocrinological investigations subsequently revealed that the two patients also had low levels of cholesterol. Karyotyping revealed a normal 46,XY karyotype for the two boys, excluding other causes of disorders of sex development due to chromosomal rearrangements. In conclusion, our study reveals that mutation in the novel SDR42E1 gene alters the steroid hormone synthesis and associated with a new syndrome we named oculocutaneous genital syndrome. In addition, this study highlights the role of SDR42E1 in the regulation of cholesterol metabolism in the maintenance of connective tissue and sexual maturation in humans.

Molecular Genetics and Pathogenesis of Ehlers-Danlos Syndrome and Related Connective Tissue Disorders

Ehlers–Danlos syndromes (EDS) are a group of heritable connective tissue disorders (HCTDs) characterized by a variable degree of skin hyperextensibility, joint hypermobility and tissue fragility. The current EDS classification distinguishes 13 subtypes and 19 different causal genes mainly involved in collagen and extracellular matrix synthesis and maintenance. EDS need to be differentiated from other HCTDs with a variable clinical overlap including Marfan syndrome and related disorders, some types of skeletal dysplasia and cutis laxa. Clinical recognition of EDS is not always straightforward and for a definite diagnosis, molecular testing can be of great assistance, especially in patients with an uncertain phenotype. Currently, the major challenging task in EDS is to unravel the molecular basis of the hypermobile EDS that is the most frequent form, and for which the diagnosis is only clinical in the absence of any definite laboratory test. This EDS subtype, as well as other EDS-reminiscent phenotypes, are currently investigated worldwide to unravel the primary genetic defect and related pathomechanisms. The research articles, case report, and reviews published in this Special Issue focus on different clinical, genetic and molecular aspects of several EDS subtypes and some related disorders, offering novel findings and future research and nosological perspectives.