Stickler-like syndrome due to a dominant negative mutation in the COL2A1 gene

The cochlear matrisome: Importance in hearing and deafness

The extracellular matrix (ECM) consists in a complex meshwork of collagens, glycoproteins, and proteoglycans, which serves a scaffolding function and provides viscoelastic properties to the tissues. ECM acts as a biomechanical support, and actively participates in cell signaling to induce tissular changes in response to environmental forces and soluble cues. Given the remarkable complexity of the inner ear architecture, its exquisite structure-function relationship, and the importance of vibration-induced stimulation of its sensory cells, ECM is instrumental to hearing. Many factors of the matrisome are involved in cochlea development, function and maintenance, as evidenced by the variety of ECM proteins associated with hereditary deafness. This review describes the structural and functional ECM components in the auditory organ and how they are modulated over time and following injury.

Characteristics of a Three-Generation Family with Stickler Syndrome Type I Carrying Two Different COL2A1 Mutations

Stickler Syndrome is typically characterized by ophthalmic manifestations including vitreous degeneration and axial lengthening that predispose to retinal detachment. Systemic findings consist of micrognathia, cleft palate, sensorineural hearing loss, and joint abnormalities. COL2A1 mutations are the most common, however, there is a lack of genotype-phenotype correlations. Retrospective, single-center case series of a three-generation family. Clinical features, surgical requirements, systemic manifestations, and genetic evaluations were collected. Eight individuals clinically displayed Stickler Syndrome, seven of whom had genetic confirmation, and two different COL2A1 mutations (c.3641delC and c.3853G>T) were identified. Both mutations affect exon 51, but display distinct phenotypes. The c.3641delC frameshift mutation resulted in high myopia and associated vitreous and retinal findings. Individuals with the c.3853G>T missense mutation exhibited joint abnormalities, but mild ocular manifestations. One individual in the third generation was biallelic heterozygous for both COL2A1 mutations and showed ocular and joint findings in addition to autism and severe developmental delay. These COL2A1 mutations exhibited distinct eye vs. joint manifestations. The molecular basis for these phenotypic differences remains unknown and demonstrates the need for deep phenotyping in patients with Stickler syndrome to correlate COL2A1 gene function and expression with ocular and systemic findings.

Severe foveal hypoplasia and macular degeneration in Stickler syndrome caused by missense mutation in COL2A1 gene

BACKGROUND

The aim is to determine the retinal changes in patients with Stickler syndrome caused by a p.R565C missense mutation of the COL2A1 gene.

METHODS

We reviewed the clinical records of 10 eyes of six patients from two families with the Stickler syndrome. The members of both families were heterozygous for the p.R565C mutation. The clinical features including the visual acuity, fundus appearances, fundus autofluorescence (FAF), optical coherence tomographic (OCT) images, and electroretinograms were examined.

RESULTS

Myopia of -12 diopters (D) to -24 D with an average of -16.8 D was observed in 9 eyes of the 5 patients. The FAF images showed different degrees of hyper and hypoautofluorescent patterns in the macula in all but the two youngest patients (7 of 9 eyes, 78%). The OCT images showed the absence of a foveal pit and destruction of the outer retinal layers in the macular area in all patients. The ellipsoid zone (EZ) in the macular region was disrupted in eight eyes (80%) of which seven were fovea sparing.

CONCLUSION

Two families with Stickler syndrome with the p.R565C mutation showed more severe foveal hypoplasia, macular degeneration, and extensive retinal degeneration. A correlation of the OCT and FAF images with the genotype is helpful in determining the prognosis of Stickler syndrome.

Proline metabolism and transport in retinal health and disease

The retina is one of the most energy-demanding tissues in the human body. Photoreceptors in the outer retina rely on nutrient support from the neighboring retinal pigment epithelium (RPE), a monolayer of epithelial cells that separate the retina and choroidal blood supply. RPE dysfunction or cell death can result in photoreceptor degeneration, leading to blindness in retinal degenerative diseases including some inherited retinal degenerations and age-related macular degeneration (AMD). In addition to having ready access to rich nutrients from blood, the RPE is also supplied with lactate from adjacent photoreceptors. Moreover, RPE can phagocytose lipid-rich outer segments for degradation and recycling on a daily basis. Recent studies show RPE cells prefer proline as a major metabolic substrate, and they are highly enriched for the proline transporter, SLC6A20. In contrast, dysfunctional or poorly differentiated RPE fails to utilize proline. RPE uses proline to fuel mitochondrial metabolism, synthesize amino acids, build the extracellular matrix, fight against oxidative stress, and sustain differentiation. Remarkably, the neural retina rarely imports proline directly, but it uptakes and utilizes intermediates and amino acids derived from proline catabolism in the RPE. Mutations of genes in proline metabolism are associated with retinal degenerative diseases, and proline supplementation is reported to improve RPE-initiated vision loss. This review will cover proline metabolism in RPE and highlight the importance of proline transport and utilization in maintaining retinal metabolism and health.

Integrated analysis of COL2A1 variant data and classification of type II collagenopathies

The COL2A1 gene encodes the alpha-1 chain of type II procollagen. Type II collagen, comprised of three identical alpha-1 chains, is the major component of cartilage. COL2A1 gene variants are the etiologies of genetic diseases, termed type II collagenopathies, with a wide spectrum of clinical presentations. To date, at least 460 distinct COL2A1 mutations, identified in 663 independent probands, and 21 definite disorders have been reported. Nevertheless, a well-defined genotype-phenotype correlation has not been established, and few hot spots of mutation have been reported. In this study, we analyzed data of COL2A1 variants and clinical information of patients obtained from the Leiden Open Variation Database 3.0, as well as the currently available relevant literature. We determined the characteristics of the COL2A1 variants and distributions of the clinical manifestations in patients, and identified four likely genotype-phenotype correlations. Moreover, we classified 21 COL2A1-related disorders into five categories, which may assist clinicians in understanding the essence of these complex phenotypes and prompt genetic screening in clinical practice.

Pathogenic gene screening in 91 Chinese patients with short stature of unknown etiology with a targeted next-generation sequencing panel

Background

Dwarfism is a common severe growth disorder, but the etiology is unclear in the majority of cases. Recombinant human growth hormone may be a treatment option, but it has limited efficacy. The currently known laboratory assays do not meet the precision requirements for clinical diagnosis. Here, we have constructed a targeted next-generation sequencing (NGS) panel of selected genes that are suspected to be associated with dwarfism for genetic screening.

Methods

Genetic screening of 91 children with short stature of unknown etiology was performed with the help of the NGS panel. All the coding regions and exon-intron boundaries of 166 genes were included in the panel. To clarify the pathogenicity of these mutations, their clinical data were reviewed and analyzed.

Results

The assay identified p.A72G, p.I282V, and p.P491S variants of the PTPN11 gene and a p.I437T variant of the SOS1 gene in 4 cases with Noonan syndrome. A frameshift mutation (p.D2407fs) of the ACAN gene was identified in a case of idiopathic short stature with moderately advanced bone age. A p.R904C variant of the COL2A1 gene was found in a patient, who was accordingly diagnosed with Stickler syndrome. Severe short stature without limb deformity was associated with a p.G11A variant of HOXD13. In addition, we evaluated evidence that a p.D401N variant of the COMP gene may cause multiple epiphyseal dysplasia.

Conclusions

Our findings suggest that syndromes, particularly Noonan syndrome, may be overlooked due to atypical clinical features. This gene panel has been verified to be effective for the rapid screening of genetic etiologies associated with short stature and for guiding precision medicine-based clinical management.

Frequent mutations of RetNet genes in eoHM: Further confirmation in 325 probands and comparison with late-onset high myopia based on exome sequencing

In our previous study, potential pathological mutations of RetNet genes were detected in 23.8% (71/298) of probands with early-onset high myopia (eoHM), based on whole exome sequencing (WES). The current study aimed to confirm this finding in an additional 325 probands with eoHM and to clarify its specificity by comparison of 195 probands with late-onset high myopia (loHM). Variants in the 234 RetNet genes were selected from whole-exome sequencing data and were filtered using multistep bioinformatics analyses. Potential pathological variants in 33 genes were detected in 76 of 325 (23.4%) probands with eoHM and 14 of 195 (7.2%) probands with loHM. Thirty-five of the 76 (46.1%) probands with eoHM had mutations in COL2A1, COL11A1, RPGR, and CACNAIF, while only 2/14 (14.3%) probands with eoHM were detected. The mutation frequency and spectrum of RetNet genes in the 325 probands with eoHM were similar to our previous study but were significantly different in 195 probands with loHM (P = 2 × 10^-6 and 0.04). Data from eoHM and loHM strongly suggest that a significant proportion of eoHM is caused by mutations in RetNet genes. These results also provide initial genetic evidence that eoHM is different from loHM. The presence of mutations in 7.2% probands with loHM raises questions about pathogenicity and the variable manifestation of some mutations. The functional studies of the mutations in question and more extensive investigations of related phenotypes in the mutation carriers and their family members may provide valuable information to address these questions.

Impact of Arginine to Cysteine Mutations in Collagen II on Protein Secretion and Cell Survival

Inherited point mutations in collagen II in humans affecting mainly cartilage are broadly classified as chondrodysplasias. Most mutations occur in the glycine (Gly) of the Gly-X-Y repeats leading to destabilization of the triple helix. Arginine to cysteine substitutions that occur at either the X or Y position within the Gly-X-Y cause different phenotypes like Stickler syndrome and congenital spondyloepiphyseal dysplasia (SEDC). We investigated the consequences of arginine to cysteine substitutions (X or Y position within the Gly-X-Y) towards the N and C terminus of the triple helix. Protein expression and its secretion trafficking were analyzed. Substitutions R75C, R134C and R704C did not alter the thermal stability with respect to wild type; R740C and R789C proteins displayed significantly reduced melting temperatures (T_m) affecting thermal stability. Additionally, R740C and R789C were susceptible to proteases; in cell culture, R789C protein was further cleaved by matrix metalloproteinases (MMPs) resulting in expression of only a truncated fragment affecting its secretion and intracellular retention. Retention of misfolded R740C and R789C proteins triggered an ER stress response leading to apoptosis of the expressing cells. Arginine to cysteine mutations towards the C-terminus of the triple helix had a deleterious effect, whereas mutations towards the N-terminus of the triple helix (R75C and R134C) and R704C had less impact.

Novel and recurrent COL11A1 and COL2A1 mutations in the Marshall-Stickler syndrome spectrum

Marshall–Stickler syndrome represents a spectrum of inherited connective tissue disorders affecting the ocular, auditory, and skeletal systems. The syndrome is caused by mutations in the COL2A1, COL11A1, COL11A2, COL9A1, and COL9A2 genes. In this study, we examined four Turkish families with Marshall–Stickler syndrome using whole-exome sequencing and identified one COL2A1 mutation and three COL11A1 mutations. Two of the COL11A1 mutations were novel. Our findings expand our knowledge of the COL11A1 mutational spectrum that causes Marshall–Stickler syndrome.

Stickler Syndrome Type 1 with Short Stature and Atypical Ocular Manifestations

Stickler syndrome or hereditary progressive arthroophthalmopathy is a heterogeneous group of collagen tissue disorders, characterized by orofacial features, ophthalmological features (high myopia, vitreoretinal degeneration, retinal detachment, and presenile cataracts), hearing impairment, mild spondyloepiphyseal dysplasia, and/or early onset arthritis. Stickler syndrome type I (ocular form) is caused by mutation in the COL2A1 gene. Ptosis and uveitis are relatively rare ophthalmological manifestations of this syndrome. We report an Indian boy having 2710C>T mutation in COL2A1 gene demonstrating short stature, ptosis, and uveitis with Stickler syndrome.

Molecular genetics of the COL2A1-related disorders

Type II collagen, comprised of three identical alpha-1(II) chains, is the major collagen synthesized by chondrocytes, and is found in articular cartilage, vitreous humour, inner ear and nucleus pulposus. Mutations in the collagen type II alpha-1 gene (COL2A1) have been reported to be responsible for a series of abnormalities, known as type II collagenopathies. To date, 16 definite disorders, inherited in an autosomal dominant or recessive pattern, have been described to be associated with the COL2A1 mutations, and at least 405 mutations ranging from point mutations to complex rearrangements have been reported, though the underlying pathogenesis remains unclear. Significant clinical heterogeneity has been reported in COL2A1-associated type II collagenopathies. In this review, we highlight current knowledge of known mutations in the COL2A1 gene for these disorders, as well as genetic animal models related to the COL2A1 gene, which may help us understand the nature of complex phenotypes and underlying pathogenesis of these conditions.

The expanding spectrum of COL2A1 gene variants IN 136 patients with a skeletal dysplasia phenotype

Heterozygous COL2A1 variants cause a wide spectrum of skeletal dysplasia termed type II collagenopathies. We assessed the impact of this gene in our French series. A decision tree was applied to select 136 probands (71 Stickler cases, 21 Spondyloepiphyseal dysplasia congenita cases, 11 Kniest dysplasia cases, and 34 other dysplasia cases) before molecular diagnosis by Sanger sequencing. We identified 66 different variants among the 71 positive patients. Among those patients, 18 belonged to multiplex families and 53 were sporadic. Most variants (38/44, 86%) were located in the triple helical domain of the collagen chain and glycine substitutions were mainly observed in severe phenotypes, whereas arginine to cysteine changes were more often encountered in moderate phenotypes. This series of skeletal dysplasia is one of the largest reported so far, adding 44 novel variants (15%) to published data. We have confirmed that about half of our Stickler patients (46%) carried a COL2A1 variant, and that the molecular spectrum was different across the phenotypes. To further address the question of genotype-phenotype correlation, we plan to screen our patients for other candidate genes using a targeted next-generation sequencing approach.

Mutation Update for COL2A1 Gene Variants Associated with Type II Collagenopathies

Mutations in the COL2A1 gene cause a spectrum of rare autosomal-dominant conditions characterized by skeletal dysplasia, short stature, and sensorial defects. An early diagnosis is critical to providing relevant patient care and follow-up, and genetic counseling to affected families. There are no recent exhaustive descriptions of the causal mutations in the literature. Here, we provide a review of COL2A1 mutations extracted from the Leiden Open Variation Database (LOVD) that we updated with data from PubMed and our own patients. Over 700 patients were recorded, harboring 415 different mutations. One-third of the mutations are dominant-negative mutations that affect the glycine residue in the G-X-Y repeats of the alpha 1 chain. These mutations disrupt the collagen triple helix and are common in achondrogenesis type II and hypochondrogenesis. The mutations resulting in a premature stop codon are found in less severe phenotypes such as Stickler syndrome. The p.(Arg275Cys) substitution is found in all patients with COL2A1-associated Czech dysplasia. LOVD-COL2A1 provides support and potential collaborative material for scientific and clinical projects aimed at elucidating phenotype-genotype correlation and differential diagnosis in patients with type II collagenopathies.

Hearing impairment in Stickler syndrome: a systematic review

Background

Stickler syndrome is a connective tissue disorder characterized by ocular, skeletal, orofacial and auditory defects. It is caused by mutations in different collagen genes, namely COL2A1, COL11A1 and COL11A2 (autosomal dominant inheritance), and COL9A1 and COL9A2 (autosomal recessive inheritance). The auditory phenotype in Stickler syndrome is inconsistently reported. Therefore we performed a systematic review of the literature to give an up-to-date overview of hearing loss in Stickler syndrome, and correlated it with the genotype.

Methods

English-language literature was reviewed through searches of PubMed and Web of Science, in order to find relevant articles describing auditory features in Stickler patients, along with genotype. Prevalences of hearing loss are calculated and correlated with the different affected genes and type of mutation.

Results

313 patients (102 families) individually described in 46 articles were included. Hearing loss was found in 62.9%, mostly mild to moderate when reported. Hearing impairment was predominantly sensorineural (67.8%). Conductive (14.1%) and mixed (18.1%) hearing loss was primarily found in young patients or patients with a palatal defect. Overall, mutations in COL11A1 (82.5%) and COL11A2 (94.1%) seem to be more frequently associated with hearing impairment than mutations in COL2A1 (52.2%).

Conclusions

Hearing impairment in patients with Stickler syndrome is common. Sensorineural hearing loss predominates, but also conductive hearing loss, especially in children and patients with a palatal defect, may occur. The distinct disease-causing collagen genes are associated with a different prevalence of hearing impairment, but still large phenotypic variation exists. Regular auditory follow-up is strongly advised, particularly because many Stickler patients are visually impaired.

Disorders of the growth plate

PURPOSE OF REVIEW

To summarize the recent advances in our understanding of the majors genes involved in chondrogenesis and their molecular mechanisms.

RECENT FINDINGS

Disorders of the growth plate and the resulting skeletal dysplasias are a consequence of defects in genes involved in various stages of the chondrocyte proliferation and differentiation process. Recent identification of disease genes has provided insights into the pathophysiology of many skeletal dysplasias.

SUMMARY

This knowledge enhances our understanding of the physiology and pathophysiology of the growth plate. Many skeletal dysplasias can now be characterized at the molecular level, allowing clinicians to provide accurate molecular diagnoses and counseling. Further research in this area will likely provide insights into possible therapeutic options for disorders of the growth plate.

R992C (p.R1192C) Substitution in collagen II alters the structure of mutant molecules and induces the unfolded protein response

We investigated the molecular bases of spondyloepiphyseal dysplasia (SED) associated with the R992C (p.R1192C) substitution in collagen II. At the protein level, we analyzed the structure and integrity of mutant molecules, and at the cellular level, we specifically studied the effects of the presence of the R992C collagen II on the biological processes taking place in host cells. Our studies demonstrated that mutant collagen II molecules were characterized by altered electrophoretic mobility, relatively low thermostability, the presence of atypical disulfide bonds, and slow rates of secretion into the extracellular space. Analyses of cellular responses to the presence of the mutant molecules showed that excessive accumulation of thermolabile collagen II was associated with the activation of an "unfolded protein response" and an increase in apoptosis of host cells. Collectively, these data suggest that molecular mechanisms of SED may be driven not only by structural changes in the architecture of extracellular collagenous matrices, but also by intracellular processes activated by the presence of mutant collagen II molecules.

The influence of pre-mRNA splicing on phenotypic modification in Stickler's syndrome and other type II collagenopathies

PURPOSE

This paper will illustrate how variation in the processing of mutant pre-mRNA can affect the phenotypic outcome of inherited disorders of type II collagen.

METHODS

Type 1 Stickler's syndrome is one of the different phenotypes resulting from mutations in COL2A1 (the type II collagenopathies). It is also the commonest, but often goes undiagnosed due to the variability of phenotypic features, which in some cases may consist of only abnormal vitreous development. Most cases of type 1 Stickler's syndrome are due to premature termination codons in the mRNA, resulting in haploinsufficiency. This leaves a conundrum as to why the disease is so variable. Using RT-PCR of illegitimate transcript and also minigenes, we have investigated how certain mutations can variably affect mRNA processing.

RESULTS

Here, we demonstrate and discuss how apparently similar mutations can have a dramatically different effect on splicing of the pre-mRNA, switching transcripts from ones which would be degraded by nonsense-mediated decay into messages that will be translated into mutant proteins that can exert a dominant-negative effect and ultimately modify the resulting phenotype.

CONCLUSION

Variability of Stickler's syndrome can, in part, be due to the variable effect that mutations have on the processing of the COL2A1 transcript.

Y-position cysteine substitution in type I collagen (α1(I) R888C/p.R1066C) is associated with osteogenesis imperfecta/Ehlers-Danlos syndrome phenotype

The most common mutations in type I collagen causing types II-IV osteogenesis imperfecta (OI) result in substitution for glycine in a Gly-Xaa-Yaa triplet by another amino acid. We delineated a Y-position substitution in a small pedigree with a combined OI/Ehlers-Danlos Syndrome (EDS) phenotype, characterized by moderately decreased DEXA z-score (-1.3 to -2.6), long bone fractures, and large-joint hyperextensibility. Affected individuals have an alpha1(I)R888C (p.R1066C) substitution in one COL1A1 allele. Polyacrylamide gel electrophoresis (PAGE) of [(3)H]-proline labeled steady-state collagen reveals slight overmodification of the alpha1(I) monomer band, much less than expected for a substitution of a neighboring glycine residue, and a faint alpha1(I) dimer. Dimers form in about 10% of proband type I collagen. Dimer formation is inefficient compared to a possible 25%, probably because the SH-side chains have less proximity in this Y-position than when substituting for a glycine. Theoretical stability calculations, differential scanning calorimetry (DSC) thermograms, and thermal denaturation curves showed only weak local destabilization from the Y-position substitution in one or two chains of a collagen helix, but greater destabilization is seen in collagen containing dimers. Y-position collagen dimers cause kinking of the helix, resulting in a register shift that is propagated the full length of the helix and causes resistance to procollagen processing by N-proteinase. Collagen containing the Y-position substitution is incorporated into matrix deposited in culture, including immaturely and maturely cross-linked fractions. In vivo, proband dermal fibrils have decreased density and increased diameter compared to controls, with occasional aggregate formation. This report on Y-position substitutions in type I collagen extends the range of phenotypes caused by nonglycine substitutions and shows that, similar to X- and Y-position substitutions in types II and III collagen, the phenotypes resulting from nonglycine substitutions in type I collagen are distinct from those caused by glycine substitutions.

The phenotypic spectrum in patients with arginine to cysteine mutations in the COL2A1 gene

BACKGROUND

The majority of COL2A1 missense mutations are substitutions of obligatory glycine residues in the triple helical domain. Only a few non-glycine missense mutations have been reported and among these, the arginine to cysteine substitutions predominate.

OBJECTIVE

To investigate in more detail the phenotype resulting from arginine to cysteine mutations in the COL2A1 gene.

METHODS

The clinical and radiographic phenotype of all patients in whom an arginine to cysteine mutation in the COL2A1 gene was identified in our laboratory, was studied and correlated with the abnormal genotype. The COL2A1 genotyping involved DHPLC analysis with subsequent sequencing of the abnormal fragments.

RESULTS

Six different mutations (R75C, R365C, R519C, R704C, R789C, R1076C) were found in 11 unrelated probands. Each mutation resulted in a rather constant and site-specific phenotype, but a perinatally lethal disorder was never observed. Spondyloarthropathy with normal stature and no ocular involvement were features of patients with the R75C, R519C, or R1076C mutation. Short third and/or fourth toes was a distinguishing feature of the R75C mutation and brachydactyly with enlarged finger joints a key feature of the R1076C substitution. Stickler dysplasia with brachydactyly was observed in patients with the R704C mutation. The R365C and R789C mutations resulted in classic Stickler dysplasia and spondyloepiphyseal dysplasia congenita (SEDC), respectively.

CONCLUSIONS

Arginine to cysteine mutations are rather infrequent COL2A1 mutations which cause a spectrum of phenotypes including classic SEDC and Stickler dysplasia, but also some unusual entities that have not yet been recognised and described as type II collagenopathies.

Prediction of collagen stability from amino acid sequence

An algorithm was derived to relate the amino acid sequence of a collagen triple helix to its thermal stability. This calculation is based on the triple helical stabilization propensities of individual residues and their intermolecular and intramolecular interactions, as quantitated by melting temperature values of host-guest peptides. Experimental melting temperature values of a number of triple helical peptides of varying length and sequence were successfully predicted by this algorithm. However, predicted T(m) values are significantly higher than experimental values when there are strings of oppositely charged residues or concentrations of like charges near the terminus. Application of the algorithm to collagen sequences highlights regions of unusually high or low stability, and these regions often correlate with biologically significant features. The prediction of stability from sequence indicates an understanding of the major forces maintaining this protein motif. The use of highly favorable KGE and KGD sequences is seen to complement the stabilizing effects of imino acids in modulating stability and may become dominant in the collagenous domains of bacterial proteins that lack hydroxyproline. The effect of single amino acid mutations in the X and Y positions can be evaluated with this algorithm. An interactive collagen stability calculator based on this algorithm is available online.

A novel retinoic acid-response element requires an enhancer element mediator for transcriptional activation

The Col11a2 gene codes for alpha2(XI), a subunit of type XI collagen that is a critical component of the cartilage extracellular matrix. The 5' regulatory region of Col11a2 was subjected to deletional analysis to detect any regulatory element in addition to the two known chondrocyte-specific enhancer elements B/C and D/E. Deletion of the region from -342 to -242 bp reduced transcriptional activity to less than 50% of wild-type, but the sequence showed no independent ability to increase transcription from a minimal promoter. When cloned downstream of the D/E enhancer, however, a subsection of the sequence nearly doubled transcriptional activity and produced an additional 3-fold activation in response to RA (retinoic acid). A 6-bp direct repeat, separated by 4 bp (a DR-4 element) near the 5'-end of this region, was found to be essential for its activity, and was further shown to bind the RA X receptor beta in electrophoretic mobility-shift assays. The present study has revealed a novel RA-response element in Col11a2 that does not interact directly with the promoter, but instead requires the D/E enhancer to mediate transcriptional activation. Proteins bound at the enhancer, therefore, would be expected to affect the transcriptional response to RA. Such a system of regulation, particularly if found to be operating in other cartilage genes, could explain the conflicting responses RA produces in chondrocytes under different experimental conditions.

Stickler syndrome type I and Stapes ankylosis

OBJECTIVE

To report a successful stapedectomy for stapedial fixation in a patient with Stickler syndrome type I (COL2A1).

SETTING

University Hospital Department for Otology, Pathology, Ophthalmology and Clinical Genetics.

STUDY DESIGN

A clinical and genetic evaluation of a mother and daughter focusing mainly on the otological, ophthalmological, histological and genetical aspects.

INTERVENTION

A stapedectomy was performed successfully.

RESULTS

Hearing impairment improved after stapedectomy. Postoperatively a shift in high-frequency threshold wa seen related to the stapedectomy. A new mutation in COL2A1 gene was dectected.

CONCLUSION

Stapedial fixation can be the cause of hearing impairment in Stickler syndrome type I (COL2A1). The hearing impairment can be improved by stapes surgery.

Collagens and collagen-related matrix components in the human and mouse eye

The three-dimensional structure of the eye plays an important role in providing a correct optical environment for vision. Much of this function is dependent on the unique structural features of ocular connective tissue, especially of the collagen types and their supramolecular structures. For example, the organization of collagen fibrils is largely responsible for transparency and refraction of cornea, lens and vitreous body, and collagens present in the sclera are largely responsible for the structural strength of the eye. Phylogenetically, most of the collagens are highly conserved between different species, which suggests that collagens also share similar functions in mice and men. Despite considerable differences between the mouse and the human eye, particularly in the proportion of the different tissue components, the difficulty of performing systematic histologic and molecular studies on the human eye has made mouse an appealing alternative to studies addressing the role of individual genes and their mutations in ocular diseases. From a genetic standpoint, the mouse has major advantages over other experimental animals as its genome is better known than that of other species and it can be manipulated by the modern techniques of genetic engineering. Furthermore, it is easy, quick and relatively cheap to produce large quantities of mice for systematic studies. Thus, transgenic techniques have made it possible to study consequences of specific mutations in genes coding for structural components of ocular connective tissues in mice. As these changes in mice have been shown to resemble those in human diseases, mouse models are likely to provide efficient tools for pathogenetic studies on human disorders affecting the extracellular matrix. This review is aimed to clarify the role of collagenous components in the mouse and human eye with a closer look at the new findings of the collagens in the cartilage and the eye, the so-called "cartilage collagens".

Clinical variability of Stickler syndrome: role of exon 2 of the collagen COL2A1 gene

Stickler syndrome (progressive arthro-ophthalmopathy) is a genetically heterogeneous disorder resulting from mutations in at least three collagen genes. The most common disease-causing gene is COL2A1, a 54-exon-containing gene coding for type II collagen. At least 17 different mutations causing Stickler syndrome have been reported in this gene. Phenotypically, it is also a variably expressed disorder in which most patients present with a wide range of eye and extraocular manifestations including auditory, skeletal, and orofacial manifestations. Some patients, however, present without clinically apparent systemic findings. This observation has led to difficulty distinguishing this Stickler phenotype from other hereditary vitreoretinal degenerations, such as Wagner syndrome and Snowflake vitreoretinal degeneration. In this regard, review of the literature indicates type II collagen exists in two forms resulting from alternative splicing of exon 2 of the COL2A1 gene. One form, designated as type IIB (short form), is preferentially expressed in adult cartilage tissue. The other form, designated as type IIA (long form), is preferentially expressed in the vitreous body of the eye. Because of this selective tissue expression, mutations in exon 2 of the COL2A1 gene have been hypothesized to produce this Stickler syndrome phenotype with minimal or absent extraocular findings. We review the evidence for families with exon 2 mutations of the collagen COL2A1 gene presenting in a distinct manner from families with mutations in the remaining 53 exons, as well as other hereditary vitreoretinal degenerations without significant systemic manifestations.

Prevalence of mitral valve prolapse in Stickler syndrome

The prevalence of mitral valve prolapse in Stickler syndrome has been reported to be much higher than in the general population. As a result, it has been recommended that all patients with Stickler syndrome undergo routine echocardiography and have antibiotic prophylaxis prior to surgery. The purpose of this study was to evaluate the prevalence of mitral valve prolapse in a large cohort of UK patients with Stickler syndrome in whom the clinical diagnosis has been confirmed by molecular genetic analysis. Probands and pedigrees were identified from the Vitreoretinal Service database according to previously published criteria. Ophthalmic, skeletal, audiometric, and orofacial features were assessed. Affected individuals underwent a full cardiological examination including auscultation and two-dimensional echocardiography. Mutation analysis of the COL2A1 and COL11A1 genes was carried out. Seventy-eight patients from 25 pedigrees were studied. Mutation analysis confirmed the clinical diagnosis in every pedigree. No patient was found to have clinical evidence of cardiovascular disease and no patient had significant mitral or other valvular prolapse on echocardiography. These data from a large cohort of UK patients with proven Stickler syndrome do not suggest an increased incidence of mitral valve prolapse over and above that found in the general population. Routine echocardiography screening and use of preoperative antibiotics are unnecessary and should be reserved for those individual cases where there is clear clinical indication.

Premature vertebral endplate ossification and mild disc degeneration in mice after inactivation of one allele belonging to the Col2a1 gene for Type II collagen

STUDY DESIGN

Skeletal tissues of mice with an inactivated allele of the Col2a1 gene for Type II collagen ("heterozygous knockout") were studied.

OBJECTIVE

To determine whether a heterozygous inactivation of the Col2a1 gene has a role in the etiology of spine disorders such as disc degeneration.

SUMMARY OF BACKGROUND DATA

Mutations in the COL2A1, COL11A1, COL11A2, and COL9A2 genes have been linked to spine disorders. However, the mechanism by which genetic factors lead to disc degeneration still are largely unknown.

METHODS

Spine tissues were studied using radiograph analyses; conventional, quantitative, and polarized light microscopy; immunohistochemistry for the major extracellular components, and in situ hybridization for procollagens alpha1(I) and alpha1(II). Voluntary running activity also was monitored in half of the mice.

RESULTS

As the findings showed, 1-month-old heterozygous knockout mice had shorter limb bones, skulls, and spines, as well as thicker and more irregular vertebral endplates, which calcified earlier than in the control mice. They also had a lower concentration of glycosaminoglycans in the anulus fibrosus, in the endplates, and in the vertebral bone than the controls. These features in the heterozygous knockout mice were compensated by the age of 15 months. However, the long bones and skulls of the mature heterozygous mice remained shorter than those of the controls. Gene-deficient mice used the running wheel less. However, physical exercise did not induce any marked structural changes in the skeleton.

CONCLUSION

Mice with heterozygous knockout of Col2a1 show subtle early skeletal manifestations that bear some resemblance to those of human spine disorders.

Osteoarthritis in children associated with a mutation in the type II procollagen gene (COL2A1)

A single-base mutation resulting in an arginine-519-cysteine (R519C) mutation of type II procollagen (COL2A1) has been shown to result in precocious osteoarthritis with mild spinal chondrodysplasia without severe foreshortening (OMIM 604864). The nature of childhood disease among affected individuals has not been described. The recent presentation of four children with this mutation allows us to provide clinical correlation. This form of premature osteoarthritis may present in childhood and should be considered in the differential diagnosis of childhood arthropathy presenting in the context of a positive family history.

Variation in the vitreous phenotype of Stickler syndrome can be caused by different amino acid substitutions in the X position of the type II collagen Gly-X-Y triple helix

Stickler syndrome is a dominantly inherited disorder characterized by arthropathy, midline clefting, hearing loss, midfacial hypoplasia, myopia, and retinal detachment. These features are highly variable both between and within families. Mutations causing the disorder have been found in the COL2A1 and COL11A1 genes. Premature termination codons in COL2A1 that result in haploinsufficiency of type II collagen are a common finding. These produce a characteristic congenital "membranous" anomaly of the vitreous of all affected individuals. Experience has shown that vitreous slit-lamp biomicroscopy can distinguish between patients with COL2A1 mutations and those with dominant negative mutations in COL11A1, who produce a different "beaded" vitreous phenotype. Here we characterize novel dominant negative mutations in COL2A1 that result in Stickler syndrome. Both alter amino acids in the X position of the Gly-X-Y triple-helical region. A recurrent R365C mutation occurred in two unrelated sporadic cases and resulted in the membranous vitreous anomaly associated with haploinsufficiency. In a large family with linkage to COL2A1, with a LOD score of 2.8, a unique L467F mutation produced a novel "afibrillar" vitreous gel devoid of all normal lamella structure. These data extend the mutation spectrum of the COL2A1 gene and help explain the basis for the different vitreous phenotypes seen in Stickler syndrome.

Classical Ehlers-Danlos syndrome caused by a mutation in type I collagen

Classical Ehlers-Danlos syndrome (EDS) is characterized by skin hyperelasticity, joint hypermobility, increased tendency to bruise, and abnormal scarring. Mutations in type V collagen, a regulator of type I collagen fibrillogenesis, have been shown to underlie this type of EDS. However, to date, mutations have been found in only a limited number of patients, which suggests genetic heterogeneity. In this article, we report two unrelated patients with typical features of classical EDS, including excessive skin fragility, in whom we found an identical arginine-->cysteine substitution in type I collagen, localized at position 134 of the alpha1(I) collagen chain. The arginine residue is highly conserved and localized in the X position of the Gly-X-Y triplet. As a consequence, intermolecular disulfide bridges are formed, resulting in type I collagen aggregates, which are retained in the cells. Whereas substitutions of glycine residues in type I collagen invariably result in osteogenesis imperfecta, substitutions of nonglycine residues in type I collagen have not yet been associated with a human disease. In contrast, arginine-->cysteine substitutions in type II collagen have been identified in a variety of chondrodysplasias. Our findings show that mutations in other fibrillar collagens can be causally involved in classical EDS and point to genetic heterogeneity of this disorder.

Clinical and molecular genetics of Stickler syndrome

Stickler syndrome is an autosomal dominant disorder with characteristic ophthalmological and orofacial features, deafness, and arthritis. Abnormalities of vitreous gel architecture are a pathognomonic feature, usually associated with high myopia which is congenital and non-progressive. There is a substantial risk of retinal detachment. Less common ophthalmological features include paravascular pigmented lattice degeneration and cataracts. Non-ocular features show great variation in expression. Children with Stickler syndrome typically have a flat midface with depressed nasal bridge, short nose, anteverted nares, and micrognathia. These features can become less pronounced with age. Midline clefting, if present, ranges in severity from a cleft of the soft palate to Pierre-Robin sequence. There is joint hypermobility which declines with age. Osteoarthritis develops typically in the third or fourth decade. Mild spondyloepiphyseal dysplasia is often apparent radiologically. Sensorineural deafness with high tone loss may be asymptomatic or mild. Occasional findings include slender extremities and long fingers. Stature and intellect are usually normal. Mitral valve prolapse was reported to be a common finding in one series but not in our experience. The majority of families with Stickler syndrome have mutations in the COL2A1 gene and show the characteristic type 1 vitreous phenotype. The remainder with the type 2 vitreous phenotype have mutations in COL11A1 or other loci yet to be identified. Mutations in COL11A2 can give rise to a syndrome with the systemic features of Stickler syndrome but no ophthalmological abnormality.