High efficiency of mutation detection in type 1 stickler syndrome using a two-stage approach: vitreoretinal assessment coupled with exon sequencing for screening COL2A1

Stickler syndrome is a genetically heterogeneous disorder that affects the ocular, skeletal, and auditory systems. To date three genes, COL2A1, COL11A1, and COL11A2, encoding the heterotypic type II/XI collagen fibrils present in vitreous and cartilage have been shown to have mutations that result in Stickler syndrome. As systemic features in this disorder are variable we have used an ophthalmic examination to differentiate those patients with a membranous vitreous phenotype associated with mutations in COL2A1, from other patients who may have mutations in other genes. Gene amplification and exon sequencing was used to screen 50 families or sporadic cases with this membranous phenotype, for mutations in COL2A1. Mutations were detected in 47 (94%) cases consisting of 166 affected and 78 unaffected individuals. We also demonstrate that the predominantly ocular form of type 1 Stickler syndrome is not confined to mutations in the alternatively spliced exon 2. Using splicing reporter constructs we demonstrate that a mutant GC donor splice site in intron 51 can be spliced normally; this contributed to the predominantly ocular phenotype in the family in which it occurred.

Type 1 Stickler syndrome: a histological and ultrastructural study of an untreated globe

AIMS

To present a histological and ultrastructural study of an untreated globe in a patient with genetically confirmed type 1 Stickler syndrome.

METHODS

Histological and electron microscopic examinations were performed on the enucleated globe from the proband of a pedigree with type 1 Stickler syndrome. Linkage analysis was carried out using polymorphic markers flanking the COL2A1 gene and the mutation was identified by direct sequencing.

RESULTS

The significant retinal abnormality was incarceration of vitreous collagen within glial strands on the inner surface of an atrophic and gliotic detached retina. The incarcerated collagenous layers contained glial cells and extended from the retina to form strands, some of which contributed to a retrolental membrane. Mutation screening detected a C to T mutation in exon 47 that inserted a premature termination codon into the reading frame of the mRNA. Sequence analysis of three of the four affected children confirmed that they were also heterozygous for the base change. The youngest child's DNA was not analysed.

CONCLUSIONS

The study represents the first evidence of abnormal interactions between pathological vitreous collagen and the inner retina in a patient with type 1 Stickler syndrome with a confirmed mutation in the COL2A1 gene.

Clinical features and surgical management of retinal detachment secondary to round retinal holes

AIMS

The majority of rhegmatogenous retinal detachments result from pathological posterior vitreous detachment (PVD) and secondary horseshoe or giant retinal tears. Retinal detachment without PVD is usually associated with either retinal dialysis or round retinal holes. This study characterises the features, surgical outcome, and incidence of bilateral involvement of detachment associated with round retinal holes.

METHODS

In all, 110 retinal detachments from 96 consecutive patients with retinal detachment secondary to round retinal holes were studied. Analysis of patient age, sex, refraction, preoperative visual acuity, presented symptoms, position and extent of detachment, number and distribution of holes present, posterior hyaloid membrane status, surgical management, outcome of surgery, and postoperative visual acuity were studied.

RESULTS

The mean age for patients was 34 years with a marked female preponderance (64%) and myopia (83%). The posterior hyaloid membrane remained attached in 95 eyes (86%). In all, 45% patients had bilateral pathology, of which 33% had 'mirror image' distribution. Detachments were predominantly shallow (93%) and slow in progression (17%). A total of 100 detachments were repaired with cryotherapy and scleral buckling, eight with cryotherapy alone, and one with laser retinopexy. In all, 99% detachments were successfully reattached with a single procedure. The mean follow-up period was 2 years. There were no instances of redetachment.

CONCLUSIONS

Round hole detachments are slowly evolving detachments with attached vitreous gel in young, predominantly female myopes. Examination of the fellow eye should be mandatory as there is a high incidence of bilateral pathology. Scleral buckling procedures remained highly effective in this selected group of patients.

A Novel Mutation ofCOL2A1Resulting in Dominantly Inherited Rhegmatogenous Retinal Detachment

PURPOSE

To determine the molecular defect in a family with autosomal dominant rhegmatogenous retinal detachment (DRRD), and to investigate missplicing as a possible phenotypic modifier of mutations in COL2A1.

METHODS

Clinical examination of the family and linkage analysis using markers flanking COL2A1 and COL11A1, the known loci for Stickler syndrome; mutation screening of COL2A1; construction of splicing reporter minigenes and transfection into cultured cells; and RT-PCR analysis of reporter specific transcripts.

RESULTS

A family with DRRD showed no systemic clinical signs (skeletal, orofacial, or auditory) usually associated with Stickler syndrome. Linkage analysis excluded COL11A1 as the disease locus but could not exclude COL2A1. Mutation screening of COL2A1 identified a novel G118R mutation in type II collagen. Transfection of minigenes carrying mutations associated with DRRD (G118R, R453X, and L467F) into cultured cells detected no missplicing of mRNA from mutant constructs.

CONCLUSIONS

Mutations outside the alternatively spliced exon 2 region of COL2A1 can also result in an ocular only phenotype. There was no evidence that missplicing modifies the phenotype of these mutations, suggesting that the minimal or absent systemic features demonstrated by the G118R and L467F mutations are the result of the biophysical changes imparted on the collagen molecule.

Hyperconvolution of the inner limiting membrane in vitreomaculopathies

BACKGROUND

This study investigates the similarities and differences between epiretinal membranes in four clinically distinct types of vitreomaculopathy. We propose a hypothesis on the origin of the predominant cell type and its potential role in causing these conditions.

METHODS

Epiretinal membranes (ERMs) surgically removed from a prospective, consecutive series of vitrectomies for macular pucker associated with an untreated peripheral horseshoe tear (MP), cellophane maculopathy (CM), stage 4 macular hole (MH) and vitreomacular traction syndrome (VMT) were examined by light microscopy and by immunocytochemistry (ICC) using antibodies marking type IV collagen, type II collagen, glial fibrillary acidic protein (GFAP), and low- and high-molecular-weight cytokeratin (MNF116). These specimens were compared with post-mortem control eyes with and without physiological posterior vitreous detachment (PVD). Light microscopy was carried out on 5-microm-thick sections cut from formalin-fixed, paraffin-embedded tissue blocks. Appropriate autoclave or enzyme pre-digestion steps were deployed to retrieve antigens for ICC. No patient had undergone previous vitreoretinal surgery or peripheral retinopexy.

RESULTS

From a series of 38 patients, (13 CM, 8 MP, 16 MH and 1 VMT) a total of 20 specimens contained sufficient tissue for histology and immunocytochemistry. All specimens contained portions of inner limiting membrane (ILM) coated by GFAP-positive cells. Specimens from patients with MP and CM exhibited hyperconvolution of the ILM, which was not found in the specimens from patients with MH or VMT or in the control eyes. Hyperconvolution was associated with increased glial cell density, GFAP staining intensity and duplication of ILM basement membrane. Three cases of ERMs from the MP group contained, in addition, cytokeratin-positive cells. In the control group; post-mortem eyes with PVDs showed patchy staining of the posterior hyaloid membrane for GFAP and type 4 collagen. Post-mortem eyes with attached gel showed weak positivity of the ILM for type 4 collagen, and a monolayer of GFAP-positive cells lined the vitreous aspect of the ILM.

CONCLUSIONS

These results indicate that glial cells are fundamentally important in the formation of ERMs found in this group of vitreomaculopathies. The hyperconvolution and duplication of the ILM in CM and MP were striking and distinctive features and suggest a mechanism by which these membranes exert tractional forces on the retina. Post-mortem control eyes contained a similar (but more dispersed) population of GFAP-positive cells in the region of the ILM, suggesting the primary aetiology for CM and MP may originate within the ILM. ERMs from MP cases may, in addition, contain cytokeratin-positive cells, of probable RPE origin.

Relationships between soil heavy metal concentration and mycorrhizal colonisation in Thymus polytrichus in northern England

A study was conducted to establish whether the wild thyme [ Thymus polytrichus A. Kerner ex Borbás ssp. britannicus (Ronn.) Kerguelen (Lamiaceae)] growing in the metal-contaminated soils along the River South Tyne, United Kingdom, is colonised by arbuscular mycorrhizal (AM) fungi, and whether the degree of colonisation increases (perhaps suggesting increasing mycorrhizal dependence) or decreases (indicating possible inhibition of AM growth) with increasing degree of soil contamination. Seasonal changes in AM colonisation were also assessed. The AM fungal communities colonising T. polytrichus were also investigated, using the polymerase chain reaction with restriction fragment length polymorphism and sequencing of fungal DNA to establish whether AM species richness varied between sites, and whether fungal ecotypes specific to sites with different amounts of metal contamination could be identified. All plants examined were heavily colonised by AM fungi, and mean percentage root length colonised did not increase significantly with increasing soil metal contamination. However, AM vesicle abundance (percentage of mycorrhizal root length containing vesicles) at the most contaminated site was significantly greater than at the other sites. No significant seasonal variation in degree of colonisation or vesicle abundance was found. Glomus was the predominant AM genus detected at all sites. The number of AM genotypes colonising T. polytrichus roots was similar at all sites but, although some were common to all sites, certain strains appeared to be specific to either the most- or the least-contaminated site. This variation in species may account for the difference in vesicle abundance between sites. The consistently heavy AM colonisation of T. polytrichus found suggests that these fungi are not inhibited by soil heavy metals at these sites, and that the host derives some benefit from its AM symbiont.

Effects of mycorrhizal colonisation on Thymus polytrichus from heavy-metal-contaminated sites in northern England

A study was performed to establish whether colonisation with arbuscular mycorrhizal (AM) fungi is beneficial to wild thyme [ Thymus polytrichus A. Kerner ex Borbás ssp. britannicus (Ronn.) Kerguelen (Lamiaceae)] growing in the heavy-metal-contaminated soils along the River South Tyne, United Kingdom. T. polytrichus plants of the same genotype were grown under controlled conditions with and without Zn contamination, and differences between AM-colonised and -uncolonised plants in mean shoot and root growth (dry weight) and Zn concentration were assessed. When grown in the heavy-metal-contaminated, low-P soil from one of the South Tyne sites, AM-colonised plants grew significantly larger than uncolonised plants; however, there was no significant difference in growth between AM and non-AM plants grown in an artificial substrate with a larger available P concentration, with or without Zn contamination. Mycorrhizal colonisation increased tissue Zn concentrations during the experiments. It is concluded that AM fungi are beneficial, if not essential, to T. polytrichus growing in the low-nutrient soils along the River South Tyne, because of their role in enhancing plant uptake of P (and possibly other nutrients). There was no evidence from this study that the fungi reduce plant uptake of heavy metals at these sites, but rather increase Zn uptake. However, the resulting tissue metal concentrations do not appear to be large enough to be detrimental to plant growth.

Apomixis in flowering plants: an overview

Apomixis is a common feature of perennial plants, which occurs in ca. 60% of the British flora, but has been largely ignored by reproductive theoreticians. Successful individuals may cover huge areas, and live to great ages, favoured by 'symmetrical' selection. Apomixis is favoured by colonizing modes, for instance post-glacially. Despite its theoretical advantages, apomixis usually coexists with sexuality, suggesting 'hidden' disadvantages. Agamospermy (apomixis by seed) is relatively uncommon, but gains from the attributes of the seed. It pays agamospermy genes, which discourage recombination, to form co-adapted linkage groups, so that they become targets for disadvantageous recessive mutant accumulation. Consequently, agamospermy genes cannot succeed in diploids and agamosperms are hybrid and highly heterotic. Agamospermous endosperm may suffer from genomic imbalance, so that nutritious ovules, which can support embryos without endosperm, may be preadapted for agamospermy. When primary endosperm nucleus fertilization ('pseudogamy') continues as a requirement for many aposporous agamosperms, selfing sex becomes preadaptive and archesporial sex remains an option. Apomictic populations can be quite variable although apomictic families are much less variable than sexuals. Only in some diplosporous species does sex disappear completely, and in those species some release of variability may persist through somatic recombination. The search for an agamospermy gene suitable for genetic modification should target fertile sexuals with a single localized agamospermy (A) gene, which therefore lack a genetic load. The A gene should coexist alongside sexuality, so that it would be easy to select seedlings of sexual and asexual origins. Plants with sporophytic agamospermy provide all these attributes.

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.

Vitreoretinopathy with phalangeal epiphyseal dysplasia, a type II collagenopathy resulting from a novel mutation in the C-propeptide region of the molecule

A large family with dominantly inherited rhegmatogenous retinal detachment, premature arthropathy, and development of phalangeal epiphyseal dysplasia, resulting in brachydactyly was linked to COL2A1, the gene encoding proalpha1(II) collagen. Mutational analysis of the gene by exon sequencing identified a novel mutation in the C-propeptide region of the molecule. The glycine to aspartic acid change occurred in a region that is highly conserved in all fibrillar collagen molecules. The resulting phenotype does not fit easily into pre-existing subgroups of the type II collagenopathies, which includes spondyloepiphyseal dysplasia, and the Kniest, Strudwick, and Stickler dysplasias.

Clinical, histological and ultrastructural studies of the posterior hyaloid membrane

AIMS

To investigate the histological, immunohistochemical and ultrastructural features of the posterior hyaloid membrane (PHM) in its naturally separated state in patients without previous surgery and slit-lamp documentation of antemortem posterior vitreous detachment (PVD).

METHODS

A prospective study was commenced in 1992 to recruit patients with physiological PVD from an unselected group of general medical inpatients and ascertain the prevalence of PVD. Postmortem specimens subsequently available were studied to analyse the clinicopathological correlation and processed using standard techniques for histology, immunohistochemistry and electron microscopy.

RESULTS

Eighty-five patients were examined with ages ranging from 68 to 98 yrs (mean 83.4 yrs). The posterior hyaloid membrane had clearly separated from the retina in 66% of eyes. Twenty-nine eyes from 15 patients were subsequently studied pathologically. The posterior hyaloid membrane exhibited a uniform cellular component, most densely populated around the Weiss' ring. The cells were characterised by oval or round nuclei, indistinct cytoplasm and were only seen within, or abutting, the weakly eosinophilic posterior hyaloid membrane. The posterior aspect of the posterior hyaloid membrane showed a convoluted appearance staining lightly with haematoxylin and eosin. The detached posterior hyaloid membrane exhibited focal positivity for GFAP and type IV collagen. Electron microscopy demonstrates both fibres and basement membrane associated with the cellular component including hemi-desmosome attachment plaques between the cells and basement membrane.

CONCLUSIONS

This study illustrates some of the structural differences between the posterior hyaloid membrane and the cortical vitreous gel it envelopes and demonstrates the presence of cells intimately associated with the posterior hyaloid membrane in its naturally separated state. We propose the cellular population integral to the PHM to be designated as laminocytes in order to emphasise their type IV collagen/basement membrane association and planar array within the membrane which separates at posterior vitreous detachment.

Molecular genetics of rhegmatogenous retinal detachment

Rhegmatogenous retinal detachment (RRD) most commonly occurs as a spontaneous event resulting from posterior vitreous detachment, typically between the ages of 40-70 yrs. It is also a feature in some inherited disorders, most commonly Stickler syndrome. The relationship between these inherited disorders and the spontaneous cases is unclear. Here in particular we review Stickler syndrome, and discuss the differential diagnosis of Stickler, Wagner and Marshall syndromes. Other rare inherited disorders associated with RRD are also briefly reviewed.

The effects of altitude, aspect, grazing and time on the proportion of cyanogenics in neighbouring populations of Trifolium repens L. (white clover)

The proportion of cyanogenic individuals of white clover amongst 200 individuals in each of 32 neighbouring populations in Northumberland, UK were recorded. Sites differed for four altitude zones, two aspects, and were either sheep pasture or ungrazed meadow. The proportion of cyanogenics showed a striking reduction with increasing altitude. Below 100 m, north-facing sites contained more cyanogenics than south-facing sites, but aspect did not affect higher sites. Grassland management had no effect on the proportion of cyanogenics. A 24-year study of one population showed highly significant changes in the proportion of cyanogenics over time, suggesting that a significant turnover of individuals occurs. In some cases, different size class cohorts varied for the proportion of cyanogenics within a year, and the same cohort varied between years. We conclude that environment at birth may determine the proportion of cyanogenics for that cohort, so that this proportion persists in that cohort as it matures. Comparisons of the proportion of cyanogenics with mean monthly averages for January minimum temperature, July maximum temperature and August rainfall showed a significant association only with mean January minima 2 1/2 years previously. We hypothesise that the 2 to 3-year cohort may predominate in this population. We suggest that winter cold and summer drought may both select against cyanogenics. Grazing by large herbivores does not favour cyanogenesis, but some invertebrate herbivory may do so. Most selection probably occurs at birth and will be greatest in populations with a high turnover.

Basilar artery aneurysm: an unusual cause of intractable hiccups

A70-year-old retired lorry driver was admitted as an emergency with a 2-day history of moderate amount of haematemesis, nausea, vomiting and intractable hiccups. He was a known hypertensive and 5 years previously had an episode of subarachnoid haemorrhage. Complications required the insertion of a Rickman reservoir shunt. Since the time of the subarachnoid haemorrhage, his hiccups had been almost constant despite taking chlorpromazine tablets 25 mg three times a day.

On examination, he had intractable hiccups. Cardiovascular, respiratory and abdominal examination were normal. Neurological examination revealed a right-sided partial third nerve palsy (with ptosis and pupillary sparing). He also had a right-sided squint operation during childhood. More detailed examination of the cranial nerves revealed mild right-sided glossopharyngeal and hypoglossal nerve palsies. Routine full blood count, blood biochemistry, chest X-ray and abdominal ultrasound scan were normal. Upper gastrointestinal endoscopy revealed a hiatus hernia with mild oesophagitis. He was started on omeprazole.

In view of the neurological findings and past history of subarachnoid bleed, a magnetic resonance imaging brain scan was performed. This revealed a large basilar artery aneurysm with a significant mass effect on the brainstem at the level of the border of fourth ventricle (Figures 1–3).

Alternative drug treatment in the form of nifedipine 10 mg three times a day was initiated instead of chlorpromazine and the hiccups resolved completely. On follow-up in the clinic a few months later, there was no recurrence of the hiccups.

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.

Seed weight and seed number affect subsequent fitness in outcrossing and selfing Primula species

Using the outcrossing Primula farinosa and its autogamous selfing relatives P. scotica, P. scandinavica and P. stricta, we compared the fitness of light and heavy seeds. Heavy seeds germinated in greater numbers and more quickly. In competition with seedlings grown from lighter seeds, heavy seeds produced larger rosettes. In P. farinosa such seedlings went on to produce more seeds, and in two populations heavier seeds, than plants from lighter seeds. After transplantation to natural populations, seedlings of P. farinosa derived from heavy seeds produced larger rosettes, more flowers and seeds than those from lighter seeds in certain populations so that seedlings born of heavy seeds were much fitter than seedlings from lighter seeds. Average seed weight varied in inverse proportion to seed number per capsule. The autogamous species produced on average about twice as many seeds per capsule as P. farinosa. In P. scotica and P. stricta this difference appears to be due in part to assured fertilization, but this high fecundity did not cause disadvantageously light seeds. As these species produced fewer capsules per scape, their overall seed production was on average no greater than for P. farinosa. P. farinosa traded-off fitness between capsules with large seed numbers, which donated more offspring to the next generation, and those with small seed numbers, whose heavy seeds would be more likely to reproduce themselves in the next generation. We conclude that low fecundity in outcrossing species might at times be advantageous.

Anaemia, diarrhoea and opportunistic infections in Fell ponies

This report summarises clinical and pathological observations on Fell pony foals with a range of signs that included ill thrift, anaemia, respiratory infection, glossal hyperkeratosis and diarrhoea. Some of the foals had normochromic, normocytic anaemia and some had low levels of plasma proteins, including immunoglobulin G. Antibiotic and supportive treatment was ineffective and all affected foals died or were killed on humane grounds. Postmortem examination of 12 foals and tissues from 2 other foals revealed a range of lesions that included glossal hyperkeratosis, typhlocolitis, intestinal cryptosporidiosis, granulomatous enteritis, proliferative and necrotising bronchiolitis consistent with adenovirus infection; lesions similar to those in the respiratory tract were present in the salivary gland and pancreas of individual foals. Lymphoid tissue was judged to be smaller than expected. These observations suggest the possibility of opportunistic infections secondary to some form of undefined immunocompromised state.

COL2A1 exon 2 mutations: relevance to the Stickler and Wagner syndromes

AIMS

To compare the clinical and molecular genetic features of two phenotypically distinct subgroups of families with type 1 Stickler syndrome.

BACKGROUND

Stickler syndrome (hereditary arthro-ophthalmopathy, McKusick Nos 108300 and 184840) is a dominantly inherited disorder of collagen connective tissue, resulting in an abnormal vitreous, myopia, and a variable degree of orofacial abnormality, deafness, and arthropathy. Stickler syndrome is the commonest inherited cause of rhegmatogenous retinal detachment in childhood with a risk of giant retinal tear (GRT) which is commonly bilateral and a frequent cause of blindness.

METHOD

Pedigrees were identified from the vitreoretinal service database and subclassified according to vitreoretinal phenotype. Ophthalmic, skeletal, auditory, and orofacial features were assessed. Linkage analysis was carried out with markers for the candidate genes COL2A1, COL11A1, and COL11A2. The COL2A1 gene was amplified as five overlapping PCR products. Direct sequencing of individual exons identified mutations.

RESULTS

Eight families exhibiting the type 1 vitreous phenotype were studied. Seven were consistent for linkage to COL2A1, with lod scores ranging from 2.1 to 0.3. In most instances linkage to COL11A1 and COL11A2 could be excluded. One family was analysed without prior linkage analysis. Three of the families exhibited a predominantly ocular phenotype with minimal or absent systemic involvement and were found to have mutations in exon 2 of COL2A1. Five other pedigrees with an identical ocular phenotype plus orofacial, auditory, and articular involvement had mutations in others regions of the COL2A1 gene. None of the pedigrees exhibited the characteristic lenticular, retinal pigment epithelial, or choroidal changes seen in Wagner syndrome.

CONCLUSIONS

These data confirm that type 1 Stickler syndrome is caused by mutations in the gene encoding type II collagen (COL2A1). In addition, data are submitted showing that mutations involving exon 2 of COL2A1 are characterised by a predominantly ocular variant of this disorder, consistent with the major form of type II procollagen in non-ocular tissues having exon 2 spliced out. Such patients are all at high risk of retinal detachment. This has important implications for counselling patients with regard to the development of systemic complications. It also emphasises the importance and reliability of the ophthalmic examination in the differential diagnosis of this predominantly ocular form of Stickler syndrome from Wagner's vitreoretinopathy.

Differentiation of murine premigratory primordial germ cells in culture

In the mouse embryo, primordial germ cells first appear in the extraembryonic mesoderm and divide rapidly while migrating to the fetal gonad. Shortly after their arrival in the gonad, germ cells sexually differentiate as proliferation ceases. Previous studies have established that primordial germ cells proliferate and migrate in feeder layer culture. To explore cellular regulation of fetal germ cell development, we have used germ cell nuclear antigen 1 (GCNA1), a marker normally expressed only in postmigratory germ cells, to investigate the developmental potency of both pre- and postmigratory cells in this culture system. We found that explanted premigratory germ cells will initiate expression of this marker and are, therefore, capable of undertaking some aspects of gonocyte differentiation without intimate exposure to the fetal gonad. We have also tested whether postmigratory gonocytes are stable in culture. As detected by either alkaline phosphatase or GCNA1, we did not detect long-term survival of either prospermatogonia or oogonia under conditions that support the survival, proliferation, and differentiation of earlier premigratory cells. These observations are consistent with an autonomous cellular mechanism governing the initial stages of gonocyte differentiation, and suggest that differentiation towards gonocytes is accompanied by a change in requirements for cell survival.

Stickler syndrome: further mutations in COL11A1 and evidence for additional locus heterogeneity

Stickler syndrome (hereditary arthro-ophthalmopathy) is a dominantly inherited connective tissue disorder with ocular, oro-facial, auditory and skeletal manifestations. It is genetically and phenotypically heterogeneous with the majority of families having mutations in the gene encoding type II collagen (COL2A1) and exhibiting a characteristic 'membranous' or type 1 vitreous phenotype. More recently a novel mutation in the gene encoding the alpha1 chain of type XI collagen (COL11A1) was reported in a Stickler syndrome pedigree with a different 'beaded' or type 2 vitreous phenotype. In the present study five more families with the type 2 vitreous phenotype were examined for linkage to four candidate genes: COL2A1, COL5A2, COL11A1 and COL11A2. Two families were linked to COL11A1 and sequencing identified mutations resulting in shortened alphal(XI) collagen chains, one via exon skipping and the other via a multiexon deletion. One of the families showed weak linkage to COL5A2 but sequencing the open reading frame failed to identify a mutation. In the remaining two families all four loci were excluded by linkage analysis. These data confirm that mutations in COL11A1 cause Stickler syndrome with the type2 vitreous phenotype and also reveal further locus heterogeneity.