Helen M. Bottomley

Mutations in LRP5 or FZD4 Underlie the Common Familial Exudative Vitreoretinopathy Locus on Chromosome 11q

Familial exudative vitreoretinopathy (FEVR) is an inherited blinding disorder of the retinal vascular system. Autosomal dominant FEVR is genetically heterogeneous, but its principal locus, EVR1, is on chromosome 11q13-q23. The gene encoding the Wnt receptor frizzled-4 (FZD4) was recently reported to be the EVR1 gene, but our mutation screen revealed fewer patients harboring mutations than expected. Here, we describe mutations in a second gene at the EVR1 locus, low-density-lipoprotein receptor-related protein 5 (LRP5), a Wnt coreceptor. This finding further underlines the significance of Wnt signaling in the vascularization of the eye and highlights the potential dangers of using multiple families to refine genetic intervals in gene-identification studies.

Recessive mutations in TSPAN12 cause retinal dysplasia and severe familial exudative vitreoretinopathy (FEVR).

PURPOSE Familial exudative vitreoretinopathy (FEVR) is an inherited disorder that disrupts the development of the retinal vasculature and can result in blindness. FEVR is genetically heterogeneous and mutations in four genes, NDP, FZD4, LRP5, and TSPAN12, encoding components of a novel ligand-receptor complex that activates the Norrin-β-catenin signaling pathway, account for approximately 50% of cases. We recently identified mutations in TSPAN12 as a cause of dominant FEVR. The purpose of this study was to identify recessive TSPAN12 mutations in FEVR patients. METHODS Mutation screening was performed by directly sequencing PCR products generated from genomic DNA with primers designed to amplify the coding sequence of TSPAN12. Splicing defects were verified by reverse transcriptase PCR of leukocyte cDNA. RESULTS TSPAN12 screening in a large dominant FEVR family unexpectedly led to the identification of homozygous mutations in severely affected family members, whereas mildly affected family members were heterozygous. Further screening in a cohort of 10 retinal dysplasia/severe FEVR patients identified an additional three cases with recessive TSPAN12 mutations. In all examined cases, single mutation carriers were mildly affected compared to patients harboring two TSPAN12 mutations. CONCLUSIONS We report for the first time recessive mutations in TSPAN12 and describe the first genetic cause for the clinical variation seen in FEVR families. Our data raise the possibility that patients with severe FEVR actually may harbor two mutant alleles, derived either from the same gene or potentially from other genes encoding components of the Norrin-β-catenin signaling pathway.

Reduced bone mineral density and hyaloid vasculature remnants in a consanguineous recessive FEVR family with a mutation in LRP5

Background/aims: Familial exudative vitreoretinopathy (FEVR) is an inherited blinding condition characterised by abnormal development of the retinal vasculature. FEVR has multiple modes of inheritance, and homozygous mutations in LRP5 have recently been reported as underlying the recessive form of this disease. The aim of this study was to examine LRP5 in a consanguineous recessive FEVR family and to clarify the eye and bone phenotype associated with recessive FEVR. Methods: All family members were examined by slit lamp biomicroscopy and indirect ophthalmoscopy. Linkage to LRP5 was determined by genotyping microsatellite markers, constructing haplotypes and calculating lod scores. Mutation screening of LRP5 was performed by polymerase chain reaction amplification of genomic DNA followed by direct sequencing. Bone mineral density (BMD) was evaluated in all family members using dual energy x ray absorptiometry (DEXA). Results: The clinical features observed in this family were consistent with a diagnosis of recessive FEVR. A homozygous LRP5 missense mutation, G550R, was identified in all affected individuals and all unaffected family members screened were heterozygous carriers of this mutation. Reduced BMD, hyaloid vasculature remnants, and nystagmus were features of the phenotype. Conclusion: Recessive mutations in LRP5 can cause FEVR with reduced BMD and hyaloid vasculature remnants. Assessment of a patient with a provisional diagnosis of FEVR should therefore include investigation of BMD, with reduced levels suggestive of an underlying LRP5 mutation.

Further evidence of genetic heterogeneity in familial exudative vitreoretinopathy; exclusion of EVR1, EVR3, and EVR4 in a large autosomal dominant pedigree

Background/aims: Familial exudative vitreoretinopathy (FEVR) is an inherited blinding condition characterised by abnormal development of the retinal vasculature. The aim of this study was to perform linkage analysis in a large family affected with FEVR to determine whether the mutation involved was in one of the three known autosomal dominant FEVR loci or in another as yet unidentified gene. Methods: Genomic DNA samples from family members were polymerase chain reaction (PCR) amplified with fluorescently tagged microsatellite markers spanning the EVR1/EVR4 locus (11q13-14) and the EVR3 locus (11p12-13). The resulting PCR products were resolved using an automated DNA sequencer and the alleles sized. These data were used to construct haplotypes across each locus and linkage analysis was performed to prove or exclude linkage. Results: The clinical evaluation in this family suggested features typical of FEVR, with deficient peripheral retinal vascularisation being the common phenotype in all affected individuals. However, linkage analysis proved that this family has a form of FEVR genetically distinct from the EVR1, EVR3 and EVR4 loci. Conclusion: The exclusion of linkage in this family to any of the known FEVR loci proves the existence of a fourth locus for autosomal dominant FEVR and shows that this rare disorder is far more heterogeneous than previously thought.

Comment on ‘cosegregation of two unlinked mutant alleles in some cases of autosomal dominant familial exudative vitreoretinopathy’

Comment on ‘cosegregation of two unlinked mutant alleles in some cases of autosomal dominant familial exudative vitreoretinopathy’