Phenotypic heterogeneity associated with a novel mutation (Gly112Glu) in the Norrie disease protein

Inherited causes of combined vision and hearing loss: clinical features and molecular genetics

Combined vision and hearing loss, also known as dual sensory impairment, can occur in several genetic conditions, including ciliopathies such as Usher and Bardet-Biedl syndrome, mitochondrial DNA disorders and systemic diseases, such as CHARGE, Stickler, Waardenburg, Alport and Alstrom syndrome. The retinal phenotype may point to the diagnosis of such disorders. Herein, we aim to provide a comprehensive review of the molecular genetics and clinical features of the most common non-chromosomal inherited disorders to cause dual sensory impairment.

Xp11.3 microdeletion causing Norrie disease and X-linked Kabuki syndrome

Purpose

To describe a novel case of Norrie disease and X-linked Kabuki syndrome caused by a microdeletion encompassing multiple genes on the X chromosome.

Observations

A 3-day-old boy born at full term had bilateral retrolental fibrovascular plaques. Surgery with lensectomy and vitrectomy revealed bilateral, closed funnel retinal detachments consistent with a clinical diagnosis of Norrie disease. In addition, the baby had congenital heart defects, hearing loss, and dysmorphic facies. His mother carried a clinical diagnosis of Kabuki syndrome. Genetic testing of the baby revealed an Xp11.3 microdeletion that included the NDP and KDM6A genes, confirming the baby had both Norrie disease and X-linked Kabuki syndrome. The mother was found via ultrawide-field fluorescein angiography to have asymptomatic peripheral retinal vascular anomalies, consistent with NDP-associated familial exudative vitreoretinopathy (FEVR).

Conclusions and importance

This is the first reported case of Norrie disease together with X-linked Kabuki syndrome. Contiguous gene deletions may explain some of the variable systemic involvement in Norrie disease.

Severe Familial Exudative Vitreoretinopathy, Congenital Hearing Loss, and Developmental Delay in a Child With Biallelic Variants in FZD4

Question

Are FZD4 variants associated with familial exudative vitreoretinopathy (FEVR) with extraocular features?

Findings

This case series included a patient with biallelic FZD4 variants with severe FEVR in infancy, congenital hearing loss, and developmental delay. Each parent was carrying 1 of the alleles and manifested mild FEVR; cell-based FZD4 receptor-activation assays determined that FZD4 function was dramatically decreased in the presence of these compound heterozygous variants.

Meaning

Biallelic variants in FZD4 can result in a severe ocular phenotype with systemic features, which may represent a novel syndrome.

Importance

Familial exudative vitreoretinopathy (FEVR) is a nonsyndromic autosomal dominant retinal disorder commonly caused by variants in the FZD4 gene. This study investigates the potential role beyond ocular abnormalities for FZD4 gene variants in patients with FEVR.

Objective

To evaluate the role of FZD4 in symptoms beyond those associated with FEVR through a patient with biallelic variants in FZD4.

Design, Setting, and Participants

This case series included the DNA testing and phenotyping of 1 patient proband and her parents, combined with signaling assays, to determine the association of patient-derived compound heterozygous variants on FZD4 signaling and biologic function.

Main Outcomes and Measures

FZD4 genes were tested using next-generation sequencing and Sanger sequencing. Cell-based assays measured the effect of the variants on FZD4 signaling.

Results

The proband presented with absent red reflexes from complete tractional retinal detachments diagnosed at 3 days of age and failed the newborn screening hearing test. Auditory brainstem response at 6 months of age showed bilateral mild to moderate high-frequency sensorineural hearing loss. The patient manifested developmental delays in speech and walking. Intravenous fluorescein angiography (IVFA) of the patient’s parents detected stage 1 FEVR. Genetic testing revealed 2 FZD4 variants in the patient, each variant found in 1 parent. Signaling assays confirmed that the presence of both variants was associated with significantly worse signaling activity compared with the heterozygous state.

Conclusions and Relevance

Results of this case series suggest that extraocular syndromic FEVR was associated with FZD4 variants. The decrease in FZD4 signaling owing to the biallelic nature of the disease resulted in hearing deficits, developmental delays, and a more severe retinal phenotype.

Hearing Function, Degeneration, and Disease: Spotlight on the Stria Vascularis

The stria vascularis (SV) is a highly vascularized tissue lining the lateral wall of the cochlea. The SV maintains cochlear fluid homeostasis, generating the endocochlear potential that is required for sound transduction. In addition, the SV acts as an important blood-labyrinth barrier, tightly regulating the passage of molecules from the blood into the cochlea. A healthy SV is therefore vital for hearing function. Degeneration of the SV is a leading cause of age-related hearing loss, and has been associated with several hearing disorders, including Norrie disease, Meniere's disease, Alport syndrome, Waardenburg syndrome, and Cytomegalovirus-induced hearing loss. Despite the SV's important role in hearing, there is still much that remains to be discovered, including cell-specific function within the SV, mechanisms of SV degeneration, and potential protective or regenerative therapies. In this review, we discuss recent discoveries elucidating the molecular regulatory networks of SV function, mechanisms underlying degeneration of the SV, and otoprotective strategies for preventing drug-induced SV damage. We also highlight recent clinical developments for treating SV-related hearing loss and discuss future research trajectories in the field.

Impact of sight and hearing loss in patients with Norrie disease: advantages of Dual Sensory clinics in patient care

Norrie disease (ND) is a rare, X-linked condition of visual and auditory impairment, often presenting with additional neurological features and developmental delays of varying severity. While all affected patients are born blind, or lose their vision in infancy, progressive sensorineural hearing loss develops in the majority of cases and is typically detected in the second decade of life. A range of additional symptoms of ND, such as seizure disorders, typically appear from a young age, but it is difficult to predict the range of symptoms ND patients will experience. After growing up without vision, hearing loss represents the greatest worry for many patients with ND, as they may lose the ability to participate in previously enjoyed activities or to communicate with others. Dual sensory loss has a physical, psychosocial and financial impact on both patients with ND and their families. Routine monitoring of the condition is required in order to identify, treat and provide support for emerging health problems, leading to a large burden of medical appointments. Many patients need to travel long distances to meet with specialists, representing a further burden on time and finances. Additionally, the rare nature of dual sensory impairment in children means that few clinical environments are designed to meet their needs. Dual Sensory clinics are multidisciplinary environments designed for sensory-impaired children and have been suggested to alleviate the impact of diseases involving sensory loss such as ND. Here, we discuss the diagnosis, monitoring and management of ND and the impact it has on paediatric patients and their caregivers. We describe the potential for dual sensory clinics to reduce disease burden through providing an appropriate clinical environment, access to multiple clinical experts in one visit, and ease of monitoring for patients with ND.

Introgression of regulatory alleles and a missense coding mutation drive plumage pattern diversity in the rock pigeon

Birds and other vertebrates display stunning variation in pigmentation patterning, yet the genes controlling this diversity remain largely unknown. Rock pigeons (Columba livia) are fundamentally one of four color pattern phenotypes, in decreasing order of melanism: T-check, checker, bar (ancestral), or barless. Using whole-genome scans, we identified NDP as a candidate gene for this variation. Allele-specific expression differences in NDP indicate cis-regulatory divergence between ancestral and melanistic alleles. Sequence comparisons suggest that derived alleles originated in the speckled pigeon (Columba guinea), providing a striking example of introgression. In contrast, barless rock pigeons have an increased incidence of vision defects and, like human families with hereditary blindness, carry start-codon mutations in NDP. In summary, we find that both coding and regulatory variation in the same gene drives wing pattern diversity, and post-domestication introgression supplied potentially advantageous melanistic alleles to feral populations of this ubiquitous urban bird.

A novel missense NDP mutation [p.(Cys93Arg)] with a manifesting carrier in an austrian family with Norrie disease

Norrie disease is a rare, X-linked genetic syndrome characterized by combined congenital blindness and progressive hearing impairment. Norrie disease is caused by alterations in the NDP gene encoding the growth factor norrin that plays a key role in vascular development and stabilization of the eye, inner ear and brain. We identified a family with 3 affected deafblind males and a single female carrier presenting with a serous retinal detachment but normal hearing. Genetic analysis revealed a novel c.277T>C missense mutation causing the substitution of a hydrophobic cysteine to a hydrophilic arginine [p.(Cys93Arg)] within the highly conserved cysteine knot domain of the norrin protein. These results should expand the scope for amniocentesis and genetic testing for Norrie disease which is gaining in importance due to novel postnatal therapeutic concepts to alleviate the devastating retinal symptoms of Norrie disease.

Screening for NDP mutations in 44 unrelated patients with familial exudative vitreoretinopathy or Norrie disease

PURPOSE

To screen mutations in the norrin (NDP) gene in 44 unrelated Chinese patients with familial exudative vitreoretinopathy (FEVR, 38 cases) or Norrie disease (6 cases) and to describe the associated phenotypes.

METHODS

Of the 44 patients, mutation in FZD4, LRP5, and TSPAN12 was excluded in 38 patients with FEVR in previous study. Sanger sequencing was used to analyze the 2 coding exons and their adjacent regions of NDP in the 44 patients. Clinical data were presented for patients with mutation.

RESULTS

NDP variants in 5 of the 6 patients with Norrie disease were identified, including a novel missense mutation (c.164G>A, p.Cys55Phe) in one patient, two known missense mutations (c.122G>A, p.Arg41Lys; c.220C>T, p.Arg74Cys) in two patients, and a gross deletion encompassing the two coding exons in two patients. Of the 5 patients, 3 had a family history and 2 were singleton cases. No mutation in NDP was detected in the 38 patients with FEVR.

CONCLUSIONS

NDP mutations are common cause of Norrie disease but might be rare cause for FEVR in Chinese.

Novel and recurrent NDP gene mutations in familial cases of Norrie disease and X-linked exudative vitreoretinopathy

PURPOSE

To present the results of molecular analysis of the NDP gene in Mexican families with Norrie disease (ND) and X-linked familial exudative vitreoretinopathy (XL-FEVR).

METHODS

Two unrelated families with ND and two with XL-FEVR were studied. Clinical diagnosis was suspected on the basis of a complete ophthalmologic examination. Molecular methods included DNA isolation from peripheral blood leucocytes, polymerase chain reaction amplification and direct nucleotide sequencing analysis of the complete coding region and exon-intron junctions of NDP. Haplotype analysis using NDP-linked microsatellites markers was performed in both ND families.

RESULTS

A novel Norrin missense mutation, p.Arg41Thr, was identified in two apparently unrelated families with ND. Haplotype analysis demonstrated that affected males in these two families shared the same ND-linked haplotype, suggesting a common origin for this novel mutation. The previously reported p.Arg121Trp and p.Arg121Gln Norrin mutations were identified in the two families with XL-FEVR.

CONCLUSION

Our results expand the mutational spectrum in ND. This is the first report of ND resulting from mutation at arginine position 41 of Norrin. Interestingly, mutations at the same residue but resulting in a different missense change were previously described in subjects with XL-FEVR (p.Arg41Lys) or persistent fetal vasculature syndrome (p.Arg41Ser), indicating that the novel p.Arg41Thr change causes a more severe retinal phenotype. Preliminary data suggest a founder effect for the ND p.Arg41Thr mutation in these two Mexican families.

Retinal vascular abnormalities and dragged maculae in a carrier with a new NDP mutation (c.268delC) that caused severe Norrie disease in the proband

Norrie disease (ND) is caused by mutations in the ND pseudoglioma (NDP) gene (MIM 300658) located at chromosome Xp11.4-p11.3. ND is characterized by abnormal retinal vascular development and vitreoretinal disorganization presenting at birth. Systemic manifestations include sensorineural deafness, progressive mental disorder, behavioral and psychological problems, growth failure, and seizures. Other vitreoretinopathies that are associated with NDP gene mutations include X-linked familial exudative vitreoretinopathy, Coats disease, persistent fetal vasculature, and retinopathy of prematurity. Phenotypic variability associated with NDP gene mutations has been well documented in affected male patients. However, there are limited data on signs in female carriers, with mild peripheral retinal abnormalities reported in both carrier and noncarrier females of families with NDP gene mutations. Here, we report a family harboring a single base-pair deletion, c.268delC, in the NDP gene causing a severe ND phenotype in the male proband and peripheral retinal vascular abnormalities with dragged maculae similar to those observed in familial exudative vitreoretinopathy in his carrier mother.

Genetic Evaluation to Establish the Diagnosis of X-Linked Familial Exudative Vitreoretinopathy

PURPOSE

To determine the usefulness of genetic analysis for confirming the diagnosis of X-linked familial exudative vitreoretinopathy (FEVR) and verifying the mode of inheritance.

METHODS

Twenty-seven consecutive patients diagnosed with FEVR were enrolled for genetic analysis. All patients underwent dilated fundus examination. A complete birth, medical, and family history was obtained at the time of examination. Patients were categorized by gender and family history in an effort to identify X-linked FEVR. Participants provided a blood sample for analysis and were evaluated for a mutation in the Norrie's disease gene (NDP) by direct sequencing.

RESULTS

Of the 27 enrolled patients, four male patients had a pedigree consistent with X-linked inheritance and 12 male patients had little or no family history. Two of these 16 patients were found to have a missense mutation in the NDP gene.

CONCLUSIONS

We found genetic testing of NDP to be helpful in confirming the diagnosis of X-linked FEVR in male patients, especially when limited family history was available. As genetic diagnostics improve, we feel that confirming diagnoses and informing patients better through genetic evaluation and consultation will become more useful in the clinical practice of ophthalmology.

SNPs: impact on gene function and phenotype

Single nucleotide polymorphism (SNP) is the simplest form of DNA variation among individuals. These simple changes can be of transition or transversion type and they occur throughout the genome at a frequency of about one in 1,000 bp. They may be responsible for the diversity among individuals, genome evolution, the most common familial traits such as curly hair, interindividual differences in drug response, and complex and common diseases such as diabetes, obesity, hypertension, and psychiatric disorders. SNPs may change the encoded amino acids (nonsynonymous) or can be silent (synonymous) or simply occur in the noncoding regions. They may influence promoter activity (gene expression), messenger RNA (mRNA) conformation (stability), and subcellular localization of mRNAs and/or proteins and hence may produce disease. Therefore, identification of numerous variations in genes and analysis of their effects may lead to a better understanding of their impact on gene function and health of an individual. This improved knowledge may provide a starting point for the development of new, useful SNP markers for medical testing and a safer individualized medication to treat the most common devastating disorders. This will revolutionize the medical field in the future. To illustrate the effect of SNPs on gene function and phenotype, this minireview focuses on evidences revealing the impact of SNPs on the development and progression of three human eye disorders (Norrie disease, familial exudative vitreoretinopathy, and retinopathy of prematurity) that have overlapping clinical manifestations.