Two new missense mutations (A105T and C110G) in the norrin gene in two Italian families with Norrie disease and familial exudative vitreoretinopathy

Clinical Exome Sequencing Identifies NDP Gene Variants in Two Chinese Families with X-Linked Norrie Disease

Purpose: To explore the genetic defects in two Chinese families with X-linked Norrie disease (ND). Methods: We analyzed two Chinese families with ND at molecular level through clinical exome sequencing and the variations were identified by Sanger sequencing. Results: Two genetic variations were found in the NDP gene by clinical exome sequencing, a partial deletion of 801 bp contained the whole exon 2 and a missense variant (164G>A) within codon 55 that resulted in the interchange of cysteine by phenylalanine. Clinical findings were more severe in the patients who presented the missense variant. Conclusion: We report two genetic variations in the NDP gene in Chinese that extend the mutational and phenotypic spectra of NDP gene, and also demonstrate the feasibility of clinical exome sequencing in application of molecular diagnosis.

Norrie disease gene polymorphism in Indonesian infants with retinopathy of prematurity

OBJECTIVE

Retinopathy of prematurity (ROP) is a major cause of blindness in newborn infants, which also occurs in low-income and middle-income countries. Why ROP progresses in some infants while it regresses in others is still presently unknown. Studies suggest that genetic factors might be involved. Mutations in the Norrie disease (ND) gene are suspected to be related to advanced ROP development. Indonesia is a country with relatively high incidence of ROP, yet the role of these genetic factors in the pathogenesis of ROP cases is still unknown. The study aimed to investigate the presence of mutations in ND on the X chromosome in infants with both non-advanced and advanced ROP in Indonesia.

METHODS AND ANALYSIS

This is a case-control study of polymorphisms in six variants within the ND gene in exon 3, C597A, L108P, R121W, A105T, V60E and C110G, in preterm newborn infants in four major hospitals in Greater Jakarta, Indonesia.

RESULTS

We included 162 preterm newborn infants. ROP was diagnosed in 83 infants, and 79 infants served as controls. Among those with ROP, 57 infants had type 2, while others had type 1. We did not find any gene polymorphisms in any of the infants with ROP nor in the control group.

CONCLUSION

We conclude that it is very unlikely that the six polymorphisms in exon 3 of the ND gene studied in this paper are involved in the development or progression of ROP in preterm infants in our population sample in Indonesia.

Genotype-Phenotype Characterization of Novel Variants in Six Italian Patients with Familial Exudative Vitreoretinopathy

Familial exudative vitreoretinopathy (FEVR) is a complex disorder characterized by incomplete development of the retinal vasculature. Here, we report the results obtained on the spectrum of genetic variations and correlated phenotypes found in a cohort of Italian FEVR patients. Eight probands (age range 7–19 years) were assessed by genetic analysis and comprehensive age-appropriate ophthalmic examination. Genetic testing investigated the genes most widely associated in literature with FEVR: FZD4, LRP5, TSPAN12, and NDP. Clinical and genetic evaluations were extended to relatives of probands positive to genetic testing. Six out of eight probands (75%) showed a genetic variation probably related to the phenotype. We identified four novel genetic variants, one variant already described in association with Norrie disease and one previously described linked to autosomal dominant FEVR. Pedigree analysis of patients led to the classification of four autosomal dominant cases of FEVR (caused by FZD4 and TSPAN12 variants) and two X-linked FEVR probands (NDP variants). None of the patients showed variants in the LRP5 gene. This study represents the largest cohort study in Italian FEVR patients. Our findings are in agreement with the previous literature confirming that FEVR is a clinically and genetically heterogeneous retinal disorder, even when it manifests in the same family.

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.

Mutations in the NDP gene: contribution to Norrie disease, familial exudative vitreoretinopathy and retinopathy of prematurity

BACKGROUND

To examine the contribution of mutations within the Norrie disease (NDP) gene to the clinically similar retinal diseases Norrie disease, X-linked familial exudative vitreoretinopathy (FEVR), Coat's disease and retinopathy of prematurity (ROP).

METHODS

A dataset comprising 13 Norrie-FEVR, one Coat's disease, 31 ROP patients and 90 ex-premature babies of <32 weeks' gestation underwent an ophthalmologic examination and were screened for mutations within the NDP gene by direct DNA sequencing, denaturing high-performance liquid chromatography or gel electrophoresis. Controls were only screened using denaturing high-performance liquid chromatography and gel electrophoresis. Confirmation of mutations identified was obtained by DNA sequencing.

RESULTS

Evidence for two novel mutations in the NDP gene was presented: Leu103Val in one FEVR patient and His43Arg in monozygotic twin Norrie disease patients. Furthermore, a previously described 14-bp deletion located in the 5' unstranslated region of the NDP gene was detected in three cases of regressed ROP. A second heterozygotic 14-bp deletion was detected in an unaffected ex-premature girl. Only two of the 13 Norrie-FEVR index cases had the full features of Norrie disease with deafness and mental retardation.

CONCLUSION

Two novel mutations within the coding region of the NDP gene were found, one associated with a severe disease phenotypes of Norrie disease and the other with FEVR. A deletion within the non-coding region was associated with only mild-regressed ROP, despite the presence of low birthweight, prematurity and exposure to oxygen. In full-term children with retinal detachment only 15% appear to have the full features of Norrie disease and this is important for counselling parents on the possible long-term outcome.

Molecular analysis of the NDP gene in two families with Norrie disease

PURPOSE

To describe the molecular defects in the Norrie disease protein (NDP) gene in two families with Norrie disease (ND).

METHODS

We analysed two families with ND at molecular level through polymerase chain reaction, DNA sequence analysis and GeneScan.

RESULTS

Two molecular defects found in the NDP gene were: a missense mutation (265C > G) within codon 97 that resulted in the interchange of arginine by proline, and a partial deletion in the untranslated 3' region of exon 3 of the NDP gene. Clinical findings were more severe in the family that presented the partial deletion. We also diagnosed the carrier status of one daughter through GeneScan; this method proved to be a useful tool for establishing female carriers of ND.

CONCLUSION

Here we report two novel mutations in the NDP gene in Mexican patients and propose that GeneScan is a viable mean of establishing ND carrier status.

Vascular defects and sensorineural deafness in a mouse model of Norrie disease

Norrie disease is an X-linked recessive syndrome of blindness, deafness, and mental retardation. A knock-out mouse model with an Ndp gene disruption was studied. We examined the hearing phenotype, including audiological, histological, and vascular evaluations. As is seen in humans, the mice had progressive hearing loss leading to profound deafness. The primary lesion was localized to the stria vascularis, which houses the main vasculature of the cochlea. Fluorescent dyes showed an abnormal vasculature in this region and eventual loss of two-thirds of the vessels. We propose that one of the principal functions of norrin in the ear is to regulate the interaction of the cochlea with its vasculature.

A C597-->A polymorphism in the Norrie disease gene is associated with advanced retinopathy of prematurity in premature Kuwaiti infants

Retinopathy of prematurity (ROP) is a retinal vascular disease which occurs in infants with a short gestational age and low birth weight and may lead to retinal detachment and blindness. In some premature infants, ROP progresses to advanced stages despite rigorous intervention, but in the majority, it spontaneously regresses before the threshold stage. Genetic factors, e.g. mutations in the Norrie disease (ND) gene, have been implicated in determining the progression of ROP to advanced stages. We have identified a novel C597A polymorphism of the ND gene; we screened this and another mutation in the ND gene, C110G, in 210 premature Kuwaiti infants using PCR-RFLP, DNA sequence analysis and DNA enzyme immunoassay hybridization to investigate their association with advanced-stage ROP. In this cohort of premature Kuwaiti newborns, 115 of 210 babies had no eye problems and served as controls, while 95 were found to have ROP. In 71 of the 95 ROP cases, the disease spontaneously regressed at or before stage 3, while in 24 of 95 ROP cases, the disease progressed to advanced stages 4 or 5. The incidence of the AA genotype of the C597A polymorphism was considerably higher in advanced-stage ROP cases (83.3%) compared to spontaneously regressing ROP cases (0%) and the normal controls (10.4%) (p < 0.0001). For the other genotypes, no significant difference was detected between the controls and ROP cases. In the case of the C110G mutation in the ND gene, no significant differences were detected between the controls and ROP cases, and the majority of subjects had a CC genotype in all three groups.

Two Thai families with Norrie disease (ND): association of two novel missense mutations with severe ND phenotype, seizures, and a manifesting carrier

We describe two Thai families with Norrie disease (ND) in three generations, including 10 affected males and one manifesting female. All affected males in each family had severely defective eye development with complete loss of vision. In addition, three male patients (one from family 1 and two from family 2) suffered from epilepsy, and one female carrier from one family manifested blindness with phthisis bulbi in her right eye. Mutation analysis of the ND gene (NDP) revealed two different novel missense mutations (L16P and S75P) that co-segregated with ND in each family, suggesting that the newly appearing proline at codon 16 or codon 75 alters the conformation of the ND protein and contributes to the severe phenotype of ND in each family. Other studies suggest that epileptic seizures or growth retardation that is associated with ND is the consequence of loss of contiguous genes, because most such patients had deletions extending beyond the Norrie locus. Our finding that the three affected males in the two families with the missense mutations had epilepsy does not support a contiguous gene effect, but favors the pleiotropism of NDP, at least as far as the epileptic manifestation is concerned. The unilateral blindness in the female carrier may have been due to non-random X-inactivation.

Retinopathy of prematurity: mutations in the Norrie disease gene and the risk of progression to advanced stages

BACKGROUND

Retinopathy of prematurity (ROP) is a retinal vascular disease that occurs in infants with short gestational age and low birth weight and may lead to retinal detachment and blindness. Missense mutations in the Norrie disease (ND) gene have been associated with the risk of progression to advanced stages in cases of ROP from the US and also in clinically similar ND and familial exudative vitreoretinopathy.

METHODS

We have screened two ND gene mutations, namely A105T and Val60Glu, by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and allele-specific PCR methods, respectively, in 210 Kuwaiti premature newborns to replicate these findings in a different ethnic group.

RESULTS

In the Kuwaiti premature newborn cohort, 115 of 210 babies had no eye problems and served as controls, while 95 were cases of ROP. In 71 of 95 ROP cases, the disease regressed spontaneously on or before stage 3, while in 24 of 95 ROP cases the disease progressed to advanced stages 4 and 5. In case of missense mutation (A105T), the AA genotype was detected in 96% of controls compared with 87% of ROP cases (NS); similarly no significant difference was found between spontaneously regressed ROP cases and those who progressed to advanced stages. For the Val60Glu mutation, no significant association was detected between the genotype and progression of ROP to advanced stages.

CONCLUSIONS

Unlike data from the US, our findings from a Kuwaiti cohort of ROP cases and controls suggest a lack of association between the two ND gene mutations (A105T and Val60Glu) and ROP and the risk of progression of the disease to advanced stages.

Molecular dissection of Norrie disease

Norrie disease (ND) is a severe form of congenital blindness accompanied by mental retardation and/or deafness in at least one third of the patients. This article summarizes advances in the molecular genetic analysis of this disease during the last 13 years, including mapping and cloning of the human gene and the generation and characterization of a mouse model. Genetic linkage studies and physical mapping strategies have assigned the ND locus to the proximal short arm of the human X chromosome. The identification of chromosomal rearrangements in several patients, such as microdeletions, enabled the isolation of the ND gene by a positional cloning approach. Numerous point mutations in this gene have been identified in three distinct clinical entities: (1) ND, (2) familial and sporadic exudative vitreoretinopathy, and (3) retinopathy of prematurity. The gene encodes a relatively small protein, consisting of 133 amino acids. The function of the gene product is yet unknown, although homologies with known proteins and molecular modelling data suggest a role in the regulation of cell interaction or differentiation processes. A mouse model has been generated to shed more light on early pathogenic events involved in ND and allelic disorders. The mouse homologous protein is highly identical (94%) to the human polypeptide. The gene is expressed in the neuronal layers of the mouse retina, the cerebellum and olfactory epithelium. Mutant mice show snowflake-like opacities within the vitreous, dysgenesis of the ganglion cell layer and occasionally degeneration of photoreceptor cells. The mouse phenotype does not include phthisis bulbi and, overall, resembles a mild form of ND. Electrophysiological studies revealed a severely altered electroretinogram b-wave. These results suggest a primary defect in the inner neuronal layers of the retina. Defects in the vitreous and photoreceptor cell layer are most likely secondary effects. Further histological, functional and molecular studies of the mouse model are needed to provide additional information on disease associated pathways.