Mizuo–Nakamura phenomenon in X-linked retinoschisis

Purpose

To determine whether the Mizuo–Nakamura phenomenon, which is an important diagnostic sign of Oguchi's disease, also occurs in patients with genetically proven X-linked retinoschisis (XLRS).

Methods

We examined three patients with a clinical and genetic diagnosis of XLRS and one patient who was clinically diagnosed with Oguchi's disease, with an emphasis on the Mizuo–Nakamura phenomenon. We obtained color fundus photographs, especially in the fully dark-adapted state, using the non-mydriatic mode on a digital retinal camera and infrared observation monitor to avoid the bleaching effects caused by the viewing light, which alters the fundus color in a short time.

Results

The Mizuo–Nakamura phenomenon was observed in all patients with molecularly proven XLRS, similar to that in the patient with Oguchi's disease. The sets of photographs were obtained in the light- and dark-adapted states using our newly devised techniques needed to witness the Mizuo-Nakamura phenomenon.

Conclusions and Importance

The Mizuo–Nakamura phenomenon was identified in three patients with genetically proven XLRS. To the best of our knowledge, this study provided the first genetic evidence of the Mizuo–Nakamura phenomenon in a patient with molecularly proven XLRS without the causative genetic abnormalities for Oguchi's disease. Our findings suggest that XLRS is responsible for the Mizuo–Nakamura phenomenon and its presence in XLRS is not a rare exception but may be a consistent manifestation of XLRS.

A point mutation in Semaphorin 4A associates with defective endosomal sorting and causes retinal degeneration

Semaphorin 4A (Sema4A) has an essential role in photoreceptor survival. In humans, mutations in Sema4A are thought to contribute to retinal degenerative diseases. Here we generate a series of knock-in mouse lines with corresponding mutations (D345H, F350C or R713Q) in the Sema4A gene and find that Sema4A^F350C causes retinal degeneration phenotypes. The F350C mutation results in abnormal localization of the Sema4A protein, leading to impaired endosomal sorting of molecules indispensable for photoreceptor survival. Additionally, protein structural modelling reveals that the side chain of the 350th amino acid is critical to retain the proper protein conformation. Furthermore, Sema4A gene transfer successfully prevents photoreceptor degeneration in Sema4A^F350C/F350C and Sema4A^−/− mice. Thus, our findings not only indicate the importance of the Sema4A protein conformation in human and mouse retina homeostasis but also identify a novel therapeutic target for retinal degenerative diseases.

Semaphorin 4A is implicated in photoreceptor survival. Nojima and colleagues generate transgenic mice with different mutations in the Sema4A gene and find that point mutation of F350 causes severe degeneration in photoreceptor cells, which can be rescued by virus-mediated gene therapy.

Two novel mutations in the EYS gene are possible major causes of autosomal recessive retinitis pigmentosa in the Japanese population

Retinitis pigmentosa (RP) is a highly heterogeneous genetic disease including autosomal recessive (ar), autosomal dominant (ad), and X-linked inheritance. Recently, arRP has been associated with mutations in EYS (Eyes shut homolog), which is a major causative gene for this disease. This study was conducted to determine the spectrum and frequency of EYS mutations in 100 Japanese arRP patients. To determine the prevalence of EYS mutations, all EYS exons were screened for mutations by polymerase chain reaction amplification, and sequence analysis was performed. We detected 67 sequence alterations in EYS, of which 21 were novel. Of these, 7 were very likely pathogenic mutations, 6 were possible pathogenic mutations, and 54 were predicted non-pathogenic sequence alterations. The minimum observed prevalence of distinct EYS mutations in our study was 18% (18/100, comprising 9 patients with 2 very likely pathogenic mutations and the remaining 9 with only one such mutation). Among these mutations, 2 novel truncating mutations, c.4957_4958insA (p.S1653KfsX2) and c.8868C>A (p.Y2956X), were identified in 16 patients and accounted for 57.1% (20/35 alleles) of the mutated alleles. Although these 2 truncating mutations were not detected in Japanese patients with adRP or Leber's congenital amaurosis, we detected them in Korean arRP patients. Similar to Japanese arRP results, the c.4957_4958insA mutation was more frequently detected than the c.8868C>A mutation. The 18% estimated prevalence of very likely pathogenic mutations in our study suggests a major involvement of EYS in the pathogenesis of arRP in the Japanese population. Mutation spectrum of EYS in 100 Japanese patients, including 13 distinct very likely and possible pathogenic mutations, was largely different from the previously reported spectrum in patients from non-Asian populations. Screening for c.4957_4958insA and c.8868C>A mutations in the EYS gene may therefore be very effective for the genetic testing and counseling of RP patients in Japan.

[A case of Oguchi disease with disappearance of golden tapetal-like fundus reflex after vitreous resection]

BACKGROUND

Oguchi disease is a form of congenital stationary night-blindness characterized by a golden tapetal fundus reflex. This reflex is known to disappear in the dark-adapted state ("Mizuo-Nakamura phenomenon"). The origin of the reflex is not clear. We report a case of Oguchi disease with the disappearance of the golden tapetal reflex after vitreous resection.

CASE

An 80-year-old man was referred for rhegmatogenous retinal detachment of the left eye. Golden tapetal reflex was observed in both eyes. Negative b-wave in the electroretinogram and mutation in the SAG gene indicated Oguchi disease. Pars plana vitrectomy and posterior hyaloid membrane peeling were performed. The tapetal reflex in the all regions of the fundus in the operated eye disappeared after the retina was reattached. The reflex partially recovered 2 years after the operation.

CONCLUSION

It is suggested that the change in vitreo-retinal interface after a vitreous operation leads to the disappearance of the reflex. This report presents a hypothesis on the origin of the golden tapetal reflex in Oguchi disease.

Use of Lectins to Enrich Mouse ES-Derived Retinal Progenitor Cells for the Purpose of Transplantation Therapy

Using the mouse ES cell line with green fluorescent protein knocked-in at the Rx locus (Rx-KI ES cell), we previously showed that photoreceptors can be efficiently obtained in defined culture conditions by enriching Rx-positive retinal progenitor cells. We aimed to explore a protocol applicable for non-Rx-labeled stem cell lines for subsequent enrichment of retinal photoreceptor precursors for transplantation. The Rx-KI ES cell line was differentiated according to the serum-free suspension conditions with serum-free suspension/Dkk1/LeftyA/serum/activin method (SFEB/DLFA) described previously. Enrichment efficacy by negative selection was compared among 20 different lectins and the lectin combination that effectively enriched the Rx-positive cells by selecting the lectin low-binding population was determined. Subsequent differentiation efficiency to photoreceptor precursors and the contamination of Nanog or Oct3/4+ cells in the culture were evaluated between the cell cultures using negative selection with lectins and Rx positive selection. The effect of cytarabine (Ara-C) for minimizing the contamination of undifferentiated cells after the selection was also studied. The combination of the lectins, wheat germ agglutinin (WGA), and Erythrina crista-galli agglutinin (ECA) enabled us to enrich the Rx-positive population by approximately twice the original Rx percentage. The selection also minimized the percentage of Oct3/4+ cells. The lectin-selected cells produced a comparable percentage of Crx/rhodopsin-positive colonies with Rx-positive selection and were differentiated into photoreceptors. The Ara-C treatment on differentiating days 24–26 decreased Nanog and Oct3/4 expression in subsequent cultures. Enrichment of Rx-positive cells using WGA and ECA was comparable to Rx-positive selection, and the method could be applied to achieve efficient photoreceptor differentiation from other ES or iPS cell lines in which the Rx gene is not marked.

Allelic copy number variation in FSCN2 detected using allele-specific genotyping and multiplex real-time PCRs

PURPOSE

Allelic copy number variation (CNV) may alter the functional effects of a heterozygous mutation. The underlying mechanisms and their roles in hereditary diseases, however, are largely unknown. In the present study an FSCN2 mutation was examined that has been reported, not only in patients with retinitis pigmentosa (RP), but also in the normal population.

METHODS

Experiments were performed to investigate the gene and allele copy numbers of FSCN2 in patients with RP who have the c.72delG mutation as well as healthy subjects with or without the mutation. A real-time PCR-based genotyping approach was established that used a real-time PCR assay to qualify the copy numbers of both the wild-type and mutant alleles of the FSCN2 gene.

RESULTS

Three patients with RP and three normal subjects had an equal ratio of the alleles. Of interest, another patient had an asymmetric allele ratio (4:1) of the copy number of the wild-type allele, compared with that of the mutant allele. These findings were further verified using quantitative assays. An allele-specific methylation assay demonstrated a random methylation pattern in the FSCN2 gene.

CONCLUSIONS

The copy numbers of the FSNC2 gene and of each allele in the mutant samples were quantified. The findings excluded the possibility that allelic CNV was associated with RP, suggesting that the c.72delG variant is not the primary cause of RP. It is not likely that the FSCN2 gene is imprinted differentially. The real-time PCR-based genotyping method developed in this study is useful for investigations of allelic asymmetries within genomic regions with CNVs.

XPG stabilizes TFIIH, allowing transactivation of nuclear receptors: implications for Cockayne syndrome in XP-G/CS patients

Mutations in the human XPG gene give rise to an inherited photosensitive disorder, xeroderma pigmentosum (XP) associated with Cockayne syndrome (XP-G/CS). The clinical features of CS in XP-G/CS patients are difficult to explain on the basis of a defect in nucleotide excision repair (NER). We found that XPG forms a stable complex with TFIIH, which is active in transcription and NER. Mutations in XPG found in XP-G/CS patient cells that prevent the association with TFIIH also resulted in the dissociation of CAK and XPD from the core TFIIH. As a consequence, the phosphorylation and transactivation of nuclear receptors were disturbed in XP-G/CS as well as xpg(-/-) MEF cells and could be restored by expression of wild-type XPG. These results provide an insight into the role of XPG in the stabilization of TFIIH and the regulation of gene expression and provide an explanation of some of the clinical features of XP-G/CS.