A Novel Mutation and Phenotypes in Phosphodiesterase 6 Deficiency

doi:10.1016/j.ajo.2008.06.017

American Journal of Ophthalmology

Volume 146, Issue 5, November 2008, Pages 780-788.e1

https://doi.org/10.1016/j.ajo.2008.06.017 Get rights and content

Purpose

To develop a systematic approach for the molecular diagnosis of retinitis pigmentosa (RP) and to report new genotype-phenotype correlations for phosphodiesterase 6 (PDE6)-based RP mutations.

Design

Clinical and molecular studies on a retrospective case series.

Methods

We screened 40 unrelated RP patients with an autosomal recessive RP microarray. Individuals with RP caused by PDE6 deficiency underwent genetic segregation and phenotype analysis.

Results

A disease-associated allele was identified in 32% of patients. Two probands (5%) had PDE6 mutations. The first proband was a compound heterozygote for known R102C and N216S alleles in PDE6A (MIM#180071). Pedigree analysis determined that the N216S variant was benign and direct sequencing discovered a novel, S303C allele. The second proband had a homozygous D600N mutation in the PDE6B gene (MIM#180072). Visual acuities of PDE6-deficient patients ranged from 20/40 to 20/200. Clinical studies showed unusual vitreomacular traction, cystoid macular edema, macular atrophy, and ring hyperfluorescence in PDE6-deficient patients. Such extensive vitreoretinal degeneration is not characteristic of photoreceptor-specific enzyme deficiencies.

Conclusion

High-throughput deoxyribonucleic acid microarray chips can be used in combination with clinical imaging to precisely characterize patients with RP. Identifying the precise mutation in RP may become the standard of care as gene therapy emerges.

Section snippets

Imaging and Psychophysical Testing

Forty unrelated individuals with autosomal recessive RP were phenotyped. If applicable, additional family members were examined. Fundus photographs, optical coherence tomography [OCT], (Stratus and Cirrus; Zeiss, Jena, Germany), and fundus autofluorescence (AF) images were reviewed. AF was obtained by illuminating the fundus with argon laser light (488 nm) and viewing the resultant fluorescence through a bandpass filter with a short wavelength cutoff at 495 nm.²² Microperimetry (MP1; Nidek

Genetic Screening

Screening of 40 unrelated patients diagnosed with autosomal recessive or sporadic RP on the arRP array resulted in identification of at least one (out of two, as per definition of a recessive disease) possible disease-associated mutation in 32.5% of individuals (13/40). The likely disease-associated variants were identified in the following genes: PDE6A (R102C and N216S), PDE6B (D600N), CRB1 (T745M and P836T), USH2A (E478D, L555V, W4149R, P1978S, G713R, E767fs, and C759F), and RPE65 (A132T).

Discussion

Genetic heterogeneity in RP makes it challenging to advise patients regarding their visual prognoses. Genotyping and comprehensive phenotyping with ERG, AF, OCT, and MP1 mapping are helpful to specify prognosis and for gene-based treatment trials.3, 14, 29 Therefore, we applied a high-throughput genotyping array platform, electrophysiological testing, imaging, and psychophysical studies to genotype and phenotype patients with PDE6-related RP.

In one patient, AF imaging revealed abnormal foci of

Stephen H. Tsang, MD, PhD, is a Bernard-Becker-AUPO-Research to Prevent Blindness scholar and an Assistant Professor in Retina Division at New York-Presbyterian Hospital studying cyclic guanosine monophosphate-phosphodiesterase (PDE6), a key component of the phototransduction cascade. Dr Tsang's team provided evidence on how light induced posttranslational modification of PDE6 regulates signaling in the living retina. Dr Tsang generated lines of transgenic mice that provide models for testing

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PDE6B encodes the beta-subunit of rod cGMP-phosphodiesterase which is a key enzyme specific for phototransduction activation in rod photoreceptors (Lamb and Pugh, 2004). Consistent with the gene function, patients show early onset of night vision loss (Jacobson et al., 2007; Tsang et al., 2008; Tatour et al., 2019). Animal models show an extremely fast degeneration of the rods.
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We analyzed protein expression in liquid vitreous biopsies from autosomal recessive (ar)RP patients with PDE6A mutations and arRP mice with Pde6ɑ mutations. Proteomic analysis of retina and vitreous samples identified molecular pathways affected at the onset of photoreceptor death. Based on affected molecular pathways, arRP mice were treated with a ketogenic diet or metabolites involved in fatty-acid synthesis, oxidative phosphorylation, and the tricarboxylic acid (TCA) cycle.
Dietary supplementation of a single metabolite, ɑ-ketoglutarate, increased docosahexaeonic acid levels, provided neuroprotection, and enhanced visual function in arRP mice. A ketogenic diet delayed photoreceptor cell loss, while vitamin B supplementation had a limited effect. Finally, desorption electrospray ionization mass spectrometry imaging (DESI-MSI) on ɑ-ketoglutarate-treated mice revealed restoration of metabolites that correlated with our proteomic findings: uridine, dihydrouridine, and thymidine (pyrimidine and purine metabolism), glutamine and glutamate (glutamine/glutamate conversion), and succinic and aconitic acid (TCA cycle).
This study demonstrates that replenishing TCA cycle metabolites via oral supplementation prolongs retinal function and provides a neuroprotective effect on the photoreceptor cells and inner retinal network.
NIH grants [R01EY026682, R01EY024665, R01EY025225, R01EY024698, R21AG050437, P30EY026877, 5P30EY019007, R01EY018213, F30EYE027986, T32GM007337, 5P30CA013696], NSF grant CHE-1734082.
Clinical findings of end-stage retinitis pigmentosa with a homozygous PDE6A variant (p.R653X)
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Phenotypes in adult RP patients with the PDE6A variants exhibit similar clinical characteristics to our proband and these previously studied cases. These characteristics include onset in early childhood, retinal degeneration with bone spicule pigmentation from mid-peripheral to peripheral retina, attenuation of peripheral retinal vessels, distinguished ERG responses of both rod and cone function, and a severely constricted visual field until reaching their 20s–30s.6,7,19–22 Furthermore, macula involvements, which are often accompanied by macular edema and atrophy, have been reported in most of these types of cases.
To report clinical and genetic features of a Japanese patient with end-stage retinitis pigmentosa (RP) caused by a homozygous PDE6A variant.
We performed comprehensive ophthalmic examinations. Whole exome sequencing analysis was used to investigate the RP patient with parental consanguinity. The pedigree included 4 RP patients in the two generations, which suggests presumed pseudo-autosomal dominant inheritance.
A PDE6A variant (p.R653X) was identified to be homozygous and disease-causing in the patient. Homozygosity mapping revealed the homozygous region including the variant and confirmation of autosomal recessive inheritance. The patient reported night blindness at 4 years of age and exhibited typical RP fundus appearance with macula involvement during the follow-up period from at the age of 52–69 years. At the age of 52, the patient exhibited a loss of visual acuity and had severely constricted visual fields, with a further gradual deterioration of her vision until she was 69 years old. At the age of 69, funduscopy showed severe chorioretinal degeneration in the area from the posterior pole to the peripheral retina.
This is the first report that the PDE6A variant (p.R653X) has been identified as one of the causes of autosomal recessive RP in the Japanese population. Longitudinal natural history/end-stage findings demonstrated early-onset and a severe RP phenotype with macula involvement when the patient was in her 50s and severe chorioretinal degenerations in her late 60s.
AAV-mediated gene therapy halts retinal degeneration in PDE6β-deficient dogs
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Établir les corrélations phénotype-génotype des RP non syndromiques dans 10 familles tunisiennes en analysant les particularités cliniques les plus fréquemment associées aux génotypes trouvés.
Étude descriptive, clinique et génétique, concernant 114 individus dont 27 sont atteints de RP non syndromique.
Nous avons identifié 7 gènes mutés : RPE65, RDH12, USHER2A, PDE6A, PDE6B, CRB1, et NR2E3. Des corrélations phénotype-génotype ont pu être dégagées notamment pour le gène RPE65 (RP de la première enfance, nystagmus, kératocône, aspect de taches blanchâtres ou de mottes pigmentaires au fond d’œil selon l’âge), le gène RDH12 (RP de l’enfant, altération sévère de l’acuité visuelle, ostéoblastes diffus à toute la rétine, œdème maculaire, épaississement maculaire à l’OCT) et le gène CRB1 (absence d’hespéranopie avec préservation de la rétine paravasculaire des gros vaisseaux).
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To evaluate the clinical phenotype of ten Tunisian families with non-syndromic retinitis pigmentosa (RP), to characterize genes and mutations causing these conditions, and to elaborate phenotype–genotype correlations.
Descriptive clinical genetic study of 114 individuals, of whom 27 are affected by non-syndromic RP. Ophthalmic examination and various visual tests were performed. DNA was analyzed using single nucleotide polymorphism, microsatellite genotyping and direct sequencing to determine the genes and mutations involved.
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RP is clinically and genetically heterogeneous. The two ultimate goals of research are to provide efficient clinical diagnostic of affected gene by phenotype–genotype correlation and to design novel treatment regimens. Our goal is to create a specific chip for our population, and then future research will focus on the identification of the remaining causal genes, the elucidation of the molecular mechanisms of disease in the retina and the development of gene therapy approaches.

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Original articleA Novel Mutation and Phenotypes in Phosphodiesterase 6 Deficiency

Purpose

Design

Methods

Results

Conclusion

Section snippets

Imaging and Psychophysical Testing

Genetic Screening

Discussion

Am J Ophthalmol

Lancet

Vision Res

Prog Retin Eye Res

Blood

Am J Ophthalmol

Retinitis pigmentosa and allied diseases: numerous diseases, genes, and inheritance patterns

Hum Mol Genet

A practical approach to retinal dystrophies

Retinal Physician

Perspective on genes and mutations causing retinitis pigmentosa

Arch Ophthalmol

Transgenic mice carrying the H258N mutation in the gene encoding the beta-subunit of phosphodiesterase-6 (PDE6B) provide a model for human congenital stationary night blindness

Hum Mutat

The effect of sildenafil citrate (Viagra) on visual sensitivity

J Vis

Role of the target enzyme in deactivation of photoreceptor g protein in vivo

Science

Retinal degeneration in mice lacking the γ subunit of rod cGMP phosphodiesterase

Science

GAP-independent termination of photoreceptor light response by excess gamma subunit of the cGMP-phosphodiesterase

J Neurosci

Removal of phosphorylation sites of {gamma} subunit of phosphodiesterase 6 alters rod light response

J Physiol

In vivo studies of the gamma subunit of retinal cGMP-phosphodiesterase with a substitution of tyrosine-84

Biochem J

Photoreceptors and photoreceptor dysfunctions

Support for the equivalent light hypothesis for RP

Nat Med

The effect of calcium channel blocker diltiazem on photoreceptor degeneration in the rhodopsin Pro213His rat

Invest Ophthalmol Vis Sci

Excessive activation of cyclic nucleotide gated channels contributes to neuronal degeneration of photoreceptors

Eur J Neurosci

Absence of photoreceptor rescue with D-cis-diltiazem in the rd mouse

Invest Ophthalmol Vis Sci

Original article
A Novel Mutation and Phenotypes in Phosphodiesterase 6 Deficiency