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Increased Plasma Cgmp in a Family with Autosomal Recessive Retinitis Pigmentosa Due to Homozygous Mutations in the PDE6A Gene

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Increased Plasma Cgmp in a Family with Autosomal Recessive Retinitis Pigmentosa Due to Homozygous Mutations in the PDE6A Gene Retina Increased Plasma cGMP in a Family With Autosomal Recessive Retinitis Pigmentosa Due to Homozygous Mutations in the PDE6A Gene Ulrika Kjellstr¨om,1 Patricia Veiga-Crespo,2 Sten Andreasson,´ 1 and Per Ekstr¨om2 1Lund University, Skane University Hospital, Department of Clinical Sciences Lund, Ophthalmology, Lund, Sweden 2Lund University, Faculty of Medicine, Department of Clinical Sciences Lund, Ophthalmology, Lund, Sweden Correspondence: Ulrika Kjellstr¨om, PURPOSE. To describe genotype and phenotype in a family with autosomal recessive retinitis Ogonkliniken¨ Sk˚anes Universitetss- pigmentosa (arRP) carrying homozygous mutations in the gene for the a-subunit of cyclic jukhus Lund, S 221 85 Lund, Swe- guanosine monophosphate (cGMP)–hydrolyzing phosphodiesterase 6 (PDE6A). Moreover, to den; compare their plasma cGMP levels to controls, exploring the possible role for cGMP in RP [email protected]. diagnostics. Submitted: May 4, 2016 Accepted: October 10, 2016 METHODS. Seven siblings and their parents were recruited. Microarray, verified by Sanger sequencing, was used for genotyping. Investigations included slit lamp and fundus Citation: Kjellstr¨om U, Veiga-Crespo P, examination, Goldmann perimetry, full-field and multifocal electroretinography (ERG), and Andreasson´ S, Ekstr¨om P. Increased optical coherence tomography (OCT). Cyclic GMP was measured with an immunoassay kit. plasma cGMP in a family with autoso- mal recessive retinitis pigmentosa due RESULTS. All siblings and their father were homozygous, and the mother heterozygous, for to homozygous mutations in the IVS6þ1G>AinPDE6A. Seven family members also carried c1532G>AinABCA4. Visual fields PDE6A gene. Invest Ophthalmol Vis were constricted with mere central remnants in older subjects and additional temporal Sci. 2016;57:6048–6057. DOI: crescents in younger subjects. Visual acuity ranged from 0.8 to amaurosis. Full-field ERGs 10.1167/iovs.16-19861 showed extinguished rod responses and minimal cone responses. Multifocal ERGs were severely reduced. Optical coherence tomography revealed either general attenuation or central macular edema. Mean plasma cGMP in patients was approximately twice that in controls. CONCLUSIONS. To our knowledge, this is the first phenotypic description of arRP due to homozygous IVS6þ1G>A mutations in PDE6A and these seem here to be associated with severe RP leading to early extinction of rod responses as well as reduced macular function. Additionally, patients showed increased plasma levels of cGMP, indicating a possible role for cGMP measurements as part of the clinical tests for this and, after further investigations, maybe other forms of RP. Keywords: cyclic GMP, electroretinography, retinitis pigmentosa etinitis pigmentosa (RP) belongs to the group of hereditary although a strategy for cases with RP mutations concerning the R retinal degenerations. The prevalence of RP is approximate- synthesis of certain lipids has been put forward.9 ly 1 in 4000 persons worldwide1 and it is a major cause of Despite the diverse genetic origin of RP, fundus findings progressive deterioration of vision and blindness among most often share a common pattern with pale optic nerve head, younger people.2 The disease typically starts with loss of night attenuated retinal blood vessels, and bone-spicule pigmenta- 1 vision in adolescence, followed by visual field constriction in tions in the midperiphery or toward the far periphery. Those young adulthood and gradual reduction of central visual acuity morphologic changes are preceded and accompanied by later in life.1 The most common mode of inheritance is photoreceptor cell death starting with rod photoreceptors autosomal recessive (ar) (50%–60% of cases), but RP can also and secondarily also engaging cones, which are considered to get damaged and die from oxidative stress,10–12 loss of be inherited in an autosomal dominant (30%–40%) or X-linked 13 3,4 metabolic and trophic support, or toxicity due to rod cell (5%–15%) way. At the moment, more than 60 genes associated 14 5 death. Indeed, alterations in antioxidative status have been with RP have been identified (see also: https://sph.uth.edu/ demonstrated in peripheral blood of RP patients,15 with retnet/home.htm; provided in the public domain by The reduced activity of superoxide dismutase 3 and increased University of Texas Health Science Center, Houston, Texas, levels of nitric oxide (NO). This imbalance in antioxidant status USA). Many of these genes code for proteins that are involved in may contribute to a poorer capacity of RP patients to cope with the phototransduction cascade, the retinoid metabolism, or in toxic oxygen metabolites.15 the maintenance of photoreceptor integrity.6 To date, RP cannot The small second messenger molecule cyclic guanosine be cured, although gene therapy has been tested for some monophosphate (cGMP) plays an important role in the retinal genetic variants.7,8 There are also no established biochemical phototransduction cascade, which is initiated by a conforma- blood markers yet to contribute to the clinical test possibilities, tional change of the photoreceptor opsins after getting hit by iovs.arvojournals.org j ISSN: 1552-5783 6048 This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Downloaded from iovs.arvojournals.org on 10/02/2021 cGMP Plasma Levels in RP Due to PDE6A Mutations IOVS j November 2016 j Vol. 57 j No. 14 j 6049 the photons of light. Subsequently, transducin levels are elevated, thereby activating the rod photoreceptor–specific cGMP–phosphodiesterase-6 (PDE6), which, in turn, catalyzes the breakdown of cGMP. Decreased cGMP levels lead to closure of cyclic-nucleotide–gated (CNG) cation channels and reduction of Ca2þ and Naþ influx,16 and the following hyperpolarization results in signal transmission to second- order neurons.17 Apart from the physiologic importance in phototransduc- tion, PDE and cGMP are likely also involved in pathologic events, since high cGMP levels, and subsequent photoreceptor cell death, are encountered in several animal RP models based on mutations of PDE6, and interestingly also as a consequence of mutations in apparently unrelated genes.18–21 The PDE6 enzyme is composed of two catalytic subunits called a (A) and b (B) and two identical inhibitory c subunits.22 Approximately FIGURE 1. Pedigree for the family. All family members except for I-2 3% to 4% of arRP cases21,23 are caused by mutations in the are homozygous for IVS6þ1G>A (c.998þ1G>A), a splice site mutation, PDE6A gene, coding for the a subunit and resulting in a in the PDE6A gene. defective PDE6 enzyme or no enzyme at all. Mutations in the PDE6B gene, coding for the b subunit of PDE6, are likewise further exploration of the possible connections between RP frequent and also lead to retinal degenerations.24–26 The PDE6 and the cGMP system in compartments outside of the retina, mutations, as well as others that lead to increased cGMP levels, not the least at the clinical level, since the possibility to use may cause an undesired opening of CNG cation channels, blood cGMP as a diagnostic tool for the disease would be most allowing continuous and pathologic Ca2þ influx with subse- welcome. To this end we here studied retinal function and quent rod photoreceptor cell death.27,28 Another possibility is morphology in a consanguineous family with arRP and that the increased cGMP levels cause cell death via activation identified their genetic errors as homozygote mutations in of cGMP-dependent protein kinase.29 Of the PDE6A and the PDA6A gene and, in some cases, also heterozygote or PDE6B mutations, the most extensive clinical descriptions so homozygote mutations in ABCA4.Thispermittedusto far come from PDE6B families, while descriptions of the describe several aspects of the ophthalmologic status of phenotype associated with PDE6A mutations are less frequent. patients with homozygote PDE6A mutations, which has not Another gene that can be associated with arRP is the ABCA4 been done to such an extent previously. In addition, it gave us gene,30–32 most often encountered in Stargardt disease33–35 but the opportunity to measure blood cGMP levels and to compare also seen in other retinal degenerations. The ABCA4 gene them to those of healthy controls, in a situation where we codes for the ABCA4 protein, an ATP-binding cassette (ABC) know that the cGMP system is involved in the disease. transporter protein located in the rim of the photoreceptor discs.36–38 ABCA4 promotes the clearance of toxic vitamin A metabolites such as N-retinylidene-phosphatidylethanolamine METHODS and phosphatidylethanolamine from the lumen of the outer segment disc membranes during phototransduction.39,40 De- Subjects and Controls fective function or absence of ABCA4 results in a buildup of the lipofuscin fluorophore N-retinylidene-N-retinylethanolamine, In this study, a large consanguineous family of Iraqi origin was which then accumulates in RPE cells, leading to RPE cell investigated. Seven siblings with nonsyndromic arRP and their death and probably secondary loss of photoreceptors.40,41 It parents were included (pedigree, Fig. 1). Four of the subjects has also been proposed that direct photoreceptor cell death (II-4–II-7) had also been examined at our Department of may precede RPE alterations.42,43 Ophthalmology, Sk˚ane University Hospital, Lund, Sweden, at a Given the fact that cGMP is an important retinal player in previous occasion in 2004. One of them, II-4, could not be general, and also in RP at least when PDE6A and PDE6B reexamined for the current study. The mean age of family mutations are concerned, clinical measurements of cGMP members was 43 6 16 years (range, 28–73 years; Table). The levels in ocular tissue or in body fluids in various retinal states study was conducted in accordance with the tenets of the are few. In the context of retinal detachment, La Heij et al.44 Declaration of Helsinki and it was approved by the Ethical have shown reduced cGMP levels in vitreous as well as Committee for Medical Research at Lund University.
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