The human PRPH2 gene is mapped to chromosome 6 and encodes a glycoprotein which includes both N and C cytoplasmic termini, four transmembrane domains, and two asymmetric extra cytosolic domains called intradiscal loop 1 and 2 [14]. The PRPH2 tetraspanin domain is involved in the process which converts a chemical phototransduction signal into an electrical one in the outer segment of the retina, so alterations of PRPH2 structure could lead to impairment of vision function, with varying degrees [15]. Previous studies have shown that the majority of mutations in PRPH2 mainly occur in intradiscal loop 2 [16–17] In our patient, the identified c.626T > A transversion mutation resulted in the substitution of a tyrosine to an aspartic acid in codon 209, which is also located in intradiscal loop 2 of exon 2. This mutation has been only reported by Birtel J, in two patients with macular dystrophies; however, no related literature was found for RP patients [8]. The deletion of codons 206–209 caused a larger in-frame deletion, and resulted in autosomal dominant RP [17]. And the deletion of codons 203–209 has been reported to lead to macular degeneration [18] In 15 RP cases reported by Keen, 8 cases involved mutations in codons 210–216. [17] Based on HGMD (http://www.hgmd.cf.ac.uk/ac/index.php), approximately 15% of reported mutations in PRPH2 involve codons 203–216. Therefore, we inferred that this region might be particularly significant for the role of PRPH2 in rods, or the peripheral retina.
Recently, researchers have demonstrated a relationship between some retinal dystrophy-causing genes and glaucoma [19]. Fernandez-Martinez L showed that the RPGRIP1 gene, which was known as retinitis pigmentosa GTPase regulator-interacting protein 1, was considered as a risk factor for primary open angle glaucoma, and that variants of RPGRIP1 might increase an individual’s susceptibility to various forms of glaucoma [20]. Micheal S reported that PRPF8 mutations were associated with both autosomal dominant RP, and adult-onset POAG [21]. RetNet genes also appear to be associated with a significant proportion of PACG, especially in probands with both PACG and RP [19]. However, the genetic correlation between PRPH2 and glaucoma has not been reported yet. In this study, the proband with the PRPH2 c.626T > A mutation showed a typical RP manifestation with close angles, yet the proband’s elder brother, lacking this novel the mutation, had a normal fundus and open angles. Therefore, we inferred that ACG was an accompanying symptom of RP, in this case, and that the PRPH2 mutation might be related to the co-occurrence of glaucoma and RP.
Based on the hereditary nature of RP and current research regarding RP and ACG, ACG is not considered as a risk factor of RP, however, some RP associated ocular manifestations, such as zonular insufficiency, nanophthalmos, or ectopic lentis, might explain the increased prevalence of ACG in RP patients [4]. Badeeb et al's study showed that a thicker and more anteriorly positioned lens could be observed in RP patients with normal axial length [3]. Several studies have shown that zonular instability was common in RP patients, which might lead to the anterior displacement of the lens [22]. All of the above manifestations could result in subsequent shallow anterior chamber and angle narrowing, and even angle closure [23]. In this study, although no lenticular tremor was observed in the proband, the abnormal shallow anterior chamber indicated the probability of the existence of zonular instability.
It is well known that a high IOP in RP patients aggravates damage of the optic nerve and defects of the visual field, resulting in devastating visual impairment in the short term [24]. Thus far, IOP control is the main treatment option for this condition. In this study, due to the unsatisfactory effect of antiglaucomatous drugs, we chose trabecular filtration surgery for the proband, and after a one-year follow-up the vision and visual field was found to be stable. However, some research has shown that the pathogenesis of both RP and glaucoma involves the immune system. Massengill MT reported that a chronic inflammatory process, mediated by Müller glial and microglial cells, could be observed in RP [25]. Ten’s research showed that the expression of IL-2, IL-6, monocyte chemoattractant protein-1 (MCP-1), and placental growth factor (PlGF) were significantly up-regulated in the intraocular fluid of RP patients, and the level of IL-8 was higher in presence of glaucoma [26]. Serious or chronic inflammation might influence the function of the trabecular meshwork and lead to an increase in IOP and damage to the optic nerve [27]. Thus, immunosuppression and neuroprotection might represent potential therapeutic targets for RP and glaucoma in the future.