CLRN1 Mutations Cause Nonsyndromic Retinitis Pigmentosa Muhammad Imran Khan, MSc, MS, 1,2 Ferry F.J. Kersten, BSc, 2,3,4,5,6 Maleeha Azam, PhD, 1,2 Rob W.J. Collin, PhD, 2,4,5 Alamdar Hussain, MSc, 1 Syed Tahir-A. Shah, MSc, 1 Jan E.E. Keunen, MD, PhD, 4 Hannie Kremer, PhD, 3,5,6 Frans P.M. Cremers, PhD, 1,2,5 Raheel Qamar, PhD, 1,7 Anneke I. den Hollander, PhD 2,4,5 Objective: To describe the mutations in the CLRN1 gene in patients from 2 consanguineous Pakistani families diagnosed with autosomal recessive retinitis pigmentosa (arRP). Design: Case-series study. Participants: Affected and unaffected individuals of 2 consanguineous Pakistani families and 90 unaffected controls from the same population. Informed consent was obtained from participants and the protocol was approved by a local institutional review board. Methods: Patients of 2 consanguineous families were genotyped with single-nucleotide polymorphism microarrays for genome-wide linkage analysis. The search for potential candidate genes within the 8-Mb overlapping homozygous region in these families revealed the presence of CLRN1, a gene previously known to cause Usher’s syndrome type III (USH3), which was analyzed by direct sequence analysis. The clinical diagnosis was based on the presence of night blindness, fundoscopic findings, and electroretinography (ERG) results. Additionally, pure tone audiometry was performed to rule out Usher’s syndrome. Main Outcome Measures: Fundoscopy, single-nucleotide polymorphism microarray, DNA sequence anal- ysis, ERG, and audiometry. Results: Sequencing of CLRN1 revealed novel missense mutations (p.Pro31Leu and p.Leu154Trp) segre- gating in 2 families. Analysis of fundus photographs indicated attenuation of the retinal vessels, and bone spicule pigmentation in the periphery of the retina. The ERG responses were indicative of a rod– cone pattern of the disease. Audiometric assessment revealed no hearing impairment, thereby excluding Usher’s syndrome. Sub- cellular localization studies demonstrated the retention of the mutant proteins in the endoplasmic reticulum, whereas the wild-type protein was mainly present at the cell membrane. Conclusions: The RP-associated mutations p.Pro31Leu and p.Leu154Trp may represent hypomorphic mutations, because the substituted amino acids located in the transmembrane domains remain polar, whereas more severe changes have been detected in patients with USH3. These data indicate that mutations in CLRN1 are associated not only with USH3, but also with nonsyndromic arRP. Financial Disclosure(s): The authors have no proprietary or commercial interest in any of the materials discussed in this article. Ophthalmology 2011;118:1444 –1448 © 2011 by the American Academy of Ophthalmology. Retinitis pigmentosa (RP, MIM#268000) consists of a group of photoreceptor degenerative disorders, initially characterized by impairment of night vision and mid- peripheral visual field loss. A hallmark feature of RP is the formation of pigmentary deposits in the retina, commonly known as bone spicules, along with mild to severe loss of rod function gauged by electroretinography (ERG). 1–3 Clinically and genetically, RP is a very heterogeneous disease. Several syndromic forms of RP exist where indi- viduals suffer from other diseases such as deafness in Ush- er’s syndrome, and obesity and renal abnormalities in Bardet–Biedl syndrome. Genetically, RP exhibits multiple modes of inheritance, and to date 48 genes and 8 loci have been described. Autosomal-recessive RP (arRP) accounts for approximately 20% to 25% of the cases; approximately 45% are isolated cases, which for the majority likely also represent autosomal recessive cases. The percentage of au- tosomal recessive cases has been found to be as high as 87% in the Pakistani population (Azam M, et al, unpublished data). Twenty-six genes have been identified for arRP, and they explain the etiology of 50% to 60% of all recessive cases. 3 These genes encode proteins that exert diverse func- tions in the retina including phototransduction, vitamin A metabolism, and ciliary transport. 3 Mutations in the USH2A gene have been reported to cause both Usher’s syndrome type II (USH2), and nonsyndromic arRP. 4–7 In the current study, we have described 2 Pakistani families with arRP caused by 2 different missense mutations in the CLRN1 gene. Mutations in this gene have previously only been associated with Usher’s syndrome type III (USH3), which is characterized by progressive postlingual hearing loss, in addition to RP of variable onset and sever- 1444 © 2011 by the American Academy of Ophthalmology ISSN 0161-6420/11/$–see front matter Published by Elsevier Inc. doi:10.1016/j.ophtha.2010.10.047