Comprehensive screening of the USH2A gene in Usher syndrome type II and non-syndromic recessive retinitis pigmentosa
Introduction
Usher syndrome (OMIM 276900) is clinically divided into three types. Patients with Usher syndrome type I (USH1) have retinitis pigmentosa (RP) and profound hearing loss and vestibular dysfunction from birth. Patients with Usher syndrome type II (USH2) have RP, mild-to-moderate congenital hearing loss, and normal vestibular function. Patients with Usher syndrome type III (USH3) have RP, adult-onset, progressive, sensorineural hearing loss, and a variable level of vestibular dysfunction (Pakarinen et al., 1995). All three types of Usher syndrome are autosomal recessively inherited. At least seven genes can cause USH1, five of which have been identified (MYO7A (Kimberling et al., 1992), USH1C (Smith et al., 1992), CDH23 (Wayne et al., 1996), PCDH15 (Wayne et al., 1997), and SANS (Weil et al., 2003)). At least three genes can cause USH2 (USH2A (Kimberling et al., 1990), USH2B (Hmani et al., 1999), and USH2C (Pieke-Dahl et al., 2000)), two of which (USH2A and USH2C) have been identified (Kimberling et al., 1995, Weston et al., 2004). Mutations in the identified USH3A gene is the only known cause of USH3 (Joensuu et al., 2001, Fields et al., 2002).
The present study considers the mutation spectrum of the USH2A gene in North America. This gene encodes usherin, a 1546-amino acid protein of unknown function (Weston et al., 2000). Mutations in the USH2A gene have been reported in patients with USH2 and also in patients with non-syndromic RP (i.e. RP without hearing loss or other non-ocular disease) (Rivolta et al., 2000, Bernal et al., 2003). Other groups have reported surveys of all 21 exons of the USH2A gene in sets of 11–57 unrelated patients with USH2 (Weston et al., 2000, Adato et al., 2000, Leroy et al., 2000, Dreyer et al., 2000). No previous group has performed a similar comprehensive mutation screen of all USH2A exons in a large set of patients with non-syndromic, recessive RP. Here we report the results of our study of all USH2A exons in a large set of patients with USH2 and another large set of patients with non-syndromic recessive RP (ARRP).
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Patient recruitment
The tenets of the Declaration of Helsinki were followed, and informed consent was obtained from all patients who participated in this study prior to donating a blood sample. Panels of 129 and 146 unrelated patients with USH2 and non-syndromic ARRP, respectively, were enrolled in this study. All patients underwent an ophthalmological examination including electroretinograms (ERGs). Patients with USH2 reported partial hearing loss, while patients with non-syndromic ARRP did not. ARRP patients had
Summary of sequence changes
Among 129 unrelated patients with USH2 and 146 unrelated patients with non-syndromic ARRP, we found 54 sequence variations. Of these, 18 were interpreted as pathogenic mutations, including 13 frameshift mutations (seven of which are novel), three nonsense mutations (two novel), and two missense mutations (Table 1). Each of the two missense mutations (C419F and C759F) was present in patients statistically significantly more frequently than in controls, based on results from this study combined
Discussion
It is noteworthy that a large proportion of patients in our study with an USH2A mutation had only one identified mutation. Specifically, out of 50 USH2 patients with identified mutations, 41 (82%) had only one identified USH2A mutation; out of 17 ARRP patients with identified mutations, 12 (71%) had only one. We are uncertain about the explanation for the high frequency of patients with only one identified mutation, since USH2 is a recessively inherited disease. It is possible that mutations
Acknowledgements
This study was supported by the National Eye Institute (EY00169, EY08683) and the Foundation Fighting Blindness, Owings Mills, MD, USA.
Since the submission of this article, another group reported that the USH2A gene has 51 additional exons at 3′ end (van Wijk et al., 2004). The frequency of USH2A mutations in these exons in our patients remains to be determined.
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