Usher syndrome type II (USH2) is an autosomal recessive disorder, characterised by moderate to severe high-frequency hearing impairment, normal balance function and progressive visual impairment due to retinitis pigmentosa. Usher syndrome type IIa, the most common subtype, is defined by mutations in the USH2A gene encoding a short and a recently discovered long usherin isoform comprising 21 and 73 exons, respectively. More than 120 different disease-causing mutations have been reported, however, most of the previous reports concern mutations restricted to exons 1-21 of the USH2A gene. To explore the spectrum of USH2A disease-causing mutations among Scandinavian USH2 cases, patients from 118 unrelated families of which 27 previously had been found to carry mutations in exons 1-21 were subjected to extensive DNA sequence analysis of the full size USH2A gene. Altogether, 122 USH2A DNA sequence alterations were identified of which 57 were predicted to be disease-causing, 7 were considered to be of uncertain pathogenicity and 58 were predicted to be benign variants. Of 36 novel pathogenic USH2A mutations 31 were located in exons 22-73, specific to the long isoform. USH2A mutations were identified in 89/118 (75.4%) families. In 79/89 (88.8%) of these families two pathogenic mutations were identified whereas in 10/89 (11.2%) families the second mutation remained unidentified. In 5/118 (4.2%) families the USH phenotype could be explained by mutations in the USH3A gene. The results presented here provide a comprehensive picture of the genetic aetiology of Usher syndrome type IIA in Scandinavia as it is known to date. (C) 2008 Wiley-Liss, Inc.

ERK (extracellular-signal-regulated kinase) 4 [MAPK (mitogen-activated protein kinase) 4] and ERK3 (MAPK6) are atypical MAPKs. One major difference between these proteins and the classical MAPKs is substitution of the conserved T-X-Y motif within the activation loop by a single phospho-acceptor site within an S-E-G motif. In the present study we report that Ser(186) of the S-E-G motif in ERK4 is phosphorylated in vivo. Kinase-dead ERK4 is also phosphorylated on Ser(186), indicating that an ERK4 kinase, rather than autophosphorylation, is responsible. Co-expression of MK5 [MAPK-activated protein kinase 5; also known as PRAK (p38-regulated/activated kinase)], a physiological target of ERK4, increases phosphorylation of Ser(186). This is not dependent on MK5 activity, but does require interaction between ERK4 and MK5 suggesting that MK5 binding either prevents ERK4 dephosphorylation or facilitates ERK4 kinase activity. ERK4 mutants in which Ser(186) is replaced with either an alanine residue or a phospho-mimetic residue (glutamate) are unable to activate MK5 and Ser(186) is also required for cytoplasmic anchoring of MK5. Both defects seem to reflect an impaired ability of the ERK4 mutants to interact with MK5. We find that there are at least two endogenous pools of wild-type ERK4. One form exhibits reduced mobility when analysed using SDS/PAGE. This is due to MK5-dependent phosphorylation and only this retarded ERK4 species is both phosphorylated on Ser(186) and co-immunoprecipitates with wild-type MK5. We conclude that binding between ERK4 and MK5 facilitates phosphorylation of Ser(186) and stabilization of the ERK4-MK5 complex. This results in phosphorylation and activation of MK5, which in turn phosphorylates ERK4 on sites other than Ser(186) resulting in the observed mobility shift.

Usher syndrome type IIa is an autosomal recessive disorder characterized by mild-to-severe hearing loss and progressive visual loss due to retinitis pigmentosa. The mutation that most commonly causes Usher syndrome type IIa is a 1-bp deletion, described as "2299delG," in the USH2A gene. The mutation has been identified in several patients from northern and southern Europe and from North America, and it has been found in single patients from South America, South Africa, and China. Various studies have reported a range of frequencies (.16-.44) among patients with Usher syndrome, depending on the geographic origin of the patients. The 2299delG mutation may be the one that most frequently causes retinitis pigmentosa in humans. Given the high frequencies and the wide geographic distribution of the mutation, it was of interest to determine whether the mutation resulted from an ancestral mutational event or represented a mutational hotspot in the USH2A gene. Haplotype analysis was performed on DNA samples from 116 unrelated patients with Usher syndrome type IIa; the patients were from 14 countries and represented 148 2299delG alleles. On the basis of six single-nucleotide polymorphisms within the USH2A gene, 12 core haplotypes were observed in a panel of normal chromosomes. However, in our analysis, only one core haplotype was found to be associated with the 2299delG mutation. The data indicate that the widespread geographic distribution of the 2299delG mutation is the result of an ancestral mutation that has spread throughout Europe and into the New World as a result of migration.

Usher syndrome type II is an autosomal recessive disorder, characterised by stable hearing impairment from childhood and progressive retinitis pigmentosa from the late teens. Mutations in the USH2A gene, located on 1q41, were recently shown to be responsible for Usher syndrome type IIa. We have investigated the molecular pathology of Usher type II by screening the USH2A gene for mutations in 31 unrelated patients from Denmark and Norway. Besides the frequent 2299delG mutation, which accounted for 44% of the disease alleles, a heterogeneous spectrum of mutations was identified. Sixteen new, putative disease-causing mutations were detected, of which 12 were private and four were shared by unrelated patients. The disease-causing mutations were scattered throughout the gene and included six nonsense and seven missense mutations, two deletions and one small insertion. In addition, six non-pathogenic polymorphisms were identified. All missense mutations resulted in major amino acid side-chain alterations. Four missense mutations affected the N-terminal part of USH2A, whereas three missense mutations affected the laminin-type epidermal growth factor-like (LE) domain. The structural consequences of the mutations affecting the LE domain are discussed in relation to the three-dimensional structure of a LE-module of the mouse laminin gamma1 chain.