F.P.M. Cremers

Association between X-linked mixed deafness and mutations in the POU domain gene POU3F4

Deafness with fixation of the stapes (DFN3) is the most frequent X-linked form of hearing impairment. The underlying gene has been localized to a 500-kilobase segment of the Xq21 band. Here, it is reported that a candidate gene for this disorder, Brain 4 (POU3F4), which encodes a transcription factor with a POU domain, maps to the same interval. In five unrelated patients with DFN3 but not in 50 normal controls, small mutations were found that result in truncation of the predicted protein or in nonconservative amino acid substitutions. These findings indicate that POU3F4 mutations are a molecular cause of DFN3.

Positional cloning of the gene for X-linked retinitis pigmentosa 2

X-linked retinitis pigmentosa (XLRP) results from mutations in at least two different loci, designated RP2 and RP3, located at Xp11.3 and Xp21.1, respectively. The RP3 gene was recently isolated by positional cloning, whereas the RP2 locus was mapped genetically to a 5-cM interval. We have screened this region for genomic rearrangements by the YAC representation hybridization (YRH) technique and detected a LINE1 (L1) insertion in one XLRP patient. The L1 retrotransposition occurred in an intron of a novel gene that consisted of five exons and encoded a polypeptide of 350 amino acids. Subsequently, nonsense, missense and frameshift mutations, as well as two small deletions, were identified in six additional patients. The predicted gene product shows homology with human cofactor C, a protein involved in the ultimate step of ß-tubulin folding. Our data provide evidence that mutations in this gene, designated RP2, are responsible for progressive retinal degeneration.

Molecular basis of choroideremia ( CHM ) : mutations involving the rab escort protein - 1 (REP - 1) gene

Choroideremia (CHM) is an X‐linked recessive eye disease that results from mutations involving the Rab escort protein‐1 (REP‐1) gene. In 18 patients deletions of different sizes have been found. Two females suffering from CHM were reported to have translocations that disrupt the REP‐1 gene. In 22 patients, small mutations have been identified. Interestingly, these are all nonsense, frameshift or splice‐site mutations; with one possible exception, missense mutations have not been found. This comprises all the known mutations in the disease. Hum Mutat 9:110–117, 1997.

The spectrum of retinal phenotypes caused by mutations in the ABCA4 gene.

BackgroundThe majority of studies on the retina-specific ATP-binding cassette transporter (ABCA4) gene have focussed on molecular genetic analysis; comparatively few studies have described the clinical aspects of ABCA4-associated retinal disorders. In this study, we demonstrate the spectrum of retinal dystrophies associated with ABCA4 gene mutations.MethodsNine well-documented patients representing distinct phenotypes in the continuum of ABCA4-related disorders were selected. All patients received an extensive ophthalmologic evaluation, including kinetic perimetry, fluorescein angiography, and electroretinography (ERG). Mutation analysis had been performed previously with the genotyping microarray (ABCR400 chip) and/or single-strand conformation polymorphism analysis in combination with direct DNA sequencing.ResultsIn all patients, at least one pathologic ABCA4 mutation was identified. Patient 10034 represented the mild end of the phenotypic spectrum, demonstrating exudative age-related macular degeneration (AMD). Patient 24481 received the diagnosis of late-onset fundus flavimaculatus (FFM), patient 15168 demonstrated the typical FFM phenotype, and patient 19504 had autosomal recessive Stargardt disease (STGD1). Patients 11302 and 7608 exhibited progression from FFM/STGD1 to cone–rod dystrophy (CRD). A more typical CRD phenotype was found in patients 15680 and 12608. Finally, the most severe ABCA4-associated phenotype was retinitis pigmentosa (RP) in patient 11366. This phenotype was characterised by extensive atrophy with almost complete loss of peripheral and central retinal functions.ConclusionWe describe nine patients during different stages of disease progression; together, these patients form a continuum of ABCA4-associated phenotypes. Besides characteristic disorders such as FFM/STGD1, CRD and RP, intermediate phenotypes may be encountered. Moreover, as the disease progresses, marked differences may be observed between initially comparable phenotypes. In contrast, distinctly different phenotypes may converge to a similar final stage, characterised by extensive chorioretinal atrophy and very low visual functions. The identified ABCA4 mutations in most, but not all, patients were compatible with the resulting phenotypes, as predicted by the genotype–phenotype model for ABCA4-associated disorders. With the advent of therapeutic options, recognition by the general ophthalmologist of the various retinal phenotypes associated with ABCA4 mutations is becoming increasingly important.

A mutation in the gamma actin 1 (ACTG1) gene causes autosomal dominant hearing loss (DFNA20/26).

Linkage analysis in a multigenerational family with autosomal dominant hearing loss yielded a chromosomal localisation of the underlying genetic defect in the DFNA20/26 locus at 17q25-qter. The 6-cM critical region harboured the γ-1-actin (ACTG1) gene, which was considered an attractive candidate gene because actins are important structural elements of the inner ear hair cells. In this study, a Thr278Ile mutation was identified in helix 9 of the modelled protein structure. The alteration of residue Thr278 is predicted to have a small but significant effect on the γ 1 actin structure owing to its close proximity to a methionine residue at position 313 in helix 11. Met313 has no space in the structure to move away. Moreover, the Thr278 residue is highly conserved throughout eukaryotic evolution. Using a known actin structure the mutation could be predicted to impair actin polymerisation. These findings strongly suggest that the Thr278Ile mutation in ACTG1 represents the first disease causing germline mutation in a cytoplasmic actin isoform.

A duplication/paracentric inversion associated with familial X-linked deafness (DFN3) suggests the presence of a regulatory element more than 400 kb upstream of the POU3F4 gene

X-linked deafness with stapes fixation (DFN3) is caused by mutations in the POU3F4 gene at Xq21.1. By employing pulsed field gel electrophoresis (PFGE) we identified a chromosomal aberration in the DNA of a DFN3 patient who did not show alterations in the open reading frame (ORF) of POU3F4. Southern blot analysis indicated that a DNA segment of 150 kb, located 170 kb proximal to the POU3F4 gene, was duplicated. Fluorescence in situ hybridization (FISH) analysis, PFGE, and detailed Southern analysis revealed that this duplication is part of a more complex rearrangement including a paracentric inversion involving the Xq21.1 region, and presumably the Xq21.3 region. Since at least two DFN3-associated minideletions are situated proximal to the duplicated segment, the inversion most likely disconnects the POU3F4 gene from a regulatory element which is located at a distance of at least 400 kb upstream of the POU3F4 gene.

A Novel Mutation Identified in the DFNA5 Gene in a Dutch Family: A Clinical and Genetic Evaluation

A novel DFNA5 mutation was found in a Dutch family, of which 37 members were examined. A nucleotide substitution was identified in the splice acceptor site of intron 7, leading to skipping of exon 8 in part of the transcripts. The mutation was found in 18 individuals. Sensorineural hearing impairment was non-syndromic and symmetric. In early life, presumably congenitally, hearing impairment amounted to 30 dB in the high frequencies. Progression was most pronounced at 1 kHz (1.8 dB/year). Speech recognition was relatively good with a phoneme score of about 50% at the age of 70. Onset age was 37 years, and recognition deteriorated by 1.3% per year. The recognition score deteriorated by 1.0% per decibel threshold increase from a mean pure-tone average (PTA at 1, 2 and 4 kHz) of 63 dB onwards. Vestibular function was generally normal. The second mutation identified in the DFNA5 gene results in hearing impairment, similar to that in the original DFNA5 family in terms of pure-tone thresholds, but with more favourable speech recognition.

Clinical and Molecular Evaluation of Probands and Family Members with Familial Exudative Vitreoretinopathy

PURPOSE To describe the ophthalmic characteristics and to identify the molecular cause of FEVR in a cohort of Dutch probands and their family members. METHODS Twenty families with familial exudative vitreoretinopathy (FEVR) comprising 83 affected and nonaffected individuals were studied. Based on the presence of an avascular zone, the clinical diagnosis was made and biometric data of the posterior pole of 57 patients and family members were obtained by the analysis of fundus photographs and compared with the data of 40 controls. The FZD4, LRP5, and NDP genes were screened for mutations in one affected individual per family. The segregation of the gene variants was studied in the corresponding families. RESULTS Forty of 83 individuals showed an avascular zone, the most evident clinical sign of FEVR, five showed major signs of FEVR, and 38 persons were not clinically affected. Compared with the control subjects the patients with FEVR had a significantly larger disc-to-macula distance and a significantly smaller optic disc. In 8 of 20 families, a FZD4 mutation was identified, in 2 a mutation in the LRP5 gene, and in 2 a mutation in the NDP gene. Three known and five novel mutations were identified. Nonpenetrance was observed in 26% of the mutation carriers. CONCLUSIONS Significant anatomic differences were identified between the eyes of patients with FEVR with an avascular zone, when compared with those of the control subjects. In patients with an avascular zone, the optic disc was smaller and the disc-to-macula distance larger than in the control subjects. In 60% of the probands, mutations were identified in one of the three known FEVR genes.

Phenotypic variations in a family with retinal dystrophy as result of different mutations in the ABCR gene

AIMS To describe two phenotypic variations of autosomal recessive retinal dystrophy occurring in a consanguineous family in a pseudodominant pattern, resulting from mutations in the ATP binding cassette transporter (ABCR) gene. METHODS Patients of this family underwent an extensive ophthalmic evaluation, including fundus photography, fluorescein angiography, and electroretinography (ERG). Genetic analysis comprised sequence analysis of the retina specific ABCR gene. RESULTS Five patients presented with decreased visual acuity in the second decade, central chorioretinal atrophy associated with a central scotoma, and severely decreased photopic and scotopic ERG responses. This clinical picture, which in our opinion resembles a cone-rod dystrophy (CRD), was associated with compound heterozygosity for IVS30+ 1g →t and IVS40+5g→a mutations in the ABCR gene. The four remaining patients presented with night blindness in the first decade because of a retinitis pigmentosa-like (RP-like) dystrophy. In addition to a pale “waxy” optic disc, attenuated retinal vessels and bone spicule deposits, a widespread chorioretinal atrophy was observed. The scotopic ERG was extinguished and the photopic ERG was severely diminished. Genetic analysis revealed a homozygous 5′ splice mutation IVS30+1g →t in the ABCRgene. CONCLUSION Mutations in the ABCR gene can cause clinical pictures resembling autosomal recessive RP and autosomal recessive CRD.

A Tyr368His RPE65 founder mutation is associated with variable expression and progression of early onset retinal dystrophy in 10 families of a genetically isolated population

Autosomal recessive retinal dystrophies cause visual impairment in approximately 1 in 4000 individuals worldwide.1 The non-syndromic forms are highly heterogeneous and can be classified into clinical subgroups, the most frequent ones being retinitis pigmentosa, cone and cone-rod dystrophies, and Leber congenital amaurosis (LCA). LCA represents the most severe phenotype with an onset of symptoms before the age of six months, visual acuity below 20/400, a searching nystagmus, sluggish pupillary reactions, and a non-detectable electroretinogram (ERG). Visual fields are usually not measurable.1 Photophobia is only occasionally reported in LCA.2 Patients with juvenile and early onset retinitis pigmentosa present with night blindness in early childhood, usually before the age of two years. They do not show searching nystagmus3 and have a relatively well preserved macular function. Central vision is often lost in the second or third decade of life.4 The cloning of more than 20 genes allows the molecular characterisation of approximately 50% of the autosomal recessive inherited retinal dystrophy cases (http://www.sph.uth.tmc.edu/Retnet5,6 ). To establish a useful clinical prognosis for patients a well defined genotype–phenotype correlation is required. Although the general population in the Netherlands is relatively outbred, there are a few examples of autosomal recessive diseases caused by Dutch founder mutations. Batten disease was found to be due to identity-by-descent in a highly inbred family.7 A frequent LDL receptor mutation originating from Dutch 17th century settlers causes familial hypercholesterolaemia in South Africa and Canada.8 The RP12 locus was mapped through linkage analysis in a genetic isolate from the northwest of the Netherlands,9 which was followed by the cloning of the underlying gene, CRB1 .10 In 1959 Schappert-Kimmijser et al described an isolated Dutch population living on a former island with a relatively high frequency of LCA.11 In 1637 this …