Abstract
Purpose: :
Usher syndrome is an autosomal recessive disorder characterized by congenital hearing impairment and retinitis pigmentosa. Usher patients in the Greek population have not been genetically characterised previously for sequence variations in any related gene. This is the first systematic study to investigate the frequency and pattern of disease-associated mutations and polymorphisms in the Usher related genes by selective exon pre-screening and DNA microarrays.
Methods: :
Exons 6 and 13 of the USH2A gene were analysed by PCR and sequencing in 17 unrelated Greek Usher patients. Further mutational analysis was performed by the Asper DNA microarray-based assay.
Results: :
The initial sequencing of exons 6 and 13 of the USH2A gene revealed a novel missense mutation, c.2296T>C, C766R (exon 13) in heterozygous state in 1 patient and the previously described 921_922dupGCCA (exon 6) in homozygous state in 2 patients. The novel sequence variation C766R was not detected in 100 controls strongly suggesting that this could be the disease-causing mutation. Further mutational analysis by DNA microarrays revealed 4 different mutations in 4 patients, 2 in the USH2A gene (G3142X, W3955X, 1 allele), 1 in the MYO7A gene (5945-1G>C splice defect) representing a novel pathogenic allele (G>C instead of the reported G>A). In the fourth patient, 3 variants were detected in both MYO7A (T1566M, 2 alleles) and USH2A (W3955X, 1 allele) genes. The polymorphism L16S in the MYO7A gene was detected in 13/14 patients probably representing a common polymorphism among Greeks. Two USH2A polymorphisms (E478D, V230M) were detected in two patients and the unknown variant A1953G in a third patient.
Conclusions: :
Our findings provide for the first time information about the frequency and pattern of mutations and polymorphisms of Usher related genes in Greek Usher patients. Selective exon sequencing revealed a novel mutation in the USH2A gene at codon 766 preceding the commonly mutated codon 767 (c.2299delG). The 2299delG mutation was not detected in any patient suggesting that it may be infrequent among the Greek patients. Microarray analysis interestingly revealed a novel intronic sequence variant in the MYO7A gene (exon 44). The detection rate by sequencing only exons 13 and 6 of the USH2A gene was 17% and by microarray analysis 23.5%, a total of 41% of the patients. Our data will be valuable for the effective genetic counselling of the patients.
Keywords: genetics • gene microarray • gene screening