Aims: This study aimed to identify the underlying genetic defect of a large Turkish X linked nystagmus (NYS) family.
Methods: Both Xp11 and Xq26 loci were tested by linkage analysis. The 12 exons and intron–exon junctions of the FRMD7 gene were screened by direct sequencing. X chromosome inactivation analysis was performed by enzymatic predigestion of DNA with a methylation-sensitive enzyme, followed by PCR of the polymorphic CAG repeat of the androgen receptor gene.
Results: The family contained 162 individuals, among whom 28 had NYS. Linkage analysis confirmed the Xq26 locus. A novel missense c.686C>G mutation, which causes the substitution of a conserved arginine at amino acid position 229 by glycine (p.R229G) in exon 8 of the FRMD7 gene, was observed. This change was not documented in 120 control individuals. The clinical findings in a female who was homozygous for the mutation were not different from those of affected heterozygous females. Skewed X inactivation was remarkable in the affected females of the family.
Conclusions: A novel p.R229G mutation in the FRMD7 gene causes the NYS phenotype, and skewed X inactivation influences the manifestation of the disease in X linked NYS females.
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Funding: This study was supported by The Hacettepe University Research Foundation (number 00-01-101-010), The Scientific and Technological Research Council of Turkey (number TUBITAK-SBAG 3334) and The International Centre for Genetic Engineering and Biotechnology (ICGEB-CRP/TUR04-01).
Competing interests: None.