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Abnormal crossing of the optic fibres shown by evoked magnetic fields in patients with ocular albinism with a novel mutation in the OA1 gene

¹ BioMag Laboratory, Helsinki University Central Hospital, Helsinki, Finland
² Helsinki Brain Research Centre, Helsinki, Finland
³ Department of Clinical Neurophysiology, Helsinki University Central Hospital, Helsinki, Finland
⁴ Department of Obstetrics and Gynecology, Helsinki University Central Hospital, Helsinki, Finland
⁵ The Folkhälsan Institute of Genetics, Department of Molecular Genetics, Helsinki, Finland
⁶ Department of Ophthalmology, Helsinki University Central Hospital, Helsinki, Finland

Correspondence to:
Correspondence to:
Tiina Alitalo
Helsinki University Central Hospital, Department of OB/GYN, Genetics, Haartmaninkatu 2, PO Box 140, Helsinki, 00029 HUS, Finland; tiina.alitalo{at}hus.fi

ABSTRACT
Aim: To perform genealogical and clinical studies in Finnish families with X linked ocular albinism (OA1), including characterisation of the potential misrouting of optic fibres by evaluating visual evoked magnetic fields (VEFs), and to determine the mutation behind the disease.

Methods: Three families with OA1 were clinically examined. VEFs were measured in two affected males and in one female carrier to characterise the cortical activation pattern after monocular visual stimulation. The neuronal sources of the VEFs were modelled with equivalent current dipoles (ECDs) in a spherical head model. All coding exons of the OA1 gene were screened for mutations by single strand conformation analysis and direct polymerase chain reaction sequencing.

Results: Genealogical studies revealed that the three families were all related. The affected males had foveal hypoplasia with reduced visual acuity varying from 20/200 to 20/50, variable nystagmus, iris transillumination, and hypopigmentation of the retinal pigment epithelium. The ECD locations corresponding to the VEFs revealed abnormal crossing of the optic fibres in both affected males, but not in the carrier female. A novel point mutation, leading to a STOP codon, was identified in the fifth exon of the OA1 gene.

Conclusions: The data indicate that the novel mutation 640C>T in the OA1 gene is the primary cause of the eye disease in the family studied. VEFs with ECD analysis was successfully used to demonstrate abnormal crossing of the optic fibres.

Abbreviations: ECD, equivalent current dipole; MEG, magnetoencephalography; OA, ocular albinism; PCR, polymerase chain reaction; RPE, retinal pigment epithelium; SSCA, single strand conformation analysis; VEF, visual evoked magnetic field; VEP, visual evoked potential

Keywords: ocular albinism; X chromosome; magnetoencephalography; evoked visual responses; mutation analysis

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