Achromatopsia Due to a Homozygous Mutation in CNGB3 Gene: Description of a Familial Case

Background. Achromatopsia (rod monochromatism, complete color blindness) is a rare inherited retinal dystrophy with a prevalence of 1/30000-50000. Characteristic clinical symptoms include color blindness, pendular nystagmus, photophobia, severely reduced visual acuity due to the absence or impairment of cone function. To date, six genes have been associated with achromatopsia: CNGB3, CNGA3, GNAT2, PDE6C, PDE6H and ATF6. Diagnosis is established based on clinical symptoms, family history and comprehensive ophthalmological evaluation. Genetic testing confirms the diagnosis and enables genetic counselting in a famtly. Although achroma­topsia is characterized by typical clini­cal symptoms, differential diagnostics includes other inherited retinal dystrophies.

Materials and methods. Based on clintcal symptoms and famtly history, a preliminary diagnosis of achro­matopsia was estabfished in two sibs. Comprehensive ophthalmological examinations, including best-coreccted visual acutty (BCVA), color vision as - sessment with Ishihara plates, slit-lamp, fundus examination, spectral domain optical coherent tomography (SD-OCT) with fundus autofluorescence images and full-field electroretinography, were performed. Peripheral blood samples for genomic DNA extraction and CNGB3 gene testing were obtained.

Results. Characteristic changes were identified in the evaluations of BCVA, colour vision, slit-lamp, fundus examination, SD-OCT, fundus autofluorescence images and full-field electroretinography. Homozygous mutation c.819_826del p.Arg274Valfs*13 was identified in CNGB3 gene. Based on literature review, differential diagnostics of clinical symptoms and ophtalmological evaluation results was performed.

Conclusions. Characteristic clini­cal symptoms, comprehensive ophtalmological evaluation, family history taking and molecular genetic testing enables establishment of achromatop­sia diagnosis and differential diagnos­tics amongst inherited retinal dystrophies. Based on the exact diagnosis, appropriate management, surveillance and genetic counseling may be instituted.