Oligocone trichromacy is part of the spectrum of CNGA3-related cone system disorders

Abstract

Purpose: To report the rare observation of CNGA3 mutation as a cause of oligocone trichromacy (OT) and present phenotypic characteristics.Methods: A 20 year old male patient underwent ophthalmological evaluation including detailed color vision assessment using Ishihara pseudoisochromatic plates, American Optical Hardy Rand Rittler plates (HRR) and Mollon-Reffin Minimalist test (MRM). Optical coherence tomography (OCT), fundus autofluorescence (FAF), visual field assessment and electrophysiological testing was also performed. The patient’s DNA was sequenced for mutations in the coding sequence of CNGA3 and CNGB3 genes.Results: Best corrected visual acuity (BCVA) was 20/50 and 20/30 in the right and left eyes respectively. His color vision was normal to Ishihara, HRR and MRM tests. Fundus appearance, FAF, OCT and Goldmann visual fields (GVF) were all normal. Humphrey visual field analysis (HVF) demonstrated reduced sensitivity and paracentral scotomas (5–20°). The full-field electroretinogram (ERG) showed normal rod responses and severely reduced cone responses. The multifocal electroretinogram (mfERG) was non-recordable above noise. Compound heterozygous mutations in exon 8 of the CNGA3 coding sequence were identified; c.1070 A > G (Tyr357Cys; novel) and c.1694 C > T (Thr565Met). Allele-specific polymerase chain reaction confirmed that the mutations were located on separate alleles. No mutations were identified in CNGB3.Conclusion: This is the second reported case of CNGA3 associated OT. Mutations in CNGA3 have previously been associated with incomplete and complete achromatopsia. This report confirms that OT forms the mildest end of the spectrum of CNGA3 related diseases.