Achromatopsia caused by novel missense mutations in the CNGA3 gene

Abstract

Aim: To identify the genetic defects in a Chinese family with achromatopsia.

Methods: A 2.5-year-old boy, who displayed nystagmus, photophobia, and hyperopia since early infancy, was clinically evaluated. To further confirm and localize the causative mutations in this family, targeted region capture and next-generation sequencing of candidate genes, such as CNGA3, CNGB3, GNAT2, PDE6C, and PDE6H were performed using a custom-made capture array.

Results: Slit-lamp examination showed no specific findings in the anterior segments. The optic discs and maculae were normal on fundoscopy. The unaffected family members reported no ocular complaints. Clinical signs and symptoms were consistent with a clinical impression of autosomal recessive achromatopsia. The results of sequence analysis revealed two novel missense mutations in CNGA3, c.633T>A (p.D211E) and c.1006G>T (p.V336F), with an autosomal recessive mode of inheritance.

Conclusion: Genetic analysis of a Chinese family confirmed the clinical diagnosis of achromatopsia. Two novel mutations were identified in CNGA3, which extended the mutation spectrum of this disorder.

Keywords: achromatopsia; genetic analysis; missense mutation.

Figure 1. A schematic model of the CNGA3 polypeptide

Cylinders imply helical structures in the protein; S1 to S6 denote transmembrane helices; dots represent the novel mutations located in functionally important domains.

Figure 2. Color fundus photographs of the posterior pole of the affected patient

A: The right eye; B: The left eye. No specific findings are seen in the optic disks and maculae. OCT images through the fovea (C: Temporal to nasal in the right eye; D: Nasal to temporal in the left eye) show neither macular atrophy nor cystoid lesions.

Figure 3. The pedigree of the family with autosomal recessive ACHM

The solid square indicates the proband who carries the two compound heterozygous mutations, p.D211E and p.V336F. The unaffected parents are heterozygous carriers of either the V336F allele or the D211E allele. Accordingly, 100% concordance of genotype and phenotype was observed.

Figure 4. Sequence analysis and identification of the novel CNGA3 mutation in the affected Chinese family

Wild-type sequence of part of exon 6 (left panel) and exon 7 (right panel) in CNGA3 gene (top row) and corresponding sequence of the affected individual (the second row), and his parents (the third and bottom rows) are shown. The arrows indicate the sites of nucleotide substitution. In the proband, two heterozygous variations, c.633T>A and c.1006G>T, are revealed, resulted in the p.D211E and p.V336F mutations. While the parents are obligate carriers of either of the two variations.

Figure 5. Multiple protein sequence alignment of CNGA3 in different species

Arrows indicate the mutated aspartic acid 211 (A) and valine 336 (B) residues which are evolutionarily highly conserved. The polypeptide sequences are NP_001289.1 CNGA3, Homo sapiens; XP_001156943.1 CNGA3, Pan troglodytes; XP_001101944.2 CNGA3, Macaca mulatta; NP_776704.1 CNGA3, Bos taurus; NP_034048.1 CNGA3, Mus musculus; NP_445947.1 CNGA3, Rattus norvegicus; NP_990552.1 CNGA3, Gallus gallus.