Achromatopsia 3

A number sign (#) is used with this entry because of evidence that the form of recessive achromatopsia present in high incidence among Pingelapese islanders, here designated achromatopsia-3 (ACHM3), is caused by homozygous or compound heterozygous mutation in the CNGB3 gene (605080), which encodes the beta subunit of the cone cyclic nucleotide-gated cation channel, on chromosome 8q21.

A form of achromatopsia designated ACHM1 was later found to be the same as ACHM3, caused by mutation in the CNGB3 gene (605080.0002).

For a general description and a discussion of genetic heterogeneity of achromatopsia, see ACHM2 (216900).

Clinical Features

Brody et al. (1970) described in Pingelapese people of the eastern Caroline Islands in the Pacific, a severe ocular abnormality manifested by horizontal pendular nystagmus, photophobia, amaurosis, colorblindness, and gradually developing cataract. From 4 to 10% of Pingelapese people are blind from infancy. Segregation analysis and equal sex distribution supported recessive inheritance. The high gene frequency was attributed to reduction in the population to about 9 surviving males by a typhoon (about 1780), combined with subsequent isolation. Whether the disorder is a form of congenital achromatopsia or a tapetoretinal degeneration with primary involvement of the cones was not clear. Carr et al. (1970) studied the same group and concluded that it is congenital complete achromatopsia. The impression of tapetoretinal degeneration was based, they thought, on severe myopia which was found in a majority of the affected persons. The disorder is nonprogressive. Compare retinal cone degeneration (180020). Maumenee (1977) concluded that Pingelapese blindness is distinct from total colorblindness (216900), mainly because of the consistent concurrence of severe myopia in the Pingelapese disease. Refraction is usually normal in total colorblindness.

Pentao et al. (1992) described a 20-year-old white female with rod monochromacy who presented with short stature (less than 5th percentile), mild developmental delay, premature puberty, small hands and feet (length less than 5th percentile), and a history of 3 consecutive first-trimester miscarriages. Cytogenetic analysis showed a 14;14 Robertsonian translocation; haplotype analysis was consistent with maternal isodisomy for all portions of chromosome 14 tested by SNP markers. The finding suggested that a form of rod monochromacy, designated ACHM1, mapped to chromosome 14; however, Wiszniewski et al. (2007) restudied this patient and identified homozygosity for a 1-bp deletion in the CNGB3 gene (1148delC; 605080.0002).

Using optical coherence tomography (OCT), Varsanyi et al. (2007) examined in vivo the anatomic structure of the retina in patients with achromatopsia and controls. In patients with achromatopsia, statistically significant reductions were found in total macular volume and in the thickness of the central retina compared with controls. Varsanyi et al. (2007) stated that a possible reason for the structural alteration is the qualitative and/or quantitative disorder of the cone photoreceptors, as the morphologic change is most expressed in the foveola.

Lee et al. (2015) studied retinal development in 10 children with achromatopsia (8 with ACHM3 and 2 with ACHM2) in comparison with 59 age-, gender-, and race-matched controls. Longitudinal data were available for 7 of the patients, with mean follow-up of 18.9 months (range, 5.3-35.5). In all of the participants with ACHM, there was evidence of foveal hypoplasia at each visit on OCT examination. A delay in migration of the photoreceptors into the central fovea was evident in the youngest patients. There was evidence of photoreceptor disruption (ellipsoid disruption and/or a hyporeflective zone) in the ACHM group, with variable severity. Retinal development occurred at a reduced rate and magnitude in children with ACHM in comparison with controls, with consequences for all retinal layers.

Inheritance

Achromatopsia-3 is an autosomal recessive disorder (Sundin et al., 2000).

Mapping

Winick et al. (1999) performed a genomewide search for linkage in 3 Pingelapese kindreds with achromatopsia. A 2-step search was used with a DNA pooling strategy, followed by genotyping of individual family members. Genetic markers that displayed a shift toward homozygosity in the affected DNA pool were used to genotype individual members of the kindreds, and an achromatopsia locus was identified at 8q21-q22. A maximal multipoint lod score of 9.5 was observed with marker D8S1707. Homozygosity was seen in 3 adjacent markers (D8S275, D8S1119, and D8S1707), whereas recombination was observed with the flanking markers D8S1757 and D8S270, defining the outer boundaries of the disease locus that spans a distance of less than 6.5 cM.

Milunsky et al. (1999) used linkage mapping to locate a gene for achromatopsia on 8q in a kindred with Irish ancestry. Five of 12 sibs were affected. A maximal multipoint lod score of 3.283 was observed near marker D8S271. Milunsky et al. (1999) raised the possibility that sailors of English/Irish descent introduced the 8q gene to the South Pacific Island of Pingelap.

Molecular Genetics

To refine the position of the ACHM3 locus by homozygosity mapping, Sundin et al. (2000) genotyped 60 affected individuals and narrowed the disease locus to a 1.4-cM interval, estimated at 2 Mb. Sundin et al. (2000) found that the genetic basis of Pingelapese achromatopsia at 8q21-q22 is a recessive point mutation in the CNGB3 gene that changes serine at residue 435 to phenylalanine in a highly conserved site in the S6 membrane-spanning domain (605080.0001). Two brothers in 1 family were found to be compound heterozygotes for 2 small frameshift deletions (e.g., 1148delC). The findings established that classic achromatopsia results from a complete loss of CNGB3 function and that this gene is not required for vital processes outside the visual system. Thus, CNGA3 and CNGB3 encode the alpha and beta subunits of a single cyclic nucleotide-gated channel that is located in the photoreceptor plasma membrane and is essential for the generation of light-evoked electrical responses in the red-, green-, and blue-sensitive cones.

Wiszniewski et al. (2007) analyzed the CNGA3, CNGB3, and GNAT2 genes in 16 unrelated patients with autosomal recessive ACHM: 10 patients had mutations in CNGB3, 3 had mutations in CNGA3, and no coding region mutations were found in 3 patients. The 1148delC mutation accounted for 75% (18/24) of disease-associated alleles and was found in 10 patients, including a homozygous female with achromatopsia previously reported by Pentao et al. (1992), who also had systemic features associated with maternal uniparental disomy (UPD) for chromosome 14. Analysis of intragenic SNPs in unrelated patients revealed transmission of a common haplotype consistent with a founder effect for the 1148delC mutation. Wiszniewski et al. (2007) concluded that CNGA3 and CNGB3 mutations are responsible for the substantial majority of achromatopsia.

Population Genetics

Among 798 individuals of south Asian origin, Lazarin et al. (2013) determined that the carrier frequency for achromatopsia caused by CNGB3 mutation was approximately 1 in 24. Among 15,798 ethnically diverse individuals screened, 162 carriers were identified (1%), for a carrier frequency of approximately 1 in 98.

Animal Model

Sidjanin et al. (2002) identified mutations in the canine homolog of CNGB3 in Alaskan malamutes and German shorthaired pointers affected with cone degeneration.

History

For a popular account of the Pingelapese people, see 'The Island of the Colorblind' by Oliver Sacks (1997).