CRDs (chronic retinal dystrophies) are inherited retinopathies of the pigmentary retinopathies group. They are less common than the primary peripheral retinopathies and macular dystrophies, but still affect more than 1/40,000 people. They usually result in legal blindness in early adulthood and are more severe than RP.

They are diagnosed by the history of night blindness in association with progressive loss of visual acuity during the day. The diagnosis is confirmed by fluorescein angiography and fundus autofluorescence (FAF) in which the central macula is usually free of lesions, while the outer and inner periphery have yellow flecks or a bull’s eye appearance. In addition, electroretinogram (ERG) shows a decrease in both rod and cone responses, with more prominent reduction of cones.

Most of the genes associated with CRDs encode proteins involved in the photoreceptor transduction process. In a few cases, mutations in these genes result in a phenotype similar to that of macular dystrophies and a small number of them cause Stargardt disease, a form of macular dystrophies that is also characterized by a ring of chromatic aberrations around the fovea.

Another group of genes are associated with a milder form of the disease. They include the genes coding for the protein PEXP and RPGR, which are related to opsin trafficking. Mutations in these genes are responsible for a number of autosomal dominant CRDs and some of the autosomal recessive RPs.

In the past, several other genes were considered to be responsible for CRDs but they were not proven to be by molecular genetic methods. However, the discovery of the mutations in RDS, ABCA4, CRX and GUC1A has changed the opinion about which genes are responsible for CRDs.

TACI-Crd2 is one of the cysteine-rich domains found in the TACI protein family. These domains are important for binding to ligands, such as APRIL and BAFF, in the TNF receptor complex. In addition, TACI-CRD2 is required for activation of the guanylate cyclase that initiates signaling in the cell.