by Dan Roberts
(Updated Aug 2008)
Cone-Rod Dystrophy (CRD) is an inherited progressive disease that causes deterioration of the cone and rod photoreceptor cells and often results in blindness. It can be found as an autosomal dominant trait, but it is usually acquired as autosomal recessive.
Symptoms of CRD are seen as decreased visual acuity in the early stages followed by loss of peripheral vision. The disease is similar to retinitis pigmentosa in this way, but there is no loss of night vision, the rate of rod and cone degeneration is equal, patterns of visual field loss are different, and the rate of rod ERG loss is significantly lower in CRD (Birch and Anderson)
There is currently no treatment or cure for CRD, but researchers have identified three genes which may eventually provide answers. Two of these are the CXR and ABCR genes. Mutations in the CXR gene cause interference in the development of embryonic photoreceptor cells (Cell, November 1997). Mutations in the recently-discovered ABCR gene (Allikmet et al.'97 Nat. Gen. 15:236-246) lead to lipofuscin accumulation in the retinal pigment epithelium (RPE). This buildup of fatty waste deposits in the RPE (as in Stargardt's disease) eventually starves the photoreceptor cells (Cell, July 1999).
Discovery of a third gene was announced in the August 2008 issue of Genome Research. Deletion in the canine gene NPHP4 (nephronophthisis 4, also known as nephroretinin), was shown to cause CRD in the standard wire-haired dachshund. Since the disease is common to both humans and canines, this discovery could lead to potential therapies. "In humans," according to the researchers, "mutations in NPHP4 have been associated with simultaneous eye and kidney disease." This study described "the first naturally occurring mutation in NPHP4 without additional kidney disease." (Study title: "A deletion in nephronophthisis 4 (NPHP4) is associated with recessive cone-rod dystrophy in standard wire-haired dachshund", Anne Caroline Wiik, et al.)
A cure may eventually come either from a process by which lipofuscin buildup can be blocked or from advances in gene replacement therapy. In the meantime, researchers have shown that people with CRD can help to slow down the progression of the disease by protecting their retinas from bright light (Weng at al. '99 Cell 98: 13-23). For more discussion on this subject, see "Stargardt's Patients Need Special Light Protection" in the MD Support Library.