Bietti’s Crystalline Dystrophy
Bietti’s crystalline dystrophy (BCD) is a rare autosomal recessive eye disease that appears to be more common in people with Asian ancestry compared to any other race or ethnicity. Depending on how the disease affects a person’s eyes, BCD is also known as Bietti crystalline corneoretinal dystrophy (BCCD), Bietti crystalline retinopathy (BCR), or Bietti tapetoretinal degeneration with marginal corneal dystrophy. In this disorder yellow or white crystal-like deposits of lipid (fat) accumulate in the retina. The deposit damages the retina, which leads to progressive vision loss and potentially blindness. The disease was named after Dr. G. B. Bietti, an Italian ophthalmologist who first described the symptoms in 1937.
Inheritance of Bietti’s Crystalline Dystrophy
To understand how a disease is inherited, it is important to understand what a gene is and its role in metabolic and molecular cell activities. A gene is a molecular unit of an organism’s heredity that codes for specific genetic traits, such as hair color and height. Each person receives one allele of the gene from his or her mother and the other from his or her father. Both alleles will determine the genetic trait. If one allele or both alleles are damaged or mutated, this can result in an abnormal trait or disease. Bietti’s crystalline dystrophy is an autosomal recessive disorder, which means the offspring must inherit the defective or mutated allele from both parents. The disorder occurs in 1 of 67,000 people and is more common in people of East Asia, especially those of Chinese and Japanese descent (Okialda et al., 2012).
Cause of the Disease
Bietti’s crystalline dystrophy is caused by the mutation of the CYP4V2 gene, which is located on chromosome 4. The CYP4V2 gene codes for members of the cytochrome P450 enzymes, which are known for breaking down various chemicals in cells. Specifically, the CYP4V2 enzyme is involved in fatty acid oxidation, which breaks down lipids (fatty acids). Even though the specific mechanism is not well understood, it is estimated that the dysfunction of the CYP4V2 enzyme in breaking down lipids affects the accumulation of lipid deposits in the retina of the eye.
Bietti’s crystalline dystrophy is characterized by the accumulation of crystals in the cornea (which covers the eye) and yellow, shiny deposits on the retina. This leads to atrophy of the retina, choriocapillaries, and choroid, which are associated with the back layers of the eye where the optic nerve is located. Patients with BCD can have some white blood cells that can be seen with crystalline deposits when viewed under the electron microscope. The crystalline deposits seem to harm only the vision. Progressive atrophy and degeneration of the retinal pigment epithelium lead to symptoms similar to other retinal degenerations like retinitis pigmentosa. Patients suffering from BCD can have reduced visual acuity, poor night vision, visual field loss, abnormal retinal electrophysiology, and often impaired color vision. Marked asymmetry between a person’s eyes is also common. Gradually, the peripheral visual field, central acuity, or both are lost, leading to blindness. The onset of the disease is usually during the second and third decades of life, but ranges can vary from early teenage years to beyond 30 years old.
Diagnosis and Treatment
The disease can be recognized by the yellow-white crystals on the retina, dysfunction of rod and cone photoreceptors on electroretinography (ERG), visual field defects, and reflective dots by spectral domain optimal coherence tomography (sdOCT). Rod and cone electroretinography tests the electrophysiology of the rod and cone cells of the retina. Persons with BCD often have reduced amplitude responses to no responses at all. Although an abnormal ERG is not required to diagnose BCD, an ERG is useful in determining the significance of the damage of the retina. To date, there is no treatment for BCD. Physicians work to reduce the damage to the retina but are not able to stop the progression of the dystrophy.
Screening and testing can be done to detect carriers of the defective gene. However, additional research is required to better understand the gene and its defect before new treatments can be developed to slow or stop the disease.
See also: Blindness; Color Blindness; Color Perception; Cones; Retina; Retinopathy; Rods; Visual System
National Eye Institute (NEI). (2009). Facts about Bietti’s crystalline dystrophy. National Eye Institute (NEI). Retrieved from https://nei.nih.gov/health/biettis/bietti
Okialda, Krystle A., Niamh B. Stover, Richard G. Weleber, & Edward J. Kelly. (2012). Bietti crystalline dystrophy. In Pagon, R.A., M. P. Adam, H. H. Ardinger, et al. (Eds.). GeneReviews® [Internet]. Seattle, WA: University of Washington, 1993—2015. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK91457/