What is CRB1 Degenerative Retinal Disease?
Mutations in the CRB1 gene can cause Retinitis Pigmentosa (RP), Lebers Congenital Amaurosis (LCA), or Cone-Rod Dystrophy.
Retina International gives a great description of RP on their website. Retinitis Pigmentosa (RP) is the name given to a group of degenerative diseases of the retina. The retina, at the back of the eye, is a thin sheet of interconnected nerve cells including the light sensitive cells (rods). It is here that light is converted into electrical signals to the brain where “seeing” takes place. In RP the rod and cone cells degenerate. Depending on the type of RP, the rate of progression varies.
Usually the rod cells are the first to be affected. They are concentrated away from the centre of vision in the retina and are responsible for seeing in dim light. Thus, one of the earliest symptoms is often night blindness followed by loss of peripheral vision leading to “tunnel vision”. Cone cells in contrast are concentrated in the centre of the retina and are responsible for brightly detailed color vision. In cases where the cone cells degenerate first, central vision becomes blurred and loss of color perception occurs. Peripheral vision is initially retained. Symptoms of RP are most often recognized in children, adolescents and young adults, with progression of the disease continuing throughout the individual’s life. Although the majority of people with RP do not suffer from associated disabilities, it does happen. Deafness associated with RP in a condition known as Usher syndrome is one such disability.
Lebers Congenital Amaurosis
LCA CRB1 is a rare inherited eye disease. It is an autosomal recessive disease meaning that both parents of the affected child are carriers of the gene mutation. There are approximately 300 LCA CRB1 cases in the US (representing 10 % of the 3000 cases of LCA), but likely there are more that have not been genetically proven as of yet. LCA is the most severe case of a classification of early onset retinal degeneration (Retinitis Pigmentosa). Most children with LCA CRB1 are born with ‘some’ functional vision but they are still considered ‘legally blind’ which means that functionally, they are blind. There is also a range of visual acuity among those with LCA CRB1. Most children with LCA CRB1 are born with nystagmus (‘dancing’ and wandering eyes), the tendency to ‘rub’ or ‘press’ their eyes, and poor eyesight in general. Typically, they have less than 10% of normal vision, and wear glasses as a baby. Since the rods in the eyes stop working first (rod cone dystrophy) seeing at night is especially difficult, as well as seeing to the sides (peripheral vision) and seeing things in motion. Most children with LCA CRB1 will have trouble moving from bright lights outside, to dim lights indoors. Children also lack depth perception and cannot ‘see’ stairs unless each step is brightly painted, more often they will ‘tap’ each stair with their foot to feel it. Children are not able to ‘see’ their family member or friends if they are standing to their left or right. Standing in front of a child with LCA CRB1 is best. Children also cannot catch a ball even if they can throw one well.
Although most children have some ‘functional’ vision at birth, LCA CRB1 is a degenerative condition. The rate of that degeneration varies within the group. Some children will experience a rapid loss of vision, while others may maintain their level of visual acuity for a while before experiencing deterioration. Most children are Braille users and cane users. Even for children with good functional vision it is important to anticipate the decline in future vision by teaching them at an early age ‘blindness-related skills’ such as Braille and using a cane.
From the National Institutes for Health www.nih.gov: Cone-rod dystrophies (CRDs) are a group of inherited eye disorders that affect both the cone and rod cells of the retina (photosenstitive receptor cells). In contrast to rod-cone dystrophies, individuals experience deterioration of the cone cells more severely than the rod cells. Initial signs and symptoms typically include decreased visual acuity when looking straight ahead (central vision loss); loss of color perception; and an abnormal sensitivity to light (photophobia). These signs are usually followed by progressive loss of peripheral vision and night blindness. Most cases occur due to mutations in any one of several genes, and CRDs can be inherited as autosomal recessive, autosomal dominant, X-linked or mitochondrial (maternally-inherited) traits.CRDs are usually non-syndromic, but they may also be part of several syndromes. Currently, there is no therapy that stops progression of the disease or restores vision; management aims at slowing the process, treating complications and helping individuals cope with the social and psychological impact of blindness.