Causes of Retinitis Pigmentosa
An estimated 100,000 people in the U.S. are living with the progressive, genetic eye disease retinitis pigmentosa (RP). RP is primarily caused by mutations (variations) in a gene inherited from one or both parents. The altered gene gives the wrong instructions to the eyes’ photoreceptor cells, telling them to make an incorrect protein or the wrong amount of protein. This imbalance causes the cells to malfunction. Mutations in dozens of genes have been associated with RP.
Genetic mutations can be passed from parent to child through one of three genetic inheritance patterns: autosomal recessive, autosomal dominant, or X-linked.
Autosomal recessive RP
In autosomal recessive RP, both parents carry one mutated gene and one normal gene, and have no symptoms themselves. In these cases the parents are called unaffected carriers. Each of their children has a 25% chance of being affected from the inheritance of a mutated gene copy from each parent. If the child only inherits a mutated gene from one parent, they will in turn be an unaffected carrier.
Autosomal dominant RP
In autosomal dominant RP, only one parent is usually affected and is the only parent with a mutated copy of the gene. A child has a 50% chance of being affected by inheriting the mutated gene. It’s unclear what role the genes of the unaffected parent play in the inheritance of the disease.
In X-linked RP, the mutated gene is located on the X chromosome. Females have two X chromosomes and can carry the disease gene on one X chromosome.
Since they have a healthy version of the gene on their other X chromosome, women who are carriers are less commonly affected. As males have only one X chromosome, they are more genetically vulnerable to X-linked diseases.
Women who carry the X-linked gene mutation have a 50% chance of passing that mutation to their daughters, who in turn become carriers, as well as a 50% chance of passing the mutation to their sons, who are likely to be directly affected by the condition. Men pass their Y chromosome to their sons, so won’t pass an X-linked disease to a male child.
Once a family member is diagnosed with RP, it is often recommended that other family members have their eyes examined by a specialist trained to identify and treat degenerative retinal disorders.
Occasionally, RP can result from other genetic conditions, medication side effects, infections, or eye injuries, but these occasions are exceedingly rare.