Norrie disease is a rare X-linked genetic disorder which causes males to be totally blind at birth or become blind in both eyes at a young age. Most males with Norrie disease experience hearing impairment and up to half of males with the disorder also have developmental delays, mental retardation or behavioral abnormalities. Norrie disease runs in families because it is passed down through one of the chromosomes that determines gender (X-chromosome).
Otherwise healthy mothers may carry a mutated gene which causes the disease. These mothers may pass the gene on to their sons, who then develop the disease. Some sons, however, will not receive the gene and will grow up to be completely healthy. There is a 50% statistical risk of a carrier mother passing on the mutated X-chromosome.
A daughter may also receive the mutated gene if her mother is a carrier. Again the statistical risk to the daughter of inheriting the mutated X-chromosome is 50%. If the daughter inherits the Norrie gene, she will become a carrier, and like her mother, have a 50% chance of passing the gene on to her own children. Because the risk of inherited the disease mutation in a daughter is 50%, half of the daughters of carriers do not even inherit the gene. These females will be completely normal and will not pass the Norrie gene on to their children.
Sometimes males with Norrie disease have children. No sons of these men will inherit the gene and, therefore, can not develop the disease. Daughters of a father with Norrie disease, on the other hand, will all be carriers of the mutated gene.
It is possible, but extremely rare, for Norrie disease to occur by spontaneous gene mutation. In this case, neither parent carries Norrie mutation and genetic risk in this family is limited to offspring of the person with the gene mutation [male or female].
Norrie disease is diagnosed if an individual has the symptoms suggesting this disorder [primarily congenital blindness]. Most often the Norrie gene mutation can be identified by DNA analysis in the affected individual. Mutations in the Norrie gene are often unique to a family and have been described throughout the extent of the Norrie gene. About 15-20% of mutations are a gene deletion involving at least some part of the Norrie gene and sometimes extending out beyond the Norrie gene to affect other adjacent genes.
Clinical genetic lab testing is available to help make the diagnosis, assess genetic risk in family members and for prenatal testing. Although Norrie disease itself does not seem to shorten lifespan, individuals with blindness, deafness and/or mental retardation may have a reduced lifespan as a result of these conditions. Norrie disease affects each individual differently, even within the same family, as there is a spectrum of symptoms and severity.
Patients who have not completely lost their vision may be treated with surgery or laser therapy in infancy. Hearing loss can be treated with hearing aids and cochlear implants. Behavioral abnormalities and retardation can be treated through counseling, medications, and care by special education professionals. Men with Norrie disease may need varying degrees of assistance from family, friends and caretakers, but lead full and rewarding lives.
Research continues to try and help us understand how these genetic mutations cause the clinical features of Norrie disease and to understand the biologic problems that lead to blindness, risk of hearing loss and those factors that may affect cognition and behavior. The hope is that in the future, with increased understanding, we will better know how to improve the clinical symptoms and support the patients and families affected by Norrie disease.