Retinitis pigmentosa (RP) is an inherited disorder that causes progressive vision loss by the damage that is caused to the retina. There are greater than 100 genetic mutations known to affect the inheritance of RP. Family history determines the mode of inheritance of retinitis pigmentosa, which describes the way a genetic disorder or trait is passed on from one generation to another.
RP can be inherited by a number of genetic abnormalities, which include: autosomal dominant, autosomal recessive or an X-linked manner.
Autosomal dominant RP
It is helpful to understand the nature of the genetic code of the human body as it determines every single characteristic of our body, and each characteristic has two genetic components. If one of these components is defective, the body usually uses the healthy component to override the defective one. However, in the event of dominate inheritance, the defective gene will prevail and cause the person to have retinitis pigmentosa.
Typically, when the dominate gene appears in a family, any family member may have retinitis pigmentosa. One parent will have RP and is the only parent that has the defected or mutated gene. Children have a 50 percent chance of inheriting the mutated gene and developing this disorder. The affected children will manifest the disease and also be capable of passing it on to their children. Even though there is a 50 percentage chance of inheriting this disease, it does not mean if the first child has the disease that the next child is safe. Each child has the same risk.
Autosomal recessive RP
A person may have the mutated gene and have no disease. If two parents each carry the mutated gene and both are free of disease, they may very likely have a child who will manifest retinitis pigmentosa since both parents are carriers of the recessive gene.
Each of their children has a 25 percent chance of inheriting two normal genes. These children will be spared the disease, and they also will not be carriers. Additionally, each child has a 25 percent chance of inheriting two defective genes. In this case, the child would have retinitis pigmentosa and also be a carrier. Each child in this family also has a 50 percent risk of inheriting one normal gene and one defective gene, which means they will be carriers just like their parents, but they will not have the disease.
Again, these percentages can be misleading, as all of the children have the same chances of receiving normal or defective genes.
Approximately one person in 80 people in the United States are carriers of this recessive gene, which also means the chance of marrying a person that is a carrier is one in 80. There is no test available at this time to determine if a person is a carrier of a recessive gene.
X-linked RP
This is another place where the genetic code plays a role. Each person is born with 46 chromosomes, half from each parent. There are two types of code that determine the sex of the individual, the X and Y chromosomes. Females always have two Xs, and males have an X and a Y. Sons are produced when the father passes on the Y chromosome, and daughters are produced when the father passes along the X chromosome. The genetic code that produces RP is always found on the X chromosome. This is the reason this disease is sometimes referred to as the “X-linked”.
Both a daughter and a son each have a 50 percent chance of getting the X chromosome with the defective gene or getting a healthy X chromosome. Daughters who inherit the defective chromosome usually carry the gene but do not get the disease. If the daughter does get the disease it is usually a milder case. Boys have a 50 percent chance of having RP, and they can pass it along to their daughters.
Rare Digenic Forms Causing PR
This is a rare genetic disorder where people have two types of mutations that cause RP. One is in the RDS gene, and there is also a mutation in the ROM1 gene. The RDS gene affects a protein in the retina. The ROD1 gene is also a protein found in the photoreceptor cells.
Genetic counseling is often used when someone is in the process of diagnosis to clinically confirm the disease. There are such a large number of mutations, that science is not able to always identify all of them.
In the near future, the hope is that 95 percent of the cases will have the underlying mutative gene identified. New strategies are being used, and great strides are being made with regard to understanding the genetic component of this disease.
