June 18, 2013

Fragile X

Normal people have 23 pairs of chromosomes. One pair is called the sex chromosomes. Males have one X chromosome and one Y chromosome. Females have two X chromosomes. Each chromosome is made up of thousands of genes, all of which have a specific function. Many genes produce a single type of protein that helps the body to accomplish a specific function.

One of the many genes on the X chromosome is the FMR1 gene. This gene codes for (produces) a protein called the FMR1 protein which is important in intellectual development. Within this gene there are repeating sequences of DNA known as CGG repeats. Normal people have between 5 to 40 CGG repeats.

Fragile X is a condition in which the number of CGG repeats in the FMR1 gene is higher than normal. Those who have 55 to 200 repeats are referred to as Fragile X carriers. Approximately 1 in 250 females and 1 in 800 males may carry the gene for Fragile X. Most of these people are normal intellectually and do not know that they are carriers. Male carriers of Fragile X are more likely than female carriers to have Fragile X-associated tremor ataxia syndrome (FXTAS). FXTAS is a condition affecting balance, tremor, and memory in carriers over age 50. Female carriers of Fragile X are more likely to develop premature ovarian insufficiency or premature ovarian failure.

Fragile X carriers are at a higher risk of having children with a larger number of repeats causing Fragile X Syndrome (FXS). FXS occurs when there are more than 200 CGG repeats. When this happens the FMR1 gene is turned off and does not produce the FMR1 protein. FXS is the most common inherited mental impairment causing in mild cases learning disabilities and in more severe cases mental retardation, autism, and autism-like behavior. FXS symptoms can include physical characteristics, behavioral deficits, and delays in motor and speech/language development. Approximately 1 in 3,600 males and 1 in 4,000 to 6,000 females have mental impairment due to FXS.

Fragile X testing by preimplantation genetic diagnosis (PGD) can be performed. These are difficult cases since women who are carriers of Fragile X are more likely to have diminished ovarian reserve and thus less likely to respond appropriately to ovarian stimulation resulting in fewer obtained eggs and fewer embryos available for testing.

If you are considering PGD or want to learn more, contact us today for a free initial consult.

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Joseph A. Hill, III, M.D.

Joseph A. Hill, III, M.D. Board-Certified in Reproductive Endocrinology and Infertility